ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05431.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTA method has been developed for analyzing the X‐ray diffraction patterns of amorphous solids, and the atomic arrangement in two simple glasses has been completely worked out. In SiO2, for example, each silicon is tetrahedrally surrounded by 4 oxygens at a distance 1.60 Å, and each oxygen is shared between two such tetrahedral groups. The mutual orientation of the two groups about their common direction of bonding is random, and hence it is a random three‐dimensional network which is built up. The configuration does not repeat at regular intervals and hence is noncrystalline; it is this feature which distinguishes the glassy state from the crystal

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05432.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe X‐ray scattering curves for vitreous SiO2and B2O3are obtained in a vacuum camera, using monochromatic Mo and Cu radiation. A Fourier analysis of the scattering curve gives directly a curve representing the distribution of neighboring atoms about any one atom. For SiO2, the distribution curve establishes difinitely the tetrahedral silicon‐oxygen network. The distribution curve for B2O3indicates a triangular coördination, each boron surrounded by three oxygens, and each oxygen shared between two borons. The interatomic distances which are found are in good agreement with the values found in crystalline silicates and borates. The importance of the new Fourier method of analysis of glass diffraction patterns is emphasized by the fact that the distribution curves which are obtained are unique, no assumptions as to structure being invo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05433.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05434.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The portion of the stable phase diagram for the ternary system CaO‐P2O5‐H2O involving calcium phosphate phases more basic than Ca(H2PO4)2has been constructed. Construction involves plotting concentrations as their fifth roots to allow phases having very different solubilities to be represented on a single diagram. Hydroxyapatite is shown to be congruently soluble. The stability region for hydroxyapatite includes compositions having Ca/P ratios extending between 1.5 and 1.67. Calcium‐deficient hydroxyapatite forms a stable invariant point with CaHPO4and a metastable invariant point with CaHPO4. 2H2O. Inspection of the diagram indicates that the compounds CaHPO4and CaPHO4.2H2O dissolve incongruently. Incongruent dissolution of either CaHPO4or CaHPO4.2H2O results in the formation of hydroxyapatite during the dissolution process. Such incongruency between these compounds and hydroxyapatite influences their microstructural relation

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05435.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

R‐curves for hot‐pressed 20 vol% ZrO2(2.5‐YPSZ and 8‐YPSZ) reinforced MoSi2composites were evaluated using the indentation/bend technique. Results indicated a modest increase in the material toughness for both composites. ExperimentalR‐curves were compared with theoretically predictedR‐curves to evaluate the critical transformation stress for the partially stabilized ZrO2composite. A high critical transformation stress value obtained was indicative of the inhibition of transformation toughening effects due to microstructural inhomogeneity in the partially stabilized ZrO2‐Mo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05436.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Composite ceramic materials based on Si3N4and ZrO2stabilized by 3 mol% Y2O3have been formed using aluminum isopropoxide as a precursor for the Al2O3sintering aid. Densification was carred out by hot‐pressing at temperatures in the range 1650° to 1800°C, and the resulting micro‐structures were related to mechanical properties as well as to oxidation behavior at 1200°C. Densification at the higher temperatures resulted in a fibrous morphology of the Si3N4matrix with consequent high room‐temperature toughness and strength. Decomposition of the ZrO2grains below the oxidized surface during oxidation introduced radial stresses in the subscalar region, and from the oxidation experiments it is suggested that the ZrO2incorporated some N during densi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05437.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Al2O3‐TiC, Al2O3‐TiC0.5N0.5, Al2O3‐WC, Al2O3‐SiC, and Al2O3‐HfB2powders were synthesized by the aluminothermic reduction of oxides in the presence of carbon or boron. The reacted powders were milled to reduce the size of agglomerates and subsequently densified without applied pressure to near‐theoretical density. Microstructures and mechanical properties of composites made from exothermically reacted powders were compared with similar ceramics made from commercially available powders. In situ sintering was possible in the Al2O3‐TiC system using a closed graphite crucible to contain reaction gases. The synthesis ofβ‐SiC at temperatures above 1400°C via the direct reaction of the elements (SHS) was compared with SiC made by the magnesiothermic reduction of SiO2in the presence of C after removing th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05438.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Residual stress profiles through mullite/alumina bodies prepared by an infiltration process were determined. In one approach, residual stress profiles were predicted by obtaining concentration profiles and treating the bodies as laminates. In a second approach, a strain gage technique in which part of the sample was ground away while recording the resulting strain was used. The results showed that residual compressive stresses existed in the surface region of all composite samples. The strain gage approach indicated values of surface compression in the range of 176 to 291 MPa, which were higher than those predicted using the laminate approach. The results were compared with changes in the mechanical properties and found to be consistent with the increases in the strength and indentation fracture toughness which resulted when mullite was added to alumina by the infiltration technique.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05439.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The resistivity ofβ”‐alumina‐based solid electrolytes is partially determined by the relative concentration of the phases present in the electrolyte. In order to determine the concentrations of the phases within such electrolytes, several methods of quantitative phase analysis using X‐ray powder diffraction data were investigated. Several analytical equations found in the literature specifically addressing the assay of phases in these systems have been found to be inconsistent. New equations, extended to accommodate the often present sodium aluminate phase, were derived and applied successfully in the calculation of the relative abundance of each phase in calcined electrolyte powders. Verification of the accuracy of these equations was obtained by comparing the results to those obtained from a Rietveld whole‐pattern fitting

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05440.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY