摘要:

TiC whiskers were synthesized via a vapor‐liquid‐solid (VLS) growth mechanism in the temperature region 1200°‐1600°C in a protecting argon atmosphere. The starting materials consisted of TiO2, carbon, nickel, and an alkali‐metal chloride, such as NaCl, used as a precursor for chlorine. The overall chemical reaction was based on a straightforward carbothermal reduction process, but the reactions that actually took place were more complex and involved formation of gaseous TiClxspecies, which were transported to the nickel catalyst. The formation of TiC whiskers depended on the processing conditions used, mixing procedure of the precursor materials applied, and properties of precursor materials chosen, e.g., their particle sizes. A route for synthesizing TiC whiskers at a yield of 80‐90 vol% was established. The whiskers were ∼1 μm in diameter and 10–30 μm in length; they were straight and had smooth surfaces. The main impurities were TiC particles, minor amounts of unreacted carbon, and remnants of th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08711.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The kinetics of thermal oxidation of H. C. Starck M‐11 and Ube SN‐E10 Si3N4powders was evaluated in the temperature range 650°‐1200°C using isothermal and nonisothermal thermogravimetric analysis, high‐resolution transmission electron microscopy, and X‐ray photoelectron spectroscopy. Between 700° and 1200°C, the isothermal kinetics was modeled equally well using the Ginstling‐Brounshtein and Zuravlev‐Lesokhin‐Tempeľman equations. Despite their similar particle sizes and surface areas, the two powders exhibited different oxidation kinetics. The activation energies for oxidation of the M‐11 and SN‐E10 powders were determined to be 400 and 540 kJ/mol, respectively, between 1000° and 1200°C, and 230 and 260 kJ/mol, respectively, between 700° and 1000°C. The parabolic rate constants for oxidation of the two powders were comparable to those reported in the literature for monolithic, chemically‐vapor‐deposited Si3N4at the higher temperatures. At lower temperatures, the oxidation kinetics of the M‐11 powder was nearly linear, whereas the kinetics of the SN‐E10

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08712.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

In the system Ni‐Co‐Si‐O, the compositions of coexisting phases participating in two‐phase equilibria between the monoxide (NiyCo1‐yO) and orthosilicate (NizCo1‐zSi0.5O2) crystalline solutions, and alloy (NixCo1‐x) and orthosilicate phase, have been determined at 1373 K. The samples were equilibrated at 1373 K under argon gas, and the equilibrium compositions of the phases present after quenching were measured by electron probe microanalysis (EPMA). A solid‐state cell incorporating yttria‐stabilized zirconia as the solid electrolyte was used to measure the oxygen chemical potential corresponding to the three‐phase equilibrium between the alloy, orthosilicate solution, and quartz as a function of alloy composition. From these measurements, activity‐composition relationships in the alloy and orthosilicate solid solutions have been derived using a new method. Activities in the silicate solution have also been obtained independently using the mixing properties of the rock‐salt solid solution available in the literature and the tie‐line information obtained in this study. Both methods yield almost identical results. All three solid solution series‐alloy, monoxide, and orthosilicate‐exhibit mild positive derivations from ideality. The excess Gibbs free energy of mixing of the orthosilicate solid solution (NizCo1−zSi0.5O2) at 1373 K ca

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08713.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Rayleigh‐scattering properties in commercial silica glasses is clarified with relation to their fabrication methods. Fused silica and synthetic silica made by the flame hydrolysis of SiCl4(VAD) method exhibit similar magnitudes of scattering, regardless of fabrication method. Synthetic silica made by flame fusion of SiCl4exhibits ∼10% lower scattering than other silica glasses. The factors affecting scattering are evaluated based on refractive index dispersion and glass transition temperature (Tg). The results suggest that the difference in Rayleigh scattering is predominantly attributable to the difference inTg, which is affected by impurities, such as OH i

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08714.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Brittle failure is classically modeled by the Weibull distribution, based on a phenomenological approach. The procedure fails if the distribution of the failure‐initiating defects varies within material batches and may lead to a wrong conclusion about the predictive power of the model. The purpose of this paper is to highlight this problem, which can occur with brittle materials such as ceramics. This study, based on the results of tensile and bending tests made on silicon nitride specimens, consists of three steps: (i) micrographic and fractographic studies are performed to determine the defect‐size density in each type of specimen and the mechanical behavior of the defects, (ii) the classical modeling with a Weibull distribution is shown to fail to predict the results, and (iii) a new approach for the identification of a fracture model is developed, based on step (i). The fracture prediction is computed from the actual defectsize distribution in each material batch, so that the procedure can be adapted to changes in the defect‐size distribution caused by the manufacturing pr

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08715.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Silicate glasses of the composition (Li2O)x(SiO2)100−x(x= 0, 20, 33.3, and 40) have been prepared and characterized by X‐ray and neutron diffraction experiments using the technique of isotopic substitution. Thus, at each composition, a set of independent diffraction patterns allows the discussion of the short‐range and medium‐range structure. For the short‐range structure, average coordination numbers and atomic‐pair distances are obtained from the pairdistribution functions. For the medium‐range structure, i.e., for atomic‐pair distances>0.5 nm, an increase of the amplitudes of the oscillations in the pair‐distribution function is observed with increases in

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08716.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The results of X‐ray and neutron diffraction experiments with (Li2O)x(SiO2)100_xglasses (x = 0, 20, 33.3, and 40) are discussed using the Reverse Monte Carlo method to produce atomic clusters, which are analyzed with respect to the O‐O distribution, the angular correlations within the first and higher coordination spheres, and the bond‐orientational multipole moments. Within Li2O‐containing silicate glasses, each lithium atom is surrounded by a ring of six SiO4tetrahedra in such a way that four oxygen atoms lie at the joints of a tetragonal cell centered by lithium. Thus, the lithium atoms increase the order of th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08717.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Barium titanate (BaTiO3) powders were prepared by a polymerized complex method based on the Pechini‐type reaction route, wherein a mixed solution of citric acid (CA), ethylene glycol (EG), and barium and titanium ions, with a molar ratio of CA:EG:Ba:Ti = 10:40:1:1, was polymerized to form a transparent resin, which was used as a precursor for BaTiO3. Characterization of the initial precursor solution of EG, CA, and barium and titanium ions by Raman scattering and13C‐NMR spectroscopy indicated that barium and titanium ions were simultaneously stabilized with CA to form a barium‐titanium mixed‐metal CA complex with a stoichiometry similar to Ba:Ti:CA = 1:1:3. Raman and13C‐NMR spectra of the liquid mixture at various reaction stages indicated that the fundamental coordination structure of the mixed‐metal complex remained almost unchanged throughout the polymerization process. X‐ray diffractometry (XRD) measurements indicated formation of pseudo‐cubic BaTiO3free from BaCO3and TiO2when the barium‐titanium polymeric precursor was heat‐treated in air at 500°C for 8 h or at 600°C for 2 h. However, the Raman spectra of the same powders indicated the formation of tetragonal (rather than cubic) BaTiO3, with traces of high‐temperature hexagonal BaTiO3stabilized at room temperature. XRD of a pyrolyzed product at 500°C for 2 h revealed a simple mixture of BaTiO3and an intermediate phase, Ba2Ti2O5.CO3. A solid‐state reaction between BaCO3and TiO2was concluded as not being responsible for the BaTiO3formation; rather, BaTiO3formed directly by thermal decomposition of the intermediate Ba2Ti2O5.CO3phase at temperatures>500°C. In addition, by Raman scattering measurements, the intermediate Ba2Ti2O5.CO3phase was found to be unstable in ambient air, yielding BaCO3as one

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08718.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Thermal shock damage in a two‐dimensional woven‐Nicalon(tm)‐fiber‐reinforced‐CVI SiC‐matrix composite was induced by water quenching and characterized by optical microscopy as a function of quench temperature difference (ΔT) and number of quench cycles. Mechanical damage generated in flexure on quenched and unquenched specimens also was characterized and compared to the thermal shock damage. The observed thermal shock damage consisted of small matrix cracks and fiber‐matrix interfacial debonding on the surface, and large interior cracks in the matrix that formed between and parallel to the fiber cloths. At low ΔTvalues, only small matrix cracks on the surface were observed, and they were related to initial decreases in Young's modulus. At higher ΔTvalues, larger cracks between the fiber cloths in the specimen interior were observed and related to decreases in the ultimate strength. Cyclic quenching resulted in progressive thermal shock damage that was consistent with Young's modu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08719.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Reaction of silica (SiO2) with triethanolamine (TEA, N(CH2CH2OH)3) and ethylene glycol (EG) under conditions (∼200°C) where byproduct water is removed resulted in the formation of the neutral silatrane glycolate complex, N(CH2CH2O)3SiOCH2CH2OH (or TEASiOCH2CH2OH) in essentially quantitative yield. Solutions of this neutral precursor in EG, when rapidly pyrolyzed and then oxidized at 500°C, formed porous ceramic powders with high specific surface areas (>500 m2/g). These powders were nitrided via ammonolysis in a fluidized‐bed reactor at temperatures of 700°‐1000°C. The resulting nitrided powders were characterized by thermal and chemical analyses, diffuse reflectance infrared spectroscopy, gas sorption, and X‐ray photoelectron spectroscopy. The apparent activation energy for the nitridation process was determined to be 54 kJ/mol. Following nitridation, the powders were amorphous and had nitrogen contents as high as 21 wt% with retained surface areas>300 m2/g at 1000°C. Under the nitridation conditions used, the extent of nitrogen incorporation correlated linearly with increases in material density. This linearity suggested that the change in density occurred primarily because of changes in coordination that occurred as trivalent nitrogen replaced divalent oxygen in the glass structure and nominally because of viscous flow. The linear density increase also suggested that pore trapping did not occur under these processing conditions. This work serves as a model for ongoing studies on the nitridation of high‐surface‐area ceramic powders produced by the rapid pyrolysis of mixed‐m

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb08720.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY