共查询到20条相似文献,搜索用时 25 毫秒
1.
Tetsuo Nakayasu Tetsuo Yamada Isao Tanaka Hirohiko Adachi Seishi Goto 《Journal of the American Ceramic Society》1997,80(10):2525-2532
First-principles molecular-orbital calculations of α- and ß-Si3 N4 with a trivalent lanthanide (Ln3+ ) ion at the interstitial site are conducted using model clusters that are composed of 41-43 atoms, neglecting lattice relaxation effects. When an interstitial Ln3+ ion is present, strong antibonding between the Ln3+ ion and Si3 N4 is found. The magnitude of the antibonding is almost the same between the alpha- and ß-Si3 N4 matrices. On the other hand, the Si-N bond around the Ln3+ ion is notably reinforced in alpha-Si3 N4 but not so much in ß-Si3 N4 . The different electronic response to the presence of the Ln3+ ion for the Si-N bond is concluded to be the origin of the different solubilities of interstitial Ln3+ ions between alpha- and ß-SiAlONs that are reported experimentally. The contribution of the electric field that is induced by the presence of a trivalent charge at the interstitial site is examined in detail; we have found that the Si-N bond strength is not simply determined by the electric field but rather in a more complex manner. 相似文献
2.
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
3.
IRVIN C. HUSEBY† HANS L. LUKAS GÜNTER PETZOW 《Journal of the American Ceramic Society》1975,58(9-10):377-380
The 1780°C isothermal section of the reciprocal quasiternary system Si3 N4 -SiO2 -BeO-Be3 N2 was investigated by the X-ray analysis of hot-pressed samples. The equilibrium relations shown involve previously known compounds and 8 newly found compounds: Be6 Si3 N8 , Be11 Si5 N14 , Be5 Si2 N6 , Be9 Si3 N10 , Be8 SiO4 N4 , Be6 O3 N2 , Be8 O5 N2 , and Be9 O6 N2 . Large solid solubility occurs in β-Si3 N4 , BeSiN2 , Be9 Si3 N10 , Be4 SiN4 , and β-Be3 N2 . Solid solubility in β-Si3 N4 extends toward Be2 SiO4 and decreases with increasing temperature from 19 mol% at 1770°C to 11.5 mol% Be2 SiO4 at 1880°C. A 4-phase isotherm, liquid +β-Si3 N4 ( ss )Si2 ON2 + BeO, exists at 1770°C. 相似文献
4.
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
5.
Fengxia Li Li Fu Xiaojian Ma Changhui Sun LianCheng Wang Chunli Guo Yitai Qian Yitai Qian 《Journal of the American Ceramic Society》2009,92(2):517-519
Starting from Si powder, NaN3 and different additives such as N -aminothiourea, iodine, or both, Si3 N4 nanomaterials were synthesized through the nitridation of silicon powder in autoclaves at 60°–190°C. As the additive was only N -aminothiourea, β-Si3 N4 nanorods and α-Si3 N4 nanoparticles were prepared at 170°C. If the additive was only iodine, α-Si3 N4 dendrites with β-Si3 N4 nanorods were obtained at 190°C. However, when both N -aminothiourea and iodine were added to the system of Si and NaN3 , the products composed of β-Si3 N4 nanorods and α, β-Si3 N4 nanoparticles could be prepared at 60°C. 相似文献
6.
Martin Krämer 《Journal of the American Ceramic Society》1993,76(6):1627-1629
α-Si3 N4 core structures within β-Si3 N4 grains have been studied by transmission electron microscopy. The grains were dispersed in an oxynitride glass which was previously melted at 1600°C. The cores were topotactically related to the as-grown β-Si3 N4 crystallites and are related to epitactical nucleation during heat treatment as the most probable mechanism. 相似文献
7.
Mikito Kitayama Kiyoshi Hirao Koji Watari Motohiro Toriyama Shuzo Kanzaki 《Journal of the American Ceramic Society》2001,84(2):353-58
β-Si3 N4 ceramics sintered with a series of rare-earth (RE = La, Nd, Gd, Y, Yb and Sc) oxide additives were fabricated by hot pressing and subsequent annealing. Their microstructures, lattice oxygen contents, and thermal conductivities were evaluated. Mean grain size increased, while lattice oxygen content decreased, and hence, thermal conductivity increased with decreasing ionic radius of the rare-earth element. In all cases, a marked change was observed in the order of ionic radius from La to Nd to Gd, and a little change was observed below them. Rare-earth oxide additives significantly influenced the thermal conductivity of β-Si3 N4 , unlike in the case of AlN. 相似文献
8.
Jianren Zeng Yoshinari Miyamoto Osamu Yamada 《Journal of the American Ceramic Society》1991,74(9):2197-2200
Fine Si3 N4 -SiC composite powders were synthesized in various SiC compositions to 46 vol% by nitriding combustion of silicon and carbon. The powders were composed of α-Si3 N4 , β-Si3 N4 , and β-SiC. The reaction analysis suggested that the SiC formation is assisted by the high reaction heat of Si nitridation. The sintered bodies consisted of uniformly dispersed grains of β-Si3 N4 , β-SiC, and a few Si2 N2 O. 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.
Youren Xu Chao M. Huang Waltraud M. Kriven Avigdor Zangvil 《Journal of the American Ceramic Society》1994,77(8):2213-2216
The microstructure of a pressureless sintered (1605°C, 90 min) O'+β' SiAlON ceramic with CeO2 doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20–30 μm long), and a continuous matrix of small rodlike β' grains (< 1.0 μm in length). Many α-Si3 N4 inclusions (0.1–0.5 μm in size) were found in the large O' grains. CeO2 -doping and its high doping level as well as the high Al2 O3 concentration were thought to be the main reasons for accelerating the reaction between the α-Si3 N4 and the Si-Al-O-N liquid to precipitate O'–SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant α–Si3 N4 from the reacting liquid, resulting in a delay in the α→β-Si3 N4 transformation. The large O' grains and the α-Si3 N4 inclusions have a pronounced effect on the strength degradation of O'+β' ceramics. 相似文献
11.
Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si- L 2,3 , Si- K , and N- K edges in α-Si3 N4 , β-Si3 N4 , spinel c -Si3 N4 , and Si2 N2 O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β-Si3 N4 and c -Si3 N4 . The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ELNES spectral calculation. 相似文献
12.
Zehn-Kun Huang Tseng-Ying Tien Tung-Sheng Yen 《Journal of the American Ceramic Society》1986,69(10):241-C
The subsolidus phase relationships in Si3 N4 –AlN–rare-earth oxide (Me2 O3 where Me=Nd, Sm, Gd, Dy, Er, and Yb) systems were studied. Solid-solution regions with the α-Si3 N4 structure were delineated along the Si3 N4 –"Me2 O3 :9AIN" joins for all of the rare-earth oxide systems studied. The solubility limits of these solid solutions increased with decreasing size of the rare-earth ions. 相似文献
13.
Wei-Wu Chen Xin-Lu Su Pei-Ling Wang Dong-Sheng Yan Yi-Bing Cheng Koji Watari 《Journal of the American Ceramic Society》2005,88(10):2955-2956
Dy-α-sialon and β-Si3 N4 materials containing Dy-oxynitride glass were hot pressed at 1800°C for 1 h. The luminescence spectra of Dy3+ in these samples were compared when excited at 350 nm. The results showed that two strong emission bands in the region 470–500 nm and 570-600 nm, associated with the 4 F9/2 →6 H15/2 and 4 F9/2 →6 H13/2 transitions of Dy3+ ions, were observed in Dy-α-sialon. However, no emission peak was detected from the β-Si3 N4 sample, despite it containing the same amount of Dy3+ cations. This proved that only the Dy3+ cations in the α-sialon structure, not those in the oxynitride glass, produce the luminescence spectrum. 相似文献
14.
The kinetics of anisotropic β-Si3 N4 grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β-Si3 N4 grains follows the empirical growth law Dn – D0 n = kt , with the exponents equaling 3 and 5 for length [001] and width [210] directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β-Si3 N4 growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperature and time. 相似文献
15.
Tzer-Shin Sheu 《Journal of the American Ceramic Society》1994,77(9):2345-2353
The in situ β-Si3 N4 /α'-SiAlON composite was studied along the Si3 N4 –Y2 O3 : 9 AlN composition line. This two phase composite was fully densified at 1780°C by hot pressing Densification curves and phase developments of the β-Si3 N4 /α'-SiAlON composite were found to vary with composition. Because of the cooperative formation of α'-Si AlON and β-Si3 N4 during its phase development, this composite had equiaxed α'-SiAlON (∼0.2 μm) and elongated β-Si3 N4 fine grains. The optimum mechanical properties of this two-phase composite were in the sample with 30–40%α', which had a flexural strength of 1100 MPa at 25°C 800 MPa at 1400°C in air, and a fracture toughness 6 Mpa·m1/2 . α'-SiAlON grains were equiaxed under a sintering condition at 1780°C or lower temperatures. Morphologies of the α°-SiAlON grains were affected by the sintering conditions. 相似文献
16.
Tetsuo Nakayasu Tetsuo Yamada Isao Tanaka Hirohiko Adachi Seichi Goto 《Journal of the American Ceramic Society》1996,79(10):2527-2532
First-principles molecular-orbital calculations using the discrete-variational Xα method have been made on model clusters of α-Si3 N4 and its solid solutions with lanthanide elements, which occupy interstitial sites in the structure. The formula is Lnχ Si12–4.5χ Al4.5χ O1.5χ N16–1.5χ (Ln = La, Nd, Gd, Dy, Ho, Er, Tm, Yb), i.e., a Ln-α-SiAION solid solution. Covalent bond strength between Si and N, evaluated by overlap population, increases because of the presence of trivalent charges at the interstitial sites. When a Ln3+ ion is present, antibondings occur between Ln orbitals and N/Si orbitals, and they depend significantly on the ionic radius of Ln3+ . The total overlap population for the whole cluster is determined by the balance of Si-N bond reinforcement and Ln-N/Si antibonding. Although no lattice relaxation around the Ln3+ ion is included in the present calculation, good correlation between maximum solubility and the total overlap population for the whole cluster is demonstrated for the first time. 相似文献
17.
β-Si3 N4 ceramics sintered with Yb2 O3 and ZrO2 were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si3 N4 raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)−1 ). Thus, this work demonstrates that it is possible for β-Si3 N4 ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder. 相似文献
18.
Gui-hua Peng Min Liang Zhen-hua Liang Qing-yu Li Wen-lan Li Qian Liu 《Journal of the American Ceramic Society》2009,92(9):2122-2124
Silicon nitride ceramics were prepared by spark plasma sintering (SPS) at temperatures of 1450°–1600°C for 3–12 min, using α-Si3 N4 powders as raw materials and MgSiN2 as sintering additives. Almost full density of the sample was achieved after sintering at 1450°C for 6 min, while there was about 80 wt%α-Si3 N4 phase left in the sintered material. α-Si3 N4 was completely transformed to β-Si3 N4 after sintering at 1500°C for 12 min. The thermal conductivity of sintered materials increased with increasing sintering temperature or holding time. Thermal conductivity of 100 W·(m·K)−1 was achieved after sintering at 1600°C for 12 min. The results imply that SPS is an effective and fast method to fabricate β-Si3 N4 ceramics with high thermal conductivity when appropriate additives are used. 相似文献
19.
Ibram Ganesh 《International Journal of Applied Ceramic Technology》2009,6(1):89-101
In this paper, a new net-shaping process, an hydrolysis-induced aqueous gelcasting (GC) (GCHAS) has been reported for consolidation of β-Si4 Al2 O2 N6 ceramics from aqueous slurries containing 48–50 vol%α-Si3 N4 , α-Al2 O3 , AlN, and Y2 O3 powders mixture. Dense ceramics of same composition were also consolidated by aqueous GC and hydrolysis assisted solidification routes. Among three techniques used, the GCHAS process was found to be superior for fabricating defect-free thin wall β-Si4 Al2 O2 N6 crucibles and tubes. Before use, the as purchased AlN powder was passivated against hydrolysis. The sintered β-Si4 Al2 O2 N6 ceramics exhibited comparable properties with those reported for similar materials in the literature. 相似文献
20.
K. HIRAGA M. HIRABAYASHI S. HAYASHI T. HIRAI 《Journal of the American Ceramic Society》1983,66(8):539-542
Microstructures of Si3 N4 -TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si3 N4 . Crystallographic orientation relations between β-Si3 N4 and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si3 N4 crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si3 N4 matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix. 相似文献