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Zone-specific temperature distribution, densification trajectory and grain-growth kinetics of microwave-hybrid-sintered and conventionally sintered Al2O3 slip casts Zone-specific temperature distribution, densification trajectory and grain-growth kinetics of...
Khalid, Muhammad Waqas; Kim, Young Il; Haq, Muhammad Aneeq; Kim, InYeong; Lee, Dongju; Kim, Bum Sung; Lee, Bin
2022-04-03 00:00:00
JOURNAL OF ASIAN CERAMIC SOCIETIES 2022, VOL. 10, NO. 2, 322–337 https://doi.org/10.1080/21870764.2022.2049997 FULL LENGTH ARTICLE Zone-specific temperature distribution, densification trajectory and grain-growth kinetics of microwave-hybrid-sintered and conventionally sintered Al O slip casts 2 3 a,b b,c a,b b,d c Muhammad Waqas Khalid , Young Il Kim , Muhammad Aneeq Haq , InYeong Kim , Dongju Lee , a,b a,b Bum Sung Kim and Bin Lee Department of Industrial Materials and Smart Manufacturing Engineering, University of Science and Technology, Daejeon, Republic of b c Korea; Korea Institute for Rare Metals, Korea Institute of Industrial Technology, Incheon, Republic of Korea; Department of Advanced Materials Engineering, Chungbuk National University, Cheongju, Republic of Korea; Department of Materials Science and Engineering, Korea University, Seoul, Republic of Korea ABSTRACT ARTICLE HISTORY Received 18 August 2021 A comparative study on the microwave hybrid sintering and conventional sintering of Al O 2 3 Accepted 2 March 2022 slip casts is reported to observe temperature distribution, grain-growth kinetics and densifica - tion in different regions of samples. It was found that the microwave hybrid sintering resulted KEYWORDS in a significantly lower temperature difference between the surface and the center as com- Slip casting; microwave pared to that in conventional sintering. Remarkably, in the microwave-hybrid-sintered samples, hybrid sintering; grain- the estimated activation energy for grain growth at the center was approximately 27% lower growth rate; activation than that at the surface in the conventionally sintered samples. The grain-growth rate in energy for grain growth; grain-size distribution microwave hybrid sintering was more than three times higher at the sample center –21 3 ((62.07 ± 0.06) × 10 m /s) than at the sample surface in the conventional sintering –21 3 ((18.75 ± 0.11) × 10 m /s). The volume diffusion for grain growth was found to be the most effective mechanism in all samples, irrespective of the sintering technique and point of observation. It is suggested that the heat-flux, as well as the microwave effect and influence of surface charge due to the electric field of the field-assisted process were the reasons for these outcomes. 1. Introduction The increasing acceptance of microwave sintering Alumina (Al O ) is one of the most economical and most for ceramics manufacturing reflects its executability as 2 3 commonly used materials in structural ceramics owing to a field-assisted process [9,10]. One of the largest points advantageous properties such as a high melting point, of interest is that irrespective of the material chemistry, corrosion resistance, wear resistance, hardness, thermal microwaves almost always decrease the sintering tem- conductivity, and biocompatibility. It has numerous appli- perature. However, most ceramic materials cannot cations and is used as a material for electronic-device absorb microwaves properly at low temperatures substrates, combustion-engine components, gears, owing to low dielectric losses, and the dielectric loss refractories, and body joints and dental implants [1–3]. increases with the sample temperature. Therefore, col- Sintering is a process of consolidating powder par- lateral heating is applied either by conventional means ticles to fabricate bulk material by mass transport [4,5]. or by using a susceptor such as silicon carbide, which is Generally, Al O powder and green bodies of desired effective for low-temperature heating with micro- 2 3 shapes are sintered into dense bodies using the con- waves [11,12]. ventional sintering process, especially on the industrial Owing to the aforementioned advantages, micro- scale. However, conventional sintering consumes large wave hybrid sintering has previously been proposed amounts of energy as Al O has a very high melting as a solution for sintering Al O with relatively low 2 3 2 3 point (2072°C), which necessitates very long proces- temperatures and time durations. With microwave sing times and high temperatures. Using conventional hybrid sintering, the shrinkage curves shift toward sintering as the powder-consolidating process, Lahiri lower temperatures relative to those for conventional et al. [6] sintered ~92% dense α–Al O at 1700°C with sintering [5,13,14]. Experimental demonstrations and 2 3 a heating cycle of 9 h, Fabris et al. [7] used a tempera- mathematical modeling have been performed in ture of 1670°C and heating cycle of more than 16 h to attempts to explain how electromagnetic fields and achieve ~98% dense Al O , and Tuan et al. [8] used diffusional transport interact with one another in this 2 3 a temperature of 1600°C and heating cycle of 6 h. field-assisted sintering process, as well as to explain CONTACT Bin Lee lbin@kitech.re.kr Korea Institute for Rare Metals, 9F, Getpearl Tower, Gaetbeol-ro, 12, Songdo-dong, Yeonsu-gu, Incheon 21999, Republic of Korea © 2022 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of The Korean Ceramic Society and The Ceramic Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. JOURNAL OF ASIAN CERAMIC SOCIETIES 323 the reasons for the outcomes [15–17]. However, the 2. Experimental procedure topic is still in debate, as the mechanisms and laws Fine α–Al O powder (99.99%, AKP–50, Sumitomo 2 3 governing microwave sintering remain unclear. Chemical, Japan) was used as the precursor. Particle Limited comparative research has been published size distribution of the α–Al O powder was measured 2 3 on the conventional and microwave sintering of Al by a particle size analyzer (LS13 320 MW, Beckman O [18–20]. Nevertheless, further comparative study is Coulter Inc., USA). The powder was ball-milled for required to clarify the phenomenon of microwave 72 h in an ethanol medium, prior to measuring the sintering. In particular, the temperature distribution, primary particle size, to remove the agglomerates. heat-flux behavior, densification, grain-growth α–Al O slip casts were fabricated, using the as- 2 3 kinetics, and their associated mechanisms on the sur- received powder, for each sintering technique (con- face as well as at center of the samples treated by ventional and microwave hybrid sintering). An Al O 2 3 microwave and conventional sintering need to be solid loading of 80 wt% (50 vol%) in deionized water investigated thoroughly. was used for the slip casting. Ammonium poly- The accurate temperature measurement during methacrylic acid solution (DARVAN® C–N, 25 wt% microwave sintering of a sample is of importance active matter, R.T. Vanderbilt Company, Inc., USA) was in exploring the mechanisms of microwave sinter- used as the dispersant, and it was added in the quan- ing. The presence of a thermocouple in tities with 0.10–0.70 wt% active matter with respect to a microwave cavity may lead to electromagnetic Al O . A planetary centrifugal mixer (ARE-500, Thinky 2 3 field perturbations [18]. The need for accurate tem- Corporation, Japan) was used to mix the slurries for perature measurement further enhances especially 10 min at 1000 rpm. The dispersant amount was opti- when a kinetic comparison with conventional sinter- mized by viscosity measurement (DV2TLV viscometer, ing is performed because such a comparison cannot Brookfield Ametek, USA) using an SC4–25 coaxial be carried out properly if the exact temperature is −1 cylindrical spindle at a shear rate of 2 s . Prior to the unknown. viscosity measurements, all slurries were subjected to To verify the temperatures at the surface and at −1 pre-shearing for 30s at a shear rate of 30s , followed the center of the sintered bodies, process tempera- by a rest condition for 1 min, in order to ensure the ture control rings (PTCR) were used. A PTCR is a ring- same rheological experience for the slurries. shaped ceramic body that is used to accurately mea- After the optimization of rheological properties, sure the temperature of the heating process taking polyvinyl alcohol (31,000–50,000 M , 87–89% hydro- place at the location of the ring. It measures the total lyzed, Sigma-Aldrich) with 20 wt% solution in deio- heat transferred to the ring via conduction, convec- nized water was added as a binder with quantity of tion, and radiation. A PTCR shrinks when exposed to 1 wt% active matter w.r.t Al O . The slurries were 2 3 heat, and the degree of shrinkage depends on the mixed again for 10 min. Debubbling was performed actual temperature at its location as well as the dura- in the same mixer using the “defoaming” mode at tion for which it is exposed to heat. The measured 1000 rpm for 20 min. Subsequently, the slurries were shrinkage is converted into the ring temperature, poured into a two-part gypsum mold with a diameter which represents the total amount of heat absorbed of 30 mm and height of 50 mm. After drying and by the ring [21,22], with the help of a chart. The shaping in the mold, the green casts were taken out temperature of a sample can be measured more of the mold and dried at 120°C for 12 h, following accurately using a combination of a PTCR and the which they were debinded at 700°C for 2 h in an thermocouple of the furnace as mentioned in electric box furnace (XY–1700 Mini, Hantech Co., Section 2. Korea). The heating rate used in drying and debinding The objective of the present study was to determine was 2°C/min. Next, the green casts were ground to how microwave hybrid sintering is different from con- a height of 15 mm, and the green density was mea- ventional sintering for the consolidation of Al O , with 2 3 sured by dimensional analysis. It was made sure that all a focus on densification behavior and grain growth the samples had the same relative green density kinetics on the surface as well as at the center of the of 58.5%. sample. To this end, α–Al O green bodies were fabri- 2 3 For accurate temperature measurements, Al O cated by slip casting, which is a commercially used 2 3 cylindrical green bodies (diameter: 40 mm; height: method to mold large bodies of any shape. The 21 mm) were fabricated via light uniaxial pressing. green bodies were sintered using conventional sinter- Three different types of PTCRs (outer ing and microwave hybrid sintering. A combination of diameter = 20 mm, inner diameter = 10 mm, PTCRs and furnace thermocouples was used to mea- thickness = 7 mm; FERRO France Sarl, France) with sure the temperatures. The grain-growth exponents, different temperature ranges were used such that rates, and activation energies were estimated, and the one ring was at center of the compact, and the other heat flux and possible mechanisms for the grain was on the surface, as shown in Figure 1. Conventional growth were explored. 324 M. W. KHALID ET AL. sintering was performed in an electric box furnace (XY– the surface and central temperatures of the sample 1700 Mini, Hantech Co., Korea), while microwave microwave hybrid sintered at the same set tempera- hybrid sintering was carried out in a single-mode- ture. Although the temperatures measured with PTCR cavity microwave furnace (SINTERMAT–1600, Unicera, may not exactly be the actual temperatures, the differ - Inc., Korea) at 2.45 GHz, with SiC granules surrounding ence between the temperatures at the center and sur- the sample for radiant heating. A heating rate of 10°C/ face for a certain sintering process, as well as that min was used up to 1550°C, with no holding time. The between the two different sintering processes could furnace thermocouples were touching the surface of be found and compared with the help of the differ - the samples in both the furnaces. After sintering, ences in the PTCR diameters. The specifications of the a vernier caliper was used to measure the diameters rings, set temperatures, and measured temperatures of the shrunk PTCRs. The measured diameter was then are given in Table 1. converted into temperature according to the chart Thus, after the temperature measurements, the provided by the PTCR manufacturer. green slip casts, without any PTCR, were placed in To reduce the errors in the measured temperature a high-purity alumina crucible that was packed with due to a slightly higher absorption of heat after the same powder as the compact to avoid contamina- a certain heating cycle ends, the surface temperature tion from the environment, and both conventional and during the conventional sintering was assumed to be microwave hybrid sintering were carried out. The fur- the set temperature since the surface was in contact nace thermocouples were in contact with the surface with the thermocouple, and the small difference in the of the samples in both the furnaces. The furnace tem- measured temperature with PTCR was subtracted. perature was set to 1550°C, and the heating rate was Then, the same difference was subtracted from the 10°C/min. For each sintering technique, five different measured temperature at the center, as well as from holding times ranging from 0–120 min were used. Figure 1. Schematic of temperature measurement for an Al O uniaxially pressed sample, using PTCR in a (a) conventional furnace 2 3 and (b) microwave furnace. JOURNAL OF ASIAN CERAMIC SOCIETIES 325 To analyze the surface (s) and center (c) points, as Agglomeration occurred owing to their fineness, schematically shown in Figure 2, the sintered samples which caused van der Waals attractive forces to be were first cut to full depth, perpendicular to the surface, dominant. However, the primary particle size seems to obtain a piece 3 mm in width along a segment contain- to be approximately 0.2 to 0.3 μm. As shown in ing point s. Subsequently, the piece was cut parallel to the Figure 3(b), the powder exhibits an average particle surface to obtain pieces with points s and c, each having size of 0.31 µm, which is in agreement with the FE-SEM a depth of 3 mm. Next, the density was analyzed using results, indicating that the ball-milling resulted in good Archimedes’ method. Ten readings were taken for every deagglomeration of the powder. Hence, all the particle sample, and the average was calculated. Phase analysis of sizes were in the colloidal range (0.1–10 µm), which is all the samples was carried out by X-ray diffraction (D8 preferred for slip casting [24]. Advance X-ray diffractometer, Bruker, Germany). The sam- It is well known that the solid loading in a slurry for ples were then polished (MetPrep 3, Allied High Tech, slip casting should be as high as possible to achieve Canada) and thermally etched in the electric box furnace a high green density [25]. Therefore, a solid loading of at 1450°C for 30 min. The heating rate was 10°C/min. 80 wt% (50 vol%) was used for the slurries prepared in Microstructural analysis was performed by field emis- this work. However, ceramic particles are prone to sion–scanning electron microscope (FE–SEM, JSM- agglomeration because of van der Waals forces, espe- 7100 F, JEOL, Japan). An image processing software pack- cially at high solid loading. Therefore, a dispersant was age (Fiji-ImageJ) [23] was used to measure the grain size needed to be used to disperse the particles. Studies distribution from around three hundred grains for each have shown that the surface properties of powders sample using the FE–SEM images. The grain size distribu- change with the addition of a dispersant; conse- tion for each sample was split into 0.5 µm bins (ranges), quently, repulsive forces, either due to the electrostatic and the results were analyzed thoroughly. repulsion originating from the overlapping of electrical For estimating the activation energies for grain double layers or due to steric hindrance resulting from growth, new green slip casts were fabricated following the absorption of large molecules, become higher than the previous method, and the samples were sintered the attractive forces, causing the particles to remain using both conventional and microwave hybrid sintering suspended in the slurry [24,25]. methods at set temperatures of 1400, 1450, and 1500°C. Figure 4 shows the optimization of the slurry rheol- No holding time was given, and a heating rate of 10°C/ ogy. No viscosity value could be obtained when dis- min was used. After cutting and polishing, the samples persant was used with active matter of 0.1–0.25 wt% were thermally etched at temperatures that were less w.r.t Al O as the slurry was too thick to measure 2 3 than the set sintering temperatures by 100°C. Next, FE- viscosity using the SC4–25 coaxial cylindrical spindle. SEM analysis was carried out to evaluate the grain size. The lowest measured viscosity of 7.60 Pa.s was obtained when the dispersant was used with an active matter of 0.5 wt% with respect to Al O . 2 3 3. Results and discussion Therefore, this quantity of the dispersant was chosen for all the samples. Since a decrease in flocculation Figure 3 shows the morphology and particle-size dis- increases the amount of liquid available for shearing, tribution of the ball-milled fine α–Al O AKP–50 pow- 2 3 the slip produces the minimum flocculation when it der particles. As shown in Figure 3(a), the powder exhibits the minimum viscosity. The viscosity particles are submicron and have random shapes. Figure 2. Schematic of an Al O slip cast sample and the points of interest (s: surface; c: center). 2 3 326 M. W. KHALID ET AL. Figure 3. (a) FE–SEM microstructure, and (b) particle-size analysis of Al O AKP–50 powder. 2 3 increased with further increase in the dispersant during conventional sintering. The details are pre- quantity. This increase in viscosity is regarded to dis- sented later along with the discussions for the densifi - turbance in particle stabilization due to the surface cation and grain-size results. tension effect of the increased quantity of disper- Phase analysis of all 20 samples, whose surfaces and sant [26]. centers were sintered by either technique, was per- Figure 5 shows the results of temperature measure- formed to check whether there was any detectable ments with PTCRs. For the set temperature of 950°C, impurity or phase change during either type of sinter- the measured temperature at the surface of the con- ing. Figure 6 shows the XRD patterns of four samples. ventionally sintered sample is considered to be correct Every sample contained an α–Al O phase similar to 2 3 as the thermocouple was in contact with the surface. the as-received powder, and no secondary phase was However, a large difference of 100°C is observed present in any of the samples. between the temperatures at the center and the sur- The variations in sintering results can be clarified by face, with the center showing a temperature of 850°C. a descriptive analysis of the microstructural evolutions Similarly, the temperature measured during conven- as a function of holding time, mode of sintering, and tional sintering was significantly higher at the surface sample region. Figure 7 shows FE–SEM microstructures than at the center for the set temperatures of 1250 and obtained from the surface and center of samples sin- 1550°C (see Table 1). tered by conventional sintering with holding times of During microwave hybrid sintering, the tempera- 0, 30, 60, 90, and 120 min. All the samples contained tures obtained from the PTCRs are higher at the center a few equiaxed and many non-equiaxed grains, with than at the surface, and those at the center are non-uniform grain-size distributions. Such a random approximately near to the set temperatures. In addi- mixture of relatively smaller and larger grains is tion, the temperature difference between the surface obtained during the sintering of Al O in air [27]. 2 3 and the center is remarkably lower than that observed The intrinsic motion of a grain boundary in a single- material system (such as the current system) is regu- lated by the diffusion of matter from the shortening grain to the enlarging grain. This implies that the grain- boundary movement is proportional to the diffusion coefficient. This diffusion coefficient is exhausted if the oxygen partial pressure is very low, which can occur in vacuum [28]. Hence, sintering of Al O in vacuum 2 3 slows down the sintering rate, impeding the amplified grain growth. In contrast, in air, no low oxygen partial pressure is present that could cause exhaustion of the oxygen diffusing through the grain boundaries and limit its motion [27]. In addition, it has been reported in other studies that during sintering in air, water vapor affects the mass transport such that the grain growth is enhanced [29,30]. The pores were very small when no holding time was applied, and they were larger in the samples that under- Figure 4. Rheology of Al O slurry prepared for slip casting. 2 3 went holding, demonstrating a pore coarsening JOURNAL OF ASIAN CERAMIC SOCIETIES 327 Figure 5. Measured temperature versus set temperature of the (a) conventionally sintered samples and (b) microwave-hybrid- sintered samples. Table 1. Temperature measurement results. Set Ring Temperature Temperature Measured Temperature Measured Temperature Measured Temperature Measured Temperature Type Range (°C) (°C) (°C) CS s (°C) CS c (°C) MS s (°C) MS c ETH 850–1100 950 950 850 930 945 STH 1130–1400 1250 1250 1217 1232 1248 HTH 1450–1750 1550 1550 1489 1525 1540 phenomenon. In a sintering process, small pores are diffusion rate, then the pores inside the grains cannot dragged by the moving grain boundary, decreasing be discharged in time and remain inside the grains, the number of pores. Where some pores shrink because forming intra-granular pores [32]. of the diffusion of matter, others grow by Ostwald For all sintering time conditions, the grains are ripening. Such pore growth usually occurs at high tem- clearly larger on the surface point than in the center. peratures and long holding times [31]. This can be Furthermore, as the holding time increases, the grains regarded as a reason for the larger pores in the samples become larger. The larger grain size on the surface that underwent holding at the sintering temperature. indicates a higher temperature and higher grain- The pores were intra-granular as well as inter- boundary mobility. granular, consistent with the results of other reported Similarly, Figure 8 shows FE–SEM microstructures studies [27,32,33]. The appearance of intra-granular obtained from the surface and center of samples sin- pores may indicate a high sintering temperature. At tered by microwave hybrid sintering with holding a high sintering temperature, if the rate of grain times of 0, 30, 60, 90, and 120 min. Similar to the boundary migration is larger than the grain boundary conventionally sintered samples, the microwave- hybrid-sintered samples also contained few equiaxed grains and many non-equiaxed grains, with non- uniform grain-size distributions. The pores are intra- granular as well as inter-granular, as expected when Al O is sintered in the presence of an electric field [34]. 2 3 In contrast to the samples sintered by conventional sintering, the microwave-hybrid-sintered samples had larger grains in center than on surface. It is also clear that the grains increase in size as the holding time increases. The larger grain size at the center indicates a higher temperature and higher grain-boundary mobility. Figure 9 shows the relative densities from the sur- face and center of the sintered samples. The relative densities of all the samples were above 95%. Irrespective of the sintering technique, all the samples showed a direct relation between the relative density and holding time. The relative densities were higher at the center than on the surface for the microwave- Figure 6. X-ray diffraction patterns of sintered Al O (s: sur- 2 3 face; c: center). hybrid-sintered samples. The relative densities were 328 M. W. KHALID ET AL. Figure 7. FE–SEM microstructures of conventionally sintered samples (s: surface; c: center). higher on the surface than at the center for the con- ventionally sintered samples. The relative density of a full sample (denoted by the symbol f) was measured between the relative densities of the surface and cen- ter regions. The measured temperatures were higher at the surface of the conventionally sintered samples than at the surface as well as at the center of the microwave-hybrid-sintered samples (Table 1). However, for a certain holding time, the relative den- sity was higher at the center as well as the surface of the microwave-hybrid-sintered sample. MS 120 c resulted in the highest relative density overall (98.7%). MS 120 s had a relative density of 98.2%. Meanwhile, CS 120 s exhibited a relative density of 97.9%, and CS 120 c had a relative density of 97.3%. The detailed reasoning will be provided together with the grain size results. Figure 9. Relative density of sintered samples (f: full compact; s: surface; c: center). Figure 8. FE–SEM microstructures of microwave-hybrid-sintered samples (s: surface; c: center). JOURNAL OF ASIAN CERAMIC SOCIETIES 329 Evidently, the grain growth is extremely sensitive to be concluded that the highest volume percentage of the heating mechanism and point of observation grains is shifted toward larger grain sizes on the sur- along the sample. Figure 10 shows quantitative proof face than at the center. In addition, the grain size of the change in grain size with respect to the sintering distribution is broader on the surface for convention- method and holding time. Figure 10(a) shows the grain ally sintered samples than at the center. size distributions from surface and center of the con- Figure 10(b) shows the grain size distributions from ventionally sintered samples. CS 0 c has grain size the surface and center of the microwave-hybrid- distribution up to only 2.5 µm. The highest volume sintered samples. MS 0 s has a grain size distribution percentage of grains (41.8%) lies in the 0.5–1 µm up to 4.5 µm, with the highest volume percentage of range for this sample. The grain size distribution dras- grains (20.5%) lying in the 0.5–1 µm range. Meanwhile, tically widens as the holding time increases. CS 90 c has MS 0 c is distributed up to 5 µm, with the highest the highest volume percentage of grains (15.0%) in the volume percentage of grains (21.1%) in the 2–2.5 µm 2–2.5 µm range. Finally, CS 120 c has the highest range. The grain size distribution significantly increases volume percentage of grains (16.5%) in the 3.5–4 µm with holding time, especially at the centers of the sam- range. The grain size distributions for both samples ples. MS 120 s and MS 120 c do not contain grains up to reach 7 µm. 1.5 µm. The grain size reaches 10 µm for MS 120 s and CS 0 s had a broader grain size distribution than CS 11 µm for MS 120 c. Thus, it can also be concluded that 0 c, with 32.0 vol% grains in the 1–1.5 µm range. The the grain size distribution is broader at the center for grain size distribution drastically increases with hold- microwave-hybrid-sintered samples than on the surface. ing time. The grain size distribution for CS 30s reaches In addition, the highest volume percentage of grains is 6 µm, whereas those for CS 60s, CS 90s, and CS 120 shifted toward larger grain sizes at the center than on s reach 7.5 µm. The highest volume percentage of the surface for a certain holding time. Comparing micro- grains also shifts to larger grain sizes with increasing wave hybrid sintering and conventional sintering, it can holding time. CS 120 s has its highest grain volume also be seen that the overall grain size distribution (on fraction (17.8%) in the 4–4.5 µm range. Thus, it can also the surface as well as at the center) is wider in the case of Figure 10. Grain-size distributions of (a) conventionally sintered samples and (b) microwave-hybrid-sintered samples, (c) average grain sizes at the surface and center points after different holding times, and (d) schematic showing the directions of the temperature gradient and grain growth during conventional and microwave sintering. 330 M. W. KHALID ET AL. microwave hybrid sintering. In addition, the increase in for the dominant diffusion mechanism. It is directly the distribution with holding time is more significant in related to the diffusion coefficient of the rate-limiting the case of microwave hybrid sintering than in the case species, which are the slowest diffusing species [40]. It of conventional sintering. is suggested that the enhanced densification in the The average grain size (Figure 10(c)) exhibits microwave-hybrid-sintered samples relative to the sur- a behavior similar to that of the relative density face of the conventionally sintered samples was (Figure 9) for both sintering techniques. For a certain caused by the microwave effect. holding time, the average grain size was larger at the The analysis of grain-growth kinetics is an important center as well as the surface of the microwave-hybrid- approach to understand the mechanisms involved in sintered samples, even though the measured tempera- the different grain-growth behaviors shown by the ture was higher at the surface of the conventionally same material (Al O ). Janney et al. [41] reported that 2 3 sintered samples. As can be seen, the average grain microwave-sintered samples showed a larger average size is larger on the surface than at the center in the grain size than conventionally sintered sample. They case of conventional sintering. The average grain size estimated the activation energies for the grain growth is larger at the center than on the surface in the case of of Al O during conventional and microwave sintering, 2 3 microwave hybrid sintering. In addition, all the sam- and the estimated activation energy for grain growth ples exhibit direct relations between the average grain during microwave sintering was 20% less than that size and holding time, i.e. as the holding time during conventional sintering because of the micro- increases, the average grain size increases, irrespective wave effect. Wang et al. [42] showed an increase in of the sintering technique. MS 120 c exhibits the lar- grain growth during microwave sintering due to the gest average grain size overall (5.7 µm). The average microwave effect. Golestani-fard et al. [19] also grain size for MS 120 s is 5.3 µm. Meanwhile, CS 120 observed an increased grain growth for microwave s has an average grain size of 4.8 µm, and the average sintering than for conventional sintering. It is sug- grain size is 4.3 µm for CS 120 c. gested that similar to the densification results, the The densification and grain growth results are enhanced grain growth for the microwave-hybrid- explained below. Microwave sintering utilizes micro- sintered samples, compared to that for the surface of wave electromagnetic radiation. Therefore, it is specu- the conventionally sintered samples, was due to the lated that the microwave electromagnetic field microwave effect. enhances the basic driving force of reducing the total The broadening of the grain size distributions with interfacial energy or provides an extra driving force holding times are attributed to the closure of open due to the ponderomotive force, which is the oscilla- porosities with an increase in the relative density. tory motion of charged particles in the presence of Grain growth elongates the open pores, thereby a coherent electromagnetic field for the diffusion of increasing the length and decreasing the diameter ions [18,35]. This leads to an accelerated mass trans- because of densification. This leads to pinching off of port via electromagnetic activation, rather than ther- the pores, resulting in closed pores. The conversion of mal activation. This phenomenon is called the open pores that act as grain-growth inhibitors into “microwave effect,” which also leads to a decrease in closed pores enhances the grain growth [4]. the activation energy for sintering [35,36]. Another means of explaining these results was sug- Previously, it was shown that compared to conven- gested by Becker et al. [43]. They proposed that during tional sintering, microwave sintering enhances the sin- electric-field-assisted sintering of ceramics, there exists terability by increasing the densification rate in Al O , a high surface area of material if there is no plastic 2 3 owing to the microwave effect [37–39]. Zuo et al. [5] deformation. The high surface area causes buildup and reported that conventionally sintered pure Al O discharge of the surface charge. As a result, particle 2 3 exhibited an activation energy for densification of sliding occurs due to surface softening, which speeds 528 ± 22 kJ/mol, which decreased to 440 ± 8 kJ/mol up densification. If there is no plastic deformation, the for microwave sintering. In another study [14], the electric field also affects the grain-growth mechanisms. activation energies for sintering were 652 and 451 kJ/ The charged defects that move to the particle surface mol for conventionally sintered and microwave- also affect the grain-boundary mobility, influencing sintered alumina, respectively, and the decrease in the grain growth. activation energy was attributed to the microwave As shown in Figure 9, the relative densities at the effect. Khalid et al. [13] reported a significantly low center are higher than those on the surface for the activation energy for densification of 246 ± 11 kJ/mol samples sintered by microwave hybrid sintering, for α–Al O , and this low activation energy was con- whereas the opposite is true for the samples sintered 2 3 sidered to be due to the microwave effect. The activa- by conventional sintering. Similarly, as is clear from tion energy for sintering in fact is the activation energy Figure 10(a–c), the average grain size is larger at the JOURNAL OF ASIAN CERAMIC SOCIETIES 331 center than on the surface in the case of microwave- and the heat energy further moves toward the core of the hybrid-sintered samples. The opposite is true in the sample via conduction. Further, a relative temperature case of conventionally sintered samples. These results drop occurs continuously from the surface toward the can be explained by the temperature gradient shown core [44,46]. Hence, the grains at the sample surface are in Figure 10(d). larger than those at the center with conventional Microwaves are absorbed volumetrically by the sam- sintering. ple, i.e. the sample starts volumetric bulk self-heating via To explore the effects of activation energy for grain molecular interactions with the electromagnetic field, growth in the current study, the newly fabricated green and the electromagnetic energy is converted into heat slip casts were sintered at set temperatures of 1400, energy to heat the sample. As the heat is generated 1450, and 1500°C without any holding time. To estimate from within the volume and radiates outwards because the temperatures that the samples were exposed to, the surface keeps losing heat to the surroundings, a heat linear intercepts of the measured values shown in flux that is directed outwards is generated. Interfaces Table 1 were obtained, as shown in Figure 11. The exhibit a significant boundary resistance to thermal estimated temperature values are presented in Table 2. transport, depending on how efficiently the phonons The microstructures of the center and surface of the can cross the interface. Therefore, heat transport across conventionally sintered samples are shown in Figure 12. an interface results in a step change in temperature Evidently, the samples experienced pore-coarsening with from one side of the interface to the other [13,44]. increase in temperature. The grains are larger at the sur- Consequently, the heat moves from the core of the face than at the center, and the grain size increases as the sample to the surface with a continuous relative tem- temperature increases. Figure 13 depicts the microstruc- perature drop. Hence, the densification and grain tures of the center and surface of the microwave-hybrid- growth are higher at the center, with the shift of grain- sintered samples. The samples experienced pore- size distribution to larger grains. coarsening with increase in temperature. Notably, the In the presence of a susceptor material (SiC was grains are larger at the center than at the surface, and used as the susceptor in this study), the sample the grain size increases as the temperature increases. absorbs radiant heat from the surroundings as well. Comparing the results shown in Figures 12 and 13, it The hybrid heating of the sample causes a more even can be seen that larger grains are produced in the micro- temperature distribution throughout the sample in the wave-hybrid-sintered samples than in the conventionally low-loss Al O , resulting in a relatively more uniform sintered ones. Figure 14 represents the quantified data for 2 3 heating throughout the sample. This reduces the mag- the average grain sizes. It can be seen that the microwave nitude of temperature gradient between the sample effect results in a larger average grain size at the center surface and the center. The curve of the parabolic temperature distribution in the sample is relatively flattened [45], which is in agreement with the results of the temperature measurements as shown in Table 1. The difference between the temperature values at the center and the surface of the microwave-hybrid- sintered samples is quite small when compared with conventional sintering. An important point to note is that despite using a susceptor, the temperature was higher at the sample center than the surface, which proposes that the heating was volumetric. It is sug- gested that the susceptor did not shield the interior of the sample from the microwave electric field, and an appreciable microwave electric field was present inside the sample that led to an influence on the mass trans- port due to the microwave effect. In the case of conventional sintering, heat moves from the heating elements to the sample surface and is Figure 11. Estimated temperatures obtained from the linear- absorbed by the sample surface. The surface heats up, intercepts of the measured temperatures. Table 2. Estimated temperatures from the measured temperatures using PTCRs. Set Temperature Estimated Temperature Estimated Temperature Estimated Temperature Estimated Temperature (°C) (°C) CS s (°C) CS c (°C) MS s (°C) MS c 1400 1400 1345 1378 1393 1450 1450 1398 1427 1442 1500 1500 1451 1477 1492 332 M. W. KHALID ET AL. Figure 12. FE–SEM images showing the microstructures of the conventionally sintered samples at different set temperatures with no holding time (s: surface; c: center). and the surface, even if the measured temperature is Comparing the conventionally sintered and lower than that at the surface of the conventionally sin- microwave-hybrid-sintered samples, it can be tered samples. noted that the activation energy for grain growth Because grain growth is a thermally activated pro- is significantly lowered when the same material (Al cess, it is assumed that the grain growth is dependent O ) is sintered in the microwave-hybrid environ- on the sintering temperature according to the follow- ment. The activation energy for grain growth at ing equation [47]: the center of the microwave-hybrid-sintered sam- ples is approximately 27% less than that at the ð Q=RTÞ D ¼ D � e (1) t 0 surface of the conventionally sintered samples. If only surfaces of the samples sintered by the two where D is the final grain size, D is the initial grain t 0 different processes are compared, then the activa- size, R is the gas constant, and T is the sintering tem- tion energy is found to be 21% less in the case of perature. Considering D to be a constant for all the microwave hybrid sintering. These results are attrib- samples, the activation energy can be obtained from uted to the microwave effect. the slope of the Arrhenius plot between the left-hand It is suggested that in the current study, slip casting side and right-hand side of Equation 1. as a wet shaping method played an important role in The Arrhenius plots are shown in Figure 15 along lowering the activation energies for the densification with the estimated activation energy values. For the as well as for the grain growth because of a higher conventionally sintered samples, the estimated activa- particle compaction and better homogeneity of parti- tion energy at the center is 152.78 ± 0.6 kJ/mol, whereas cle coordination [19]. In addition, it is suggested that that at the surface is 134.60 ± 1.4 kJ/mol. The activation the sintering aids present in the as-received Al O 2 3 energy at the surface of the microwave-hybrid-sintered powder helped to decrease the activation energies samples is estimated to be 106.15 ± 0.7 kJ/mol, whereas for the grain growth further. that at the center is 98.70 ± 0.4 kJ/mol. JOURNAL OF ASIAN CERAMIC SOCIETIES 333 Figure 13. FE–SEM images showing the microstructures of microwave-hybrid-sintered samples at different set temperatures with no holding time (s: surface; c: center). Figure 14. Average grain sizes at the surface and center points after sintering at different estimated temperatures. The grain growth rate is another important para- a grain-growth constant depending on the grain- meter of grain-growth kinetics that can be calculated growth mechanism. For a single-phase system, the from a kinetic law of the following form [48]: grain-growth constant typically varies between 2 and 4. An n value of 2 corresponds to grain-growth by grain n n D D ¼ Kt (2) boundary diffusion, 3 corresponds to volume diffusion, where K is the grain growth rate, D is the grain size and 4 means grain-growth by surface diffusion. For after the holding time, D is the grain size in the grain growth by surface diffusion, a high volume per- absence of holding, t is the holding time, and n is centage of free surface (porosity) is required [48]. As all 334 M. W. KHALID ET AL. Figure 15. Estimated activation energy for grain-growth at the (a) surface and (b) center of the conventionally sintered samples, and that at the (c) surface and (d) center of the microwave-hybrid-sintered samples. –21 3 the samples sintered at the set temperature of 1550°C sintered samples was (62.07 ± 0.06) × 10 m /s, have relative densities greater than 95%, the possibility whereas that on the surface was lower at –21 3 of grain growth by surface diffusion can be ruled out. (49.77 ± 0.13) × 10 m /s. In contrast, for convention- n n Figure 16(a) represents the plots of D – D versus ally sintered samples, the grain-growth rate on the sur- –21 3 holding time t, where the n value taken was 2. The face was high, at (18.75 ± 0.11) × 10 m /s, whereas –21 3 linear fits for CS s, CS c, and MS c exhibit linear regres- that at the center was low, at (9.18 ± 0.10) × 10 m /s. sion coefficients of 0.97, and that shown by MS s is Hence, microwave hybrid sintering resulted in n n 0.96. Figure 16(b) presents the plots of D – D versus a remarkably enhanced grain-growth rate compared to holding time, where the n value taken was 3. The linear conventional sintering. This rate was more than three fits for CS s, CS c, and MS c exhibit linear regression times higher at the centers of the microwave-hybrid- coefficients of 0.99, and that shown by MS s is 0.98. sintered samples than on the surfaces of the conven- Although the two n values do not bring a big differ - tionally sintered samples. In addition, it was more than ence, an n value of 3 gives R values closer to 1. Thus, it five times higher on the surface with microwave sinter- is suggested that for all the samples; both those that ing than at the center with conventional sintering. were conventionally sintered and those that were The microwave-hybrid-sintered samples showed microwave-hybrid-sintered, and both on the surface a higher grain-growth rate than conventionally sintered and at the center, grain growth by volume diffusion samples, further confirming that Al O grains grow fas- 2 3 was more dominant than grain boundary diffusion. ter with microwave hybrid sintering than with conven- A grain-growth constant of 3 has also previously tional sintering. These data confirm that the microwave been shown for the grain-growth analysis of Al O for effect not only decreases the activation energy for grain 2 3 conventional as well as microwave sintering [31,41]. growth, but also accelerates grain boundary migration The grain-growth rate (K) for conventionally sintered during microwave hybrid sintering. and microwave-hybrid-sintered Al O was calculated Diffusion mechanisms for densification may be differ - 2 3 3 3 from the slopes of graphs plotted between D – D ent between conventional and microwave sintering [41], and the holding time. There was indeed a difference in but diffusion mechanisms should not be confused with the grain-growth rate between the sintering techniques grain-growth mechanisms as all the samples showed and for different points of observation. The grain- same dominant grain-growth mechanism (volume diffu - growth rate at the center of the microwave-hybrid- sion). The structural changes in grains during grain JOURNAL OF ASIAN CERAMIC SOCIETIES 335 Figure 16. Grain-growth mechanism estimation with grain-growth constants n of (a) 2 and (b) 3, and (c) grain-growth rate (K) for conventionally sintered and microwave-hybrid-sintered Al O 2 3. growth under conventional and microwave hybrid sin- The temperature measurements revealed that when tering were similar, and the difference was in the grain fired at a certain set temperature, the temperature was size. Hence, the postulation that the same dominant significantly lower at the sample center than at the grain-growth kinetics mechanism exists in both conven- surface in the case of conventional sintering; a set tional and microwave sintering is supported by the simi- temperature of 1550°C resulted in a temperature of larity of microstructural evolutions, as shown in Figures 7, 1489°C at the center. In the case of microwave hybrid 8, 12, and 13. The postulation is also supported by the sintering, a set temperature of 1550°C resulted in observation of dominant cubic grain-growth kinetics in a temperature of 1525°C at the surface and 1540°C at both the sintering methods, as shown in Figure 16: the the center. These results indicate that despite using same value of n, providing an excellent fit for both a susceptor, the resulting heat flux was directed different sintering techniques. Thus, it is suggested that outwards. the difference in grain growth observed during the two The highest relative density achieved was 98.70% at sintering processes was due to the difference in the rate the center of the sample subjected to microwave sin- of material transfer, i.e. the diffusivities of the controlling tering with a holding time of 120 min. The former species, rather than the mechanism involved. sintering technique resulted in higher relative densities at the center, while the latter technique resulted in higher relative densities on the surface, with 97.88% being the highest relative density for conventional 4. Conclusion sintering with a holding time of 120 min. For a certain In this study, slip casting of α–Al O was performed, 2 3 set temperature, the microwave hybrid sintering and the samples were subjected to conventional and resulted in relative densities higher than those microwave hybrid sintering. The sintering tempera- obtained from the conventional sintering, despite tures were accurately measured using PTCRs. The a lower actual temperature at the surface and center microstructural evolutions, densification behaviors, than that at the surface of the conventionally sintered grain-size distributions, and grain-growth kinetics at samples. The highest volume fraction of the grains the surface and the center of the samples were studied shifted to a larger grain-size range at the surface than and compared. at the center for the conventionally sintered samples, 336 M. W. KHALID ET AL. with CS 120 s showing 17.76 vol% grains in the 4– [5] Zuo F, Badev A, Saunier S, et al. Microwave versus conventional sintering: estimate of the apparent acti- 4.5 μm grain-size range. The outcomes were opposite vation energy for densification of α-alumina and zinc in the case of the microwave-hybrid-sintered samples, oxide. J Eur Ceram Soc. 2014;34:3103–3110. with MS 120 c showing 14.40 vol% grains in the 5– [6] Lahiri S, Sinhamahapatra S, Tripathi HS, et al. 5.5 μm grain-size range. Rationalizing the role of magnesia and titania on sin- The estimated activation energy for grain growth tering of α-alumina. Ceram Int. 2016;42:15405–15413. [7] Fabris DCN, Polla MB, Acordi J, et al. 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Effects of the in the presence of microwave electromagnetic fields. susceptor dielectric properties on the microwave sinter- ing of alumina. J Am Ceram Soc. 2013;96:3728–3736. [12] Khalid MW, Kim Y Il, Haq MA, et al. Densification behavior of microwave hybrid sintered Al O bimodal powder 2 3 Acknowledgments mixtures and comparison with 3D modeling and simulation. Int J Refract Met Hard Mater. 2021;99:105586. The authors gratefully acknowledge the National Research [13] Khalid MW, Kim Y Il, Haq MA, et al. Microwave hybrid Foundation of Korea (NRF), Ministry of Education, Ministry of sintering of Al O and Al O –ZrO composites, and Science and ICT (MSIT), Korea, for the financial support under 2 3 2 3 2 effects of ZrO crystal structure on mechanical proper- project number NRF-2018R1D1A1B07043025. The authors ties, thermal properties, and sintering kinetics. Ceram also thank the Ministry of Trade, Industry & Energy (MOTIE), Int. 2020;46:9002–9010. Korea, for the financial support through the program [14] Mizuno M, Obata S, Takayama S, et al. Sintering of “Industrial strategic technology development program” alumina by 2.45 GHz microwave heating. J Eur Ceram under project number 20011520. Soc. 2004;24:387–391. [15] Rybakov KI, Olevsky EA, Krikun EV. Microwave sinter- ing: fundamentals and modeling. J Am Ceram Soc. Disclosure statement 2013;96:1003–1020. [16] Bhattacharya M, Basak T. Susceptor-assisted enhanced No potential conflict of interest was reported by the microwave processing of ceramics - a review. Crit Rev author(s). Solid State Mater Sci. 2017;42:433–469. [17] Singh S, Gupta D, Jain V, et al. Microwave processing of materials and applications in manufacturing Funding industries: a review. Mater Manuf Process. 2015;30 (1):1–29. This work was supported by the Ministry of Trade, Industry [18] Zuoa F, Saunier S, Marinel S, et al. 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