摘要:

The dielectric constants and dielectric loss values of two naturally occurring cordierite single crystals were determined at 1 MHz using a two‐terminal method and empirically determined edge corrections. The results are as follows. Cordierite 1: κ′a= 5.653, tan δ= 0.009; κ′b= 6.106, tan δ= 0.003; κ′c= 5.658, tan δ= 0.010. Cordierite 2: κ′a= 5.765, tan δ= 0.0006; κ′b= 6.334, tan δ= 0.0007; κ′c= 5.807, tan δ= 0.0007. The agreement between measured dielectric polarizabilities as determined from the Clausius–Mosotti equation and those calculated from the sum of oxide polarizabilities according to αD(mineral) =ΣαD(oxides) neglecting the channel H2O and CO2for these two cordierite samples is ∼20%. Inclusion of the effects of water and CO2assuming mobile H2O molecules with no correlation between their dipoles improves the agreement to ∼2%. Comparison of dielectric constants of manufactured cordierite ceramics shows a discrepancy between typical observed values of 4.5–5.0 and a theoretical value of 4.0 calculated from the Clausius–Mosotti equation. In certain cases, this discrepancy can be accounted for by the presence of high‐κ′ impurities, but in others

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05591.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Fine‐grained β‐Ca2SiO4containing small amounts of sodium was fabricated as an analogue to tetragonal zirconia polycrystals (TZP) in order to study the stress‐induced β→γ transformation. This avoided the problems associated with the fabrication and evaluation of composites containing β‐Ca2SiO4. The microstructure of dense β‐Ca2SiO4exhibited severe intergranular strains and twin‐terminating microcracks as seen by TEM. The β‐phase twin widths were quantitatively correlated with grain sizes giving an average ratio of 0.04. Stress‐induced transformation was observed on ground surfaces but not on fracture surfaces. The stress–strain behavior and the mechanical properties were consistent with stress‐induced microcracking and microcrack coalescence. The elastic modulus of fully dense β phase

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05592.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca2SiO4) were studied by TEM. Three types of superlattice structures were observed in the α′Land β phases. Almost all β grains were twinned and strained. Symmetry‐related domain structures inherited from previous high‐temperature transformations were observed in β grains. Both the α→α′Hand α′L→β transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the β→γ transformation is proposed to be strains and cation charge repulsions in the β structure. This mechanism can be displacive, but it needs to overcome a compara

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05593.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Effects of the concentration of ZrOCl2, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high‐purity ZrSiO4fine powders from ZrOCl2.8H2O (0.2Mto 1.7M) and equimolar colloidal SiO2using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO4from the high‐purity ZrSiO4powders have been also investigated. Single‐phase ZrSiO4fine powders were synthesized at 1300°C by forming ZrSiO4precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO4precursor gels to ZrSiO4powders from concentrations other than 0.4MZrOCl2.8H2O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO2and SiO2was depressed and the propagation and increase of ZrSiO4nuclei in the gels were accelerated. The density of the ZrSiO4sintered bodies, manufactured by firing the ZrSiO4compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at hig

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05594.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca2(SiO3)(OH)2) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low‐crystalline β‐Ca2SiO4, which is stable at room temperature and has a remarkably large specific surface area of about 7 m2/g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topot

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05595.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Pourbaix diagrams are shown to conceptually explain dissolution of glasses in terms of their thermodynamic stability and surface layer formation. To predict the long‐term effects of ground‐water contact on the durability of nuclear waste glass in a geologic repository, the nature of the glass dissolution process has been extensively examined. Hydration thermodynamics has been used to quantify the role of glass composition and the effect of solution pH. The glass compositions examined vary widely in composition and include natural, lunar, medieval, and nuclear waste glasses, as well as some glass‐ceramics. The known effects of solution pH and oxidation potential (Eh) on glass dissolution are empirically described by thermodynamically calculated Pourbaix diagrams. Statistical analysis of over 300 glass durability tests demonstrates that the Pourbaix diagram can be quantified because of (1) the colinearity of the hydration free energy with solution pH defined by the Nernst equation and (2) the colinearity of the solution pH with the concentration of dissolved silicon and boron in the solution defined by the pH dependence of the ion activities. Construction of Pourbaix diagrams to describe glass dissolution serves to unify and categorize various existing experimental glass dissolution

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05596.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A realistic computer model is presented for calculating the time‐dependent volume fraction transformed during the devitrification of glasses, assuming the classical theory of nucleation and continuous growth. Time‐ and cluster‐dependent nucleation rates are calculated by modeling directly the evolving cluster distribution. Statistical overlap in the volume fraction transformed is taken into account using the standard Johnson–Mehl–Avrami formalism. Devitrification behavior under isothermal and nonisothermal conditions is described. The model is used to demonstrate that the recent suggestion by Ray and Day,1that nonisothermal DSC studies can be used to determine the temperature for the peak nucleation rate, is qualitatively correct for lithium disilicate, the glass inv

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05597.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The strength of a commericially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long‐term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150°, 1260°, and 1370°C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated by allowing individual creep tests to continue until specimen failure. The applicability of the four‐point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150°C, failure was controlled by a slow crack growth mechanism. At 1260° and 1370°C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman–Gra

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05598.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The morphology of surfaces of several ceramic materials has been examined using transmission electron microscopy. The approach used was to prepare a sample for examination in the microscope, carefully clean it, and then heat‐treat it. In the case of the oxides studied (alumina and spinel) the samples were heated in air; the non‐oxides (α‐SiC andβ‐SiC) were annealed under vacuum. The morphology in all but one case was such that the surface faceted parallel to the nearest low‐index plane to give well‐defined terraces; these were separated by ledges which also tended to facet parallel to the traces of low‐index planes. The exception was the {1100} alumina surface, which appears to be unstable in air at temperatures close to 1400°C. A computer program using a multislice approach was used to estimate the height of the steps on the (0001) surface; the step heights appear to be multiples of theclattice parameter. A reconstruction of this surface as a result of this heat treatment

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05599.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

TiB2‐particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.1–3Commercially available TiB2powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB2particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (>98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC‐particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05600.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY