Cooling-rate effects on the depletion layer of BaTiO3 positive temperature coefficient of resistance (PTCR) were analysed using d.c. resistivity and a.c. complex impedance spectroscopy. As the cooling rates decreased,...
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Cooling-rate effects on the depletion layer of BaTiO3 positive temperature coefficient of resistance (PTCR) were analysed using d.c. resistivity and a.c. complex impedance spectroscopy. As the cooling rates decreased, the d.c. resistivity of the sample increased. A.c. complex impedance spectra showed that the observed d.c. resistivity increase was mainly due to the grain-boundary resistivity increase. The grain resistivity also exhibited relatively weak PTCR behaviour. The built-in potential and the depletion-layer width were analysed using the a.c. complex impedance data. Slow cooling rate produced higher built-in potential and large depletion-layer width. The relationship between the built-in potential and the depletion-layer width at different temperatures was analysed and used to explain the observed grain-boundary resistivity change. The resistivity decrease in the low-temperature region was due to the depletion-layer width decrease. The resistivity jump over the temperature was due to both the built-in potential and the depletion-layer width changes.
The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 1. min in a flowing Ar at...
The effects of alumina on the densification of boron carbide and related reaction phenomena in alumina-doped B4C were studied. Pressureless sintering was conducted at various temperatures for 1. min in a flowing Ar atmosphere. The addition of alumina improved the densification of boron carbide. Maximum density of 96% theoretical was obtained with the 3 wt% alumina-doped B4C sintered at 21.0-degrees-C. Abnormal (or exaggerated) grain growth was observed in the specimen containing more than 4 wt% alumina. In the B4C-Al2O3 reaction couples, good wetting of the liquid phase on the boron carbide grains was observed. X-ray diffraction and Auger electron spectra showed that the AlB1.C2 phase was formed by the reaction between boron carbide and alumina. It is suggested that these phenomena promote the densification of boron carbide.
ZrO2-Y2O3 ceramics with varying Bi2O3 contents were prepared and their microstructures and electrical conductivities investigated. The phase stability of cubic fluorite zirconia was disturbed by the introduction of Bi...
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ZrO2-Y2O3 ceramics with varying Bi2O3 contents were prepared and their microstructures and electrical conductivities investigated. The phase stability of cubic fluorite zirconia was disturbed by the introduction of Bi2O3 and tetragonal or monoclinic second phases appeared. The effect of the second phases on the intragrain and the grain boundary conductivities was investigated in the 300-550-degrees-C range using complex plane analysis in the frequency range of 5 Hz to 1. MHz. It showed that conductivity data could readily be interpreted in terms of possible physical models and electrical equivalent circuits. Tetragonal phases had a small positive influence on the intragrain conductivity. The grain 9boundary resistivity could be diminished by discrete monoclinic second phases which offered more conductive intergranular contacts.
Sol-gel processed lanthanum-modified lead zirconate titanate (PLZT) thin films consisting of two different perovskite phase contents were fabricated on indium tin oxide coated Corning 7059 glass substrates with two di...
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Sol-gel processed lanthanum-modified lead zirconate titanate (PLZT) thin films consisting of two different perovskite phase contents were fabricated on indium tin oxide coated Corning 7059 glass substrates with two different heating schedules: direct insertion at 650-degrees-C for 30 min and at 500-degrees-C for 2 h. Optical transmittance spectra, polarization versus electric field curves, relative dielectric constant versus frequency and capacitance versus d.c. bias voltage curves of the samples were investigated. The samples showed a good transparency of over 70% and interference oscillation. A thin film consisting of mainly perovskite phase showed a slim loop hysteresis in the polarization versus electric field curve and in the capacitance versus d.c. bias voltage curve, indicating the presence of ferroelectric domains, but a film consisting of mainly pyrochlore phase did not. The dielectric constant and loss factor of the thin film consisting of mainly perovskite phase were about 90 and about 0.2, respectively, at relatively low frequency and showed dispersion of the dipolar polarization of permanent dipole moment in the ferroelectric perovskite phase in the frequency range between 1. kHz and 1.MHz.
In the present work, the effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites were investigated. The sedimentation treatment was designed to make a difference to t...
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In the present work, the effects of sedimentation treatment on the mechanical properties of SiC whisker-reinforced Si3N4 composites were investigated. The sedimentation treatment was designed to make a difference to the raw materials, especially to remove large agglomerates and impurities. The treatment improved the fracture toughness of the whisker reinforced composites.
The phase diagram and thermodynamic data for the In-Ga-Sb system are fit with a model for the LII-V liquid phases, in which the enthalpy and excess entropy of mixing are quartic functions of the atomic fraction and th...
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The phase diagram and thermodynamic data for the In-Ga-Sb system are fit with a model for the LII-V liquid phases, in which the enthalpy and excess entropy of mixing are quartic functions of the atomic fraction and the enthalpy of mixing is a quadratic function of temperature. The Gibbs energy, enthalpy, and entropy of formation of the LII-V compounds are obtained as functions of temperature from the heat capacities and the standard enthalpy and entropy of formation at 298 K. The fits are quantitatively satisfactory and better than any hitherto obtained, particularly for the m-V liquidus lines and the ternary liquid. The partial pressures of Sb, and Sb-4 are calculated along the three phase curves of In1.uGauSb(s), and relations given to obtain the relative chemical potentials of In and/or Ga are from these. The enthalpy of mixing of the m-v liquids is calculated for a few hundred degrees above the compound melting points, where presently there are no data. Characteristics of an improved model are postulated.
Monodispersed spherical SiC gel powders were synthesized by hydrolysis and condensation of phenyltrimethoxysilane (PTMS) or a mixture of PTMS and tetraethylorthosilicate (TEOS) from a system of silane-H2O-catalyst to ...
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Monodispersed spherical SiC gel powders were synthesized by hydrolysis and condensation of phenyltrimethoxysilane (PTMS) or a mixture of PTMS and tetraethylorthosilicate (TEOS) from a system of silane-H2O-catalyst to which no co-solvent such as alcohol was added. The experiments were conducted through two routes;base catalysed and acid base catalysed routes. In each process, excess water was used as a dispersing medium in addition to a hydrolysing agent. In the base-catalysed route, monodispersed spherical gel powders were obtained regardless of NH4OH content used in this experiment when the concentration of silane and the molar ratio of TEOS to PTMS were less than 0.5 mol l-1.and 0.5, respectively. In the acid base catalysed route, polydispersed powders were produced when the concentration of silane exceeded 0.25 mol l-1. When heated above 1.00-degrees-C, the initially monodispersed powders sustained their shape in the PTMS TEOS system, but bulk and fibre phase were produced in the PTMS system.
Dense TiB2/TiC composites were fabricated by the directional reaction of molten titanium with boron carbide preform. The reaction between pure molten titanium and boron carbide preform could not progress due to reacti...
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Dense TiB2/TiC composites were fabricated by the directional reaction of molten titanium with boron carbide preform. The reaction between pure molten titanium and boron carbide preform could not progress due to reaction choking. However, when a few weight per cent of nickel were added in the titanium, the reaction progressed continuously and resulted in TiB2/TiC composites. A gradient of grain sizes was observed in the reaction products. The processing temperature affected the microstructure of the reaction products rather than the reaction rate. The degree of grain-size gradient in the reaction product increased with the processing temperature.
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