Electrical properties of annealed a-SiC:H thin films

In the present work the dependence of electrical properties of a-SiC:H thin films on annealing temperature, T_a, has been extensively studied. From the measurements of dark dc electrical conductivity, σ_D, in the high temperature range (from 283 up to 493 K), was found that the conductivity activation energy, E_a, is invariant for T_a ⩽ 673 K and equal to 0.64 eV, whereas for T_a from 673 up to 873 K, E_a increases at about 0.2 eV reaching to a maximum value 0.85 eV at T_a = 873 K, suggesting the optimum material quality. This behavior of E_a as a function of T_a is mainly attributed to relaxation of the strain in the amorphous network, which is possibly combined with weak hydrogen emission for temperatures up to 873 K. For further increase of T_a (>873 K) the phenomenon of hydrogen emission, causes rapid decrease of E_a down to 0.24 eV at T_a = 998 K, deteriorating the material quality. These results are also supported by the measurements of dark dc electrical conductivity in the low temperature range (from 133 up to 283 K), where the dependence of the density of gap states at the Fermi level, N(E_F), on annealing temperature presents the minimum value at T_a = 873 K. The Meyer–Nelder rule was found to hold for the a-SiC:H thin films for annealing temperatures up to 873 K. Finally, the dependence of dark dc electrical conductivity at room temperature, σ_DRT, on T_a showed to reflect directly the dependence of E_a on T_a.     

X-ray diffraction measurements for liquid Ge–Si alloys using synchrotron radiation

Energy dispersive X-ray diffraction measurements have been carried out for liquid Ge_1−xSi_x alloys (x = 0.0, 0.3, 0.5, 1.0) using synchrotron radiation at SPring-8. We measured the X-ray diffraction spectra of liquid Ge and Si up to a high temperature range, (liquid Ge from 1270 to1870 K and liquid Si from 1680 to 2020 K), liquid Ge_0.7Si_0.3 at 1620 K, and liquid Ge_0.5Si_0.5 at 1540, 1590, 1670 and 1720 K. The total structure factors of the liquid Ge–Si alloys have a characteristic shoulder on the high-wave-vector side of the first peak. We deduced a pair distribution function from the Fourier transform of the observed structure factor, which was weakly dependent on the temperature. The nearest-neighbor coordination number of liquid Ge–Si alloys is close to that of pure liquid Ge and Si. The first peak of the pair distribution function moved to a shorter distance with increasing Si concentration. These results may indicate that the atomic radii of the Si and Ge atoms in the pure liquid are preserved in the liquid alloys.     

Silica polymorphs,glass and melt: An in situ high temperature XAS study at the Si K-edge

High temperature X-ray absorption spectra at the Si K-edge were obtained for SiO₂ quartz from room temperature up to 2030 K. Important modifications are observed for the XANES spectra. These change are related to rearrangements of the SiO₄ tetrahedra beyond the short-range correlations. To interpret these spectral evolutions, SiO₂ polymorph samples were observed at room temperature and XANES calculations using FDMNES were performed. Very strong differences are shown between the different polymorphs even between α and β phases for which only small displacive angle rotations of the SiO₄ tetrahadra occurs. Therefore the quartz α to β transition could be identified at its expected temperature, 842 K. A badly defined transition toward β-cristobalite is observed between 1670 and 1940 K. The dynamics of this totally reconstructive transition was further investigated on heat treated cherts. Finally the liquid is reached around 2000 K. Many similarities were observed on SiO₂ between its glass at room temperature, β-cristobalite and liquid at high temperature.     

Octahedral fluoride glasses: Raman spectra and structure of niobium pentafluoride

Raman spectroscopy is used to characterize the NbF₅ phases in the temperature range 80–500 K. A new clear glass is formed by quenching the melt to liquid nitrogen temperatures having a glass transition at ～206 K and crystallization at ～233 K. For all phases including the melt, the glass, the supercooled liquid, the crystalline solid and the gas, the Raman spectra show a rather common high frequency band at ～760 cm^?1 which is attributed to the Nb–F terminal frequency of partially bridged ‘NbF₆’ octahedra. Based on the systematics of the Raman spectra for all phases and the literature physicochemical data a model is proposed for the glass and the liquid phases where ‘NbF₆’ octahedral bridged in cis and/or trans configurations form a variety of cyclic and/or chain structures which intermix building up the overall structure. At exceptionally low energies (<11 cm^?1) a rather weak in intensity Boson peak is observed in the glass which shifts to even lower energies with increasing temperature. Librational and/or tortional motions of the bridged octahedra participating in the glass structure are possible candidates for the origin of this peak.     

Experimental evidence against the existence of an ideal glass transition

The absolute liquid heat capacity of poly(α-methyl styrene) was determined at temperatures far below T_g and T_K in previous work by use of pentamer/polymer athermal mixtures. Here the data is compared to data compiled by Wunderlich and coworkers from 0 K to above T_g in order to obtain the absolute entropy for the polymer in its equilibrium state at temperatures as much as 180 K below the glass temperature or 130 K below the Kauzmann temperature. The results provide no evidence of a second-order transition or of a smeared transition in the entropy. In addition, we find no evidence that the entropy would become negative at a finite temperature.     

NMR in soft materials: A study of DMPC/DHPC bicellar system

The DMPC/DHPC bicellar system at the molar ratio of 2.8:1 has been characterised by measurements of self-diffusion coefficient (using PFGSE and PFGSTE NMR sequences), differential scanning calorimetry (DSC) and small angle scattering of synchrotron radiation (SAXS). The DSC curve shows only one endothermic peak characterised by the peak temperature T_peak = 295.7 K and the onset temperature T_onset = 290.1 K. This peak can be assigned to the nematic to smectic phase transition. Below the phase transition temperature, NMR diffusion experiments indicate a two-exponential decay of the spin echo amplitude allowing two diffusion coefficients D₁ and D₂ to be extracted from the experimental data. The maximum size (D_max) of the bicelle determined from SAXS data using the pair distance distribution function p(r) is 11.2 nm and the bilayer thickness is 5 nm.     

The shoulder in the second peak of the pair correlation function of superheated liquid Fe80B20 alloy

By referencing to a series of superheated liquid Fe_100 ? xB_x (x = 0 to 100) alloys, the existence of the shoulder in the second peak of the pair correlation functions of liquid Fe₈₀B₂₀ alloy is confirmed. The shoulder is caused by the unequal opportunity of connection modes rather than the occurrence of particular types of B-centered clusters. Our model predicts that vertex sharing and face sharing of the B-centered structure units are the two significant connection modes responsible for the shoulder. This picture may fill the gap in understanding the widely different shapes of the second peak in superheated liquid and amorphous Fe₈₀B₂₀ alloy.     

Structural properties of liquid Au–Si and Au–Ge alloys with deep eutectic region

Au–Si and Au–Ge alloys have a deep eutectic region around 19 at.% Si and 28 at.% Ge, respectively. The metallic glass was prepared by quenching around 25 at.% Si near the eutectic composition for the Au–Si alloys [2] which has stimulated many researchers to study on the structural properties of such liquid alloys. However their results were different from each other, which seems due to the experimental difficulties from heavy absorption of both X-ray and neutron beams by Au atom in diffraction experiments. X-ray diffraction measurements have been carried out for liquid Au–Si and Au–Ge alloys around the eutectic region by the transmission method using high-energy X-rays to investigate the atomic arrangements in the liquid state. From the temperature dependence of the observed structure factors, the partial pair correlation and the detailed atomic arrangements will be discussed by Reverse Monte Carlo analysis. The produced atomic arrangements around the eutectic region suggest a substitutional structure and also an increase in liquid density with decreasing temperature.     

Beta radiation induced thermoluminescence in pure ZrO2 prepared by sol–gel

Pure ZrO₂ powders were prepared by the sol–gel method from zirconium oxychloride. For hydrolysis and peptization aqua ammonia and nitric acid were used. The obtained hydrogel was converted to xerogel by aging in air at room temperature and subsequent drying at 373 K for 30 min. This initial xerogel was calcinated in air at different temperatures in the range from 873 to 1273 K for 30 min. Beta irradiated powder exhibit four thermoluminescence peaks with maxima at 340, 415, 470 and 540 K. To clarify the structure of the glow curves a thermal cleaning procedure was used. The initial rise method and deconvolution of the glow curves recorded after preheats were applied to determine the peaks kinetic parameters. All the peaks are well described by one set of the kinetic parameters. The kinetics orders are equal to 2.0, 1.4, 1.7, 1.3 and the activation energies are equal to 0.65, 0.78, 1.1, 1.2 for the increasing peak temperatures, respectively. The calcination temperature strongly influences the sensitivity of ZrO₂. The dependences of the peak areas on the calcination temperature are fitted well with straight lines in the Arrhenius coordinates with the activation energy of 1.7 eV that can be related with migration of oxygen vacancies.     

Electrical properties of SrBi2(Nb0.7V0.3)2O9−δ in the SrO–Bi2O3–0.7Nb2O5–0.3V2O5–Li2B4O7 glass system

Glasses in the system (100 − x)Li₂B₄O₇–x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (where x = 10, 30 and 50, in molar ratio) were fabricated via melt quenching technique. The compositional dependence of the glass transition (T_g) and crystallization (T_cr) temperatures was determined by differential thermal analysis. The as-quenched glasses on heat-treatment at 783 K for 6 h yielded monophasic crystalline strontium bismuth niobate doped with vanadium (SrBi₂(Nb_0.7V_0.3)₂O_9−δ (SBVN)) in lithium borate (Li₂B₄O₇ (LBO)) glass matrix. The formation of nanocrystalline layered perovskite SBVN phase was preceded by the fluorite phase as established by both the X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The dielectric constants for both the as-quenched glass and glass–nanocrystal composite increased with increasing temperature in the 300–873 K range, exhibiting a maximum in the vicinity of the crystallization temperature of the host glass matrix. The electrical behavior of the glasses and glass–nanocrystal composites was characterized using impedance spectroscopy.     

X-ray diffraction measurement of liquid As2Se3 by using third-generation synchrotron radiation

X-ray diffraction (XD) measurements of liquid As₂Se₃ were carried out in the temperature range up to 1600 °C where the temperature is well beyond the semiconductor to metal (SC–M) transition temperature around 1000 °C. The measurements were done by using third-generation synchrotron radiation at SPring-8 and the obtained structure factors have been much improved compared to previous in house XD measurements with regard to the momentum transfer range and the data statistics. The deduced pair distribution functions show that with increasing temperature, the position of the first peak does not change within the errorbar and the coordination number gradually decreases up to 1600 °C irrespective of the SC–M transition. These results coincide with those of the first-principle molecular dynamics simulation.     

Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.     

Polarons in boron doped iron phosphate glasses

The electrical conductivity and dielectric properties of xB₂O₃–(40 ? x)Fe₂O₃–60P₂O₅ (x = 6–20, mol%) glasses were investigated in the frequency range from 0.01 Hz to 1 MHz and the temperature range from 303 K to 523 K. At temperatures below 523 K an ac conductivity and the dielectric constant follow the universal dielectric response (UDR), being typical for hopping or tunneling of localized charge carriers. A detailed analysis of the temperature dependence of the UDR parameter s in terms of the theoretical model for tunneling of small polarons revealed that below 523 K this mechanism governs the charge transport in these glasses. The comparison of the values of characteristic coefficients W_∞ and α determined by two different methods confirms the polaronic behavior of boron doped iron phosphate glasses.     

Microscopic collective dynamics in liquid tellurium

The spectra of collective excitations of liquid tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (T_m = 723 K) and at ∼1000 K. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods.     

Magnetic properties of liquid 3d transition metal–Si alloys

The magnetic susceptibility has been measured for liquid transition metal–Si alloys (TM_1−cSi_c, TM = Ni, Co, Fe, Mn) as a function of temperature. The magnetic susceptibilities of liquid Ni_1−cSi_c with c ⩾ 0.3, liquid Co_1−cSi_c with c ⩾ 0.4 and liquid Fe_1−cSi_c with c ⩾ 0.6 were found to be almost independent of temperature, which suggests that the Ni, Co and Fe ions on the Si side are in the non-magnetic state. Liquid Ni_1−cSi_c with c ⩽ 0.2, liquid Co_1−cSi_c with c ⩽ 0.3 and liquid Fe_1−cSi_c with c ⩽ 0.5 show the Curie–Weiss behavior with a reasonable value of effective Bohr magnetic number. Liquid Mn_1−cSi_c with c ⩽ 0.2 and c ⩾ 0.6 were found to be in their non-magnetic state. However, the Curie–Weiss behavior was observed for liquid Mn_1−cSi_c with c = 0.3, 0.4 and 0.5. The magnetic susceptibility of their liquid alloys in the non-magnetic state has been studied by using Anderson model. The density of 3d states at the Fermi level can be estimated from the data of magnetic susceptibility.     

Measurement of diffusion coefficients of Au in liquid Ag with the shear cell technique

Diffusion coefficients of Au in liquid Ag were measured at the temperatures 1300 K and 1500 K by using the shear cell technique. The adopted technique was a modified long capillary method which had a diffusion couple configuration. Obtained diffusion coefficients of Au in liquid Ag were respectively 2.30 × 10⁻⁹ m² s⁻¹ at 1300 K and 3.16 × 10⁻⁹ m² s⁻¹ at 1500 K, which were in good agreement with calculations based on the hard sphere mixture model.     

Liquid–liquid phase separation in sodium borosilicate glass: Ordering phenomena in particle arrangement

A structure of 13.9Na₂O · 36.0B₂O₃ · 50.1SiO₂ glass heat-treated at 610 °C for 5 h (glass G1) and 10 h (glass G2) was studied by stereological method on the basis of electron microscopy data obtained from plane sections of samples. It has been found that liquid phase separation in the course of heat treatment leads to formation of spherical particles with the volume fractions 0.047 (G1) and 0.065 (G2). It is argued that the stage of growth takes place in the glass G1 and the Ostwald ripening stage occurs in the glass G2. The analyses of pair correlation functions of particle traces on a secant plane indicate elements of ordering in mutual arrangement of particles for the both glasses G1 and G2. This conclusion is confirmed by results of numerical simulation. The experimental data on the pair correlation functions are in agreement with the results of calculations presented in the literature. It is noted that the ordering elements in mutual arrangement of particles play a key role in light scattering and other physical properties of material.     

Effect of crystallization behavior on the oxidation resistance of a Zr–Al–Cu metallic glass below the crystallization temperature

The crystallization behavior of Zr_65.0Al_7.5Cu_27.5 (at.%) metallic glass with 753 and 1053 K annealing treatment and its effect on oxidation resistance around the supercooled liquid region at 623 and 663 K was studied. The crystalline phase of bct-Zr₂Cu precipitates for the specimen annealed at 753 K was observed, while duplex structures of bct-Zr₂Cu and Zr₂Al formed in the specimen annealed at 1053 K. The oxidation resistance of the specimen depended on the amount of crystalline precipitates. Regardless of the exposure temperature, the annealed specimens showed higher oxidation resistance than the melt-spun one, especially for the specimen annealed at 1053 K. The formation of numerous crystalline phases of bct-Zr₂Cu and Zr₂Al from the matrix was responsible for improving the oxidation resistance due to their higher oxidation resistance and promotion of the development of Al₂O₃ and oxides of copper. The oxide constituents of the amorphous alloy after long exposure depended on the temperature. The oxide was composed of a large amount of CuO, some tetragonal and monoclinic-ZrO₂ and a minor amount of Cu₂O as well as a slight amount of Al₂O₃ for the melt-spun specimen during exposure at 623 K. Under the 663 K exposure, however, the oxide state of Cu³⁺ in the scale was also detected.     

Magnetic studies of intermetallic compounds Al3R (Al11R3) both in the solid and liquid states

The magnetic susceptibility of Al₃R and Al₁₁R₃ (R = Ce, Pr, Sm, Gd, Dy, Ho, Yb) compounds is investigated over wide temperature (300–1900 K) and field (0.2–1.2 T) ranges. It is found that microinclusions of Al₂R can exist in the samples. They are not detectable in ordinary X-ray diffraction but give a significant contribution to the magnetic properties. The average effective magnetic moment per rare-earth atom changes through the rare-earth metals line reaching its maximum for holmium but rather lower than for the pure rare-earth metals. It means that rare-earth elements form covalent bonds with aluminum. These bonds remain in the liquid phase and begin to break down after melting point of Al₂R only. The transition into a true solution state of the melt is somewhere above 1900 K.     

Tensile properties of ZrCu-based bulk metallic glasses at ambient and cryogenic temperatures

The tensile behaviors of a series of (Zr_47.5Cu_47.5Al₅)_1 ? x(Zr₈₀Nb₂₀)_x (x = 0, 0.05, 0.10, 0.15) bulk metallic glasses were studied at ambient and cryogenic (77 K) temperatures. It is found that the tensile strength of the alloys increases as the temperature decreases from 298 K to 77 K. The maximum enhancement is 15.7%, and the toughness of these alloys does not deteriorate at low temperatures. We demonstrate that the higher energy required to raise the temperature in the shear bands from the cryogenic temperature to glass transition temperature is the origin of the tensile strength enhancement at low temperatures.