摘要:

Strontium and calcium pyroniobates were prepared by a sol–gel process, using strontium/calcium metal and niobium ethoxide as precursors. The formation of Sr2Nb2O7occurred at 750°C via an intermediate perovskite phase of composition close to Sr0.82NbO3. The crystallization of Ca2Nb2O7occurred at 600°C directly without any intermediate phases. Sintered Sr2Nb2O7and Ca2Nb2O7pellets showed a preferred grain orientation. Microstructural studies revealed an increase in grain growth and associated orientation with sintering temperat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05491.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Detailed surface compositional analysis has been conducted on 12 simulated nuclear waste forms leached for two years in the brine environment of the Waste Isolation Pilot Plant (WIPP). Secondary ion mass spectrometry (SIMS) of both cations and anions has identified characteristic leaching and precipitation zones within the leached layers on all specimens. The thickness of the leached layers is found to range from approximately 0.3 to 2.7 μm and the total element loss from 80 to 625 mmol/m2, indicating that durability in brine differs by less than an order of magnitude between the different glasses. The in‐depth profiles of anionic elements indicate the significant roles of carbon, fluorine, and chlorine in the formation of surface phases. Also recorded and discussed are elemental SIMS images of sputter crater tapers, providing additional documentation of leaching subzon

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05492.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The Hall coefficient, and hence the charge carrier density, of the system of Y1Ba2Cu3Oxwas measured at room temperature as a function of oxygen content,x, in its entire homogeneity range (6

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05493.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

β‐Si3N4whisker‐reinforced β′SiAlON composites were fabricated by extrusion and densified, using pressureless sintering. Whisker alignment was observed in both the green and sintered microstructures. SEM analysis of polished, sintered samples showed a microstructure consisting of the original β‐Si3N4whiskers in a matrix of fine SiAlON grains. SEM of plasma‐etched samples and TEM analysis showed that the whiskers, as a result of grain growth, consisted of two phases, a core and a sheath layer. X‐ray mapping and EDS analysis revealed that the core material contained no trace of Al, confirming the presence of original β‐Si3N4whiskers. The composition of the sheath was qualitatively identical to that of the fine β′ SiAlON grains in the matrix. The sheath was thus formed by the precipitation of the β′SiAlON during liquid‐phase sintering and led to substantial growth of the whiskers. Microdiffraction showed that the β′SiAlON grew epitaxially on the β‐Si3N4whiskers, resulting in

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05494.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A study of the mechanical characteristics of a unidirectional fiber–reinforced calcium aluminosilicate matrix composite has been conducted. The properties have been related to the individual properties of the matrix, the fibers, and the interfaces, as well as the residual stress, using available models of matrix cracking and fiber fracture. Comparisons have been made with lithium aluminosilicate matrix composites. Predictions of initial matrix cracking and of ultimate strength using the models are found to correlate well with the measured values. However, deficiencies have been noted in the ability of the models to predict the evolution of matrix cracks, plus associated changes in the modulu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05495.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Slip casting and filter pressing are ceramic forming processes which can be affected by the migration of fine particles during filtration. Fine particles can be carried along with the filtrate and deposited within the ceramic compact and /or filter medium, thereby clogging and reducing the permeabilities of the porous media. This in turn affects the growth rate as well as the porosity of the compact. Evidence of clogging of the ceramic compact and filter medium was obtained by (1) SEM analysis of the compact and filter media, (2) surface area measurements of cross sections of compacts, and (3) measurements of the thickness of compacts as a function of casting time.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05496.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A fine‐grained superplastic β′‐silicon nitride solid solution (SiAlON) was found to develop elongated grains and a pronounced texture during tensile deformation at 1550°C. The texture development is well‐described by a geometrical model of grain rotation in accordance with the strain field. Once aligned, grains can then grow with little constraint due to impingement and often coalesce into each other. With the above microstructural development, the stressstrain curve displayed unusually strong strain hardening characteristics due to a fiber–reinforcement effect of the aligned silicon nitride grains on the glass–containing matrix. By extending the rheological model of Chen and Yoon and considering these microstructural evolutions, we are able to simulate the deforma

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05497.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Workpiece and abrasive‐tip temperatures are measured during the grinding of silicon nitride, zirconia, sapphire, and Ni‐Zn ferrite with diamond abrasives. The measurements are carried out using a multiple‐element infrared sensor having a time constant of ∼7 μs and a spot size as small as 30 μm in diameter; the temperature measurements were made for both single‐point and full‐wheel grinding. A simple analytical model is used to predict the abrasive‐tip and work‐surface temperatures, the subsurface temperature of the workpiece, and the cooling of the abrasive grains between cutting. The predictions of the model agree reasonably well with experiments. The model requires as inputs only the grinding forces, grinding process variables, and the physical properties of the abrasive and workpiece materials. It is shown that, in view of the excellent thermal properties of the diamond abrasive, the surface temperature of the ceramic workpieces is essentially independent of the thermal propertie

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05498.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The synthesis of nanometer‐sized α‐Al2O3−M composites (nanocomposites) is described for M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si, and Fe alloys. The synthesis is performed by solid‐state reduction reaction which occurs during room‐temperature ball‐milling of mixtures of a metal oxide (MxOy) and aluminum (Al2y/3). A complete reduction is reached for milling times of ≅ 1.5 h in a planetary mill. The crystallite sizes of alumina and metal obatined, for powders processed in a vibratory mill, are in gener

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05499.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Dispite a few studies on the Cu─O eutectic bonding of copper and alumina, the mechanism of Cu─Al2O3bonding has been unclear to the present, especially with respect to the formation of a reaction phase. The present work reveals that peel strngth decreases and recovers with the concentration of oxygen at the interface, and that the recovery process is rate‐controlled by the diffusion of oxygen in copper. There is a thin, 5‐nm‐thick, reaction layer between the copper and the alumina, which is likely to be amorpous. Incorporating XPS data of the interface, thermodynamic considerations suggest that the reaction phase is CuAlO2. The variation of peel strength is discussed and related to the stability of CuAlO2and oxygen at the

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05500.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY