Experimental determination and thermodynamic modeling of phase equilibria in the Cu–Cr system

Liquidus temperatures in the Cu–Cr system at compositions of 10.0–72.7 at.% Cr were determined using electromagnetic levitation melting. The present data agree with the prediction of a recent thermodynamic study of the system for compositions up to 20.0 at.% Cr. However, they show large and positive deviations for other compositions. Microscopic studies reveal that compositions between 10.0 and 50.5 at.% Cr solidified into a dendritic microstructure, whereas those between 55.9 and 72.7 at.% Cr solidified into a droplet-shaped microstructure. The microstructure of the latter type provides direct evidence for the existence of a stable miscibility gap over Cr-rich compositions. Phase equilibria in the Cu–Cr system were calculated using the CALPHAD method. A novel phase diagram was proposed for the Cu–Cr system, which shows a monotectic reaction between compositions of 50.8 and 83.2 at.% Cr at an invariant temperature of 2020 ± 22 K. The novel phase diagram has reduced the discrepancies between the literature data.     

The influence of neutron irradiation on the thermal conductivity of aluminum in the range 5–50 K

Measurements of thermal conductivity of 6N to 3N pure aluminum in the temperature range 5–50 K subjected to fast neutron irradiation, with exposures of 10¹³ and 10¹⁶ n · cm^–2, are reported. The thermal conductivity maximum was found to shift towards higher temperatures with an increase in the fast neutron irradiation exposure. At high temperatures, a departure from Wilson's theory was observed, which may be attributed to the existence of additional electron scattering mechanisms. An increase in both ideal and residual thermal resistivity components with an increase in the radiation exposure was noted.Nomenclature I ₅ (/t) Debye integral of the fifth order - –m slope of the straight line that crosses maximum thermal conductivity values - n exponent in ideal thermal resistivity component - T _m temperature corresponding to maximum thermal conductivity - W _e total electronic thermal resistivity - W _i ideal thermal resistivity - W ₀ residual thermal resistivity - ideal thermal resistivity coefficient in Eq. (4) - ideal thermal resistivity coefficient in Eq. (1) - constant related to the ideal part of thermal resistivity in Eq. (2) - () ideal thermal resistivity coefficient depending on irradiation exposure - () residual thermal resistivity coefficient depending on irradiation exposure - thermal conductivity - _m maximum thermal conductivity - Debye characteristic temperature - irradiation exposure     

Determination of the thermal conductivity of polypyrrole over the temperature range 280–335 K

Samples of polypyrrole were synthesised under galvanostatic conditions to produce films possessing a range of electrical conductivity from 10^–3 to 10 S cm^–1. The electrical and thermal conductivity of these films has been determined between 280 and 335 K. The electrical conductivity was measured using a four probe technique calibrated against ASTM D4496-87. Thermal conductivity was determined from measurements of thermal diffusivity, specific heat and density. Thermal diffusivity was determined using a modified a.c. calorimetry technique, while differential scanning calorimetry (DSC) was used to determine specific heat. The polymer's density was measured using Archimedes' principle. The results were used to calculate the Lorenz number of polypyrrole. A comparison of the predicted behaviour and experimental results was made. Thermal conductivity is found to be large compared to that predicted from the electrical conductivity measurements on low conductivity films. Molecular vibration effects are found to be non-trivial and experimental means for measuring their contribution are mentioned. While polypyrrole has been regarded as a synthetic metal the thermal conductivity results show this classification is wrong.     

Experimental determination of nickel-rich corner of Ni–Al–Ta phase diagram

Abstract

Liquid–solid and solid–state phase equilibria have been studied in the Ni–NiAl–Ni₃Ta triangle of the Ni–AI–Ta system, using a combination of several experimental techniques. Five primary phases occur in this region, including the ternary compound, π, Ni₆TaAI, which enters into equilibrium with each of the other four. Another compound, Ni₈Ta, forms in the solid state by decomposition of Ta–rich Ni solid–solutions and occurs in equilibrium with the γ, π, and δ (Ni₃ Ta) phases. The extent of these different phase fields has been determined at 1250°C and particular attention has been paid to the γ-γ′ solvus surface which has been shown to be accurately described by a second–order polynomial function of the atomic concentrations.

MST/321     

Investigation of thermal conductivity and thermal diffusivity of liquid bismuth within the temperature range of 545–970 K

Thermal conductivity and thermal diffusivity of liquid bismuth within the temperature range from 545 K up to 970 K are investigated by the laser flash method. The measurement errors are equal to (3.5–4.5)%. Approximating equations are obtained, and the reference tables are presented for the temperature dependencies of the properties. The measurement results are compared to the published data available. The temperature dependence of the Lorentz number is calculated up to 970 K.     

Experimental investigation of acoustic properties of titanium alloys in the temperature range of 20–1000°C

Titanium and titanium-based alloys, which are widely used in various sectors of the national economy, require deep and versatile investigations of their physico-mechanical properties in a wide temperature range. Numerous abnormal physical phenomena are observed in titanium-based alloys at high temperatures, especially in the region of polymorphic transformation. In particular, in addition to significant structural variations, which influence the strength properties of the final products, near such transformations the titanium alloys (especially with a fine structure) have a tendency to superplastic deformation, which is widely applied in modern technology. Among the physical characteristics, which provide extensive information about the structural and physico-mechanical properties of titanium alloys, are the temperature expansion and acoustic properties (in particular, the speed of ultrasound, information about the temperature dependence of which is unavailable for the majority of engineering materials), which allow the Young modulus for these materials to be calculated.     

Experimental investigation of phase equilibria in the Cu–Ni–Zr system

Phase equilibria in the Cu–Ni–Zr ternary system have been measured through alloy sampling combined with diffusion couple approach. According to the phase relations identified with electron probe microanalysis and X-ray diffraction techniques, isothermal sections at both 1073 and 1293 K were constructed. It is evident that remarkable ternary solubility occurs in almost all binary intermetallic phases at both temperatures. The formerly reported ternary compounds T1 (Cu_20–40Ni_40–60Zr₂₀) and T2 (Cu_20–25Ni_60–65Zr₁₅) were not verified in this work. No other ternary compound was detected. In addition, continuous dissolution between Cu₁₀Zr₇ and Ni₁₀Zr₇ at 1073 K was observed.     

Computational analysis of the thermal conductivity of the carbon–carbon composite materials

Experimental data for carbon–carbon constituent materials are combined with a three-dimensional stationary heat-transfer finite element analysis to compute the average transverse and longitudinal thermal conductivities in carbon–carbon composites. Particular attention is given in elucidating the roles of various micro-structural defects such as de-bonded fiber/matrix interfaces, cracks and voids on thermal conductivity in these materials. In addition, the effect of the fiber precursor material is explored by analyzing PAN-based and pitch-based carbon fibers, both in the same type pitch-based carbon matrix. The finite element analysis is carried out at two distinct length scales: (a) a micro scale comparable with the diameter of carbon fibers and (b) a macro scale comparable with the thickness of carbon–carbon composite structures used in the thermal protection systems for space vehicles. The results obtain at room temperature are quite consistent with their experimental counterparts. At high temperatures, the model predicts that the contributions of gas-phase conduction and radiation within the micro-structural defects can significantly increase the transverse thermal conductivity of the carbon–carbon composites.     

Investigation of the heat conductivity of niobium in the temperature range 0.05–23 K

We report thermal conductivity measurements on a single-crystal niobium specimen of resistivity ratio 33,000 over the temperature range 0.05–23 K in the superconducting state and above 9.1 K in the normal state. The axis of the niobium rod was [110] oriented. The surface roughness was varied by sandblasting of the sample. The values of the thermal conductivity in the range from the lowest temperatures up to the maximal value covered a range of six orders of magnitude (=2×10^–5 W cm^–1 K^–1 at 50 mK to =22 W cm^–1 K^–1 at 9 K). Above 2 K the results for the untreated and the sandblasted sample are in accord, whereas below 2 K the influence of the sample surface is discernible. The various conduction and scattering mechanisms are discussed.     

On the thermal conductivity of Cu–Zr/diamond composites

Interfaces and close proximity between the diamond and the metal matrix are very important for their thermal conductance performance. Matrix-alloying is a useful approach to greatly enhance the interfacial bonding and thermal conductivity. In this study, the copper–diamond (Cu/Dia) composites with addition of 0.8, 1.2 and 2.4 wt.% zirconium (Zr) are prepared to investigate the influence of minor addition of Zr on the microstructure and thermal conductivity of the composites. The thermal conductivity of the composites is analyzed both experimentally and theoretically. It is demonstrated that moderate interfacial modification due to the Zr added is beneficial to improve the thermal conductivity of the Cu/Dia composites.     

Experimental phase diagram of the dodecane–tridecane system as phase change material in cold storage

Integrating thermal storage with phase change materials (PCMs) in refrigeration and air conditioning processes enables energy performance improvements. Herein, the experimental phase diagram of the alkane system dodecane–tridecane (C₁₂H₂₆–C₁₃H₂₈) is evaluated to find PCMs for freezing applications. For that, the temperature-history method was coupled with a Tammann plot analysis. The obtained C₁₂H₂₆–C₁₃H₂₈ phase diagram indicated a congruent minimum-melting solid solution and polymorphs. The minimum-melting liquidus and the polymorphs identified here agree with previous literature. However, the system does not represent a eutectic, as previously was proposed. The minimum-melting composition is here identified within 15–20 mol% C₁₃H₂₈ compositions. The 17.7 mol% C₁₃H₂₈ is the narrowest minimum-melting composition among those analyzed, melting and freezing between −16 and −12 °C and between −17 and −15 °C, with the enthalpies 185 kJ kg⁻¹ and 165 kJ kg⁻¹; no supercooling; and only minor hysteresis. Hence, this blend has potential as a PCM in freezing refrigeration applications.     

Refinement of the V-O phase diagram in the range 25–50 at % oxygen

The boundaries of the V₁₄O₆ + V _x O _z two-phase region in the V-O system at temperatures from ? 1050 to ? 1650 K have been determined experimentally. The V-O phase diagram has been refined in the range 25–50 at % oxygen using structural and microstructural data for vanadium oxides containing less than 50 at % oxygen in conjunction with earlier results. The possibility of ordering of cubic vanadium monoxide has been examined.     

Experimental investigation on the influence of high temperature on viscosity,thermal conductivity and absorbance of ammonia–water nanofluids

To select the optimal ammonia–water nanofluids and apply to ammonia–water absorption refrigeration systems (AARS), this paper investigated the influence of heating on viscosity, thermal conductivity and absorbance of binary nanofluids. The hysteresis phenomenon was observed after heating at high temperature which is rarely reported in the literature. Experimental results show that most of nanofluids' thermal conductivity increased by about 3–12% after heating. However, their viscosities increased by as much as 15% to 25% except the γ-TiO₂ ammonia–water nanofluid, which was reduced by 2% to 7%. This study also shows that the trend of viscosity is consistent with the absorbance. Due to fact that the thermal conductivity of γ-TiO₂/NH₃–H₂O mixture increased after heating, while the viscosity decreased, even if the concentration of the base liquid is 12.5% or 25%, therefore it is the optimal choice for practical research in AARS at present.     

Experimental investigation and thermodynamic calculation of the phase equilibria in the Cu–Mo–Ni ternary system

The phase equilibria at 900 °C, 1000 °C, 1100 °C, 1200 °C and 1300 °C in the Cu–Mo–Ni system were experimentally determined by means of optical microscopy (OM), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD) on the equilibrated alloys. The experimental results firstly found that the fcc-type miscibility gap exists at 900 °C, 1000 °C, 1100 °C and 1200 °C in the Cu–Mo–Ni system, and the solubility of Cu in the MoNi phase at 900 °C, 1000 °C, 1100 °C and 1200 °C are about 0.5 at.%, 1.5 at.%, 1.7 at.% and 4.0 at.%, respectively. The as-cast Cu₂₀Mo₂₀Ni₆₀ (at.%), Cu₂₀Mo₃₀Ni₅₀ (at.%), Cu₁₀Mo₆₀Ni₃₀ (at.%), Cu₇₀Mo₁₀Ni₆₀ (at.%), Cu₂₀Mo₆₀Ni₂₀ (at.%) and Cu₈₀Mo₁₀Ni₁₀ (at.%) alloys appear the separated macroscopic morphologies, which are caused by the liquid phase separation on cooling, while the as-cast Cu₁₀Mo₂₅Ni₆₅ (at.%), Cu₃₂Mo₅Ni₆₃ (at.%) and Cu_30.7Mo_6.3Ni₆₃ (at.%) alloys show the homogenous microscopic morphologies. On the basis of the experimental data investigated by the present and previous works, the phase equilibria in the Cu–Mo–Ni system were thermodynamically assessed by using CALPHAD (Calculation of Phase Diagrams) method, and a consistent set of the thermodynamic parameters leading to reasonable agreement between the calculated results and experimental data was obtained.     

Heat capacity of rare-earth stannates in the range 350–1000 K

R₂Sn₂O₇ (R = Pr–Lu) rare-earth stannates with the pyrochlore structure have been synthesized by solid-state reactions, by firing stoichiometric mixtures of SnO₂ and R₂O₃ in air at 1473 K. The high-temperature heat capacity of the rare-earth stannates has been determined by differential scanning calorimetry in the temperature range 350 to 1000 K, and the Raman spectra of polycrystalline Tb₂Sn₂O₇ and Dy₂Sn₂O₇ samples have been measured.     

Oxidation of ion-implanted niobium in the 300–700°C temperature range

Ion implantation of different species was shown to have a beneficial influence on the thermal oxidation kinetics of niobium in pure oxygen at temperatures below 500°C. The implants were chosen with regard to their affinity for oxygen compared to that of niobium and their solubility in niobium. The effects of the treatment was to delay the appearence of the linear catastrophic kinetics. The mechanisms involved are discussed.