Thermodynamic behaviour of undercooled melts

The Gibbs free energy difference (ΔG) between the undercooled liquid and the equilibrium solid phases has been studied for the various kinds of glass forming melts such as metallic, molecular and oxides melts using the hole theory of liquids and an excellent agreement is found between calculated and experimental values of ΔG. The study is made for non-glass forming melts also. The temperature dependence of enthalpy difference (ΔH) and entropy difference (ΔS) between the two phases, liquid and solid, has also been studied. The Kauzmann temperature (T ₀) has been estimated using the expression for ΔS and a linear relation is found between the reduced glass transition temperature (T _g/T _m) and (T ₀)/T _m). The residual entropy (ΔS _R) has been estimated for glass forming melts and an attempt is made to correlate ΔS _R,T _g,T ₀, andT _m which play a very important role in the study of glass forming melts.     

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part IThe deformation of three aluminum alloys at 473 K to 833 K

Constant-load indentation tests were performed on wrought-2024, P/M-2024, and wrought-1100 aluminum alloys to assess the capability of the microindentation testing technique for measuring the high-temperature deformation rate controlling parameters of these alloys. The three alloys all display threshold indentation stress σ_th below which the indentation strain rate ε_ind approaches zero. The nominal inter-obstacle spacing, ℓ*, calculated from σ_th, increases with temperature in a way that is consistent with the known temperature dependence of the inter-particle spacing and dislocation cell size. The measured activation energy ΔG_o of ɛ_ind increases with temperature but remains within the range that is typical of deformation that occurs by dislocation glide limited by weak particles or dislocation/dislocation interactions. The three alloys tested show different trends of ΔG_o versus ℓ* and the trends are consistent with the known temperature dependence of the obstacles to dislocation glide. This study demonstrates that high-temperature indentation tests are sufficiently precise to detect changes in the operative deformation parameters between different alloys of the same general composition. This lays the groundwork for the use of this technique as a general tool for studying the local high-temperature deformation of a wide range of metal-based systems.     

Viscous and thermodynamic behaviour of glass-forming organic liquids

The free energy difference (ΔG) between an undercooled liquid and its corresponding equilibrium solid has been evaluated on the basis of a method involving Taylor series expansion of ΔG around its value at the equilibrium melting temperature. The resultant expression is shown to be capable of correctly estimating ΔG at temperatures as low as the glass transition temperature. The method is then enlarged to obtain the configurational entropy and used in conjunction with the Adam and Gibbs model to derive a novel expression for the viscosity of undercooled liquids. Most commonly used expressions for the temperature dependence of viscosity are shown to be approximations of the equation obtained in this study.     

The effects of temperature gradient and growth rate on the microstructure of directionally solidified Sn–3.5Ag eutectic solder

The mechanical properties (microhardness, tensile strength) of alloys are controlled by their microstructure, which depends strongly on temperature gradient (G) and growth rate (V). Thus, it is important to understand the relationships among G, V and microstructure (rod eutectic) of Sn–Ag solders. The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate, V (16.5 μm/s) at different temperature gradients, G (1.43–4.28 K/mm) and with a constant temperature gradient, G (3.93 K/mm) at different growth rates, V (8.3–500 μm/s) in a Bridgman–type directional solidification furnace. The rod spacings (λ) have been measured from both longitudinal section (parallel to the growth direction, λ _L ) and transverse section (perpendicular to the growth direction, λ _T ) of the samples. The undercooling values (ΔT) were calculated by using V, λ and system parameters (K ₁ and K ₂). It was found that the values of λ (λ _T , λ _L ) decrease while V and G are increasing. The relationships between rod spacing and solidification parameters (G and V) were obtained by linear regression analysis. The dependences of eutectic spacings λ on undercooling (ΔT) are also analyzed. λ² V, λΔT, ΔTV ^−0.5 and ΔTG ^−0.5 values were determined by using λ, ΔT, V and G values. The results obtained in this work are compared with the Jackson–Hunt eutectic theory and the similar experimental works. The experimental l_\textT ² \textV \lambda_{\text{T}}^{ 2} {\text{V}} value (159.3 μm³/s) is slightly lower than the result 174.6 μm³/s calculated from Jackson–Hunt eutectic theory.     

Coulomb Interaction Effect in Quantum Transport

We study the quantum Anderson model in the presence of the finite Coulomb interaction U using the t-matrix approximation. The energy of the d-electron level increases as Uln (U/πΔ) for strong Coulomb interaction U and finite level-width Δ. In the case of small U/Δ parameter this approximation gives the results from the perturbation theory. The spin susceptibility is calculated as a function of U and compared with the results obtained by renormalized perturbation approximation. The results can be used to study the effect of the finite Coulomb interaction in the quantum transport.     

Adsorption of hydrogen atoms and oxygen molecules on zinc oxide and titanium oxide on the surface conductivity and work function

A change in the substrate work function has been calculated using the Anderson-Newns model for H/ZnO, O₂/ZnO, and O₂/TiO₂ systems in the interval of submonolayer adsorbate coverages (0 ≤ Θ ≤ 1). Using the relation ΔG/Δϕ = const obtained previously, where ΔG and Δϕ are changes in the surface conductivity and work function, the values of ΔG(Θ) are evaluated, which show a good agreement with the experimental data.     

An exact study of pairing fluctuations and phase diagrams in four-site Hubbard Nanoclusters

Charge-spin separation and pairing fluctuations and pseudogaps are studied using the analytical eigenvalues of the four-site Hubbard Nanoclusters with the grand canonical and canonical ensemble approaches in a multidimensional parameter space of temperature (T), magnetic field (h), on-site interaction (U), chemical potential (μ), and number of electrons (N). The electron charge energy gap, with one hole off half filling, corresponds to an excitonic particle–hole pair binding energy Δ ^e-h > 0 at U > U _c and vanishes at a critical parameter U _c = 4.584. For U < U _c, particle–particle pair binding is found with a pairing energy Δ^p > 0. In addition, for U ⩽ U _c we find an electron pair binding instability at finite temperature near N ≈ 3, which manifests a possible pairing mechanism, a precursor to superconductivity, in small clusters. The resulting phase diagram, consisting of charge and spin pseudogaps, hole pairing near 1/8th filling with hole-rich and hole-poor regions in the ensemble of Nanoclusters, closely resembles the phase diagrams in the family of doped high-T _c-cuprates.     

Thermodynamics of rare-scattered element In<Superscript>3+</Superscript> adsorption by nanometer titanium oxide

A new method using nano-TiO₂ as a solid-phase extractant has been developed for the determination of trace amounts of indium. The adsorption pH curves, adsorption isotherms, and adsorption capacities were obtained. It was found that the adsorption rate of In³⁺ was more than 95% at pH 3.0–5.0. The adsorption reaches equilibrium in a short time and follows a Langmuir isotherm. From the results of thermodynamic analysis, the adsorption constant K _C , standard free energy ΔG ⁰, standard enthalpy ΔH ⁰, and standard entropy ΔS ⁰ of the adsorption process were calculated. The standard thermodynamic function changes were ΔG ⁰ < 0, ΔH ⁰ > 0, and ΔS ⁰ > 0, which indicated that the adsorption was a spontaneous increasing enthalpy and entropy driving process. The positive value of ΔS ⁰ for adsorption could be explained by the release of the attached water molecules from In³⁺ into solution as In³⁺ adsorbed on substrate and contributed to the increase in the degree of freedom at the solid-solution interface during the adsorption process. Fourier transformation infrared spectrum and x-ray diffraction investigated the structure of nano-TiO₂. The results and the thermodynamic data indicated that In³⁺ adsorption onto nano-TiO₂ was characterized by physical adsorption. The text was submitted by the authors in English.     

Entropy Flow of Chemical Oscillating Reactions Involving Amino Acids

Variations of H⁺, Mn²⁺, and BrO₃^-{{\rm BrO}_3^-} during chemical oscillating reactions involving amino acids were investigated using H₂SO₄–BrO₃^-{{\rm BrO}_3^{-}} –Mn²⁺–acetone as an oscillator, and the thermodynamic functions (ΔS, ΔH, and ΔG) were calculated. The results indicated that the oscillating reaction is a process from disorder to order. In the oscillating system, the heat provided by the environment is used to increase the internal energy and entropy. When external positive entropy flow could make the system reach negative entropy flow, the system starts to oscillate and to maintain oscillations by a stable entropy.     

Isosbestic Points in the Spectral Function of Correlated Electrons

We investigate the properties of the spectral function A(ω,U) of correlated electrons within the Hubbard model and dynamical mean-field theory. Curves of A(ω,U) versus ω for different values of the interaction U are found to intersect near the band-edges of the non-interacting system. For a wide range of U the crossing points are located within a sharply confined region. The precise location of these “isosbestic points” depends on details of the non-interacting band structure. Isosbestic points of dynamic quantities therefore provide valuable insights into microscopic energy scales of correlated systems. This paper is dedicated to Hilbert von L?hneysen on the occasion of his 60th birthday.     

Conductometric Study of Some Metal Halides in Glycerol + Water Mixtures

Electrolytic conductivities of potassium halides, KX (X = Cl⁻, Br⁻, I⁻) have been investigated in 10, 20, and 30 mass% glycerol + H₂O mixtures at 298.0, 308.0, and 318.0 K. The conductance data have been analyzed by the Fuoss-conductance–concentration equation in terms of the limiting molar conductance (Λ⁰), the association constant (K _A ), and the distance of closest approach of ion (R). The association constant (K _A ) tends to increase in the order: 10 mass% < 20 mass% < 30 mass% glycerol + water mixtures, while it decreases with temperature. Thermodynamic parameters ΔH ⁰, ΔG ⁰, and ΔS ⁰ are obtained and discussed. Also, Walden products (Λ⁰η) are reported. The results have been interpreted in terms of ion–solvent interactions and structural changes in the mixed solvents.     

Study of the impact of HgO addition and low-field magnetic relaxation behaviour in granular high-T c superconductors

A series of high-T _c superconductors have been prepared with the HgO addition/substitution. Significant improvement in theT _conset as well asT _c0 was observed in all the cases. Substitution of Hg at the Sr site and Ba site in the case of (Bi, Pb)-Sr-Ca-Cu-O and Y-Ba-Cu-O systems, respectively over a range 0·01–0·6 at% helps in constructing an entire spectrum: improvement ofT _c0 up to 0·4 at % in the case of Bi-system and up to 0·03 at % in the case of Y123 system and slight drop inT _c thereafter. Such improvement is the result of abundant supply of highly reactive nascent oxygen all through the bulk. HgO decomposes and provides oxygen which helps in maintaining proper oxygen stoichiometry throughout the bulk. No Hg or Hg-based impure phases were observed in the X-ray diffraction spectra. Low-field (10–100 Oe) magnetic relaxation studies reveal faster relaxation of the intergranular critical state in the case of silver added samples as the grain boundary coupling energyE _J becomes quite uniform across the entire bulk which leads to smaller flux pinning energy. The distribution of the pinning energy is evaluated from the observed relaxation pattern and is found to be narrower in the case of silver added samples. It was also observed that the transportJ _c ∼ exp[−ΔT _c/T _c0] and the flux pinning energyU ∼ ΔT _c, where ΔT _c is the transition width and is a measure of the inhomogeneity within the sample. Such relationships may help in devising a strategy for achieving highJ _c, highU yet low ΔT _c. Silver addition turns out to be an effective tool in tailoring the sample properties depending on requirement.     

Magnetostriction and microstructure of the melt-spun Fe83Ga17 alloy ribbons

Fe₈₃Ga₁₇ alloy ribbons with four thicknesses were prepared by the melt-spun method. The measured results show that magnetostriction is related to the thickness of the ribbons, and its maximum value, −2100 ppm, has been obtained in the ribbon with a thickness of 75 μm. The results of the X-ray diffraction (XRD) and the differential scanning caloric (DSC) show that a phase with DO₃ structure emerges in those ribbons with the thicknesses of 45, 55 and 75 μm, and a phase transition of A2 + DO₃ → A2 occurred at 669°C for the ribbon with the thickness of 75 μm. The increase in the cooling rate can restrain the precipitation of the Ga-rich phase in the ribbon. It is considered that the giant magnetostriction of ribbons originates from the A2 + special DO₃ structure and shape magnetic anisotropy. Translated from Acta Metallurgica Sinica, 2006, 42(2): 177–180 [译自: 金属学报]     

On the α-relaxation in bulk polyoxymethylene

Values quoted forΔH _α, the activation energy of the high temperatureα-relaxation in polyoxymethylene (POM), range from 20 to 92 kcal mol⁻¹. This paper seeks to rationalize the discrepancy by remeasuringΔH _α using time-temperature superposition of torsional creep and dynamic compliances for a POM specimen annealed at 160° C. Superposition of loss compliance curvesJ″ (ω, T) is possible over the range 20 to 120° C but creep compliance curvesJ(t, T) fail to superpose above about 70° C. The creep anomaly is explained in terms of the McCrum-Morris reduction equations in which the unrelaxed complianceJ _U ^T increases with temperature more rapidly than the relaxed complianceJ _R ^T . The activation energyΔH _α has a constant value of 21±1 kcal mol⁻¹ below about 70° C. Above about 70° C,ΔH _α increases steadily up to 33±2 kcal mol⁻¹ at 120° C.     

Vortex State Microwave Resistivity in Tl-2212 Thin Films

We present measurements of the field induced changes in the 47 GHz complex resistivity, Δρ~(H, T), in Tl₂Ba₂CaCu₂O_8+x (TBCCO) thin films with T _c ≃ 105 K, prepared on CeO₂ buffered sapphire substrates. At low fields (μ₀ H < 10 mT) a very small irreversible feature is present, suggesting a little role of intergranular phenomena. Above that level Δρ~(H, T exhibits a superlinear dependence with the field, as opposed to the expected (at high frequencies) quasilinear behaviour. We observe a crossover between predominantly imaginary to predominantly real (dissipative) response with increasing temperature and/or field. In addition, we find the clear scaling property Δρ~(H, T = Δρ~[H/H ^* (T)], where the scaling field H ^∗ (T) maps closely the melting field measured in single crystals. We discuss our microwave results in terms of loss of flux lines rigidity.     

Sound Velocity Measurements in the Low and the High Field Phases of the Nuclear-Ordered bcc Solid 3He in Magnetic Fields

We have measured the temperature and magnetic-field dependences of the sound velocity for one longitudinal and two transverse waves in the low field phase (LFP) and the high field phase (HFP) of nuclear spin ordered bcc solid ³He crystals with a single magnetic domain along the melting curve. From sound velocity measurements for various crystal orientations as a function of the sound propagation direction, we determined the elastic stiffness constants, c _ij (T,B). In the LFP with tetragonal symmetry for the nuclear spin structure, we extracted six nuclear spin elastic stiffness constants Δc _ij ^ℓ (T,0.06 T) from the temperature dependence of the sound velocity at 0.06 T and Δc _ij ^ℓ (0.5 mK,B) from the magnetic-field dependence of sound velocity at 0.5 mK. In the HFP with cubic symmetry for the nuclear spin structure, we extracted three Δc _ij ^h (T,0.50 T) at 0.50 T and Δc _ij ^h (0.5 mK,B) at 0.5 mK. At the first-order magnetic phase transition from the LFP to the HFP at the lower critical field B _c1, large jumps in sound velocities were observed for various crystal directions and we extracted three . Using the thermodynamic relation between Δc _ij and the change in the internal energy for the exchange interaction in this system, ΔU _ex(T,B), Δc _ij are related to the generalized second-order Grüneisen constants Γ _ij ^X ≡∂ ²ln X/∂ ε _i ∂ ε _j as Δc _ij (T,B)=Γ _ij ^X ΔU _ex(T,B), where X represents some physical quantity which depends on the molar volume and ε _j is the j-th component of a strain tensor. In the LFP, the Δc _ij ^ℓ (T,0.06 T) were proportional to T ⁴, and Δc _ij ^ℓ (0.5 mK,B) were proportional to B ². We extracted for the spin wave velocity in the LFP, s ^ℓ , from Δc _ij ^ℓ (T,0.06 T) and for the inverse susceptibility, 1/χ ^ℓ from Δc _ij ^ℓ (0.5 mK,B). In the HFP, Δc _ij ^h (T,0.50 T) were proportional to T ⁴ and Δc _ij ^h (0.5 mK,ΔB) were proportional to ΔB(≡B−B _c1). We obtained for the spin wave velocity in the HFP, s ^h , from Δc _ij ^h (T,0.50 T) and for B _c1 from Δc _ij ^h (0.5 mK,ΔB). The values obtained for and were compared with the Multiple Spin Exchange model (MSE) with three parameters by using analytic expressions for s ^ℓ and χ ^ℓ . The three-parameter MSE does not agree with the observed Δc _ij in the LFP.      

Surface roughness and mechanical properties of melt-spun Al-Cu ribbons

Al, Al-5.23, Al-13.46 and Al-33 wt% Cu ribbons with different thicknesses,t, have been prepared by chilled block melt spinning under different processing conditions. Surface roughness, taken as peak to valley values,R _t, measured on both the substrate and air sides of the ribbon, shows great dependence on substrate velocity,v, ejection gas pressure,P, nozzle height above the substrate,H, and a lesser dependence on substrate thermal conductivity,k, and melt superheat, T. The bulk density over all the zones shows an increase over conventionally cast alloys of 7.4% for Al-33 wt% Cu and about 3% for aluminium. Microhardness,H _n, measurements inside each of the three microstructure zones, which were identified across the ribbon section, show variation witht asH _n =H _o t ^–m, where the constantH _o depends on the microstructure zone andm depends on the alloy composition. The zone at the substrate side has the highestH _n values and that at the air side has the lowest ones. The number of bending cycles to fracture, which is taken as a measure of ductility, increased with ribbon thickness and decreased with copper content.     

Use of ellipsometric measurements for high-sensitivity monitoring of surface temperature

An analysis is made of the possibility of developing a high-sensitivity ellipsometric method of monitoring the surface temperature of samples in ultrahigh vacuum. The method is based on the temperature dependence of the phase thickness of the layer being studied. Measurements made for ZnTe layers show that the sensitivity coefficients for the ellipsometric angles ψ and Δ are 0.1 and 0.5° respectively per degree temperature. This is more than an order of magnitude higher than the sensitivity of ellipsometric methods of measuring temperature used conventionally. Pis’ma Zh. Tekh. Fiz. 25, 65–71 (July 26, 1999)     

Glass transition and fragility of telluro-vanadate glasses containing antimony oxide

The activation energy (ΔH ^*) of the glass transition and the heating-rate dependence of the glass transition temperature (T _g) of V₂O₅–Sb₂O₃–TeO₂ glasses were determined using differential scanning calorimetry technique. Non-isothermal measurements were performed at different heating rates φ (=3, 6, 9, 10, 13 K/min). The heating rate dependence of T _g was used to investigate the applicability of different theoretical models describing the glass transition. The application of Moynihan and Kissinger et al. models to the present data led to different values of (ΔH ^*) at each different heating-rate regions. This behavior was attributed to the strong heating rate dependence of the activation energy of the process. The fragility parameter (m = ΔH ^*/RT _g) were ≲90, suggesting that these glasses may be classified as strong glasses. The viscosity, η, calculated at a few selected temperatures near the glass transition region increased with increasing Sb₂O₃ content at any given temperature, which is also expected. Also the compositional dependence of T _g and ΔH ^* was investigated.     

Theoretical Study of Specific Heat, Density of States and Free Energy of Itinerant Ferromagnetic Superconductor URhGe

Following the equation of motion method and Green’s function technique, the coexistence of itinerant ferromagnetism (FM) and superconductivity (SC) is investigated in a single band homogeneous system. Self-consistent equations for superconducting order parameter (Δ) and magnetic order parameter (Δ_FM) are derived. It is shown that there generally exists a coexistent (Δ≠0 and Δ_FM≠0) solutions to the coupled equations of the order parameter in the temperature range 0<T<min (T _C,T _FM) where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. Expressions for the specific heat, density of states and free energy are derived. The specific heat has a linear temperature dependence at low temperatures as opposed to the exponential decrease in the BCS theory. The density of states for a finite Δ_FM increases as opposed to that of a standard ferromagnetic metal. The free energy shows that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and therefore is realized at low enough temperature. The theory is applied to explain the observations of URhGe. The agreement between theory and experimental results is quite satisfactory.