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1.
Ultrafine (<0.1 μm) high-purity θ-Al2 O3 powder containing 3–17.5 mol%α-Al2 O3 seeds was used to investigate the kinetics and microstructural evolution of the θ-Al2 O3 to α-Al2 O3 transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al2 O3 lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al2 O3 sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm. 相似文献
2.
Mark I. Jones Maria-Cecilia Valecillos Kiyoshi Hirao Motohiro Toriyama 《Journal of the American Ceramic Society》2001,84(10):2424-2426
Si3 N4 powders were sintered using a 28 GHz gyrotron source, with Y2 O3 , Al2 O3 , and MgO as sintering aids, in an attempt to investigate the effect of microwave radiation on densification behavior. The microwave-sintered samples were compared with identical samples produced by conventional pressureless sintering. The effect of sintering on the microstructural development and grain growth of the samples was assessed using scanning electron microscopy. Phase transformation behavior was assessed using X-ray diffractometry. In the microwave-sintered samples, densification and α→β transformation occurred at temperatures ∼200°C lower than those of the conventionally sintered samples. More importantly, at comparable stages of densification, the microstructures of the microwave-sintered and conventionally sintered samples were significantly different, with the microwave-sintered samples showing the development of elongated β grains at a much earlier stage of the α→β transformation. It was concluded that the effect of microwave radiation on sintering was not simply a decrease in sintering temperatures, but in possibly a different sintering mechanism, clearly related to localized heating within the grain-boundary phase. 相似文献
3.
Novel Two-Step Sintering Process to Obtain a Bimodal Microstructure in Silicon Nitride 总被引:5,自引:0,他引:5
Hai-Doo Kim Byung-Dong Han Dong-Soo Park Byong-Taek Lee Paul F. Becher 《Journal of the American Ceramic Society》2002,85(1):245-252
A two-step sintering process is described in which the first step suppresses densification while allowing the α-to-β phase transformation to proceed, and the second step, at higher temperatures, promotes densification and grain growth. This process allows one to obtain a bimodal microstructure in Si3 N4 without using β-Si3 N4 seed crystals. A carbothermal reduction process was used in the first step to modify the densification and transformation rates of the compacts consisting of Si3 N4 , Y2 O3 , Al2 O3 , and a carbon mixture. The carbothermal reduction process reduces the oxygen:nitrogen ratio of the Y-Si-Al-O-N glass that forms, which leads to the precipitation of crystalline oxynitride phases, in particular, the apatite phase. Precipitation of the apatite phase reduces the amount of liquid phase and retards the densification process up to 1750°C; however, the α-to-β phase transformation is not hindered. This results in the distribution of large β-nuclei in a porous fine-grained β-Si3 N4 matrix. Above 1750°C, liquid formed by the melting of apatite resulted in a rapid increase in densification rates, and the larger β-nuclei also grew rapidly, which promoted the development of a bimodal microstructure. 相似文献
4.
Kyeong-Sik Cho Young-Wook Kim † Heon-Jin Choi June-Gunn Lee 《Journal of the American Ceramic Society》1996,79(6):1711-1713
A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A12 O3 and Y2 O3 by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m1/2 , approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m1/2 ). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites. 相似文献
5.
Thommy Ekström Harald Herbertsson Matthew James Ian Fleck 《Journal of the American Ceramic Society》1994,77(12):3087-3092
β-sialon and Nd2 O3 -doped α-sialon materials of varying composition were prepared by sintering at 1775° and 1825°C and by glass-encapsulated hot isostatic pressing at 1700°C. Composites were also prepared by adding 2–20 wt% ZrO2 (3 mol% Nd2 O3 ) or 2–20 wt% ZrN to the β-sialon and α-sialon matrix, respectively. Neodymium was found to be a fairly poor α-sialon stabilizer even within the α-phase solid solution area, and addition of ZrN further inhibited the formation of the α-sialon phase. A decrease in Vickers hardness and an increase in toughness with increasing content of ZrO2 (Nd2 O3 ) or ZrN were seen in both the HIPed β-sialon/ZrO2 (Nd2 O3 ) composites and the HIPed Nd2 O3 -stabiIized α-sialons with ZrN additions. 相似文献
6.
α - Al2 O3 nanopowders with mean particle sizes of 10, 15, 48, and 80 nm synthesized by the doped α-Al2 O3 seed polyacrylamide gel method were used to sinter bulk Al2 O3 nanoceramics. The relative density of the Al2 O3 nanoceramics increases with increasing compaction pressure on the green compacts and decreasing mean particle size of the starting α-Al2 O3 nanopowders. The densification and fast grain growth of the Al2 O3 nanoceramics occur in different temperature ranges. The Al2 O3 nanoceramics with an average grain size of 70 nm and a relative density of 95% were obtained by a two-step sintering method. The densification and the suppression of the grain growth are achieved by exploiting the difference in kinetics between grain-boundary diffusion and grain-boundary migration. The densification was realized by the slower grain-boundary diffusion without promoting grain growth in second-step sintering. 相似文献
7.
Ya-Wen Li Pei-Ling Wang Wei-Wu Chen Yi-Bing Cheng Dong-Sheng Yan 《Journal of the American Ceramic Society》2002,85(10):2545-2549
Two calcium-doped α-SiAlON compositions (Ca0.6 Si10.2 Al1.8− O0.6 N15.4 and Ca1.8 Si6.6 Al5.4 O1.8 N14.2 ) were prepared by hot pressing at 1600° and 1500°C, respectively, for complete phase transformation from α-Si3 N4 to α-SiAlON. Both samples were subsequently fired at different temperatures for different periods of time to study the grain growth of α-SiAlON. Elongated α-SiAlON grains were developed in both samples at high temperatures. The kinetics of grain growth was investigated based on the variations in length and width of the α-SiAlON grains under different sintering conditions. Different growth rates were found between the length and width directions of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development. 相似文献
8.
Jian-Feng Yang Tatsuki Ohji Koichi Niihara 《Journal of the American Ceramic Society》2000,83(8):2094-2096
The present study investigates the influence of the content of Y2 O3 –Al2 O3 sintering additive on the sintering behavior and microstructure of Si3 N4 ceramics. The Y2 O3 :Al2 O3 ratio was fixed at 5:2, and sintering was conducted at temperatures of 1300°–1900°C. Increased sintering-additive content enhanced densification via particle rearrangement; however, phase transformation and grain growth were unaffected by additive content. After phase transformation was almost complete, a substantial decrease in density was identified, which resulted from the impingement of rodlike β-Si3 N4 grain growth. Phase transformation and grain growth were concluded to occur through a solution–reprecipitation mechanism that was controlled by the interfacial reaction. 相似文献
9.
Dong-Duk Lee Suk-Joong L. Kang Gunter Petzow Duk N. Yoon 《Journal of the American Ceramic Society》1990,73(3):767-769
By using α-Si3 N4 and β-Si3 N4 starting powders with similar particle size and distribution, the effect of α-β (β') phase transition on densification and microstructure is investigated during the liquid-phase sintering of 82Si3 N4 ·9Al2 O3 ·9Y2 O3 (wt%) and 80Si3 N4 ·13Al2 O3 ·5AIN·5AIN·2Y2 O3 . When α-Si3 N4 powder is used, the grains become elongated, apparently hindering the densification process. Hence, the phase transition does not enhance the densification. 相似文献
10.
Branko Matovic Georg Rixecker Fritz Aldinger 《Journal of the American Ceramic Society》2004,87(4):546-549
This paper deals with the densification and phase transformation during pressureless sintering of Si3 N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2 O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si3 N4 to β-Si3 N4 taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si3 N4 to β-Si3 N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2 O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li2 O−Y2 O3 system can provide very effective low-temperature sintering additives for silicon nitride. 相似文献
11.
Seeding of the Reaction-Bonded Aluminum Oxide Process 总被引:1,自引:0,他引:1
The effect of the initial α-Al2 O3 particle size in the reaction-bonded aluminum oxide (RBAO) process on the phase transformation of aluminum-derived γ-Al2 O3 to α-Al2 O3 , and subsequently densification, was investigated. It has been demonstrated that if the initial α-Al2 O3 particles are fine (∼0.2 μm, i.e., 2.9 × 1014 γ-Al2 O3 particles/cm3 ), then they seed the phase transformation. The fine α-Al2 O3 decreases the transformation temperature to ∼962°C and results in a finer microstructure. The smaller particle size of the seeded RBAO decreases the sintering temperature to as low as ∼1135°C. The results confirm that seeding can be utilized to improve phase transformations and densification and subsequently to tailor final microstructures in RBAO-derived ceramics. 相似文献
12.
Peter den Exter Louis Winnubst Theo H. P. Leuwerink Anthonie J. Burggraaf 《Journal of the American Ceramic Society》1994,77(9):2376-2380
The densification behavior of ZrO2 (+ 3 mol% Y2 O3 )/85 wt% Al2 O3 powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively. 相似文献
13.
Isao Tanaka Giuseppe Pezzotti Taira Okamoto Yoshinari Miyamoto Mitue Koizumi 《Journal of the American Ceramic Society》1989,72(9):1656-1660
Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO2 . Above 1730°C, the densification and β-grain elongation accelerated concurrently because of the appearance of liquid SiO2 . However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high-temperature strength. 相似文献
14.
Dense Sic ceramics were obtained by pressureless sintering of β-Sic and α-Sic powders as starting materials using Al2 O3 -Y2 O3 additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from α-SiC powder was composed of equiaxed grains, whereas SiC obtained from α-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the β→α phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using α-SiC powder increased slightly from 4.4 to 5.7 MPa.m1/2 with holding time, that is, increased grain size. In the case of the sintered SiC using β-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa.m1/2 with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains. 相似文献
15.
The phase relations in and physical properties of SiAlON ceramics prepared by the hot isostatic pressing technique or by pressureless sintering with a sintering aid are reviewed. The sintering aid used is an AIN and Al2 O3 mixture either pure or in combination with Y2 O3 and/or various rare-earth oxides. Special attention is paid to the amount and type of phase(s) formed and to how properties such as hardness (HV10), fracture toughness ( K IC ), and oxidation resistance vary with the sintering aid used for different types of SiAlON materials, i.e., O'-, almost monophasic α-, pure β-, and mixed (α+β)-SiAION Ceramics. 相似文献
16.
Mikito Kitayama Kiyoshi Hirao Shuzo Kanzaki 《Journal of the American Ceramic Society》2006,89(8):2612-2618
Fully dense Si3 N4 sintered with rare-earth (RE=La, Nd, Gd, and Yb) oxide additives were fabricated by hot pressing. They were mainly composed of the α-phase, which enabled the investigation of the α–β transformation kinetics without the effect of porosity. The phase transformation kinetics was found to be linear. The activation energies for the phase transformation were 677, 546, 487, and 424 kJ/mol for RE=La, Nd, Gd, and Yb, respectively, and showed a good correlation with the ionic radii of RE atoms, which was discussed in conjunction with the segregation behaviors of RE atoms at the interface. 相似文献
17.
The sinter forging behavior of α-Al2 O3 seeded and unseeded nanocrystalline θ-Al2 O3 was investigated as a function of temperature, stress, and strain rate. Seeded samples exhibited the highest degree of plastic deformation during the θ- to α-AI2 O3 phase transformation. As a result, microstructure control, increased densification, and a higher degree of transformation were obtained. A uniform microstructure of 150 nm α-Al2 O3 grains developed, reaching 57% relative density after sintering 1.5 wt%α-Al2 O3 seeded samples for 30 min at 1060°C. When sinter forged at 0.25 mm/min to 63 MPa and 1060°C for 30 min large deformations during the phase transformation increased the relative density to 74%. When the stress was increased to 235 MPa (1060°C, 30 min), 99.7% dense α-Al2 O3 with a grain size of 230 nm was obtained. By increasing the sinter forging temperature to 1150°C, 99.5% relative density was achieved at 190 MPa for 30 min. 相似文献
18.
Suxing Wu Helen M. Chan Martin P. Harmer 《Journal of the American Ceramic Society》2005,88(9):2369-2373
Tantalum (V) oxide (Ta2 O5 ) has potential applications as part of an environmental barrier coating system for Si3 N4 -based turbine components. However, at elevated temperatures, Ta2 O5 undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta2 O5 microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X-ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta2 O5 , transformation of the low-temperature β-phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta2 O5 was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO4 second and phase particles were seen to be highly effective at inhibiting grain growth. 相似文献
19.
Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2 O3 Additions 总被引:2,自引:1,他引:1
Jian-Feng Yang Guo-Jun Zhang Ji-Hong She Tatsuki Ohji Shuzo Kanzaki 《Journal of the American Ceramic Society》2004,87(9):1714-1719
Addition of Y2 O3 as a sintering additive to porous β-SiAlON (Si6− z Al z O z N8− z , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y2 O3 were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y2 O3 -free ceramics of the same chemical composition. Although this level of Y2 O3 addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y2 O3 -free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y2 O3 addition the supposed formation of Y-α-SiAlON. 相似文献
20.
Grain growth in a high-purity ZnO and for the same ZnO with Bi2 O3 additions from 0.5 to 4 wt% was studied for sintering from 900° to 1400°C in air. The results are discussed and compared with previous studies in terms of the phenomenological kinetic grain growth expression: G n — G n 0 = K 0 t exp(— Q/RT ). For the pure ZnO, the grain growth exponent or n value was observed to be 3 while the apparent activation energy was 224 ± 16 kJ/mol. These parameters substantiate the Gupta and Coble conclusion of a Zn2+ lattice diffusion mechanism. Additions of Bi2 O3 to promote liquidphase sintering increased the ZnO grain size and the grain growth exponent to about 5, but reduced the apparent activation energy to about 150 kJ/mol, independent of Bi2 O3 content. The preexponential term K 0 was also independent of Bi2 O3 content. It is concluded that the grain growth of ZnO in liquid-phase-sintered ZnO-Bi2 O3 ceramics is controlled by the phase boundary reaction of the solid ZnO grains and the Bi2 O3 -rich liquid phase. 相似文献