Ga as an additive in the As2Te3 glass

Results of measurements of the mean atomic volume (V), the glass transition temperature (T _g), the activation energy for glass transition (E _t) and the d. c. electrical conductivity () are reported and discussed for ten glass compositions of the Ga–As–Te system. The glasses studied can be represented as Ga _x (As_0.4Te_0.6)_100–x glasses, with the additive Ga ranging from 0 to 12 atomic percent (at.%) in the parent As₂Te₃glass. In the Ga _x (As_0.4Te_0.6)_100–x glasses, changes in slope are observed in the V, T _g, E _t, and other electronic properties, at the composition with a Ga content of 2 at.%. The results are compared with those obtained on introduction of Ag and Cu to the As₂Te₃and the [0.5As₂Te₃–0.5As₂Se₃] glasses. Analysis of the data suggest formation of GaAs, Ga₂Te₃and excess Te structural units (s.u.) in lieu of some of the original As₂Te₃s.u., for addition of Ga up to 2 at.% to the parent As₂Te₃glass; for higher Ga contents, formation of GaAs, GaTe and excess Te s.u. are indicated.     

Structural and optical properties of CdGa2Se4 thin films

CdGa₂Se₄ thin films were prepared by vapour deposition onto either room temperature or preheated quartz and glass substrates (T _S) or they were deposited at room temperature and then annealed at about (T _A) 623 K. The films thus prepared were crystalline with a thiogallate tetragonal structure. The optical constants (the refractive index n and the absorption index, k) were determined for CdGa₂Se₄ films deposited onto quartz substrates held at either room temperature or at T _S = 573 K. These constants were also determined for preannealed films (T _A = 623 K). Plots of (hv)² = f(hv) and (hv)^1/2 = g(hv) were linear, indicating the existence of both direct and indirect optical transitions. It was found that the values of E _g ^d and E _g ^ind for as-deposited CdGa₂Se₄ films were 2.46 and 1.91 eV, respectively. The corresponding values for the annealed films and the films deposited at T _S = 573 K were 2.56 and 2.06 eV, respectively.     

Kinematical studies of the glass transition in glassy Se80 ? xTe20Sbx

The present paper reports kinematic studies of the glass transition in glassy Se_80-xTe₂₀Sb_x alloys using the differential scanning calorimetric technique. The glass transition temperature (T_g) is found to increase monotonically with the heating rate. T_g also increases with the increase of Sb concentration in ternary Se_80-xTe₂₀Sb_x system. From the heating rate dependence of glass transition temperature, the activation energy ( E_t) for the relaxation time controlling the structural enthalpy, is calculated. The composition dependence of T_g and E_t is discussed in terms of the structure of the Se-Te-Sb glassy system.     

Ultrasonic attenuation of longitudinal and shear waves in superconducting lead

Longitudinal and shear wave ultrasonic attenuations have been measured in high-purity Pb on two single crystals obtained from the same ingot. The measurements were done at low temperatures, at different frequencies, and in transverse magnetic fields, up to a field of 7.3 kG. The propagation directions in the two crystals were along [100] and [110]. For some propagation and polarization directions the _s / _n ratio is found to be frequency-independent, while for others, large divergences in the _s / _n ratios at different frequencies are observed. A sharp decrease of _s / _n nearT _c is observed for a particular longitudinal wave propagation, but not in any shear wave propagation. In some cases _s / _n is found to be abnormally high and this feature is associated with a peak in attenuation _n and a relatively high _n at 7.2 K. None of the _s / _n curves fits closely to any BCS energy gap. For longitudinal waves the high magnetic field (H) dependence of the normal state attenuation was found to agree qualitatively with the free electron theory for propagation along [100], but not for propagation along [110]. For shear waves the high-field attenuations do not extrapolate to zero asH tends to infinity. For all propagation and polarization directions the high-field attenuations show 1/H ² field dependence.     

Evolution of transient thermal processes in layer counterflow apparatuses and heat exchangers

Results are given of an analytic investigation of transient processes inside counterflow apparatuses and heat exchangers with temperature disturbance in one of the heat carriers at the entry to the apparatus.Notation =(t–t₀)/(T₀–t₀),=(T–t₀)/(T₀ s-t₀) relative temperatures - t, T temperatures of material and gas respectively - t₀, T₀ same for the initial state - Z=[ _Vm₁/c(1–w/w_g)] [–(y₀–y)/w_g] dimensionless time - m₁=1/(1+Bi/) solidity coefficient - B₁=( _FR/) Biot number - _{F V} heat-exchange coefficients referred to 1 m² surface and 1 m³ layer - R depth of heat penetration in a portion - portion heat conductivity coefficient - shape coefficient (=0 for a plate,=1 for a cylinder,=2 for a sphere) - c, C_g heat capacities of material and gas respectively - , _g volumetric masses - w, W_g flow velocities of material and gas - y distance from the point of entry to the heating heat carrier - y₀ heat-exchanger length - Y= _Vm₁y/W_gC_{g g} dimensionless coordinate - m=cw/C_g– _gW_g water equivalent ratio Deceased.Translated from Inzhenerno-Fizicheskii Zhurnal, vol. 20, No. 5, pp. 832–840, May, 1971.     

Thermal Expansion and Isothermal Compressibility of TlIn1 – xNdxSe2 (0 ≤ x≤ 0.08) Crystals

The thermal expansion coefficient () and isothermal compressibility ( _T ) of TlIn_{1 – x} Nd _x Se₂(0 x 0.08) crystals were measured between 77 and 400 K. In the range 77–160 K, both and _T increase with temperature, the increase in being much steeper. At higher temperatures, and _T change very little. The observed composition dependences of and _T are interpreted in terms of energy-band structure.     

Isotope effect and tTc suppression in d-wave superconductors

We have studied the isotope effect on the superconducting transition temperatureT _c ofd-wave pairing instabilities due to antiferromagnetic spin fluctuation exchange. We found that models with conventional coupling toharmonic phonon degrees of freedom and reasonable limits on coupling strength and phonon spectra, give values for the isotope exponent which are at least one order of magnitude smaller than typical observed values of in the cuprates. Models with coupling to non-linear tunneling modes in a stronglyanharmonic multi-well lattice potential and realistic coupling strengths have effectively larger isotopic mass dependences andmay therefore account for the observed magnitudes of.This work was supported by the National Science Foundation (Grant DMR-8913878 and DMR-9215123) and by the University of Georgia Office of the Vice President for Research. Computing support from UCNS, University of Georgia, and NCSA, University of Illinois, is gratefully acknowledged.     

Synthesis of amorphous and metastable Ti40Al60 alloys by mechanical alloying of elemental powders

Ti₄₀Al₆₀ amorphous and metastable alloys have been prepared by mechanical alloying (MA), under controlled milling conditions in a planetary mill. Three different quantities of kinetic energy at the collision instant have been achieved by using balls of different size, _b = 5, 8 and 12 mm, keeping constant all other device parameters. Assuming the collision between the balls and the vial walls to be inelastic, during the early stage of alloying, the amount of energy transferred to the trapped powder could be estimated. The experimental results show that the milling with balls of diameter _b = 5 or 8 mm leads to a solid-state amorphization of the Ti₄₀Al₆₀ mixture, through the attainment of a supersaturated solid solution of aluminium into -titanium. Otherwise, the milling causes the nucleation of the A1-fcc disordered form of the TiAl intermetallic compound. The end products of MA-induced solid-state reaction (SSR) have been ascribed to the different temperature reached by the powder during each collision and to the reaction time scale for the formation of the amorphous phase, t _a, and for the nucleation of the non-equilibrium intermetallic compound, t _d. Differential scanning calorimetry has indicated that the crystallization of amorphous samples follows a two-step reaction. At a temperature T _c400 °C, the amorphous phase crystallizes into the A1 -fcc. TiAl phase having a measured heat of crystallization of 6.2 kJ(g at)^–1. Upon further heating, the system undergoes A1 L1_o reordering transition with an enthalpy release of about 3.2 kJ (g at)^–1.     

Temperature dependence and microwave enhancement of the critical current of superconducting microbridges

The temperature dependence of superconducting tin, thin-film, narrow bridges has been investigated. NearT _c the critical current varies with (1 – T/T_c ), with a distribution of values between 1 and 3/2. It is suggested that the values can be used to characterize the bridges. The microwave enhancement ofI _c andT _c was measured and these data were compared with a recent theory by Christiansenet al. on the microbridges. We found that their theory gives a correct prediction for the Dayem effect in the bulklike bridges (with =3/2) while their theoretical values for the junction-like bridges (with =1) are not in agreement with our data.Work supported in part by the National Science Foundation.     

Hexagonal (superlattice) form of Ga2Te3

Gallium telluride (Ga₂Te₃) was synthesized at different temperatures (850 to 460° C) using different cooling rates. Materials synthesized at higher temperatures (including quenched materials from the melt) always yielded zinc-blende lattice with well resolved _{1 2} doublet X-ray powder diffraction lines. In the material synthesized at lower temperature ( 460°C), we obtained additional (superlattice) lines as reported by Newman and Cundall [4]. It was possible to index these reflections not only on an orthorhombic unit cell (a=0.417, b=2.360, c=1.252 nm) but also on cubic (a=1.7678 nm) and hexagonal (a=0.832, c=3.065 nm) unit cells. To us, the hexagonal cell appears to be more realistic. If sufficient time is given to reach equilibrium, the whole of the zinc-blende form of Ga₂Te₃ is transformed to the hexagonal form. It has been further observed that conversion of the hexagonal into the cubic form and vice versa can be brought about by heating the material at temperatures greater or less than 460° C, respectively. Lastly, the zinc-blende phase of Ga₂Te₃ is metastable and slowly transforms to hexagonal form at room temperature.     

In situ electrical conductivity and amorphous-crystalline transition in vacuum-deposited thin films of Se20Te80 alloy

In situ electrical conductivity measurements have been carried out on vacuum-deposited thin films of Se₂₀Te₈₀ alloy during heating and cooling cycles. The electrical conductivity and X-ray diffraction studies show that the as-grown Se₂₀Te₈₀ films are amorphous and, upon heating, undergo an irreversible amorphous-crystalline transition between 315 and 350 K. The observation that the as-grown thin films (deposited at room temperature on glass substrates) are amorphous is in contrast to earlier observations by other workers who find that they are polycrystalline. Above the transition temperature, the electrical conductivity of the polycrystalline Se₂₀Te₈₀ films changes as an exponential function of reciprocal temperature. The amorphous-crystalline transition in Se₂₀Te₈₀ thin films is broad, with films of high initial resistance having lower transition temperatures and low-resistance films having higher transition temperatures. The observation of a broad transition in the case of the present Se₂₀Te₈₀ thin films has to be contrasted with our earlier observations of sharp transitions in the case of Se₈₀Te₂₀ and Se₅₀Te₅₀ thin films.     

Investigation of thermoelectric power in indium sesquitelluride(In2Te3) thin films

Polycrystalline and stoichiometric thin films of indium sesquitelluride (In₂Te₃) belonging to the -phase were prepared on glass substrates by flash evaporation technique at a constant temperature of 473 K. The thermoelectric power of these p-type -In₂Te₃ thin films was determined as a function of temperature of the hot end of the films and also of film thickness. It was found that the thermoelectric power is nearly independent of temperature and a possible reason for this behavior has been given. The dependence of the thermoelectric power on the reciprocal thickness of the films has also been discussed on the basis of the size effect theories.     

Thermal characterization of glassy Se70Te20M10 using DSC technique

Differential scanning calorimetry (DSC) has been employed to study the phase transformation in glassy Se₇₀Te₂₀M₁₀ (M = Ag, Cd, Sb) to understand the glass forming tendency (GFT) and rate of crystallization. The difference (T _c–T _g) (indicator of GFT and thermal stability) has been determined by DSC thermograms for each sample. The values of T _c and T _g are found to depend on GFT. The rate constant K (indicator of rate of amorphous to crystalline phase transformation) has been evaluated using method of Augis and Bennett. The parameter K is also found to be related with (T _c–T _g).     

Anisotropy of the superconductive to normal transition in tantalum-nitrogen single crystals

Systematic experiments on the transition to the normal state in superconducting tantalum single crystals doped with interstitially dissolved nitrogen impurities are presented. On the low- side of the magnetic phase diagram, where at some low temperature 0 <T ^* <T _ca transition from type I to type II superconductivity occurs, a new effect has been observed, as reported briefly elsewhere: because of the correlation ofH _{c 2} with crystal directions, at fixed temperatures nearT ^*, whether the sample is a type I or a type II superconductor depends on which crystal direction is oriented parallel to the external field. In the type II superconducting range all samples display anH _{c 2} anisotropy effect, which is commonly observed in cubic superconductors, even in the extreme dirty limit (~12). However, the second anisotropy coefficient (l = 6 component) is found to be negligibly small in all tantalum samples.     

Thermal properties of new molybdenum oxyfluoride glasses

New oxyfluoride glasses were prepared in the form MoO₃-BaF₂-xRF where RF is alkali fluoride (LiF, NaF and mixed NaF–LiF) with x = 5, 10, 15, 20, 25, 30 and 35 mol%. The densities of the studied glasses were measured and the molar volumes were calculated. Thermal properties like glass transition temperature T _g, the onset of crystallization temperature T _x , crystallization temperature T _c and melting temperature T _m were determined by using the differential scanning calorimetry (DSC) technique and from which the glass stability S and the glass forming tendency K _g were calculated. Values of thermal expansion coefficient and specific heat C _p of the present glasses were also measured. All the above properties are correlated with composition alkali fluoride RF content and structure of the glass.     

Relative diffusion resistance in drying wheat

An apparatus is described for examining various methods of convective drying.Notation tan=N_I drying rate in the first period - tan =(dW^c/d)_II drying rate in the second period - drying time - W _e ^c equilibrium water content - W^c water content of grain on dry mass - N^*=(1/N_I)(dW^c/d) dimensionless drying rate - T_sur surface temperature - T_a ambient temperature - T_w wet-bulb temperature - A,, experimental coefficients Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 31, No. 5, pp. 839–843, November, 1976.     

On the determination of the thermal diffusion factor from column measurements

A new method for experimental determination of the thermal diffusion factor _T for binary gas mixtures with a thermal diffusion column (TDC) is developed, based on A. M. Rozen's equation of TDC. The experimental results for _T are obtained in a reduced form in this approximation. An experimental reference point, determined in the same TDC with a standard gas mixture, is used for the transformation of the results for _T in absolute units. The proposed method is applicable for arbitrary gas mixtures, irrespective of the mass difference of the components.     

Seeding effects on crystallization temperatures of cordierite glass powder

The role of solid state epitaxy in the crystallization of nanocomposite cordierite glass to glass ceramic was investigated. The use of isostructural (-cordierite) seeds in cordierite glass led to a lowering in the crystallization temperature to form glass ceramic by about 50 °C compared to the unseeded glass. The use of non-isostructural seeds such as ZrO₂ and TiO₂ did not lower the crystallization temperature of cordierite glass to glass ceramic, and in the case of the TiO₂-seeded glass the crystallization temperature increased by about 50 °C compared to the unseeded-cordierite glass. The lowering in crystallization temperature by-cordierite seeding can be attributed to the nucleation and epitaxial growth mechanism.     

Specific heat measurements of intercalation compounds M x TiS2 (M=3d transition metals) using ac calorimetry technique

Specific heats of 3d transition metal intercalates of 1T-CdI₂-type TiS₂, M _x TiS₂ (M=V, Cr, Mn, Fe, Co, and Ni; 0x1), have been measured in the temperature range 1.6–300 K using an ac calorimetry technique. The electronic specific heat coefficient (2–100 mJ/mole K²) and the Debye temperature _D (240–430 K) are found to depend on the guest 3d metals and their concentrations. All the intercalates show anomalous specific heat at low temperatures following an – lnT dependence ( and are constants), as found in dilute alloys.     

Characterization of p-In2Se3 thin films

Indium selenide thin films were deposited onto glass substrates kept at 150 °C by thermal evaporation of -In₂Se₃. Some of the films were annealed at 150 °C and 200 °C and they all were found to exhibit p-type conductivity without intentional doping. Scanning electron microscopy (SEM) established that the films have an atomic content of In₅₁Se₄₉. X-ray diffraction (XRD) indicated that the as-grown films were amorphous in nature and became polycrystalline -In₂Se₃ films after annealing. The analysis of conductivity temperature-dependence measurements in the range 320–100 K revealed that thermal excitation and thermionic emission of the carriers are the predominant conduction mechanisms above 200 K in the amorphous and polycrystalline samples, respectively. The carrier transport below 200 K is due to variable range hopping in all the samples. Hall measurements revealed that the mobility of the polycrystalline films is limited by the scattering of the charged carriers through the grain boundaries above 200 K. © 2001 Kluwer Academic Publishers