Microwave Sintering of Alumina at 2.45 GHz

Kristen H. Brosnan,* Gary L. Messing,** and Dinesh K. Agrawal*Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University,

University Park, Pennsylvania 16802

The sintering kinetics and microstructural evolution of alu-mina tubes (�17 mm length, �9 mm inner diameter, and �11mm outer diameter) were studied by conventional and micro-wave heating at 2.45 GHz. Temperature during microwaveheating was measured with an infrared pyrometer and wascalibrated to �10°C. With no hold at sintering temperature,microwave-sintered samples reached 95% density at 1350°Cversus 1600°C for conventionally heated samples. The activa-tion energy for microwave sintering was 85 � 10 kJ/mol,whereas the activation energy for conventionally sinteredsamples was 520 � 14 kJ/mol. Despite the difference intemperature, grains grew from �1.0 �m at 86% density to�2.6 �m at 98% density for both conventionally sintered andmicrowave-sintered samples. The grain size/density trajectorywas independent of the heating source. It is concluded that theenhanced densification with microwave heating is not a con-sequence of fast-firing and therefore is not a result in thechange in the relative rates of surface and grain boundarydiffusion in the presence of microwave energy.

I. Introduction

MICROWAVE heating results in lower energy costs and de-creased processing times for many industrial processes. In

ceramics, it can be used for sintering, synthesis, drying, ordebinding. Because of heat losses from the surface, the thermalgradients obtained by microwave heating are reverse to thoseobserved in conventional heating. That is, the core temperature ofthe microwave-heated sample is higher than the surroundingenvironment. By combining radiant heating and microwave heat-ing, the thermal gradients can be substantially reduced to obtain auniformly heated sample.1

Tian, Johnson, and Brodwin fast-fired MgO-doped alumina(Baikalox CR-30, Baikowski International Corp., Charlotte, NC)at 1700°C for 12 min in a 2.45 GHz microwave furnace to obtain99% and 99.8% densities for 0.8 and 1.9 �m final grain sizes,respectively. They claimed that the “enhanced sintering” (orlowered sintering time) was a result of the activation of grainboundary and lattice diffusion before surface diffusion couldcoarsen the microstructure and reduce the driving force forsintering.2 This explanation is similar to that of Harmer and Brook,who explained better densification of alumina by fast-firing.3

However, the microwave-sintered microstructures were not com-pared with conventionally sintered samples in the work by Tian etal. and thus it is not possible to know whether their hypothesis iscorrect.2

Janney and Kimrey sintered �-alumina (AKP-50, SumitomoChemical Co., Ltd., Osaka, Japan) in a microwave at 28 GHz at�250°C lower than conventional sintering. Densification up to98.2% density occurred after 60 min at 1200°C. They reported thatthe average pore size of the microwave-sintered samples wassmaller than the average pore size of samples conventionallysintered (for a given density). This could have been a result of lesscoarsening in the microwave-sintered samples.4

Ritzhaupt-Kleissl et al. showed that heating alumina withmicrowave energy lowered the sintering temperature and hold timeto obtain similar density samples as obtained by conventionalsintering. The grain size was reported to be 1 order of magnitudefiner, to 1–2 �m, when sintered at 1550°C in a microwavefurnace.5 However, these grain size results are relative to thesintering of alumina at 1620°C for 2 h in a conventional furnace.Significant grain growth probably occurred as a result of oversin-tering in the conventional furnace.

Katz and Blake achieved 99% dense alumina using �-Al2O3

(AKP-50, Sumitomo Chemical Co., Ltd., Osaka, Japan) as astarting material. The total processing time by microwave heatingwas over 100 min and the final sintering temperature was over1400°C. The final grain sizes were between 5 and 50 �m.6

Cheng et al. obtained 99.9% dense, transparent MgO-dopedAl2O3 (CR-10, Baikowski International Corp.) by microwavesintering at 1700°C for 30 min in ultrapure hydrogen. The averagegrain size for these samples was �30 �m.7

Other researchers have reported that the microstructure evolu-tion in microwave and conventional sintering are not the same, andthat microwave sintering produces finer-grained alumina.2,4,5

Accurate temperature measurement has been a serious issue inmicrowave sintering. It is important to note that while some research-ers provided detailed descriptions of temperature measurement tech-niques for microwave heating, there are many who do not. Importantproperties of any temperature measurement technique used in amicrowave furnace are that it should not change the microwave fieldor disturb the thermal distribution in the sample being measured.8,9

Optical fiber thermometers sometimes have a metallic coatingat the tip that can alter the microwave field where the temperatureis being measured. Pyrometry methods require emissivity data forthe material to be sintered to measure temperature accurately. Theadvantage of optical techniques is that they are noncontact, andtherefore the microwave field or the thermal distribution of thesample is not disturbed.

Pert et al. determined that using a single-color or two-colorpyrometer in a microwave furnace would give moderate toexcellent accuracy, while use of a thermocouple in a microwavefurnace has unproven accuracy. Also, the effects of microwavefield disturbances by a thermocouple are enhanced when thesample is a low- to medium-loss dielectric. For accurate temper-ature measurement, Pert et al. suggest calibration of noncontactmethods against a thermocouple in the absence of microwaves.8

Bruce et al. used an infrared pyrometer to measure temperatureduring microwave sintering. The authors corrected for thewavelength-dependent absorption of the Lucite window, used as aviewing port in the microwave furnace, by calibrating the pyrom-eter in a conventional furnace with a window.9 By adjusting for theconditions of the microwave (such as the Lucite window) and

L. Johnson—contributing editor

Manuscript No. 186633. Received September 30, 2002; approved March 19, 2003.Presented at the 105th Annual Meeting of the American Ceramic Society, St.

Louis, MO, May 1, 2002 (Basic Science Division, Paper No. AMB.1-F-06-2002).This work was supported by the NSF Particulate Materials Center.*Member, American Ceramic Society.**Fellow, American Ceramic Society.

J. Am. Ceram. Soc., 86 [8] 1307–12 (2003)

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calibrating the pyrometer with a conventional furnace using athermocouple, these authors demonstrated accurate and reproduc-ible temperature measurements.

While microwave heating holds promise for sintering alumina,there are some misrepresentations of its value relative to conven-tionally heated samples. This is because of the tendency tocompare samples after significantly different heating conditionsand inaccuracies in temperature measurements. In this paper,infrared pyrometry was used for accurate temperature measure-ments. Tubular samples were used to avoid the reverse temperaturegradient and therefore the temperature of the sample measured byinfrared pyrometry was likely the temperature of the entire sample.The densification and grain growth of microwave-heated andconventionally heated alumina are compared.

II. Experimental Procedure

Commercially produced alumina tubes (Osram Sylvania Prod-ucts, Inc., Beverly, MA) were obtained for this study. The tubeshad outer and inner diameters of �11 and �9 mm, respectively,and were cut to �17 mm lengths. The samples were comprised ofan organic binder and high-purity alumina (Baikalox CR-30,Baikowski International Corp.). The alumina was �65 wt%�-alumina and 35 wt% transitional �-alumina (Baikalox ProductInformation, Baikowski International Corp., Charlotte, NC, 2000).The alumina was doped with 350 ppm Y2O3 and 500 ppm MgO.Before sintering the binder was removed by heating at 5°C/min to650°C with no dwell in a conventional furnace. The average greendensity of the tubes after binder removal was 1.63 � 0.01 g/cm3 ata 95% confidence level (40.85 � 0.35% theoretical density at a95% confidence level) as determined from the dimensions andweight of 141 samples.

Samples were sintered in a 17 cm � 19 cm � 20 cmconventional furnace (Thermolyne 46100 High Temperature Fur-nace, Barnstead International, Dubuque, IA) heated with MoSi2heating elements. Sintering was also conducted in a homebuilt 47cm � 61 cm � 64 cm multimode 6 kW, 2.45 GHz microwavefurnace. All samples were sintered in air at temperatures rangingfrom 900° to 1600°C in various temperature intervals. Isothermalsintering kinetics were studied from 1150° to 1350°C for micro-wave heating and from 1350° to 1600°C for conventional heating.The heating rate for conventional sintering was maintained at10°C/min. The heating rate for the microwave furnace is unknownbelow 900°C, due to the limitations of the infrared pyrometer.After 10 min of microwave power, the temperature of the samplesreached 900°C and the heating rate to higher sintering tempera-tures was �45–60°C/min. It should be noted that only one samplewas used for each time and temperature condition reported inSection III.

The input power for microwave heating was adjusted manually tocontrol the sintering temperature and heating rate. The samples wereplaced in a crucible surrounded by an insulating casket made fromaluminosilicate fibers (Fiberfrax Duraboard 3000, Unifrax Corp.,Niagara Falls, NY). Figure 1 shows the design of the insulating casketfor microwave sintering. A hole of 1.3 cm diameter was drilled intothe top cover of the casket for infrared pyrometry. SiC powder (SiC99%, Alfa Aesar, Word Hill, MA) of 10–20 �m particle size wasused as a microwave absorber. The SiC powder was placed in a 100mL alumina crucible (No. 65505 AD998 High Alumina Crucible,CoorsTek, Golden, CO) containing the 40 mL sample crucible (No.65535 AD998 High Alumina Crucible, CoorsTek, Golden, CO). TheSiC powder was used as a low-temperature microwave susceptor topreheat the alumina samples because alumina is a poor microwaveabsorber at room temperature.

In this study, all temperatures in the microwave furnace weremeasured with a digital infrared pyrometer (Minolta/Land Cyclops152, Land Infrared, Bristol, PA). The pyrometer requires the userto manually set an emissivity value for the sample being measuredas well as manual focusing. The emissivity of the alumina sampleswas calibrated against samples heated in a conventional furnace.For calibration, the samples were heated to temperatures from 900°

to 1400°C and held for 30 min in a conventional furnace. Thetemperature for the calibration in the conventional furnace wasmeasured with a thermocouple and pyrometric shrinkage disks(Tempchek, Orton Ceramics, Westerville, OH) to determine theemissivity values of the samples as a function of temperature.Conditions of the microwave were also simulated, including usingthe same crucible to hold the samples. The measured emissivitydid not vary significantly with increasing temperature. Emissivitiesof 0.56 � 0.03 at a 95% confidence level were used for allmicrowave sintering experiments. The accuracy of temperaturemeasurement with the infrared pyrometer was determined to bewithin �10°C of the temperature measured with an S-typethermocouple in conventional sintering experiments. An accesshole (choke tube) located in the center of the top of the microwavecavity was used for pyrometry measurements. The choke tube(1.27 cm diameter, 5.60 cm length) prevented microwave leakageyet was large enough to allow accurate temperature measurementfor the pyrometer.

Scanning electron microscopy (SEM) (Philips XL20, PhilipsElectronic Instruments Co., Mahwah, NJ) was used to analyzemicrostructure evolution. For dense samples (�92% density),cross sections of the samples were polished and thermally etchedin air at 50°C below the sintering temperature for 30 min. Grainsizes were measured for these samples using the line-interceptmethod and multiplied by 1.56 to obtain the grain size.10 Forsamples that were below 92% density, grain sizes were measuredfrom fracture surfaces. Fracture surfaces displayed intergranularfracture so no multiplication factor was necessary. Densities weredetermined using the Archimedes method.

III. Results

The densities of the microwave-sintered and conventionallysintered tube samples, with no hold at temperature, are shown inFig. 2. The microwave-sintered samples reached almost fulldensity by 1400°C, whereas the conventionally sintered samples

Fig. 1. Insulating casket design for microwave sintering.

1308 Journal of the American Ceramic Society—Brosnan et al. Vol. 86, No. 8

were only 52% dense. Densification of microwave-sintered sam-ples started as early as 1100°C, whereas densification started at1300°C with conventional heating. An �250°C shift from theconventional sintering curve is observed in Fig. 2, which is similarto observations of lowered sintering temperature for microwavesintering of alumina at 28 GHz by Janney and Kimrey.4

The microstructures of conventionally sintered and microwave-sintered samples are shown in Fig. 3. The sample conventionallysintered to 1400°C shows very little densification and no coarsening(Fig. 3(a)). In contrast, the sample microwave-sintered to 1400°Cshows remarkable densification and grain growth (Fig. 3(c)). Themicrostructure of the microwave-sintered sample to 1400°C is verysimilar in density and microstructure as Tian et al. reported forMgO-doped alumina (Baikalox CR-30, Baikowski InternationalCorp.) microwave-sintered at temperatures �1600°C.2 Thissuggests that their temperature measurement using opticalpyrometry during microwave sintering was inaccurate.

Figures 4(a) and (b) show the isothermal densification for theconventionally sintered and microwave-sintered samples. At1350°C near full density was reached with microwave sintering inless than 50 min, while at this temperature the sample reached only62% density after 50 min by conventional sintering. Only at1600°C did the sample sinter quickly (�50 min) to full density inthe conventional furnace. Conventional sintering at lower temper-atures (�1350°C) did not result in significant densification evenafter 1000 min. At 1150°C, microwave-sintered alumina sampleachieved �95% density in less than 200 min.

The microstructure evolution of alumina conventionally sintered at1500°C is shown in Fig. 5. After 10 min at 1500°C, the sample is only79.7% dense (Fig. 5(a)). After 60 min at 1500°C, the sample is 92.2%dense and has a grain size of 1.2 �m, as seen in the fracture surfacein Fig. 5(b). After 480 min at 1500°C, the sample reached 98.6%density and the grain size was 2.8 �m. Overall, the conventionallysintered samples displayed normal grain growth with pores generallylocated on the grain boundaries (Fig. 5(c)).

The microstructure evolution for samples microwave-sintered at1200°C is shown in Fig. 6. After 70 min, the sample is 86.2% denseand has a grain size of about 1.0 �m (Fig. 6(a)). After 100 min, thesample is 95.6% dense and has a grain size of about 1.8 �m (Fig.6(b)). When the sample was microwave heated for 120 min, it was97.9% dense with a grain size of about 2.7 �m (Fig. 6(c)). Overall, themicrowave-sintered samples exhibited grain structures similar to theconventionally sintered samples of the same relative density.

The grain size is plotted as a function of relative density todetermine whether there is a difference in the grain growthtrajectories of the microwave-sintered and conventionally sinteredsamples. The relative grain sizes of samples microwave-sintered to

1200°C and conventionally sintered to 1500°C are plotted againsttheir relative densities in Fig. 7. It is clear that there is minimalgrain growth until samples are 90% dense, and then the data showa gradual, nonlinear increase in grain size with relative density.The dashed line is an extrapolation of the grain size data to thesample at 82% density with a grain size of 1 �m. The plot alsoshows that there is no noticeable difference in the grain growthtrajectories for microwave-sintered and conventionally sintered

Fig. 2. Density of microwave-sintered and conventionally sintered alu-mina samples as a function of sintering temperature with zero soak time.

Fig. 3. Microstructures of sintered alumina samples: (a) conventionallyheated to 1400°C, no hold, 52% dense; (b) conventionally heated to1600°C, no hold, 92% dense; (c) microwave-heated to 1400°C, no hold,98% dense.

August 2003 Microwave Sintering of Alumina at 2.45 GHz 1309

samples. The microwave-heated and conventionally heated sam-ples both have high densification rates relative to the grain growthrates. The grain sizes of �98% dense samples are only 3 timeslarger than the grain size of �90% dense samples.

Densification is a thermally activated process. The activationenergy for sintering is related to the densification rate by thefollowing equation:

dr/dt � Ce�Ea/RT� (1)

where dr/dt is the densification rate, C is a function of grain sizeand density, Ea is the activation energy for the rate-limitingdensification mechanism, T is the sintering temperature, and R isthe gas constant. The activation energy can be obtained from anArrhenius plot of the data from Fig. 4. The slope of the plot is–Ea/R. The densification rates at 0.80, 0.85, and 0.90 relativedensities for microwave and conventional sintering were used forthe activation energy analysis. Densification rates were taken forthese densities because densification occurs with minimal graingrowth (see Fig. 7). As a result, the driving force for densificationshould be the same because the samples have essentially the samegrain size. Three values for activation energy (from ln ( r/ t) vs

1/T plot) were obtained for both microwave and conventionalsintering. For each relative density, lines fit the data to R2 valuesof �0.95. Activation energies were determined from the slopes ofthe lines and reported based on three activation energy values eachfor microwave and conventional sintering. The activation energiesfor densification were calculated to be 85 � 10 kJ/mol at a 95%confidence level for microwave sintering and 520 � 14 kJ/mol ata 95% confidence level for conventional sintering.

IV. Discussion

Interestingly, both microwave-heated and conventionallyheated samples have the same grain size over the density range of86%–98% density despite the difference in sintering temperature.This suggests that the reason given by Tian et al. for betterdensification by microwave heating is not true for the material ofthis study since the degree of coarsening was the same for bothheating methods. Recall, however, that the alumina in this studywas doped with MgO and Y2O3 and only MgO was used as adopant in the study by Tian et al.2 The low degree of coarseningin this study may be a result of solute drag by the codopants.

Fig. 4. (a) Densification kinetics for conventionally sintered samples at various temperatures. (b) Densification kinetics for microwave-sintered samples atvarious temperatures.

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The driving force for pressureless sintering is the reduction insurface free energy of the system. In this study, the driving forcefor microwave-sintered and conventionally sintered samples is thesame throughout densification, as shown in Fig. 7. Thus, theenhanced densification of microwave-heated samples at lowersintering temperatures must be a result of enhanced diffusion ordensification by another mechanism.

Previous studies about the densification of MgO-dopedalumina report activation energies of 440 –578 kJ/mol forconventionally heated samples.3,11–13 In comparison, the con-ventionally heated samples of this study had an activationenergy of 520 kJ/mol. This result suggests that despite thecodoping in the alumina of this study, these materials sinter bysimilar mechanisms.

Previous studies in microwave densification of MgO-dopedalumina report activation energies of 170 –240 kJ/mol, which

are significantly less than reported for conventionally heatedaluminas.4,13,14 In contrast, the codoped alumina in this studyhad an activation energy of 85 kJ/mol for microwave-heatedsamples. In this study, the activation energy at 2.45 GHz iscloser to the 170 kJ/mol activation energy for microwavedensification at 28 GHz reported by Janney et al.15 Janney et al.suggested that part of the activation energy decrease could bedue to enhanced diffusion of oxygen. However, the differencein activation energy measured for enhanced diffusion is stillmuch less than the total difference measured for microwavesintering of polycrystalline alumina.15

The difference in the previously reported activation energiesfor microwave sintering and the results of this study may be dueto the presence of Y2O3 doping. Yttrium has been reported tosegregate to the grain boundaries in alumina and then precipi-tate as yttrium aluminum garnet (YAG) when a critical grain

Fig. 5. Microstructure evolution of samples conventionally sintered at1500°C for (a) 10, (b) 60, and (c) 480 min. Fig. 6. Microstructure evolution of samples microwave-sintered at

1200°C for (a) 70, (b) 100, and (c) 120 min.

August 2003 Microwave Sintering of Alumina at 2.45 GHz 1311

size is obtained in conventional heating.16 To date, however,there is a lack of significant literature on the effect ofmicrowave heating on Y2O3-doped and codoped aluminas. It isimpossible to conclude from the results of this study that thecodopants may account for the lower activation energy com-pared with other studies for densification by microwave heat-ing, but this should be investigated in a future study.

V. Summary

The sintering kinetics and the microstructural evolution ofmicrowave-sintered and conventionally sintered alumina tubeswere compared. To ensure accurate temperature measurement formicrowave heating, temperature was measured with an infraredpyrometer in the microwave furnace and was calibrated against aconventionally heated furnace.

Microwave sintering of the samples resulted in faster processingtimes and lower sintering temperature than conventional sintering.Samples microwave-sintered at 1400°C, with no hold time, were 98%dense. Similarly, samples microwave-heated at 1200°C for 120 minwere 98% dense. The grain growth for both microwave and conven-tional sintering followed similar grain growth trajectories with densi-fication. Thus, a fast-firing argument does not explain the enhanceddensification by microwave heating for the materials in this study.Note that the fastest heating rate in this study was 45°–60°C/min andthat at higher heating rates with other materials that couple withmicrowaves microstructural effects may be observed.

The apparent activation energy for microwave densification was85 � 10 kJ/mol, which is less than the measured value of 520 �14 kJ/mol for conventional sintering. The lower activation energyfor microwave sintering compared with conventional sintering isqualitatively in agreement with other studies in microwave sinter-ing. The significant difference in activation energy is believed tobe a result of enhanced diffusion, but the mechanistic differencecannot be determined from the data in this study.

Acknowledgment

The authors wish to express their thanks to Osram Sylvania for supplying aluminatubes.

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Fig. 7. Grain growth trajectories of microwave-sintered and conventionally sintered samples.

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