Electrical and microstructural properties of Bi2−x Sb x Te and Bi2−x Sb x Te2 foils obtained by the ultrarapid quenching process

This paper is concerned with the feasibility and reproducibility of the ultrarapid quenching process used to fabricate Bi_2–x Sb _x Te and Bi_2–x Sb _x Te₂ alloys for thermoelectric applications. Microstructural properties of the materials, obtained in the shape of foils, were studied concerning the phase analysis, cell parameters, texture, and microstructure observations. The Bi_2–x Sb _x Te alloys were found to have the (2 0 3) texture. The (2 0 4) texture, with an additional (1 1 0) component for x values greater than 0.4, was predominant for Bi_2–x Sb _x Te₂ foils. The electrical properties of these materials were then characterized by measuring the Seebeck coefficient, Hall coefficient, and electrical resistivity. It was found that Bi_2–x Sb _x Te foils changed from n- to p-type for an x-value of about 1.2. A maximum Seebeck coefficient, ||, of 36×10^–6 V K^–1 was measured for Bi₂Te. In the case of Bi_2–x Sb _x Te₂ foils, the change from n- to p-type was observed for an x value of about 1. A maximum Seebeck coefficient, ||, of 32×10^–6 V K^–1, was measured for Bi_1.4Sb_0.6Te₂. Measurements of the temperature-dependent electrical resistivity, Hall and Seebeck coefficients of the foils were carried out and the analysis revealed a semi-metallic behavior.     

Non fermi liquid ground states in strongly correlated f-electron materials

Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for three f-electron systems: M_1–xU_xPd₃ (M = Sc, Y), U_1–xTh_xPd₂Al₃, and UCu_5–xPd_x. The emerging systematics of NFL behavior in f-electron systems, based on the present sample of nearly ten f-electron systems, is updated. Many of the f-electron systems exhibit the following temperature dependences of the electrical resistivity p, specific heat C, and magnetic susceptibility for T T₀, where T_o is a characteristic temperature: P(T) 1 –aT/T ₀, where a < 0 or > 0, C(T)/T (-1/T _o) In (T/bT ₀), and (T) 1 –c(T/T_o)^1/2. In several of the f-electron systems, the characteristic temperature T_o can be identified with the Kondo temperature T_k.     

Heat transfer in plane film formation

A mathematical model is obtained for the process of cooling with formation of a planar film. The solution obtained is verified experimentally.Notation mean axial velocity gradient - v_x current axial velocity - v_o initial polymer velocity - v₁ sampling velocity - K draw ratio - deformation rate tensor - x, y, z spatial coordinates - X, Y dimensionless coordinates - L() differential operator - T temperature - T_o initial temperature - T_c temperature of surrounding medium - dimensionless temperature - dimensionless temperature averaged over film thickness - thermal-diffusivity coefficient - 2_o initial film thickness - thermal conductivity - heat-transfer coefficient - f(X) distance function - Bi Biot criterion, Bi_o, Biot criterion calculated for initial film thickness - Gz* modified Graetz criterion - V dimensionless velocity - ₁, ₂, ₃ heat-transfer coefficients produced by radiation, free convection, and forced convection - v_c, _c mean velocity and film half-thickness in formation zone - T₁ calculated temperature value - T₂ experimental temperature value - l formation zone length Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 5, pp. 854–858, November, 1979.     

Electrical properties of BaSnO3 in substitution of antimony for tin and lanthanum for barium

Polycrystalline materials of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– were prepared. Substitutional solubilities of antimony for tin and lanthanum for barium, respectively, in BaSnO₃ were obtained to be x=0.18 for BaSn_1–x Sb _x O_3– and y<0.052 for Ba_1-y La _y SnO_3–. The X-ray photoemission spectroscopy measurements showed the valence of antimony and tin is mixed in our samples of BaSn_1–x Sb _x O_3–. At lower temperature, magnetic susceptibilities of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– satisfy the Curie law, indicating the existence of non-interacting localized electrons at the Sn⁴⁺ site, and forming a Sn⁴⁺+e^– state in these systems. By substitution of antimony and lanthanum in BaSnO₃, the conductive properties are semiconductor-like. To explain this conductive behaviour, three types of mechanism were taken into consideration.     

Flow visualization glass-ceramic: Preliminary experimental and modelling results

A glass-ceramic material was developed to act as a flow visualization material. Preliminary experiments indicate that aperiodic, thermally induced, convective flows can be sustained at normal processing conditions. These flows and the stress and temperature gradients induced are most likely responsible for the anomalous behaviour seen in these materials and the difficulties encountered in their development and in their production on industrial and experimental scales. A simple model describing the dynamics of variable-viscosity fluids was developed and was shown to be in qualitative agreement with more sophisticated models as well as with experimental results. The model was shown to simulate the dependence of the critical Rayleigh number for the onset of convection on the viscous properties of the fluid at low T, and also to simulate quenching behaviour when the temperature differences were high.Nomenclature C _p Heat capacity - D, E, F Expansion coefficients - H Height of the roll cell - Pr Prandtl number - R _a Rayleigh number - R _c Critical Rayleigh number for the onset of convection in a constant-viscosity fluid - S Dimensionless stream function - T Temperature - T _m Mean temperature - T ₀ Bottom surface temperature - T _r Reference temperature - a Aspect ratio of cell - g Acceleration due to gravity - k Thermal conductivity - k ₁ Function related to ²v/T ² - k ₂ Function related to ⁴v/T ⁴ - r Rayleigh number ratioR _a/R _c - t Time - w Dimensionless vertical coordinate - w _m Mean cell height - x Horizontal coordinate - y Dimensionless horizontal coordinate - z Vertical coordinate - , Constants - _t Thermal expansion coefficient - Constant in viscosity function - T Temperature difference between top and bottom surfaces - _i Viscosity coefficients - Kinematic viscosity - _m Mean kinematic viscosity - Dimensionless kinematic viscosity - Thermal diffusivity - Non-linear temperature function - Dimensionless non-linear temperature function - _o - Stream function - Dimensionless time - Eigenvalues     

On strong Dickson pseudoprimes

It is known that the Lucas sequenceV _n(,c)=aⁿ + bⁿ,a, b being the roots ofx ² – x + c=0 equals the Dickson polynomial .^n–2i Lidl, Müller and Oswald recently defined a number b to be a strong Dickson pseudoprime to the parameterc (shortlysDpp(c)) if [itg_n(b, c)b modn for all b. These numbers seem to be very appropriate for a fast probabilistic prime number test. In generalizing results of the above mentioned authors a criterion is derived for an odd composite number to be ansDpp(c) for fixedc. Furthermore the optimal parameterc for the prime number test is determined.     

Transport properties of helium near the liquid-vapor critical point. II. Thermal conductivity of a3He-4He mixture

Measurements of the thermal conductivity are reported for an 80%³He-20%⁴He mixture above the critical point along several isotherms and near-critical isochores, using the same techniques and apparatus described for a study of³He. Using again the assumption that the observed conductivity can be decomposed into a sum of a regular and a singular contribution _reg and _sing, it is shown that along two near-critical isochores, _sing diverges. In particular, along the isochore showing the largest at the phase transition, the divergence is nearly the same as for³He and can be roughly characterized by a simple power law (T-T_c)^– with 0.58. This observation is contrary to predictions that foresee _sing0 asT _c is approached. The relaxation times characterizing the attainment of steady state conditions after switching the heat flux on and off show a similar behaviour as a function of reduced temperature as do those for pure³He. This result might indicate a substantial coupling between concentration and entropy diffusion. In the Appendix, the correlation length for³He nearT _c is calculated from heat conductivity, viscosity, and specific heat data.Work supported by Grant DMR 8024056 of the National Science Foundation.     

Microstructural,electrical and magnetic properties of NdBa2Cu3O7−δ prepared from BaO2

NdBa₂Cu₃O_7– was synthesized in oxygen from a stoichiometric mixture of Nd₂O₃, BaO₂ and CuO between 890 and 1070 C. The resulting material was composed of small crystals of NdBa₂Cu₃O_7– (2 to 10 m) and of BaCuO₂ as a secondary phase. The effect of the synthesis temperature on the normal-state resistivity, the critical current density and the magnetic susceptibility was investigated. The highestJ _c values (175 A cm^–2) were measured in samples treated between 1010 and 1050 C.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

Growth and Properties of Double-Sided YBCO Films Deposited by Off-Axis Sputtering for Microwave Applications

The double-sided YBa₂Cu₃O _y (YBCO) films were grown on LaAlO₃ (LAO) substrates by an off-axis magnetron sputtering system with two 2-inch sputtering guns. High-quality YBCO films within a 50-mm-diameter area were obtained. The homogeneous YBCO films revealed transition temperature T _c(R=0) of 86K and a critical current density J _c (zero field) of 1.5×10⁶ A/cm² at 77 K. Thesurface impedance of YBCO films was measured using a probe-coupling type microstripline resonator method. The surface resistance R _S of 1.8 was obtained at 77 K around 2180 MHz. Additionally, a HTS three pole bandpass filter was fabricated and revealed a good microwave characteristic. The influence of surface roughness on microwave properties of YBCO films was discussed.     

Metal-Insulator Transition and Cu K-Edge XANES Studies for Pr1.85Ce0.15CuO4+δ High-Tc Superconductors

Metal-insulator transition near oxygen content parameter –0.018 was observed for electron-doped Pr_1.85Ce_0.15CuO₄₊ (–0.0030.03) cuprates. Cu K-edge X-ray absorption near-edge structure (XANES) studies with nearly identical threshold edge energy E₀ of 8980.8 eV indicate a Cu formal valence smaller than 2 for all samples, which is consistent with the estimated Cu valence of 1.84 for 20.5 K superconductor Pr_1.85Ce_0.15CuO_3.997 and 1.91 for Pr_1.85Ce_0.15CuO_4.03 insulator. The XANES spectrum reflects the Cu 3d ⁿ character where low energy peak A ₁ reflects the 3d ¹⁰ configuration of Cu(I) oxidation state and A ₂ peak reflects the 3d¹⁰ ( for a oxygen ligand hole) configuration for Cu(II) oxidation state. The variation of energy separation E(A ₂–A ₁) is consistent with the observed metal-insulator transition, increases sharply from 2.42 eV for Pr_1.85Ce_0.15CuO_4.018 insulator to 2.74 eV for 15 K underdoped superconductor Pr_1.85Ce_0.15CuO_4.015.     

Theory of periodically driven,current-carrying,superconducting filaments. I. Spatially homogeneous states

Starting from the nonequilibrium theory of dirty superconductors in the Ginzburg-Landau regime, spatially homogeneous states with an applied currentI=I ₀+I ₁ cos (t) are considered. Expressions for the linear response (I₁ small) valid up to high frequencies (k _BT_c) are derived and evaluated analytically for the experimentally important case of smallI ₀ and ₀(T). Then the nonlinear response is treated for frequencies with _E1. Interesting new behavior is found for frequencies ₀ 1, where ₀ is essentially the GL relaxation time.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

Application of a new iterative method for elastic-plastic stress analysis

A new iterative method for elastic-plastic stress analysis based on a new approximation of the constitutive equations is proposed and compared with standard methods on the accuracy and the computational time in a test problem. The proposed method appears to be better than the conventional methods on the accuracy and comparable with others on the computational time. Also the present method is applied to a crack problem and the results are compared with experimental ones. The agreement of both results are satisfactory.List of symbols u = (u ₁, u ₂) displacements u ^(H) = u ⁽ⁿ⁺¹⁾ - u ⁽ⁿ⁾ u _k ⁽ⁿ⁾ = u _(k ^{(n + 1)} - u ⁽ⁿ⁾ (n, k = 0, 1, 2, ...) - = ₁₁, ₂₂, ₁₂) stresses - = (₁₁, ₂₂, ₁₂) strains - = (₁₁, ₂₂, ₁₂) center of yield surface - D elastic coeffficient matrix, C = D ^–1 - von Mises yield function. The initial yielding is given by f() = _Y - f {f/} - ^* transposed f - H hardening parameter (assumed to be a positive constant for kinematic hardening problems) - time derivative of - [K] total elastic stiffness matrix - T traction vector - = [B] relation between nodal displacements and strains     

Parafluidity in helium near the λ-transition and the generalized time-dependent Landau theory

A phenomenological theory of parafluidity, i.e., an enhancement of fluidity due to order-parameter fluctuations, is presented for helium near the transition. The generalized time-dependent Landau theory of second-order phase transitions is reviewed in general and is applied to the superfluid transition in helium as a particular example. In helium, it is found that parafluidity is manifested in the divergences of the mass diffusivity , the thermal conductivity , the first-sound amplitude attenuation ||^–1, and the second-sound dampling , which are all consistent with the dynamic scaling hypothesis. Here a characteristic relaxation time ₀ ||^–1 is used, where =(T–T _c )/T _c andT _c is the transition temperature. Although there are not enough experimental data to confirm our formulas, the present approach is seen to agree in order of magnitude with available experiments. Finally, the sound absorption above a ferromagnetic transition is calculated by adding a diffusion term to the generalized time-dependent Landau equation. The result thus obtained agrees in order of magnitude with experiments in nickel.Supported in part by the National Science Foundation and the Horace H. Rackham School of Graduate Studies.     

Compressive creep of Si3N4/MgO alloys

The compressive creep behaviour of four compositions within the Si₃N₄-Mg₂SiO₄-Si₂N₂O compatibility triangle were studied in air at 1400° C. Strain rate ( ) versus stress () was analysed to determine the stress exponent, n ( ). Cavitation during creep was determined by precise (sink-float) density measurements. Compositions close to the Si₃N₄-Si₂N₂O tie line exhibited no cavitation and had n1, whereas compositions close to the Si₃N₄-Mg₂SiO₂ tie line exhibited extensive cavitation and had n2. Test results are interpreted in terms of the volume fraction of the viscous phase present.     

Creep of CoO single crystals

Cobalt monoxide single crystals having a [100] orientation were creep tested in compression over ranges of temperature, stress and oxygen pressure. The creep curves were S-shaped and only the inflection creep rate, ₂, was analysed. In the range of 1000 to 1200° C, 850 to 1700 psi and 10^–3 to 1 atm oxygen, ₂ was given by ₂=A po ₂ ^{0.45 7.1}exp(– Q _c /RT) where Q _c =87±6 kcal/mol at 0.01 atm O ₂ and 100±16 kcal/mol at 1 atm O ₂. Slip occurred on two orthogonal {011} 0¯11 slip systems. The presence of subboundaries was observed by optical and transmission electron microscopy. It is suggested that the creep rate is controlled by oxygen diffusion.     

Fractographic analysis of vitreous calcia-alumina eutectic fibres produced by inviscid melt spinning (IMS)

The mechanical properties of inviscid melt spun (IMS) CaO-Al₂O₃ (46.5 wt % CaO-53.5 wt % Al₂O₃) eutectic fibres were examined by fractographic analysis as well as four-point bending and micro-indentation. The averaged fracture strength and elastic modulus values of the IMS Calcia-Alumina (CA) fibre were determined to be 460 MPa and 99.8 GPa, respectively by using four-point bending tests. The inner mirror constant (M) was determined to be 2.39 MPa·m^1/2 by using the plot of the fracture strength (_f) obtained from the bending tests as a function of r ^–1/2, where r is the inner mirror radius measured from scanning electron microscopy (SEM) on the fractured CA fibres. The flaw-to-mirror ratio (c/r) for the CA fibre was calculated to be 111.24. Also, the critical flaw size (c) of the CA fibre was estimated to be 2.35 m. The averaged elastic modulus value from Knoop micro-indentation was determined to be 102.5 GPa which is in good agreement with that from the four-point bending tests.     

The formation of metal antimonates by mechanical milling and the conversion of α-Sb2O4 to β-Sb2O4

The mechanical milling of -Fe₂O₃ or V₂O₅ with Sb₂O₃ results in the formation of small particle FeSbO₄ or VSbO₄. Milling also induces the conversion of cubic Sb₂O₃ to the orthorhombic modification and the transformation of -Sb₂O₄ to -Sb₂O₄. The milling-induced - to -Sb₂O₄ phase transition is even more facile in the presence of vanadium or iron.     

Optimal choice of descent steps in gradient methods of solution of inverse heat-conduction problems

Modifications are proposed for the methods of steepest descent and conjugate gradients for the solution of multiparameter inverse problems in heat conduction.Notation A, B, L linear operators - u element of the solution space U - f exact initial data - f error in the initial data - value of the error in the initial data - A^–1 inverse operator - u^(k)() k-th derivative of the function u - _i() polynomials of degree i–1 - A^*, B^*, L^* operators conjugate to the operators A, B, L - J(g) discrepancy functional - J'g gradient of the discrepancy functional - _n ⁱ depth of descent with respect to the i-th component of the antigradient of the discrepancy in the n-th iteration - _m length of the observation interval Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 2, pp. 264–269, August, 1980.