摘要:

The indentation behavior (Vickers) of Single‐crystal MgO was studied as a function of temperature (20° to 1000°C). Indentations were made on the {110} plane, with the indents oriented such that one indent diagonal was parallel to the 〈001〉 direction. Using etchant techniques, the dislocation etch pit structures were examined both in the plane of the indentation and in cross section. All the observed slip traces were found to be consistent with primary slip ({110}〈110〉), with no evidence of secondary slip, even at 1000°C. Radial cracking was observed only at the pair of indent corners joined by the indent diagonal parallel to 〈001〉. The crack length increased with temperature (T) for indentations conducted atT<800°C. For indents made at 800°C or higher, however, no cracking occurred. These results are discussed both with respect to an existing slip‐induced crack nucleation model, and the change in crack driving force and toughness with ind

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07218.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A morphous solids belonging to the systems Al2O3–Me2O (Me = Na, K) and Al2O3–B2O3were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850° to 900°C for the compositions Al6MexO(9+0.5x)(x≅ 1; Me = Na, K) and Al6‐xBxO9(1x3). The resultant solids were stable only within a difinite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X‐ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in thePbam(no. 55) space group. The present results indicate that these silica‐free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+, K+) or (2) substitution of B for Al in some of the tetrahed

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07219.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The interfacial debond characteristics of sputtered Al2O3and Y2O3, coating were assessed in a γ‐TiAl/Nb composite. The preferred debond coating identified for this composite system was Y2O3, which also protected the ductile reinforcing phase from reaction with the matrix. Tension tests on a model laminate specimen were performed to characterize the toughening behavior and showed that interfacial debonding promoted extensive plastic deformation of the ductile phase over a large volume. The coatings were characterized to understand the debonding behavior. Comparison is made with other metal/oxide syst

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07220.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The corrosion in Ar–2% CI2gas mixtures of four low‐cost SiC‐based materials suitable for use in high‐temperature heat exchangers has been invesigated. The oxygen potential was controlled by addition of O2or H2at 1000°C. Little attack was observed in the reducing environment composed of Ar–2% Cl2–1% H2or the oxidizing environment composed of Ar–2% Cl2–20% O2but all of the materials were subject to active corrosion at intermediate oxygen potentials. Selective attack of the free silicon phase was obsrved for the siliconized silicon carbide materials. The severity of the active oxidation and the oxygen potentital at which the corrosion changed from active to passive were affected by the nature of the sintering aids used

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07221.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The equations properly describing the diffusion–controlled reaction of a spherical particle to form a concentric spherical shell of product are set up then solved numerically and their predictions compared with those of the commonly used approximate models due to jander, Ginstling and Brounshtein, and Valensi or Carter. It is shown that the quasi‐steady‐state Ginstling and Brounshtein model correctly describes the kinetics of the process when the solute is of low solubility but becomes increasingly inaccurate as solubility increases; the Jander model gives almost identical predictions up to about 60% degree of converison. There is thus a range of problems of practical interst for which either the Jander or Ginstling and Brounshtein model gives an acceptable description of the shape of at least the early stages of the degree of reaction versus time curve, but these simple models do not then give the correct values of, for example, diffusivity from such curve fitting. The simple models are nevertheless suitable for determining whether a reaction is controlled by diffusion through the product layer or by reaction at the interface, provided that some important assumptions are sati

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07222.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The kinetic parameters of nucleation and crystal growth of high‐quartz solid solution in multicomponent lithium aluminosilicate glasses doped with various transition‐metalions were studied by nonisothermal DTA. The crystallization of glasses nucleated at different temperatures was carried out, and plots of the DTA peak versus the nucleation temperatures were used to determine the maximum nucleation rate temperature. Peak temperature data of nucleated samples at varying heating rates (5–20 K/min) were used to determine the activation energy for crystallization via the JMA equation. The temperature of maximum nucleation rate depends greatly on the doped transition‐ metal ions present. The activation energy for crystallization obtained for undoped glass or glasses doped with Fe2O3is of the same order as that already published, and the Avrami exponent is consistent with predominantly three‐dimensional crystal growth. The much higher activation energy values for glasses doped with CoO could be a consequence of two crystallization processes proceeding simul

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07223.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Compositional variation within the Pb(Mg1/3Nb2/3) O3–PbTiO3–(Ba, Sr)TiO3(hereafter PMN–PT–BT,ST) ternary (6.4% PT% 14.1%, 1.25% BT,ST% 2.5%) results in major changes in induced strain and hysteresis. For the 1.25% BT family, the increase in strain correlates with an increase inTmax, while the dielectric loss is uncorrelated with hysteresis and strain. In addition, weak field aging (which is not reset by application of field) shows little effect on strain and hysteresis for drive fields of>0.2 MV/m. The vary narrow polarization‐fields loops (virgin curvesnearly indistinguishable from subsequent cycles) show that weak‐field permittivity is a good approximation to the high‐field permittivity. is a good approximation to the high‐field Permittivity. Although these data clarify the frequency (Tmaxis linearly dependent on the logarithm of the frequency) effect on weak‐field dielectric behavior, they do not directly address the question of meaningful extrapolation of high‐field strain with frequency. In particular, the question remains as to whether the high‐field permittivity and strain are frequency dependent. In future papers we will address this question by a combination of measurement techniques as f

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07224.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Ceramics in the (1 –x)[(1 –y)Pb(Mg1/3Nb2/3)O3·yPbTiO3] ·xMeTiO3system, where Me is Sr or Ba, exhibit very large electrostrictive strains at reasonable drive fields. However, the optimum use temperature and frequency vary with the particular composition used. As relaxor ceramics, each composition has a broad transition from electrostrictive to partially piezoelectric behavior. The transition temperature (Tt) can be roughly determined from strain or polarization properties; however, it can be more quantitatively determined from the effective electro‐mechanicalQ. A plot of induced transverse strain/induced polarization squared (effectiveQ12) as a function of temperature shows a sharp and unmistakable change in slope—this definesTt. The slope of induced transverse strain/polarization (effective g31) also shows a change in slope atTt, although this is more gradual than that of effectiveQ. The indicatedTtcorrelates with those found from measurement of strain and pol

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07225.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Fine, homogeneous, dual‐phase Ag–YBa2Cu3O7–xcomposite powders were prepared by a simple colloidal sol–gel coprecipitation technique that was previously used for synthesis of single‐phase YBa2Cu3O7–x. Samples containing up to 60 wt% silver were synthesized. Silver neither reacted with nor degraded the YBa2Cu3O7–x. The presence of silver was found to aid the oxygenation of the precursor during calcination to form orthorhombic YBa2Cu3O7–x. Measurements made by ac magnetic susceptibility showed no significant degradation in transition temperature for samples containing up t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07226.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A very pure magnesium dititanate, MgTi2O5, is obtained from a precipitate calcined at 480° to 500°C for 2 h in a dynamic atmosphere. This precipitate is prepared at room temperature by the coprecipitation of a sulfuric solution, Ti(IV)–Mg(II), in a molar ratio of 2:1, with an aqueous solution of sodium hydroxide at pH

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb07227.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY