摘要:

The coexistence of the cubic fluorite and tetragonal phases in rapidly quenched samples was studied in the ZrO2‐MO1.5systems for M = Sc, In, Y, and rare earths (R). Spontaneous transformation from metastable cubic phase was triggered at room temperature by a mechanical force. Isolated tetragonal platelets in the cubic matrix were bounded by [101] habit planes and contained anti‐phase boundaries. The tetragonality decreased with stabilizer content and vanished at around 18 mol% for M = Y and R, 23 mol% for M = Sc, and 25 mol% for M = In, all at room temperature. With increasing temperature, the tetragonality initially increased because of anisotropic thermal expansion, then decreased rapidly, after reaching a maximum, as the temperature for the tetragonal‐to‐cubic transformation was approached. Being a first‐order martensitic transformation, the cubic‐to‐tetragonal transformation is accompanied by a discontinuous change of tetragonality and a hysteresis loop as the temperature or composition passes through the equi

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05546.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

29Si MAS‐NMR measurements have been made on a series of lithium borosilicate glasses of general compositionRLi2O.B2O3·KSiO2. At low alkali contents (R<1), the29Si resonance envelope is broadened and indicates a distribution of Si sites. AsRincreases above 1, the FWHM of the29Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the11B NMR studies of these same glasses, the29Si NMR data are quantitatively best‐fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins atR= 0. Models of sharing developed from the11B NMR studies of these glasses, where proportional sharing above a certain fixed (independent ofK) or variable (dependent onK) minimumR0, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unk

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05547.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

A pure, acicular lead titanate (PbTiO3) fine powder with a white color has been prepared by hydrothermal synthesis. It is a new phase of PbTiO3withI4 symmetry, cell parameters ofa= 12.358 Å andb= 14.541 Å, and a density of 6.80 g.cm−3. The influences of pH (12.5 to 14.4), Pb/Ti ratio (1.0 to 1.6) in the feedstock, reaction temperature (130° to 230°C), time (0.25 to 4 h), starting materials, and additives on the formation of acicular PbTiO3under hydrothermal conditions have been investigated. The acicular PbTiO3withI4 symmetry, referred to as the PX phase, can be converted to the perovskite‐type (PE phase) of PbTiO3at about 605°C while its acicular morphology is essentially unchanged. The preferable conditions for preparing pure acicular PX‐phase PbTiO3are that the pH is 13.0 to 14.0, Pb/Ti ratio is>1.3, reaction temperature is 170° to 200°C, time is 0.5 to 1.0 h, titanium butoxide (Ti[O(CH2)3CH3]4) is the starting material, and poly(vinyl alcohol) is an additive. The acicular grain of the PX phase is usually less than 100 nm in diameter and more than 1000

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05548.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

By focusing on the similarities between the three stages of sintering, a single equation is derived that quantifies sintering as a continuous process from beginning to end. The microstructure is characterized by two separate parameters representing geometry and scale. The dimensionless geometry parameter, denoted T, comprises five scaling factors that relate specific microstructural featuers (e.g., surface curvature) to the scale (grain diameter). Calculations of T from experimental data show (a) agreement with computer simulations of initial‐stage sintering, (b) the effect of surface diffusion on T, and (c) changes in T with microstructural evolution during sintering. Application of the model to the design of firing schedules and the study of microstructural geometry effects on sintering is discusse

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05549.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Three‐layer samples in the SiC‐AlN system with outer layers richer in SiC, as well as monolithic samples of uniform composition, were fabricated by hot‐pressing. The strain gage technique, previously described, was used to estimate residual compressive stress in the outer layers developed because of the difference in coefficients of thermal expansion between inner and outer layers. Bar‐shaped samples were indented using a Vickers indenter under loads as high as 20 kg (196 N) on outer layers and were fractured in three‐point bending. Three‐layer samples with outer layers under compression exhibited increased resistance to the contact‐induced damage in comparison to the monoc

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05550.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The formation of superconducting phases in the Bi(Pb)─Sr─Ca─Cu─O system has been systematically investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, ICP‐AES, fourprobe dc resistance, and ac susceptibility. Starting compositions, firing temperature, and the duration of heat treatment, together with the atmosphere, were found to be critical in determining the preferred formation of the 2223 phase. This paper reports the effect of the initial chemical composition, emphasizing the importance of compositional control in the synthesis of the single 2223 phase. It has been shown that, with a correct starting composition and predetermined synthesis conditions, single 2223 phase can be obtained without intergrowth by the 2212 and other impurity crystalline phases. The optimum starting composition for the preferred growth of the 2223 phase was identified as being Bi1.7Pb0.3+ySr2Ca2Cu3Ox(y= 0.1), with excess Pb added in order to compensate for its loss at high temperatures. The effect of Pb doping and excess Cu on the phase formation in the Bi oxide based superconducting system is

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05551.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The formation mechanism, the temperature range for the growth, and the thermal stability of the 2223 phase in Bi(Pb)─Sr─Ca─Cu─O have been investigated using DTA/TG, XRD, SEM/EDAX, TEM, EPMA, four‐probe resistance and acsusceptibility measurement. The formation of the 2223 phase was found to follow a dissolution–precipitation process. A 2212 phase first reacts with the liquid phase formed via an incongruent melting of the Ca2PbO4phase, and a dissolution of CaO and CuO takes place. The 2201 phase, which has the largest negative free energy, is then precipitated from the melt; the nucleation and growth of the 2223 phase are subsequently developed by the reaction between the 2201 phase precipitates and ions of Ca2+and Cu2+present in the liquid phase. The 2223 phase is formed at temperatures in the range 827°CT<856°C. The optimum temperatureTffor the formation of 2223 phase is 845°± 5°C. The 2223 phase is thermodynamically unstable at temperat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05552.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The formation of second phases during the preparation of the 2223 phase and their stability in the Bi system under various annealing temperatures and atmospheres have been studied. The 2201 precipitates developed at ∼830°C, and their conversion to the 2223 phase can be completed in the temperature range 810°C ≤T<830°C. A Ca2PbO4‐like phase can precipitate from the liquid phase at ∼830°C during cooling. A (Sr,Ca)14Cu24O41phase is usually found accompanying the synthesis of the 2223 phase. This secondary phase is stable in an oxidizing atmosphere but can be eliminated by annealing under a low oxygen atmosphere or by choosing a suitable starting composition and set of sintering conditions. The precipitation of Ca2PbO4‐like phase can be avoided by using a relatively fast cooling rate. Unlike the YBa2Cu3Oxsuperconductor, the 2223 phase can be stable under a wide range of atmospheres, such as argon, ai

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05553.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Thin films of titania have been prepared by spin coating on fused silica, Si(100), and rutile(110), starting with a sol–gel process. The alkoxide solution was chelated with diisopropanolamine, and the resulting precursor solution was hydrolyzed prior to coating. Oriented rutile films were obtained on fused silica and Si(100), while epitaxially oriented film was formed on rutile (110). X‐ray diffraction results indicated that the as‐deposited films transformed to rutile via anatase with increasing temperature. The phase transformation temperature was found to be dependent on the substrate, and it was in general higher on the substrates than that observed for the gel powder. Microstructural studies revealed that these films consisted of finely dispersed grains of 0.05 to 0.15 μm i

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05554.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Composite densification was studied by performing slip casting and sintering experiments on an Al2O3matrix and Si3N4whisker system. Even though all the slip‐cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore‐size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker‐associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si3N4affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the compo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb05555.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY