Key Engineering Materials Vols. 317-318

Abstract: The target of this work is to investigate the effect of small additions of SiO2 or CaO on the sintering behavior and the microstructure of an ultrapure α-alumina compound. The sintering behavior has been investigated through extensive dilatometric study. SiO2 additions lead to a significant decrease in shrinkage rate during the intermediate stage of sintering whereas CaO is beneficent to densification. It has been found that during this stage which corresponds to the maximum of densification rate, grain boundaries diffusion controls densification through oxygen vacancies. The study of the densification behavior under different atmospheres help us to explain the role of the additives in agreement with electroneutrality equations. S.E.M. investigations confirm the well know correlation between doping and heterogeneous microstructures. After doping with SiO2 or CaO, abnormal grain growth appears at temperatures corresponding to the lowest eutectics given by Al2O3-SiO2 or Al2O3-CaO phase diagrams. H.R.T.E.M. observations show that below the critical temperatures for abnormal grain growth, additives enrichment is observed near grain boundaries (GBs). Above these temperatures, glassy phase for SiO2-doping and calciumhexaluminate (CA6) for CaO-doping are present at grain boundaries.

Abstract: In order to fabricate an alumina ceramics with high density at low sintering temperature, nanosized γ–Al2O3 powders with average size of 9.7 nm were added to microsized γ–Al2O3 powders with 2 #m and they were well mixed. Its sintering behavior was studied in the temperature range of 1000oC to 1300oC and in holding time from 1 hour to 10 hours. Compacted samples with a different mixed ratio of nanosized and microsized Al2O3 powders (N/M ratio) were prepared and pressured at 1 GPa in a uniaxial direction. The phase transformation from γ–Al2O3 to α–Al2O3 takes place at 1100oC for 1hour sintering in all compacted samples. This rate is increased with increasing N/M ratio. The relative density varied from 70% to 95% depending on temperature and N/M ratio. With increasing sintering temperature from 1000oC to 1300oC, it was changed from 70% to 93%. Especially, the relative density was enhanced about 9% higher than that of only microsized sample by only 10 wt% addition of nanosized powders.

Abstract: The Master Sintering Curve (MSC) is quite useful for analyzing the shrinkage behavior of ceramics. It is possible to compare shrinkage behavior using MSCs that are obtained from different firing profiles. In this study, shrinkage behavior during sintering of green bodies of several kinds of Al2O3 based ceramics were evaluated, using an electric furnace equipped with a dilatometer to be controlled based on the MSC theory. Although all of the samples shrank monotonically, shrinkage behavior depended on the additive and heating rate. The MSC theory was applied to analyze shrinkage behavior. As a result, a different MSC could be obtained in Al2O3 with and without the addition of MgO. In the pure Al2O3, a single MSC could be obtained from shrinkage curves by firing at a heating rate of 7.5-20oC/min, though the shrinkage curve at a heating rate of 3-5oC/min did not correspond with the MSC. In contrast, shrinkage curves at heating rate of 5-20oC/min were converged in the case of the MgO doped Al2O3 to obtain a unique MSC independent of firing profile. Apparent activation energy for sintering was estimated as 555 kJ/mol in the pure Al2O3 and 880 kJ/mol in the MgO doped Al2O3. The firing profile to obtain a requested sintering shrinkage curve was predicted from the resultant MSC. A comparison between the predicted and the experimental shrinkage curves, showed good consistency, thus confirming that it is possible to control shrinkage behavior using the MSC.

Abstract: Consolidation of pyrolyzed powders has been tried by hot isostatic pressing (HIP) without sintering additives, in order to obtain dense non-oxide ceramic bulk materials derived from polymer precursors. Si1.0C1.6N1.3 ceramic powders were derived from a polyvinylsilazane polymer. The polymer was thermally crosslinked at 250oC and pyrolyzed at 1050oC under Ar atmosphere. The pyrolyzed powders were die-pressed into rectangular bars at room temperature and densified by HIP at 1400oC-900 MPa and 1500oC-950 MPa. Dense ceramic monolith, in which pores were not observed by optical microscopy, was obtained by the HIP consolidation at 1500oC-950 MPa. The microstructure of the ceramic monolith was a nano-composite structure consisted of α-Si3N4 and graphite phases. In the compression tests of the HIP-treated sample, slight plastic deformation was observed at 1400 and 1500oC in spite of high compressive stress over 1000 MPa. On the other hand, the sample showed a compressive strain of about 7% at 1000 MPa at 1600oC. The compressive strain of about 11% was achieved at 1700oC.

Abstract: Co-firing of multiple materials results in the formation of internal stress due to the difference in shrinkage behavior. This internal stress causes retardation of sintering, crack formation and/or de-lamination at the interface. To reduce internal stress in layered structures, homogeneous pressing via centrifuge has been attempted. The effect of centrifugal pressing was demonstrated by film sintering, in which crack suppression and void removal by the present process were revealed. Owing to the homogeneous microstructure, the thermal stability of the film was significantly improved.

Abstract: We sintered α (6H)- and β(3C)-SiC powders using an Al-B-C additive. SiC powders were densified to > 98% of the theoretical density from 1950 to 2150oC with 0.67-2.7 mass % AlB2 and 2.0 mass % C. Sintering temperatures are 150-200 oC lower than the conventional. During sintering, 6H polytype in α-SiC powder was partly transformed to 4H. α-SiC powder grew moderately into plate-shaped grains. β-/SiC powder was completely transformed to 6H and subsequently 4H with large grain growth. Low-temperature sintering enabled us to use hot isostatic pressing resulting in pore-free SiC materials.

Abstract: Zr2Al3C5 has been successfully synthesized via solid state reaction between Al, ZrC and carbon powder at 1600
in vacuum. This complex carbide has very strong bond between metal atoms and carbon atoms. Thus, this material has a potential to be utilized as structural materials. Some properties of Zr2Al3C5 powder from solid-state reaction in vacuum had been tested. It was found that this powder was completely oxidized in air at 900
1 h, and can be hydrated in moist air. These drawbacks might come from the high reactivity of the powder due to synthesis in vacuum. Zr2Al3C5 powder from solid state reaction in vacuum was sintered at various temperatures from 1500
to 2000
under vacuum with pulse electric current sintering (PECS) and pressureless sintering. Zr2Al3C5 started to sinter at 1500
and got partially dense from 1700
. Physical properties and mechanical properties of this material were investigated and discussed.

Abstract: Lanthania (La2O3) and zirconia (ZrO2) powders in ethanol based suspension were mechanochemically treated in a planetary ball mill for 12 hours at 200 rpm, dried and sintered at various temperatures from 400
to 1500
. Particles in nanometer sizes are produced after milling. X-ray diffractometry results show the formation of single phase lanthanum zirconate on subsequent heat treatment for 1h at 1500
. Phase evolution based on the intensities of the XRD plots and BET surface area analysis indicates three stages of crystallization: below 800
, between 800
and ~1100
, and above 1100
where reflections of La2Zr2O7 with pyrochlore structure are increased with further heating. Only endothermic energy peaks are observed in the differential thermal analysis (DTA) curve of the milled powders, which could be attributed to the reactions involving dehydroxylation, decarboxylation and complete disintegration of ethanol. This indicates that probably, lanthanum zirconate has grown on sintering at high temperatures from the very fine particles produced by mechanochemical activation during milling. Thermogravimetric analysis has recorded a total weight loss of ~9% from the original weight of the milled powder on sintering at 1500
. The values of the surface area of the powders are found to decrease while the crystallite size of La2Zr2O7 are increased with increasing temperature.

Abstract: The effect of Al2O3 addition on sinterability of Tetragonal Zirconia Polycrystal powders including 3mol% Y2O3 (3Y-TZP) was investigated. Each 3Y-TZP powder dispersed with 0.3, 0.6, 0.9 and 1.2wt.% of Al2O3 was prepared by the spray-drying method. The prepared powders were pressed into a disk type and sintered at 1350
, 1400
, 1450
and 1500
for 2 hours in the air. Resultant microstructures and mechanical properties of specimens were investigated by using Vickers/Micro hardness Tester, FE-SEM and XRD. Most of the specimens showed high relative density over 99% and a higher fracture toughness than pure 3Y-TZP. Al2O3 particles dispersed in 3Y-TZP microstructure depressed grain growth of 3Y-TZP by the pinning effect. Increase in fracture toughness of 3Y-TZP was explained by the crack deflection due to dispersed Al2O3 particles.

Abstract: 12 mol%CeO2 based composites were fabricated by a pressureless sintering technique and post-HIP treatments. The mixture of 12CeO2-ZrO2/3Y2O3-ZrO2 (= 80 wt%/20 wt%) powders from various methods were prepared by a ball milling technique and consequently sintered at 1400
in air atmosphere and post-HIP treatment at 1350
. The 12Ce-ZrO2/Y-ZrO2 composites sintered from mixture of 12CeO2-ZrO2 powder and 3Y2O3-ZrO2 powders possessed the high strength over 1300 MPa and also high fracture toughness over 11 MPam1/2. Furthermore, the significant inhibition of the ZrO2 phase transformation of tetragonal to monoclinic phase under an aqueous hydrothermal condition at 150
was obtained for these 12Ce-ZrO2/Y-ZrO2 composites prepared from this method. Microstructural observation was in detail performed by TEM and EDS.