The effect of the temperature on the reduction kinetics of oxidized Ni(111) surfaces by hydrogen

The reduction kinetics of oxidized Ni(111) surfaces are measured in situ with ellipsometry in a temperature range between 450 and 675 K. The reaction rate is proportional to the square root of the hydrogen pressure below reduction temperatures of 525 K. The rate limiting step is the reaction between chemisorbed oxygen and dissociated hydrogen and has an experimental activation energy of 57 ± 7 kJ/mol. This reaction takes only place on the oxide free part of the Ni surface. Above 600 K, the reaction rate is proportional to the hydrogen pressure. The rate limiting step is the formation of water and has no experimental activation energy. At temperatures above 600 K the distribution of oxygen throughout the Ni crystal has a large effect on the reduction curves. A new reduction model is proposed that describes all the observed curves satisfactorily.     

Synthesis, crystal growth and characterization of a metal-organic nonlinear optical tri-allylthiourea mercury chloride single crystals

Metal-organic nonlinear optical material tri-allylthiourea mercury chloride single crystal was grown using low temperature solution growth method. The growth parameters such as pH and temperature were optimized for getting colorless single crystals. The X-ray rocking curve measurement shows that the grown crystal was free from internal structural grain boundaries. The structural, spectral, optical and thermal properties of the grown specimen were studied using various characterization techniques. The defects present in the grown crystals were studied with chemical etching analysis. The results show that the grown crystals are suitable candidates for nonlinear optical applications.     

An equation of state for alkali halide crystals

Nonparametric equations of state are obtained for NaBr, KBr, RbCl, and RbBr crystals with B1 and B2 lattices over a wide pressure range. Binding energy, lattice parameters, and free crystal compressibility are calculated together with phase-transition pressures and volume change on transition from B1 to B2 lattice. Calculated results agree well with experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–105, January. 1976.     

Na-doped optical Germanium bulk crystals

In an effort to develop a material for infrared (IR) optics with improved parameters, bulk crystals of optical germanium doped with Na have been first grown and studied. Single-crystalline and coarse-crystalline Ge:Na boules of different shapes and dimensions, up to 10 kg by weight, have been grown. Sodium was incorporated into the Ge crystal during the crystal growing from the melt. Despite the fact that Na contamination in the source material was not strictly controlled, the density of Na in the grown crystals determined by the neutron activation analysis as well as by the glow discharge mass spectrometry did not exceed 10¹⁵ cm^?3. Just this value may be supposed to be close to the solubility limit of Na incorporated in Ge in the course of bulk crystal growth. A first demonstration of donor behavior of Na in bulk Ge crystals is made by means of a thermoelectric type of testing. An interstitial location of Na impurity has been verified by experiments on donor drift in the dc electric field. The crystals are grown with free electron density in the range from 5?10¹³ to 4?10¹⁴ cm^?3 which is optimal for using Ge crystals as an optical material for fabricating passive elements of the IR technique. A comparison between the properties of Ge:Na crystals and Ge crystals doped with Sb, a conventional impurity in optical germanium, grown under the same technological conditions and from the same intrinsic Ge as a source material, revealed a number of advantages of Ge:Na crystals; among them, the higher transparency in the IR region, smaller radiation scattering and higher regular optical transmission, lower dislocation density, more uniform distribution of electrical and optical characteristics over the crystal volume, the identity of optical parameters in the single-crystalline, and coarse-crystalline boules. No degradation of optical elements fabricated from Ge:Na crystals was detected in the course of their commercial application, starting from 1998.     

The phase coexistence method to obtain surface free energies and nucleation barriers: a brief review

ABSTRACT

A recently developed method where one analyses the finite size effects associated with liquid–solid phase equilibria including vapour–crystal coexistence is briefly reviewed. It is shown that the estimation of the chemical potential of the vapour surrounding the crystal as function of the crystal volume yields information on the bulk coexistence conditions, when an extrapolation to the thermodynamic limit is performed. Estimating the pressure of the fluid surrounding the crystal nucleus in the finite simulation box and the volume of this nucleus that coexists with the fluid in thermal equilibrium, an estimate for the total surface excess free energy can be obtained, which to a very good approximation is independent of the size of the simulation box. The free energy barrier against homogeneous nucleation of crystals thus can be estimated as a function of the nucleus volume. Monte Carlo simulations for the soft effective Asakura–Oosawa model of colloid-polymer mixtures which form face-centered cubic colloidal crystals are used to exemplify this method, computing the surface excess free energy of these crystals over a wide range of crystal volumes, without the need to characterise the non-spherical crystal shape. A possible extension of these concepts to heterogeneous nucleation is also briefly discussed.     

Photoluminescence spectra of the AgGaTe2 single crystals doped with hydrogen

The photoluminescence spectra of single crystals of the ternary AgGaTe₂ compound obtained by the Bridgman-Stockbarger method are studied before and after bombardment by the 100-eV hydrogen ions of dose 10¹⁵ cm^?2. The spectra were detected in the temperature range from 10 to 300 K at various powers of laser excitation. The spectra exhibit emission bands related to the donor-acceptor recombination, free, and bound excitons. The intensity of these bands in crystals doped with hydrogen is higher than that in undoped crystals. This is explained by the passivation of defects in a crystal lattice by hydrogen. The binding energy of a free exciton and the band gap in AgGaTe₂ crystals are calculated at 10 K.     

Mo衬底生长金刚石薄膜表面晶形研究

 用HFCVD方法在Mo衬底上进行了金刚石薄膜生长研究，观察到在不同的反应压强条件下，金刚石薄膜晶粒的三种形貌，并且讨论了反应条件对晶粒形貌的影响以及菜花状大晶粒的构成机理。     

Melting kinetics in polymers

In polymers, it is possible to obtain single chain forming single crystals. It is feasible to melt these crystals by simple consecutive detachment of chain segments from the crystalline substrate and its diffusion into the melt. However, complication in the melting process occurs when the chain in the process of detachment from the surface is shared between different crystals. Experimentally, a clear distinction in different melting processes is observed, by the differences in the activation energies required for the consecutive detachment of chain segments or of segments having topological constraints. The consecutive detachment of free chain segments starts at the melting temperature predicted from the Gibbs-Thomson equation, whereas higher temperature or time is required if the chain has to overcome the constraints.     

Optical,dielectric and ferroelectric behaviour on doped lithium sulphate crystals

Optically transparent single crystals of urea doped with lithium sulphate (ULS), thiourea doped with lithium sulphate (TLS) and cupric chloride doped with lithium sulphate (CuLS) were grown in aqueous solution by slow evaporation technique at room temperature. Single crystal X-ray diffraction analysis confirmed the changes in the lattice parameters of the ULS, TLS and CuLS. The lattice parameters and the quality of doped crystals were confirmed by powder X-ray diffraction studies. The functional groups present in the ULS, TLS and CuLS crystals were determined qualitatively by using Fourier transform infrared (FTIR) spectroscopy. Optical absorption studies revealed that doped crystals acquire very low absorption in the entire visible region. The energy dispersive X-ray analysis (EDAX) gives the chemical composition of the grown crystal. CHN analysis confirmed the presence of carbon, hydrogen and nitrogen in the added dopants. The dielectric constant and dielectric loss of the doped crystal were studied as a function of frequency and temperature and the ferroelectric property of the crystal was confirmed by dielectric studies.     

Vibrational spectra and structural studies of nonlinear optical crystal ammonium D,L‐tartrate: a density functional theoretical approach

Single crystals of ammonium D , L ‐tartrate, a potential nonlinear optical (NLO) material of interest, were grown by the slow evaporation technique. The crystal structure was determined by single‐crystal X‐ray diffraction. Fourier transform infrared and Raman spectra of the crystallized molecule were recorded and analyzed. The geometry, intermolecular hydrogen bonding, first hyperpolarizability and harmonic vibrational wavenumbers were calculated with the help of B3LYP density functional theory method. The red shift of hydroxyl and NH₄⁺ stretching wavenumbers indicate the formation of inter‐ and intramolecular hydrogen bonding. Simultaneous activation of CH stretching wavenumbers shows the presence of intramolecular charge transfer in the molecule. Natural bond orbital analysis was carried out to demonstrate the various inter‐ and intramolecular interactions that are responsible for the stabilization of this molecule, leading to high NLO activity. Copyright © 2010 John Wiley & Sons, Ltd.     

Pressure Effect on Molecular and Lattice Dynamics in Pyridinium Nitrate

Measurements of spin-lattice relaxation time T 1 , second moment M 2 and neutron scattering have been performed for a polycrystalline sample of pyridinium nitrate as a function of hydrostatic pressure and temperature. The structure of this compound has also been analysed by the HF/6-31 G method. The results of the measurements and calculations have confirmed that reorientation of the pyridinium cation takes place between the potential minima of different values, and the asymmetry parameter decreases with increasing temperature. The temperature dependence of the asymmetry parameter is modified by the pressure applied. The presence of the hydrogen bond in this compound implies a very small activation volume and is responsible that the crystal does not undergo a phase transition to the disordered phase.     

Liquid Phase Kinetic and Thermodynamic Studies on Promoted Nickel Catalysts used for H/D Exchange

Successful choice of the suitable catalyst composition for hydrogen/deuterium exchange reaction between hydrogen and water requires understanding of chemical kinetics as well as surface properties and activity in both liquid- and vapour phases. The present study deals with the thermodynamic and kinetic behaviour of a simple liquid phase reaction on the surface of nickel catalyst promoted with zirconium oxide used for the H/D exchange. This study helps understand the condensation of water on the catalyst surface during operation which leads to its poisoning. In this specific case water molecules are responsible for selective poisoning of this type of catalysts. The reaction rate constant, the increase in Gibb,s free energy, the enthalpy, the entropy of activation and the energy of activation were calculated.     

Ab initio study of structural and mechanical property of solid molecular hydrogens

Ab initio calculations based on density functional theory (DFT) were performed to investigate the structural and the elastic properties of solid molecular hydrogens (H₂). The influence of molecular axes of H₂ on structural relative stabilities of hexagonal close-packed (hcp) and face-centered cubic (fcc) structured hydrogen molecular crystals were systematically investigated. Our results indicate that for hcp structures, disordered hydrogen molecule structure is more stable, while for fcc structures, Pa3 hydrogen molecular crystal is most stable. The cohesive energy of fcc H₂ crystal was found to be lower than hcp. The mechanical properties of fcc and hcp hydrogen molecular crystals were obtained, with results consistent with previous theoretical calculations. In addition, the effects of zero point energy (ZPE) and van der Waals (vdW) correction on the cohesive energy and the stability of hydrogen molecular crystals were systematically studied and discussed.     

Atomistic study on the activation enthalpies for interface mobility and boundary diffusion in an interface-controlled phase transformation

A multi-lattice kinetic Monte Carlo atomistic simulation method has been used to simulate the austenite to ferrite interface-controlled transformation in pure iron. By performing simulations with different amounts of “free volume” at the interface, quantitative relations between the activation energies for interface mobility, boundary self diffusion and bulk self diffusion were investigated. The effect of different interface orientations on the distribution of free volume over the interface, as determined by interface crystallography, and its consequences for interface mobility activation energy was also evaluated. The essential role of thermal fluctuations in free volume distribution at the interface has been shown. The activation enthalpy for boundary diffusion is shown to be larger than the interface mobility activation enthalpy.     

Nucleation kinetics,solubility, growth and characterization of novel semiorganic crystal—Bisthiourea zinc formate

Single crystals of bisthiourea zinc formate were grown by the low temperature solution growth technique. The nucleation parameters such as solubility, induction period, interfacial energy and metastable zone width, radius of critical nucleus, critical free energy barrier, number of molecules in the critical nucleus and energy per unit volume have been evaluated. Metastable and induction period values were determined experimentally in order to optimize the growth parameters. Transparent good quality single crystals are characterized by single crystal XRD, powder XRD and FTIR analysis. Optical and photoluminescence studies have been carried out for the grown crystal. The results have been discussed in detail.     

To the theory of isotope effect in the thermodynamics of “Classical” crystals

Based on general expressions for quantum contributions to the free energy of a statistical system, an approach is developed making it possible to carry out a complete theoretical analysis of the equilibrium thermodynamic characteristics of argon-type crystals as a function of the isotopic mass. The theory is based on independently calculated (in fact, without fitting parameters) one-particle and two-particle distribution functions of the classical crystal and permits one to quantitatively describe the data on the lattice parameters of isotope crystals obtained from numerical experiments. It is pointed out that real experiments are needed to study the isotope effect in crystals of noble gases, especially in view of the fact that, according to the literature data, xenon undergoes an fcc-hcp phase transition under increased pressure.     

Tellurium recrystallization under microgravity conditions and the resulting properties of samples

Three experiments on the tellurium recrystallization by a modified Bridgman method were performed under microgravity conditions on board the Mir orbital space laboratory using a ChSK-1 Kristallizator furnace. The physical properties of samples were studied, including the final crystal structure, the distribution of impurities and defects, and the charge carrier concentration and mobility. The results were compared to the analogous parameters of crystals remelted using the same method under the normal gravity conditions. It is established that the samples recrystallized in a close volume under the on-board microgravity conditions “break off” from the container walls and touch the walls only in a few points. This circumstance gives rise to special effects, such as the growth of crystals with a free surface and deep supercooling. Study of the distribution of electrically active impurities over the length of ingots shows evidence of the presence of thermocapillary convective flows in the melt under the microgravity conditions. The flows tend to increase upon separation of the melt from the container walls. The contributions due to impurities and electrically active structural defects to the charge carrier distribution are taken into account. The single-crystal sample obtained upon the partial recrystallization of tellurium in a close container volume under the on-board microgravity conditions exhibits the electrical characteristics comparable to those of a crystal grown by the Czochralski technique under the normal gravity conditions.     

Crystal growth and dielectric, mechanical, electrical and ferroelectric characterization of n-bromo succinimide doped triglycine sulphate crystals

Single crystals of triglycine sulphate (TGS) doped with n-bromo succinimide (NBS) were grown at ambient temperature by the slow evaporation technique. An aqueous solution containing 1-20 mol% of n-bromo succinimide as dopant was used for the growth of NBSTGS crystals. The incorporation of NBS in TGS crystals has been qualitatively confirmed by FTIR spectral data. The effect of the dopant on morphology and crystal properties was investigated. The cell parameters of the doped crystal were determined by the powder X-ray diffraction technique. The dielectric constant of NBS doped TGS crystal was calculated along the ferroelectric direction over the temperature range of 30-60 °C. The dielectric constant of NBSTGS crystals decrease with the increase in NBS concentration and considerable shift in the phase transition temperature (T_C) towards the higher temperature observed. Pyroelectric studies on doped TGS were carried out to determine the pyroelectric coefficient. The emergence of internal bias field due to doping was studied by collecting P-E hysteresis data. Temperature dependence of DC conductivity of the doped crystals was studied and gradual increase in the conductivity with the increase of dopant concentration was observed. The activation energy (ΔE) calculated was found to be lower in both the ferroelectric and the paraelectric phases for doped crystals compared to that of pure TGS. The micro-hardness studies were carried out at room temperature on thin plates cut perpendicular to the b-axis. Less doped TGS crystals show higher hardness values compared to pure TGS. Piezoelectric measurements were also carried out on 010 plates of doped TGS crystals at room temperature.     

尼龙6在高压退火中的晶体生长

 利用六面顶压机制备了高压退火的尼龙6样品,通过宽角X射线衍射（WAXD）、差示扫描量热（DSC）和红外光谱等手段,探索了高压退火对尼龙6结晶度以及晶体结构的变化。研究发现,在高压退火过程中尼龙6分子链段运动受到压力的限制,材料中晶体的形成主要依靠转氨基反应实现。     

Experimental studies on the spectral sensitization of photoconductivity on zinc-oxide crystals

The photoconductivity at the prism-surfaces(1$\text{1}$00) of ZnO-crystals was studied before and after adsorption of a merocyanine-dye under well defined experimental conditions in ultrahigh vacuum.The undyed crystal exhibited a pronounced photoconductivity only for fundamental absorption (UV-light, band-band excitation). The dyed crystal showed a very enhanced photoconductivity in the visible surmounting even the values measured for band-band light. The two-component system merocyanine dye/ZnO-surface is thus well suited for studies of spectral sensitization.The influence of different parameters was studied, e.g. pretreatment of the surface with oxygen or hydrogen (variation of surface conductivity) and the effect of temperature between 90 and 298 K. The conductivity of the crystals in the dark may be lowered or increased during dye-deposition. The spectrally sensitized photoconductivity was found negative or positive. No decrease was found for samples with low surface conductivity before dyeing.An activation energy of 34 meV was deduced from the temperature dependence of the initial slope of the sensitized photoconductivity. The sign of the photosignal may change between 90 and 298 K. Pretreatment or treatment of the dyed surface with oxygen or atomic hydrogen did not influence the spectral distribution of photoconductivity in the sensitization region.