Thermophysical properties of zinc gallate

Complex spinel oxides AB₂O₄ are inorganic materials, which, due to their refractoriness and a whole range of functional properties, are widely in demand in various fields of the power industry and electronics. This paper reports on the results of an experimental study of the thermophysical properties of zinc gallate, which is a promising material for optoelectronics, solid-state lighting and catalytic and biomedical applications. Highly dispersed single-phase ZnGa₂O₄ powder was synthesised by gel combustion. As a result of high-temperature sintering of pressed powder blanks, ZnGa₂O₄ ceramics (≈95% of the theoretical) were produced. According to SEM, the material has a dense microstructure consisting of micron-sized spinel grains. The heat capacity of the material produced was investigated by relaxation, adiabatic and differential scanning calorimetry. By combining experimental data in the range of 8.7–1276 K, a single consistent temperature dependence of ZnGa₂O₄ heat capacity was obtained for the first time. The analysis of the curve obtained showed no signs of phase transitions in the studied temperature range. Based on the results of calorimetric measurements within the range 0 K–1276 K, the temperature dependences of heat capacity, entropy, enthalpy change and Gibbs energy function were calculated. The absolute entropy of ZnGa₂O₄ spinel at 298.15 K was 120.23 J/(K·mol). The thermal diffusivity of ZnGa₂O₄ ceramics measured by laser flash method was 2.6 mm²/s at 300 K, decreasing to 0.46 mm²/s at 1174 K. Its thermal conductivity calculated using experimental data on heat capacity and thermal diffusivity at room temperature was 7.6 W/(m·K). The obtained data are of interest in terms of improving materials based on ZnGa₂O₄, which are promising for use in light-emitting diodes and new-generation power semiconductor devices.     

Thermal expansion and heat capacities of holmium and erbium orthotantalates ceramics

Monoclinic M-orthotantalates of holmium and erbium ceramics were obtained by co-precipitation and annealing at 1773 K and characterized by X-ray powder diffraction, scanning electron microscopy, and chemical analysis. Using the high-temperature X-ray diffraction method, the temperature dependence of the monoclinic lattice parameters of both ceramics were determined at range 298-1273 K. The molar heat capacity of M-HoTaO₄ and M-ErTaO₄ were measured by adiabatic (7-350 K) and differential scanning (320-1350 K) calorimetry. Standard thermodynamic function (entropy, enthalpy change, and reduced Gibbs energy) were calculated on the smoothed values of molar capacity, without taking into account contribution of low-temperature interval: 0-7 K for M-HoTaO₄ and 0-8 K for M-ErTaO₄. The general shape of the anomalous Schottky contribution to the molar heat capacity was determined.     

Crystal Structure and Thermodynamic Properties of Barium–Thulium Bismuthate with Perovskite Structure

A new sample of triple oxides—barium–thulium bismuthate Ba₂TmBiO₆ with perovskite structure—has been synthesized by ceramic technology. The unit cell parameters of the sample and the coefficients of thermal expansion have been determined by the method of X‐ray analysis. Thermal properties of the sample have been studied by DTA over the temperature range of 293–1473 K. The heat capacity of the compound has been measured over the range of 5–320 K by vacuum adiabatic calorimetry. Thermodynamic functions of the compound have been determined based on the heat capacity data.     

Magnetocaloric properties of Gd2MoO6 prepared by a simple and fast method

The experimental study of the specific heat, magnetic susceptibility, and magnetization of Gd₂MoO₆ powder sample was performed in the temperature range between 0.4 K and 300 K in the magnetic fields up to 9 T. Powder sample was prepared via nonconventional mechanochemical/thermal process from powdered oxide precursors. Magnetic ion Gd³⁺ with spin S = 7/2 is responsible for the magnetic properties. The specific heat study in zero magnetic field revealed two anomalies; a phase transition to the ordered state at T_N1 ≈ 0.98 K and a smaller maximum at T_N2 ≈ 0.6 K. Relatively high magnetic density 6.85 g/cm³ of Gd₂MoO₆ predetermines this compound as potential magnetocaloric material with a high cooling performance at cryogenic temperatures. A large conventional magnetocaloric effect was found around 3 K with magnetic entropy change ?ΔS_max ≈ 44 J/(kg K) (300 mJ/(Kcm³)) for magnetic field change from 9 T down to zero with a refrigerant capacity of 464 J/kg. Maximal temperature change -ΔT_ad ≈ 23.8 K was found for initial temperature T_init ≈ 29.9 K for the mentioned change of magnetic field. Alternating susceptibility measurements revealed the presence of two relaxation channels in Gd₂MoO₆ existing in two different time scales ≈ 10^-3 s and ≈ 1 s.     

Molecular motions in poly(dimethyl siloxane) oligomers and polymers

The glass transition temperatures of fifteen samples of poly(dimethyl siloxane) polymers and oligomers have been measured by differential scanning calorimetry. Polymers with M_n>2400 exhibit an asymptotic value of T_g(∞)=148K, but for shorter chains the T_g is lower. The thermomechanical behaviour has also been examined using a torsional braid analyser and this has served to confirm the chain length dependence of T_g. A sub-glass transition, located in the temperature range 80–120K for short chain samples, has been attributed to methyl group rotation.     

Transitions from solid to liquid in isotactic polystyrene studied by thermal analysis and X-ray scattering

A three-phase model, comprising mobile amorphous fraction (MAF), rigid amorphous fraction (RAF) and crystalline fraction (C), has been applied to interpret the thermal transitions and structure of cold-crystallized isotactic polystyrene (iPS) from below the glass transition temperature, T_g, to above the melting point. Quenched amorphous iPS films were isothermally crystallized at different temperatures for 12 h. The fraction of crystalline phase, ?_c, was measured by differential scanning calorimetry (DSC), wide angle X-ray scattering and Fourier Transform infrared spectroscopy. The fraction of the mobile amorphous phase, ?_MAF, was obtained from the heat capacity increment at the glass transition temperature. In the three-phase model, the fraction of the rigid amorphous phase, ?_RAF, was found from 1−?_MAF−?_c. Specific heat capacity measurements by standard DSC confirm that the experimental baseline heat capacity conforms to a three-phase model for temperatures ranging from below T_g, up to the relaxation of RAF. The relaxation of RAF appears as a sigmoidal change in heat capacity accompanied by excess enthalpy, in which solid-like RAF is converted to an identical amount of liquid-like MAF.At temperatures above the relaxation of RAF, either one or two major crystal melting endotherms are observed in standard DSC, dependent upon crystallization temperature. However, using quasi-isothermal temperature modulated DSC, we always observed two reversing melting endotherms. The effects of annealing on iPS structure during the quasi-isothermal measurement were assessed using small angle X-ray scattering (SAXS). Combining the DSC and SAXS results, a model for the melting of iPS lamellae at low heating rates is presented.     

On the theory of the decomposition of a metastable gas hydrate

Methane hydrate decomposition at atmospheric pressure in the overheated state relative to the equilibrium temperature (T _S = 193 K) at positive (T ₀ > 273 K) and negative (T ₀ < 273 K) temperatures is discussed with reference to available experimental data. Two temperature ranges (193 K < T ₀ < 240 K and 240 K ≤ T ₀ < 273 K) arte distinguished at negative temperatures, and one temperature range (T ₀ > 273 K) at positive temperatures. For the lower range of negative temperatures, it is accepted in the construction of the theoretical model that the major factors determining the intensity of gas hydrate decomposition into ice and gas are Arrhenius-type kinetics and conductive heat transfer. Two schemes, namely, frontal and bulk ones are considered. For the upper range of negative temperatures, where an anomalous preservation effect is observed in experiments, it is assumed in the theoretical model that the release of the gas from the hydrate is controlled by the diffusion mechanism of gas transport through the solid phase or through the surface ice crust. For the positive temperatures, it is accepted that the decomposition rate is determined by the heat flux through the draining water film that has resulted from hydrate decomposition. Calculations have been carried out for different initial and boundary temperatures, and the results of the calculations have been analyzed and have been compared to available experimental data.     

Conformational effects on isobaric and constant volume specific heats in polymers

An equation was derived for the isobaric specific heat change ΔC_p at T_g for polymers by use of the configurational partition function normalized per unit volume for the polymer liquid. Eliminating the contributions of the conformational and free volume specific heat changes, the remaining contribution to ΔC_p, relating to the thermal wave-length and the excess specific heat change, was considered thermodynamically. In order to investigate the causes of the abrupt change in C_p at T_g as observed in many polymers, for PE, PET, nylon-6 and PP, the conformational specific heat C^c was calculated in the temperature range from 50 K to melting temperature. The characteristic temperature dependence of C^c with a peak temperature of 100 K for PE, PET and nylon-6, 250 K near T_g (263 K) for i-PP and about 320 K for s-PP was obtained. At T_g of PE, PET and nylon-6, no peculiar change in C^c was found. The abrupt change in C_p at T_g may be attributed almost entirely to the change in the free volume specific heat C^f.     

Non-isothermal elimination process in the solid state of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene] studied with MTDSC and TGA

The elimination process of n-alkyl-sulphinyl precursor polymers towards conjugated poly[2-(3′,7′-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene], or OC₁C₁₀-PPV, was studied with modulated temperature differential scanning calorimetry (MTDSC) and thermogravimetric analysis (TGA), with a focus on the subsequent reactions of the elimination products. The latter reactions were monitored using the non-reversing heat flow and the heat capacity (C_p) measured in non-isothermal MTDSC experiments. The disproportionation reaction occurs in a temperature range between 85 and 135 °C and is seen as an increase in C_p. Water and elimination products released during the elimination reaction act as plasticizers and lower the T_g. TGA experiments show that the temperature, film thickness, and the eliminated group play an important role on the diffusion and evaporation of the elimination products. The elimination products can further decompose and interact with the conjugated system to form undesirable crosslinks (network formation) in a temperature range of 140-160 °C.     

The effect of free stream concentration on heat and binary mass transfer with thermodynamic coupling in forced convection on a flat plate

Simultaneous heat and mass transfer, which arises from injecting a gas into a flowing external stream, has been studied analytically for a flat plate geometry. The studies are centered around the effect of free stream concentration of the injected gas on the thermodynamic coupling, over a wide range of T_w/T_e.Exact and linearized approximate solutions were obtained numerically. It was found that the thermal diffusion effect on mass transfer becomes increasingly important as the free stream concentration increases and T_w/T_e departs from unity. On the other hand the diffusion thermo effect on heat transfer was found to be the most important when the free stream concentration is zero and as T_w/T_e, approaches unity.     

The electrochemistry of silver in KOH at elevated temperatures—IV. AC impedance study

The ac impedance properties of silver in 1 mol kg^?1 KOH were studied under potentiostatic conditions over the temperature range 295–478 K. Measurements were obtained over the frequency range 0.5 Hz–5 kHz, and were analyzed in terms of equivalent in terms of equivalent circuits. It was shown that provision for surface roughness is required for a satisfactory explanation of the impedance data. Values for the double layer capacity and the concentration AgO^? ions at the electrode surface could generally then be obtained. At potentials corresponding to the formation of Ag₂O, it is demonstrated that both diffusion in solution and diffusion in the oxide film are operative. At elevated temperatures, the rate of growth of Ag₂O centres is controlled partly by diffusion in the oxide and party by a reaction step at the growing interfaces of the centers. The subsequent formation of Ag₂O₂, at all temperatures in the range studied, is interpreted on the basis of an electrocrystallisation impedance.     

Investigation of the compatibility of butadiene—acrylonitrile copolymers with poly(vinyl chloride)

Various methods were used to study the compatibility of butadiene-acrylonitrile copolymers with poly(vinyl chloride). These blends were investigated by phase contrast microscopy, differential scanning calorimetry and torsion pendulum analysis. We conclude that the copolymers are compatible with poly(vinyl chloride) in all PVC compositions within the range 23–45% acrylonitrile. These blends exhibit a single T_g in the torsion pendulum studies and differential scanning calorimetry studies and follow a Fox expression in the variation of T_g with composition. Experimental densities are also higher than those calculated assuming volume additivity, implying better packing and a negative heat of mixing leading to molecular compatibility.     

Influence of cooling rate on the heat capacity and thermal transitions of amorphous polyhexene-1

Heat capacity measurements have been made on an amorphous polyhexene-1 sample between 20 and 300K, with an adiabatic calorimeter giving a low dispersion on the C_p curve (0·2–0·4%). The measured sample was prepared by a new method increasing its thermal diffusivity. The heat capacities were measured for different cooling rates (250 K/h and 0·15 K/h) through the glass transition. This large difference of the cooling rates did not change T_g very much (215·5 to 213·5K), but a peak was added to the usual ΔC_p at T_g. An anomalous behaviour was observed at 80K, which is consistent with an experiment described elsewhere. Around 80K dielectric and mechanical relaxations were found in other experiments. In conclusion, the importance of the glass transition is pointed out, its effect being visible down to the lowest temperatures.     

Effect of heat pretreatment and strain rate on tensile properties of polycarbonate sheet

The effects of heat pretreatment and ambient gas (air and vacuum) on selected properties of the polycarbonate sheet have been studied. Changes in tensile properties as functions of heat pretreatment temperature (up to 160°C) and strain rate (wide range of 1.7 × 10^?4 ? 13.1 m/sec = 0.29 ? 2.3 × 10⁴ %/sec) were determined and these are discussed in relation to changes in differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) data. The performance characteristics of the present tensile testing are obtained over a wide range of extension rates without changing the mode of deformation and the shape of the test pieces. It was suggested from the experimental results that heat pretreatment below the glass transition temperature (T_g) causes ordered molecular domains to grow on the free surfaces of the sheet, consisting of thermally deteriorated macromolecules and possessing lower crazing stresses (exhibiting more brittle mechanical responses, leading to the decrease in breaking strain and energy). The effect of annealing above T_g on the tensile properties, and on the results of DSC and GPC, could not be precisely understood.     

The effect of molecular weight and casting solvent on the miscibility of polystyrene-poly(α-methyl styrene) blends

The glass transition temperatures (T_g) have been measured for blends of polystyrene and poly(α-methyl styrene) in the molecular weight ranges: polystyrene, 2030 < M < 250 000, and poly(α-methyl styrene), 17 000 < M < 250 000. The presence of either one T_g or two has been used as a criterion to determine the miscibility of each blend over a range of compositions, and the T_gs were obtained from measurements of the temperature dependence of the heat capacities of the blends. A sinlge T_g was observed over the entire composition range for blends in which the component molecular weights were both less than 70 000 g mol^?1, when these were cast from toluene solutions. When propylene oxide solutions were used to prepare the blends, this limit dropped to M = 50 000 g mol^?1. By using the appearance of two T_gs as an indication that phase separation had taken place, it was possible to establish regions of miscibility and immiscibility as a function of casting solvent and molecular weight for both components. The change in heat capacity at the glass transition was measured and it was found that for miscible blends the heat capacity changes are similar to the calculated values, but for immiscible systems the measured change is smaller than expected.     

Effect of Ba concentration on phase stability and mechanical and thermal properties of La2Mo2O9

Ba-substituted La₂Mo₂O₉ ((La_1−xBa_x)₂Mo₂O_9−δ, x = 0–0.12) was prepared and the thermal and mechanical properties were evaluated. The thermal expansion coefficients (TECs) were determined from high-temperature X-ray diffraction (XRD) analysis. Phase transition in La₂Mo₂O₉ was suppressed via substitution of Ba for La, as demonstrated by differential scanning calorimetry (DSC) analysis. The mechanical properties, such as the bulk modulus, shear modulus, Young’s modulus, compressibility, and Debye temperature were evaluated from the measured sound velocities. The thermal conductivity was evaluated from the thermal diffusivity, heat capacity, and density in the temperature range from room temperature to 1073 K. The thermal conductivity decreased with increasing Ba content. Theoretical calculations based on the Klemens–Callaway model were performed to analyze the thermal conductivity, and the results suggest that the reduction of the thermal conductivity was mainly attributed to oxygen defects in the anion sublattice of La₂Mo₂O₉.