Mechanisms of dendrites occurrence during crystallization: Features of the ice crystals formation

Dendrites formation in the course of crystallization presents very general phenomenon, which is analyzed in details via the example of ice crystals growth in deionized water. Neutral molecules of water on the surface are combined into the double electric layer (DEL) of oriented dipoles; its field reorients approaching dipoles with observable radio-emission in the range of 150 kHz. The predominant attraction of oriented dipoles to points of gradients of this field induces dendrites growth from them, e.g., formation of characteristic form of snowflakes at free movement of clusters through saturated vapor in atmosphere. The constant electric field strengthens DELs' field and the growth of dendrites. Described phenomena should appear at crystallization of various substances with dipole molecules, features of radio-emission can allow the monitoring of certain processes in atmosphere and in technological processes. Crystallization of particles without constant moments can be stimulated by DELs of another nature with attraction of virtual moments of particles to gradients of fields and corresponding dendrites formation.     

Comparative analysis of Tm-Ho:KYF4 laser crystals

A spectroscopic characterization of Tm-Ho:KYF₄ crystals with different doping levels is presented. A comparative investigation based on diode-pumped 2-μm laser experiments shows that the best trade-off in terms of cw output power characteristics and wavelength tuning ranges is obtained with 5% Tm-0.5% Ho active crystal. PACS 42.55Xi; 42.55Rz; 42.60Pk; 42.70Hj; 42.60Lh     

Evolution of crystallization structure during deep deformation and annealing of single crystals of austenitic carbon steel

The distribution of alloying elements, impurities, and secondary phases in axial and interaxial regions was studied for single crystals of the austenitic carbon steel, 04Kh17N14M3BG2, with the classical cross-shaped dendritic structure. It was shown that the interaxial regions are enriched with chromium, molybdenum, and carbon and contain precipitates of Cr₂₃C₆ and Mo₂C. The chemical and phase inhomogeneities, connected with dendritic crystallization, are stable with respect to the effect of deep deformation and annealing, and in the finished product evolve into bands with a different content of impurities and second phase particles.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 62–66, November, 1991.     

Dynamic aspect of bacteriorhodopsin as a typical membrane protein as revealed by site-directed solid-state 13C NMR

We demonstrate here a general feature of dynamic aspect of membrane proteins as revealed by site-directed 13C NMR studies on bacteriorhodopsin (bR) as a typical membrane protein and a variety of mutants at ambient temperature. 13C NMR signals of [3-13C]Ala- or [1-13C]Val-labeled proteins were assigned regio-specifically with reference to the data of the conformation-dependent 13C chemical shifts from model polypeptides, followed by site-specific assignment based on site-directed mutants. Revealed picture of membrane protein at ambient temperature is not static in contrast to anticipation from crystalline structures but flexible enough to undergo a variety of local fluctuations with frequencies from 10(2) to 10(8)Hz, as pointed out already. This picture was further refined by taking into account of residue-specific dynamics of interfacial domains between the surface and inner part of the transmembrane helices and conformational fluctuation induced by the presence of a kinked structure. The residue-specific dynamics of the former was revealed by observation of broadened or suppressed peaks from the interfacial domains caused by acquisition of internal fluctuation motions interfered with frequencies of proton decoupling or magic angle spinning. The presence of such suppressed peaks due to molecular fluctuations in the interfacial domains was further confirmed by insensitivity of the peak-intensities from the interfacial domains in spite of the presence of accelerated relaxation rate to nearby residues from surface bound Mn2+ ion. Further, conformational change of the transmembrane alpha-helix F due to a plausible kinked structure at Pro 186 was confirmed in view of specific displacements of Ala 184 and Val 187 13C NMR peaks from chemically synthesized [3-13C]Ala(184)-, [1-13C]Val(187)-labeled wild type and P186L mutant of transmembrane fragment F(164-194) incorporated into lipid bilayer. It is emphasized that the observed displacement of [3-13C]-labeled Ala 184 peak at 17.4 ppm in the presence of kinked structure in this model peptide is consistent with that of intact protein at 17.27 ppm.     

Comparative analysis of the search procedures for surface acoustic wave solutions in piezoelectric crystals

Various search procedures for finding the global extremum of a multivariate target function (TF) necessary for calculating the characteristics of surface acoustic waves (SAW) and leaky surface acoustic waves (LSAW) in crystals are analyzed. The search procedures aimed at determining the optimal orientations for SAW in crystals are considered. A comparative analysis of the promising methods for finding the global extremum of the TF is performed.     

Unifying the crystallization behavior of hexagonal and square crystals with the phase-field-crystal model

By employing the phase-field-crystal models, the atomic crystallization process of hexagonal and square crystals is investigated with the emphasis on the growth mechanism and morphological change. A unified regime describing the crystallization behavior of both crystals is obtained with the thermodynamic driving force varying. By increasing the driving force, both crystals(in the steady-state) transform from a faceted polygon to an apex-bulged polygon, and then into a symmetric dendrite. For the faceted polygon, the interface advances by a layer-by-layer(LL) mode while for the apex-bulged polygonal and the dendritic crystals, it first adopts the LL mode and then transits into the multi-layer(ML)mode in the later stage. In particular, a shift of the nucleation sites from the face center to the area around the crystal tips is detected in the early growth stage of both crystals and is rationalized in terms of the relation between the crystal size and the driving force distribution. Finally, a parameter characterizing the complex shape change of square crystal is introduced.     

Comparative studies of the features of the formation of impurity heterogeneity in GaSb:Te crystals in the case of directed crystallization under space and ground conditions

The results of comparative studies concerned with the formation of impurity heterogeneity in GaSb:Te crystals grown at the POLIZON facility by the Bridgman method under space and ground conditions are presented. Crystallization is carried out in the same temperature-time modes of the heaters: on board the Foton-M 3 spacecraft in space and by the vertical Bridgman method under conditions of weakened thermogravitational convection on Earth. In both cases, Marangoni convection is eliminated, and the effect of vibration microaccelerations on the ampoule that contains the melt is minimized. The microuniformity of the dopant distribution in the crystal areas that are recrystallized by the Bridgman method is higher than that in the seed grown by the Czochralski method. Microuniformity is higher under space conditions than under ground conditions with weakened thermogravitational convection. Spectral Fourier analysis of the spreading resistance distribution R _s reveals the characteristic periods of heat and mass transfer processes in the melts under ground and space conditions.     

Comparative analysis of the magnetization processes of the Gd3Ni and Gd3Co single crystals

The investigations and comparative analysis of the magnetization processes for Gd₃Ni and Gd₃Co single-crystal compounds are presented. The possible magnetic structures for both compounds are proposed and the features of the field-induced metamagnetic transitions are discussed on the basis of the possible magnetic structures. The anomalous behavior of the Gd₃Ni compound observed above the magnetic ordering temperature is very likely caused by the appearance of the nickel atoms magnetic moment and field-induced magnetic ordering.     

The fluid phenomena in the crystallization of the protein crystal

This paper reports that an optical diagnostic system consisting of Mach-Zehnder interferometer with a phase shift device and image processor has been used for study of the kinetics of protein crystal growing process. The crystallization process of protein crystal by vapour diffusion is investigated. The interference fringes are observed in real time. The present experiment demonstrates that the diffusion and the sedimentation influence the crystallization of protein crystal which grows in solution, and the concentration capillary convection associated with surface tension occurs at the vicinity of free surface of the protein mother liquor, and directly affects on the outcome of protein crystallization. So far the detailed analysis and the important role of the fluid phenomena in protein crystallization have been discussed a little in both space- and ground-based crystal growth experiments. It is also found that these fluid phenomena affect the outcome of protein crystallization, regular growth, and crystal quality. This may explain the fact that many results of space-based investigation do not show overall improvement.     

X-ray topography characterization of the structural response of Ge(Ga) crystals to variation in gravity force vector orientation during crystallization

The influence of crystallization process disturbances connected with the variation of the crystallization front orientation relative to the gravity force vector (characteristic for crystal growth under conditions of microgravity onboard space vehicles) on the real crystal structure has been investigated by plane-wave X-ray topography. It has been found that these disturbances can result in a local disorder in the impurity distribution in the form of microsegregation growth striations. Quantitative estimations of the amplitude for the composition variation based on the analysis of contrast in growth striation images obtained by the method of plane-wave X-ray topography have been carried out.     

Thermal hysteresis during the melting and crystallization of macroobjects

Different thermal hysteresis processes associated with macroobjects have been analyzed using heating and cooling thermograms with regard to liquid–solid phase transformations. Two types of thermal hysteresis for equilibrium and nonequilibrium crystallizations have been revealed. The paths of Gibbs energy variation are shown for both cases.     

Comparative thermal analysis of Nd- and Yb-doped YAG and KGdW laser crystals under diode- and flashlamp-pumping

Temperature distribution in the Nd- and Yb- doped YAG and KGdW laser crystals under flashlamp- and diode-pumping was characterized by means of finite element analysis. For KGdW, two laser crystal orientations were considered for light propagation along the N_p and N_g optical indicatrix axes, taking into account the anisotropy of thermal conductivity coefficient. The influence of the cooling conditions, pump spot size and dopant concentration on the temperature distribution was analyzed. For flashlamp-pumping conditions, the applicability of the quasi-steady-state model is discussed. The main concerns in the thermal management of KGdW laser host is the relatively low thermal conductivity that results in poor cooling and significant absorption coefficients under diode pumping that result in highly non-uniform volumetric heat deposition. “Athermal” N_g-cut KGdW crystal was found to produce higher temperature gradients that the “standard” N_p-cut one, that should results in higher internal stresses and higher probability of thermally-induced cracks.     

Femtosecond laser-induced crystallization of protein in gel medium

We succeeded in generating femtosecond laser-induced crystallization of hen egg-white lysozyme (HEWL) in the irradiated area by suppressing convection of the solution using a highly concentrated gel. When a laser pulse, whose energy was above the threshold energy for bubble formation, was focused along with a linear scanning of the stage, HEWL crystallization was enhanced at the surface of bubbles on the irradiated line. The relation between the bubble formation process and the crystallization is discussed.     

Features in the kinetics of lateral zone crystallization of semiconductor crystals in a temperature gradient field

Theoretical and experimental investigations of the kinetics of a lateral zone crystallization process in a temperature gradient field are given. Kinetic rules for the method are established which allow one to control the rate of the process and the configuration of the crystallization channel.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 67–71, January, 1996.     

Changes in the photoluminescence spectrum near twin boundaries in ZnTe crystals produced by rapid crystallization

{111} ZnTe crystals with various densities of twin boundaries in the growth direction were produced at ～670°C by the chemical vapor deposition method with the vapor environment offset toward an excess of Zn. Defects are formed in conical crystallites (up to 5 mm in height and with lateral dimensions of 10–500 μm at the bottom and up to 2 mm at the top) due to instabilities in the crystallization front, which arise because of convection-type heat and mass exchange in the oversaturated vapor medium. The influence of twin boundaries on the distribution of chemical impurities and the electronic spectrum of ZnTe was studied using x-ray diffractometry, scanning electron microscopy, and low-temperature photoluminescence (PL). It is found that rapid low-temperature growth of [111] ZnTe polycrystals from the vapor phase with an excessive Zn content favors the intensive formation of rotation and reflection twins. The incoherent [111] boundary of reflection twins is conductive to the separation and accumulation of impurities. In the regions of a crystal with a high density of reflection twins, exciton-impurity complexes (I _C , I _X ) and a Y strip, which is usually related to extended defects (dislocations, twins, crystallite boundaries), are found in the low-temperature PL spectra. Additional studies show that I _X is related to excitons trapped by neutral isoelectronic or charged defects and that I _C is probably due to an impurity of group IV of the Periodic Table.     

对比分析三角Pr~(3+)中心在CsCdBr_3和GdCl_3晶体中的能级分裂

以Racah的群表示论和Slater的波函数理论为基础,分别建立了4f2组态离子Pr~(3+)在三角晶体场C3v和C3h中的91×91完全能量矩阵,并对Pr~(3+)离子掺杂在卤化物CsCdBr_3和GdCl_3中的Stark能级做了计算与分析.结果显示计算值与实验值吻合很好,表明在分析由稀土离子掺杂体系的能级分裂时,完全能量矩阵方法是有效的方法.此外,将两种具有不同点群对称的体系的能级分裂情况作了比较,结果显示六阶晶体场参量对能级分裂的影响是不能忽略的,而且CsCdBr_3:Pr~(3+)和GdCl_3:Pr~(3+)将产生不同程度的畸变.     

对比分析三角Pr3+中心在CsCdBr3和GdCl3晶体中的能级分裂

以Racah的群表示论和Slater的波函数理论为基础,分别建立了4f2组态离子Pr3+在三角晶体场C3v和C3h中的91×91完全能量矩阵,并对Pr3+离子掺杂在卤化物CsCdBr3和GdCl3中的Stark能级做了计算与分析. 结果显示计算值与实验值吻合很好,表明在分析由稀土离子掺杂体系的能级分裂时,完全能量矩阵方法是有效的方法. 此外,将两种具有不同点群对称的体系的能级分裂情况作了比较,结果显示六阶晶体场参量对能级分裂的影响是不能忽略的,而且CsCdBr3:Pr3+和GdCl3:Pr3+将产生不同程度的畸变.     

Formation of the structure of gold nanoclusters during crystallization

The structure formation in gold nanoparticles 1.6–5.0 nm in diameter is studied by molecular dynamics simulation using a tight-binding potential. The simulation shows that the initial fcc phase in small Au clusters transforms into other structural modifications as temperature changes. As the cluster size increases, the transition temperature shifts toward the melting temperature of the cluster. The effect of various crystallization conditions on the formation of the internal structure of gold nanoclusters is studied in terms of microcanonical and canonical ensembles. The stability boundaries of various crystalline isomers are analyzed. The obtained dependences are compared with the corresponding data obtained for copper and nickel nanoparticles. The structure formation during crystallization is found to be characterized by a clear effect of the particle size on the stability of a certain isomer modification. Nickel and copper clusters are shown to exhibit common features in the formation of their structural properties, whereas gold clusters demonstrate much more complex behavior.     

Controlling polymorphism during the crystallization of an atomic fluid

We use molecular dynamics simulations to shed light on polymorph selection during the crystallization of the Lennard-Jones fluid. By varying pressure at fixed supercooling, we form large crystallites either of the stable face centered cubic form or of the metastable body centered cubic form and even fine-tune the fractions of stable and metastable polymorphs in the crystallite. We demonstrate that the conditions of crystallization, leading to large bcc crystallites, lie within the occurrence domain of the metastable bcc polymorph. We also find that the predominantly fcc crystallites contain a notable amount of the hexagonal close packed form, due to the cross nucleation of the hcp form on the fcc form. By varying temperature at fixed pressure, we prevent cross nucleation and form pure fcc crystallites. Our results reveal that polymorph selection may take place, and be controlled, during the growth step.