Improvement of thermoelectric properties of WO3 ceramics by NiO addition

The thermoelectric properties of tungsten trioxide (WO₃) ceramics doped with nickel oxide (NiO) were investigated from 323 up to 1,023 K. The results revealed that doping WO₃ with NiO could promote the grain growth and the density. There was a second phase (NiWO₄) segregation at the grain boundaries in the samples containing more than 1.0 mol% NiO, which inhibited the further grain growth. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the NiO content. As for the power factor (σS ² ), the 1.0 mol% sample has the maximum value of the power factor which is 0.55 μW m^?1 K^?2 at 1,023 K.     

Equilibrium vacancy parameters and limit temperature of nonequilibrium melting in osmium

For osmium, based on experimental data on enthalpy and the averaged heat capacity obtained by the method of mixing within the temperature range of 1150?C2960 K, for the first time the parameters of the equilibrium vacancies for this metal have been determined: the vacancy formation energy E = 1.8 eV, the vacancy concentration at melting c = 3.3%, and the vacancy formation entropy S = 25.6 J/(mol K). The limit temperature of the onset of nonequilibrium melting of osmium T _m = 4256 K and its relative value T _lim/T _m = 1.30 has been calculated.     

A new method for estimating the isothermal devitrification and crystallization of mold powder slags from non-isothermal DSC data

The rate of formation of crystalline phases from liquid and glassy mold powder slags is of foremost importance in the performance of molds used for continuous casting of steel. This study shows how the Induction Period (of Šimon and Kolman) and the Kissinger methods can be combined in a kinetic model to evaluate the isothermal rate of formation of crystalline phases from thermo-analytical data – onset temperature, T_i, peak maximum temperature, T_m, shape index, S, and conversion at peak maximum, x_m – collected at various linear heating and cooling rates. The diagram of the extent of isothermal transformation as a function of time calculated for a commercial mold powder, used for casting low carbon steels, shows good agreement with the degree of transformation observed in photomicrographs of glass disks devitrified isothermally, at several temperatures for different times. Additionally, qualitative and quantitative X-ray diffraction results obtained at room-temperature from glass powder samples treated isothermally and quenched also show good accord with the degree of transformation predicted with the kinetic model developed in this work.     

EBSD analysis of {10–12} twinning activity in Mg–3Al–1Zn alloy during compression

The {10–12} twinning activity of Mg–3Al–1Zn magnesium alloy during uniaxial compression at room temperature has been investigated by electron backscatter diffraction. The results indicated that the twinning activity was closely related with two angles: one was the angle between the c-axis and the compression direction and the other was the angle between the a-axis and the titling direction in the basal plane for a given relation between the c-axis and the compression direction. These two parameters can be used to explain which twinning variant will operate under the given strain path. For the grains containing a single {10–12} twinning variant, the {10–12} twinning variant occurred in a wide range of Schmid factor values (0 < Schmid factor < 0.5) and the Schmid factor rank of 1 or 2 was the most commonly observed. By contrast, for the grains containing two {10–12} twinning variants, the {10–12} twinning activity exhibited a stronger orientation dependence and the combinations of Schmid factor ranks 1–3 and 1–2 were the most commonly observed.     

The dependence of the temperature of crystal-liquid phase transition on the size and shape of simple nanocrystal

The dependence of melting temperature T _m on the size and shape of an n -dimensional nanocrystal of elementary single-component substance is studied. The nanocrystal has the form of an n-dimensional parallelepiped with a square base. The ratio of the length of side rib to the length of base rib (which is equal to f) defines the form of the system. It is demonstrated that, if the surface pressure is ignored, the value of T _m decreases with isomorphic (f = const) decrease in the size of nanocrystal. In so doing, the more the value of form parameter f deviates from unity, the more appreciable the size dependence of T _m will be. However, if the surface pressure (“Laplace pressure”) is taken into account in the case of decrease in size, the value of T _m may vary significantly. In so doing, if the surface pressure compresses the nanocrystal, this leads to an increase in T _m when its size decreases. In the case of stretching of nanocrystal by surface pressure, the drop of T _m with isomorphic decrease in size increases. It is demonstrated that the surface pressure may both attenuate (at low values of T _m ) and intensify (at high values of T _m ) the dimensional oscillation of melting temperature. This variation of oscillation of dimensional dependence will be most pronounced for substances with high values of Grueneisen parameter. The variation of melting temperature with decreasing crystal dimension n is studied. It is demonstrated that, in the case of isomorphic decrease in the size of nanocrystal, the coefficient of thermal expansion at melting temperature increases and reaches a maximum, and the specific energies of activation processes and the surface energy decrease and reach a minimum.     

Dependence of the crystal-liquid phase transition temperature on the size and shape of a nanocrystal

The dependence of the crystal-liquid phase transition (melting) temperature T _m on the size and shape of a nanocrystal with a free surface has been studied proceeding from the Lindemann criterion. It is shown that, with neglect of the surface pressure, the T _m value decreases with a reduction in the isomorphous nanocrystal size. The greater the nanocrystal shape deviation from cubic, the more pronounced the size dependence of T _m. With allowance for the surface pressure, the T _m value increases with decreasing nanocrystal size if the arising pressure compresses the crystal. If the surface pressure produces an extension of the crystal, the decrease in T _m with decreasing size becomes more pronounced. The surface pressure can both suppress and enhance the size-dependent T _m variations.     

High Vortex Depinning Temperatures in YBCO Films with BZO Nanorods

The Bose glass theory for the vortex matter in superconductors with correlated disorder predicts the depinning of vortices due to the renormalization of the vortex pinning barriers by thermal fluctuations. For YB₂Cu₃O₇ (YBCO) in external magnetic fields H oriented along the columnar pins generated by various techniques theoretical estimates give a depinning temperature T _dp very close to the critical temperature T _c (T _dp～0.95T _c), whereas the results of standard magnetization relaxation experiments are repeatedly interpreted in terms of a much lower T _dp (～0.5T _c). We investigated the temperature T variation of the normalized magnetization relaxation rate S for YBCO thin films containing BaZrO₃ (BZO) nanorods preferentially oriented along the c axis, with H along the nanorods. The nonmonotonous S(T) variation below the matching field observed up to close to T _c does not support a low T _dp. The often considered S(T) maximum occurring at relatively low T (which was connected to a disappointing T _dp) is related to the occurrence of thermomagnetic instabilities. We show that the accommodation of vortices to the columnar pins in the presence of the T dependent macroscopic currents induced in the sample is signaled by a pronounced S(T) deep located at high T, in agreement with a T _dp close to T _c. By increasing the film thickness and using the substrate decoration the BZO nanorods splay out, leading to the inhibition of (detrimental) vortex excitations involving double vortex kink or superkink formation, characteristic for high-quality thin films and single crystals with columnar pins along the c axis.     

Upper limit ofT c for the copper oxides

The value of the critical temperature of the cuprates correlates with the doping level and is affected by the interplay of two competing factors: (1) the increase in carrier concentration, and (2) the pair-breaking effect of magnetic impurities. An analysis of the temperature dependence of the critical field leads to the conclusion that magnetic impurities are present even in a sample with the maximum observed value ofT _c. A new parameter, “intrinsic”T _c (T _cintr), which is its value in the absence of magnetic impurities, is introduced. The maximum value ofT _cintr, which corresponds to the maximum doping level, appears to be similar for different cuprates and to be equal to 160–170 K. This is the upper limit ofT _c in the cuprates.     

The High Pressure Superconductivity of CaLi2 Compound: The Thermodynamic Properties

The thermodynamic properties of the superconducting state in CaLi₂ at 60 GPa have been described in the paper. The numerical analysis has been carried out in the framework of the Eliashberg formalism. It has been shown that: (i) the critical value of the Coulomb pseudopotential is equal to 0.20, which corresponds to the value of 1795 meV for the Coulomb potential; (ii) the critical temperature (T _C ) cannot be correctly calculated by using the Allen-Dynes (AD) formula; (iii) the dimensionless ratios: $T_{C}C^{N} (T_{C} )/H^{2}_{C} (0 )$ , (C ^S (T _C )?C ^N (T _C ))/C ^N (T _C ) and 2Δ(0)/k _B T _C take the non-BCS values: 0.157, 1.78 and 3.85, respectively. The symbol C ^N represents the specific heat in the normal state, C ^S denotes the specific heat in the superconducting state, H _C (0) is the thermodynamic critical field near the temperature of zero Kelvin, and Δ(0) is the order parameter; (iv) the ratio of the electron effective mass ( $m^{\star}_{e}$ ) to the electron band mass (m _e ) assumes a high value, in the whole range of the temperature, where the superconducting state exists. The maximum of $m^{\star}_{e}/m_{e}$ is equal to 2.15 for T=T _C .     

Two-phase structure of ultra-thin La-Sr-MnO films

The structural, electrical and magnetic properties of ultra-thin La_0.83Sr_0.17MnO₃ (LSMO) films, deposited on NdGaO₃ substrate by using the MOCVD technique, were studied. The film thickness d varied in the range from 4 to 140 nm. X-ray and RHEED measurements demonstrated that the films had a two-phase structure. One phase had an orthorhombic face centred structure (a = 0.406 nm and c = 0.46 nm), while the other one had a cubic perovskite-like structure with a = 0.388 nm. Low field dc resistance and magnetization vs. temperature dependences were investigated in the temperature range from 5 to 300 K using a conventional four-probe method and a SQUID magnetometer. It was found that the temperature of the resistivity maximum, T_m, increases with increasing film thickness and that the value of the Curie temperature T_C estimated from the temperature dependence of magnetization is very close to T_m. Modelling of the remanent magnetization vs. temperature dependence based on a two-phase model was in agreement with experimental results. This model also explains the T_m shift to lower temperatures with decreasing film thickness.     

Photoluminescence, photoacoustic and Raman spectra of zinc oxide films grown by LP-MOCVD using diethylzinc and water as precursors

Polycrystalline zinc oxide (ZnO) films were grown on alkali-free glass substrates by low pressure metalorganic chemical vapor deposition (LP-MOCVD) using diethylzinc (DEZn) and water (H₂O) as precursors. Photoluminescence (PL) spectra of the films were composed of the near-band-edge (NBE) emission at ~ 380 nm and the orange band (OB) emission at ~ 600 nm. Variations of the intensity ratio of the NBE emission to the OB emission (I_NBE/I_OB) and the photon energy of the knee on the photoacoustic spectrum (PA edge) as a function of substrate temperature (T_S) could be divided into two regions at the boundary temperature (T_B). Below T_B, the NBE emission exhibited the tail extending to the higher energy than the bandgap of ZnO and the I_NBE/I_OB value became smaller with increasing T_S. In addition, the PA edge shifted towards lower energies with increasing T_S. Concerning the Raman results, these tendencies seem to have some relations with the decrease in concentration of the secondary phase Zn(OH)₂ with increasing T_S. Above T_B, however, the I_NBE/I_OB became larger and the PA edge shifted towards higher energies with increasing T_S, which is probably due to the removal of defect levels related to the excess oxygen atoms.     

Transport properties of Nd1.85Ce0.15 δ single crystals: The narrow-band model

The anisotropy of the resistivity and thermoelectric power (TEP)S of Nd_1.85Ce_0.15CuO_4?δ single crystal (T _c =17 K) has been investigated. In the temperature rangeT _c <T<300 K the ratioρ _c/ρ_ab≈10⁴ and the dependencesρ _ab (T) andρ _c (T) change from quadratic to linear atT～200 K. The dependencesS _ab (T) andS _c (T) reach a maximum atT>T _c and then decrease almost linearly with increasing temperature, changing sign from positive to negative nearT～ 150 K. The features of the resistivity and TEP temperature dependences (the lawρ∝T ² changing toρ∝T, the change in the sign of S with temperature, and the low TEP anisotropy at largeρ anisotropy) have been interpreted in the framework of the narrow-band model.     

High-pressure studies on Tc and crystal structure of iron chalcogenide superconductors

The superconducting transition temperature, T_c, in iron-based solids can be enhanced by applied pressure: T_c increases from 8 to 37 K for the 11-type FeSe when the pressure is raised from 0 to 4 GPa. High-pressure studies can elucidate the mechanism of superconductivity in such novel materials. In this paper, we present a high-pressure study of Fe(Se_1−xTe_x) and Fe(Se_1−xS_x). In the case of Fe(Se_1−xTe_x), the maximum T_c under high pressure did not exceed the T_c of FeSe, which can be attributed to the structural transition to the monoclinic phase. For Fe(Se_1−xS_x) (0 < x < 0.3), T_c exhibited a significant increase with pressure; however, the maximum T_c under high pressure did not exceed the T_c of FeSe. This may be due to the disorder induced by substituting S for Se, which is similar to the pressure effect on T_c for the 1111-type superconductor Ca(Fe_1−xCo_x)AsF. The T_c of Fe(Se_1−xS_x) showed a complex behavior below 1 GPa, first decreasing and then increasing with increasing pressure. From high-pressure x-ray diffraction measurements, the T_c (P) curve was correlated with the local structural parameter.     

Deformation behaviour of single crystals of InP in uniaxial compression

The deformation characteristics of indium phosphide (InP) single crystals under uniaxial compression have been examined as a function of strain rate, temperature and orientation. It has been shown that at temperatures below 0.55T _m (T _m=melting point; 1335 K) the material fractures in a brittle manner whereas at higher temperatures, within the range 0.55 to 0.71T _m, plastic deformation occurs by both slip and deformation twinning; above 0.71T _m, slip alone is the operative deformation mechanism. The observed operative slip systems are of the type {1 1 1} 0 1 1 which are characteristic of most Group IIIb-Group Vb compounds. Deformation twinning occurs predominantly on {1 1 1} planes but some activity is also observed on planes of the type {3 4 5}.     

The formation of fine relaxation cracks along the compressive maximum shear crack

The formation of fine short relaxation cracks frequently found along the cyclically propagating 45° compressive maximum shear crack were analyzed. Comparison was made of the results computed from two different fracture criteria, the minimum strain energy density (MSED) and the maximum tensile (tangential) stress (MTS). The measured directions θ₀ of the relaxation cracks were in excellent accord with both the theoretical MSED directions and the MTS directions. The computed MSED crack directions were predicated on two conditions: 1) the strain energy density factor was minimum, S _min, and 2) the volume change component S _v of S _min was greater than the distortion component S _d. Such requirements correspond to crack directions with locally maximum tensile tangential stresses.     

Anisotropic Magnetocaloric Effect in Single-crystalline Pr0.52Sr0.48MnO3

We report the magnetocaloric effect (MCE) in a Pr_0.52Sr_0.48MnO₃ single crystal estimated from the isothermal magnetization curve using the Maxwell relation. Isothermal magnetization curves are measured over the range 20 K to 320 K where the field was applied parallel (??) and perpendicular (??) to the [110] direction of the perovskite structure with Pbnm space group. A peak in the temperature (T) dependence of magnetic entropy change (??S _M) with a fairly large negative value (???3.3 J/kg?K) is observed at 275 K close to the Curie temperature (T _C) for a change in field of ??H=40 kOe. The ?? and ?? components of ??S _M deviate from each other below ??260 K and an inverse MCE is observed below ??150 K. We note that the Landau theory of phase transitions satisfactorily explains the ??S _M vs T plot around the second-order transition at T _C.     

Crystal orientation dependence of the out-of-plane dielectric properties for barium stannate titanate thin films

The orientation-dependent out-of-plane dielectric properties of barium stannate titanate (Ba(Sn_0.15Ti_0.85)O₃ (BTS)) thin films prepared by sol–gel method were investigated. Films with (1 0 0), (1 1 0), and (1 1 1) orientation were grown on LaNiO₃-buffered (1 0 0), (1 1 0), and (1 1 1) LaAlO₃ (LAO) single-crystal substrates, respectively. The different temperature of the dielectric constant maximum (T_m) of the BTS thin films with different orientation was believed to be attributing to stress inside the films. Films with the (1 1 1) orientation had higher relative dielectric constant and tunability than (1 0 0)- and (1 1 0)-oriented films. This difference in dielectric properties in these three kinds of oriented BTS films may be attributed to change in the direction and magnitude of electric polarization in orientation engineered BTS films and stress in the films.     

Properties of electron-beam-evaporated tin oxide films

Transparent conducting thin films of tin oxide were prepared by electron beam evaporation of sintered pellets of SnO₂ under controlled conditions. Variations in such parameters as the substrate temperature T_s and the post-deposition annealing temperature T_A and time t_A were studied. Structural, electrical and optical properties were measured to characterize the films. The film structure changed gradually from amorphous to crystalline (SnO phase) as T_S was varied from 150 to 350 °C. A sharp decrease in the room temperature resistivity together with the growth of a crystalline phase occured in the as-deposited films at T_S ≈ 250°C. On annealing in air (T_A = 550°C, t_A = 2 h) a radical structural transformation from amorphous to crystalline occurred with a sharp fall in resistivity for T_S ⩽ 225°C. For T_S = 350°C the lowest resistivity achieved for the undoped annealed films was 6.6×10^-3 Ω cm, the average visible transmittance was about 90% and the structure was characteristic of pure SnO₂.     

Adsorption of mercury cation on chemically modified clay

A montmorillonite clay (M) sample from the Amazon region, Brazil, was intercalated with pyridine (Py), dimethyl sulfoxide (DS) and 3-aminopropyltriethoxysilane (APS). The chemically modified montmorillonite (M_P/APS) sample showed modification of its physical-chemical properties including: specific area 41.39 m² g⁻¹ (M) to 198.45 m² g⁻¹ (M_P/APS). Solid-state ²⁹Si CPMAS/NMR of the silylated montmorillonite samples showed Q² and Q³ signals as well as T² and T³ signals. The appearance of T² and T³ signals can be attributed to the grafting of APS to the interlayer surface silanol groups. The natural and modified clays were used for mercury cation adsorption from aqueous solutions at room temperature and pH 3.0. The energetic effects (Δ_intH°, Δ_intG° and Δ_intS°) caused by mercury cation adsorption were determined through calorimetric titrations.     

Calculated neutron air kerma strength conversion factors for a generically encapsulated Cf-252 brachytherapy source

The ²⁵²Cf neutron air kerma strength conversion factor (S_KN/m_Cf) is a parameter needed to convert the radionuclide mass (μg) provided by Oak Ridge National Laboratory into neutron air kerma strength required by modern clinical brachytherapy dosimetry formalisms indicated by Task Group No. 43 of the American Association of Physicists in Medicine (AAPM). The impact of currently used or proposed encapsulating materials for ²⁵²Cf brachytherapy sources (Pt/Ir-10%, 316L stainless steel, nitinol, and Zircaloy-2) on S_KN/m_Cf was calculated and results were fit to linear equations. Only for substantial encapsulation thicknesses, did S_KN/m_Cf decrease, while the impact of source encapsulation composition is increasingly negligible as Z increases. These findings are explained on the basis of the non-relativistic kinematics governing the majority of ²⁵²Cf neutron interactions. Neutron kerma and energy spectra results calculated herein using MCNP were compared with results of Colvett et al. and Rivard et al.