Chemical States of Copper and Tin in Copper Glazes Fired under Various AtmospheresWakamatsu, Mitsuru; Takeuchi, Nobuyuki; Nagai, Hiroki; Ishida, Shingo
doi: 10.1111/j.1151-2916.1989.tb05946.xpmid: N/A
R—O, N—R, and O—R firing operations produce copper red glazes resulting from the formations of Cu2O, metallic copper, and both Cu2O and metallic copper, respectively. The symbols O, N, and R denote oxidizing, neutral, and reducing atmospheres, respectively, and in the set of symbols, the former corresponds to the heating atmosphere and the latter to the successive cooling atmosphere. All other firing operations such as R—R and R—N fail to produce red glazes. The formation mechanisms of metallic copper and Cu2O are postulated from a microscopic viewpoint.
Crack Growth Resistance of Textured AluminaSalem, Janathan A.; Shannon, John L.; Brad, Richard C.
doi: 10.1111/j.1151-2916.1989.tb05947.xpmid: N/A
The crack growth resistance of a textured, extruded alumina body was compared with that of an isotropic, isopressed body of similar grain size, density, and chemistry. R‐curve levels reflected the preferred orientation; however, R‐curve slopes (dKIR/dΔa) were the same in all instances, implying a similar crack growth resistive mechanism. Three orthongonal orientations of crack growth in the two structures exhibited similar forms of KIR versus Δa curves, for which a schematic diagram for polycrystalline ceramics is proposed.
Admittance—Frequency Response in Zinc Oxide Varistor CeramicsAlim, Mohammad A.
doi: 10.1111/j.1151-2916.1989.tb05948.xpmid: N/A
The lumped parameter/complex plane analysis technique revealed several contributions to the terminal admittance of the ZnO—Bi2O3 based varistor grain‐boundary ac response. The terminal capacitance has been elucidated via the multiple trapping phenomena, a barrier layer polarization, and a resonance effect in the frequency range 10−2≤f≤ 109 Hz. The characterization of the trapping relaxation behavior near ∼ 105 Hz (∼ 10−6 s) provided a better understanding of a previously reported loss‐peak. The possible nonuniformity in this trapping activity associated with its conductance term observed via the depression angle of a semicircular relaxation in the complex capacitance (C*) plane has been postulated.
Predicting the Grain‐Size Distributions in High‐Density, High‐Purity Alumina CeramicsVenkataraman, Kollengode S.; DiMilia, Robert A.
doi: 10.1111/j.1151-2916.1989.tb05949.xpmid: N/A
This paper discusses the image‐analyzer‐based grain‐size distributions (GSDs) of fully densified ceramics obtained from pressure casting a high‐purity alumina powder, develops an algorithm for predicting the GSDs as a function of sintering time and temperature, and compares of the GSDs thus predicted with those experimentally observed. The GSD data for all sintered specimens, when nondimensionalized in terms of the median grain size, reduced to a single self‐preserving GSD curve. The median grain was predicted as a function of sintering time and temperature using the classical kinetics equation. The GSDs predicted using the algorithm developed tallied well with those that were experimentally obtained.
Low‐Temperature Sintering of Seeded Sol—Gel‐Derived, ZrO2‐Toughened Al2O3 CompositesMessing, Gary L.; Kumagai, Masato
doi: 10.1111/j.1151-2916.1989.tb05950.xpmid: N/A
Seeding a mixture of boehmite (AIOOH) and colloidal ZrO2 with α‐alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ‐to α‐alumina transformation, homogeneous mixing, and the uniform structure of the sol‐gel system. Complete retention of pure ZrO2 in the tetragonal phase was obtained to 14 vol% ZTA because of the low‐temperature sintering. The critical grain size for tetragonal ZrO2 was determined to be ∼0.4 μm for the 14 vol% ZrO2—Al2O3 composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.
Temperature Dependence of Infrared Absorption Spectra of Hydroxyl Groups in Soda Germanate GlassesHosono, Hideo; Abe, Yoshihiro
doi: 10.1111/j.1151-2916.1989.tb05951.xpmid: N/A
Infrared absorption spectra of hydroxyl stretching vibrations in soda germanate glasses were measured over the range from ambient temperature up to about their softening point. The spectrum of 5Na2O · 95GeO2 glass was found to consist of two absorption bands through an analysis of the spectral changes with temperature. One is located around 3550 cm−1, similar to the band seen in GeO2 glass. Another is centered around 3300 cm−1, also seen in 28Na2O · 72GeO2 glass. From the OH band position and the negative charge on the acceptor oxygen, the bonding configuration responsible for the latter band is considered to be an OH group forming a hydrogen bond with an oxygen bridging a 4Ge and a 6Ge (superscript: coordination number). The equilibrium between the two types of hydroxyl groups giving the above two bands was almost frozen at a temperature far below the Tg of the host glass.
Effect of Creep Damage on the Tensile Creep Behavior of a Siliconized Silicon CarbideCarroll, Daniel F.; Tressler, Richard E.
doi: 10.1111/j.1151-2916.1989.tb05952.xpmid: N/A
The tensile creep behavior of a siliconized silicon carbide was investigated in air, under applied stresses of 103 to 172 MPa for the temperature range of 1100° to 1200°C. At 1100°C, the steady‐state stress exponent for creep was approximately 4 under applied stresses less than the threshold for creep damage (132 MPa). At applied stresses greater than the threshold stress for creep damage, the stress exponent increased to approximately 10. The activation energy for steady‐state creep at 103 MPa was approximately 175 kJ/mol for the temperature range of 1100° to 1200°C. Under applied stresses of 137 and 172 MPa, the activation energy for creep increased to 210 and 350 kJ/mol, respectively, for the same temperature range. Creep deformation in the siliconized silicon carbide below the threshold stress for creep damage was determined to be controlled by dislocation processes in the silicon phase. At applied stresses above the threshold stress for creep damage, creep damage enhanced the rate of deformation, resulting in an increased stress exponent and activation energy for creep. The contribution of creep damage to the deformation process was shown to increase the stress exponent from 4 to 10.
Quantitative Analysis of Brittle Fracture Surfaces Using Fractal GeometryMecholsky, J. J.; Passoja, D. E.; Feinberg‐Ringel, K. S.
doi: 10.1111/j.1151-2916.1989.tb05954.xpmid: N/A
Fractal geometry is a non‐Euclidean geometry which has been developed to analyze irregular or fractional shapes. In this paper, fracture in ceramic materials is analyzed as a fractal process. This means that fracture is viewed as a self‐similar process. We have examined the fracture surfaces of six different alumina materials and five glass‐ceramics, with different microstructures, to test for fractal behavior. Slit island analysis and Fourier transform methods were used to determine the fractal dimension, D, of successively sectioned fracture surfaces. We found a correlation between increasing the fractional part of the fractal dimension and increasing toughness. In other words, as the toughness increasing the fracture surface increases in roughness. However, more than just a measure of roughness, the applicability of fractal geometry to fracture implies a mechanism for generation of the fracture surface. The results presented here imply that brittle fracture is a fractal process; this means that we should be able to determine processes on the atomic scale by observing the macroscopic scale by finding the generator shape and the scheme for generation inherent in the fractal process.