Lattice dynamics and specific heat of Al-Si and Al-Ge solid solutions

A simplified treatment has been proposed to study quantitatively the lattice dynamics of Al-Si and Al-Ge alloy systems by solid-solutioning under pressure. The volume and electron density effects on the lattice dynamics of the pure constituents aluminium, silicon or germanium are considered, and the phonon dispersion relations of the local and band modes were obtained. Then, the concentration dependence of the local and band mode frequencies were calculated for the Al_1–x Si _x and Al_1–x Ge _x systems. Using the local and band mode frequencies, the lattice specific heat at constant volume was determined theoretically, and results obtained for the temperature-dependent specific heat of matrix aluminium were found to be in good agreement with the experimental data. The concentration dependence of the specific heat could then be predicted quantitatively for Al_1–x Si _x and Al_1–x Ge _x alloy systems.     

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     

Correlation between thermal diffusivity and activation energy of ordering of lead free solder alloys Sn65?xAg25Sb10Cux rapidly solidified from molten state

Internal friction Q^–1, thermal diffusivity D_th, and thermal properties of the melts of Sn_65–xAg₂₅Sb₁₀ Cu_x (0–2.5 wt.%) solder alloys are investigated and studied by means of vibration resonance technique and calibrated differential thermal analysis respectively. The thermal analysis technique is used to measure the temperature dependence of the specific heat, C_p, the solidus temperature, T_s, the liquidus temperature T₁, and the activation energy of ordering U. It is found that T_s and T₁ show strong dependence on copper content. The activation energy of ordering U show critical dependence on the amount of the copper added. A typical dependence of D_th and U for all studied alloys is obtained. The formation of intermetallic compound of AgSb and Sb phases in the range of composition investigated causes a pronounced increase in the electrical resistivity.     

Decay of laminar flow inside a retarded rotating cylindrical vessel

Summary The effect of finite retardation of a rotating cylindrical vessel on the decay process of viscous fluid flow inside the vessel is considered analytically. Exact solution for instantaneous velocity profiles is presented. An instability layer is defined in accordance with Rayleight's criterion. The thickness of this layer is derived and dependence on retardation is discussed.Nomenclature a cylinder radius - D _inst instability thickness, dimensionless - j _n n-th zero of Bessel functionJ ₁,n=1, 2, ... - r, R dimensional and dimensionless radial co-ordinate - R _inst (1–D _inst) - s complex Laplace variable - t, T dimensional and dimensionless time - t ₀,T ₀ dimensional and dimensionless time constant - U complement velocity=R–V - U ^* auxiliary velocity, see text - V tangential velocity, dimensionless - kinematic viscosity - initial angular speed of cylinder - circulation With 4 Figures     

Microstructures and thermoelectric properties of p-type pseudo-binary AgxBi0.5Sb1.5−xTe3 (x = 0.05–0.4) alloys prepared by cold pressing

The p-type pseudo-binary Ag_xBi_0.5Sb_1.5−xTe₃ (x = 0.05–0.4) alloys were prepared by cold pressing. The thermal conductivities (κ) were calculated from the values of heat capacities, densities and thermal diffusivities measured, and range approximately from 0.66 to 0.56 (W K^− 1 m^− 1) for the Ag_xBi_0.5Sb_1.5−xTe₃ alloy with molar fraction x being 0.4. Combining with the electrical properties obtained in the previous study, the maximum dimensionless figure of merit ZT of 1.1 was obtained at the temperature of 558 K.     

Space-charge-limited current analysis in amorphous InSe thin films

The study of space-charge-limited currents (SCLC) in amorphous InSe thin films is presented. The temperature-dependent current–voltage (J–V) measurements were carried out for TO/a-InSe/Au sandwich structures in the range of 200–320 K. For all samples, ohmic behavior was observed up to an electric field strength of about 2×10⁵ V cm^–1. From the temperature dependence of conductivity data, the position of the thermal equilibrium Fermi level E _fo is determined as 250 meV above the valence band E _v. At higher electric field strength values in the SCLC regions, the proportionality constant of voltage changes is between 2 and 2.9 with temperature. The analysis of J–V characteristics using the SCLC method and analytical approach for the determination of density of states (DOS) in the energy range of 190–250 meV shows that DOS changes between 3.8×10¹⁷–1.7×10¹⁸ eV^–1cm^–3 with energy. The energy distribution of DOS is temperature independent indicating that the SCLC in these amorphous films is related to the bulk, not to the surface layer between the contact and the film.     

The Density of UO2–ZrO2 Alloys

The density of a UO₂–ZrO₂ melt (atomic ratio U/Zr = 1.528) is experimentally measured by a pycnometric method in the temperature range of 2973–3373 K. The found temperature dependence of density has the form (T) = (7.0 ± 0.01) – (4.5 ± 0.4) × 10^–4 × (T – 2973 K), g/cm³. The temperature dependence measured enables one to calculate the values of the density of UO₂–ZrO₂ melts depending on the temperature and composition for any atomic ratio U/Zr.     

Temperature effects in the fracture of PMMA

Experiments are described in which the fracture toughness,K _c, of PMMA has been determined in the temperature range –190 to + 80° C and over the crack speed range of 10^–2 to 10² mm sec^–1. Single edge notch tension was used for instability measurements but the other data were obtained using the double torsion method. In the range –80 to + 80°C the variations inK _c may be described in terms of modulus changes and a constant crack opening displacement criterion. Crack instabilities are correlated with isothermal-adiabatic transitions at the crack tip. Below –80° C there is an inverted rate dependence associated with thermal effects during post-instability crack propagation.     

Dielectric properties and polarizability of molybdenum tellurite glasses

The dielectric behaviour of the [TeO₂]_1–x [MoO₃] _x , x=0.2, 0.3 and 0.45 mol%, glassy system is reported for the temperature range 300–573 K and the frequency range of 0.1–10 kHz. Both the static and high frequency dielectric constants for these binary tellurite glasses decrease with increasing MoO₃ content. The temperature dependence of the dielectric constants of these glasses are positive. The frequency dependence of the dielectric constant identifies a frequency dependence which does not show a flattening at low frequency. The room temperature static dielectric polarizability is discussed in terms of the MoO₃ concentration. The temperature dependence of the dielectric constant has been analysed in terms of the temperature changes of both volume and polarizability and also a volume change of the polarizability.     

Transport properties of InSe x flash evaporated thin films

Thin films of InSe _x were obtained by vacuum evaporation of polycrystalline materials onto substrates at moderate temperatures,T _s. Electrical properties of films grown from different stoichiometries of flash source materials are reported in this work. The temperature dependence of the conductivities shows two conduction regimes. The low temperature regime exhibits aT ^–1/4 conductivity dependence which fits well, using the Mott model, with an average localized states density value ofN(E _F ) 8×10¹⁸cm^–3eV^–1. Hall measurements as a function of temperature show that the predominant conduction mechanism is scattering by grain boundaries in polycystalline films.     

Activated hopping of the hydroxide ion through electrodeposited thin films of electrochromic cobalt oxyhydroxide

Thin films of cobalt(II) oxyhydroxide on platinum have been prepared by electrodeposition. The chemical diffusion coefficient D of the hydroxide ion through the material has a marked temperature dependence, showing a simple activated behaviour in the range 273–292 K. The activation energy to ionic movement was rather large at 54.5 kJ mol^–1 which, it is proposed, is ascribable to ion hopping. When spins have changed alignment, at the Néel transition temperature T _Nr the activation energy more than doubled to 125 kJ mol^–1; and the diffusion coefficients obtained above this temperature also increased dramatically. It is proposed that structural changes occur in the electrodeposited oxide at about T _Nr hence the change in activation energy.     

Magneto-Transport Properties of p-Type Zn1?xMnxTe:N Grown by MBE

The magneto-transport properties of nitrogen-doped p-Zn_1–x Mn _x Te films grown by MBE were studied, with an emphasis on the dependence on the Mn composition and the hole concentration. The Mn composition x and hole concentration p were varied in the range of x = 0.026–0.068 and p = 8.2 × 10¹⁸–2.6 × 10¹⁹ cm^–3. In the measurement of magnetoresistance (MR), a large negative component was observed at low temperatures. In the dependence on the Mn composition, it was found the magnitude of the negative component of MR became larger and survived up to higher temperatures in the sample with a higher Mn composition. The observed negative MR is attributed to the localization of holes due to the magnetic polaron formation. The origin of the dependence on the Mn composition is discussed.     

Thermal properties of solid ND3

The heat capacity (2–197 K), thermal expansion (23–160 K), and thermal conductivity (30–120 K) of solid deuteroammonia have been studied, and the enthalpy of fusionH _tr and temperatureT _tr corresponding to the triple point of ND ₃ determined. Comparison is given of the thermal properties of NH ₃ and ND ₃ , and the temperature dependence of the properties studied is discussed.     

The influence of surface kinetics in modelling chemical vapour deposition processes in porous preforms

The isothermal chemical vapour infiltration (ICVI) process is a well known technique for the production of composites and the surface modification of porous preforms. Mathematical modelling of the process can provide a better understanding of the influence of individual process parameters on the deposition characteristics such as final porosity or deposition profiles in the pore network. The influence of different rate expressions for several binary compounds on the ICVI process is discussed. Experimental work is used to validate the importance of correct kinetic expressions in a continuous ICVI model for cylindrical pores. The predicted infiltration characteristics are compared with experimental results. The final densification and Thiele modulus, i.e. a number which is a measure for the diffusion limitations in a pore, are used for the evaluation of the presented model, and conditions are given for an optimal densification of a porous preform by the ICVI process for several binary compounds. The deposition profiles as predicted by the model calculations are in agreement with the experimentally determined deposition profiles of TiN and TiC in small tubes. Moreover, it can be concluded that the shape of the deposition profiles is determined by the heterogeneous reaction kinetics. There is only a qualitative agreement between the predicted densification and measured densification for the synthesis of TiN and TiB₂ in sintered porous alumina. This mismatch can be explained in terms of a complexity of the pore network and differences in reaction kinetics. Model calculations reveal that there is a scattering for the predicted residual porosity as a function of the Thiele modulus for TiN. Moreover, this Thiele modulus can not fully account for the changes in densification at different temperatures. Given these uncertainties it is likely that a residual porosity of less than one percent can be obtained if the Thiele modulus is smaller than 1 × 10^–4. However, a CVI process with such a small Thiele modulus will not be practical, because of the concomitant long process times. Therefore, more precise conditions for the individual process parameters, i.e. concentration, reactor pressure, and temperature are deduced from the model calculations.Nomenclature a, b, c reaction order constants - C _i(x, t) concentration of species i at axial position x and time t (mole m^–3) - C _i ^o bulk concentration of species i (mole m^–3) - C _i ^* (x, t) dimensionless concentration of species i at axial position x and time t - D _e(x, t) effective diffusion coefficient at axial position x and time t (m²s^–1) - D _ij(x, t) binary diffusion coefficient (m²s^–1) - D _K(x, t) Knudsen diffusion coefficient at position x and time t (m²s^–1) - F correction factor for effective diffusion coefficient - k growth rate constant (ms^–1(m³mole^–1)^a+b-1) - K _i adsorption-desorption equilibrium constant (m³mole^–1) - L length of a pore (m) - M _i molecular weight of species i (g mole^–1) - M _ij harmonic mean of the molecular weights of species i andj (g mole^–1) - M _s molecular weight of deposit (g mole^–1) - m _t measured mass increase (g) - n _i stoichiometric number - P reactor pressure (Pa) - R(C _i) growth rate (mole(m^–2s^–1)) - r(x, t) pore radius at position x and time t (m) - r ^o initial pore radius (m) - r ^* dimensionless pore radius - S geometrical surface area (m²) - s ^t fraction of free titanium sites at the surface of TiN - s ⁿ fraction of free nitrogen sites at the surface of TiN - T temperature (K) - t time (s) - t _p process time (s) - U K _HCl/(K _{H 2} C _{H 2})^1/2 (m³ mole^–1) - V volume of alumina substrate (m³) - W K _TiCl3(m³ mole^–1) - X volume of infiltrated deposit relative to initial pore volume - x axial distance (m) - x ^* dimensionless axial distance - z number of time steps - dummy variable for integration - porosity of sintered porous alumina substrate - ratio of the volume over the surface area perpendicular to the flux (m) - density deposit (kg m^–3) - _ij a characteristic length (Å) - tortuosity factor of substrate - Thiele modulus - _D collision integral     

On the analysis of the thermal diffusivity measurement method with modulated heat input

The present paper proposes a simplified way to analyze thermal diffusivity experiments in which the phase shift is measured between the modulations of the temperatures on either face of a disk-shaped sample. The direct application of complex numbers mathematics avoids the use of the cumbersome formulae which hitherto have hampered a wider confirmation of the method and which restricted the range of the phase lag to an angle of 180°. The algorithm exposed makes it more practical to refine the analysis, which may lead to a higher accuracy and a wider use of the method. The origins of some possible errors in the calculated results are briefly reviewed.Nomenclature a Thermal diffusivity, m² · s^–1 - c Index denoting a constant part, dimensionless - c _l, c ₀ Inverse extrapolation length, m^–1 - C _p Specific heat, J · kg^–1 · K^–1 - f Modulation frequency, Hz - l Thickness of disk-shaped sample, m - Q _c Equilibrium energy per unit surface deposited on surface x=l, W · m^–2 - Q _m(t) Energy of modulation per unit surface deposited on surface x=l, W · m^–2 - Q(t) Total energy per unit surface deposited on surface x=l, W · m^–2 - q Complex energy modulation amplitude, W · m^–2 - T _l Equilibrium temperature of heated surface, K - t ₀ Equilibrium temperature of nonheated surface, K - T(x, t) Total temperature of any plane at distance x and at time t, K - T _m(x, t) Modulation temperature at any distance x and at time t, K - t Time, s - x Distance perpendicular to the specimen's surface and with the nonheated surface as the reference, m - Thermal linear expansion coefficient, dimensionless - Intermediary parameter, m^–2 - Phase difference between heated and nonheated specimen face, radian - ₀ Phase difference between energy modulation and nonheated face, radian - _l Phase difference between energy modulation and heated face, radian - Total emissivity, dimensionless - _s Spectral emissivity, dimensionless - Temperature, amplitude of modulated part argument, K - Thermal conductivity, W · m^–1 · K^–1 - Density, kg · m^–3 - Stefan-Boltzmann constant, 5.66961×10^–8W · m^–2 · K^–4 - Angular frequency=2f, s^–1     

Cross Sections of Excitation of Singlet F-Series of Barium Atom by Electron Impact

The excitation of singlet F-levels of barium atom by electron impact is experimentally studied. Manifestations of strong perturbations in the 6s5d ¹ D ₂–6snf ¹ F ^º ₃ spectral series are observed. The dependence of the excitation cross sections on the principal quantum number of the upper level in the 6s5d ¹ D ₂–5dnp ¹ F ^º ₃ spectral series follows the power law in the range n = 6–8. The obtained results are compared with those of the previous experiment.     

Applications of Judd–Ofelt theory to praseodymium and samarium ions in phosphate glass

The Judd–Ofelt (J–O) theory has had a remarkable success in the characterization of radiative transitions in lanthanide doped solids. The purpose of this paper is to review the applications of the J–O theory to the Pr³⁺ and Sm³⁺ ions and to use this system as an occasion to appraise its validity and to clarify its limits. In this paper we dwell at length on the absorption and luminescence measurements of two glass samples doped with Pr³⁺ and Sm³⁺, because they are basic for the J–O treatment. The results obtained for the J–O theory application to phosphate glasses doped with Pr³⁺ and Sm³⁺ present two undesirable outcomes: (1) a positive value of parameter Ω₂ and (2) large uncertainties with which the three Judd–Ofelt parameters were obtained. The validity of the J–O theory for intensity analysis was also tested for Sm³⁺ doped in phosphate glass. The resulting Ω₂ was much lower than Ω₄. The first set parameters were obtained using all the levels for which f_exp. was available. The second set parameter values were evaluated without the ⁶F_1/2 and ⁶H_15/2 levels. The Ω_2,4,6 values given in these two sets clearly suggest that particular care should be taken while evaluating the Judd–Ofelt parameters as well as when these parameters are compared to other systems due to their strong dependence on the nature of levels.     

Physical properties of a new cuprate superconductor Pr2Ba4Cu7O15−δ

We present studies of the thermal, magnetic, and electrical transport properties of reduced polycrystalline Pr₂Ba₄Cu₇O_15−δ (Pr247) showing a superconducting transition at T_c=10–16 K, and compare them with those of as-sintered non-superconducting Pr247. The electrical resistivity in the normal state exhibited T² dependence up to approximately 150 K. A clear specific heat anomaly was observed at T_c for Pr247 reduced in a vacuum for 24 h, proving the bulk nature of the superconducting state. By the reduction treatment, the magnetic ordering temperature T_N of Pr moments decreased from 16 to 11 K, and the entropy associated with the ordering increased, while the effective paramagnetic moments obtained from the DC magnetic susceptibility varied from 2.72 to 3.13μ_B. The sign of Hall coefficient changed from positive to negative with decreasing temperature in the normal state of a superconducting Pr247, while that of the as-sintered one was positive down to 5 K. The electrical resistivity under high magnetic fields was found to exhibit T^α dependence (α=0.08–0.4) at low temperatures. A possibility of superconductivity in the so-called CuO double chains is discussed.     

High-Temperature Limit for the O2-Catalyzed Ortho-Para Conversion Rate in Solid Hydrogen

Ortho-para conversion in solid hydrogen doped with small concentration of molecular oxygen (100 and 1000 ppm) was studied by pulsed NMR in the temperature range 4.2–10.9 K. Conversion rate was mainly determined by the paramagnetic oxygen impurities. The catalyzed conversion rate constant increases with temperature in the range 4.2–8 K and becomes independent of temperature above 8 K. The high-temperature limit is 1.15(8) s ^–1 . The catalyzed conversion rate was shown to become temperature independent when the ortho-H ₂ diffusion time _d falls below the conversion time _cc in the first coordination sphere of O ₂ impurity. The obtained value of _cc =10.5(8) s was used to identify the position of O ₂ in H ₂ lattice as substitutional. The temperature dependence of the ortho-H ₂ diffusion rate near the high temperature limit was determined.     

Internal friction in Se-Sb glasses

The internal friction,O ^–1, as a function of temperature has been investigated for Se_100–x Sb _x systems, wherex=7.5, 12.5 and 17.5 at%, using the magnetostrictive pulse-echo method in the range of vibrational frequencies 50–100 kHz. Two well-defined peaks appeared, characterizing this glass. The first peak covers the temperature range 320–330 K, where the peak position has been shifted to the higher temperature as Sb content increases. This peak indicates the glass transition temperature,T _g, of the tested glassy samples. The second peak was detected in the temperature range 360–378 K. The position of this peak has been shifted to a lower temperature as Sb content increases. This peak is attributed to the crystallization temperature,T _c, of each glassy sample tested. The peak height of both of the above peaks decreased as the Sb content increased. Also, the appearance of these two peaks was affected by annealing.