Ferromagnetic-to-Paramagnetic Phase Transition of MnAs Studied by Calorimetry and Magnetic Measurements

Manganese monoarsenide samples have been prepared by the sealed-ampule technique and characterized by X-ray diffraction, differential thermal analysis, and scanning electron microscopy. The hexagonal- to-orthorhombic phase transition of MnAs has been studied using differential scanning calorimetry (DSC) and magnetic measurements. The enthalpy and temperature range of the transition have been determined to be ΔH =–5.6 J/g and 312.5–319 K, respectively. The enthalpy and temperature range of the transition are influenced by the quality of the samples. The samples containing inclusions of the metastable, orthorhombic phase have a lower enthalpy and broader temperature range of the magnetostructural transformation of manganese monoarsenide. It has been demonstrated that DSC is an effective tool for assessing the quality of MnAs samples. Temperature dependences of specific magnetization and magnetic permeability for MnAs lend support to the DSC results. From these data, the Curie temperature of MnAs has been determined to be 40°C, in good agreement with previously reported data.     

Thermophysical Properties of Solid Phase Hafnium at High Temperatures

The thermophysical properties of several hafnium samples with a content of zirconium below 1% were experimentally studied over a wide temperature range. The specific heat capacity and specific electrical resistivity were measured from 300 to 2340 K, the hemispherical total emissivity from 1000 to 2130 K, while the thermal diffusivity was measured in the range from 300 to 1470 K. The thermal conductivity and Lorentz number were computed from measured properties for the range from 300 to 1470 K. The specific heat capacity, specific electrical resistivity, and hemispherical total emissivity were measured by subsecond pulse calorimetry, and the thermal diffusivity using the laser flash method. Samples in the form of a thin rod or wire, and in the form of a thin disk were used in the first and second methods, respectively. For data reduction and computation of relevant parameters, recent literature values of the linear thermal expansion were used. The results are compared with literature data and discussed.     

Heat capacity and thermodynamic functions of GaSe from 300 to 700 K

The heat capacity of ?-GaSe has been determined by differential scanning calorimetry in the temperature range 300–700 K. Smoothed heat capacity data have been used to evaluate the thermodynamic functions of gallium monoselenide (entropy, enthalpy increment, and reduced Gibbs energy).     

Simultaneous Measurement of Thermophysical Properties and Dielectric Properties of PZT-Based Ferroelectric Ceramics by Thermal Radiation Calorimetry

Simultaneous measurements of thermophysical properties and dielectric properties have been performed for PZT-based ferroelectric ceramics. An apparatus based on thermal radiation calorimetry was used in the present measurements. Anomalies in the thermophysical properties were observed near the ferroelectric-to-paraelectric phase transition temperature. The anomalous peak was at almost the same temperature as the inflection point of the dielectric constant. It was found that modification of PZT with increasing Nb, Mg, Zn, and Sr causes a decrease of the Curie temperature and an increase of the hysteresis phenomena for the phase transition, and the values of the thermal conductivity increase with temperature similar to amorphous materials.     

Microsecond Laser Polarimetry for Emissivity Measurements on Liquid Metals at High Temperatures—Application to Tantalum

The aim of this work was to determine accurate and reliable thermophysical properties of liquid tantalum from melting up to temperatures of 5000 K. Temperature measurements on pulse-heated liquid metal samples reported by different authors have always been performed under the assumption of a constant emissivity over the whole liquid range because of the lack of data for liquid metals. The uncertainty in temperature measurement is reduced in this work by the direct measurement of emissivity during the experiments. The emissivity measurements are performed by linking a laser polarimetry technique with the established method for performing high speed measurements on liquid tantalum samples at high temperatures during microsecond pulse-heating experiments. A set of improved thermophysical properties for liquid tantalum, such as temperature dependences of normal spectral emissivity at 684.5 nm, heat capacity, enthalpy, electrical resistivity, thermal diffusivity, and thermal conductivity, was obtained.     

Thermophysical Properties of W–Re Alloys Above the Melting Region

In earlier experiments we have studied pure elements with a fast pulse heating technique to obtain thermophysical properties of the liquid state. We report here results for thermophysical properties such as specific heat and dependences among enthalpy, electrical resistivity, and temperature, for four W–Re alloys (3.95, 21.03, 23.84, and 30.82 at % of Re) in a wide temperature range covering solid and liquid states. Thermal conductivity is calculated using the Wiedemann–Franz law for the liquid alloy, as.well as data for thermal diffusivity for the beginning of the liquid phase. Additionally, data for the entire temperature range studied have been analyzed in comparison with those of the constituent elements, tungsten and rhenium, since both metals have been studied previously with the same experimental technique. Such information is of interest in the field of metallurgy since W–Re alloys of low Re content in the region of mutual component solubility in the solid state are widely used as thermocouple materials for the purposes of high-temperature thermometry.     

Drop Calorimetry Studies on 9Cr–1W–0.23V–0.06Ta–0.09C Reduced Activation Steel

The temperature dependence of enthalpy increment (H _T − H ₂₉₈) of 9 mass% Cr–1 mass% W–0.23 mass% V–0.06 mass% Ta–0.09 mass% C reduced activation steel has been measured by inverse drop calorimetry in the temperature range 400 K to 1273 K. A critical comparison of present isothermal enthalpy measurements with the results of our previous dynamic calorimetry studies has been made to reveal clearly the occurrence of various diffusional phase transformations that occur at high temperature. These phase changes are marked by the presence of distinct inflections or cusps in an overall nonlinear variation of enthalpy values with temperature. The principal thermal relaxation step of the martensitic microstructure obtained through quenching from the high-temperature γ-austenite phase is observed around 793 K. The ferromagnetic-to-paramagnetic transition of the α-ferrite phase is found to occur at 1015 K. The equilibrium values of γ-austenite start (Ae ₁) and finish (Ae ₃) temperatures are found to be 1063 K and 1148 K, respectively. A value of 12 J · g⁻¹ has been estimated for Δ°H ^α→γ the latent heat associated with the α → γ transformation. The measured enthalpy increment variation of the α-ferrite phase with temperature has been fitted to a suitable empirical function to estimate the temperature-dependent values of the specific heat. A comparison of the drop calorimetry-based indirect estimate of the specific heat with the direct differential scanning calorimetry-based values revealed that the drop calorimetry estimates are systematically lower than its dynamic calorimetry counterpart. This difference is attributed to the fact that, under finite heating rate conditions that are typical of dynamic calorimetry, measurements are made under nonequilibrium conditions. Notwithstanding this limitation, there is a good overall agreement between the two C _p values and also among the phase transformation temperatures so that a reliable assessment of thermal properties and phase transformation characteristics of reduced activation steel can be determined by a combined analysis of the results of drop and differential scanning calorimetry.     

Low-temperature heat capacity and thermodynamic functions of DyVO4

The heat capacity of dysprosium orthovanadate has been determined by adiabatic calorimetry in the range 6.12–343.26 K. The present and earlier data have been used to calculate the thermodynamic functions of DyVO₄ in the temperature range 0–350 K. We have determined the absolute entropy and Gibbs energy of formation of dysprosium orthovanadate: S ⁰(298.15 K) = 148.34 ± 0.11 J/(mol K), Δ_f G ⁰(298.15 K) = ?1671.6 ± 2.1 kJ/mol. An anomaly has been detected at temperatures below 42 K, due to the Jahn-Teller phase transformation (T _C = 14.42 K). We have determined the thermodynamic characteristics of the transformations in the temperature range 0–42.63 K.     

Calorimetric Study of Rubidium Hexafluoroarsenate at Elevated Temperatures

The heat capacity of rubidium hexafluoroarsenate, RbAsF₆, was measured from 300 to 700 K by differential scanning calorimetry. The results indicate that the structural transformation from the rhombohedral to the cubic phase occurs in the range 305–436 K through an intermediate stable phase and reaches isothermal completion in a narrow temperature range of 407–409 K. The enthalpy of the transformation is 10.90 ± 0.03 kJ/mol, and its entropy is 26.39 ± 0.08 J/(K mol) = Rln24, which suggests that this structural transition is of the order–disorder type. The heat capacity data are used to evaluate the thermodynamic properties of RbAsF₆ in the range 300–700 K.     

Calorimetric techniques for the evaluation of thermal efficiencies of shape memory alloys

The latent heat and entropy changes of NiTi shape memory effect (SME) alloys have been evaluated by three different calorimetric techniques; adiabatic calorimetry, differential scanning calorimetry and a Clapeyron analysis of isothermal stress-strain data. It is found that these techniques provide consistent estimates for the enthalpy and entropy to within 20% for NiTi and noble metal SME alloys. From published thermodynamic data for SME alloys, thermal efficiencies were calculated based on an ideal SME heat engine cycle. It was found that NiTi provides the maximum thermal efficiency with the highest temperature transformation range.     

Thermophysical Properties of a Quaternary Refrigerant Mixture: Comparison of Dynamic Light Scattering Measurements with a Simple Prediction Method

Dynamic light scattering (DLS) has been used for the measurement of several thermophysical properties of a quaternary refrigerant mixture R-125/143a/32/134a in its liquid phase under saturation conditions. The thermal diffusivity and sound speed have been obtained by light scattering from bulk fluids over a temperature range from about 293 K up to the liquid–vapor critical point. By applying the method of DLS to a liquid–vapor interface, also called surface light scattering (SLS), the saturated liquid kinematic viscosity and surface tension can be determined simultaneously. These properties have been measured from about 243 to 343 K. The results are discussed in comparison with literature data and with a simple prediction method based on the mass-weighted properties of the pure components, expressed as functions of the reduced temperature. Once again, the simple prediction method was shown to be applicable for the calculation of different transport and other thermophysical properties of multicomponent refrigerant mixtures and this with sufficiently high accuracy for technical practice. Moreover, the input data for the simple prediction scheme can be reduced without loss of accuracy by treating binary or ternary mixtures as a subset of the multicomponent mixture.     

High-temperature heat capacity and vibrational spectra of Eu<Subscript>2</Subscript>Sn<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Eu₂Sn₂O₇ compound has been prepared by solid-state reaction (by sequentially firing a stoichiometric mixture of Eu₂O₃ and SnO₂ in air at 1273 and 1473 K) and its heat capacity has been determined by differential scanning calorimetry in the temperature range 370–1000 K. The heat capacity data have been used to evaluate the thermodynamic properties of europium stannate: enthalpy increment H°(T)–H°(370 K), entropy change S°(T)–S°(370 K), and reduced Gibbs energy Ф°(T). Raman spectra of Eu₂Sn₂O₇ polycrystals with the pyrochlore structure have been measured in the range 200–1200 cm^–1.     

Thermophysical Properties of Solid Phase Zirconium at High Temperatures

This paper presents experimental results on the thermophysical properties of relatively pure polycrystalline zirconium samples in the solid phase from room temperature up to near the melting point. The specific heat capacity and specific electrical resistivity were measured from 290 to 1970 K, the hemispherical total emissivity from 1400 to 2000 K, the normal spectral emissivity from 1480 to 1940 K, and the thermal diffusivity in the range from 290 to 1470 K. From these data, the thermal conductivity and Lorentz number were computed in the range from 290 to 1470 K. For necessary corrections the most recent values of the linear thermal expansion from the literature have been used. Subsecond pulse calorimetry for measuring heat capacity, specific electrical resistivity, and both emissivities and the laser flash method for measuring thermal diffusivity were applied. Samples in the form of a thin rod and in the form of a thin disk were used in the first and second methods, respectively. Measurement uncertainties were generally about 3% for heat capacity, 1.6% for specific electrical resistivity, 3–10% for the two emissivities, and from less than 1% up to 6% for thermal diffusivity. All the results are discussed in reference to available literature data.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.     

High-temperature thermodynamic properties of LuPO<Subscript>4</Subscript>

The high-temperature enthalpy of lutetium orthophosphate has been determined as a function of temperature in the range 432.92–1744.58 K using drop calorimetry. The present and earlier experimental data have been used to calculate temperature-dependent heat capacity of LuPO₄ in the range 1–1750 K.     

Effect of Ag doping on structural, electrical and dielectric properties of TiO2 nanoparticles synthesized by a low temperature hydrothermal method

Silver (Ag) doped thermally stable TiO₂ nanoparticles in the anatase phase have been synthesized by a low temperature hydrothermal method. The formation of anatase phase has been investigated by X-ray diffraction. Thermogravimetry and differential scanning calorimetry have been used for thermal studies. Scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy have been used for the morphology and composition studies. Surface areas were studied by the Brunauer-Emmett-Teller method. Dielectric properties were studied for dopant levels of 0.25, 0.5 and 1.0?wt% at 300?K in the frequency range of 42?Hz–5?MHz. The crystallite size, alternating current (AC) conductivity and dielectric properties of undoped TiO₂ nanoparticles were greatly affected by doping with Ag. At high frequencies, the materials showed high AC Conductivity and low dielectric constant.     

Determination of thermophysical properties of 45% Pb‒55% Bi alloy. Thermodynamic simulation

The thermophysical properties of 45% Pb?55% Bi alloy were studied using thermodynamic simulation. Vapor pressure, partial pressures of the components of vapor, heat capacity, entropy, enthalpy, and the thermal conductivity of an alloy depending on temperature were determined. A comparison of the results from published data was made.     

Characterization of Thermal Conductivity of Carbon Fibers at Temperatures as Low as 10 K

Due to their excellent thermal structural properties, carbon fibers have been extensively applied in the fields of mechanical and aerospace engineering. Many studies have been conducted at room temperature or higher, yet the literature gives very few data on the measurement of thermophysical properties at very low temperatures. The thermal diffusivity of two kinds of carbon fibers was measured by the transient electrothermal technique at very low temperatures (down to 10 K) in the work, the ρCp followed the Debye T3 law dependence quite accurately in agreement with theoretical investigations at very low temperatures(<?10 K). The thermal diffusivity of carbon fiber almost increases linearly with decreasing temperature, but when the temperature drops down below 90 K, the thermal diffusivity rapidly decreases. The thermal expansion mismatch between the different materials present in carbon fibers may influence the change of thermal diffusivity. The relationship between the impurity concentration and the thermal transport of the carbon fibers is discussed.     

锗酸铋（Bi4Ge3O12）单晶热物理性质的研究

本文用不同的测试技术和方法测定了锗酸铋(BGO)单晶的比热(300～800K)热膨胀系数(100～1100K)和导温系数(140～700K),进而导出了 BGO 单晶不同温度下的导热系数、定容比热、德拜温度和格虑内森数。本文还对 BGO 单晶热物理性质的变化规律作了理论解释。     

Combined DSC and Pulse-Heating Measurements of Electrical Resistivity and Enthalpy of Platinum,Iron, and Nickel

Measurements of the enthalpy, electrical resistivity, and specific heat capacity as a function of temperature starting from the solid state up into the liquid phase for Fe, Ni, and Pt are presented. Two different measurement approaches have been used within this work: an ohmic pulse-heating technique, which allows – among others – the measurement of enthalpy, specific heat capacity, and electrical resistivity up to the end of the stable liquid phase, and a differential–scanning–calorimetry technique (DSC) which enables determination of specific heat capacity from near room temperature up to 1500 K. The microsecond ohmic pulse-heating technique uses heating rates up to 10⁸ K·s^–1 and thus is a dynamic measurement, whereas the differential–scanning–calorimetry technique uses heating rates of typically 20 K·min^–1 and can be considered as a quasi-static process. Despite the different heating rates both methods give good agreement of the thermophysical data within the stated uncertainties of each experiment. Results on the metals Fe, Ni, and Pt are reported. The enthalpy and resistivity data are presented as a function of temperature and compared to literature values.     

Thermophysical Properties of Binary Mixtures of R125 + R143a in Comparison with a Simple Prediction Method

The thermal diffusivity and sound speed of binary refrigerant mixtures of R143a (1,1,1-trifluoroethane) and R125 (pentafluoroethane) have been determined for both the saturated liquid and vapor phase using dynamic light scattering (DLS). Measurements were performed for four quite different mixture compositions over a wide temperature range from 293 to 345 K approaching the vapor-liquid critical point. The results obtained corroborate the usefulness of a simple prediction method for the determination of different thermophysical properties of multicomponent mixtures in the two-phase region up to the critical point. Besides the information on the properties for the pure components, the successful application of the prediction method is also based on an exact knowledge of the critical temperature. The composition dependence of the critical temperature has been determined by observation of the vanishing meniscus between liquid and vapor phases. The mixture results are discussed in detail and compared with available literature data.