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1.
Yuji Iwamoto Wolfgang Völger Edwin Kroke Ralf Riedel Tomohiro Saitou Katsuyuki Matsunaga 《Journal of the American Ceramic Society》2001,84(10):2170-2178
The crystallization behavior of organometallic-precursor-derived amorphous Si-C-N ceramics was investigated under N2 atmosphere using X-ray diffractometry (XRD), transmission electron microscopy (TEM), and solid-state 29 Si nuclear magnetic resonance (NMR) spectroscopy. Amorphous Si-C-N ceramics with a C/Si atomic ratio in the range of 0.34–1.13 were prepared using polycarbosilane-polysilazane blends, single-source polysilazanes, and single-source polysilylcarbodiimides. The XRD study indicated that the crystallization temperature of Si3 N4 increased consistently with the C/Si atomic ratio and reached 1500°C at C/Si atomic ratios ranging from 0.53 to 1.13. This temperature was 300°C higher than that of the carbon-free amorphous Si-N material. In contrast, the SiC crystallization temperature showed no clear relation with the C/Si atomic ratio. The TEM and NMR analyses revealed that the crystallization of amorphous Si-C-N was governed by carbon content, chemical homogeneity, and molecular structure of the amorphous Si-C-N network. 相似文献
2.
Jin-Joo Park Osamu Komura Akira Yamakawa Koichi Niihara 《Journal of the American Ceramic Society》1998,81(9):2253-2260
An amorphous Si-C-N powder with Y2 O3 and Al2 O3 powder as sintering additives was hot-pressed at 1900°C for 120 min in a nitrogen atmosphere. Changes in the crystalline phases and microstructure of the amorphous Si-C-N powder during sintering were investigated by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The defects at the fracture origins of the sintered bodies after bending tests also were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). XRD showed that alpha-Si3 N4 was formed initially from the amorphous Si-C-N by 1530°C, which then transformed to ß-Si3 N4 at 1600°C. Also, a slight formation of crystalline SiC occurred during the transformation from alpha- to ß-Si3 N4 , and it increased after the transformation was completed at 1900°C. TEM revealed that many SiC nanoparticles were incorporated into ß-Si3 N4 grains after the transformation from alpha- to ß-Si3 N4 at 1600°C. They were located at the triple points of the grain boundaries of ß-Si3 N4 after continued Si3 N4 grain growth at 1900°C. Besides the SiC nanoparticles, large agglomerations of carbon or SiC particles of 20-60 µm size were observed by SEM and EPMA at the fracture origins of the sintered bodies after the bending tests. 相似文献
3.
Kean W. Chew Alan Sellinger Richard M. Laine 《Journal of the American Ceramic Society》1999,82(4):857-866
AlN-SiC-particle-reinforced composites have been prepared at lessthan equal to1400°C using submicrometer AlN, -325 mesh alpha-SiC particles, and polymethylsilane (PMS; -(CH3 SiH) n -) via a polymer infiltration and pyrolysis (PIP) process. PMS is an organometallic SiC polymer precursor that can be modified with 16 wt% cross-linking aid to provide mPMS. mPMS converts to nanocrystalline β-SiC with >80% ceramic yield (1000°C in argon) with some excess (<5 wt%) graphitic carbon. mPMS has been used successfully as a nonfugitive binder for AlN-SiC compacts. Densities of 2.5 versus 2.1 g/cm3 have been obtained after nine PIP cycles for disk-shaped compacts formulated with and without mPMS binder, respectively. alpha-SiC seeds crystallization of β-SiC derived from mPMS at temperatures as low as 1000°C. Some evidence suggests that AlN-SiC solid solutions form at particle/matrix interfaces. 相似文献
4.
The infiltration of liquid Fe3 Si (mp of ∼1300°C), Fe5 Si3 (mp of ∼1210°C), and FeSi (mp of ∼1410°C) into SiC powder preforms was performed at various infiltration temperatures for 60 min under either argon flow or dynamic vacuum. The amount of infiltration under various infiltration conditions was studied as a function of infiltration temperature. For the preforms as-pressed from raw SiC powder, the amount of infiltration of the three silicides under argon flow was independent of their melting points, but suddenly increased within a common temperature range from 1450° to 1550°C. Thermodynamic analyses indicated that the common temperature range corresponded to the temperature at which the SiO2 on the surface of the SiC particles was decreased under argon flow. Infrared spectroscopy showed SiO2 on the surfaces of as-received SiC powder particles, but not on the surfaces of the SiC powder particles fired under argon at 1600°C. The amount of infiltration of the as-pressed SiC under vacuum and of fired SiC under argon and vacuum exhibited an obvious dependence on the silicide melting points. This was attributed to the SiO2 reduction taking place at temperatures lower than the melting points of the silicides. The amount of infiltration was then controlled by the melt viscosity. 相似文献
5.
Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy Study of Nonstoichiometric Silicon-Carbon-Oxygen Glasses 总被引:2,自引:0,他引:2
Christian Turquat Hans-Joachim Kleebe G. Gregori Steffen Walter Gian D. Sorarù 《Journal of the American Ceramic Society》2001,84(10):2189-2196
Crystallization behavior of Si-C-O glasses in the temperature range of 1000°–1400°C was investigated using transmission electron microscopy (TEM) in conjunction with electron energy-loss spectroscopy (EELS). Si-C-O glasses were prepared by pyrolysis of polysiloxane networks obtained from homogeneous mixtures of triethoxysilane, TH , and methyldiethoxysilane, DH . Si-C-O glass composition depended on the molar ratio of the precursors utilized. At a ratio of TH /DH = 1, the formation of a carbon-rich glass was observed, whereas a ratio of TH /DH = 9 yielded a Si-C-O glass with excess free silicon. Both materials were amorphous at 1000°C, but showed a distinct difference in crystallization behavior on annealing at high temperature. Although TH /DH = 1 revealed a small volume fraction of SiC precipitates in addition to a very small amount of residual free carbon at 1400°C, TH /DH = 9 showed, in addition to SiC crystallites, numerous larger silicon precipitates (20–50 nm), even at 1200°C. Both materials underwent a phase separation process, SiC x O2(1-x ) → x SiC + (1 - x )SiO2 , when annealed at temperatures exceeding 1200°C. 相似文献
6.
Synthesis of Silicon Nitride/Silicon Carbide Nanocomposite Powders through Partial Reduction of Silicon Nitride by Pyrolyzed Carbon 总被引:1,自引:0,他引:1
An alternative method to incorporate nanometer-sized silicon carbide (SiC) particles into silicon nitride (Si3 N4 ) powder was proposed and investigated experimentally. Novolac-type phenolic resin was dissolved in ethanol and mixed with Si3 N4 powder. After drying and curing, the resin was converted to reactive carbon via pyrolysis. Si3 N4 powder was partially reduced carbothermally using the pyrolyzed carbon, and nanometer-sized SiC particles were produced in situ at 1530°-1610°C in atmospheric nitrogen. At temperatures <1550°C, the reduction rate was low and the SiC particles were very small; no SiC whiskers or barlike SiC was observed. At 1600°C, the reduction rate was high and the reaction was close to completion after only 10 min, with the appearance of SiC whiskers as well as curved, barlike, and equiaxial SiC, all of which were dozens of nanometers in diameter; this size is greater than that at observed temperatures <1550°C. A longer soaking time at 1600°C led to agglomerates. SiC particles were close to the surface of the Si3 N4 particles. The SiC content could be adjusted by changing the carbon content before reduction and the reduction temperature. A reaction mechanism that involved the decomposition of Si3 N4 has been proposed. 相似文献
7.
Most of the chemistry-based preparation routes for bismuth titanate (BIT) involve calcination at elevated temperatures in order to realize precursor-to-ceramic conversion. In a completely different approach using an amorphous BIT hydroxide precursor, nanocrystalline particles of layered perovskite BIT are synthesized by mechanical activation, skipping the detrimental crystallite coarsening and particle aggregation encountered at high temperatures. Mechanical activation leads to nucleation and steady growth of BIT crystallites in the amorphous precursor matrix, while Bi2 O3 is involved as an intermediate transitional phase. The activation-derived BIT particles demonstrate a rounded morphology of ∼50 nm in size. This is in contrast to the BIT derived from calcination of the coprecipitated precursor at 600°C that is dominated by coarsened platelike particles. The former is sintered to a density of >95% theoretical at 875°C for 2 h, leading to a dielectric constant of ∼1260 when measured at 1 MHz and the Curie temperature of 646°C. 相似文献
8.
Huiwen Xu Keith J. Bowman Elliott B. Slamovich 《Journal of the American Ceramic Society》2003,86(10):1815-1817
Bismuth titanate was synthesized under hydrothermal conditions from an amorphous bismuth–titanium precursor gel. The gel was formed by mixing a bismuth acetate complex with titanium butoxide and then adding the solution dropwise into 6 M NaOH. The resulting gel suspension was reacted under hydrothermal conditions at temperatures ranging from 160° to 200°C to form crystalline bismuth titanate. The gel crystallization kinetics increased with temperature, which resulted in 100% crystalline bismuth titanate in 5 h at 200°C. Wavelength-dispersive spectroscopy data indicated that sodium was incorporated into bismuth titanate during processing, and X-ray diffractometry suggested that the powder was composed of the Bi5 Ti4 O15 phase. Transmission electron microscopy micrographs showed that the gel particles decomposed to 100–200 nm crystalline bismuth titanate particles during hydrothermal processing. 相似文献
9.
Glass in the MgO-Li2 O-A12 O3 -SiO2 system was observed to crystallize readily at temperatures from 700° to 900°C. The primary crystalline phase evolved was Li2 Si2 O5 , and the secondary phase evolved was Li2 SiO3 . The glass was amorphous after heating in air at 1050°C for 30 min. The addition of 0.5 wt% SiC powder resulted in the crystallization of Li2 SiO3 during heating in air at 1050°C for 30 min. It was suggested that the difference in crystallization behavior with Sic addition was due to dissolution of Sic into the oxide glass. 相似文献
10.
Based on the RBAO technology, low-shrinkage mullite/SiC/ Al2 O3 /ZrO2 composites were fabricated. A powder mixture of 40 vol% Al, 30 vol% A12 O3 and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO2 wear debris into the mixture. The precursor powder was isopressed at 300–900 MPa and heattreated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al2 O3 completely, while SiC oxidizes to SiO2 only on its surface. Fast densification (at >1300°C) and mullite formation (at 1400°C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49–55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa. m1/2 increased to 890 MPa and 6 MPa. m1/2 , respectively. 相似文献
11.
Tanguy Rouxel Fumihiro Wakai Kansei Izaki 《Journal of the American Ceramic Society》1992,75(9):2363-2372
Si3 N4 /SiC composites are ceramic materials that exhibit excellent performance for high-temperature applications. Prepared from an ultrafine amorphous Si-C-N powder, sintered materials are constituted mainly of a β -Si3 N4 matrix with SiC inclusions and have a very small grain size (less than 1 μm). Such a microstructure is propitious for superplastic forming. Superplasticity has been studied in tension, from 1550° to 1650°C, under nitrogen atmosphere. Elongations over 100% have been achieved. In many cases, at the highest temperatures and slowest strain rates, materials are damaged by different processes, including microcracking, cavitation, and chemical decomposition. A map of the most suitable (strain-rate/temperature) domain has been established. It allows the prevention of any structural alteration by selecting carefully the testing conditions. Since specimens suffered considerable strain-induced hardening, sources for this phenomenon are examined. Although the experiments have involved high temperature and extensive strain, neither static nor dynamic grain growth has occurred. Crystallization of the amorphous grain-boundary phase, which is reported in most cases, may be invoked. However, based on microstructural observations, it is not the unique origin for flow hardening. 相似文献
12.
A carbothermal reaction of silica–phenol resin hybrid gels prepared from a two-step sol–gel process was conducted in atmospheric nitrogen. The gels were first pyrolyzed into homogeneous silica–carbon mixtures during heating and subsequently underwent a carbothermal reaction at higher temperatures. Using a gel-derived precursor with a C/SiO2 molar ratio higher than 3.0, Si3 N4 /SiC nanocomposite powders were produced at 1500°–1550°C, above the Si3 N4 –SiC boundary temperature. The predominant phase was Si3 N4 at 1500°C, and SiC at 1550°C. The Si3 N4 and SiC phase contents were adjustable by varying the temperature in this narrow range. The phase contents could also be adjusted by changing the starting carbon contents, or by its combination with varying reaction temperature. A two-stage process, i.e., a reaction first at 1550°C and then at 1500°C, offered another means of simple and effective control of the phase composition: the Si3 N4 and SiC contents varied almost linearly with the variation of the holding time at 1550°C. The SiC was nanosized (∼13 nm, Scherrer method) formed via a solid–gas reaction, while the Si3 N4 has two morphologies: elongated microsized crystals and nanosized crystallites, with the former crystallized via a gaseous reaction, and the latter formed via a solid–gas reaction. The addition of a Si3 N4 powder as a seed to the starting gel effectively reduced the size of the Si3 N4 produced. 相似文献
13.
Fiona C. M. Woudenberg Wiebke F. C. Sager Johan E. ten Elshof Henk Verweij 《Journal of the American Ceramic Society》2004,87(8):1430-1435
Nonagglomerated spherical ZrO2 particles of 5–8 nm size were made by emulsion precipitation. Their crystallization and film-forming characteristics were investigated and compared with nanosized ZrO2 powders obtained by sol–gel precipitation. High-temperature X-ray diffraction indicated that the emulsion-derived particles are amorphous and crystallize at 500°C into tetragonal zirconia, which is stable up to 1000°C. Crystallite growth from 5–20 nm occurred between 500°–900°C. Films of 6–75 nm thickness were made by spreading, spin coating, and controlled deposition techniques and annealed at 500°–600°C. The occurrence of t -ZrO2 in the emulsion-precipitated powder is explained by the low degree of agglomeration and the corresponding low coarsening on heating to 500°–800°C, whereas the agglomerated state of the sol–gel precipitate powder favors the occurrence of the monoclinic form of zirconia under similar conditions. 相似文献
14.
Submicron hafnium oxide powder prepared by hydrolytic decomposition of alkoxides was studied. The particle size range of this powder was 10 to 50 Å. Emission spectrographic analysis of the powder after it was calcined at 250°C for 0.5 h indicated a purity of >99.995%. Up to 320°C, the powder showed no crystallinity by X-ray analysis. The amorphous HfO2 was isothermally aged at 5° to 10°C intervals between 200° and 500°C. X-ray diffraction patterns indicate a sharp transition from an amorphous state to the monoclinic phase at 325°C. High-temperature X-ray studies and DTA suggest nucleation and growth of small crystallites at 420°C leading to conversion to monoclinic HfO2 at 480°C. BET surface area measurements and TGA of the powders were also conducted. A powder which transformed at 325°C to the monoclinic phase was isothermally aged below 325°C for 150 h without change. 相似文献
15.
Grain-oriented Bi2 WO6 ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at <800°C, the Bi2 WO6 crystallites are 3∼5 μ m in size and, at >850°C, =100 μm. After dissolving away the salt matrix, the Bi2 WO6 particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical. 相似文献
16.
Radoslaw A. Wach Masaki Sugimoto Masahito Yoshikawa 《Journal of the American Ceramic Society》2007,90(1):275-278
Silicon carbide (SiC) ceramic coating was developed from precursor polymer blend of polycarbosilane and polyvinylsilane on porous alumina substrate by radiation curing. The polymers were crosslinked with oxygen present in the atmosphere during irradiation and pyrolyzed at 850°C in order to convert the polymer into SiC ceramics. Fabricated SiC film was used as a membrane for gas separation, achieving high separation ratios of 206 for H2 and 241 for He over the nitrogen at 250°C. 相似文献
17.
Nithyanantham Thangamani Ashutosh S. Gandhi Vikram Jayaram Atul H. Chokshi 《Journal of the American Ceramic Society》2005,88(10):2696-2701
This study examined pressure consolidation of amorphous Al2 O3 –15 mol% Y2 O3 powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al2 O3 phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders. 相似文献
18.
New Strategies for Preparing NanoSized Silicon Nitride Ceramics 总被引:2,自引:0,他引:2
Xin Xu Toshiyuki Nishimura Naoto Hirosaki Rong-Jun Xie Yinchun Zhu Yoshinobu Yamamoto Hidehiko Tanaka 《Journal of the American Ceramic Society》2005,88(4):934-937
We report the preparation of nanosized silicon nitride (Si3 N4 ) ceramics via high-energy mechanical milling and subsequent spark plasma sintering. A starting powder mixture consisting of ultrafine β-Si3 N4 and sintering additives of 5-mol% Y2 O3 and 2-mol% Al2 O3 was prepared by high-energy mechanical milling. After milling, the powder mixture was mostly transformed into a non-equilibrium amorphous phase containing a large quantity of well-dispersed nanocrystalline β-Si3 N4 particles. This powder precursor was then consolidated by spark plasma sintering at a temperature as low as 1600°C for 5 min at a heating rate of 300°C/min. The fully densified sample consisted of homogeneous nano-Si3 N4 grains with an average diameter of about 70 nm, which led to noticeable high-temperature ductility and elevated hardness. 相似文献
19.
Kyoung R. Han Hee Jin Koo Mee-Jeong Hong Chang Sup Lim 《Journal of the American Ceramic Society》2000,83(4):971-973
A simple way to prepare phase-pure, submicrometer PbTiO3 powder was tried by precipitation. Precipitation was carried out in an aqueous PbO slurry using aqueous TiCl4 and dilute NH4 OH at pH 9.5 ± 0.1. The TG/DSC curves of the PT precursor show weight loss of ∼7% and two exotherms at 492° and 522°C. They are attributed to the crystallization of tetragonal PbTiO3 . XRD shows that tetragonal PbTiO3 can be obtained by heat treatment around 500°C via a noncrystalline state. SEM shows that the PbTiO3 powder calcined at 600°C for 1 h is well crystallized and in the range of 100—300 nm. 相似文献
20.
Julin Wan Matthew J. Gasch Amiya K. Mukherjee 《Journal of the American Ceramic Society》2003,86(3):526-528
Starting with Si-C-N(-O) amorphous powders, and using the electric field assisted sintering (EFAS) technique, silicon nitride/silicon carbide nanocomposites were fabricated with yttria as an additive. It was found that the material could be sintered in a relatively short time (10 min at 1600°C) to satisfactory densities (2.96–3.09 g/cm3 ) using 1–8 wt% yttria. With decreasing yttria content, the ratio of SiC to Si3 N4 increased, whereas the grain size decreased from ∼150 nm to as small as 38 nm. This offers an attractive way to make nano-nanocomposites of silicon nitride and silicon carbide. 相似文献