摘要:

An analysis of the use of temperature profiles in the determination of the kinetic parameters of self‐sustaining combustion synthesis is provided. Two mathematical approaches are discussed. In the first approach, the analysis focuses on the initial portion of the temperature profile and thus emphasizes the kinetics of the initial stages (η≲ 1.0) of this process. The second approach emphasizes both the initial and final stages of the process (η∼ 1.0). With the simultaneous measurements of the velocity of the combustion wave, temperature profile analysis provides values for the effective thermal diffusivity of the re

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05461.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The temperature profiles associated with the passing of a self‐propagating wave in the combustion synthesis of zirconium boride were investigated. The use of these profiles in the determination of the kinetics of the associated synthesis reaction was investigated. From profile analysis an activation energy of 144.5 ± 15 kJ · mol−1was calculated and was found to agree very well with the corresponding value obtained from wave velocity measurements, 148.6 ± 12 kJ · mol−1. From these analyses, the thermal diffusivity of the reactants was also calculated. The thermal diffusivity decreased slightly with increasing combustion temperature, with the values ranging from 2.10 × 10−3to 1.79 × 10−3over the range of 2609 to 2909

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05462.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

In order to clarify the mechanism of mechanochemical polishing of SiC with Cr2O3abrasive, 6H‐wurtzite single‐crystal specimens were dry‐polished. A significant anisotropic polishing rate difference was found between Si(0001) and C(0001) surfaces. The C(0001) surface was removed 10 times as fast. Polished surfaces were observed from cross‐sectional and plan‐view directions by high‐voltage TEM. There was no trace of mechanical effects such as residual strain or scratches. The polishing debris was analyzed by X‐ray diffraction, high‐resolution TEM, and analytical TEM. No crystalline phases were identified from X‐ray diffraction patterns except for Cr2O3, while it was found from TEM observation that a large amount of an amorphous phase consisting of Si, C, and O was contained in the debris. These results indicated that the surface of SiC was removed mechanochemically by the aid of a catalytic oxidatio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05463.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The role of high‐purity silica in the fracture of Si3N4at high temperatures has been investigated. The flexural strength at 1400°C was found to be greater than that at room temperature. Little plastic deformation was observed even when 10 wt% SiO2was added and the strain rate was decreased 2 orders of magnitude from that for a standard bend test. Microstructural observations revealed that the glassy phase was localized at intergranular pockets when SiO2additions were ≤ 10 wt%. High strength at 1400°C despite the presence of a fairly large amount of glassy phase is attributed to a high cavitation threshold in such glassy pockets consisting of high‐purity SiO2. However, the deformation behavior changed abruptly for SiO2additions of 10 and 20 wt%, which is explained by the morphological change of the glassy phase to thicker intergranular layers which allow macroscopic visco

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05464.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Perovskite‐type (La1‐xSrx)MnO3(0x0.3) was synthesized through the sol–gel process at low temperature (400° to 500°C). Poly(acrylic acid) (PAA) was used to make a gel from an aqueous solution of lanthanum, strontium, and manganese nitrates. The particle‐diameter distribution of the manganites had a maximum value of 0.3 to 0.7μm, and a specific surface area of about 17.5 to

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05465.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The presently developed two‐stage process involves diping the prefired porous disks ofn‐BaTiO3in nonaqueous solutions containing Al‐buty rate, Ti‐isopropoxide, and tetraethyl silicate and subsequent sintering. This leads to uniform distribution of the grain‐boundary layer (GBL) modifiers (Al2O3+ TiO2+ SiO2) and better control of the grain size as well as the positive temperature coefficient of resistivity characteristics. The technique is particularly suited for GBL modifiers in low concentrati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05466.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A pure Mode I fracture specimen and test procedure has been developed which provides extended, stable, through‐thickness crack growth in ceramics and other brittle, nonmetallic materials. Fixed displacement loading, applied at the crack mouth, promotes stable crack extension by reducing the stored elastic strain energy. Extremely fine control of applied displacements is achieved by utilizing the Poisson expansion of a compressively loaded cylindrical pin. Stable cracks have been successfully grown in soda‐lime glass and monolithic Al2O3for lengths in excess of 20 mm without uncontrollable catastrophic fail

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05467.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The absorption coefficient values of several silicate glasses in the IR radiation range were determined. Four methods were employed in this study: (1) direct transmission by CO2laser, (2) direct transmission through ultrathin sections using an IR spectrometer, (3) glass particle dispersions in KBr pressed pellets, and (4) IR reflection spectral analysis. Wide variations in values were observed for some of these techniques. The reasons for the variations were explored, and evidence is presented to support the conclusion that the reflection measurements and the low‐power transmission technique yield accurate values. The pellet technique was found to possess too many experimental variables which could not be controlled and gave erroneous data. The damage produced by direct transmission of high‐power radiation was severe and introduced effects which gave spurious resu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05468.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

High‐purity mullite powders (3Al2O3.2SiO2) have been prepared using a high‐temperature aerosol decomposition technique yielding submicrometer (0.6 μm average) particles of spherical morphology with no hard agglomerates using aluminum nitrate and fumed silica as precursors. Depending upon the reaction conditions used, the powders range from amorphous to crystalline with no evidence of secondary‐phase formation. This mullite synthesis approach has the advantages of not requiring postsynthesis milling, the ability to use a wide range of precursor systems, and enhanced control over chemical homogeneity and particle size

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05469.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

When sintered Al2O3is annealed with CaMgSiAlO glass at 1600°C, polyhedral MgAl2O4grains form and glass pockets are entrapped within the grains. After annealing for 13 h at 1600°C, the liquid pockets show a regular octahedron shape which is expected to represent the equilibrium shape. All grain surfaces in contact with the glass matrix show the same shape. The small grains, which must be shrinking, thus have the equilibrium shape, because their shrinkage shape is is identical to the equilibrium and growth shape. However, the octahedral shape also represents the growth shape for the growing large grains. The grains also form grain boundaries with neighboring grain

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05470.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY