摘要:

Bi2O3‐based cubic solid solutions containing 20% Er2O3undergo transformation to a rhombohedral phase when annealed at temperatures 635°C. This transformation is generally very sluggish and is accompanied by a decrease in conductivity. The kinetics of this transformation were enhanced by the addition of CaO and suppressed by the addition of ZrO2. The time constants for transformation kinetics at 600°C for CaO‐doped and undoped samples were ∼55 and ∼330 h, respectively, and the incubation periods ranged between ∼20 and ∼2000 h depending upon the dopant type, its concentration, and the temperature. This result is rationalized on the premise that cation interstitials are more mobile compared to cation vacancies. The same samples, originally cubic of CaF2type, when annealed below about 600°C for a short period of time (a few hours) undergo a degradation in conductivity without a change in XRD patterns. The kinetics of this conductivity decay were observed to be significantly faster than the kinetics of cubic → rhombohedral phase transformation. A similar degradation of conductivity was also observed in samples containing other stabilizers such as Y2O3, Dy2O3, and Yb2O3. The kinetics of conductivity decay without the formation of the rhombohedral phase were also found to depend upon the presence of aliovalent dopants. Specifically, ZrO2suppressed this decay. Electron diffraction showed the formation of a superstructure in samples annealed at temperatures<600°C. Analysis of the diffraction patterns suggests that the structure corresponds to a doubling of the unit cell with the ordering of cations responsible for the origin of the superstructure. The decay in conductivity in the ordered state is attributed to the expected differences in bonding between oxygen ions and the two different c

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03961.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AlN and Al2OC, both of 2H structure, form a homogeneous solid solution at elevated temperatures. Annealing at lower temperatures leads to the formation of two solid solutions, one AlN‐rich and the other Al2OC‐rich. Samples of near equimolar composition form disk‐shaped precipitates upon annealing at temperatures below ∼1900°C. In the present work, samples of several compositions were fabricated by hot‐pressing mixtures of AlN, Al4C3and Al2O3. Lattice parameters of the resulting solid solutions were measured by X‐ray diffraction. Samples of an equimolar composition were annealed at 1500°C for various periods of time up to a maximum of 512 h. Disk‐shaped precipitates with their axes along the [0001] direction were initially coherent. In the coherent stage, thealattice parameters of the precipitate and the matrix were identical because of coherent fitting of lattice planes. The shape of the precipitates and their orientation were in accord with predictions based on theoretical calculations of coherency strain energy. Using the experimentally measured values of the aspect ratios, the precipitate/matrix interfacial energy for coherent precipitates was estimated to be on the order of a few mJ/m2. Long‐term annealing led to the loss of coherency as manifested by the formation of inter

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03962.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The dielectric function of the ordered Ba(Mg1/3Ta2/3)O3ceramics was investigated by the infrared reflectance spectra taken over the 50–4000‐cm−1range. The detailed crystal structure of the specimen was examined by the Rietveld method. The space group of trigonalD3d3and the degree of long‐range order of 0.97 for Mg and Ta atomic arrangement were confirmed. The reflectance spectra were analyzed on the basis of the four‐parameter semiquantum model assuming 16 infrared active vibrational modes allowed for the relatedD3d3structure. The lowest‐frequency optical mode was found at 60 cm−1, which can be assumed to involve the motions of the heavy

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03963.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The electrical potential difference between the interface and the bulk in TiO2is obtained as a function of temperature, oxygen pressure, and acceptor or donor doping from a space charge model that explicitly includes the high‐temperature lattice defect chemistry. Using defect equilibrium constants for TiO2from previous literature studies, it is shown that for a space charge determined by ionic defect equilibration with the interface, the potential is negative in undoped and acceptor‐doped TiO2and positive at high donor concentrations. The isoelectric point lies in the donor‐doped regime at high temperatures due to the contribution of defects from reduction, even for fairly oxidizing amb

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03964.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A quantitative study of space charge solute segregation at grain boundaries in TiO2is conducted, using a new STEM method for the measurement of aliovalent solute accumulation. It is shown that the electrostatic potential at grain boundaries can be varied in sign and magnitude with doping, oxygen pressure, and temperature, and that the isoelectric point lies in slightly donor‐doped compositions for samples annealed in air. The experimental results closely fit the space charge model in Part I. Space charge solute segregation is found even in defect regimes of high electron concentration. Approximately one in ten grain boundaries are “special” in exhibiting no detectable segregation; in one such instance a twin boundary is identified. Among boundaries with significant amounts of segregation, clear differences in potential also exist. From the potential determined in acceptor‐ and donor‐doped compositions, the Frenkel energy (assumed to be lower than the Schottky energy in TiO2) can be separated into its individual terms. An average value for the titanium vacancy formation energy ofgvTi= 2.4 eV and an upper limit to the titanium interstitial formation energy ofgTii= 2.6 eV are

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03965.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Green ceramic fibers of Al2O3, Si3N4, Ce‐ZrO2, and SiC were prepared by suspension dry spinning of powderloaded polymer solutions. The fiber‐forming polymer was ethyl methacrylate dissolved in methyl ethyl ketone. The experimental conditions for fiber formation were defined for the alumina system. Thread lengths measured as a function of extrusion rate showed a transition from capillary‐limited fracture to continuous fibers in agreement with Ziabicki's theory of thread formation. The conditions for spinnability were correlated to rheological parameters. Spinnable Al2O3dopes were elastic fluids with a high viscosity (60 Pa·s at 0.46 s−1) under the conditions of th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03966.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Cobalt(II) oxide (CoO) has been prepared in the zincblende structure by the decomposition of cobalt(II) acetate under vacuum at 290°C. Calorimetric experiments have shown that the zinc‐blende structure is energetically 37 ± 10 kJ/mol less stable than the common rock‐salt form of CoO. This value is in good agreement with the value recently reported by Grimes and Lagerlof of 38 kJ/mol found by a lattice energy calcu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03967.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The compression creep test is analyzed with respect to the friction at the loaded ends of the specimen. An analytical and a finite‐element solution performed for large deformations reveals that friction leads to a strain‐independent reduction of the creep rate which only depends on the stress exponent of the creep law and the geometry of the specimen. The assumed material behavior is a time‐dependent, isochoric, nonlinear viscosity. Compression creep tests on 3‐mol%‐yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP) have been analyzed. The determination of a stress exponent corresponding to the decreasing stress state in the specimen during large deformations results in a higher resolution of the stress–strain‐rate relationship derived from a certain number of creep tests. For 3Y‐TZP the compression creep tests reveal a decreasing stress exponent wit

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03968.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The constrained sintering of films on substrates leads to a reduction in densification rate and may lead to processing flaws. This paper reports on a study of damage and cracking in sintering films, with particular emphasis on the growth of preexisting cracks. Experiments have been conducted with glass and polycrystalline Al2O3films on various substrates. The effect of important variables (viz., film thickness, crack length, and friction with the substrate) on crack growth is reported. The experiments with glass films show that cracking occurs above a critical film thickness which is in good quantitative agreement with a recent analysis for this problem. In the case of Al2O3films, we observe a diffuse damage zone ahead of cracks. Crack growth occurs by the coalescence of microcracks with each other and with the main crack. Some possible reasons for this difference between the glass and Al2O3films are presented. As a model for diffuse damage, the stability of a sintering film under spatial variations in constitutive parameters is analyzed. It is shown that the film is unstable to small perturbations only in the early stages of densification, and that for viscous sintering the films are usually kinetically stable.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03969.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A thin plate of TeO2glass of 5.0 × 4.0 × 0.25 mm3size, which was large enough for various optical measurements, was obtained by a rapid quenching method. The linear refractive index was measured as a function of wavelength from 486.1 to 1000 nm. The refractive index at 486.1 nm was as high as 2.239. The optical energy band gap was estimated as 3.37 eV from the optical absorption spectrum. The third‐order nonlinear optical susceptibility, χ(3), was determined by the third‐harmonic generation (THG) method. The χ(3)value was as high as 1.4 × 10−12esu, about 50 times as large as that of SiO2glass. The results are discussed based on Lines' model in which an influence of cationic emptyd‐orbitals on the nonlinear properties was taken

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03970.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY