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.     

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.     

Study of Na2O-SiO2-Fe2O2-Y2O3 glass by Mössbauer and EPR spectroscopy

Mössbauer, EPR and magnetization experiments have been carried out on the glass composition Na₂O-SiO₂-Fe₂O₂-Y₂O₃ in which yttrium iron garnet (YIG) can be precipitated by suitable heat-treatment. The measurements have been carried out on the as-quenched sample as well as samples heat-treated for 4 h at 400, 500, 600, 650, 700, 750, 810 and 850 C. Mössbauer spectra from the as-quenched sample as well as the first six samples showed a quadrupole splitting while the last two samples, as well as the sample heat-treated (i) by a two-stage process at 600 C and then at 750 C for 4 h each, and (ii) at 700 C for 40 h, showed a hyperfine as well as quadrupole splitting. The behaviour of the isomer shift (IS) and quadrupole splitting (E) with the heat-treatment temperature show significant changes at the glass transition and crystallization temperatures. The Mössbauer data have been found consistent with optical and electron micrographs which show a large variation in particle size of the precipitated magnetic phase. This has been further correlated with EPR and magnetization data.     

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.     

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.     

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.     

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.     

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.     

90‡ domain reversal in Pb(Zr x Ti1−x )O3 ceramics

A simple and direct method has been proposed, which may be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. Using this method, the 90 domain reorientation in the Pb(Zr _x Ti_1–x )O₃ ceramic under an electric field was examined through the X-ray diffraction analysis. It was found that polarization switching in the PZT ceramic with a composition near the morphotropic phase boundary, is predominantly controlled by the two successive 90 domain processes rather than only the 180 domain reversal process. Experimental results also indicate that the coercive field of ferroelectric ceramics is related to the cooperative deformation associated with each grain. This cooperative deformation arises from the 90 domain-reversal process.     

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.     

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.     

Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Microstructural,electrical and magnetic properties of NdBa2Cu3O7−δ prepared from BaO2

NdBa₂Cu₃O_7– was synthesized in oxygen from a stoichiometric mixture of Nd₂O₃, BaO₂ and CuO between 890 and 1070 C. The resulting material was composed of small crystals of NdBa₂Cu₃O_7– (2 to 10 m) and of BaCuO₂ as a secondary phase. The effect of the synthesis temperature on the normal-state resistivity, the critical current density and the magnetic susceptibility was investigated. The highestJ _c values (175 A cm^–2) were measured in samples treated between 1010 and 1050 C.     

Chebyshev approximation for the roots of the equation BiWГ(μ)=μVГ(μ)

A minimax approximation, uniform for Bi [0, ), is developed for the roots of the equation BiW()=V(), by means of Chebyshev polynomials.Translated from Inzhenero-Fizicheskii Zhurnal, Vol. 28, No. 4, pp. 710–715, April, 1975.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Surface and grain boundary analysis of doped zirconia ceramics studied by AES and XPS

The surface- and grain boundary composition of Y, Ce and Ti doped zirconia were studied by X-ray Photoelectron Spectroscopy and Auger Electron Spectroscopy/Scanning Auger Microscopy. The grain boundaries and free surfaces showed the same enrichment levels. After heat treatment 1000 C all yttria doped samples showed yttrium enrichment. In the ZrO₂-Y₂O₃ system the yttrium enrichment did not depend on the bulk concentration and amounted 30–34 mol% YO_1.5 in all cases. As a consequence the segregation factor increases with decreasing solute concentration in the bulk. The thickness of the segregation layer was about 2–4 nm. In the ternary Y doped systems yttrium is the main segregant. In ceria-doped tetragonal zirconia polycrystals (Ce-TZP) systems significant segregation of cerium starts atT1300C and is mainly attributed to Ce³⁺. In Y,Ti-TZP systems also strong segregation of Ti⁴⁺ occurs. The absolute value of the increase of the surface concentration in fine grained material is smaller than in coarse grained material. This is mainly due to depletion of the bulk.     

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.     

Barium titanate fabricated from fur-fibres

Long-fibrous barium titanate (BaTiO₃) particles were prepared by a hydrothermal reaction of potassium titanate hydrate (2K₂O·11TiO₂·3H₂O) and barium hydroxide (Ba(OH)₂). Effects of preparation conditions on crystal structure and powder morphology were examined. Fur-fibres of K₂O·4TiO₂, 1–10 mm long and 1–100 m in diameter, were obtained by heating a mixture of K₂CO₃ and TiO₂ powders at 1000 C for 100 h. Keeping the fur-fibres in ion-exchanged water for 4 days gave fur-fibres of 2K₂O·11TiO₂·3H₂O). Long-fibrous BaTiO₃, with fibres 100m–1mm long and 1–10 m in diameter, was obtained by a hydrothermal reaction of the hydrate and Ba(OH)₂ (Ba/Ti ratio of 1) at 150 C for 24 h. As-prepared long-fibrous BaTiO₃ was composed of fine crystallites (average size about 270 nm) of cubic phase. The cubic phase and morphology of fur-fibres were maintained up to 1250 C, but heat treatment at 1300 C brought about a growth of crystallites to a few micrometers and a phase transformation to tetragonal phase. It was found that the hydrothermal reaction was effective in producing crystalline BaTiO₃ powder at a low temperature of 150 C.     

Crystallization of amorphous Fe78B13Si9

Crystallization of amorphous Fe₇₈B₁₃Si₉ has been investigated using a combination of differential scanning calorimetry (DSC) and conventional and high-resolution transmission electron microscopy. The crystallization mechanisms and crystalline products are sensitive to the annealing temperature. At 450C, crystallization takes place by the growth of b c c -Fe (Si) dendrites, while at 510 and 515C there are three simultaneous reactions to form dendritic b c c -Fe (Si), elliptical crystals of b c t Fe₃B and lamellar eutectic spherulites of b c c -Fe (Si) and b c t Fe₃B. Quantitative TEM shows that the b c c -Fe (Si) dendrites and b c c -Fe (Si)-b c t Fe₃B spherulites both form with constant nucleation and growth rates, in agreement with previous. DSC measurements of an Avrami exponent of 4.