共查询到20条相似文献,搜索用时 15 毫秒
1.
Beta-type CVD-Si3 N4 plates (up to 1.1 mm thick) have been prepared by adding TiCl4 vapor to the system SiCl4 -NH3 -H2 at deposition temperatures of 1350° to 1450°C, while α-type or amorphous CVD-Si3 N4 was obtained without TiCl4 vapor at the same deposition temperature. Three to four wt % 777V was included in the β-type CVD-Si3 N4 matrix. The density, preferred orientation, and lattice parameters of β-type CVD-Si3 N4 were examined. 相似文献
2.
DELLA M. ROY 《Journal of the American Ceramic Society》1958,41(8):293-299
The nature of the low-temperature inversions γ-α' and α'-β was investigated by various techniques: hydrothermal and "dry" quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X-ray diffractometer patterns obtained at elevated temperatures, "static" pressure techniques, and infrared absorption spectrometry. A revised energy-temperature diagram is presented for Ca2 SiO4 , with the transition γ' to α' taking place at about 725°C. and the α'-β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ-α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat-of-transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO2 -3H2 O (X), 6CaO 3SiO2 .H2 O (Y), and 9CaO-6SiO2 H2 O (Z), were found to be stable above 700°C. at H2 O pressures greater than 7500 lb. per sq. in. 相似文献
3.
A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y3 Al5 O12 (YAG) coatings. Polycrystalline single-phase Y3 Al5 O12 coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y3 Al5 O12 coatings. The microstructure of the Y3 Al5 O12 coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications. 相似文献
4.
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
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.
Thermal decomposition of silicon diimide, Si(NH)2 , in vacuum resulted in very-high-purity, fine-particle-size, amorphous Si3 N4 powders. The amorphous powder was isothermally aged at 50° to 100° intervals from 1000° to 1500°C for phase identification. Examination of ir spectra and X-ray diffraction patterns indicated a slow and gradual transition from an amorphous material to a crystalline α-phase occurring at 1200°C for >4 h and/or 1300° to 1400°C for 2 h. As the temperature was increased to ≥1450°C for 2 h, the crystalline β-phase was observed. Phase nucleation and crystallite morphology in this system were studied by electron microscopy and electron diffraction combined with TG as functions of temperature for the inorganic polymer starting materials. Powders prepared in this manner with 4 wt% Mg3 N2 added as a sintering aid were hot-pressed to high-density fine-grained bodies with uniform microstructures. The optimum hot-pressing condition was 1650°C for 1 h. Silicon concentration steadily increased as the hot-pressing temperature or time was increased. A method for chemical etching for high-density fine-grained Si3 N4 is described. Electrical measurements between room temperature and ∼500°C indicated dielectric constant and tan δ values of 8.3±0.03 and 0.65±0.05×10−2 , respectively. 相似文献
7.
The composition Zn2.33 Sb0.67 O4 (or Zn7 Sb2 O12 ) exists in two polymorphic forms. The thermodynamically stable, low-temperature orthorhombic β form transforms to the high-temperature cubic α-polymorph with a spinel structure at 1225°±25°C. The transformation is fully reversible but slower in the α→β direction and therefore, it is easy to preserve the high-temperature α-polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the α-polymorph directly at low temperatures, e.g., 900°C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the α-polymorph has been confirmed by Rietveld refinement of X-ray powder diffraction data to be an inverse spinel, (Zn)[Sb2/3 Zn4/3 ]O4 , in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn. 相似文献
8.
Cubic-pyrochlore-structure α-phase and low-symmetry-structure ß-phase are the basic phases of dielectric ceramics that are based on Bi2 O3 -ZnO-Nb2 O5 (BZN). In this paper, the melting behaviors of the α-phase and the ß-phase have been explored using differential thermal analysis and X-ray diffractometry. The results show that BZN-based α-phase is a congruent compound; its congruent melting point is 1190°C. BZN-based ß-phase is an incongruent compound that dissociates to form α-phase and a liquid phase, with an absorption of heat, at its incongruent melting point (peritectic temperature) of 1100°C. Upon cooling, the α-phase reacts with the liquid phase to form the ß-phase via peritectic reaction. Upon quenching, the BZN-based cubic-fluorite phase precipitates from the liquid phase. 相似文献
9.
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
10.
The microstructure and humidity-sensitive characteristics of α -Fe2 O3 porous ceramic were investigated. Microporous α -Fe2 O3 powders were obtained by controlling the topotactic decomposition reaction of α -FeOOH. Water vapor adsorption thermogravimetrical experiments were carried out in the relative humidity (rh) range 0% to 95% on the α -Fe2 O3 powder and the 900°C sintered compact. The microstructure was investigated by SEM, TEM, Hg intrusion, and N2 adsorption porosimetry techniques. The humidity sensitivity was investigated by the impedance measurements technique in 0% to 95% rh on the compacts sintered at 50°C steps in the 850° to 1100°C range. Humidity response was found to be affected by the microstructure, i.e., the characteristics of the precursor powders and sintering temperatures. 相似文献
11.
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
12.
New data obtained on the join Ca2 SiO4 -CaMgSiO4 established a limit of crystalline solubility of Mg in α-Ca2 SiO4 corresponding to the composition Ca1.90 Mg0.10 SiO4 at 1575°C. The α-α'Ca2 SiO4 inversion temperature is lowered from 1447° to 1400°C by Mg substitution in the lattice. α'-Ca2 SiO4 takes Mg into its lattice up to the composition Ca1.94 Mg0.06 SiO4 at 1400°C and to Ca1.96 Mg0.04 SiO4 at 900°C. A new phase (T) reported previously by Gutt, with the approximate composition Ca1.70 Mg0.30 SiO4 , was stable between 979° and 1381°C, and should be stable at liquidus temperatures in multicomponent systems involving CaO–MgO–SiO2 . 相似文献
13.
Complete solid solubility was demonstrated to occur between LiAlGeO4 and the low temperature form of Li AlSiO2 (a-eucryptite). Hydrother-mal preparation was necessary for the silicate-rich compositions. Under atmospheric pressure, about 65 mole % LiAlGeO4 entered the β-eucryβ-tite phase at 1150°C, but solid solutions containing more than 25 mole % LiAlGeO4 exsolved if held at lower temperatures. Directional thermal expansion data were obtained by X-ray diffraction methods on both α- and β-eucryptite and their solid solutions. Substitution of Ge4+ for Si4+ produced no significant difference in the thermal expansion coefficients in the α and β phases. An increase in the lattice parameters in the a and c directions took place as expected when Ge4+ (0.53 A) was substituted for Si4+ (0.39 A). 相似文献
14.
Chunli Guo Zheng Xing Xiaojian Ma Liqiang Xu Yitai Qian 《Journal of the American Ceramic Society》2008,91(5):1725-1728
Silicon nitride nanowires or nanorods have been synthesized from SiCl4 , NaN3 , and metallic Mg at temperatures ranging from 200° to 300°C. X-ray powder diffraction patterns indicated that the as-obtained products were mainly β-Si3 N4 . Scanning electron microscope and high-resolution transmission electronic microscopy showed that the samples mostly consisted of Si3 N4 nanowires or nanorods. As metallic iron powder was used, α-Si3 N4 was mainly formed at 250°C. 相似文献
15.
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
16.
Yong W. Bae Woo Y. Lee David P. Stinton 《Journal of the American Ceramic Society》1995,78(5):1297-1300
Crystalline β-Ta2 O5 coatings were deposited on hot-isostatically-pressed Si3 N4 by reacting TaCl5 with H2 and CO2 in the temperature range of 1000°–1300°C and at a pressure of 660 Pa. The Ta2 O5 coatings generally consisted of wellcoalesced 2–3 μm grains, resulting in the formation of a nonporous coating morphology. However, the presence of microcracks on the as-deposited surface was consistently observed. The surface morphology, texture, and growth rate of the coatings were examined as a function of deposition parameters. 相似文献
17.
Jana Bezjak Aleksander Renik Botjan Janar Philippe Boullay Ivana Radosavljevi Evans Danilo Suvorov 《Journal of the American Ceramic Society》2009,92(8):1806-1812
Polymorphic phase transitions in Ba4 Nb2 O9 were studied by thermal analyses, high-temperature transmission electron microscopy and X-ray powder diffractometry. Two stable polymorphs were isolated, low-temperature α-modification and high-temperature γ-modification, with the endothermic phase transition at 1176°C. The α→γ transformation is accompanied by the formation of a 120° domain structure, which is a consequence of hexagonal→orthorhombic unit cell reconstruction. Reheating the presintered γ-Ba4 Nb2 O9 results in the formation of a metastable γ'-modification (formerly known as β-polymorph) in the temperature range between 360° and 585°C, before the γ→α transformation at 800°C. Above ∼490°C Ba4 Nb2 O9 becomes moderately sensitive to a loss of BaO. In air the surface of Ba4 Nb2 O9 grains decomposes to nanocrystalline Ba5 Nb4 O15 and BaO, which instantly reacts with atmospheric CO2 to form BaCO3 . Surface reaction delays γ→α transformation up to 866°C in air. In vacuum the loss of BaO is even more enhanced and consequently the formation of minor Ba3 Nb2 O8 phase is observed above 1150°C. 相似文献
18.
Masahiro Tatsumisago Akihiro Taniguchi Tsutomu Minami 《Journal of the American Ceramic Society》1993,76(1):235-237
Superionic conductor α-AgI, which is stable only above 147°C, was successfully frozen at ambient temperature in AgI─Ag2 O─Mx Oy (Mx Oy = WO3 , V2 O5 ) glass matrices by a twin-roller-quenching technique. The system with WO3 , provided the larger composition regions where α-AgI was frozen at ambient temperature, compared to the system with V2 O5 . The matrix glasses with higher glass-transition temperatures had a stronger effect in depressing the α–β transformation of AgI. The α-AgI-frozen samples exhibited extremely large conductivities of 3 × 10−2 -5 × 10−2 S.cm−1 at 25°C. 相似文献
19.
Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al2 O3 · 5SiO2 ) and spodumene (Li2 O · Al2 O3 · 4SiO2 ) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed. 相似文献
20.
The reciprocal salt pair Sr2 SiO4 -Sr2 GeO4 -Ba2 GeO4 -Ba2 SiO4 was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K2 SO4 structure type (α') occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr2 SiO4 was found which converts reversibly to the high-temperature α'-modification at 85°C. The enthalpy of the β-α' transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C. 相似文献