Birth-and-spread growth on the Kossel and a non-Kossel surface

A birth-and-spread growth model is derived for an anisotropic crystal surface and fitted to growth rate data obtained from Monte Carlo simulations of an isotropic and anisotropic Kossel (0 0 1) surface and a non-Kossel (0 0 1) surface. Only the step free energy is used as a fit parameter. All growth rate sets are nicely fitted by the new expression and the fitted values of the step free energy are in the physically relevant regime.     

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.     

Crystallization behavior and microstructure of (CaO·ZrO2·SiO2)–Cr2O3 based glasses

The effects of chromium oxide on the crystallization behavior of glass compositions in the calcium, zirconium and silicon oxides system were investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopic. Results indicate that crystallization is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 1.0 mol% of doping oxide. The effect of heating rate on differential thermal analysis curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E_c for the different crystalline phases. Activation energy (E_c) was found to be 490 ± 5 kJ/mol for 5.0 mol% chromium oxide in glasses. The most suitable nucleation temperature was determined as 810 °C for the above mentioned glass. The results of this study have highlighted that a small percentage of chromium oxide strongly affects the crystal formation thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallization of calcium, zirconium and silicon oxides glasses.     

Determination of the relaxation behavior in time and frequency domains of a dipolar chain with Glauber dynamics using Monte Carlo simulation

In this paper, we have determined the relaxation behavior of one dimensional dipolar chain with the Glauber dynamics both in time and frequency domains using Monte Carlo simulation. Two algorithms are developed to simulate the decay function for a single dipole and for the whole dipolar chain in the time domain. MC simulations of the whole dipolar chain show that both exponential and KWW type behaviors for the decay function ?(t) = exp[?(t/τ)^β] 0 < β < 1 are possible. The frequency domain transformation is also computed by using the Fourier transform of the corresponding time domain response of the decay function which is obtained by MC simulations. We have determined the components of the normalized complex dielectric permittivity. The observed behavior of loss curves are in full agreement with experiments performed on glass-forming materials in which two loss curves – called α- and β-relaxation processes – are observed.     

Characterizations of pulsed laser deposited SiC thin films

Thin films of silicon carbide (SiC) were prepared using pulsed laser deposition (PLD) on Si(1 0 0) substrates at a temperature of 370 °C. Various structural characterizations showed the development of short-range SiC precipitates in the films. These films were annealed isochronally at temperatures of 800 °C, 1000 °C and 1200 °C for 2 h under an inert environment. Thermally induced crystalline ordering of SiC into β-SiC phase was investigated by X-ray diffraction (XRD), Raman spectroscopy and Fourier transforms infrared (FTIR) spectroscopic measurements. In addition to the crystallization of SiC films, high temperature annealing resulted in the dissolution of carbon clusters found in the as-grown films.     

Boron removal from metallurgical-grade silicon using lithium containing slag

The possibility of extracting boron impurity from metallurgical-grade silicon by lithium containing slag refining for solar cell application was investigated. The distribution coefficients of boron between CaO–SiO₂–Li₂O and CaO–SiO₂–LiF slags were examined. The boron content in the refined silicon was studied under different conditions of mass ratio of slag to silicon and refining time. The results showed that a small amount of Li₂O or LiF had significant effect on the distribution coefficients of boron. The removal of boron impurity using CaO–SiO₂–LiF system was more effective than using CaO–SiO₂–Li₂O system. Values of boron content in the refined silicon did not decrease significantly when the mass ratio exceeded 4, while it had been obviously falling with the increase in refining time. When the mass ratio of CaO–SiO₂–Li₂O slag to metallurgical-grade silicon was 4, the boron content in silicon was successfully reduced from 2.2 × 10^? 5 to 1.3 × 10^? 6 after slag refining for 2 h at 1823 K.     

Density and structure of undercooled molten silicon using synchrotron radiation combined with an electromagnetic levitation technique

X-ray diffraction and density measurements have been simultaneously performed to investigate the atomic structure of molten silicon in the undercooled temperature region with the use of an electromagnetic levitation technique. The density was obtained from the levitated sample shape by a non-contact method based on an image analysis technique. From structural analysis of the undercooled molten silicon, the first nearest neighbor coordination number and interatomic distance were determined to be about 5 and 2.48 Å, respectively. There are no temperature dependence of both the first nearest neighbor coordination number and interatomic distance, but a small change observed on the side of the first peak with the structure factor in the temperature range of 1550–1900 K. From these results, we conclude that the short-range order based on tetrahedral bonds of the undercooled molten silicon did not change with the degree of undercooling, however, the medium-range order was changed by the degree of undercooling.     

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80 ? x)% SiO₂ (10 + x / 2)% B₂O₃ (10 + x / 2)% Na₂O, where 5 < x < 30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

Photo-crystallization in a-Se imaging targets: Raman studies of competing effects

Photo-induced crystallization of a-Se is investigated by Raman spectroscopy as a function of temperature (250–340 K) and exposure time in thin-film structures used as targets in high-gain avalanche rushing photoconductor (HARP) video cameras. The Stokes-to-Antistokes ratio is monitored to obtain the local temperature T_loc at the laser spot; fluxes (632 nm) of 17 and 10 W/cm² are used. We find a rich temperature behavior that reflects the competition of changes in viscosity and strain, and defines four distinct regimes. No photo-crystallization is seen for T_loc below 260 K, nor in a 15 K range around T_g ～ 310 K. For T_loc in the regime 260–302 K the initial rate of crystal growth after onset of photo-crystallization is temperature independent, whereas for T_loc > 318 K the growth rate is thermally enhanced. Our results are in qualitative accord with a theory by Stephens treating the effects of local strain on the secondary growth of crystalline nuclei in a-Se. We conclude that the observed growth rate between 260 and 302 K is driven by local strain, and that relaxation of this strain near T_g suppresses crystal growth until thermally assisted processes accelerate the photo-crystallization at higher temperatures.     

Thermal annealing of a-Si/Au superlattice thin films

The superlattice films, which consist of amorphous silicon (a-Si) and amorphous gold (Au), were prepared by ultra-high vacuum evaporation system. The first layer was grown a-Si with a thickness of 4.2 nm and the second layer was grown Au with a thickness of 0.8 nm. Thermal annealing was performed at 473, 673, and 873 K, respectively. The structural properties of the films were investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), and Raman scattering spectroscopy. The electrical property was assessed by the temperature dependence of electrical conductivity. A crystallization of Si and a forming of Au nanoparticles were observed in all of the annealing films. The crystalline volume fraction reached 70% by annealing time for 15 min. An average diameter of the Au nanoparticles embedded in Si matrix also increased with increasing the annealing temperature. At annealing temperature above 873 K, Au atoms migrated toward the film surface. It was observed that the electrical conductivity changed in several temperatures.     

Extended free radical network from condensed cyanuric chloride with p-phenylenediamine: An EPR/FMR study

A covalent layered network was obtained by condensation of cyanuric chloride with bridging paraphenylenediamine. The local chemical environment of the layered solid can be changed by a redox reaction to obtain new reconstructed derivatives. A blue product was obtained by treating an alcoholic dispersion of the layered solid with ferric nitrate or potassium persulfate, indicating the possible formation of an extended free radical. When iron nitrate was used as oxidant, the temperature-dependent magnetic resonance spectra were measured in the 290–4 K region. The magnetic resonance measurements showed the coexistence at room temperature of two spectra arising from two different magnetic centers: a narrow line centered at g = 2.0038(1) with linewidth of ΔH = 7.42(2) G (free radical) and a broad line centered at g = 2.254(1) with linewidth of ΔH = 1300(5) G (magnetic iron-oxide cluster). A new sample was prepared so that the broader line was more intense. The temperature dependence of the magnetic resonance lines was subject to intense changes in both cases. The integrated intensities decreased with decreasing temperatures in both spectra in the high temperature region. This type of behavior is similar to that of magnetic nanoparticles in non-magnetic matrices. Upon reducing the temperature with the gradient of ΔH_r/ΔT = 1.5(1) G/K, the resonance field of the broad line was shifted towards lower magnetic fields, while the narrow line was shifted towards higher magnetic fields with ΔH_r/ΔT = 0.020(1) G/K. The linewidth of the broader line increased with decreasing temperature, while the narrow line remained almost constant. The magnetic iron-oxide clusters could produce an internal magnetic field acting on free radicals. This field could compel free radicals to form a magnetic ordered state at high temperatures.     

2H NMR studies of supercooled and glassy aspirin

Acetyl salicylic acid, deuterated at the methyl group, was investigated using ²H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin–lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.     

Network structure of multi-component sodium borosilicate glasses by neutron diffraction

Neutron diffraction structure study has been performed on multi-component sodium borosilicate based waste glasses with the composition of (65 − x)SiO_2. · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂, x = 5–15 mol%. The maximum momentum transfer of the experimental structure factor was 30 Å⁻¹, which made available to determine the distribution function with high r-space resolution. Reverse Monte Carlo modelling was applied to calculate several partial atomic pair correlation functions, nearest neighbor distances and coordination numbers have been revealed. The characteristic features of Si–O and Si–Si distributions are similar for all glassy samples, suggesting that the Si–O network consisting of tetrahedral SiO₄ units is highly stable even in the multi-component glasses. The B–O correlations proved to be fairly complex, two distinct first neighbor distances are present at 1.40 Å and 1.60 Å, the latter equals the Si–O distance. Coordination number distribution analyzes has revealed 3 and four-coordinated boron atoms. The O–O distribution suggests a network configuration consisting of boron rich and silicon rich regions. Our findings are consistent with a structure model where the boron rich network contains mostly trigonal BO₃ units, and the silicon rich network is formed by a mixed continuous network of ^[4]Si–O–Si^[4] with several different ^[4]B–O–Si^[4] and ^[3]B–O–Si^[4] linkages.     

Regenerator performance below 4 K in Tm-based bulk metallic glasses

The main obstacle for reaching low temperatures below 4 K using cryocoolers was the inefficiency of the regenerator materials. We report that large volumetric specific heat below 4 K in a wide temperature range has been obtained in Tm-based bulk metallic glasses, and the peak temperature of specific heat peak is tunable. We show that the amorphization of rare-earth based crystalline alloys and the lower magnetic transition temperature of spin glass behavior in the glasses result in the specific heat anomaly below 4 K. The large and tunable specific heat anomaly below 4 K indicates the potential regenerator performance of the glassy materials for sub-4 K cryocoolers.     

Structural characterization of silicon nanocrystals from amorphous silicon oxide materials

Silicon nanocrystals have been produced by disproportionation reaction of bulk silicon monoxide at temperatures higher than 1073 K. More specific, samples annealed at 1123, 1173, 1223 and 1323 K as well as the starting material SiO have been examined. X-ray diffraction, high-resolution transmission electron microscopy and infrared spectroscopy have been employed in order to investigate the structure of the produced silicon nanocrystals. Photoluminescence measurements reveal a three band emission with maxima positioned at 1.33, 1.52 and 1.67 eV. The intensity of the photoluminescence emission increases with the annealing temperature exponentially. This fact can be directly correlated with the disproportionation reaction which results in bigger amounts of silicon nanocrystals by increasing the annealing temperature and is discussed here. Also a possible explanation is given for the origin of each emission band.     

Properties of thin-film silicon solar cells at very high irradiance

The focussed beam of a low-power helium–neon laser is used to study accelerated light-induced degradation (Staebler–Wronski effect) and high steady-state photocarrier generation rates in amorphous and microcrystalline silicon thin-film solar cells, at up to 13 MW m^? 2 irradiance. Even at these high power densities, COMSOL® simulations indicate that heat diffusion into the substrate, aided by spreading conduction via the Ag back-contact, restricts the temperature rise to less than 14 °C. Short-circuit current may be measured directly, and the I–V characteristic estimated by taking into account shunting by the inactive part of the cell. The improved resistance to degradation of microcrystalline silicon cells is shown to persist to high irradiance. Computer simulations of an amorphous silicon solar cell are presented that are consistent with measured un-degraded and degraded properties, and offer insight into prevailing defect creation processes and carrier recombination mechanisms.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.     

Sinter-crystallization of a glass obtained from basaltic tuffs

Glass-ceramic materials, obtained by sinter-crystallization of melted alkaline-olivine basaltic tuffs, were investigated. The kinetics of bulk crystallization was evaluated by differential thermal analysis (DTA) at different heating rates. The phase formation and the sintering behavior of glass powders (<75 μm) were studied in air and in nitrogen atmospheres by DTA and dilatometry, respectively. The crystalline phases formed were identified by X-ray diffraction. The DTA traces showed an unusual phase formation behavior with a higher crystallization trend for the bulk samples. The crystallization activation energy was evaluated as 590 ± 20 kJ/mol in the range 1080–1110 K. A value of about 3 of the Avrami constant, corresponding to three-dimensional growth on a fixed number of nuclei, was evaluated by Ozawa and Augis–Bennet methods. The densification at low-temperatures is reduced by the intensive crystallization process in both air and nitrogen atmospheres. The sintering starts again at 1150–1250 K. In air atmospheres, due to the FeO oxidation, the sintering temperature increases and the percentage of formed crystal phase decreases by 15–20%.