Phase stability and oxygen transport characteristics of yttria- and niobia-stabilized bismuth oxide

As a part of an overall study to explore the potential application of stabilized Bi₂O₃ as oxygen separator in various electrochemical systems, an investigation of the stability and transport characteristics of yttria- and niobia-stabilized bismuth oxide was undertaken. Polycrystalline Bi₂O₃ samples containing 25mol % Y₂O₃ were fabricated by pressureless sintering powder compacts at 1000 C in air. Samples containing 15mol % Nb₂O₅ were also fabricated by pressureless sintering at 900 C in air. The resulting samples were dense and of an equiaxed microstructure with grain size in the range from 28m for the yttria-stabilized and 42m for the niobia-stabilized materials, respectively. X-ray diffraction of the as-sintered specimens showed them to be single phase with CaF₂-type structure. Ionic conductivity was measured by an a.c. technique over a wide range of temperatures. It was observed that the ionic conductivity of the yttria-stabilized bismuth oxide was greater than that of the niobia-stabilized one.The specimens subsequently were annealed over a range of temperatures between 600 C and 700 C for up to several days. X-ray diffraction traces taken on the Y₂O₃-stabilized samples indicated that the original cubic solid solution had decomposed. The decomposition of the yttria-stabilized samples was also accompanied by the occurrence of exaggerated grain growth. The observed decomposition is not in agreement with the phase diagram available in the literature, according to which the cubic phase should be stable over the range of temperatures the samples were annealed in the present study. By contrast, Nb₂O₅-stabilized Bi₂O₃ remained cubic, although it appeared to have dissociated into two cubic solid solutions of slightly differing lattice parameters. There was no perceptible change in the grain size of the niobia-stabilized samples.Several electrolyte tubes made of the yttria- and niobia-stabilized bismuth oxide were electrolytically tested under a d.c. mode with silver electrodes. In tubes made of the yttriastabilized material, the current density decreased with time (under a constant applied voltage) at 650 C and at 700C but did not at 700C consistent with the observation that the material did not decompose at 700 C but did at 650 C. At 600 C, the rate of decrease was slower than at 650 C indicating that the kinetics of phase decomposition is probably slower at 600 C. In the niobia-stabilized tubes the decrease in the current density was lower. This decrease is probably related to the apparent formation of two cubic solid solutions of slightly differing compositions.The present work shows that the published phase diagram of the Y₂O₃-Bi₂O₃ system is incorrect. The present results also suggest that for application to temperatures as low as 650 C (and possibly lower), electrolytes made with Nb₂O₅ as the stabilizer are preferable.     

An etch rate study on thermally annealed SiO2 films deposited in a TEOS-LPCVD system

The etch rate behaviour of tetraethylorthosilicate (TEOS)-SiO₂ films was investigated as a function of annealing parameters (time, temperature and ambient pressure). The etch rate of TEOS-SiO₂ films depends strongly on annealing pressure within the temperature range 750 to 900 C, while the etch-rate behaviour of films thermally annealed at 1000 C is mainly controlled by the thermally activated rearrangements of SiO₄ tetrahedra from as-deposited films in a closed structure to that of thermally grown SiO₂ films. The etch-rate behaviour of thermally annealed TEOS-SiO₂ films is interpreted in terms of the chemical change of the film structure.     

Strength characterization of yttria-doped sintered silicon nitride

The flexural strength of yttria-doped sintered silicon nitride was evaluated as a function of temperature (20 to 1300 C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 900 C, in which the mode of crack propagation was primarily transgranular. Above 1000 C, the strength (fracture stress) decreased considerably due to the presence of subcritical or slow crack growth which occurred intergranularly. This material did not show a static oxidation problem in short-term (100 h) tests in the low-temperature regime (600 to 1000 C) as has been observed in other yttria-doped silicon nitrides. Flexural-stress rupture testing in the temperature range 800 to 1200 C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature.     

Phase transformations in the thermotropic liquid crystal polymer: 60/40 PABA/PET

Microstructures observed in 60/40 PABA/PET co-polyester in transmitted polarized light are reported. The microstructure changes as a function of temperature. Between 190 and 340 C the optical textures are similar to those seen in small molecule liquid crystals in the smectic C modification; above 340 C the textures are typical of nematic structures. At 420 C the specimen is totally isotropic and begins to degrade. Rapid cooling to below 190 C can quench in the high temperature phases, including the isotropic one. DSC traces show endotherms identifiable with the onset of mobility at 190 C, the transition from smectic C to nematic-like textures at 340 C and the development of the isotropic phase in the range 350 to 420 C. The smectic C to nematic transition in texture is associated with the appearance of a transient microstructure, known as a myelin texture, and reported here for the first time in a liquid crystal polymer.     

Observations on the mechanical properties of nickel oxide scales

The mechanical properties of NiO scales produced by the complete oxidation of high-purity (grade-1) Ni and commercial-purity (grade-A) Ni have been investigated at 700 to 1000 C. The modulus of elasticity of both grades of oxide decreased with increasing temperature, whereas the modulus of rupture for grade-A oxide exhibited a maximum at 850 C and that for grade-1 oxide decreased with increasing temperature. At 700 C, elastic deformation to fracture occurred with both oxides, whereas, at temperatures 850 C, plasticity was also observed. The plasticity of grade-1 oxide was 3 times greater than that of grade-A oxide.Creep behaviour of the oxides was studied at 900 and 1000 C. Primary and secondary creep was observed and, in both oxides, the creep rates increased with increasing temperature and load. The creep rate of grade-1 oxide was 10 to 20 times greater than that for grade-A oxide.     

Investigation of lithium,magnesium-aluminosilicate solid solutions crystallized in some cordierite-zircon ceramics containing lithium

The crystallization of Li,Mg-aluminosilicate solid solutions in lithium-bearing zircon and/or cordierite-based ceramics is studied as a function of firing temperature up to 1500 C. X-ray diffraction, electron microprobe analysis and microscopic techniques were used for the identification of the studied phases. Keatite solid solution (ss) coexists with zircon and/or cordierite at temperatures up to 1300 C. On raising the temperature to 1400–1500 C,-quartz ss is crystallized at the expense of both keatite and cordierite with the precipitation of spinel on cooling from such high temperatures. In bodies containing zircon, the dissociation of zircon at 1400–1500 C into its constituents epitaxially enhances the nucleation of-quartz ss crystallization.     

Electrical and optical properties of vacuum-evaporated CdS films

1 m CdS films for the window layer of CdS/CulnSe₂ solar cells have been prepared by vacuum evaporation at various deposition conditions. Deposition rates were 0.73 and 3.3 nms^–1, and substrate temperature ranged from 50 to 225 C. The effect of the deposition conditions on the properties of CdS films was investigated by measuring electrical resistivity, optical transmittance and reflectance.The resistivity of the evaporated CdS films strongly decreased as substrate temperature decreased and the films with high deposition rate showed lower resistivity compared to the films with low deposition rate. Interestingly, the combination of high deposition rate and very low substrate temperature resulted in an increase of resistivity. The optical transmittance of CdS films increased as substrate temperature decreased and then decreased with further decrease in substrate temperature. The transmittance strongly depended on deposition rate at low substrate temperature (<100C), while it was independent of deposition rate at high substrate temperature (>100C). In particular, high transmittance can be extended to lower substrate temperature by reducing deposition rate. Low optical reflectance can be obtained by lowering substrate temperature. The results indicate that CdS films of low resistivity and high transmittance can be produced by vacuum evaporation at low substrate temperature and low deposition rate.     

Time-dependent structural modifications in tin oxide thin films under environmental conditions

Time-dependent crystalline structure and surface morphology of transparent conducting thin films of undoped tin oxide have been studied under environmental conditions by X-ray diffractometry and scanning electron microscopy. Tin oxide thin films were produced, via chemical vapour deposition, in three batches; two batches with single deposition at substrate temperatures of 400 C and 560 C, respectively, and the third batch produced by double deposition firstly at 560 C and then at 400 C substrate temperature. It is found that the crystallinity of the as-grown tin oxide thin films produced by single deposition degrades with shelf-life period and an amorphous phase is developed under environmental conditions. In the tin oxide films produced by double deposition there was no degradation in the crystallinity of the films, but a change in the preferential crystallite orientation was observed. The implications are discussed.     

The kinetics of hot-pressing for undoped and donor-doped BaTiO3 ceramics

The kinetics of hot-pressing for undoped and donor-doped BaTiO₃ of high purity have been studied at 1050 C and at 2.3 to 27.6 MPa in an oxidizing atmosphere. The stress exponent and grain size exponent of the densification rate are indicative of control of densification by a grain boundary diffusion mechanism. The activation enthalpy (400 kJ mol^–1) for the densification process has been measured over the range of 1050 to 1 200 C. No significant influence of the donor is observed on the densification of BaTiO₃ until the late stages of sintering.     

Thermal transformation of hydrated ferric oxide gel and preparation of ultrafine oxide

Hydrated ferric oxide gel has been prepared by addition of a solution of ferric ammonium sulphate to that of sodium hydroxide kept at room temperature. Thermal analysis of the gel shows an endothermic peak at 130 C and three exothermic peaks at 210, 315 and 430 C. The sequence of transformation has also been investigated by thermogravimetry, X-ray diffraction, infra-red spectroscopy and surface-area measurement. The gel is found to consist of -FeOOH·nH₂O as the primary particle, which crystallizes at around 210 C and subsequently transforms to -Fe₂O₃ at around 315 C. The initial crystallite size of the oxide formed, is about 700 nm. However, it increases significantly ( 3000 nm) accompanied by an exotherm when the oxide is heated beyond 400 C.     

Plastic deformation of polycrystalline zirconium carbide

The compressive yield strength of arc melted, polycrystalline zirconium carbide has been found to vary from 77 kg mm² at 1200C to 19 kg mm² at 1800C. Yield drops were observed with plastic strain-rates greater than 3×10^–3sec^–1 but not with slower strainrates. Strain-rate change experiments yielded values for the strain-rate sensitivity parameterm which range from 6.5 to 1500C to 3.8 at 1800C, and the productm ^*(T) was found to decrease linearly with increasing temperature. The deformation rate results are consistent with the Kelly-Rowcliffe model in which the diffusion of carbon assists the motion of dislocations.     

Radial distribution functions of non-crystalline polymers and their application to the structural analysis of PMMA

The use of Radial Distribution Functions (RDFs) in the determination of the structure of non-crystalline polymers is briefly reviewed. Particular aspects of the procedure for preparing RDFs from X-ray scattering are discussed in detail; namely the employment of an energy dispersive detector to remove the Compton component of the scattered X-rays and the application of the method of sampled transforms. A RDF is presented for atactic polymethylmethacrylate (PMMA) and its precision and reliability are discussed. It is analysed by comparison with RDFs calculated from computer-generated atom co-ordinates for isolated lengths of PMMA chains in different conformations. Methods are introduced by which the calculated RDFs are smeared to account for random disorder in the real chain and normalized so that, despite the finite range of the model, they can be immediately compared with the difference RDF which is directly obtained by transforming the data. Comparison between experimental and calculated RDFs shows that reasonable agreement is only obtained for a very limited range of conformations corresponding to sequences of backbone bond rotation angles of (10, 10, –10, –10) and the bond angles alternately 110 and 128. The form of the RDF appears very sensitive to important aspects of the molecular structure. The results both confirm and refine an earlier proposal from this laboratory which was based on comparisons between experimental and calculated functions in reciprocal space.     

Methanation reactivity of carbon deposited directly from CO2 on to the oxygen-deficient magnetite

The methanation reactivity of surface carbon deposited from CO₂ on the oxygen-deficient magnetite was studied by the isothermal methanation reaction and the temperature programmed surface reaction (TPSR). In the methanation reaction with H₂ gas, in a closed system at 150–200C, active carbon, which was not observed by the TPSR measurements, was found. About 20% out of the deposited carbon was in the form of atomic carbon and readily converted into CH₄ (5 min) above 150C with H₂, and about 80% at 300C. At 350C these atomic carbons were transformed into polymerized carbons, which was less reactive for methanation.     

Preparation of lithium aluminosilicate glass-ceramic monolith from metal alkoxide solution

Glass-ceramic monoliths with a composition of Li₂O·Al₂O₃·4SiO₂ have been synthesized by the sol-gel technique using metal alkoxides as starting materials. Heating dried gel monoliths of about 36 mm diameter and height to appropriate temperatures gave glass-ceramic monoliths of about 20 mm diameter and height without the occurrence of cracks, while a melt-derived glass body of the same composition was cracked or softened on heating. The glass-ceramic monoliths obtained by heating gel monoliths at 1000 C for 40 h were a porous body, whose bulk density and porosity were 1.62 g cm^–3 and 34%, respectively. On heating gel monoliths, -eucryptite crystals were first precipitated around 750 C, followed by precipitation of -spodumene crystals at 830 C. At higher temperatures the latter grew at the expense of the former phase. The crystallized specimens exhibited very low thermal expansion coefficient ranging from –13-12×10^–7 C^–1 at temperatures from room temperature to 700 C depending on the heat-treatment temperature of gels, indicating that a porous glass-ceramic monolith with ultra-low thermal expansion can be prepared using the present sol-gel method.     

Temperature-dependent change of Cu-O bond in La2CuO4 and YBa2Cu3O7

Valence-band spectra of La₂CuO₄ and YBa₂Cu₃O₇ were obtained by using X-ray photoemission varying the temperature of the measurement (35, 260, 300, and 500 C). In La₂CuO₄ the broad band centred around 4eV splits into two peaks at 260 C. At both 35 and 500 C the spectra show almost similar shape. In YBa₂Cu₃O₇ the broad peak centred around 4eV splits into two peaks at 500 C. Below 300 C the spectra show almost similar shape. These splittings of the valence-band spectra may be due to the rearrangement of the crystal structure accompanying the phase transition from the orthorhombic to tetragonal symmetry.     

Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

The effective surface energy of brittle materials

The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture, _I . The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process, _F . The two estimates of _I give consistent values, and there is no systematic variation of _I with notch depth. Values of _F decrease with increasing notch depth as the fracture process becomes more controlled. For alumina _{I F} . For PMMA and glass _I > _F because of a multiplicity of crack sources during fracture initiation. For graphite _I < _F because of subsidiary cracking as fracture proceeds.     

Sintering mechanisms and microstructural development of coprecipitated mullite

Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al₂O₃+22 wt% SiO₂ (high-Al₂O₃ material) and 72 wt% Al₂O₃+28 wt% SiO₂ (low-Al₂O₃ material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al₂O₃ materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al₂O₃ compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al₂O₃/SiO₂ ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al₂O₃ were produced from high-Al₂O₃ powders calcined between 600 and 1100 C. Low-Al₂O₃ sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al₂O₃ materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al₂O₃ compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries.     

Wetting of nickel by silver

Sessile drop experiments of molten silver and nickel were performed in air and helium at 970 C. A NiO layer formed at the interface in air; silver formed a 90 contact angle. In helium silver formed a 9 contact angle on nickel. The role of solution reactions in forming these angles is discussed.     

Evaporated cadmium telluride films on steel foil substrates

Adherent CdTe films were vacuum-evaporated on steel foil substrates heated to between 240 C and 320 C. As-deposited films were p-type and had a preferred (111) orientation with an average grain size of 0.6 m. To control the electrical properties of the films, gold or tellurium interlayers were deposited between the steel and the CdTe. A tellurium interlayer caused larger as-deposited grains than either the gold or bare steel. Annealing of CdTe films at 550 C increased the grain size and tended to transform p-type CdTe into n-type. X-ray fluorescence analysis indicated that annealing had caused iron to diffuse into the CdTe, which may have caused the type reversion.