Measurement of the heat capacity of molybdenum (Standard Reference Material) in the range 1500–2800 K

Measurement of the heat capacity of molybdenum (Standard Reference Material 781 of the National Bureau of Standards) in the temperature range 1500–2800 K by a subsecond-duration, pulse-heating technique is described. The results of the measurements on three specimens are in agreement within 0.6%. The heat capacity of molybdenum in the temperature range 1500–2800 K based on the present results is expressed by the following function (standard deviation =0.5%): C _p =–3.0429+4.7215×10^–2 T–2.3139×10^–5 T ²+4.7090× 10^–9 T ³ where T is in K and C _p is in J · mol^–1 · K^–1. The inaccuracy of the reported results is estimated to be not more than 3%.     

Deformability enhancement in ultra-fine grained, Ar-contained W compacts by TiC additions up to 1.1%

Nano-sized Ar bubbles give negative influence on the fracture resistance and occurrence of superplasticity in ultra-fine grained (UFG) W–TiC compacts. In order to enhance deformability in UFG, Ar-contained W–TiC compacts, effects of TiC addition on the high-temperature deformation behavior were examined. W–TiC compacts with TiC additions of 0, 0.25, 0.5, 0.8 and 1.1 wt% were fabricated by mechanical alloying in a purified Ar atmosphere and hot isostatic pressing. Tensile tests were conducted at 1673–1973 K (0.45–0.54 T_m, T_m: melting point of W) at initial strain rates from 5 × 10⁻⁵ to 5 × 10⁻³ s⁻¹. It is found that as TiC addition increases, the elongation to fracture significantly increases, e.g., from 3 to 7% for W–0 and 0.25TiC/Ar to above 160% for W–1.1TiC/Ar when tested at 1873 and 1973 K at 5 × 10⁻⁴ s⁻¹. The flow stress takes a peak at 0.25%TiC and decreases to a nearly constant level at 0.5–1.1%TiC. The ranges of the strain rate sensitivity of flow stress, m, and the activation energy for deformation, Q, with TiC additions are 0.17–0.30 and 310–600 kJ/mol, respectively. The observed effects of the TiC additions on the tensile properties are discussed.     

High thermal expansion KAlSiO4 ceramic

Orthorhombic kalsilite (KAlSiO₄) was prepared by solid-state reaction from K₂CO₃, Al₂O₃, and SiO₂. The axial thermal expansion coefficients of the orthorhombic kalsilite were 1.6×10^–5°C^–1 for the a-axis, 1.6×10^–5°C^–1 for the b-axis, 2.8×10^–5°C^–1 for the c-axis, and 2.0×10^–5°C^–1 for the average from room temperature to 1000°C. A high thermal expansion ceramic consisting of the orthorhombic kalsilite was prepared by sintering. The densification was promoted by adding Li₂CO₃. The KAlSiO₄ ceramic sintered at 1200°C for 2 h with 5 wt% Li₂CO₃ had a bending strength of 65 MPa and linear thermal expansion coefficient of 2.2×10^–5 °C^–1 from room temperature to 600°C.     

Thermal conductivity of ethane from 290 to 600 K at pressures up to 700 bar,including the critical region

The paper presents thermal conductivity measurements of ethane over the temperature range of 290–600 K at pressures to 700 bar including the critical region with maximum uncertainty of 0.7 to 3% obtained with a transient line source instrument. A correlation of the data is presented and used to prepare tables of recommended values that are accurate to within 2.5% in the experimental range except near saturation, and in the critical region, where the anomalous thermal conductivity values are predicted to within 5%.Nomenclature a _k , b _ij , b _k , c _i Parameters of the regression model, k=0 to n, i=0 to m, j=0 to n - P Pressure, (MPa or bar) - Q _l Heat flux per unit length (mW · m^–1) - t Time, s - T Temperature, K - T _cr Critical temperature, K - T _r Reduced temperature = T/T _cr - T _w Temperature rise of wire between times t ₁ and t ₂ K - T ^* Reduced temperature difference (T–T _cr)/T _cr - Thermal conductivity, mW · m^–1 · K^–1 - ₁ Thermal conductivity at 1 bar, mW · m^–1 · K^–1 - _bg Background thermal conductivity, mW · m^–1 · K^–1 - _cr Thermal conductivity anomaly, mW · m^–1 · K^–1 - _e Excess thermal conductivity, mW · m^–1 · K^–1 - Density, g · cm^–3 - _cr Critical density, g · cm^–3 - _r Reduced density, = / _cr - ^* Reduced density difference =(- _cr)/ _cr     

Alloying (In,Mn)As and (Ga,Mn)As: Ferromagnetic (In,Ga,Mn)As Lattice-Matched to InP

An effect of alloying two ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As on the ferromagnetic properties of resultant (In,Ga,Mn)As alloys is reported. For conditions close to lattice-matching to InP substrates, y = 0.53 in (In _y Ga_1–y )_1–x Mn _x As, ferromagnetism up to Curie temperatures T _C = 100–110 K could be achieved for a Mn composition x = 0.13. Trends in the Curie temperature in (In,Ga,Mn)As are compared with (Ga,Mn)As and (In,Mn)As as a function of Mn content. Hole concentrations determined from magnetotransport, taking into account the anomalous Hall contribution to Hall resistance, gives p/Mn = 0.03 ratio to Mn composition in metallic case for x = 0.13. We mention the possible role of chemical ordering (short range) of Mn impurity atoms on hole concentration and, consequently, for the ferromagnetic properties.     

Investigation of Microinhomogeneities in a-SiC r :H Films Using the Chemical Induction Model

Amorphous hydrogenated silicon–carbon (a-SiC _r :H) films grown by decomposing silane–methane mixtures in a low-frequency (55 kHz) glow discharge at different methane concentrations are studied by IR spectroscopy. The absorption band in the range 1850–2300 cm^–1 is decomposed into four Gaussian components, and the results are compared with calculations in the chemical induction model. It is found that the carbon atoms are nonuniformly distributed in the nearest neighbor environment of the SiH groups in the form of HSi–Si_{3 – n} C _n (n= 0–3) structures. The random bonding model is used to evaluate the probability of formation of each HSi–Si_{3 – n} C _n structure as a function of the C/Si ratio. Comparison with experimental data points to an inhomogeneous microstructure of the films.     

Microwave Conductivity of Superconducting and Insulating Fullerides

Superconductivity of A₃C₆₀ (A=K and Rb) is destroyed by the intercalation of neutral ammonia molecules, and is replaced by the antiferromagnetic ground state. These properties suggest the importance of the electronic correlation in fullerides, which have been believed to be simple BCS superconductors. By using a microwave cavity perturbation technique, we confirmed that the conductivity of the antiferromagnet (NH₃)K_3–x Rb _x C₆₀ at 200 K is already 3–4 orders of magnitude smaller than those of superconductors, K₃C₆₀ and (NR₃) _x NaRb₂C₆₀, and that the antiferromagnetic compounds are insulators without metal-insulator transition at the Néel temperature. These results indicate that the Mott–Hubbard transition in the A₃C₆₀ systems is driven by a reduction of lattice symmetry from face-centered-cubic (fcc) to face-centered-orthorhombic (fco), rather than by the magnetic ordering.     

Thermodynamic properties by levitation calorimetry — V. High-temperature heat content of liquid gallium

The heat content (enthalpy) of liquid gallium relative to the supercooled liquid state at 298.15 K has been measured by levitation calorimetry over the temperature range 1412–1630 K. Thermal energy increments were determined using an aluminum block calorimeter of conventional design. The sharp decrease of C _p with increasing temperature observed just above the melting point does not persist up to the high temperatures of the present work. When combined with recent laser-flash calorimetry results from the literature, the present work indicates that C _p is 26.46 ± 0.71 J · g-atom^–1 · K^–1 over the temperature range 587–1630 K.Paper presented at the Japan-United States Joint Seminar on Thermophysical Properties, October 24–26, 1983, Tokyo, Japan.     

Molar heat capacity at constant volume for air from 67 to 300 K at pressures to 35 MPa

Measurements of the molar heat capacity at constant volumeC _v for air were conducted with an adiabatic calorimeter. Temperatures ranged from 67 to 300 K, and pressures ranged up to 35 MPa. Measurements were conducted at 17 densities which ranged from gas to highly compressed liquid states. In total, 227C _v values were obtained. The air sample was prepared gravimetrically from research purity gases resulting in a mole fraction composition of 0.78112 N₂ + 0.20966 O₂ + 0.00922 Ar. The primary sources of uncertainty are the estimated temperature rise and the estimated quantity of substance in the calorimeter. Overall, the uncertainty (± 2) of theC _v values is estimated to be less than ± 2% for the gas and ±0.5% for the liquid.Nomenclature C _v Molar heat capacity at constant volume, J · mol^–1 K^–1 - C _v ⁰ Molar heat capacity in the ideal-gas state, J · mol^–1 · K^–1 - V _bomb Volume of the calorimeter containing sample, cm³ - P Pressure, MPa - P Pressure rise during a heating interval, MPa - T Temperature, K - T ₁,T ₂ Temperature at start and end of heating interval, K - T Temperature rise during a heating interval, K - Q Calorimetric heat energy input to bomb and sample, J - Q ₀ Calorimetric heat energy input to empty bomb, J - N Moles of substance in the calorimeter, mol - Fluid density, mol · dm^–3     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

The effect of molecular weight on slow crack growth in linear polyethylene homopolymers

The rate of initiation and growth of cracks in linear high-density polyethylene with different molecular weights was observed in single-edge-notched tensile specimens under plane strain condition as a function of applied stress, notch depth and temperature. The initial rates of crack initiation all have the form of C ^m a ₀ ⁿ exp (–Q/RT) or AK ^pexp (–Q/RT) where = stress, a ₀ = notch depth and K= stress intensity factor. For the different molecular weights, m, n, P and Q are almost the same where m=5, n=2, P=4.7 and Q=115 kJ mol^–1, but the constants C and A varied as (¯M _w–¯M _c)^–1 where ¯M_c is a limiting molecular weight for sudden fracture. A molecular model based on tie-molecules has been used to explain the dependence on ¯M _w. The effect of ¯M _w on the fast-fracture strength at low temperature and the relationship to tie-molecules have also been investigated. Quantitative relationships between the concentration of tie-molecules and the fracture behaviour have been obtained.     

Cyclotron Resonance of Electrons and Holes in Paramagnetic and Ferromagnetic InMnAs-Based Films and Heterostructures

We have studied the cyclotron resonance of electrons and holes in various types of InMnAs-based structures at ultrahigh magnetic fields. Our observations, in conjunction with an eight-band effective mass model including the s–d and p–d exchange interactions with Mn d-electrons, unambiguously suggest the existence of s-like and p-like delocalized carriers in all samples studied. The samples studied include Paramagnetic n-type In_1–x Mn _x As films (x 0.12) grown on GaAs, ferromagnetic p-type In_1–x Mn _x As films (x 0.025) grown on GaAs with Curie temperatures (T _C) > 5 K, paramagnetic n-type In_1–x Mn _x As/InAs superlattices, ferromagnetic p-type In_1–x Mn _x As/GaSb heterostructures (x 0.09) with T _C = 30-60 K, and ferromagnetic (In_0.53Ga_0.47)_1–x Mn _x As/In_0.53Ga_0.47As heterostructures (x 0.05) grown on InP with T _C up to 120 K.     

Thermal properties of high-temperature superconductors

The specific heat and thermal conductivity measurements of YBa₂Cu₃O_7– high-T _c superconductors were performed by an a.c. calorimetry method. Investigations of the specific heat of YBa₂Cu₃O_7– ceramics in magnetic fields show that an increase in the magnetic field reduces the jump in the specific heat, broadens the transition region, and shifts the transition temperature downward by about 0.5 K, Temperature dependence of the specific heat of a YBa₂Cu₃O_7– high-T _c superconducting ceramic reveals that fluctuation affect the specific heat near the superconducting transition, Critical exponents = = 0.5, the critical amplitudesC ⁺ =C ^– = 0.5 J · mol^–1 K^–1, the space dimensionalityd = 3, and the number of components in the order parametern = 3 is calculated, The specific heat and the along-c-axis thermal conductivity of YBa₂Cu,₃O_7– single crystal were simultaneously measured.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Contributions to the heat capacity of solid molybdenum in the range 300–2890 K

An analysis has been made of contributions to the heat capacity of Mo, with a special examination of the effect of the formation of vacancies near the melting point. Literature values of the heat capacity at constant pressure C _P were fitted to a polynomial. Using recent measurements of the velocity of sound at high temperature and literature data of the coefficient of expansion, the dilation correction was made to C _P to obtain the heat capacity at constant volume C _V . This heat capacity was taken to consist only of independent contributions from electron excitations (C _VE), harmonic lattice vibrations (C _VH), anharmonic lattice vibrations (C _VA), and the formation of vacancies (C _VV). Three models of C _VE (free electron, band theory, and electron-photon) have been used to calculate the electronic contribution, and an examination of the results indicates that the electron-phonon model is the best. C _VH is assumed to be given by the Debye model, with a single Debye temperature. Thus, the excess heat capacity C _VEX= C _V -C _VE -C _VH is taken as equal to (C _VA +C _VV ), where C _VA is linear with temperature (C _VA=A T), and we have fitted the values of C _VEX to determine the values of A and the energy and entropy of formation of vacancies which give the best fit. The anharmonic contribution is positive. The energy of vacancy formation is 100,000 J · mol^–1, in agreement with estimates by Kraftmakher from C _P data. The entropy of formation is 11.6 J · mol^–1 · K^–1. The concentration of vacancies at the melting point (2890 K) is calculated to be 6.3%.     

Non-contact measurements of thermophysical properties of niobium at high temperature

Four thermophysical properties of both solid and liquid niobium have been measured using the vacuum version of the electrostatic levitation furnace developed by the National Space Development Agency of Japan. These properties are the density, the thermal expansion coefficient, the constant pressure heat capacity, and the hemispherical total emissivity. For the first time, we report these thermophysical quantities of niobium in its solid as well as in liquid state over a wide temperature range, including the undercooled state. Over the 2340 K to 2900 K temperature span, the density of the liquid can be expressed as _L (T) = 7.95 × 10³ – 0.23 (T – T _m)(kg · m^–3) with T _m = 2742 K, yielding a volume expansion coefficient _L(T) = 2.89 × 10^–5 (K^–1). Similarly, over the 1500 K to 2740 K temperature range, the density of the solid can be expressed as _s(T) = 8.26 × 10³ – 0.14(T – T _m)(kg · m^–3), giving a volume expansion coefficient _s(T) = 1.69 × 10^–5 (K^–1). The constant pressure heat capacity of the liquid phase could be estimated as C _PL(T) = 40.6 + 1.45 × 10^–3 (T – T _m) (J · mol^–1 · K^–1) if the hemispherical total emissivity of the liquid phase remains constant at 0.25 over the temperature range. Over the 1500 K to 2740 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as _TS(T) = 0.23 + 5.81 × 10^–5 (T – T _m). The enthalpy of fusion has also been calculated as 29.1 kJ · mol^–1.     

Thermophysical Properties of Containerless Liquid Iron up to 2500 K

Thermophysical properties of high temperature liquid iron heated with a CO₂ laser have been determined in an aerodynamic levitation device equipped with a high-speed camera and a three-wavelength pyrometer. Characteristic curves of the free cooling and heating of the drop can be used to determine the same apparent emissivity of solid and liquid iron and to calibrate pyrometers based on the known value of the melting point of iron, i.e., 1808 K. Examination of the recalescence of undercooled liquid iron and further solidification are used to obtain the ratio of the melting enthalpy versus the heat capacity of liquid iron as . The surface tension was determined from an analysis of the vibrations of liquid drops. Results are accurately described by (mJm^–2)=(1888±31)–(0.285±0.015) (T–T _m ) between 1750 K (undercooled liquid) and 2500 K. The density of liquid iron has been deduced from the image size and the mass of the liquid iron drops.     

Thermodynamic Properties and Decomposition of Lithium Hexafluoroarsenate, LiAsF6

The heat capacity of lithium hexafluoroarsenate is determined in the temperature range 50–750 K by adiabatic and differential scanning calorimetry techniques. The thermodynamic properties of LiAsF₆ under standard conditions are evaluated: C _p ⁰(298.15 K) = 162.5 ± 0.3 J/(K mol), S ⁰(298.15 K) = 173.4 ± 0.4 J/(K mol), ⁰(298.15 K) = 81.69 ± 0.20 J/(K mol), and H ⁰(298.15 K) – H ⁰(0) = 27340 ± 60 J/mol. The C _p(T) curve is found to contain a lambda-type anomaly with a peak at 535.0 ± 0.5 K, which is due to the structural transformation from the low-temperature, rhombohedral phase to the high-temperature, cubic phase. The enthalpy and entropy of this transformation are 5.29 ± 0.27 kJ/mol and 10.30 ± 0.53 J/(K mol), respectively. The thermal decomposition of LiAsF₆ is studied. It is found that LiAsF₆ decomposes in the range 715–820 K. The heat of decomposition, determined in the range 765–820 K using a sealed crucible and equal to the internal energy change U _r(T), is 31.64 ± 0.08 kJ/mol.     

Thermal conductivity of air in the range 312 to 373 K and 0.1 to 24 MPa

We have used the transient hot-wire technique to make absolute measurements of the thermal conductivity of dry, CO₂-free air in the temperature range from 312 to 373 K and at pressures of up to 24 MPa. The precision of the data is typically ±0.1%, and the overall absolute uncertainty is thought to be less than 0.5%. The data may be expressed, within their uncertainty, by polynomials of second degree in the density. The values at zero-density agree with other reported data to within their combined uncertainties. The excess thermal conductivity as a function of density is found to be independent of the temperature in the experimental range. The excess values at the higher densities are lower than those reported in earlier work.Nomenclature Thermal conductivity, mW · m^–1 · K^–1 - Density, kg · m^–3 - C _p Specific heat capacity at constant pressure, J · kg^–1 · K^–1 - T Absolute temperature, K - q Heat input per unit wire length, W · m^–1 - t Time, s - K(=/C _p) Thermal diffusivity, m² · s^–1 - a Wire radius, m - Euler's constant (=0.5772 ) - p _c Critical pressure, MPa - T _c Critical temperature, K - _c Critical density, kg · m^–3 - R Gas constant (=8.314 J · mol^–1 · K^–1) - V _c Critical volume, m³ · mol^–1 - Z _c(=p _c V _c/RT _c) Critical compressibility factor     

A theoretical model of the manfacture of reaction-bonded silicon nitride with particular emphasis on the effect of ambient reaction temperature and compact size

An analysis of the exothermic, irreversible silicon-nitrogen reaction, based on the particle-pellet model, is presented using mixed type boundary conditions to represent external resistances. The mathematical model incorporates a sharp cut-off in the reaction and takes into account its transient behaviour. The resulting system of partial differential equations is solved numerically using an explicit finite difference scheme. The effects of varying the ambient reaction temperature and compact size on the temperature distribution inside the nitriding compact and on the solid product formation rate, are examined. The results obtained are in acceptable agreement with previous experimental research by other workers, which illustrates how the model adequately represents the silicon-nitrogen reaction.An investigatory report on the validity of the Arrhenius equation for determining the thermal activation energy of this reaction is also presented. a characteristic dimensions of the compact,m - C concentration of the gas within the compact, mol m^–3 - C _A molar concentration of the product in the compact, mol m^–3 - C _AO maximum concentration of the product, 1.6×10⁴ mol m^–3 - C _f concentration of the gas surrounding the compact, mol m^–3 - C _p specific heat of solid reactant, 1250 J kg^–1K^–1 - D _e effective diffusion coefficient, 2×10^–6 m² sec^–1 - E activation energy of reaction, 5.5×10⁵ J mol^–1 - H heat of reaction, –7.5×10⁵ J mol^–1 - h heat transfer coefficient, Wm^–2 K^–1 - K _e effective thermal conductivity, 6.6 Wm^–1 K^–1 - /n derivative normal to the surface of the compact - r(C, T) reaction rate per unit area, kg m^–2 sec^–1 - r _p mean particle size,m - s _g specific area of the compact, 7.142×10² m² kg^–1 - T absolute temperature within the compact, K - T _c temperature at the centre of the compact, K - T _f initial and surrounding temperature, K - T _s temperature at the surface of the compact, K - t time, sec - u dimensionless concentration of the gas in the compact - U _A C _A/C _AO - V ^* dimensionless space - v dimensionless temperature in the compact - X conversion factor, defined by Equation 7 - x dimensionless space variable within the compact - void fraction of the compact, x0.4 - _b bulk density of the compact, 1400 kg m^–3 - dimensionless time     

Thermal Expansion and Isothermal Compressibility of TlGaS2

The thermal expansion and isothermal compressibility of TlGaS₂ were measured between 77 K and room temperature. No anomalies in the temperature dependences of these properties were detected. The experimental data were used to evaluate the Debye temperature, rms dynamic atomic displacements, the difference between the specific heats at constant pressure and constant volume (C _p – C _V), and the Grüneisen parameter. The appreciable discrepancy between the C _p – C _V values calculated using thermodynamic relations and an empirical formula is attributed to the pronounced anisotropy of TlGaS₂.