共查询到20条相似文献,搜索用时 46 毫秒
1.
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
2.
L. J. BOWEN T. G. CARRUTHERS R. J. BROOK 《Journal of the American Ceramic Society》1978,61(7-8):335-359
The rates of densification and phase transformation undergone by α-Si3 N4 during hot-pressing in the presence of Y2 O3 , Y2 O3 −2SiO2 , and Li2 0−2Si02 as additives were studied. Although these systems behave less simply than MgO-doped Si3 N4 , the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si3 N4 phase. 相似文献
3.
A distinct bimodal microstructure has been obtained in a Si3 N4 –BAS (barium aluminum silicate) ceramic-matrix composite by pressureless sintering. It is shown that the addition of coarse β-Si3 N4 seeds causes abnormal grain growth in this composite, and hence encourages the formation of a bimodal microstructure. This abnormal grain growth is due to the nature of the heterogeneous nucleation mechanism in Si3 N4 α-to-β phase transformation, and is promoted by the transformation. After complete phase transformation, further abnormal grain growth is comparably slow and governed by the Ostwald ripening mechanism. Therefore, a stable bimodal microstructure can be easily achieved by pressureless sintering. 相似文献
4.
Silicon nitride (Si3 N4 ) ceramics, prepared with Y2 O3 and Al2 O3 sintering additives, have been densified in air at temperatures of up to 1750°C using a conventional MoSi2 element furnace. At the highest sintering temperatures, densities in excess of 98% of theoretical have been achieved for materials prepared with a combined sintering addition of 12 wt% Y2 O3 and 3 wt% Al2 O3 . Densification is accompanied by a small weight gain (typically <1–2 wt%), because of limited passive oxidation of the sample. Complete α- to β-Si3 N4 transformation can be achieved at temperatures above 1650°C, although a low volume fraction of Si2 N2 O is also observed to form below 1750°C. Partial crystallization of the residual grain-boundary glassy phase was also apparent, with β-Y2 Si2 O7 being noted in the majority of samples. The microstructures of the sintered materials exhibited typical β-Si3 N4 elongated grain morphologies, indicating potential for low-cost processing of in situ toughened Si3 N4 -based ceramics. 相似文献
5.
Jianren Zeng Isao Tanaka Yoshinari Miyamoto Osamu Yamada† Koichi Niihara 《Journal of the American Ceramic Society》1992,75(1):148-152
The influence of SiO2 addition on the densification and microstructural development of high-purity Si3 N4 during hot isostatic pressing (HIP) was studied. During HIP, densification was promoted, but the phase transformation from α -Si3 N4 to β -Si3 N4 was impeded by SiO2 . Analysis using a simple model shows that the enhanced densification was mainly due to the viscous flow of SiO2 . The microstructure changed remarkably at between 10 and 20 wt% SiO2 additions. Analysis of the phase transformation kinetics suggests that the diffusion of Si3 N4 through SiO2 glass is the ratecontrolling step for the transformation. 相似文献
6.
Pulsed Electric Current Sintering of Silicon Nitride 总被引:1,自引:0,他引:1
Motohiro Suganuma Yasunori Kitagawa Shigetaka Wada Norimitsu Murayama 《Journal of the American Ceramic Society》2003,86(3):387-394
Pulsed electric current sintering (PECS) has been used to densify α-Si3 N4 powder doped with oxide additives of Y2 O3 and Al2 O3 . A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS. 相似文献
7.
Gas-pressure sintering of α-Si3 N4 was carried out at 1850 ° to 2000°C in 980-kPa N2 . The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si3 N4 powder was similar to that in the materials from β-Si3 N4 powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si3 N4 ceramics. 相似文献
8.
Jun-Qi Li Fa Luo Dong-Mei Zhu Wan-Cheng Zhou 《Journal of the American Ceramic Society》2007,90(6):1950-1952
The influence of phase formation on the dielectric properties of silicon nitride (Si3 N4 ) ceramics, which were produced by pressureless sintering with additives in MgO–Al2 O3 –SiO2 system, was investigated. It seems that the difference in the dielectric properties of Si3 N4 ceramics sintered at different temperatures was mainly due to the difference of the relative content of α-Si3 N4 , β-Si3 N4 , and the intermediate product (Si2 N2 O) in the samples. Compared with α-Si3 N4 and Si2 N2 O, β-Si3 N4 is believed to be a major factor influencing the dielectric constant. The high-dielectric constant of β-Si3 N4 could be attributed to the ionic relaxation polarization. 相似文献
9.
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
10.
C. Greskovich 《Journal of the American Ceramic Society》1981,64(2):31-C-
The development of microstructure in hot-pressed Sia N4 was studiehd for a typical Si3 N4 powder with and without BeSiN2 as a densification aid. The effect of hot-pressing temperature on density, α- to β-Si3 N4 conversion and specific surface area showed that BeSiN2 appears to increase the mobility of the system by enhancing densification, α- to β-Si3 N4 transformation, and grain growth at temperatures between 1450° and 1800°. These processes appear to occur in the presence of a liquid phase. 相似文献
11.
Solid-liquid equilibria at 1750°C and subsolidus phase relations in the system Si3 N4 −AlN-SiO2 −Al2 O3 were determined for the composition region bounded by the β-Si3 N4 solid solution line and silica-alumina join X-ray diffraction and optical microscopy were used to determine the phases present in specimens cooled rapidly after equilibration. The extent of a single liquid-phase region and the tie lines for the βsolid solution + liquid field at 1750°C were established from quantitative X-ray diffractometry and lattice parameter measurements of βsolid solutions in equilibrium with liquid. The results were corroborated by optical microscopy and melting behavior observations. A new composition, Si12 Al18 O3 9N8 , is suggested for the x1 phase. The lowest melting temperature in the system is ≅ 1480°C and the corresponding composition is 10 eq% Al-90 eq%O. 相似文献
12.
Hisayuki Suematsu Mamoru Mitomo Terence E. Mitchell John J. Petrovic Osamu Fukunaga Naoki Ohashi 《Journal of the American Ceramic Society》1997,80(3):615-620
Single crystals of α-Si3 N4 were annealed at 2000°–2150°C. The β phase was detected after annealing at 2150°C only when the crystals were surrounded by MgO·3Al2 O3 or Y2 O3 powders. On the other hand, no evidence of the α–β transformation was found when the crystals were annealed without additives. The solution–precipitation mechanism was concluded to be the dominant factor in the α–β transformation of Si3 N4 . 相似文献
13.
The rate of dissolution of β-Si3 N4 into an Mg-Si-O-N glass was measured by working with a composition in the ternary system Si3 N4 -SiO2 -MgO such that Si2 N2 O rather than β-Si3 N4 was the equilibrium phase. Dissolution was driven by the chemical reaction Si3 N4 (c)+SiO2 ( l )→Si2 N2 O(c). Analysis of the kinetic data, in view of the morphology of the dissolving phase (Si3 N4 ) and the precipitating phase (Si2 N2 O), led to the conclusion that the dissolution rate was controlled by reaction at the crystal/glass interface of the Si3 N4 , crystals. The process appears to have a fairly constant activation energy, equal to 621 ±40 kJ-mol−1 , at T=1573 to 1723 K. This large activation energy is believed to reflect the sum of two quantities: the heat of solution of β-Si3 N4 hi the glass and the activation enthalpy for jumps of the slower-moving species across the crystal/glass interface. The data reported should be useful for interpreting creep and densification experiments with MgO-fluxed Si3 N4 . 相似文献
14.
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
15.
The densification behavior of Si3 N4 containing MgO was studied in detail. It was concluded that MgO forms a liquid phase (most likely a magnesium silicate). This liquid wets and allows atomic transfer of Si3 N4 . Evidence of a second-phase material between the Si3 N4 grains was obtained through etching studies. Transformation of α- to β-Si3 N4 during hot-pressing is not necessary for densification. 相似文献
16.
Jian-Feng Yang Yoshihisa Beppu Guo-Jun Zhang Tatsuki Ohji Shuzo Kanzaki 《Journal of the American Ceramic Society》2002,85(7):1879-1881
Single-phase β'-SiAlON (Si6− z Al z O z N8− z , z = 0–4.2) ceramics with porous structure have been prepared by pressureless sintering of powder mixtures of á-Si3 N4 , AlN, and Al2 O3 of the SiAlON compositions. A solution of AlN and Al2 O3 into Si3 N4 resulted in the β'-SiAlON, and full densification was prohibited because no other sintering additives were used. Relative densities ranging from 50%–90% were adjusted with the z -value and sintering temperature. The results of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy analyses indicated that single-phase β'-SiAlON free from a grain boundary glassy phase could be obtained. Both grain and pore sizes increased with increasing z -value. Low z -value resulted in a relatively high flexural strength. 相似文献
17.
Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si3 N4 -Al2 O3 , containing β-sialon, X phase, β-Si3 N4 , and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si4+ migration for the temperature range studied. Pure X phase shows ionic conduction by Si4+ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si3 N4 -Al2 O3 containing β-sialon, β-Si3 N4 X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively. 相似文献
18.
L. K. V. LOU T. E. MITCHELL A. H. HEUER 《Journal of the American Ceramic Society》1978,61(9-10):392-396
Impurity phases in commercial hot-pressed Si3 N4 were investigated using transmission electron microscopy. In addition to the dominant, β-Si3 N4 phase, small amounts of Si2 N2 O, SiC, and WC were found. Significantly, a continuous grain-boundary phase was observed in the ∼ 25 high-angle boundaries examined. This film is ∼ 10 Å thick between, β-Si3 N4 grains and ∼ 30 Å thick between Si2 N2 O and β-Si3 N4 grains. 相似文献
19.
Xinwen Zhu Tohru S. Suzuki Tetsuo Uchikoshi Yoshio Sakka 《Journal of the American Ceramic Society》2008,91(2):620-623
This paper reports the texturing behavior of β-sialon by strong magnetic field alignment (SMFA) during slip casting, followed by reaction pressureless sintering, using either α or β-Si3 N4 , Al2 O3 , and AlN as the starting materials. It is found that the β-Si3 N4 crystal exhibits a substantially stronger orientation ability than the α-Si3 N4 crystal regardless of the Si3 N4 raw powders in the magnetic field of 12 T. The β-raw powder produces a highly a , b -axis-oriented β-Si3 N4 green body with a Lotgering orientation factor of up to 0.97. During sintering, the β-raw powder allows the a , b -axis-oriented β-sialon to retain the Lotgering orientation factor similar to and even higher than that of β-Si3 N4 in the green body. In contrast, the α-raw powder leads to a faster transformation rate of α/β-Si3 N4 to β-sialon but a substantially lower texture in β-sialon. The results indicate that the use of the β-raw powder is more efficient for producing highly textured β-sialon via SMFA than that of the α-raw powder as well as the prolonged sintering. 相似文献
20.
The subsolidus phase diagram of the quasiternary system Si3 N4 -AlN-Y2 O3 was established. In this system α-Si3 N4 forms a solid solution with 0.1Y2 O3 : 0.9 AIN. The solubility limits are represented by Y0.33 Si10.5 Al1.5 O0.5 N15.5 and Y0.67 Si9 A13 ON15 . At 1700°C an equilibrium exists between β-Si3 N4 and this solid solution. 相似文献