Vibronic states in La{2-x}BaxCuO4BaxCuO4

The temperature dependences of the resistivity R(T) and the thermoelectric power α(itT) under different hydrostatic pressuresP for the system La_{2-x}BavCuO₄, 0.11 ≤ x < -1.6, are reported and interpreted as evidence for strong electron coupling to oxygen vibrations along the CuO-Cu bond axes of the CuO₂ sheets. The sensitivity of the transport properties to the bending Φ of the (180‡-Φ) Cu-O-Cu bond angle and their insensitivity to long-range magnetic order indicate that the segregation into hole-rich and magnetic stripe domains is driven primarily by electron-lattice interactions and not by electron-electron interactions alone.     

Study on hydration of Portland cement with fly ash using electrical measurement

The electrical resistivity method was used for measuring the electrical resistivity of cement paste incorporating with fly ash. The study found that the bulk electrical resistivity ρ(t) was a function of the solution electrical resistivity ρ0(t) and porosity Φ. A two-component model was proposed, in which ρ(t) was dominated by ρ0(t) at the early period of hydration and dominated by Φ at a later period. The porosity formation curve was derived from the 2-point method. A logarithmic equation ρ(t)=K^*In(t/t₀) was proposed to express the electrical resistivity development with time after hardening, where K represents the rate of hydration, and the pastes with fly ash had a lower K. The setting times and compressive strengths were tested and the results were compared with the electrical properties.     

Metricas riemanianas asociadas a M-divergencias

Resumen Tras plantear las métricas diferenciales asociadas a M-divergencias para funciones de densidad de probabilidad pertenecientes a la misma familia de funciones paramétricas, consideramos para las funciones Φ_α(t)=t ^α la relación entre las matrices que definen las métricas y las matrices α-informativas. Obtenemos, en segundo lugar, las funciones Φ(t) que determinan M-divergencias invariantes, a nivel diferencial, frente a cambios no singulares de parámetros y variables aleatorias. Observamos, finalmente, que las funciones que proporcionan invariancia aseguran un incremento en el valor de la distancia al a?adir variables aleatorias estocásticamente independientes.      

Revised heat capacity and thermodynamic functions of GdVO<Subscript>4</Subscript>

The heat capacity of GdVO₄ has been determined by adiabatic calorimetry in the range 5–345 K. The present experimental data and earlier results have been used to evaluate the thermodynamic functions of gadolinium orthovanadate (C _p ⁰(T), S ⁰(T), H ⁰(T) − H ⁰(0), and Φ⁰(T)) as functions of temperature (5–350 K). Its Gibbs energy of formation is determined to be Δ_f G ⁰(GdVO₄, 298.15 K) = −1684.5 ± 1.6 kJ/mol.     

Thermodynamic properties of Cu<Subscript>5</Subscript>SmSe<Subscript>4</Subscript>

The heat capacity of Cu₅SmSe₄ has been measured from 80 to 300 K. The results have been used to assess the main thermodynamic functions of Cu₅SmSe₄: entropy (S ⁰(T) − S ⁰(0)), enthalpy increment (H ⁰(T) − H ⁰(0)), and reduced Gibbs energy (Φ⁰(T)).     

Thermodynamic functions of ScVO<Subscript>4</Subscript> at temperatures from 0 to 350 K

The heat capacity of ScVO₄ has been determined by adiabatic calorimetry at temperatures from 14.52 to 347.13 K, and smoothed heat capacity data have been used to evaluate its thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy). At 298.15 K, the thermodynamic functions of scandium orthovanadate are C _p ⁰(298.15 K) = 110.5 ± 0.1 J/(mol K), S ⁰(298.15 K) = 110.9 ± 0.1 J/(mol K), H ⁰(298.15 K) − H ⁰(0) = 18.53 ± 0.01 kJ/mol, and Φ⁰(298.15 K) = −[G ⁰(298.15 K)/298.15] = 48.75 ± 0.12 J/(mol K). The calculated Gibbs energy of formation of scandium orthovanadate from its constituent elements is Δ_f G ⁰(ScVO₄, 298.15 K) = −1644.0 ± 2.2 kJ/mol.     

High Dynamic Range SQUID Readout for Frequency-Domain Multiplexers

A 16-SQUID array has been designed and fabricated, which shows 0.12 μΦ ₀ Hz^−1/2 flux noise at 4.2 K. The readout amplifier based on a cryogenic silicon-germanium bipolar transistor employs short-delay negative flux feedback and reaches 7 MHz bandwith for a 1 Φ _0p-p signal. The −1 dB compression is reached approximately at 4.2 Φ _0p-p amplitude when the signal frequency is 1 MHz. In the feedback mode the flux noise is anomalously increased to 0.35 μΦ ₀ Hz^−1/2.      

Non-uniform temperature gradients and thermal stresses produced by a moving heat flux applied on a hollow sphere

This work presents numerical analyses of transient temperature and thermally-induced stress distributions in a hollow steel sphere heated by a moving uniform heat source applied on a certain zenithal segment (the heated zenithal segment, Θ _H ) of its outer surface (the processed surface) under stagnant ambient conditions. Along the process, the moving heat source (MHS) moves angularly from the first zenithal segment to the last zenithal segment on the processed surface with a constant angular speed, ω, and then returns backward to the first zenithal segment with the same speed. It is assumed that the inner surface is heat-isolated and that the outer surface except the heated segment is under stagnant ambient conditions. The numerical calculations are performed individually for a wide range of thermal conductivity, λ, of steel and for the different Θ _H s. The maximum effective thermal stress ratio calculated as per the heat flux intensity (q ₀) can be reduced in considerable amounts. By increasing λ(∼75%) and ω(∼63%) the maximum effective thermal stress ratio calculated can be significantly reduced.     

Numerical study of effect of oxygen fraction on local entropy generation in a methane-air burner

This study considers numerical simulation of the combustion of methane with air, including oxygen and nitrogen, in a burner and the numerical solution of local entropy generation rate due to high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (Φ) and oxygen percentage (γ ) on combustion and entropy generation rates are investigated for different Φ (from 0.5 to 1.0) andγ values (from 10 to 30%). Combustion is simulated for the fuel mass flow rate resulting in the same heat transfer rate (Q)y to the combustion chamber in each case. Numerical calculation of combustion is performed individually for all cases with the use of the Fluent CFD code. Furthermore, a computer program has been developed to calculate the volumetric entropy generation rate and the other thermodynamic parameters numerically by using the results of the calculations performed with the FLUENT code. The predictions show that the increase of Φ (or the decrease of λ) significantly reduces the reaction rate levels. Average temperature in the combustion chamber increases by about 70 and 35% with increase ofγ (from 10 to 30%) and Φ (from 0.5 to 1.0) respectively. With increase ofγ from 10 to 30%, volumetric local entropy generation rate decreases by about 9 and 4% for Φ = 0.5 and 1.0 respectively, while total entropy generation rate decreases exponentially and the merit numbers increase. The ratio of the rates useful energy transfer to irreversibility therefore improves as the oxygen percentage increases     

Mixed-mode crack growth in ductile thin-sheet materials under combined in-plane and out-of-plane loading

Ductile thin-sheet structures, such as fuselage skin or automobile panels, are widely used in engineering applications. These structures often-times are subjected to mixed mode (I/II/III) loading, with stable crack growth observed prior to final fracture. To characterize specific specimen deformations during stable tearing, a series of mixed-mode I/III stable tearing experiments with highly ductile thin-sheet aluminum alloy and steel specimens have been measured by using three-dimensional digital image correlation (3D-DIC). Measurements include (a) specimen’s deformed shape and 3D full-field surface displacement fields, (b) load-crack extension response and (c) crack path during stable tearing, (d) angular and radial distributions of strains and (e) the mixed mode crack-opening displacement (COD, measured at 1-mm from crack tip along crack surface) variation as a function of crack extension. Results indicate that for both aluminum alloy and steel at all mixed-mode I/III loading conditions (Φ = 30°, 60° and 90°), the crack tip fields have almost identical angular and radial polar strain distributions. The mixed mode I/III fields were different from those observed for the nominal Mode I loading case (Φ = 0°). The effect of the Mode III loading component is that it lowers the magnitude of the dominant strain component ε _θθ ahead of the growing crack tip and increases the singularity of the strain as compared with that in the mode I case. In addition, measurements indicate that the average mixed mode I/III stable COD for AL6061-T6 (GM6208 steel) is 4×(3×) greater than the average Mode I stable COD.     

Occurrence of Superconductivity and Magnetism in Nominally Undoped LaOFeAs

Superconductivity in a LaOFeAs system is known to get introduced by F-doping (LaO_1−x F _x FeAs) even under ambient conditions and oxygen deficiency (LaO_1−x FeAs) under high pressure conditions. Hitherto unreported, superconductivity in F-free undoped LaOFeAs samples is observed and confirmed for the first time by various characterization tools–resistive ρ(T,B), magnetic M(T,B) and modulated microwave absorption (MMA) measurements. The ρ(T) at B=0 shows a clear superconducting transition with an onset at T _con∼17 K and a tail-like behavior when R goes to zero at T _c0∼8 K. In the presence of B, the superconducting transition shifts to lower T with a rate ∼−5.5 and −1.65 T/K, depending on whether the ρ(T) has dropped to 90% or 5% of its normal state value, respectively. M(T) in zero field cooling at B=10 mT shows diamagnetic downturn below at ∼12 K. At T<T _c0 the change in MMA with B shows a low field (B∼1 mT) peak, which vanishes at T>T _c0, indicating the presence of weak link superconducting networks in the sample. The sample shows a complex electrical and magnetic behavior in the normal state. For instance, ρ(T) reveals a weak SDW-like anomaly at T∼132 K along with a resistivity minimum at T _min∼78 K. M(T) also shows the presence of a magnetic anomaly at T∼130 K. Both below and above T _con, presence of an additional ferromagnetic component is observed in the isothermal M(B) loop measurements. The superconducting and normal state features of our sample are compared with other undoped and doped LaOFeAs systems reported in the literature.     

Viscoelastic properties of hybrid copolymers based on methacryloxypropyl-grafted nanosilica and methyl methacrylate

The linear viscoelastic behavior of “model” hybrid materials based on methyl methacrylate and methacryloxypropyl-grafted nanosilica was investigated. As unique features, the materials under study present an excellent dispersion of silica within the polymer matrix and are almost free of uncross-linked chains. In addition, very progressive changes in network architecture are available, resulting from changes in particle diameter, d, volume fraction of filler, Φ, number of methacryloyl units grafted per surface unit of silica particle, n, and nature of the grafting agent. The influence of these parameters on the characteristics of the mechanically active relaxations α and β was examined. Emphasis was put on the storage modulus, E′, on the loss modulus, E′′, and on their dependence on filler volume fraction. E′′ values were shown to simply account for the reduction of the mechanical energy lost within the material, in connection to the occurrence of polymer molecular motions. Analysis of E′ variations as a function of Φ was based on the theoretical models available in the literature to account for the contribution of the spherical filler particles. In the glassy state, Kerner’s and Christensen and Lo’s models yielded comparable results. In the rubbery state, Guth and Gold’s model was shown to prevail on Kerner’s model.     

Correlated one-body density matrix of boson superfluids

The correlated density matrix theory is employed and further developed to analyze the one-body density matrix ρ₁(|r ₁-r ₂|) of the normal and superfluid phases of a strongly interacting Bose system at non-zero temperature. The approach continues the formal development described in an earlier article and is based on a suitable trial ansatz for the many-body density matrixW(R, R′)∼Φ(R) Q(R, R′) Φ(R′) with the wave function Φ and incoherence factorQ incorporating the essential statistical and dynamical correlations. Special attention is given to the appearance of off-diagonal long-range order in function ρ₁(|r ₁-r ₂|) and its relation to the condensation strength B_cc characterizing the degree of coherence in the superfluid phase. We derive a number of structural relations that have counterparts in known results on ρ₁ in the Jastrow variational theory of the Bose ground state. We discuss Bose-Einstein condensation and make contact to Landau's phenomenological theory of continuous phase transitions. Numerical estimates are presented on the condensation strength and the condensate fraction of liquid⁴He as functions of the temperature.     

Limitation of a giant amplification surge by photoinduced space charge accompanying an increase in the concentration of recombination centers in intrinsic threshold photoresistors with extracting contacts

An analysis is made of the dependence of Ĝ on V under conditions of a giant surge of the photoelectric gain Ĝ as the concentration of recombination impurities N increases, where V is the applied voltage and Ĝ is the value of G at the maximum point of the function G(N). It is shown that as a result of the space charge induced by the optical radiation the function Ĝ(V) exhibits strongly nonmonotonic behavior. The optimum voltage V _opt applied to the sample is determined for which Ĝ(V) reaches the maximum Ĝ _max. This nonmonotonic dependence of Ĝ on V does not correlate with heating of carriers or the lattice or by injection of charge from the contacts. Pis’ma Zh. Tekh. Fiz. 25, 1–8 (March 12, 1999)     

Slow Dynamics of Nonequilibrium Density Fluctuations in Concentrated Hard-Sphere Suspensions

The coupled diffusion equations recently proposed for concentrated hard-sphere suspensions of interacting Brownian particles, the nonlinear deterministic diffusion equation with the self-diffusion coefficient D _S((x, t)) for the average local volume fraction (x, t), and the linear stochastic diffusion equation with D _S((x, t)) for the density fluctuations n(x, t) are numerically solved under a spatially inhomogeneous, nonequilibrium initial state. Thus, in a supercooled region where < _g, the slow evolution of the cluster-like glassy domains with (x, t) _g and the slow relaxation of the nonequilibrium density fluctuations are shown to be caused by the dynamic singularity of the self-diffusion coefficient, D _S((x, t)) (1–(x, t)/_g)², where is a particle volume fraction, _g = (4/3)³/(7 ln 3 – 8 ln 2 + 2) is the colloidal glass transition volume fraction, and is the crossover volume fraction.     

A modified collocation method and a penalty formulation for enforcing the essential boundary conditions in the element free Galerkin method

The Element free Galerkin method, which is based on the Moving Least Squares approximation, requires only nodal data and no element connectivity, and therefore is more flexible than the conventional finite element method. Direct imposition of essential boundary conditions for the element free Galerkin (EFG) method is always difficult because the shape functions from the Moving Least Squares approximation do not have the delta function property. In the prior literature, a direct collocation of the fictitious nodal values & u circ; used as undetermined coefficients in the MLS approximation, u ^h (x) [u ^h (x)=Φ·& u circ;], was used to enforce the essential boundary conditions. A modified collocation method using the actual nodal values of the trial function u ^h (x) is presented here, to enforce the essential boundary conditions. This modified collocation method is more consistent with the variational basis of the EFG method. Alternatively, a penalty formulation for easily imposing the essential boundary conditions in the EFG method with the MLS approximation is also presented. The present penalty formulation yields a symmetric positive definite system stiffness matrix. Numerical examples show that the present penalty method does not exhibit any volumetric locking and retains high rates of convergence for both displacements and strain energy. The penalty method is easy to implement as compared to the Lagrange multiplier method, which increases the number of degrees of freedom and yields a non-positive definite system matrix.     

Barrier characteristics of Pt/Ru Schottky contacts on <Emphasis Type="Italic">n</Emphasis>-type GaN based on <Emphasis Type="Italic">I–V–T</Emphasis> and <Emphasis Type="Italic">C–V–T</Emphasis> measurements

We have investigated the current–voltage (I–V) and capacitance–voltage (C–V) characteristics of Ru/Pt/n-GaN Schottky diodes in the temperature range 100–420 K. The calculated values of barrier height and ideality factor for the Ru/Pt/n-GaN Schottky diode are 0·73 eV and 1·4 at 420 K, 0·18 eV and 4·2 at 100 K, respectively. The zero-bias barrier height (Φ_b0) calculated from I–V characteristics is found to be increased and the ideality factor (n) decreased with increasing temperature. Such a behaviour of Φ_b0 and n is attributed to Schottky barrier (SB) inhomogeneities by assuming a Gaussian distribution (GD) of barrier heights (BHs) at the metal/semiconductor interface. The current–voltage–temperature (I–V–T) characteristics of the Ru/Pt/n-GaN Schottky diode have shown a double Gaussian distribution having mean barrier heights ( [`(F)]<sub>\textb0</sub> {\bar{{\Phi}}_{\text{b}0}} ) of 1·001 eV and 0·4701 eV and standard deviations (σ <sub>0</sub>) of 0·1491 V and 0·0708 V, respectively. The modified ln (J<sub>0</sub> /T<sup>2</sup> )-( q<sup>2</sup>s <sub>0</sub><sup>2</sup>/2k<sup>2</sup>T<sup>2</sup> ){ln} ({{J}_{0} /{T}^{2}} )-( {{q}^{2}{\sigma} _{0}^{2}/{2}{k}^{2}{T}^{2}} ) vs 10<sup>3</sup>/T plot gives [`(F)]_\textb0 \bar{{\Phi}}_{\text{b}0} and Richardson constant values as 0·99 eV and 0·47 eV, and 27·83 and 10·29 A/cm²K², respectively without using the temperature coefficient of the barrier height. The difference between the apparent barrier heights (BHs) evaluated from the I–V and C–V methods has been attributed to the existence of Schottky barrier height inhomogeneities.     

Pinning Energy of High-Tc (Bi, Pb)2Sr2Ca2Cu3Oy

The flux pinning potential energy of silver (Ag) sheathed high-T _c Bi-2223 tapes determined from measured current densities, J(T,H), is reported in this paper. Measurements of the transport current density, J, of the Ag sheathed Bi-2223 tapes were performed with an applied magnetic field directions parallel (H‖ab) and perpendicular (H‖c) to the tape’s board surface from 0 to 3 T, at temperatures from 23 to 77 K. To obtain the pinning potential energy of the Bi-2223 superconductor, an empirical scaling of J(T,H) is introduced in this study. Under the flux dynamics, the effective pinning energy, U _eff(T,H,J), can be formulated using the measured current density, J(T,H). Furthermore, the pinning potential energies, U(T,H), for both H‖ab and H‖c were quantitatively determined by the scaling of J(T,H). The two energies, U(T,H), exhibit as a function that is inversely proportional to both temperature and logarithmic magnetic field. The magnitude of U(T,H) for H‖ab is three or more times larger than that for H‖c under the considered conditions. The U(T,H) is sensitive to the temperature, but insensitive to the magnetic field. This characteristic of insensitiveness towards magnetic field is an advantage for applications on the conditions of high current density, high magnetic field, and low temperature.     

Testing the hypothesis of a general linear model using nonparametric regression estimation

Summary Given the modelY _i =m(χ _i )+ɛi,whereE(ɛ _i) =0,X _i ≠Ci=1, ...,n, andC is ap-dimensional compact set, we have designed a new method for testing the hypothesis that the regression function follows a general linear model,m(·) ∈ {m _θ(·) =A ^t (·)θ}_{θ∈Θ⊂ℛq} , withA a function fromℜ ^p toℜ ^q. The statistic, denoted ΔASE, used fortesting the given hypothesis is defined to be the difference between the average squared errors (ASE) associated with the non-parametric estimator ofm and the minimum distance parametric estimator ofm. The asymptotic normality of both ΔASE and the minimum distance estimators is proved under general conditions. Alternative bootstrap versions of ΔASE are also considered.     

Thermoanalytical characterization of epoxy matrix-glass microballoon syntactic foams

Syntactic foams are finding new applications where their thermal stability and high temperature response are important. Therefore, the high temperature response of these advanced composites needs to be characterized and correlated with various material parameters. The present study is aimed at evaluating the effect of microballoon (hollow particle) volume fraction (Φ) and wall thickness (w) on thermoanalytical characteristics of epoxy matrix syntactic foams containing glass microballoons. These composites are characterized to determine the glass transition temperature (T _g), the weight loss, and the char yield. It is observed that T _g decreases and the char yield increases due to the presence of microballoons in the resin. The T _g is increased with an increase in Φ but is not significantly affected by w. The thermal stability is increased by increasing w and is relatively less sensitive to Φ. Understanding the relations between thermal properties of syntactic foams, the microballoon wall thickness, and microballoon volume fraction will help in developing syntactic foams optimized for mechanical as well as thermal characteristics. Due to the increased interest in functionally graded syntactic foams containing a gradient in microballoon volume fraction or wall thickness, the results of the present study are helpful in better tailoring these materials for given applications.