SiC-ZrO2 複合材料のマイクロ波加熱における ZrO2 粒子分散が及ぼす影響 Lydia Anggraini1)、豊田
翔平 1)、藤原 弘 2)、飴山 惠 3)
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EFFECT OF MICROWAVE HEATING ON ZrO2 DISPERSION IN SiC-ZrO2 COMPOSITE Lydia Anggraini1), Shohei Toyota1), Hiroshi Fujiwara2) and Kei Ameyama3)
Microwave heating has been used to evaluate the zirconia dispersion in the silicon carbide-zirconia composite. During the evaluation of ZrO2 dispersions, the SiC with various ZrO2 contents and milling time were prepared. The temperatures of the microwave were controlled by the forward power of 0.8kW and measured by an infrared radiation thermometer. The final compacts and the microstructures observed were compared to those of spark plasma and conventionally electric furnace heated materials. The ZrO2 dispersion surrounding the outer surface of the SiC has been obtained for each specimen with and without mechanically milled using microwave heating. From the result of microstructure change, the relationship of ZrO2 fraction area and heating time revealed that microwave heating could prevent the grain coarsening occurring during a constant 1273K temperature with a 1.80ks longest heating time. Keywords: ZrO2, dispersion, microwave, SiC-ZrO2, microstructure Email:
[email protected] (Kei Ameyama)
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立命館大学グローバル・イノベーション研究機構、3) 立命館大学理工学部機械工学科 1) 2)
3)
立命館大学院理工学研究科機械システムコース、
Graduate School of Science and Engineering, Ritsumeikan University
Ritsumeikan Global Innovation Research Organization (R-GIRO), Ritsumeikan University
Department of Mechanical Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
1. Introduction Microwave technology initially was only targeted to food production. Nowadays, interest has been growing in the usages of microwave energy to powder metallurgy processing i.e. sintering. Developments in materials processing using microwave technology has demonstrated many advantages; are a few to mention energy saving, products with better properties and substantial savings in processing cycle times1,2). The microwave heating is a type of presureless sintering (MWS) process which is almost similar to the conventional electric furnace sintering (EFS) of a powder compact without applied pressure. The unique heating technique in that it heats materials directly can be achieved by microwave3). In contrast to the EFS techniques in which the surface of a body is initially heated by radiation and heat are subsequently conducted to the center of the body3). Additionally, many researchers have shown that various kinds of powder materials including ceramics can be sintered by microwaves at lower temperatures for shorter times than EFS4-8). They also reported that the potential of applicators in MWS maintains the nanometer grain size in the compacted specimens8,9), which have rapid heating rates than those required by EFS. Therefore, increasing evidence exists that the grain size measurement on the MWS and its effect on the densification has been studied before8-14). As yet, the effect of the particle dispersion of zirconia (ZrO2) in the silicon carbide–zirconia (SiC-ZrO2) composite during MWS and its comparison to other sintering methods such as spark plasma sintering (SPS) and EFS have not been reported before. During MWS, we also evaluated the microstructure changes on the SiC-ZrO2 composite. In order to prove whether microwave sintering process altered the grain growth, the relationships between heating time and ZrO2 fraction area are discussed in detailed.
2. Experimental Procedures 2.1 Preparation of SiC-ZrO2 Powder SiC powder of 2~3m was mechanically milled with 1m ZrO2 powder using a WC-Co ball and pot diameter of 5 and 60mm, respectively. The MM process was carried out using a Super Misuni vibration milling machine. This MM conducted a vibrating frequency of 125Hz. The powder to ball weight ratio was 1:5. In order to evaluate the ZrO2 dispersion in the SiC-ZrO2 composite, various compositions of ZrO2 i.e. 10mass% and 50mass% were used and mechanically milled for 36ks and 144ks, respectively. In addition, the mixture powder or without milled (0ks) of SiC with 50mass%ZrO2 were also prepared to compare the ZrO2 dispersion during MWS, SPS and EFS. The microstructure of all powders was characterized by scanning electron microscope (SEM). Furthermore, these powders were applied to the cold compaction process with 50MPa pressure and 0.30ks holding time before applying to the MWS and EFS.
2.2 Microwave and Other Sintering Conditions The experimental setup is schematized in Fig. 1 (IDX, MSS-TE0004, Japan) with a single-mode applicator and linked to a magnetron that delivers a variable forward power of up to 2kW at a frequency of 2.45GHz. Heating was performed in N2 gas flow. The field was tuned via a three-stub tuner. The heated materials were located inside the quartz crucible and processed in a flowing nitrogen atmosphere. They were set upon a quartz plate and surrounded by a silica tube for thermal insulation, as shown in Fig. 1(a). The alumina stage with an 18.50mm in diameter was used as the susceptor. Silica and alumina are readily absorbing microwaves, cause it to heat up. This difference in microwave
absorptions can be generating a temperature distribution, resulting in local heating15-17).
Fig. 1 Schematics of single-mode microwave 2.45 GHz applicator.
The temperature measurement on microwave was set on the center surfaces of the specimen inside the silica pot and above the susceptor are continuously measured by an infrared radiation thermometer camera (Chino, IR-CAI), which is located 170mm behind the applicator, as shown in Fig. 1(b). This camera converts infrared radiation from the surfaces facing it into an image that depicts temperature value to each pixel and can also calculate average values in prescribed zones of the image. The necessary input is the emissivity of the heated materials, which may be difficult to evaluate with accuracy, at a high temperature, in particular. The temperature measurements were first performed along the different electric (E) and magnetic (H)-field applied to the initial SiC and ZrO2 ceramics. The maximum setting of the specimens on the E-field plunger was set to 142mm and 102.25mm for the H-field. In this evaluation of different effect E and H-field, the forward power was fixed to 800W. This is based on our investigation of the temperature measurement of the initial SiC and ZrO2 powders compact where increased up to 800 and 900W powers, respectively. In our investigations, the temperature measurements were suddenly decreased starting 900W for SiC and 1000W for ZrO2 due to rise in the energy. Therefore, the 800W applied power was chosen for the evaluation of the effect of E and H-field on the initial SiC and ZrO2 powders. The E-field, SiC and ZrO2 ceramic powders could be sintered unlike the H-field, this indicates that the initial ceramics such as SiC and ZrO2 are the proto-typical ceramic materials with excellent dielectric properties. SiC has lower dielectric loss than ZrO218). However, the dielectric loss of SiC is higher than Al 2O3. This evidenced in Agrawal et al.19) has reported that Al2O3 material is not easy to heat up by microwave, especially at lower temperature. Since the dielectric loss of Al 2O3 increases with temperature, microwave heating of Al2O3 becomes more efficient at high temperature19). In SiC ceramic, absorb microwave radiation more efficiently at room temperature. The transparent Al2O3 can greatly enhance the interaction of the system with microwaves. In the magnetic field, all specimens cannot be heated at all under the same microwave power for same exposing time due to SiC, ZrO2 and Al2O3 are non magnetic materials. This demonstrates that the E-field at maximum position on microwave can be used to the ZrO2 dispersion and microstructure change evaluation. The evaluation of ZrO2 dispersions in the SiC-ZrO2 composite by microwave were carried out at 1273K temperature. The temperature on MWS was directly increased from 2s to around 40s sintering time. In addition to MWS, the other sintering methods such as SPS and EFS were used in order to compare the particle dispersion of ZrO2 ceramic in the SiC-ZrO2 ceramics composite. SPS is a type of pressure sintering process also known as a field assisted sintering
technique (FAST) which sintering technique that allows to produce fully dense materials within minutes20) while applying high heating rates and short times. The milled SiC-ZrO2 powder was loaded in a graphite die (10 mm diameter) and subjected to vacuum in a Dr. Sinter 1020 apparatus with 50MPa uniaxial pressure. SPS temperature was measured using an IR-AH thermometer. Temperature during SPS was increased from 60s and reached to the peak sintering temperature 1273K, at around 100s of the sintering time. For the conventional EFS, the milled powder was pre-compacted in 10mm diameter stainless steel die with 50MPa pressure, and sintered at 1273K temperature. In conventional EFS, sintering temperature was measured using an R-type thermocouple and gradually increased 1K from 1s to 30s, sintering time. Then, the temperature was increasing faster until at around 160s, sintering time. The comparison of heating profile of MWS, SPS and EFS are shown in Fig. 2.
Fig. 2 Heating profile of (a) MWS, (b) SPS and (c) EFS. The temperature measured by IR for MWS, SPS and by thermocouple for EFS.
3. Results and Discussion 3.1 Starting Powders Fig. 3 (a) and (b) shows the appearance of the initial SiC and ZrO2 powders, respectively. The initial SiC powder shows the irregular shapes with 2~3m in diameter and initial ZrO2 shows the agglomerate particles with 1m in diameter. The appearance of the SiC with 10mass%ZrO2 powders changed after milling for 36ks as shown in Fig. 3 (c), a SiC surface surrounded by ZrO2 powders, partially. The results of 50mass%ZrO2 are shown in Fig. 3 (d) and (e). The more dispersion of the ZrO2 powders is almost totally surrounded on the surface of SiC powder after mechanically milled for 144ks as shown in (d). Furthermore, in (e) shows a manually mixtured powder, without mechanical milling, the figure shows that both SiC and ZrO2 powders on the spreading apart and only few ZrO2 powders attached the SiC surface.
3.2 Dispersion of ZrO2 Powders Compact in SiC- ZrO2 Composite Table 1 shows the SEM micrographs of the dispersion of the compacted ZrO2 powders in the SiC-ZrO2 composite specimens sintered by microwave, spark plasma and conventional heating at 1273K. The SEM micrographs were observed on the cross section area of the specimen. Here, the bright irregular phases are ZrO2 powders and the dark phases are SiC powders, due to photographic contrast, the phases of SiC-ZrO2 composite appear brighter. The dispersion of ZrO2 can be easily recognized on the microstructure of the microwave specimens. As can be seen from
Fig. 3 SEM micrographs of (a) and (b) initial SiC and ZrO2, (c)-(e) MM of SiC-10mass%ZrO2 36ks, 50mass%ZrO2 144ks and 0ks, respectively.
Table 1, on MWS microstructure, the size of the SiC powders considerably decreased with increasing ZrO 2 content and milling time. On the other hand, the compacted mixed powders without mechanically milled, the size of the SiC powders was the same as the lowest content of ZrO2 at 36ks milling time. The dispersion morphology on MWS specimens of 10mass%ZrO2 at MM 36ks and 50mass%ZrO2 without mechanical milling in the SiC-ZrO2 composite was forming the slightly discontinuous structure on the ZrO2 powders. With 50mass%ZrO2 mechanically milled for 144ks, the homogeneously dispersed, like the network structure of ZrO2 surrounding several SiC powdered was clearly observed. The dispersion of the ZrO2 powders in the homogeneously dispersed structure spread larger than other microstructures. This suggested that the MM time, growth of the ZrO2 contents and the microwave heating at 1273K temperature have an important role in the formation of evenly dispersed homogeneous structure, because there is the local heating occurs in the single mode applicator. In these ceramics composite, the single mode microwave heated specimens, the most sintered parts are located in the top section of the specimen (in average, 200 m diameter at 1273K, partially), and almost no sintered part in the center and the bottom sections of the specimen (in average, 700 m height and diameter, at 1273K, respectively). Whereas the un-sintered part appearance is like a pile of powders after the cold compaction process, where almost no inter particle bond. In order to compare the microstructure of SiC-ZrO2 at 1273K temperature, the spark plasma and conventional electric furnace heating were used. The microstructure of spark plasma shows the partially sintered in whole compact area. As can be seen the backscattered electron image of SPS specimens in Table 1. The sintered area of the SPS microstructure of MM SiC-10mass%ZrO2 36ks and MM SiC-50mass%ZrO2 144ks show that the ZrO2 dispersion surrounds the SiC was not form unlike on the microwave. The SPS microstructure of MM SiC-50mass%ZrO2 0ks show relatively coarse. In a separate investigation, on the SPS of SiC-50mass%ZrO2 mechanically milled for 144ks, the homogeneously dispersed structure of ZrO2 surrounding several SiC powders were clearly observed at a higher temperature of 1773K
with 50MPa and 0.60ks applied pressure and holding time, respectively. However, the dispersion of ZrO2 on SPS was a predominantly heterogeneously dispersed with 10mass%ZrO2 and 50mass%ZrO2 without mechanical milling at 1773K temperature. Among the three routines, conventional EFS seem relatively coarse and porous which shows to the pile of powders after applied pressure and the inter particle bonding was appearing on partially. In addition, the uniformity of a microstructure in whole EFS compact area was obtained.
Table 1. Dispersion of ZrO2 powders in SiC-ZrO2 composites
Fig. 4 shows the heating profile of microstructure changes. The microstructure change on the microwave sintered SiC-ZrO2 composite materials were examined by controlling the power at 1273K and with gradually increasing sintering time from 0.18, 0.36, 0.60 and 1.80ks. After each heating time, the microwave power was turned off and the silica pot was cooled in room temperature. The sinter button is activated manually to start and stop with a determined sintering time that has been pre-defined. Before this examination, the four specimens have been prepared by the same cold compaction condition and applied to the microwave, alternately. Along the microstructure changes examination, the specimens were also set on the E-field maximum with the plunger length is 142mm from the reflection plate to the applicator outer wall. Microstructure observations were performed on the cross section of each specimen by SEM. Fig. 5 (a)-(d) shows the results of microstructure change on MWS. The result obtained from microstructure change at t1=0.18 ks, was that the powder dispersion of both SiC and ZrO2 were scattered throughout the composite. At t2=0.36 ks, the ZrO2 dispersion is started to condense and the closing of pores can be observed with SiC powders forming a liquid phase. This phenomenon occurs because of the melting of SiC at ~2273 K and ZrO2 at ~2973 K18). Thus, the sintering
temperature of SiC is expected to be lower than ZrO 2. The MWS of all powders were carried out at ~1273 K. So the composite SiC powders sintered first and surrounded the ZrO2 powders as shown on the illustration in Fig. 6. The black phases are SiC powders and grey phases are ZrO2. The white patches are the porosity in the composite. Finally, at longer sintering time t3=0.60 ks and t4=1.80 ks, compact of SiC surrounded by ZrO 2 become clear and completed. In addition, the ZrO2 and SiC powder fraction areas on these microstructures were calculated by using image analysis (AnalySIS FIVE, digital imaging solution). From the computation of the ZrO2 and SiC powder fraction areas the amount of ZrO2 and SiC powder fraction areas are not increases, at the longer sintering time which is shown in Fig. 7. This result proves the advantages of MWS are not only as time and energy efficient but also to prevent from powder growth or coarsening.
Fig. 4 Heating profile of MWS for evaluation of microstructure
Fig. 5 SEM micrographs of SiC-50mass%ZrO2 microstructure
changes at (a) 0.18, (b) 0.36, (c) 0.60 and (d) 1.80 ks
changes at (a) 0.18, (b) 0.36, (c) 0.60 and (d) 1.80 ks by MWS.
Fig. 6 Schematic illustration of microstructure changes at
Fig. 7 Relationship of ZrO2 fraction area and sintering time
(a) 0.18, (b) 0.36, (c) 0.60 and (d) 1.80 ks by MWS.
during microstructure changes on MWS.
4. Conclusions The dispersion of ZrO2 powders in the SiC-ZrO2 composite has been evaluated in a single-mode microwave applicator system operated at 2.45 GHz. In order to compare the effect of ZrO2 dispersion, spark plasma and electric furnace heating were also used. The microstructure change on the microwave sintered SiC-ZrO2 composite materials
were also examined at various sintering time i.e. from 0.18ks to 1.80ks. The results obtained from the evaluation of the ZrO2 dispersion in SiC-ZrO2 composite with or without milling, during microwave heating at 1273K temperature indicated that in the formation of ZrO2 dispersed surrounds the SiC or like the network structure were effective. Whereas the ZrO2 dispersion surrounds the SiC compact after heating by SPS and conventional EFS were not effective. From the examination of the microstructure changes resulting that the powder dispersion of both SiC and ZrO2 were scattered throughout the composite, and then ZrO2 dispersion was starting to condense with increasing sintering time. Finally, at the longest sintering time, the powder growth was not found on the SiC-ZrO2 powders. This evidence revealed that the advantages of MWS are not only as time and energy efficient but also to prevent from powder growth or coarsening.
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