Porous ceramics with controllable properties prepared by proteinfoaming-consolidation method

Ahmad Fadli • Iis Sopyan

Published online: 11 March 2010

� Springer Science+Business Media, LLC 2010

Abstract A new protein foaming-consolidation method

for preparing porous alumina was developed using egg

yolk both as consolidating and foaming agent. This method

allows the control of properties of porous alumina not only

by varying alumina-to-yolk ratio but also by managing the

foaming process. After drying, the green bodies were

burned at 600 �C for 1 h to remove the pore creating agent,

followed by sintering at 1,550 �C for 2 h. The porous

alumina ceramics with pore sizes of 25–1,000 lm and

relative density of 29–50% were obtained. The compres-

sive strength of the sintered samples varied within the

range of 1.1–5.7 MPa, corresponding to porosity of 40–

71%. The addition of dispersant with different concentra-

tion into alumina slurries shifted the rheological properties

from shear thinning behavior to a Newtonian fluid, which

resulted in changes in the pore sizes of the resulting

ceramics. The main advantages of the process are the

simplicity of the process and the low-cost processing

equipment/materials needed. These results have opened a

novel preparative way for porous ceramics especially alu-

mina-based porous materials designed for biomedical

applications.

Keywords Protein foaming-consolidation � Egg yolk �Alumina � Porous ceramics � Compressive strength �Porosity

1 Introduction

Porous ceramics have become important materials for

various applications such as catalyst, porous burners,

membrane, kiln furniture, and bioceramics [1–5]. Each

application needs specific features and properties of porous

ceramics; they may differ in terms of porosity, strength,

pore size distribution, pore morphology and pore connec-

tivity. For effective thermal insulation, for example, it is

favorable to have closed porosity whereas for filters and

membranes open porosity is a must. In the bioceramics

field it is desirable to use porous ceramic implants with

certain porosity to promote integration with biological

tissues [6].

Processes like replication method and the foaming

method have been used for preparation of porous ceramics.

Schwartzwalder and Somers [7] reported the replication of

polymer foams, as one of the first manufacturing tech-

niques developed for producing ceramics with controlled

macroporosity. They started using polymeric sponges as

templates to prepare ceramic cellular structures of various

pore sizes, porosities, and chemical compositions. In the

polymeric replica approach, a highly porous polymeric

sponge (typically polyurethane) is initially soaked into a

ceramic suspension until the internal pore cavities are filled

with suspension. The polymeric sponge is then removed

from the suspension and the excess slurry squeezed out.

This is followed by firing to completely vaporize and burn

out the organic material as well as to vitrify the ceramics.

In the foaming method, porous ceramics are prepared by

mixing ceramic powder with organic materials as a pore-

forming agent, which are followed by combustion of the

mixture, leaving pores within the ceramic body. This

technique allows easy and fast production of highly porous

ceramics materials with generally less open and dense

A. Fadli � I. Sopyan (&)

Department of Manufacturing and Materials Engineering,

Faculty of Engineering, International Islamic University

Malaysia, 50728 Kuala Lumpur, Malaysia

e-mail: sopyan@iiu.edu.my

A. Fadli

Department of Chemical Engineering, Faculty of Engineering,

Riau University, Pekanbaru 28293, Indonesia

123

J Porous Mater (2011) 18:195–203

DOI 10.1007/s10934-010-9370-8

struts which results in lower permeability but higher

strength [8].

Lyckfeldt et al. reported a starch consolidation method for

preparation of porous ceramics. In this process, native and

chemically modified potato starch granules were incorpo-

rated into aqueous ceramic powder suspensions and the

slurry was further consolidated into near-net-shape bodies by

heating in the temperature range of 60–80 �C. The process

produces porous ceramics with 30–70% porosities and pore

size depends on the size of starch granules which is nearly

50 lm [9]. Preparation of porous ceramics with hierarchical

structure has also been reported using foaming and consol-

idation method. In this case, a ceramic powder suspension

containing cassava starch is foamed by using a liquid

detergent followed by consolidated in a microwave at 400 W

[10]. The usages of ovalbumine as foaming agent have been

tried by some researcher groups [11]. However, due to its

high foaming capacity, it was hardly to be suitable to make

porous ceramics with controlled porosity.

Recently, there is an increasing attention in using pro-

teins as additives to make porous materials. Proteins are

high molecular compounds, which are formally understood

as products of amino acids condensation. The main chain

of protein molecule is characterized by covalent peptide

bonds, while their (conformation) is stabilized by weak,

mostly non-covalent bonds. A change in the situation of

these bonds leads to a change in physical and chemical

properties of the molecule. The thermal activated loss of

this structure is called denaturation. Proteins are prone to

foaming because of their amphilic character [12]. When

adding a protein to a ceramic slip through a mixing oper-

ation, air bubbles are introduced, and the protein molecules

are adsorbed at the interface between air and water via

hydrophobic areas, and a partial unfolding (surface dena-

turation) occurs. The increase in the surface tension caused

by protein adsorption facilitates the formation of the new

interfaces and more bubbles are created. The ability of

protein molecules to form and stabilize foam depends on

the diffusion rate and denaturation ability [12].

Garrn et al. [13] reported the usage of protein (Bovine

Serum Albumine, BSA) as binders for producing alumina

foams. The combination of foaming and increased stiffness

gave a stable protein-ceramic foam structure. A fine cel-

lular foam structure of approximately 50–300 lm cell

diameters and 8–20% relative densities was obtained [13].

We have succeeded in developing a novel method for

preparation of porous alumina using egg yolk as both

consolidating and foaming agent [14]. Egg yolk is a

complex association of water (50%), lipids (33%) and

proteins (17%) [15]. It has been well known that the lipids

phase in egg yolk would reduce the foaming capacity of

protein in making pores. Previously, we have reported that

the porosities and compressive strengths of the porous

alumina were controlled by adjusting the drying tempera-

ture of slurry [16]. In this report we wish to present the

effect of foaming process condition and composition

between yolk and alumina powder on physical properties

of the obtained porous alumina.

2 Experimental

2.1 Materials

The starting materials used to prepare the porous ceramics

were a commercial alumina powder (Sigma–Aldrich, Inc.

USA), with an average particle size of 0.25 lm (measured

using a Malvern Instruments nanosizer, NanoS model) and

a specific surface area of 0.39 m2/g (measured by N2

adsorption method on a Quantachrome surface area ana-

lyzer, Autosorb-1 model). Scanning electron microscope

(SEM) measurements showed that most of the particles

have irregular size and shape. Protein used was yolk that

freshly isolated from chicken egg. Darvan 821 A (R. T.

Vanderbilt, Norwalk, CT) was selected as the dispersant

because of its ability in stabilizing dispersion of alumina in

protein -water suspension [11].

2.2 Preparation of porous ceramics

The flow chart in Fig. 1 describes an overview of the

process. Slurries were prepared by dispersing the alumina

powder and yolk with an alumina-to- yolk ratios of 1.00,

0.83, 0.75, and 0.65 in weight. The slurries were magnet-

ically stirred in a beaker glass for 3 h with a rate of

150 rpm. Into the slurry, the dispersant of 0.01–0.05 wt%

concentrations was added. The slurries were cast in

cylindrical open stainless steel mold (ø & 1.3 mm,

h & 1.4 mm). Covering the molds with castor oil made

demolding easier. Thermal foaming-consolidating was

done in an air oven (Memmert, 100-800 model) at range

temperatures of 110–180 �C for 1 h. The yolks were

removed by burn-out in a SiC furnace (Protherm, PLF 160/

5 model) at 600� for a period of 1 h at 10 �C/min rate.

Then heating was continued at a rate of 2 �C/min up to

1,550 �C ended by 2 h dwell time at the temperature.

2.3 Determination of foaming capacity

The foaming capacity of slurries was evaluated by mea-

suring the change in volume of slurry as a function of

drying time. 10 mL slurries contained in a 100 mL glass

measuring cylinder was placed in a temperature-controller

air oven for 60 min. The temperature was varied between

100 and 180 �C. The change in slurries volume was

monitored for every specified time intervals. The foaming

196 J Porous Mater (2011) 18:195–203

123

capacity is evaluated in terms of the volume ratio of

foamed slurry to the original one [17].

2.4 Characterization

The rheological property of the slurries was measured in a

ThermoHaake VT 550 viscotester with a measuring system

of concentric cylinders using sensor cone type of SV-DIN.

Thermogravimetry analysis was carried out in a Perkin

Elmer TG/DTA (Pyris Diamond model) to understand the

details of the yolk burn-out in ambient air at a heating rate

of 10 �C/min. The apparent density of sintered samples

obtained was measured in Electronic densimeter (Alfa

Mirage, MD300S model). The theoretical density of fully

densified alumina (3.98 g/cm3) was used as the reference to

calculate the total volume fraction of porosity. The pore

size, interconnection among pore and also the grain

structure were examined using scanning electron micros-

copy (JEOL, 5600 model) and field emission scanning

electron microscopy (JEOL, JSM 6700 F model). The

mechanical strength of the porous bodies was measured

using a universal testing machine (Lloyd, LR10 K plus

model) through diametrical compression on samples of 3/2

height-to-width ratio. Five scaffold specimens were used to

determine the average maximum compressive strength.

3 Results and discussion

3.1 Rheological behavior

The viscosity of the slurries were measured at various shear

rates after 150 rpm for 3 h. Figure 2 shows the influence of

alumina-to-yolk ratio on the viscosity alumina slurries. For

comparison purpose, the viscosity of alumina suspension

with dispersant addition is also plotted in Fig. 2. It can be

seen clearly that the slurries without dispersant addition

showed pseudoplastic flow behavior. Though the slurries

showed small difference in viscosity (9.3–12.4 Pa s) at low

shear rate (10 s-1), the viscosity values of the slurries are

close at high shear rate. Pseudoplasticity in ceramics

slurries usually arises because of the existence of an

interparticle network, which undergoes a gradual break-

down with increasing shear rate, causing the typically

observed decrease in viscosity of slurries. It can be also

seen from Fig. 2 that addition of dispersant into slurries

with 1.00 alumina-to-yolk ratio altered flow behavior from

shear thinning to a Newtonian fluid. A viscosity of as high

as 12.4 Pa s was observed for the slurry without dispersant

content at low shear rate (10 s-1) but it decreased signifi-

cantly to 2.0 Pa s when 0.05 wt% dispersant was added.

The viscosity value of the all slurries at high shear rate

(700 s-1) was in the range 0.9–1.9 Pa s and it was still

suitable for casting.

The pseudoplasticity in ceramic can be characterized by

non-Newtonian index n calculated according to the power

law model as per the following equation [18]:

g ¼ kcn�1 ð1Þ

where g is the viscosity of the slurry, c the applied shear

rate, k and n are the consistency factor and non-Newtonian

index, respectively. Highly shear thinning slurries, with a

strong inter-particle network, show a rapid decrease in

viscosity with an increase in shear rate, corresponding to a

lower value on the non-Newtonian index, n, and slurries

with weak or no inter-particle network become closer

to the Newtonian behaviour with n values approaching

1.0 [18]. However, in suspensions containing yolk the

Fig. 1 Preparation process of porous alumina ceramics by protein

foaming-consolidation method

Fig. 2 Plots showing the change in viscosity of alumina slurries as a

function of shear rate at various mixing conditions of alumina and

yolk

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123

pseudoplasticity arises not only from gradual breakdown of

the inter-particle network of ceramic grains but also from

alignment of the yolk molecule at high shear rates.

The calculated n index for the 1.00 alumina-to-yolk ratio

slurries with 0.03–0.05 wt% dispersant is in range of 0.71–

0.82, thus indicating a behavior closer to Newtonian

(Table 1). On the other hand, n values for slurries without

dispersant addition and varied yolk are in the range of

0.32–0.51 indicating the shear thinning behavior. The

calculated k parameter, which is a consistency factor in

the power law model, decreased almost linearly with the

increasing dispersant concentrations but increased with the

increasing yolk contents in the ceramic suspension.

3.2 Foaming of slurry

Figure 3 shows the effect of drying temperature on volume

increase of slurry at 1.00 alumina mass-to- yolk ratio

without dispersant addition. It is well known that heat-

treatment can induce protein denaturation, thus leading to

changes in the functional properties of the treated proteins

[19]. During the process of denaturation, secondary va-

lency bonds such as hydrogen bridge bonds, ionic bonds,

hydrophobic bonds and disulphide bridges are partially

modified. In consequence of the partially or totally

unfolding, larger random coil structures are formed.

Between these unfolded protein chains new bonds are

statistically formed. This leads to consolidation and a

decrease in solubility [12].

Generally, foaming of slurry took place in three stages:

pre-heating of slurry without increasing volume, foaming

stage with the strongly increased until reaches a maximal

value and then undergoes a plateau (stabilizing stage).

During the pre-heating period, the structural properties of

protein were induced by heating, leading to changes in the

functional properties. Subsequently, during foam forma-

tion, proteins will (1) diffuse to the air–water interface,

concentrate and reduce surface tension; (2) partially unfold

and reorient such that the polar groups are directed towards

the polar phase; and (3) interact with one another via non-

covalent interactions and potentially covalent bonds to

form a continuous viscoelastic film. For effective foam

formation, proteins must be capable of migrating and ori-

entating rapidly to form encapsulating films around the gas

bubbles to prevent destabilization. Foams may destabilize

due to: (1) diffusion of gas from small to large bubbles or

to the atmosphere, termed Ostwald ripening or dispropor-

tionation; (2) drainage of liquid from and through the foam

layer due to gravity; and (3) coalescence of bubbles due to

instability of the film between them [20].

The amphiphilic character of protein molecule causes a

decrease in surface tension, thus leading to possibly better

foaming properties [20, 21]. The foaming capacity values

determined from volume increase of slurry was 2.2, 1.97

and 1.7 v/v for slurry alumina-to-yolk ratios 0.65, 0.83 and

1.00 respectively. The foaming ability of slurries became

intensive with the increased yolk content.

The volume increases for slurry without and with 0.01%

dispersant addition at 60 min drying are presented in

Table 2. The foaming experiments involving slurries con-

taining dispersant (Darvan 821 A) showed that the foaming

capacity increased with dispersant addition of 0.01 wt% in

the slurry in the range of drying temperature 110–180 �C.

Compared to the slurries without dispersant which pro-

duces volume increase in the range 1.0–2.0 (v/v) the slur-

ries containing dispersant of 0.01% has improved foaming

capacity from 1.3 to 2.2 (v/v). It could be deducted that

dispersant molecules will decrease the viscosity of slurry

(see Fig. 2) and accelerate transfer proteins from interior of

the slurry towards the newly created surface, thus

decreasing the surface tensions and increasing the foaming

capacity.

The foam stability is the ability to retain air for a certain

period of time [22]. For certain drying time, the surface

tensions minimal value, hence the foaming capacity keeps

steady and stable. Foam stability relates to the stability

lamellae and the ability to retain the gas for a given time.

Foam stability is a reflection of foam film characteristics

such as impermeability to gas, viscoelastic characteristics

and mechanical strength of the film [20].

Table 1 The parameters n and k as a function of yolk and dispersant

contents in alumina suspensions

Alumina-to-yolk mass

ratio (w/w)

Dispersant

concentration (%)

k n

0.65 0 74.2 0.32

0.75 0 47.3 0.41

1.00 0 37.6 0.51

1.00 0.03 8.5 0.71

1.00 0.04 3.9 0.81

1.00 0.05 3.1 0.82

Fig. 3 The effect of drying temperature on volume increase of slurry

at 1.00 alumina mass-to-yolk ratio without dispersant addition

198 J Porous Mater (2011) 18:195–203

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The stability of the foam relates to the decrease of foam

volume with time. We found that the green bodies after

demolding would be collapse when it dried less than

20 min time, whereas after drying time more than 20 min,

the green bodies would be steady. A photograph of typical

green bodies produced from the suspension after 13 and

40 min drying is shown in Fig. 4.

3.3 Burn-out

In general, foamed green body is easy to debinder than

dense body, for it contains abundant cells and channels to

let organic components occupy and escape. Thermal

analysis was used to optimize the burn-out schedule of the

foamed body.

Figure 5 shows a TGA curve for yolk carried out in air.

The figure revealed that the first drop in TG appeared at

*100 �C with a 51% weight loss which is due to water

evaporation. The second drop occurs in the range of

*100–340 �C and experienced ca 24% weight loss which

attributed to the removal of lipids. In the temperature range

of 340–550 �C, the decomposition of proteins occurred

with a weight loss of 25%. From the graph, it is also evi-

dent that at 550 �C, the yolk was completely burned out.

Thus, the heating was set up to 600 �C with a dwell time of

1 h to allow ample time for the complete burnout of the

yolk for creating pores.

3.4 Sintering

Prepared alumina samples after sintering were performed

without significant deformations which indicated good

homogeneity of the materials. Figure 6 shows three sin-

tered porous alumina samples with cylindrical shape.

The relative density and shrinkage of sintered samples

using slurry without dispersant addition is listed in Table 3.

Sintered ceramics prepared in this work exhibited relative

densities in the range 29–50%, increased with varying

alumina content. It can be explained that high alumina

substance of the slurry resulted in high viscosity, hence a

decrease in foaming capacity. In addition, high solid frac-

tion itself also lead to high density of cell struts.

Furthermore, with the increase of yolk content, the

sintered alumina shrunk increasingly. The shrinkage of

samples changed from 29 to 40% when the alumina-to-

yolk ratio decreased from 1.00 to 0.65. A substantial

shrinkage occurred as the proteins were removed.

3.5 Microstucture

Figure 7 shows the FESEM pictures of macrostructures of

porous alumina prepared from different slurry composi-

tions. It shows the macropores of 50–600 lm diameters in

average (Fig. 7a, b).

Figure 7c and d show the FESEM images of the

microstructure of sample after sintering at 1,550 �C. As the

amount of yolk increased more micropores are found, thus

showing poorer densification of particles. It could be

deducted that high yolk content resulted in low viscosity,

which corresponded to high foaming capacity and low

density. Moreover, the grain size of porous alumina walls

Table 2 The effect of the addition of 0.01 wt% dispersant into the

slurry on foaming capacity at 60 min drying

Drying

temperature (�C)

Volume increase (v/v)

Added with 0.01%

dispersant

Without

dispersant

100 1.30 1.09

110 1.55 1.32

160 1.81 1.78

180 2.19 2.00

Fig. 4 Comparison between green bodies of porous alumina with

1.00 alumina-to-yolk mass ratio and 0.01 wt% dispersant concentra-

tion after drying times at a 13 and b 40 min

Fig. 5 TG curve of egg yolk

J Porous Mater (2011) 18:195–203 199

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shown irregular and in some parts small particles adsorb on

the large grains. There is much bonding area among grains.

It is well known that more bonding area usually leads to

higher strength between particles; consequently the frac-

ture of alumina bodies mainly happens at the particles

boundary.

In particular, the effect of drying time on the pore size of

alumina bodies has been investigated using SEM analysis.

From the macrostructure images indicated that the pore

size of alumina bodies increased with drying time. The

slurries were dried at 180 �C with alumina to yolk mass

ratio of 1.00. It is clear that the pore size is determined by

drying time; hence higher drying time leads to higher

foaming capacity (see Fig. 3) and corresponding with

bigger pore size.

Figure 8 shows top view SEM images of sintered alu-

mina porous bodies with the green bodies dried at tem-

peratures of 110 (a), 150 (b) and 180 �C (c). As the drying

temperature increased, the pore size of porous alumina

bodies increased as well. Sintered porous alumina prepared

in this work showed an increase in pore size from 25 to

375 lm as drying temperature increased from 110 to

180 �C. It can be explained that slurry drying at low

temperatures resulted in low foaming capacity, hence poor

pore generation. When dried at 110 �C, the alumina slurry

was slightly foamed with volume increasing of 1.1 (v/v; see

Fig. 3), thus the created pores of sintered bodies were

mainly attributed to protein removal (Fig. 8a). As a result,

pores of small sizes and ununiform shape are generated. On

the other hand, when dried at 150 and 180 �C, protein

foaming took place well and produced most pores with

spherical shape in the porous bodies (Fig. 8b, c).

Figure 8d shows side view SEM image of sintered

samples prepared using the slurry dried at 180 �C. We can

see that pores sizes is in the range of 50–200 lm and it

indicated that the alumina body has open pore with good

interconnectivity. In tissue engineering, particularly poros-

ity, pore size distribution, pore morphology and orientation,

as well as the degree of pore interconnectivity significantly

affect bone penetration in macropore of implants, thus

mediating implant-tissue osseointegration [23].

Dissimilarity in pore distribution was found in samples;

the upper part of porous bodies shows higher porosity than

the lower part due to insufficient slurry stability during the

drying process. The stability of slurry was not fully

achieved because alumina particles tend to agglomerate

and to precipitate over slurry foaming process. Conse-

quently, slurry in upper part which containing high yolk

was more exhaustively foamed than the lower part.

Generally the foaming process has to be carried out prior

to the stiffening of the protein. At low temperatures

the material has still a remaining extensibility which allows

the deformation of the bodies. This is comparable with the

moment of dough rupture in the process of baking bread,

which is caused by similar mechanism [24]. Thus, control

of drying process is crucial for producing foams of high

structural integrity and good mechanical properties. The

dispersant concentration played an important role in

determining the microstructure of the sintered porous alu-

mina, as shown in Fig. 9.

All samples were prepared using slurries with alumina-

to-yolk ratio of 1.00 and then dried at 180 �C. It can be

seen that the pore size of alumina bodies increased with

dispersant concentration. As the dispersant content was

increased, the pores became bigger, more interconnected

with less dense and thinner pore walls, important factors

that influence the mechanical properties of the porous

materials. Besides, the morphology of pores was altered

from spherical to polygonal shape. The pore size of sin-

tered bodies was in the range of 200–1,000 lm. It can be

explained that low viscosity of the slurry resulted in high

foaming capacity, hence increased the pore generation.

Samples produced by the above technique have porosity up

to 78% when 0.01wt.% dispersant was added. The apparent

densities of sintered alumina as a function of dispersant

addition with constant alumina to yolk mass ratio were

found in the range 0.7–1.8 g/cm3. These results suggest

that porous alumina can be a potential candidate for

floating microcarrier application especially in a bioreactor

cell culture.

Fig. 6 The sintered porous alumina ceramics with cylindrical shape

made by protein foaming-consolidation method

Table 3 Relative density and shrinkage of sintered alumina at vari-

ous alumina-to-yolk mass ratio without dispersant addition

Mass ratio of

alumina-to-yolk

(w/w)

Shrinkage

(vol%)

Relative

density (%)

0.65 39.9 29.4

0.75 36.4 47.2

0.83 32.5 49.5

1.00 29.3 45.2

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Fig. 7 FESEM cross-section of porous ceramics for alumina-to-yolk mass ratios of a 0.75 and b 0.65 and its grain morphology of c 0.75 and

d 0.65

Fig. 8 Top view SEM images of sintered alumina porous bodies with the green bodies dried at temperatures of 110 (a), 150 (b) and 180 �C (c).

Side view image for 180 �C drying is shown in (d)

J Porous Mater (2011) 18:195–203 201

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3.6 Mechanical strength

In order to evaluate mechanical properties of the samples,

compressive strength tests were conducted. The compres-

sive strength remarkably increased from 1.07 to 5.72 MPa

with the decreasing porosity from 71 to 40 vol.%, as shown

in Fig. 10. Many studies have been made to establish a

relationship between strength and microstructural proper-

ties such as pore structure and pore size distribution. The

strength of a porous ceramic is strongly affected by the

strength of the ceramic wall (or strut) and the surface flaws

on the strut. Porosity is also considered to have a significant

impact on compressive strength.

The results of compressive strength measurement were

fit to three mathematical models given in Table 4, to

investigate the relationship between pore structure and

compressive strength. In these equations, ‘‘rc’’ is defined as

the strength of a porous material. ‘‘rc0’’ is the theoretical

strength of a material at zero porosity. Porosity is repre-

sented with ‘‘n’’ and ‘‘m’’ is a constant. Figure 10 shows

plots of porosity and compressive strength relationship for

the results of compressive strength measurements and the

three models. A smooth connection has been obtained

between relationship from experiment results and the

mathematical models. The error percentage of Balshin,

Ryshkewitch, and Hasselman models were 20, 25 and 18%,

respectively. Based on all models and the experiment facts,

Fig. 9 SEM micrograph of sintered samples prepared using slurries with added dispersant concentrations of: a 0.01, b 0.03, c 0.04 and d 0.05

wt%

Fig. 10 Relationship between porosity and compressive strength

Table 4 Mathematical models between porosity and compressive

strength [25]

Model Empirical equation

Balshin rc = rc0(1-n)m

Ryshkewitch rc = rc0 exp(-m�n)

Hasselman rc = rc0–m�n

202 J Porous Mater (2011) 18:195–203

123

it can be deduced that the compressive strength of porous

ceramics decrease with an increase in porosity, which were

represented well by our results.

4 Conclusions

A novel method to make porous alumina ceramics with

controllable properties using egg yolk both as consolidat-

ing and foaming agent was successfully developed. The

physical properties of the ceramic like pore sizes, shrink-

age, porosities and compressive strength were controlled

by altering initial yolk contents employed in the slurry and

foaming process condition. Low alumina-to-yolk ratio in

slurry resulted in low viscosity, thus providing high

foaming capacity and consequently manufacturing of big-

ger pore sizes. The slurry drying at high temperatures

resulted in high foaming capacity and bigger pore sizes as

well as privileged pore generation. The use of different

dispersant concentrations in 1.00 alumina-to-yolk ratio

slurries transmitted the rheological conditions, which

resulted in changes in the pore sizes and shapes. Depending

on dispersant concentration (0.01–0.05 wt%) in the alu-

mina slurry, the density ranged from 0.7 to 1.8 g/cm3 and

the mean pore size varied from 200 to 1,000 lm, while the

pore shapes altered from spherical to polygonal. The

compressive strength of the porous bodies increased from

1.1 MPa at 40% porosity to 5.7 MPa at 71% porosity. A

good correlation has been obtained between the relation-

ship from experiment data and other mathematical models.

The present method is applicable to fabricate a floating

microcarrier for cell culture in a bioreactor.

Acknowledgments The authors would like to thank Faculty of

Engineering and Research Management Center, International Islamic

University Malaysia for providing the financial support under the

research project No. EDW B 0901-197.

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