摘要:

The local structural arrangement of ions around a substitutional Cr3+ion in sapphire (ruby) has been studied experimentally using extended X‐ray absorption fine structure (EXAFS) in the vicinity of the Cr absorption edge. The findings are compared with an ionic model (Mott‐Littleton) computation using two sets of pairwise potentials. Both the EXAFS results and the computations reveal that when Cr substitutes for an Al ion in sapphire, the surrounding ions relax to an arrangement similar to that for Cr in α‐Cr2O3. Also, most of the structural relaxation is accommodated by the first shell of oxygen and aluminum ions around the substituted Cr3+ion. The computations also indicate that with applied pressure (tensile and compressive) the ionic positions change self‐similarly and in proportion to the macroscopi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07873.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The formation of mullite was investigated using microcomposite powders which consist of α‐alumina cores and amorphous silica coatings. Differential thermal analysis and X‐ray diffraction showed that the mullitization reaction was endothermic. In contrast, mullite forms exothermically in samples prepared by sol‐gel processing. The results are shown to be consistent with available thermodynamic data for mullite formation from different alumina and silica

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07874.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The microstructures of ferroelectric glass‐ceramics derived from (Pb,Sr,Ba)Nb2O6solid solutions have been investigated systematically using electron microscopy (TEM, SEM) and X‐ray diffraction (XRD). These data, together with the measured dielectric properties of the materials, have been correlated with both the sintering/devitrification conditions and compositional changes to the base glass. The low incidence of ferroelectric domains suggests that an increase in permittivity results from an increase in internal stress during grain growth. These domains were observed only in a few tetragonal structured crystals with grain sizes>1 μm. Their compositions were typically Sr‐rich and Ba‐depleted. For a given devitrification/crystallization temperature, the grain compositions and sizes are affected significantly by relatively minor compositional changes to the base glass network formers. The diffuse temperature dependence of permittivity is attributed to the fine‐grain (submicrometer) microstrucutres and the compositional range of solid solutions which arise during devit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07875.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Ceramic‐metal composites have been made to near net‐shape by reactive penetration of dense ceramic preforms by molten Al. In contrast to processes involving physical infiltration of a porous medium, this process works with dense ceramic preforms. Ceramic‐metal composite formation by reactive metal penetration is driven by a strongly negative Gibbs energy for reaction. For Al, the general form of the reaction is (x + 2)Al + (3/y)MOy, → Al2O3+ M3Alx, where MOy, is an oxide that is wet by molten Al. In low Po2atmospheres and at temperatures above about 900°C, molten Al reduces mullite to produce Al2O3and silicon. The Al/mullite reaction has a ΔGr°(1200 K) of −1014 kJ/mol and, if the mullite is fully dense, the theoretical volume change on reaction is less than 1%. Experiments with commercial mullite containing a silicate grain boundary phase average less than 2% volume change on reaction. In the Al/mullite system, reactive metal penetration produces a fine‐grained alumina skeleton with an interspersed metal phase. With enough excess aluminum, mutually interpenetrating ceramic‐metal composites are produced. Properties measurements show that ceramic‐metal composites produced by reactive metal penetration of mullite by Al have a Young's modulus and hardness similar to that of Al2O3, with improved fracture toughness ranging from 5 to 9 Mpa·m1/2. For penetration times less than 1 h, reaction layer thickness varies as the square root of time, which allows ceramic‐metal composite coatings to be fabricated by controlling the penetration time. Thermodynamic calculations indicate that other compositions also are candidates forin situreaction synthesis, which suggests that reactive metal penetration may be a general route to composite synthesis with the prospect for near

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07876.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

This study focuses on interfacial bonding between intergranular silicon carbide particles and an alumina matrix, to determine the creep inhibition mechanism of alumina/ silicon carbide nanocomposites. It is revealed that the silicon carbide/alumina interface possesses much stronger bonding than the alumina/alumina interface through three approaches: investigation of fracture toughness and fracture mode and consideration of internal thermal stresses acting at grain boundaries, estimation of equilibrium thickness of intergranular glassy films by force balance, and direct observation of grain boundaries by TEM. The rigid bonding of alumina/silicon carbide interfaces causes inhibition of vacancy nucleation and annihilation at the interfaces, causing remarkably improved creep resistance of the nanocomposite.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07877.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

New fabrication techniques were developed for uniaxial creep specimens of high‐strength, high elastic modulus, pitch‐based, carbon filaments, with a matrix‐free test section. Experiments show that pitch‐based carbon filaments with a tensile modulus of 896 GPa can exhibit, under high stress, large creep deformation without necking. The stress exponent for the pitch‐based fibers in the Dorn equation is much lower than that for PAN‐based fibers, while the activation energy is the same. The activation energies and activation volumes determined from the experiments suggest mechanisms of creep

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07878.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Crack growth in silica optical fibers was measured directly in a wide range of crack velocities da/dt(10−13

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07879.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

The temperature dependence of the fracture toughness of ceramics exhibiting duplex microstructures was studied relative to their single‐phase constituents using two test methods: bend testing of chevron‐notched beams, and the indentation‐crack‐length technique. The two materials systems studied were Al2O3:c‐ZrO2(Y) and A12O3:Y3A1SO12(YAG), and the testing temperature ranged from room temperature to 1200°C. The study showed that in both systems the duplex materials showed higher toughness values than their single‐phase constituents above 800°C. This result was attributed to the contribution of low‐energy interphase boundaries to the overall composite toughness. Indentation crack length measurements gave toughness values and trends comparable to those determined by the chevronnotched beam method. By comparing the results of the two test methods it was possible to demonstrate that the indentation calibration constant (ξ) shows no significant temperature or material dependence. For the zirconia‐containing materials, however, indentation at elevated temperatures is accompanied by significant localized plasticity, which suppressed the radial cracking. Under such conditions, some caution is warranted, since localized plasticity can lead to an overestimation of the

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07880.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Strain development in several notched ceramic‐matrix composites (CMCs) has been monitored over a broad range of load. This was achieved by using phase‐shifting moire interferometry, which provides a map of the surface strains. A sequence of fringe patterns was used to chart the evolution of strain redistribution as a function of load. The ensuing strains were related to the micromechanical damage mechanisms. Stress concentrations were estimated from the strains by using stress/strain curves. Implications for the notch sensitivity of CMCs are discus

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07881.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

In this paper, the tensile properties of cement‐based composites containing random discontinuous steel fibers are reported. Direct tensile tests were performed to study the effects of fiber length (hence fiber aspect ratio), interfacial bonding, and processing conditions on composite properties. Composite tensile strength and ductility are highlighted and discusse

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1996.tb07882.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY