COMPARATIVE STUDY OF MICROWAVE AND CONVENTIONAL

SINTERING OF GLASS METAL COMPOSITES

_______________

A Thesis

Presented to the

Faculty of

San Diego State University

_______________

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

in

Mechanical Engineering

_______________

by

Gajendra Pendse

Spring 2011

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Copyright © 2011

by

Gajendra Pendse

All Rights Reserved

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DEDICATION

This thesis is dedicated to my father, who taught me that the best kind of knowledge

is that which is learned for its own sake. This work is dedicated to my mother, who taught me

that even the largest task can be accomplished if it is done one step at a time. It is also

dedicated to my sister who taught me that belief comes first.

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ABSTRACT OF THE THESIS

Comparative Study of Microwave and Conventional Sintering of Glass Metal Composites

by Gajendra Pendse

Master of Science in Mechanical Engineering San Diego State University, 2011

Sintering is a useful technology in the present world. The applications of the sintered products range from automotive and aerospace parts to the basic household parts. An attempt has been made to study the science of sintering through its practical application. This thesis aims to clarify some of the important issues related to two sintering processes namely conventional and microwave sintering processes. There have been few studies which have included comparative analysis of conventional and microwave sintering process.

The present study includes the study of microwave and conventional sintering of a composite made from glass and copper powders. The differences in the two processes are an object of study. Most of the sintering process parameters are discussed as wells as the properties of the sintered products.

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TABLE OF CONTENTS

PAGE

ABSTRACT ...............................................................................................................................v

LIST OF TABLES ................................................................................................................. viii

LIST OF FIGURES ................................................................................................................. ix

ACKNOWLEDGEMENTS ..................................................................................................... xi

CHAPTER

1 INTRODUCTION .........................................................................................................1 

1.1 Sintering .............................................................................................................1 

1.2 Fundamentals of Sintering .................................................................................2 

1.3 Conventional Sintering Process .........................................................................6 

1.4 Microwave Sintering Process ............................................................................7 

1.5 Composites .........................................................................................................8 

1.6 Sintering of Composites .....................................................................................9 

1.7 Sintering of Glass ...............................................................................................9 

1.8 Conventional Sintering of Glass-Metal Composites........................................11 

1.9 Microwave Sintering of Glass .........................................................................13 

1.10 Limitations of Existing Research Activities ..................................................15 

1.11 Research Objectives .......................................................................................17 

2 EXPERIMENTAL PROCEDURE ..............................................................................21 

2.1 Materials Used .................................................................................................21 

2.2 Equipment Used ...............................................................................................23 

2.3 Conventional Sintering Procedure ...................................................................29 

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2.4 Microwave Sintering Experimental Procedure ................................................34 

2.5 Density Measurement Technique ....................................................................40 

2.6 Characterization Process ..................................................................................40 

3 RESULTS ....................................................................................................................41 

4 COMPARATIVE ANALYSIS ....................................................................................51 

4.1 Density Comparison.........................................................................................56 

4.2 Microstructure Comparison .............................................................................58 

4.3 Process Duration Comparison ..........................................................................61 

5 CONCLUSIONS..........................................................................................................63 

6 SCIENTIFIC AND ENGINEERING NOVELTY OF THE CONDUCTED RESEARCH .................................................................................................................66 

7 FUTURE SCOPE.........................................................................................................67 

REFERENCES ........................................................................................................................68 

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LIST OF TABLES

PAGE

Table 2.1. Physical Properties of BSG Powder .......................................................................22 

Table 2.2. Electrical Properties of BSG Powder ......................................................................22 

Table 2.3 Chemical Durability of BSG Powder ......................................................................22 

Table 2.4. Particle Size Data ....................................................................................................23 

Table 2.5. Temperature Results Using Thermocouple ............................................................40 

Table 3.1. Results of Conventional Sintering Process (Sample 1) ..........................................42 

Table 3.2. Results of Conventional Sintering Process (Sample 2) ..........................................43 

Table 3.3. Results of Conventional Sintering Process (Sample 3) ..........................................44 

Table 3.4. Results of Microwave Sintering Process (Sample 1)..............................................45 

Table 3.5. Results of Microwave Sintering Process (Sample 2)..............................................46 

Table 3.6 Results of Microwave Sintering Process (Sample 3)...............................................47 

Table 4.1. Density Comparison ...............................................................................................52 

Table 4.2 Microstructure Comparison .....................................................................................53 

Table 4.3. Process Duration Comparison ................................................................................56 

ix

LIST OF FIGURES

PAGE

Figure 1.1. Sintering process. ....................................................................................................4 

Figure 1.2. Flowchart showing research objectives and research tasks. ..................................20 

Figure 2.1. Vacuum baking oven. ............................................................................................24 

Figure 2.2. Magneto stirrer equipment. ...................................................................................25 

Figure 2.3. Carver Hydraulic Press. .........................................................................................26 

Figure 2.4. Microwave sintering oven. ....................................................................................26 

Figure 2.5. NEY series 2 furnace. ............................................................................................27 

Figure 2.6. Theta dilatometer. ..................................................................................................28 

Figure 2.7. Infrared pyrometer. ................................................................................................29 

Figure 2.8. Scanning electron microscope. ..............................................................................30 

Figure 2.9. Microwave sintering setup with aluminosilicate refractory lining. .......................37 

Figure 2.10. Microwave oven and pyrometer alignment. ........................................................38 

Figure 2.11. Microwave oven and thermocouple setup. ..........................................................39 

Figure 3.1. Temperature profile for sample 1 (5 vol % of Copper). ........................................48 

Figure 3.2. Temperature profile for sample 2 (10 vol % of Copper). ......................................48 

Figure 3.3. Temperature profile for sample 3 (20 vol % of Copper). ......................................49 

Figure 3.4 Dilatometer shrinkage curve for sample 1 (5 vol % of Copper). ...........................49 

Figure 3.5. Shrinkage curve for sample 2 (10 vol % of Copper.) ............................................50 

Figure 3.6. Shrinkage curve for sample 3(20 vol % of Copper). .............................................50 

Figure 4.1. Microwave and conventional sintered chart comparison. .....................................58 

Figure 4.2. Relative density vs. composite concentration of Cu by volume (%). ...................59 

x

Figure 4.3. Temperature vs. time for microwave and conventional sintering processes. ........62 

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ACKNOWLEDGEMENTS

I wish to thank my advisor Dr. Eugene Olevsky for the support and direction that he

has given me. I would also like to acknowledge his guidance and support throughout the

duration of this research. I want to thank my fellow lab members for all their help and

support. I would also like to thank Dr. Sam Kassegne and Dr. Ege Engin for serving on this

graduate committee.

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CHAPTER 1

INTRODUCTION

The current research in the field of sintering is an important part of the research

activities aimed at the development of new materials. The main objective of this thesis can be

understood more clearly if the process of Sintering is known in its most basic and general

form. So let us start from the question what is sintering? Sintering is the technique of

applying thermal energy to metal and ceramic powders to produce high density products.

It involves the formation of the coherent bond between the particles of the material

powder as heat is supplied to the material powders. An important thing to note is that in solid

state sintering this bond is formed without the melting of the material involved in the process.

The ISO defines Sintering as “The thermal treatment of a powder or compact at a

temperature below the melting point of the main constituent, for the purpose of increasing its

strength by bonding together of the particles.” [1].

Sintering is a process in powder metallurgy which renders consolidated products.

There is still some debate over the fundamental governing factors of sintering process. This

process however does not always result in a fully dense finished product. Some secondary

treatment processes can be further applied to the end product.

1.1 SINTERING

The science of sintering process is accompanied by two major phenomena’s at any

given time and they are strengthening and densification. A powder compact after undergoing

sintering process has a better densified microstructure as well as good strength. Another

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question which might arise with respect to the importance of the uses of the sintered product

compared to other process products are the mechanical properties. The sintered product

increases mechanical and chemical qualities as well as durability due to which it can be used

for many purposes.

There are many ways to carry out the sintering process, different type of furnaces

give different type of heating. The amount of thermal energy supplied is usually below the

melting point of the powder which is to be processed. The thermal energy supplied can be

monitored and varied to give control over the sintered product. This independence to vary the

input conditions such as temperature as well as pressure benefits the overall sintering process

in turn increasing the quality of the product.

Sintering helps in reducing the porosity of the material powder used. Porosity is

defined as the ratio of the volume of all void spaces to the total volume of the compact. It is

highly desired that porosity remain as close to zero as possible. This helps to strengthen the

compact as the pores can reduce the strength of the compact. Another criteria to be

considered is the particle size. The total elimination of porosity with high inter particle bond

strength is the prime purpose and result of any sintering process.

1.2 FUNDAMENTALS OF SINTERING

The fundamentals that govern what really happens to the green compact undergoing

the sintering process has been a topic of research for many years. Green compact is the state

of the powder before it undergoes sintering process. Thus the particle behavior at and before

the sintering temperature is a cause of curiosity for the scientists. There have been many

theories developed over the years after the very first one developed by Ya. I. Frenkel [2].

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Ya. I. Frenkel established the first physical theory of sintering. His study concludes

that due to surface tension the powder particles adhere to each other resulting in decrease of

pores as well as consolidation of the powder particles. Other researchers such as B. Ya. Pines

[3]and G. C. Kucyunski [4] used the basic concept of Frenkel to explain diffusion driven

sintering. The atomic movements give rise to different mechanisms such as surface diffusion,

grain boundary diffusion, evaporation and condensation, plastic deformation.

Over the years the science of sintering has developed rapidly. The Sintering process

can be mainly characterized by the phase involved in the process. It can be liquid phase

sintering, solid state sintering, viscous flow sintering or transient liquid phase sintering. Also

when parameter such as pressure is varied during the sintering process the process is termed

pressure assisted sintering. Both the pressure assisted as well as pressure less sintering

processes have been used for the production of a large number of products.

Other process parameters include time, temperature and furnace atmosphere as shown

by Suk Joong L Kang [5]. Each have their own effect on the sintering process. Also the

overall output of the sintering process can be controlled by varying the material variables

such as particle size, particle shape, particle structure and particle composition and most

importantly green density. All these variables affect the sinter quality as well as the rate of

sintering.

Another important aspect affecting the strength of the processed powder product is

the size of the grains as shown in the study of M N Rahaman [6]. The grain size should not

be above a desired level to give better strength to the product. Further the physics of sintering

process has to be studied carefully to understand the science of sintering. The basic concepts

behind the sintering science are explained as we go further.

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The physics of sintering gives us a good understanding about what really drives the sintering

process. There are some driving forces which govern the sintering process for nearly any

system. They include particle surface curvature and applied pressure in case of pressure –

assisted sintering. The reduction in surface area causes a reduction in surface energy. This

results in the lowering of the free energy system thus causing the particles to adhere to each

other.

Neck growth is another important phenomenon in sintering. It is caused due to the

transfer of mass between two particles. The neck size at various stages affects the physics of

sintering process to a large extent. It can either expedite or slower the process. The grain

boundary and powder defects also affect the sintering process as well as sintered product.

Figure 1.1 shows the process of sintering in three stages. Particles are joined together by the

forces. In the second stage the neck growth is evident Substech [7]. We can see how the

porosity is eliminated in successive stages of sintering as shown in the Figure 1.1.

Figure 1.1. Sintering process.

Grain growth in the sintered product is highly undesirable. Grain growth negatively

changes the properties of the sintered material. Thus the essence of sintering is the increase

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of density but decrease of grain growth. Grain growth occurs due to the atomic motion across

grain boundaries. The nature of grain growth depends upon the nature of the curved surface,

whether it is a convex or a concave surface. Grain boundaries move towards the center of

curvature in convex surfaces. There are two ways to control the grain growth in a sintering

process. The grain growth can be reduced by addition of a dopant. Also it can be reduced by

reducing temperature.

Sintering is an irreversible process. The mechanisms of mass transport in sintering are

surface diffusion, lattice diffusion from the surface; lattice diffusion from grain boundary,

vapor transport, grain boundary diffusion and plastic flow. Some mechanisms such as

volume diffusion, grain boundary diffusion, plastic flow and viscous flow induce shrinkage

on the compact while others such as surface diffusion, evaporation condensation do not

induce shrinkage in the compact. During the sintering process different mechanisms are

involved at different point of time. The size of the particles involved also affects the

mechanism of sintering process.

Modeling of the existing sintering processes is extremely important. There have been

well established models for all the sintering processes. The models developed till now have

been 2 stage or 3 stage explaining the initial stage, intermediate stage and final stage of

sintering. Only some models include more than 3 stages. Some basic criteria for the modeling

of the sintering processes include the particle to particle interaction, grain growth,

microstructural development, pore sizes.

Sintering in the modern day can be carried out in many ways. Some of the ways to

achieve this process is by using a commercially available furnace, a microwave, spark

plasma sintering machine, selective laser sintering machine. Microwave sintering process is a

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very useful process; the microwaves used can range in frequency and power. Another way is

by conventional sintering method in which any commercially available furnace can be used.

Again the furnaces can vary in the temperature range. One of the most advanced sintering

machines is the spark plasma sintering machine. It makes use of a pulsed DC current which

is directly passed through a graphite die and the powder compact to be sintered

simultaneously. This is a very fast and a powerful approach. Another way of carrying out the

sintering process is with the help of a selective laser sintering machine. In this process a very

powerful laser is used to sinter the powder compact. Similar to selective laser sintering is the

direct metal laser sintering. Also rapid prototyping makes use of laser sintering.

1.3 CONVENTIONAL SINTERING PROCESS

As mentioned earlier, the sintering of a powder compact is only possible by applying

some thermal energy. When the source of this thermal energy is any type of commercially

available furnace, the process is termed as conventional sintering. Usually in conventional

sintering the temperature can be controlled as well as the shrinkage can be monitored. All

these controls help in achieving the maximum density, obtaining the required grain size of

the material powder is frequently a challenge.

In case of conventional sintering the heating of the powders causes the thermal

energy transfer via conduction. This process has been used to sinter almost all ceramic and

metal powders. In conventional sintering the heating of the material is only external and not

internal. The atmosphere used for the sintering process can also be controlled in the

experiments inside the conventional furnaces. Besides Air the different types of atmospheres

used can be Argon, Nitrogen, and Hydrogen etc.

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The conventional sintering process can be controlled by modifying various factors.

The controlling factors of the conventional sintering process are the material composition,

particle size distribution, temperature and time.

1.4 MICROWAVE SINTERING PROCESS

When the microwave energy is the source of the thermal energy needed to processing

of a powder, the technique is termed as microwave sintering. Microwave sintering is now

widely applied. The fundamental properties that drive the microwave sintering process are

still under research. However the basic common understanding is that the electromagnetic

energy of the microwaves is converted into thermal energy resulting in the internal heating of

the powder. It can be said that the heating of the powders is volumetric.

Usually a 2.45 Ghz microwave furnace is sufficient to sinter most of the material

powders available. The frequency of these microwave electromagnetic waves range from

400MHz to 60 GHz. Microwave susceptors are used as they rapidly couple with microwaves

and heat fast, thereby heating the powder to be sintered. These susceptors are often made

using silicon carbide material as it readily couples with microwaves and increases the initial

rate of heating. The atmosphere can also be controlled in the case of microwave sintering

process. The rate of sintering increases to a great extent because of the microwave effect.

Also densification can occur in a short period of time.

A proven capability of the microwave sintering process is the sintering of metal

powders. As we all know the metal powders are used in the aerospace, transport, defense and

other related areas. In the study of K Saitou [8] single mode microwave radiation has been

used for the sintering of iron, cobalt, nickel, copper and stainless steel powders. The purpose

of study was to observe the densification behavior of these materials.

8

The need for the better, faster densified as well as high toughness materials

incentivizes the usage of microwave sintered products. However till now there have not been

many efforts to model the microwave sintering process. As the relationship between the

metals and the electromagnetic waves are complex there have not been established any basic

principles as well as models of this process, hence the analysis of the process becomes

difficult.

As mentioned above, there have been few efforts in this area. David Clark and Diane

C Folz have studied the microwave processing of materials [9]. The study also includes the

microwave processing of glasses. Fundamentals of microwave processing technology have

also been discussed. Also modeling of the microwave sintering has been performed as shown

by a study by Buchelnikov, Luzgin, Anzulevich, Bychkov, Yoshikawa, Sato and Inoue [10].

There have been many findings with supported evidence that sintering in the metals

and ceramics is carried out by solid state diffusion process. Microwave Sintering of all

metals such as nickel, tin, titanium, aluminum, copper, steel, iron have been studied

individually. For instance the study of Kristen H. Brosnan, Gary L. Messing and Dinesh K.

Agrawal [11] describes the microwave sintering of pure alumina at 2. 45 GHz. In the study

sintering kinetics and microstructure evolution have been analyzed.

1.5 COMPOSITES

This study concerns the sintering of composites. Composites are combinations of two

or more materials with different physical and distinct chemical properties. Composites are

usually man-made materials. The logic behind using composites is simple and that is the

combination of the better qualities of different materials. The sintering of composites will

give a very densified, hard and chemically durable final product which can be used in many

9

different areas. Also composites are prepared to diminish poor qualities or the disadvantages

one or more material can have in the specific area of use. The next section describes research

work done in sintering composite powders.

1.6 SINTERING OF COMPOSITES

Sintering of composites has fascinated researchers for years as composites give a

mixture of the advantages of the both the component materials involved and a better final

product is achieved. Again the sintering of different metal combinations have been studied in

this area. The study of Agarwal, Cheng and Roy [12] involves the analysis of the sintering of

cermet’s. Cermet’s are combinations of ceramic and metal materials. There are many such

detailed analysis of the composite sintering according to their specific use in various areas.

1.7 SINTERING OF GLASS

Because of the great practical importance of the use of sintered glass, the research

study of the kinetics of the sintering of glass has been performed over the years. The

sintering of glass has been a subject of study on a few but consistent occasions in this field.

Sintered Glass materials are used for a variety of purposes individually as well as in the

composite form. The physics of sintering of glass has been studied for a long period of time.

G C Kuczynski first established the model for the process of sintering of glass [13]. The

study shows that the sintering of glass alone is possible by viscous flow mechanism. This

observation was made in the study for glass spheres. Kingery and Berg [14] showed that the

initial rate of sintering of glass powders is directly proportional to the surface tension and

inversely proportional to the particle size and the viscosity of glass. Mackenzie and

Shuttleworth [15] arrived at a similar result in their research involving the study of the

sintering of glass.

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Cutler and Heinrichsen [16] showed that glass particles having high specific surface

densify five times faster than the spherical glass particles. In another study [17] Czarwinski

showed that during the sintering of glass, the grains adhere to one another due to heat

treatment. The study shows that the process is carried out in two stages. In the first stage the

glass powders transform into a low strength porous materials. In the second stage density

becomes same as that of monolithic glass.

From the study of Shelyubskii and T. M Moiseeva [18], it is again very obvious that

the analysis of viscosity of glass is of very high importance. For the use in the electrovacuum

industry the study of sintering of glass shows that the viscosity analysis has been an

important criteria for the manufacturing of products in this industry .There have been few

studies of sintering of glass metal composites with shrinkage being one of the objectives of

the study. The study of Bocaccini and Olevsky [19] gives an account of the anisotropic

shrinkage of glass particles during the sintering process.

From the study of A. E. Shilo, E. K. Bondarev, and S. A. Kukharenko, [20], it has

been shown in their investigative research of low melting glass powders for the abrasive

industry that glass material can be used as a binder for these abrasive tools which are made of

superhard materials. There are few other areas in which the sintered glass products can be

applied but due to its brittle nature the usage remains limited. For this reason it is important

to carry research on composites involving glass.

Sintered glass usually lacks strength. Another limited property of glass is its low

ductility. This limits the use of the sintered glass in many industries and hence glass is used

in combination with many metals and ceramics to increase their use to a large extent. Glass

metal composites have their applications in many areas such as. The sintering of glass has

11

been improved as well as hindered the densification process by adding metal to glass. The

cause of failure is that densification is hard to achieve even in some metal-glass mixtures.

The chapter ahead gives good understanding of the research conducted till now in case of

microwave and conventional processes involving glass metal composites.

1.8 CONVENTIONAL SINTERING OF GLASS-METAL

COMPOSITES

Study of sintering behavior of glass along with Zirconia fibres has been performed by

M. J. Pascual, A. Dura´ n and L. Pascual [21]. The zirconia fibres have been added to

increase the mechanical properties of the glass composites. The corrosion resistance of the

composite is effectively increased making it a good sealing material. The equipment used for

the experimental process was a conventional electric furnace. Viscosity of sintered material

was another important aspect which was studied. The viscosity was high enough to avoid the

flow of the material. These are the perfect conditions for the sealing material. The use of

these composites in general involve joining and packaging of electronic devices, film

resistors etc.

The study of fracture characteristics of glass metal composites is done by DLouhy,

Boccaccini and Rheinish [22]. Using conventional sintering techniques the study shows the

sample preparation procedure as well as the study of the sintered microstructure and

mechanical properties of the metal –glass composites, the metal used was vanadium. Also

fracture behavior analysis was conducted in this study.

The study of compaction and sintering of glass alumina composites by Ray and

Tiwari [23] shows the investigation of the sintering of lead borosilicate glass after addition of

alumina. Conventional furnace was used for the sintering process to be carried out. The study

describes the rearrangement of particles due to the heating effect. Also the sintering

12

parameters such as relative sintered density and percentage of diametrical shrinkage have

been analyzed. Also the effect of sintering temperature and thermal expansion coefficient

have been studied. The microstructure of the sintered composites has been also the subject of

study.

In another study by Eberstein, Reinsch, Müller, Deubener and Schiller [24] sintering

of glass matrix composites with small rigid inclusions has been the subject of research. These

composites find their applications as co-fired ceramics. The study incudes the particle size

distribution. The sintering process was carried out in a conventional furnace. The effects of

the small rigid inclusions on the shrinkage of the sintered sample have been studied.

Another study by F. Beloivan, G. V. Isakhanov, I. D. Radomyselskii and N. I.

Shcherban [25] shows the analysis of the ductility of glass-metal materials. The study

concludes that glass activates the sintering process for the metal.

Research carried out by I. D Radomyselskii and N I Scherban [26] describes the

sintering process in glass metal composites. The study involves liquid phase sintering as the

glass softens and reacts with the metallic part triggering a sintering reaction. It has been

concluded that Glass activates the shrinkage process during the sintering process.

Relationship between sintering temperature and shrinkage has also been developed. Acid

content in the glass affects the sintering process. More acid content causes more shrinkage.

Vlasyuk, R.Z. along with I.D Radomysel’skii [27] also describes the behavior of glass

in the study of sintering of glass metal materials. The study was conducted with many

different type of glasses. The investigative analysis concluded that shrinkage occurs in two

stages. During the first stage shrinkage is brought about by the surface tension forces at the

metal and glass interface and glass promotes shrinkage. However during the second stage the

13

glass hinders the shrinkage process by internal pressure. This internal pressure can be

decreased by the increase in the porosity. Another conclusion has been made that oxygen

enhances the shrinkage of the glass-metal composites.

Our literature survey indicates numerous studies made in the area of the sintering of

glass- metal composites. The foci of these studies have been viscosity of the glass, the

behavior of glass, the overall shrinkage of the metal glass composites. Porosity has been also

a subject of study as it does affect the sintering process to a large extent. For most of the

studies the experiments have been carried out in a conventional furnace. So a significant

understanding of the kinetics of the sintering of glass metal composites has been achieved.

The next section describes the research carried out in the microwave sintering of glass metal

composites.

1.9 MICROWAVE SINTERING OF GLASS

The microwave sintering of metal powders has been introduced more than a decade

ago. For instance the research carried out by Roy, Agarwal, Cheng and Gedevanishvili [28]

included the microwave sintering of full metal powders. Microstructure analysis of the

sintered composites has been conducted by P. Veronesi, C. Leonelli, G. C. Pellacani and A.

R. Boccaccini [29]. This study was based on a unique microstructure obtained from glass

metal composites by microwave- assisted heat treatments. Gas evolution formation in the

glass metals has been thoroughly studied. Also the causes of the entrapment of the pores have

been analyzed and concluded that the pore shape was mostly spherical. All these factors give

rise to a unique microstructure evolution in the sintered composite.

The research of E. J. Minaya, A. R. Boccaccini, P. Veronesi c, V. Cannillo, C.

Leonelli [30] involved the conventional and microwave sintering of various glass-metal

14

composites. The glass used was borosilicate glass. The metals used in the research were

tungsten, molybdenum, tin, nickel, iron and aluminum. The powders were pre-pressed and

pellets were prepared for the sintering process experiments. The goal was to determine the

microwave heating parameters for the composites when they reach maximum densification.

Some composites such as those containing Iron densified extremely well. Others such as

Nickel and Tin could not be densified by microwave sintering. Also conventional sintering

was performed for similar composites.

In another study by E. J. Minaya, P. Veronesi, V. Cannillob, C. Leonellib, A. R.

Boccaccini, “ [31] microwave sintering of glass metal composites has been studied. Pore

analysis has been the main aim of this study. As the pores have a direct relation with the

density, the control of the pore size has been the subject of research in this study. The glass

used has been soda borosilicate glass. The results of the study show that high shock

resistance composites can be produced which are usable for thermal protection systems. The

fracture toughness is increased by the addition of the metallic fibers to the glass matrix. The

metals used in this case were Molybdenum and tungsten.

Another study by D. V. Louzguine-Luzgin, G. Q. Xie, S. Li, A. Inoue, N. Yoshikawa,

K. Mashiko S. Taniguchi M. Sato [32] on the sintering of metallic glasses, showed that

microwave heating considerably promoted crystallization and at the same time reduced the

crystallization temperature of these composites. The metal used was Iron, also in this case it

was seen that the sintering was carried out due to the softening of the metallic glasses. The

particles attached to each other at a very low temperature.

The research study in the area of metal fibre reinforced glass matrix using microwave

radiation by E. J. Minay, A. R. Boccaccini, P. Veronesi, V. Cannillo and C. Leonelli [33]

15

showed the usage of metal for raising the temperature of the glass. Similarly to earlier

remarks, metals helped in coupling with the microwaves and intensified the initial stage of

sintering. Another goal achieved was that the susceptors were not required in the microwave

heating of the composite material. Another study by L. Chen, C. Leonelli, T. Manfredini, C.

Siligardi, [34] shows the sintering of glass – ceramic composite by microwave heating.

1.10 LIMITATIONS OF EXISTING RESEARCH ACTIVITIES

The previous sections describe the research conducted in the area of conventional and

microwave sintering of glass metal composites. The following is an account of the

shortcomings of the research study in this area. These limitations serve as an incentive for

our research study in this area. There have been almost no comparative studies for both the

conventional and microwave sintering of glass-metal composites.

Some previous studies have analyzed the conventional sintering of glass metal

composites, while others have independently concentrated on microwave sintering of glass

metal composites. Very few have concentrated their research on both conventional and

microwave sintering of composites such as a study by E. Brevala, J. P. Chenga, D. K.

Agrawal, P. Gigla, M. Dennis, R. Roy, A. J. Papworth [35], which shows the comparison

between microwave and conventional sintering of W/Co composites. Different sintering

mechanisms have been compared in the research. The study of the kinetics of sintering can

be completed only by analyzing the process for the same composite material in both the

conventional and the microwave furnaces. The other limitation includes the scarcity of the

studies comparing densities of the final compacts by both processes. There have been very

few studies which have addressed this issue

16

The other important issue for comparison is the microstructure comparison. Again

very few studies have addressed this matter. The microstructure examination of two similar

compacts sintered by conventional and microwave process is essential. It gives a good

understanding into the qualitative analysis of the sintered properties of the glass metal

composites. After density and microstructure the next important parameter is the duration of

each of the processes, conventional and microwave. It is common knowledge that the

microwave sintering process takes less time than the conventional sintering process. It is

therefore obvious that microwave sintering is time saving but is needed to show whether this

present comparative study benefits. Further limitations in terms of the final product

properties include pore comparative analysis of the sintered powder compacts obtained by

conventional and microwave process.

Almost all the research has been concentrated on the sintering of various glass metal

combinations for one chosen concentration of the metal in the glass metal composites. Does

the metal concentration variation in the glass mixture exhibit any different sintered qualities

have not been thoroughly examined. The investigations of glass metal only by microwave

sintering cannot give us a conclusion as to whether the microwave sintering process is a

better option than conventional for the particular powder compact. Microwave heating

provides for rapid means of heating as compared to conventional sintering but the method in

which it heats the powder compact is still not very clear. There has been almost no defining

model of the microwave sintering process.

Two completely different thermal energy sources drive the microwave and

conventional sintering process so there has to be some final product difference. Also the

material heating itself is different due to the application of different heating sources. This

17

difference in heating can cause shape changes for one process and not for the other. This can

only be observed when the composite is sintered by both the microwave and conventional

furnaces and the results are compared. This has not been the topic of research in many

studies involving sintering of glass-metal composites.

The limitations of the previously conducted studies in this area show that there has

been a shortage of research involving comparative analyses of microwave and conventional

process of sintering with respect to the microstructure and density of the powder sample. The

other shortcoming in the previously conducted research is that the constitutive behavior of

the material could not be identified for microwave sintering. Taking into consideration all the

limitations of the conducted studies in this area we identify the purpose of the present

research project as comparative analysis of the Sintering of glass and copper composite in the

microwave and conventional furnaces.

1.11 RESEARCH OBJECTIVES

The main aim of this thesis is the comparative study of the sintering of the glass

copper composite with different copper concentrations processed in both microwave and

conventional furnace. The research objective of this study includes the analysis of the

specimen’s density evolution for the two different processes which can give a fair idea as to

which process is better in terms of sintering outcomes.

It is known that conventional heating takes longer time than microwave heating to

sinter the product completely. The quality of the sintered products however, should be

compared to determine the more efficient sintering process.

Another research objective is the microstructure examination. In depth microstructure

analysis of the sintered compact specimen obtained from both microwave and conventional

18

process has to be done. In a conventional furnace the added metals may somewhat increase

the heating rate of glass powders. In microwave furnace metals will help in generating a lot

of internal volumetric heating of the glass metal composite. The microstructure

characterization should be carried out by a Scanning Electron Microscopy. Also a

comparative analysis should be conducted in terms of the compact shape distortion and the

strength of the compact at the end of the process. To check for any defects in the final

sintered compact is of prime importance too.

The sample preparation for both microwave and conventional sintering process

should be the same in order to keep the green density as similar as possible. The glass used

for the research purpose is borosilicate glass as the sintering properties are well known for

this particular type of glass. Also a lot of research has been concentrated on borosilicate

glasses by Boccaccini and Co workers [23] where the characteristics of borosilicate glass

with reinforced metal particles have been studied. In this study the glass used is

commercially available Borosilicate glass. Usually the glass used for the study of sintering of

glass metal composites has been borosilicate glass due to its low melting point as well as a

low coefficient of expansion. Also it provides greater chemical durability. It is also thermal

shock resistant.

An additional reason behind using borosilicate glass for research purposes is that

there are very low material stresses induced due to small temperature changes. Hence it is

rather resistant to thermal fracture. The metal used in the present study is copper as it has not

been investigated before in glass-metal composite sintering.

The experimental procedure and the related practical considerations are described in

the further chapters. The composite composition used is by volume 5%, 10% and 20 % of

19

copper in the content of glass copper composites. As mentioned before very few studies have

taken a variation of concentration of the same metal in the metal glass composition into

consideration. This is the reason to conduct a thorough study of the effect of the

concentration of metal in the glass based composites. The particle size used for the

borosilicate glass is 22 microns, the particle size has been kept as small as possible as finer

grain size helps in better densification and effectively a better sintering process for both the

conventional as well as the microwave sintering.

All other material considerations have been kept the same for both the processes,

conventional as well as microwave. It is necessary for a comparative study to have same

usage of input, otherwise it becomes difficult to compare the process. The green compact was

prepared by uniaxial pressing for both the microwave as well as the conventional sintering

proceses.

The common applications found for these composites are the systems where high

corrosion resistance is required. They are also used for commercial thermal and electronic

applications. These composites are good materials for the electronic circuit boards. The

chapters further shall give a full and comprehensive data of the research carried out. The flow

chart in Figure 1.2 shows the research objectives and corresponding research tasks.

20

Figure 1.2. Flowchart showing research objectives and research tasks.

  Comparative Analysis of 

Sintered Density. 

Process Duration effects on 

the sintered glass‐metal 

composites. 

Micro‐structural analysis of 

the sintered glass‐metal 

composites. 

Performance of 

Microwave and 

Conventional Sintering 

Procedure.  

Density Measurement 

Using Archimedes 

Technique.  

Study the effect of 

Variation of 

Concentration on 

density of the glass‐Cu 

specimens.  

Measurement of 

process duration.  

Study the Impact of 

high and slow heating 

rates on the mass 

transport mechanisms 

of the sintered glass‐Cu 

composite.  

Study of the effect of 

time on the grain‐pore 

relationship.  

Characterization of the 

sintered specimens 

obtained from 

conventional and 

microwave sintering 

processes using an SEM 

Microscope.  

Comparative Analysis of 

the characterized 

specimens.  

 

RESEARCH OBJECTIVES  RESEARCH TASKS

21

CHAPTER 2

EXPERIMENTAL PROCEDURE

2.1 MATERIALS USED

The materials used for the sintering experiments are the commercially available

borosilicate glass powder and copper powder.

Borosilicate Glass Powder

German glassmaker Otto Schott was the first to develop borosilicate glass. Today

borosilicate glass is known as Pyrex as introduced by Corning Glass Works. The main

constituents of borosilicate glass powder are silica and boron oxide. The important property

of this glass powder is its resistance to thermal shock. It is known to have a very low thermal

expansion coefficient. The usage of borosilicate glass includes kitchen glassware, laboratory

test tubes and beakers. Glassblowing form of lampworking is another important usage of this

glass. Jewelry and telescope mirrors are also manufactured using this glass. In the

semiconductor industry the glass is used for the development of the micro-electrical

mechanical systems.

The type of borosilicate powder used is SCQ-3 and Table 2.1 shows the physical

properties The chemical composition of the used borosilicate glass powder was 70% silica,

10% boron oxide, 8% sodium oxide 8% potassium oxide, and 1% calcium oxide. The

electrical properties of the BSG powder used is shown in Table 2.2. The chemical Durability

of BSG powder is shown in Table 2.3. The mean particle size of the glass powder is about 20

microns as shown in the Table 2. 4. The mean particle size of the glass powder is about 20

microns as shown in the Table 2. 4. The thermal expansion is 55 x 10-7/ °C while Thermal

22

Table 2.1. Physical Properties of BSG Powder

Softening Point 710 ° C

Annealing Point 520 ° C

Strain Point 479 °C

Thermal Expansion (RT-300°C ) 55 x 10-7/ °C

Thermal Contraction (520-25°C) 71 x 10-7 /°C

Density 2. 308 gm/cc

Refractive Index 1. 49

Table 2.2. Electrical Properties of BSG Powder

Volume Resistivity, Log ρ ohm-cm @ 250 °C - 7. 2 @ 350 °C - 5. 7

Dielectric Constant (25°C @ 1 MHz) K -5. 8

Power Factor (25°C @ 1 MHz) Δ% - 0. 66

Table 2.3. Chemical Durability of BSG Powder

USP XVIII ml N/50 H2SO4 - 1. 3

SEALING RANGE 950 - 1000°C for 30 minutes

Contraction (520-25°C) 71 x 10-7 /°C, Density of the glass powder is 2. 308 gm/cc while the

refractive index of the powder is 1. 49. Borosilicate glass powders (BSG) used in the process

is from SEM-COM Company, Inc. The type of borosilicate powder used is SCQ-3. The glass

is a medium expansion glass which seals to tantalum. The following tables give a good idea

of both the physical and chemical constituents of the powders used for sintering purposes.

23

Table 2.4. Particle Size Data

% Tile Size (µm)

10. 00 3. 14

20. 00 6. 03

30. 00 9. 91

40. 00 14. 90

50. 00 20. 95

60. 00 27. 15

70. 00 33. 48

80. 00 40. 90

90. 00 52. 60

95. 00 66. 25

Copper Powder

The copper powder has been manufactured by Cerac Speciality Inorganics. It is

typically 99% pure. The particles of the powder are copper metal. The copper powder used to

make the composite compact has an average particle size of – 325 mesh.

2.2 EQUIPMENT USED

The process of sintering involves the use of various equipment including microwave

furnace, conventional furnace as well as the dilatometer. The other equipment’s used in the

process are the baking oven used to de-humidify the powders, the Carver hydraulic pressing

machine to prepare the green compacts. Magnetic stirrer used for preparing the mixture of

24

the powders. Also the scanning electron microscope (SEM) is used for characterization

purposes. All the equipment is briefly described below.

Vacuum Baking Oven

For baking the powders a commercially available vacuum baking oven is used. It is

utilized to reduce the moisture from the powder. The oven manufacturer is Precision

Scientific. The model used is 19. The temperature range is from 35 degree Celsius to 200

degree Celsius. Vacuum range of the model is from 1 atm. to 30 in. Hg (762 mm Hg). The

Figure 2.1 shows the vacuum baking oven used.

Figure 2.1. Vacuum baking oven.

Magnetic-Stirring Equipment

For the powder mixing process, magnetic stirring equipment from Econostir

Laboratory Craftsmen as shown in Figure 2.2 has been used. The magnets used are 30 gram

Alnico V Bar magnet. The power consumption of the equipment is 45 W, 120 volts, 60Hz.

25

Figure 2.2. Magneto stirrer equipment.

Hydraulic Press

For making a green compact of the composite material the Carver Hydraulic Press as

shown in Figure 2.3 has been used. The press model no is 3925. The Carver uniaxial pressing

uses the hydraulic force of a fluid for applying pressure to make a compact. The Press

consists of a 25 ton hydraulic unit and maximum Ram strike of six and a half inches.

Microwave Oven

For microwave sintering purposes a commercially available microwave oven as

shown in Figure 2.4 has been used. The microwave version is Panasonic Genius Sensor. The

oven uses inverter technology. This oven operates at a power of 1300 W. The oven is a

consistent source of microwave energy.

Conventional Furnace

For initial tests on the samples before the use of dilatometry a conventional furnace as

shown in Figure 2.5 has been used. The furnace is NEY SERIES 2 - 1350 single door

industrial oven. The furnace can go up to high temperatures of approximately 1600 degree

Celsius. It includes digital control.

26

Figure 2.3. Carver Hydraulic Press.

Figure 2.4. Microwave sintering oven.

27

Figure 2.5. NEY series 2 furnace.

Dilatometer

For conventional sintering experiments a dilatometer as shown in Figure 2.6 has been

used. The dilatometer has been manufactured by theta industries, Inc. It is a horizontal

dilatometer with temperatures up to 1600 degree Celsius. The dilatometer consists of the

following parts and accessories

1. Measuring Head

2. Specimen Holder

3. Furnace

4. Temperature Control

5. Signal Conditioner

6. Computer

7. Software

Thermocouple

For temperature measurement a K type thermocouple has been used. The

thermocouple is a rugged thermocouple from Omega Co. The model number and version of

28

Figure 2.6. Theta dilatometer.

the thermocouple is omega clad TJ36-CAXL-14U-24. The thermocouple can approximately

go upto a temperature of 1300 degree Celsius.

Infrared Pyrometer

Infrared pyrometer also has been used for temperature measurement. The infrared

pyrometer used is manufactured by Impac in collaboration with Mikron Infrared Inc is

shown in Figure 2.7. The pyrometer version used is MI-P 140. This pyrometer is a non-

contact infrared pyrometer.

Weight Measurement

The weight measurement for the entire project has been done using the weight

balance manufactured by OHAUS Corporation.

Scanning Electron Microscope

29

Figure 2.7. Infrared pyrometer.

For characterization Scanning Electron Microscope (SEM) as shown in Figure 2.8 has

been used. The Scanning Electron Microscope is manufactured by HITACHI co. The model

number is S 2700.

The other equipment used for the sample characterization preparation were the saw to

cut the sample, polisher and the mounting Press.

2.3 CONVENTIONAL SINTERING PROCEDURE

The conventional sintering experiments have been performed using the theta

dilatometer. The whole experimental process first involves trial tests and then the final

experiments. A commercially available NEY furnace has been used for the conventional

sintering trial tests. For performing conventional sintering experiments, the first step is to

bake the glass and copper powders separately in a baking oven. The baking oven is

essentially an oven which can create vacuum inside it with simultaneous heating the

30

Figure 2.8. Scanning electron microscope.

powders. The purpose is to remove any moisture out of the powders so it does not affect the

sintering process. The baking was carried out for around 2 hours at a temperature of about

200 degree Celsius and the powders were kept in the vacuum till their removal for the actual

experimental process before sample preparation. Also the powders cannot be mixed before

the baking process in the vacuum oven as they might reactively interact inside the oven due

to the substantial amount of heat supplied which is around 200 degree Celsius.

Once the glass and copper powders are baked individually inside the vacuum baking

oven, the next step of the experimental procedure to be performed is to prepare the green

sample pellets from the mixture of the two powders. To achieve this first both the powders

have to be mixed. For the mixture of finely distributed copper particles in the glass powder,

the copper and glass powder are first mixed together in volumetric ratio. The mixture

prepared for some trial tests and some final experiments contained 5 vol % copper, 10 vol %

copper as well as 20 vol % copper. The mixing of the powders was done in a glass beaker.

31

To ensure that the two powders will thoroughly mix, they were first mixed in a glass beaker

by hand with the help of a stirrer. Stirring by hand was done for about 5-7 minutes.

After stirring by hand the mixture of the two powders was subjected to the magnetic

stirring mechanism as shown in Figure 2.4. The mixing of powders is an integral part for the

proper sintering of any composite powder compact. Also to maintain a good quality of

sintering the mixing should be taken care of properly. In the magneto stirrer mixing

procedure, the two powders to be mixed together are put in a small plastic bottle which can

be closed with a cap on it. Two small cylindrical magnets are placed inside this plastic case.

The whole assembly is then placed on the top of a mounting which has access to electric

supply. As soon as the switch is turned on, the magnetic stirrers start to vibrate and rotate in a

clockwise direction mixing the powders. This process is carried on for duration of 30 minutes

to ensure thorough mixing of the powders.

The next step of the experimental procedure is the preparation of the pellet which is

going to be sintered. The compacts used in the conventional sintering process are prepared in

the Carver hydraulic uniaxial press machine as shown in Figure 2.3.

The usage of the Carver press is as follows. The mixture of the two powders which is

already prepared in a desired ratio is now poured into a die having a diameter of

approximately 12. 5 mm. After pouring the powder into the die, the punch is slowly pushed

inside the die while the outer sleeve is slid over the assembly of die and the punch. The

whole assembly is put in the press. Pressure of about ten thousand pounds is exerted by the

use of a handle. The action is performed till the desired level of pressure is achieved to

prepare a sufficiently solid compact. The pressure is applied for around five minutes. Then

slowly the pressure is released. Any sudden pressure changes would create irregularities in

32

the compact thereby increasing probability of cracking of the compact. The green density

obtained for the samples was around forty percent. The diameter of the pellets produced was

fixed and was same as the diameter of the die (12. 5 mm). The green density can be

calculated using the dimensions and the weight of the pellets. The pellets are of cylindrical

shape.

After releasing the pressure the compact is removed carefully from the die. Carefully

pressure is applied on the inner punch so that it pushes the compact out on to a soft surface.

In case of glass, due to its brittle nature care has to be taken as in some cases the compacts

produced are difficult to handle.

Conventional Sintering Experiments

In case of conventional sintering, first step was to perform tests in the conventional

furnace to know the melting temperature as well as to assess the sintering temperature. The

samples were then placed in a conventional furnace as shown in the figure below. This

furnace is a high temperature furnace from NEY.

The samples produced were put through trial tests in the NEY Series 2 furnace. The

furnace was programmed to heat the samples at the rate of 25 degrees Celsius per 5 min, so it

took more than 2 hours for the sample to reach high temperatures.

The samples were kept inside the furnace and the rate of heating was set at 25 degree

Celsius per 5 min. The next step was to hold the temperature at a temperature range of 600 to

735 individually for about 5 minutes. The samples were then allowed to cool. Inside the

furnace the sample was kept on an alumina base as shown in the figure. The alumina base

was kept on two refractory bricks for the maintenance of adequate thermal energy inside the

furnace. The alumina base is kept for the protection of the inner parts of the conventional

furnace in case of the melting of the compact. The density of these test samples was

33

measured using the Archimedes technique. The final sintering process of these samples was

carried out in the theta horizontal dilatometer.

The purpose of the pre sintering tests is to find a close range of temperature to the

melting point. It was observed that the glass from the composite starts to soften at a very high

rate at and above 700 degree Celsius. Softening point is not the melting point but close to the

melting point. So the closest temperature which could be achieved in solid state sintering was

650 degree Celsius. Similar samples were now ready to be sintered in the dilatometer.

A dilatometer is a scientific instrument for measuring thermal expansion or

contraction. In this device the samples to be sintered are placed in a horizontal or vertical

direction. Then as heat is supplied, the dilatometer measures the change in the volume of the

sample.

The actual experimental procedure to carry out the sintering process in the

dilatometer is discussed below. The specimen tube holder is the place where the sample is

located during sintering. The dilatometer which is used for the experiments is a horizontal

dilatometer as seen in the Figure 2.6. The specimen holder can slide inside the outer tube.

Now the sample is placed in the specimen holder at the end of the tube inside the cylindrical

casing. The push rod’s movement is corresponds to the linear shrinkage recorded by the

measuring head.

The push rod is arranged in such a way that it barely touches the sample. At one end

of the push rod a measuring head is located. This head has to be zero calibrated before

starting the experiment as it can give wrong shrinkage results if not properly checked. Then

the whole assembly is inserted inside the tube furnace. The joint of the two tubes is tightened

with a C clamp. The whole assembly is connected to a computer interface. The computer can

34

control the input temperature as well as get the output shrinkage from the dilatometer

equipment. The temperature controller takes care of the temperature management of the

entire process.

The whole temperature profile to be set as well as the experimental conditions can be

monitored using the program known as DILASOFT which is controlling this dilatometer. For

the experimental purpose the temperature profile was set to heat at a rate of 20 degree Celsius

for every five minutes upto 650 degree celsius, then holding the temperature at 650 degree

celsius for about 90 minutes and then cooling at a rate of 20 degree celsisus per five minutes.

For selecting the temperature profile, the heat increment, the holding time and temperature

and the cooling time are specified. The whole process was performed for different

concentrations of copper powder in the composite. As stated before firstly 5 vol % copper

was used in the composite mixture, then 10 vol % copper was used while finally 20 vol %

copper was made use of in the composite sintering.

After cooling the sample is carefully removed from the specimen holder. The density

measurements are conducted. The density determination is made with the help of

Archimedes technique which proves very beneficial. Further the sample is cut and

characterized with a scanning electron microscope.

2.4 MICROWAVE SINTERING EXPERIMENTAL

PROCEDURE

The experimental work for the microwave sintering process was performed in a 1300

Watt Panasonic microwave machine. Similar mixing technique has been used for the

microwave sintering experiments as the one previously explained regarding the baking and

mixing technique for the preparation of the powder compact in the conventional sintering

experiments. The glass and copper powders were first separately baked in a baking oven for

35

about 2 hours. The powders were then mixed by hand in a glass beaker. Further the powders

were subjected to the magnetic stirring. Finally after the mixing process was finished, the

powders were transformed into pellets of same size and diameter as prepared for the

conventional sintering experiments with the help of the Carver uniaxial hydraulic Pressing.

The pressure applied has been kept the same as before (ten thousand pounds.)

Before the experiments were performed in the microwave, it was necessary to carry

out tests in the microwave furnace to determine the point at which the sample melts. Test

runs were performed for the glass copper composites to determine approximately the heating

rate of these two materials in the microwave to be used. Also it was important to know at

which maximum temperature to sinter the composite. It was a challenge to measure the

temperature of the composite during sintering process in the microwave as the microwave

door obstructs the pyrometer measurement of temperature. It was important to maintain the

similar conditions which will are present during the actual sintering experiments, during

microwave sintering.

In the microwave sintering it was observed that the glass in the composite starts to

soften after 25 minutes. However the thermal energy produced inside the microwave furnace

depends largely on the placement of the silicon carbide susceptors as well as the use of

aluminosilicate refractory lining. Now comes the most important part of the procedure which

is the actual sintering experiments to be carried out inside the microwave oven. The

microwave oven used is a commercially available 1300 Watt with a frequency of 2. 45 Ghz .

The prepared compact is kept in the microwave at the center of the plate which is located

inside the microwave. The compact is now covered with a cylindrical aluminosilicate

refractory lining which helps to prevent the thermal energy from escaping outside. This

36

cylindrical lining has a hole on its side for the pyrometer to measure the temperature. It also

has a hole on the top of it for the thermocouple to measure the temperature of the compact

during pyrometer calibration.

As mentioned before the sample is placed inside the microwave oven on the plate as

shown in the Figure 2.9. It is surrounded by the aluminosilicate refractory lining. Two silicon

carbide susceptors are closely placed without touching the composite sample. The task of the

susceptors is to increase the initial rate of heating as they couple with the microwave readily.

These susceptors supply heat to the compact quickly to increase the thermal energy needed to

sinter the compact. Thus with an enclosed refractory lining and silicon carbide susceptors a

substantial amount of thermal energy is created inside the microwave for sintering the

compact. The sample is heated for about 20 minutes. The sample is sintered after a period of

twenty minutes as shown by the microstructure analysis. The sample is allowed to cool for

some time.

After cooling the sample is removed and further subjected to density measurement

and characterization process. The refractory lining helps in conserving the energy inside the

microwave. It functions as a heat energy leakage protector. Thus the maximum thermal

energy is concentrated on the compact. This is necessary for the rapid sintering of the

material compact. A number of compacts were processed and characterized to understand the

sintering process as well as the effect of heating time on the compacts. After sintering is

finished the sintered compact is then allowed to cool. First the compacts containing 5 vol %

copper were experimented with. Then the compacts containing 10 vol % copper were used.

Finally compacts containing 20 vol % copper were used.

37

Figure 2.9. Microwave sintering setup with aluminosilicate refractory lining.

Temperature Measurement For Microwave Sintering Processing

There are limitations for using a thermocouple during the actual sintering process as it

affects the sintering process. Hence an Infrared non contact pyrometer has been used to

record the temperature during the actual sintering process. But due to the limitations of the

pyrometer to record temperature through the tinted microwave door, the pyrometer has been

calibrated using a thermocouple. Therefore the temperature measurement for the microwave

sintering process has been carried out in 2 steps. In the first step pyrometer records

temperature of the compact during the actual sintering process. In the next step the pyrometer

is further calibrated with the help of a thermocouple.

Temperature Measurement Using a Pyrometer

The Pyrometer temperature recording process is briefly described as follows. The

pyrometer microwave assembly can be seen in the Figure 2.10 which helps to understand the

38

Figure 2.10. Microwave oven and pyrometer alignment.

actual temperature measurement process. As the microwave oven is turned on, heat starts to

build up inside the oven. Simultaneously the pyrometer starts reading the temperature of the

compact. Also before starting the experimental process, the lens of the pyrometer is focused

on the compact. The refractory lining has a small hole in it for the infrared pyrometer to

record the temperature of the heated surface of the compact. The hole on the refractory

lining is in alignment with the pyrometer lens spot and the compact as shown below in the

Figure 2.10. The pyrometer is connected to the computer to record the temperature. The

temperature profile is recorded by the computer for the samples sintered in the microwave

oven and can be captured from the computer using the pyrometer software.

Pyrometer Calibration using a thermocouple

The pyrometer does not give an accurate reading due to the microwave furnace door.

The pyrometer was further calibrated by using a thermocouple. For the purpose of

temperature measurement and pyrometer calibration an omega thermocouple was used. The

39

microwave oven consists of a small opening at the top of its body for thermocouple to be

inserted through it to measure temperature of the heated composite. But there is also a fixture

for the hole to be covered when the microwave sintering process is in progress.

The thermocouple was inserted from the top of the microwave furnace down to the

specimen through the hole in the aluminosilicate refractory lining. Also a part of the opening

was covered to reduce thermal losses. The whole assembly was first set up as shown in

Figure 2.11 before the sintering experiment was started. By simulating the conditions of the

actual sintering process, the temperature measurement by the thermocouple was recorded.

Figure 2.11. Microwave oven and thermocouple setup.

When the microwave was turned on and the specimen starts heating up, the

thermocouple also starts reading temperature of this heated specimen. At the same time the

pyrometer also records the temperature. The temperature measurement for the compact was

done under similar conditions which were used for actual sintering experiments. The heating

profiles are the ones created by the pyrometer. Table 2.5 has been plotted at an interval of

40

Table 2.5. Temperature Results Using Thermocouple

Pyrometer Temperature Reading of the

composite (ºC)

Thermocouple Reading of the composite

(ºC)

200 0

300 80

400 170

500 240

600 420

700 530

800 620

every hundred degrees. The left hand side shows the pyrometer reading of the composite and

the right side values indicate the corresponding temperature values recorded by the

thermocouple for the same compact.

The next step is the density measurement of the compact. This step has to be

performed before the characterization step as the sample has to be cut during

characterization. The density is measured by the Archimedes density measurement technique.

2.5 DENSITY MEASUREMENT TECHNIQUE

Same procedure for density measurement applies for the conventional sintering

experiments. The results are reported in the next chapter.

2.6 CHARACTERIZATION PROCESS

Further the characterization of the sintered sample is done with the help of a scanning

electron microscope. For the characterization process the samples are cut at various cross

sections to observe the porosity and the inter-particle interaction. The characterization results

for the microwave and conventional sintering are reported in the next chapter.

41

CHAPTER 3

RESULTS

Tables 3.1 to 3.6 show the relative density as calculated by Archimedes Technique

for the sintered samples obtained from both the processes. Also the table includes the

duration of the sintering process for each individual sample. The left hand side shows the

microstructure images of the sintered samples. The magnification used for the SEM images

in all cases was the same. The main aim was to look at the inter particle interaction of the

samples. Also porosity can be assessed from the SEM images

Pyrometer Temperature Profiles for Microwave Sintering Experiments

Figures 3.1 to 3.3 show the pyrometer temperature profile for the three samples

sintered in the microwave oven. The temperature profile for sample 1 (5 vol % of Cu) is

shown in Figure 3.1. The temperature profile for sample 2 (10 vol % of Cu) is shown in

Figure 3.2. The temperature profile for sample 3 (20 vol % of Cu) is shown in Figure 3.3.

The shrinkage curves have been plotted for each specimen sintered in the dilatometer.

Figure 3.4 corresponds to the specimen consisting of 5% of copper by volume. Figure 3.5

corresponds to the specimen consisting of 10% of copper by volume. Figure 3.6 corresponds

to the specimen consisting of 20% of copper by volume. The graphs have been plotted for the

push rod position vs. Time at specific interval.

42

Tab

le 3

.1. R

esu

lts

of C

onve

nti

onal

Sin

teri

ng

Pro

cess

(S

amp

le 1

)

S

EM

imag

e

Tim

e (m

inut

es)

Init

ial G

rain

Siz

e (µ

m)

Rel

ativ

e D

ensi

ty

(%)

Bor

osil

icat

e

Gla

ss

Cop

per

(5 %

) In

itia

l F

inal

1

00 µ

m

150

21

1-44

40

87

Por

esC

u

Incl

usio

ns

Gla

ss

43

Tab

le 3

.2. R

esu

lts

of C

onve

nti

onal

Sin

teri

ng

Pro

cess

(S

amp

le 2

)

SE

M im

age

Tim

e (m

inut

es)

Init

ial G

rain

Siz

e (µ

m)

Rel

ativ

e D

ensi

ty

(%)

Bor

osil

icat

e

Gla

ss

Cop

per

(10

%)

Init

ial

Fin

al

10

0 µ

m

150

20

1-44

39

83

Cu

Incl

usio

ns

Gla

ss

Mat

rix

P

ores

44

Tab

le 3

.3. R

esu

lts

of C

onve

nti

onal

Sin

teri

ng

Pro

cess

(S

amp

le 3

)

SE

M im

age

Tim

e (m

inut

es)

Init

ial G

rain

Siz

e (µ

m)

Rel

ativ

e D

ensi

ty

(%)

Bor

osil

icat

e

Gla

ss

Cop

per

(20

%)

Init

ial

Fin

al

10

0 µ

m

100

2

1

1-4

4

41

83

Cu

Incl

usio

ns

Gla

ss

Mat

rx

Por

es

45

Tab

le 3

.4. R

esu

lts

of M

icro

wav

e S

inte

rin

g P

roce

ss (

Sam

ple

1)

SE

M im

age

Tim

e (m

inut

es)

Init

ial G

rain

Siz

e (µ

m)

Rel

ativ

e D

ensi

ty

(%)

Bor

osil

icat

e G

lass

C

oppe

r(5

%)

Init

ial

Fin

al

100

µm

20

21

1-44

40

85

Cu

Incl

usio

ns

Por

es

Gla

ss

Mat

rix

46

Tab

le 3

.5. R

esu

lts

of M

icro

wav

e S

inte

rin

g P

roce

ss (

Sam

ple

2)

SE

M im

age

T

ime

(min

utes

)In

itia

l Gra

in S

ize

(µm

) R

elat

ive

Den

sity

(%

) B

oros

ilic

ate

Gla

ss

Cop

per

(10%

) In

itia

lF

inal

700

µm

20

21

1-44

40

93

Gla

ss

Mat

rix

Por

es

Cu

Incl

usio

n

47

Tab

le 3

.6 R

esu

lts

of M

icro

wav

e S

inte

rin

g P

roce

ss (

Sam

ple

3)

SE

M im

age

T

ime

(min

utes

) In

itia

l Gra

in S

ize

(µm

) R

elat

ive

Den

sity

(%

) B

oros

ilic

ate

G

lass

C

oppe

r (2

0%)

Init

ial

Fin

al

100

µm

19

2

1

1-4

4

39

82

Cu

Incl

usio

nP

ores

Gla

ss

Mat

rix

48

Figure 3.1. Temperature profile for sample 1 (5 vol % of Copper).

Figure 3.2. Temperature profile for sample 2 (10 vol % of Copper).

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25

Tem

per

atu

re (

°C

)

Time (mins)

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25

Temperature (°C)

Time (mins)

49

Figure 3.3. Temperature profile for sample 3 (20 vol % of Copper).

Figure 3.4. Dilatometer shrinkage curve for sample 1 (5 vol % of Copper).

0

100

200

300

400

500

600

700

800

900

0 5 10 15 20 25

Tem

per

atu

re (

°C)

Time (mins)

‐0.5

‐0.45

‐0.4

‐0.35

‐0.3

‐0.25

‐0.2

‐0.15

‐0.1

‐0.05

0

0 10 20 30 40 50 60 70 80 90

Pu

sh R

od

Po

siti

on

(m

m)

Time (mins)

50

Figure 3.5. Shrinkage curve for sample 2 (10 vol % of Copper.)

Figure 3.6. Shrinkage curve for sample 3(20 vol % of Copper).

‐0.6

‐0.5

‐0.4

‐0.3

‐0.2

‐0.1

0

0 10 20 30 40 50 60 70 80

Pu

sh R

od

Po

siti

on

(m

m)

Time (mins)

‐0.5

‐0.45

‐0.4

‐0.35

‐0.3

‐0.25

‐0.2

‐0.15

‐0.1

‐0.05

0

0 10 20 30 40 50 60 70 80 90

Po

siti

on

Ro

d P

osi

tio

n (

mm

)

Time (mins)

51

CHAPTER 4

COMPARATIVE ANALYSIS

The purpose of sintering the glass copper composites by conventional and microwave

sintering machines is to compare the results obtained from both the procedures. The

comparison of results consists of 3 steps namely, the comparison of microstructure, the

comparison of the duration of each procedure as well as the comparison of the densities of

the final sintered products. The comparison of the two processes (conventional and

microwave) is done on the same platform. It means that the same methods have been applied

to create the samples used for both processes. The height and the metal phase concentrations

have been maintained the same for a better and a fair comparative analysis.

As stated earlier the conventional and microwave sintering processes have not been

compared for the chosen glass copper. Previously conducted works have tried either one of

the process, and even if they have tried both processes, they have not done used a

comparative approach. The conventional sintering approach has been studied over the years

and the fundamental mechanisms that govern the conventional sintering process have been

formulated. Same is not the case with microwave sintering process. Even though the

microwave sintering process has been studied a number of works, the actual fundamental

mechanisms that govern the microwave sintering process are still a matter of research. The

comparative analysis carried out in this research project should be helpful in bringing more

understanding of the microwave sintering physical basis.

The results of the conventional and microwave sintering process with respect to time

are given in the Tables 4.1, 4.2 and 4.3 respectively.

52

Table 4.1. Density Comparison

Sample

Number

Microwave Sintering

Conventional Sintering

(Relative

Density (% )

Time

(mins)

Temp

( °C )

(Relative

Density (% )

Time

(mins)

Temp

(°C )

Sample 1

(5 % Copper)

85

19

650 87

150

650

Sample 2

(10 %

Copper)

93 20

650

83

150

650

Sample 3

(20

%Copper)

83

20

650

82

100

650

53

Tab

le 4

.2 M

icro

stru

ctu

re C

omp

aris

on

Sam

ple

Num

ber

Mic

row

ave

Sin

teri

ng P

roce

ss

(Mic

rost

ruct

ure

imag

e us

ing

SE

M)

Con

vent

iona

l Sin

teri

ng P

roce

ss

(Mic

rost

ruct

ure

imag

e us

ing

SE

M)

Sam

ple

1

Tim

e ta

ken

to s

inte

r –

19 m

ins

Tem

pera

ture

– 6

50 °

C

Tim

e ta

ken

to s

inte

r –

150

min

s

Tem

pera

ture

– 6

50 °

C

(tab

le c

onti

nu

es)

Cop

per

incl

usio

ns e

mbe

dded

in

gla

ss m

atri

x

Cop

per

incl

usio

ns

surr

ound

ed b

y po

res.

54

Tab

le 4

.2. (

con

tin

ued

) S

ampl

e N

umbe

r M

icro

wav

e S

inte

ring

Pro

cess

(M

icro

stru

ctur

e im

age

usin

g S

EM

) C

onve

ntio

nal S

inte

ring

Pro

cess

(M

icro

stru

ctur

e im

age

usin

g S

EM

)

Sam

ple

2

Tim

e ta

ken

to s

inte

r –

20

Tem

pera

ture

– 6

50 °

C

Tim

e ta

ken

to s

inte

r –

150

Tem

pera

ture

- 6

50 °

C

(tab

le c

onti

nu

es)

Cop

per

incl

usio

ns e

mbe

dded

in g

lass

mat

rix

Cop

per

incl

usio

ns

surr

ound

ed b

y po

res.

55

Tab

le 4

.2. (

con

tin

ued

) S

ampl

e

No

Mic

row

ave

Sin

tere

d S

peci

men

C

onve

ntio

nall

y S

inte

red

Spe

cim

en

Sam

ple

3

Tim

e ta

ken

to s

inte

r –

20

Tem

pera

ture

– 6

50 °

C

Tim

e ta

ken

to s

inte

r –

100

Tem

pera

ture

- 6

50 °

C

Cop

per

incl

usio

ns e

mbe

dded

in g

lass

mat

rix

Cop

per

incl

usio

ns

surr

ound

ed b

y po

res.

56

Table 4.3. Process Duration Comparison

Sample Number Microwave Sintering Process

Conventional Sintering

Process

Time

(mins)

Density Time

(minutes)

Density

Sample 1

(5 % Copper)

19

85

150

87

Sample 2

(10 % Copper)

20

93

150

83

Sample 3

(20 % copper)

20

83

100

82

4.1 DENSITY COMPARISON

The microstructure pictures show that there are no crack formations in any of the

sintered samples. There were no other shape defects found in the sintered samples. For both

processes i. e conventional and microwave sintering processes the samples showed some

oxidation. But this was not the case for all samples But rather only for a few in the test

experiments. This also shows that oxidation was not affected by time. Indeed, both the

samples were sintered in air and conventional sintering process takes more time than the

57

microwave process. Nevertheless, the oxide formation was seen for both the processes in

extremely few cases.

For the comparison of the porosity, it is necessary to find the density of the samples

which have been sintered through both the processes. The density calculation was done by

the Archimedes technique.

Table 4.2 shows the difference in the densities. All the 3 samples varied in density for

both the processes. In the case of the first specimen, the density of the specimen obtained by

the conventional sintering process is more than the microwave sintering-processed sample. In

case of the second specimen, the density of microwave sintered process is much higher than

the conventionally sintered process. In case of third specimen, the density of specimens

sintered by both the processes is almost the same with microwave sintered specimen having

higher density than conventional sintered specimen.

The bar graph in Figure 4.1 shows the relative densities plotted for microwave and

conventional sintered specimens for their respective composite concentrations. The blue bar

indicates the microwave sintered density and the red bar indicates conventional sintered

density. As seen from the graphs the sintered density of the first specimen containing 5 vol %

of copper obtained from conventional sintering process is higher than the specimen obtained

from microwave sintering process. However in case of the second (Cu 10%vol) and third

samples (Cu 20% vol) the microwave sintered density is higher than the conventional

sintered density. The graph in Figure 4.2 has been plotted for the relative density of the

sintered specimen vs. Copper concentration by volume (%) in each sintered sample. As it can

be seen from the graph the conventional sintered density decreases as copper concentration in

the composite increases. In case of microwave sintering process, the relative density of the

58

Figure 4.1. Microwave and conventional sintered chart comparison.

sintered sample is highest for the second specimen consisting of 10 vol % of copper and

lowest for the third specimen consisting of 20 vol % of copper. It can be seen from the

microwave sintering process graph that there is a competition between the localized heating

which causes densification of glass and the rigid inclusions which act as an obstacle for this

densification process. As the amount of rigid inclusions increases in the composite, they

provide resistance to the bonding of glass.

4.2 MICROSTRUCTURE COMPARISON

Table 4.2 shows the difference in microstructures of both microwave and

conventional sintered samples. The sample 1 contains 5 vol % copper. As it is seen there is

76

78

80

82

84

86

88

90

92

94

1 2 3

85

93

83

87

8382

Rel

ativ

e D

ensi

ty (

%)

Glass-Cu Composite Specimens

Microwave Sintered Density

Conventionally Sintered Density

59

F

igu

re 4

.2. R

elat

ive

den

sity

vs.

com

pos

ite

con

cen

trat

ion

of

Cu

by

volu

me

(%).

80

82

84

86

88

90

92

94

05

10

15

20

25

Relative Density (%)

Co

mp

osi

te C

on

cen

trat

ion

o

f C

u b

y V

olu

me

(%)

Mic

row

ave

Sin

tere

d

Con

vent

iona

lly

Sin

tere

d

60

more porosity in the microwave sintered samples than in the conventionally sintered sample.

The conventionally sintered sample is to the right while the microwave sintered sample is to

the left. In this case it is seen that the conventionally sintered sample has less porosity and the

particle size is bigger. That is not the case in the sample which has been sintered by the

microwave process. In this case the particles have combined less with each other resulting in

a large amount of small pores. The pore sizes although small are in a greater quantity. Also it

can be seen that the microwave sintered sample has a superior appearance as against the

conventionally sintered sample.

The next sample contains 10 vol % of copper. The Table 4.2 shows microwave

sintered sample on the left hand side while the conventionally sintered sample is on the right

hand side. Again we find the same pattern in this case too. In the case of conventional

sintering the porosity is very low. So we can see that the conventional sintering process has

again given a better sample. The next sample contains 20 % of copper in it. This time we find

that the microwave sintered sample matches with the conventionally sintered sample. The

porosity in both the cases is low and the particles have combined with one another to a large

extent.

From the microstructure images it can be seen that in case of microwave sintered specimens the copper inclusions are completely embedded inside the glass matrix.

In case of conventionally sintered specimens the copper inclusions are surrounded by pores in all the specimen microstructures.

This can be explained with the formation of hot and cold spots inside the compact. In

case of microwave sintering due to the electromagnetic radiations, the copper particles gets

heated up faster than the glass particles thus creating a hot spot in those regions of the

powder compact. Comparatively the glass particles are cooler in the compact during the

microwave sintering process. This makes energy transfer possible from high temperature

61

regions to low temperature region. Essentially the densification might rise due to these

particular hot spots. This can result in more intense heating and early activation of viscous

flow sintering in the glass matrix.

4.3 PROCESS DURATION COMPARISON

The Table 4.3 shows the time taken in minutes for the composite to sinter by

conventional and microwave sintering process. As seen the time taken to sinter the composite

samples in the microwave was around 20 minutes for each of the three specimens. But in the

case of conventional sintering process the time taken to sinter the composite samples was

around 180 minutes. This shows the differences in duration of sintering. The difference is

160 minutes which is a huge difference. Microwave sinters the composite more rapidly than

conventional sintering. This shows how powerful microwave sintering is. Also throws a light

on the potential of microwave sintering in the product manufacturing industries in the present

world.

It should be noted that, the process duration for conventional sintering was defined by

us. However the conventional sintering technique could not consolidate the specimen and the

density was lower. Hence to solve this issue the holding time was increased. This shows a

limitation in the heating method of conventional sintering. Even though it provides heating to

the specimen in a consistent manner, it cannot match the rapid heating as provided by

microwave oven. Meanwhile the microwave sintering process could consolidate the

specimen to a great extent. Even if the idea behind conventional sintering is to provide heat

for a longer duration for high quality of sintering, it looks like the nature of the heat produced

in the specimen is less effective in comparison with the microwave heating of the specimen.

62

Another explanation for the better densification in microwave sintering is the Impact of High

and Slow Heating Rates as shown in Figure 4.3 on mass transport mechanism.

Figure 4.3. Temperature vs. time for microwave and conventional sintering processes.

Sintering of glass takes place by viscous flow mechanism. In a sintering process the

first mechanism to be activated is surface diffusion. After surface diffusion the sintering

takes place by viscous flow and grain boundary diffusion processes respectively. The impact

of heating a composite at a very high rate for a short period of time can change the sequence

of operation of the mass transport mechanisms as seen from the study of Eugene A Olevsky

et al [36]. In microwave sintering this can lead to the simultaneous activities of surface

diffusion, viscous flow and grain boundary diffusion thus intensifying the heating process.

However for conventional sintering the mass transport mechanisms will have more time

leading to less intensity of the densification process.

Microwave

Sintering

Conventional Sintering

Process

63

CHAPTER 5

CONCLUSIONS

The comparative analysis of the microwave and conventional sintering has been the

main objective of this thesis. The focus of research was on the three parameters namely

density of the sintered product, the duration of the sintering process and the microstructure of

the sintered material. The results of comparison as seen in the previous chapters give a good

idea as to which sintering process is more effective.

The questions which are to be answered are as follows

Which process is more reliable with respect to time consumption?

Which process gives higher density products?

How is the surface appearance of the final sintered product?

What observations can be made from microstructure analysis?

Which process has higher probability of giving defective products?

At microstructure level what is nature of porosity and grain growth?

Which process is more suitable for a manufacturing environment?

The first criterion to be studied was duration of the sintering process. As the results

show for each sample the conventional sintering process has taken much more time than the

microwave sintering process. The process has taken around 3 hours to give a sintered

product. While the microwave sintering process took around 20 minutes to give a 93 % dense

sintered product. In most cases due to unconsolidated final product, the holding time was

increased. This is one of the drawbacks concerning the heating method of conventional

sintering process.

64

The duration of conventional sintering process is a very formidable hindrance in the

industrial work culture where production has to be fast. This makes microwave sintering

much effective as the time taken to sinter the same composite samples was found to be less

than the time taken to sinter the composite via conventional route. This is a substantial

difference giving microwave sintering an edge over conventional sintering.

The next criterion to be studied was the density of the sintered product. This is the

most important criterion as the industry demands high density high strength products. The

quality of the product is of prime importance. High density products have huge demand in

the manufacturing of automobile and other such mechanical parts. It was seen that for

majority of the samples the density achieved was higher in case of microwave sintering

process. The highest density of 93 % was obtained in a specimen sintered by the microwave

process. Thus it can be concluded that the microwave sintering process was much more

effective than the conventional sintering process.

The sintered samples showed some differences in the surface appearance. The

microwave sintered sample showed smoother surface than the conventional sintered sample.

The microwave sintered sample had a superior appearance than the conventionally sintered

sample. This might be due to the copper particles on the surface of the specimen might get

highly heated than the rest of the composite as they are continuously and consistently heated

at a very high rate in case of microwave sintering.

The microstructure analysis of the sintered sample was another important criterion

which was studied in this thesis. As seen from the images conventionally sintered samples

have shown more porosity than the microwave sintered samples. We can conclude that

65

microwave sintering process has been a more effective process in comparison with the

conventionally sintered process for the sintering of glass metal composites.

There were no other surface defects in the products obtained from microwave as well

as conventionally sintered specimens. There were no crack formations in both the cases as

well. The appearance of the microwave sintered sample is better than the conventionally

sintered sample. The conclusions of this study can be listed in the following manner:

The comparative analysis concludes that densification in microwave sintered glass-copper composites was better and much higher for two specimens than the conventionally sintered composites.

It is also seen that the duration of the process affects the densification process as density in conventional sintering is dependent on time. The grain growth and porosity can only increase with increased time. However in microwave sintering the density has been found to be independent of time.

Metals do have a positive effect in sintering glass in a microwave oven as against sintering glass in conventional furnace. The internal volumetric heating of the composite in the microwave oven can trigger high quality sintering of glass.

The microstructure images give some evidences of metals hindering the conventional sintering process. The study thus concludes that variation of concentration affected the densification of composite in conventional sintering. The density decreased with increase in volume fraction of Cu inclusions. However Microwave sintering densification was not affected negatively by different volume percent of Cu.

The surface appearance of the microwave sintered specimens was better than the conventionally sintered specimens.

The study concludes that microwave sintering process can be used for manufacturing glass-metal composite components more effectively than conventional sintering process.

66

CHAPTER 6

SCIENTIFIC AND ENGINEERING NOVELTY OF

THE CONDUCTED RESEARCH

The research carried out has been novel from scientific and engineering point of view.

The achievement of these tasks has been listed below.

Sintering study of Glass and Copper composites has not been carried out extensively. In fact this study is one of the first research activities in this area.

The comparative study between the conventional and microwave sintering of glass copper composites has not been performed previously.

The study provides some conclusive evidence with respect to the microwave and conventional sintering processes used to sinter glass copper composite specimens.

The study also shows micro structural analysis of the conventional and microwave sintered specimens.

Also copper is a highly conductive metal. Very few studies have concentrated on the microwave sintering of glass and a highly conductive metal like copper.

Most importantly this study has been thorough and comprehensive as sintering analysis is based upon different concentrations of copper inclusions in the glass matrix

67

CHAPTER 7

FUTURE SCOPE

There are some important areas in which the work on this thesis can be expanded.

The following list outlines some of these areas

Comparing Microwave and Conventional Sintering processes having the same heating rates.

Finite element modeling of thermal stresses occurring due to the mismatch of the coefficient of thermal expansion and shrinkage rates of glass copper composite sintering.

Comparing Microwave and Conventional Sintering mass flow mechanisms especially diffusional and viscous flow mechanisms

Comparative Studies of neck growth in glass copper composites.

Even though the comparative study has been attempted in this thesis, it can be made

more substantial by in depth microstructure analysis. The main shortcoming of the present

approach is that the conventional and microwave sintering processes with different heating

rates have been compared. Therefore such a comparison is not very complete. In future one

should design a process comparison when both sintering approaches will have same

temperature history (heating rates).

Subsequent study of the sintering mechanism which operates in both the process can

give better knowledge and understanding of sintering science. This detailed analysis can give

more information regarding the material properties of the sintered products. Also it can give

large amount of information with respect to the strength of the inter-particle contacts in the

final sintered sample. It can also help in improving the process control parameters. This is

why the fundamentals of any process should be absolutely clear.

68

REFERENCES

[1] Boccaccini, A. R., and G. Ondracek. “Viscous Sintering of Non-Spherical Borosilicate Glass Powder.” Glastechnische Berichte - Glass Science and Technology 65 (1992): 73-78.

[2] Frenkel, J. “Viscous Flow of Crystalline Bodies under the Action of Surface Tension.” Journal of Physics 9 (1945): 385–391.

[3] Pines, B. Ya. “Mechanics of Sintering.” Journal of Technical Physics 16 (1946): 137.

[4] Kuczynski, G. “Self-Diffusion in Sintering of Metallic Particles.” Transactions of the American Institute of Mining, Metallurgical and Petroleum Engineers 185 (1949): 169–178.

[5] Kang, Suk-Joong L. Densification, Grain Growth and Microstructure. Oxford: Elsevier Butterworth-Heinemann, 2005.

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