Temperature effects on the properties of Ge thin films

The effects of substrate temperature (T_s) on the properties of vacuum evaporated p-type Ge thin films have been investigated for 25s<400°C. Increase in the substrate temperature improves the crystallinity and increases the grain size resulting a gradual change from amorphous to polycrystalline structure which was attained above a substrate temperature of 225°C. Low resistive (1×10^–2 ohm-cm) and high mobility (280 cm²/V·s) films were obtained at T_s=400°C. It has been observed that the conduction mechanism in polycrystalline films was dominated successively by hopping, tunneling and thermionic emission as the sample temperature was increased from 40 to 400 K. In amorphous samples, conduction was described in terms of different hopping mechanisms.     

Barium–Strontium and Nickel–Cobalt Solid-State Interdiffusion between Hexagonal W-Ferrites BaM2Fe16O27and SrM2Fe16O27 (M = Ni2+, Co2+)

The Ni, Co, Ba, and Sr profiles in the diffusion zones produced between hexagonal W-ferrites (BaCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/SrNi₂Fe₁₆O₂₇, SrCo₂Fe₁₆O₂₇/BaCo₂Fe₁₆O₂₇, and SrCo₂Fe₁₆O₂₇/BaNi₂Fe₁₆O₂₇) by annealing at 1520 K were used to evaluate the interdiffusion coefficients of the cations involved by the Boltzmann–Matano method over the whole composition range.     

Densities of Sulfur Hexafluoride and Dinitrogen Monoxide over a Wide Temperature and Pressure Range in the Sub- and Supercritical States

Densities of sulfur hexafluoride (SF₆) and dinitrogen monoxide (N₂O) have been measured with a fully computer-controlled high-temperature high-pressure vibrating tube densimeter system in the sub- and supercritical states. The uncertainty in density measurement was estimated to be between ±0.2 and ±0.3kg·m^–3 depending on the temperature. With respect to accuracy, reliability, suitability, and time consumption, this system has significant advantages for measuring PT properties in the compressed liquid and supercritical states. The densities were measured for temperatures from 273 to 623K and at pressures up to 30MPa for SF₆ (442 data points) and from 273 to 473K and up to 40MPa for N₂O (251 data points), which encompassed density ranges between 142.9 and 1778.5kg·m^–3 for SF₆ and between 124.4 and 1051.1kg·m^–3 for N₂O. Furthermore, the liquid densities of SF₆ and N₂O were correlated with a new three-dimensional density correlation system (TRIDEN) and the complete set of PT data in the sub- and supercritical states were correlated with a virial-type equation of state. For checking the accuracy and suitability of the vibrating tube densimeter system, the experimental densities of SF₆ were compared with published data and with the results of a reference equation of state.     

X-ray-Induced Paramagnetic Centers in SiO2Xerogels

SiO₂xerogels prepared by a sol–gel process followed by sintering at 350–1100°C were irradiated with 30-keV x-rays at a dose rate of 80 R/s. The room-temperature electron spin resonance (ESR) spectra of the xerogels showed signals arising from radiation-induced intrinsic defects (E" centers and terminal oxygens) and organic radicals. By recording ESR spectra in a vacuum of 10^–2Pa, surface E" centers differing in relaxation time from the volume centers were identified for the first time. The ESR signal from the E" centers was measured as a function of x-ray dose. The possible mechanisms for the formation of E" centers under x-ray irradiation are discussed.     

Coherent Brillouin Spectroscopy in Solid Parahydrogen

We applied coherent Brillouin spectroscopy to solid parahydrogen, and measured the Brillouin spectra of longitudinal acoustic modes at 5.6K. It was found that the linewidth of these spectra is 1.5MHz. From the observed Brillouin shift and the crystal orientation, the elastic stiffness was determined as C ₁₁=0.355±0.016GPa and C ₃₃=0.432±0.022GPa.     

CuGaTe2–CuAlTe2 System

It was shown by x-ray diffraction and differential thermal analysis that CuGaTe₂ and CuAlTe₂ form a continuous series of solid solutions. CuGaTe₂, CuAlTe₂, and CuAl _x Ga_{1 – x} Te₂ crystals consisting of large blocks were grown by the horizontal Bridgman process, and their thermal expansion and near-edge transmission and reflection spectra were measured. The thermal expansion coefficient was shown to vary linearly across the solid-solution series, while the band gap is a nonlinear function of composition.     

High-Pressure Phase Transitions of M2O5(M = V, Nb, Ta) and Thermal Stability of New Polymorphs

The stability regions of various M₂O₅(M = V, Nb, Ta) polymorphs were studied by quenching samples from 600–1300°C at pressures in the range 5.0–8.0 GPa. Above 7.0 GPa, Nb₂O₅and Ta₂O₅were found to have a new polymorph (Zphase) in which the metal atoms are in sevenfold coordination. In addition, the samples contained the high-pressure polymorph B-M₂O₅ with the rutile-related structure. Differential thermal analysis at atmospheric pressure showed a weak, broad exotherm, indicating that the transformation of Z-M₂O₅into other phases begins at 100–150°C and reaches completion at 400°C in Nb₂O₅and 550°C in Ta₂O₅. At p 8.0 GPa and t> 750°C, a new high-pressure phase B-V₂O₅, isostructural with B-Nb₂O₅, was identified (a= 11.9640(6) Å, b= 4.6986(3) Å, c= 5.3249(3) Å, = 104.338(4)°, V= 290.01 ų, Z= 4, sp. gr. C2/c). At atmospheric pressure, B-V₂O₅transforms into -V₂O₅, with two strong exothermic peaks at 230 and 270°C. Experiments in the pressure range 5.0–8.0 GPa confirmed that the high-pressure phase -V₂O₅has a broad stability region.     

The Transfer of Matrix Toughness to Composite Mode I Interlaminar Fracture Toughness in Glass-Fibre/vinyl Ester Composites

The transfer of matrix toughness to composite mode I interlaminar fracture toughness (G _Ic ) has been investigated in unidirectional glass-fibre reinforced composites with brittle and rubber-toughened vinyl ester matrices. Single-edge-notch bend (SENB) and double cantilever beam (DCB) specimens were used for matrix and composite G _Ic characteristion, respectively. The initial crack opening displacement rate was used as the parameter for comparison of G _Ic results. Matrix G _Ic was completely transferred to composite G _Ic for crack initiation (G _Ic-init) in the brittle-matrix composites, but in the toughened composites transfer was only partial due to the presence of fibres. The conclusion is that the maximum contribution to energy absorption by the matrix is more accurately reflected by G _Ic-init, and should be used for further assessment of the enhancing effect of fibre bridging during steady-state crack propagation, instead of matrix G _Ic . A plot of composite G _Ic for steady-state crack propagation, G _Ic-prop versus G _Ic-init indicates that the enhancing effect of fibre bridging is greater in the toughened composites. This enhancement is related to a larger deformation zone size in the toughened matrices.     

The separation of the fracture energy in metallic materials

The total plastic strain energy which is consumed during fracture of a plain-sided CT specimen is separated into several components. These are the energies required for deforming the specimen until the point of fracture initiation, for forming the flat-fracture surfaces, for forming the shear-lip fracture surfaces, and for the lateral contraction and the blunting at the side-surfaces, W _lat. Characteristic crack growth resistance terms, R _flat and R _slant, are determined describing the energies dissipated in a unit area of flat-fracture and slant-fracture surface, respectively. R _flat is further subdivided into the term R _surf, to form the micro-ductile fracture surface, and into the subsurface term, R _sub, which produces the global crack opening angle. Two different approaches are used to determine the fracture energy components. The first approach is a single-specimen technique for recording the total crack growth resistance (also called energy dissipation rate). Plain-sided and side-grooved specimens are tested. The second approach rests on the fact that the local plastic deformation energy can be evaluated from the shape of the fracture surfaces. A digital image analysis system is used to generate height models from stereophotograms of corresponding fracture surface regions on the two specimen halves. Two materials are investigated: a solution annealed maraging steel V720 and a nitrogen alloyed ferritic-austenitic duplex steel A905. For the steel V720 the following values are measured: J _i=65kJ/m², R _surf=20kJ/m², R _flat=280kJ/m², R _slant=1000kJ/m², W _lat=30J. For the steel A905 which has no shear lips, the measured values are: J _i=190kJ/m², R _flat=1000kJ/m², and W _lat=45J. Apart from materials characterization, these values could be useful for predicting the influence of specimen geometry and size on the crack growth resistance curves. Key words: Elastic-plastic fracture mechanics, fracture energy, energy dissipation rate, fracture surface analysis.     

Fractal Surfaces of ZrO2 , WO3 , and CeO2 Powders

The surface fractal properties of ZrO₂, WO₃, and CeO₂ powders prepared by the thermal decomposition of ZrO(NO₃)₂, (NH₄)₄W₅O₁₇, and (NH₄)₂Ce(NO₃)₆, respectively, were studied by mercury porosimetry. The results demonstrate that these oxides may, in principle, have fractal surfaces owing to topochemical processes of the type A(s) B(s) + C(g). The surface fractal dimension of individual crystallites and their aggregates are determined.     

Corrosion of Iron in a Carbonate-Bicarbonate Solution. Part 1. Crystallographic Analysis of Passive Films

Visible passive films are formed on the surface of 1010 steel at 325K in a solution of 1N NaHCO₃ + 1N Na₂CO₃ at anodic potentials up to –320mV_NHE inclusively. For potentials below –400mV, they are dull and contain FeCO₃. For –(390–320)mV, the films are thin blackish bright. Most probably, the diffraction peaks are caused by Fe₃O₄–Fe₂O₃. For –(430–400)mV, the films are dull blackish. Probably, this color is caused by a composite of magnetite and siderite.     

Application of Raman in phase equilibrium studies: the structures of substitutional solid solutions of KNO3 by RbNO3

Raman spectroscopy was employed to investigate the KNO₃-RbNO₃ system. Raman studies indicated that Rb⁺ may substitute K⁺ up to 67 mol% in the solid solutions of the KNO₃ II structure (denoted as K_1–x Rb_xNO₃ (KII)) and up to 80 mol% in the solid solutions of the KNO₃ III structure (denoted as K_1+x Rb_xNO₃ (KIII)). The substitutional crystals retained the transitions of I to III to II of KNO₃ on cooling and the metastable property of KNO₃ III at room temperature. It was found that rubidium nitrate had a considerable tendency to have the structure of potassium nitrate. This accounts for the fact that larger rubidium ions can replace many more smaller potassium ions in the nitrate than vice versa. When the concentration of rubidium ions was more than 67 mol%, the substituted crystals underwent the mixed structural phase transition of the KNO₃ III structure to the RbNO₃ IV structure, and K_1–x Rb_xNO₃ (KIII) seemed to consist of disordered R3m microstructure.     

Electrical Conductivity of Composite Materials Based on Fine-Particle Silicon near the Metal–Insulator Transition

Si/SiO₂ composites containing 17, 19, and 21 wt % crystalline B-doped Si particles (near the insulator–metal transition) are studied by impedance spectroscopy. The results indicate that, at low frequencies, the nonlinear variation of the real part of electrical conductivity, Re, with applied dc voltage V for the composite containing 17 wt % Si (dielectric properties) is due to the current being limited by the space charge buildup at traps in the silica layers between Si particles. At high frequencies, the nonlinear Re(V) behavior is due to tunneling through the contacts between the particles.     

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from to -SiC reveals a dependency on the initial -content and the sintering atmosphere. The transformation rate decreases with an increasing -content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure -SiC starting powders when the additive system consists of 10.34 wt% Y2O3 and 2.95 wt% AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial /-ratio (>1:9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.     

Synthesis, crystal structure and X-ray powder diffraction data of the phosphor matrix 4SrO·7Al2O3

The white phosphor matrix 4SrO·7Al₂O₃ has been synthesized by firing the appropriate mixture of SrCO₃, Al(OH)₃ and H₃BO₃ in the molar ratios 1:3.5:0.135 at 1300°C for 4–7 h. The crystal structure of 4SrO·7Al₂O₃ has been determined as a orthorhombic Pmma space group with a=24.7451(2)Å, b=8.4735(6)Å, c=4.8808(1)Å, V=1023.41(3)ų, Z=2, and D=3.66 g cm^–3 by the Rietveld analysis. The refinement figures of merit are R_p=8.26, R_wp=11.60, R_bragg=4.44 and s=2.61 for 844 reflections with 2<119.94°. And the corresponding X-ray powder diffraction data are presented for search/match analysis.     

Preparation of LiTaO3 , LiNbO3 , and NaNbO3 from Peroxide Solutions

A procedure is described for preparing peroxide solutions of LiTaO₃ and LiNbO₃. It is shown that the use of peroxide solutions makes it possible to obtain stoichiometric LiTaO₃, LiNbO₃, and NaNbO₃ and Li _x Na_{1 – x} Nb₃ solid solutions.     

Dielectric and Magnetic Properties of Pb(Fe1 /2Nb1 /2)O3–Pb(Fe2 /3W1 /3)O3Solid Solutions

Pb(Fe_1/2Nb_1/2)O₃–Pb(Fe_2/3W_1/3)O₃solid solutions were characterized by dielectric measurements at low frequencies and in the microwave range and magnetic measurements at high frequencies. The observed microwave dispersion was tentatively attributed to the domain mechanism of polarization. The obtained results suggest that the solid solutions studied are potential microwave-absorbing materials.     

Crystal Structure of (Ti,Mo)12P7and Refined Phase Equilibria in the Ti–Mo–P System

A new ternary compound of composition Ti_3.8Mo_8.2P_7.0 was identified, and its structure was determined by single-crystal x-ray diffraction: sp. gr. P6¯, a= 1.67821(6) nm, c= 0.33196(2) nm; R _F = 0.073 and R _w = 0.078 for 1359 independent reflections with F _hkl > 4(F _hkl). The phase equilibria in the Ti–Mo–P system were refined in the region 0–25 mol % Ti and 33–40 mol % P.     

The piezoresistance coefficients of copper and copper-nickel alloys

This paper attempts to further a better understanding of the piezoresistance coefficients by studying the piezoresistive effects in copper and copper-nickel alloys. The experimental evidence of isotropic piezoresistance coefficients (₁₁=₁₂) has been obtained for the annealed copper and copper-nickel alloys. The piezoresistance coefficients of the cold-worked copper and Cu₆₀Ni₄₀ alloy are of the tensor character (₁₁₁₂). A physical explanation has been given to the change of the ( _ij ) tensor.     

Phase Transitions in the Gross–Neveu Model on a Sphere for High-T c Materials

Phase transitions were studied in the 3D Gross–Neveu model on a sphere at T 0. The zeta function of the Dirac operator of the model was represented with a double sum, and its derivative was calculated using recurrent formulas for generalized Epstein–Hurwitz functions. The effective potential Vwas determined as a function of the field, temperature T, and inverse radius of curvature r. Phase transitions first-order in temperature at R= 0 (T _c = 1/2ln2) and second-order in curvature R= 2/r ² at T= 0 (r _c = 1/1.5) were identified. For the general case, T 0 and R 0, the critical phase dependence T _c(r _c) was obtained, and the V(, T, r) surface was constructed. External stimuli such as temperature and curvature were shown to restore the symmetry of the system in the 3DGross–Neveu model.