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.     

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.     

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.     

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.     

Antimicrobial and antifungal agents derived from clay minerals

Montmorillonite supported by Ag⁺-chelate of hypoxanthine was synthesized and heat-treated at 200 C intervals below 800 C under nitrogen. These samples were subjected to examinations of the structure and properties including antimicrobial activity. Hypoxanthine itself did not possess antimicrobial activity to bothStaphylococcus aureus andEscherichia coli, but montmorillonites supported by the Ag⁺-chelate exhibited activity. Hypoxanthine in montmorillonite decomposed at 400–600 C; and Ag metal particles deposited after heating to 600 C and grew with raising heating temperature. The Ag⁺-chelate montmorillonite showed clear antimicrobial activity even after heating to 800 C.     

Preparation of rod-shaped BaTiO3 powder

Rod-shaped BaTiO₃ powder particles have been prepared from rod-shaped TiO₂ ·nH₂O and BaCO₃ in molten chloride. The morphology of BaTiO₃ particles was studied referring to the effects of the chemical species of the starting titanium compound, amount of chloride, particle size of the titanium compound and reaction conditions, and the preparation condition of rod-shaped BaTiO₃ has been determined: i.e., large TiO₂ ·nH₂O particles were heated at 700°C in molten salt with an equal amount of BaTiO₃. This condition was effective in suppressing the formation of BaTiO₃ by a solution-precipitation process as well as the deformation of either TiO₂ ·nH₂O or BaTiO₃, which are responsible for the formation of equiaxed BaTiO₃ particles. The obtained rod-shaped BaTiO₃ particles had a cubic symmetry. Electron diffraction analysis showed that the following topotactic relation is retained; 0 1 0_{potassium tetratitanate} 0 1 0_{hydrated titania} 1 0 0_anatase 1 0 0_{barium titanate}     

Microstructures and microwave dielectric characteristics of ceramics with the composition BaO·Nd2O3·5TiO2

The microstructures and the microwave dielectric characteristics of BaO·Nd₂O₃·5TiO₂ ceramics were determined as a function of the sintering conditions. The ceramics were confirmed to have a ternary orthorhombic major phase with minor amounts of Nd₂Ti₂O₇ and TiO₂, and nearly equiaxed fine structures and homogeneous columnar structures were obtained at sintering temperatures of 1340 C and 1370 C, respectively, while discontinuous and excess grain growth were observed at sintering temperatures 1350–1360 C and 1380 C, due to the formation of a liquid phase. The desired value of Qf was achieved in ceramics with fine-grained and/or homogeneous microstructures, and the discontinuous and excess grain growth was found to be seriously harmful to the dielectric properties.     

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.     

Microstructures and subcritical crack growth in oxidized hot-pressed Si3N4

The microstructure of the oxide scales, primarily the size, distribution, and density of the pits, was characterized in hot-pressed Si₃N₄ oxidized at different temperatures from 1300 to 1450 C. These microstructural features and the chemical changes in Si₃N₄ due to oxidation were related to the elevated-temperature subcritical crack-growth (SCG) behaviour. Oxidation at 1375 C for 240h resulted in a measurable improvement in SCG over that in as-hot-pressed and 1300 C-oxidized Si₃N₄.     

Influence of core-shell structured grain on dielectric properties of cerium-modified barium titanate

The dielectric properties and chemical inhomogenity of BaTiO₃ ceramics sintered with additions of CeO₂:1.5TiO₂ were investigated using TEM XRD and EDS. Grains had three regions: grain shell severely doped with cerium, grain core BaTiO₃ of greater purity, and a concentration gradient region where Curie temperatures were distributed. With decreasing temperatures, the volume fraction of ferroelectric domains in the concentration gradient region increased. Addition of CeO₂:1.5TiO₂ to BaTiO₃ decreased the tetragonality (c/a ratio) and the grain size, and lowered the Curie temperature by approximately 30 C per mol%. Even a small addition of 0.005 mol CeO₂:1.5TiO₂ to BaTiO₃ reduced the average grain size from 25 m to less than 1.25 m and grains of an average grain size were completely doped with cerium, which explained why the 130 C ferroelectric transition peak did not exist. The inhomogenity of the cerium concentration had a large influence on the dielectric temperature characteristic.     

Direct current electrical conductivity of gamma ferric oxide

The study of the direct current electrical conductivity, , of freshly prepared -Fe₂O₃ and that of a sample stored for seven days in static air suggests that -Fe₂O₃ adsorbs oxygen and water from the atmosphere. From infra-red spectra it is deduced that the absorbed water in -Fe₂O₃ is present as the physically adsorbed water and as lattice water. The adsorbed oxygen and physically adsorbed water are removed by heating to 100 C, while the lattice water remains in -Fe₂O₃ even up to 280 C. The removal of lattice water is associated with a decomposition during which some of the hydrogen formed occupies the vacancy sites. This suggested formation of the hydrogen ferrite phase is based on the kink in the log against T ^–1 curve observed at 177 C. This kink is very well resolved for a sample equilibrated at 100 C in normal atmosphere, and the measurements of above 100 C of this sample are done in an N₂ atmosphere. The suggestion that the hydrogen ferrite phase is formed has been substantiated by comparison of the X-ray diffraction patterns of -Fe₂O₃ heated under the different atmospheres. From the log against T ^–1 plot for a sample heated under a nitrogen atmosphere the activation energy is small (< 0.05 eV) up to 215 C, and it is comparatively large (0.95 eV) above 215 C. These results suggest a hopping mechanism for the direct current electrical conductivity of -Fe₂O₃. This suggestion has been substantiated by data of the temperature variation of Seebeck voltage.     

Preparation of ceramic coatings from pre-ceramic precursors

SiC-coatings derived from pyrolysis of polycarbosilane layers on stainless steel and mild steel substrates have been studied using a combination of scanning electron microscopy (SEM), energy dispersive X-ray analysis, X-ray photoelectron spectroscopy and ultramicrohardness techniques. The coatings on mild steel plates at firing temperatures of 700C are cracked but uncracked SiC coatings can be formed on stainless steel substrates at 700–800C. X-ray photoelectron studies show that these coatings are covered with a layer of SiO₂ and contain graphitic carbon, while Rutherford backscattering studies indicate inhomogeneities in the coating layer due to mixing of substrate and coating components at the interface between the two. Ultramicrohardness results indicate the SiC/stainless steel coatings formed at 800C are softer than the equivalent SiC coatings formed on alumina substrates at 1100C. Above 800C, a possible combination of both thermal expansion mismatch and CrN formation, which causes the growth of chromium-rich nodules in the stainless steel, serve to disrupt and ultimately destroy the coherence of the SiC coatings. The use of sol-gel-derived SiO₂ coatings as a barrier does not prevent the destruction of the SiC coating by this mechanism.     

Oxidation behaviour of Β-sialon fabricated from α-Si3N4 and aluminium iso-propoxide

-sialon with z=0.5 was fabricated by hot pressing of a spray-dried mixture of -Si₃N₄ and aluminium iso-propoxide solution. The oxidation behaviour of this -sialon was investigated, comparing it with commercial -sialon containing Y₂O₃ as a sintering aid. Oxidation tests were carried out at 1200 and 1400C for 25 to 200 h in air. The oxide layer of aluminium isopropoxide-derived -sialon was thin, dense, smooth and homogeneous without bubbles and cracks. The strength after oxidation at 1400C for 200 h was about 800 MN m^–2, almost the same value as before oxidation. The oxide layer of Y₂O₃-doped -sialon was thick and inhomogeneous, containing many bubbles, cracks and grown needle-like crystallites (Y₂Si₂O₇). The strength after oxidation at 1200C for 200 h fell to 1/2(440 MN m^–2) because of pit formation in the oxide layer, and at 1400C for 200 h fell to 1/4(200 MN m^–2) because of severe swelling and flaking of the oxide layer. The high oxidation resistance of aluminium iso-propoxide derived -sialon was mainly due to its homogeneous microstructure and freedom from foreign constituents such as Y₂O₃.     

Effect of nickel on microwave dielectric properties of Ba(Mg1/3Ta2/3)O3

The dielectric and physical properties of the complex perovskite Ba(Mg_1/3Ta_2/3)O₃ system in which magnesium was substituted for nickel from 0.03–0.67 mol%, were investigated in the temperature range 20–110C, and the frequency range 10.5–14.5 GHz. As the nickel content was increased, the dielectric constant, the degree of ordering, and the unloaded Q decreased. The temperature dependence of the dielectric constant and the temperature coefficient of resonant frequency of the specimens annealed at 1500C for 20 h were found to be greater than those of the specimens sintered at 1650C for 2 h. These results are due to the increase in the density, the increase in grain size, and the lattice distortion.     

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.     

The crystallization of diabase glass

The crystallization behaviour of diabase glass at elevated temperatures was studied in samples prepared by melting the diabase rock. DTA and X-ray analyses revealed the crystallization of diopside (CaO · MgO · 2SiO₂) at 865 C and anorthite (CaO · Al₂O₃ · 2SiO₂) at 1060 C. Further, the kinetics of crystallization of diopside were studied. The phenomenological Johnson-Mehl-Avrami equation was used and the exponentn=3/2 determined from the dependency of the volume fraction of the crystal phase (diopside) on time. The activation energy of crystallization of diabase glass 248 kJ mol^–1 was estimated on the basis of DTA measurements carried out at different heating rates and found to be in good agreement with literature data for similar glass.     

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.     

Deformation in spinel

Stoichiometric MgAl₂O₄ spinel was deformed in compression at temperatures from 1790 to 1895 C and the dislocation structures analysed by transmission electron microscopy. {1 1 1}1 1 0 slip was observed on both the primary and cross-slip systems, and there was much secondary slip as well; all six 1 1 0 Burgers vectors were present in electron micrographs. This secondary slip leads to very high work-hardening rates, approximately/70 at 1790 C, where is the shear modulus. Since it is known that deformation in nonstoichiometric (alumina-rich) spinel crystals occurs by {1 1 0}1 1 0 slip, the electrostatic and geometric aspects of 1/4 110 dislocations moving on {1 1 1} and {1 1 0} planes are considered in some detail. It is porposed that the octahedral cation vacancies present in non-stoichiometric spinel diffuse to dislocations during deformation and thus favour {1 1 0} slip.     

Synthesis and characterization of a high-temperature oxide lubricant

PbMoO₄ is a potential solid lubricant for use at elevated temperatures in oxidizing environments. Pulsed laser deposition (PLD) was utilized to grow thin films of this material because it allows good control over film chemistry and crystallinity. Films were grown at different substrate temperatures in vacuum and in partial pressures of oxygen. The chemistry and crystallinity of the films were evaluated using X-ray photoelectron spectroscopy, Raman spectroscopy, and glancing angle X-ray diffraction. Friction coefficients and wear lives were measured using a ball-on-flat tribometer at room temperature and 700C. Films deposited in vacuum, at room temperature and at 300C, were oxygen deficient. To adjust chemistry and crystallinity, films were grown in a partial pressure of oxygen (i.e. 6.7×10^–1 Pa). Stoichiometric, crystalline films of PbMoO₄ were produced when films were grown at 300C in this environment. PbMoO₄ films were lubricious ( = 0.35) and long lived at 700C, but at room temperature had high friction and failed quickly. The properties of the films grown at the different conditions are discussed.     

Temperature dependence of the piezoelectric,mechanical, and dielectric characteristics of rubber-elastic piezoelectric composites

New rubber-elastic piezoelectric composite materials are investigated by a resonance technique in the temperature range from –60C to +80C. A slight change in the bulk piezoelectric parameters is noted in the interval 5–70C g _v = 60–75 mVm/N). A theoretical physical model is proposed for calculating the piezoelectric characteristics. A piezoelectric cable and large-area piezoelectric sheets have been developed using the new materials.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 54–57, June, 1995.