Crystallization of a germanium melt in an external electric field

Data are presented on experimental studies of the influence of an external electric field on crystallization of a germanium melt under the layer of a B₂O₃ flux. It has been found out that with the field supercoolings of the melt sharply change. This effect is due to the change of the number of active nucleation centres at the germanium – B₂O₃ flux interface. The maximum supercoolings of the germanium melt ΔT⁻ = 190 K were obtained when a negative potential was connected to germanium. The dependences of supercooling on preliminary melt overheating were measured.     

Crystallization behavior of binary metallic glasses containing Pt

Binary MPt glasses were made by rapid quenching from liquid, where M is Ti, Zr, and Hf. Glass forming compositions were found near eutectics in TiPt, ZrPt and HfPt. Isothermal crystallization for Zr₇₅Pt₂₅ was studied at a temperature range from 780 to 792 K. The non-isothermal transformation was measured by differential scanning calorimetry with heating rates from 5 to 160 K/min. Activation energy for crystallization was obtained from the non-isothermal heating, which is consistent with that from the isothermal annealing.     

Peculiarities of the initial transient process of binary melt crystallization

The time dependence of the crystallization rate V(t) in the transient process that occurs during crystal pulling into a cold zone at a constant rate W has been investigated within the one-dimensional time-dependent model of binary melt solidification using numerical and analytical approaches. It is shown that the character of the dependence V(t) in this process is mainly determined by the parameter B _w = V _c /W, where V _c is the crystallization rate at which the constitutional supercooling zone is formed in the melt. At B _w ≥ 1, the function V(t) monotonically approaches its steady-state value W, while at B _w < 1 V(t) is generally nonmonotonic. The nonmonotonic character of V(t) can manifest itself both as aperiodic damping and in the form of damping oscillations. At B _w ∼ 0.1, the vibrational amplitude of the crystallization rate in the initial transient process may reach several tens of a percent of W.     

Crystallization of beryllian indialite from its own melt under oxidizing conditions

It is established that beryllian indialite (ideal formula Mg₂BeAl₂Si₆O₁₈) is formed spontaneously under oxidizing conditions in a wide range of relatively high cooling rates at small overheatings of its own melt and short overheating times. The significant difference in the growth rates of the prism and pinacoid faces of beryllian indialite from its own melt (0.08 and 0.40 mm/h, respectively) obtained by the Steber method is explained by the fact that the prism grows according to the tangential mechanism, while the pinacoid grows in the normal way. The face effect in these crystals manifests itself in the enrichment of the prism with beryllium and silicon as opposed to pinacoid.     

Crystallization kinetics of lithium niobate films from limited volume of melt solution

The growth kinetics of LiNbO₃ films from a limited volume of melt solution is observed. We developed crystallization models describing the character of mass transfer in the liquid phase for isothermal and non-isothermal epitaxy. Analytical expressions have been derived connecting the film thickness with the system growth parameters. The experimental values being in good agreement with the calculated ones.     

Investigation of the stability of the flat crystallization front of a binary melt by a variational method

The stability of the flat crystallization front of a dilute binary melt is investigated within the two-dimensional model of solidification taking into account the latent heat of fusion and the difference between thermal conductivities of the solid and liquid phases by the method of integral functionals with an unknown region of integration. The stationary problem of impurity diffusion in a crystallizing melt and heat propagation in a limited region Ω is solved within the second-order approximation in the amplitude of deviation from a flat crystallization front. An expression for the functional is obtained whose value is proportional to the energy dissipated in the region Ω owing to heat and mass transfer processes. The results obtained are compared with the literature data on the stability of the flat crystallization front of a binary melt.     

Crystallization rate control for alkali halide crystal growth by the VGF technique with a skull layer

This study describes the ability to use the melt‐level control for stabilization of the crystallization rate during NaI crystal growth by the VGF technique with a skull layer. It is shown that a conventional linear decrease of the heater temperature leads to a nonuniform crystallization rate and deterioration of crystal quality. A method and algorithm of temperature control for the stabilization of the crystallization rate during crystal growth is proposed. The series of growth experiments with NaI(Tl) crystals proved the efficiency of this approach and ability to obtain scintillators with high registration efficiency, about 6.3% energy resolution for a ¹³⁷Cs (662 keV) source.     

Crystal growth rate from the AgNO3-Dimethylsulfoxide melt

The growth rate of AgNO₃·DMSO crystals from its melt and concentrated aqueous solutions was studied in the dependence on supercooling (ΔT = 0.5–65°C) by microscopic technique. The experimental results were analyzed according to the formalism proposed for describing growth kinetics from melts.     

Simplified version of calculation of the composition distribution along a single crystal of a binary solid solution obtained by pulling from a feeding melt

The possibility of obtaining single crystals of binary solid solutions exhibiting strong segregation upon crystallization by pulling from a feeding melt with the use of a crucible shaped as a truncated cone and a feeding ingot of complex shape is shown. The composition distribution along the crystal length is found by solving the continuity equation for the second component flux under certain initial and boundary conditions. It is shown that single crystals can be obtained in which the second-component concentration in the stationary mode exceeds the corresponding value in the feeding ingot. The method developed is applied to Ge-Si solid solutions.     

Effect of the growth rate on thermal conditions during the growth of single crystals with melt feeding

An attempt is made to establish a correlation between the radial and axial growth rates and the change in the conditions of heat transfer from a growing crystal to the atmosphere of the water-cooled vacuum furnace for the growth of large alkali halide single crystals. It is found experimentally that an increase in the growth rate leads to an increase in the automatic compensation of the melt temperature by the main heater. In this case, the thickness of the layer of melt condensate on the end face and the lateral surface of the crystal decreases. It is revealed that the possibility of growing infinitely long ingots in the presence of intense melt evaporation is restricted by the possibilities of the heat transport through the boundary between the furnace atmosphere and the cooled furnace walls, onto which melt condensate deposits.     

The melt temperature change influence on the crystallization rate in czochralski growth system

This paper opens a series of publications about growth system parameters change influence on the crystallization rate. The influence of crucible melt temperature fluctuations and mechanical instability will be investigated. In approximation of one-dimensional heat transfer the behaviour of crystallization rate in case of crucible melt temperature small change is investigated in this paper. As an example characteristic time values and system sensitivity are given for fluorite and silicon crystals.     

Crystallization of a niobium phosphate glass

Niobium phosphate glasses with composition 37P₂O₅ · 23K₂O · 40Nb₂O₅ are stable in relation to crystallization during the heating process, exhibit a low critical cooling rate, and are potentially good for nuclear wasteforms. The crystallization of these glasses was evaluated by optical microscopy after proper heat treatments, showing that surface crystallization is the main process occurring during the heat treatment. Two main crystalline phases were observed. These crystalline phases were KNb₃O₈ and K₃NbP₂O₉. Surface crystal growth rates were measured in the temperature range of 806–972 °C (T_g = 683 °C) for both crystalline phases. Apparent crystallization enthalpies were determined through the Arrhenius plots of lnU vs. 1/T. The enthalpies are 496 kJ/mol and 513 kJ/mol for each crystalline phase, respectively. The surface density of nucleation sites (N_s) on 3 μm diamond paste polished surfaces is (2.4 ± 0.7) × 10⁸ nuclei/m² for one crystalline phase and (9.8 ± 0.8) × 10⁹ nuclei/m² for the other crystalline phase, when revealed at 838 °C/17.5 h, and these values show a slight variation depending on the time and the temperature. At the tested temperatures, only one crystal phase appeared inside the volume, and a volume density of nucleation sites N_v = 5 × 10⁶ nuclei/m³ was measured.     

On the dependence of the critical crystallization rate on the initial impurity concentration in melt

The dependence of the critical crystallization rate on the initial impurity concentration in melt is derived by determining the condition at which a nonplanar solution to the stationary diffusion problem arises. It is suggested that the conditions at which defects arise at the interface differ from those obtained when crystallization becomes stationary. The initial transient process of binary melt solidification under crystal pulling at a constant rate has been studied numerically within a 1D model. It is shown that the character of the time dependence of the crystallization rate is determined by the ratio of the crystal pulling rate to solidification rate V _c, when a constitutional supercooling zone is formed in the melt.     

The order-disorder transformation at supercooled melt/crystal transition region of binary melts (I) the master equation

Binary melts crystallizing with almost perfectly ordered structures if exposed to conditions close to thermodynamic equilibrium exhibit an increasing tendency to form metastable crystals with increasing disorder when these alloys crystallize on the growth conditions of supercooling. Based on the model of a two-phase transition zone separating a growing crystal from its non-solid surrounding a kinetic master equation is formulated describing the kinetics of competitive atomic exchange processes within the transition region. The theory considers the case of simple cubic structure with equal particle numbers of two components that form a NaCl-type lattice at the state of perfect order. In a subsequent paper solutions of the master equation obtained for steady-state conditions are discussed.     

The order-disorder transformation at supercooled melt/crystal transition regions of binary melts (II). The steady-state solution

In a preceding paper a kinetic master equation has been derived describing the kinetics of atomic exchange processes that occur within the transition region separating a growing binary crystal and its nonsolid surrounding. For simple cubic structures, with the lattice sites being occupied by equal particle numbers of two components (in the case of perfect order corresponding to an NaCl-type lattice) solutions for the steady-state conditions have been derived. According to the solutions the long-range order parameter is related to the atomic interaction energies, the arrival rate of particles at the crystal surface from the melt, and to temperature. A critical supercooling is predicted for the transition from a partly crdered structure to a disordered structure of the crystals growing from the melt. At the critical supercooling the temperature dependence of the crystallization rate reveals a characteristic change.     

Pattern formation mechanism of a periodically faceted interface during crystallization ofSi

We investigated the pattern formation mechanism of a periodically faceted crystal–melt interface during the crystallization of Si by in situ observation. It was directly proved that spacing between the reentrants of adjacent zigzag facets increases with the unification of adjacent facets when a facet with a higher growth velocity catches up with the one with a lower growth velocity. The spacing becomes stable after unification, and the stable spacing was found to increase with increase in growth velocity. The experimental results was discussed by taking the negative temperature gradient in front of the growth interface into account.     

Crystallization with oils: a new dimension in macromolecular crystal growth

The crystal growth of biological macromolecules is a complicated process involving numerous parameters. This paper presents an approach which employs the use of oil as a major aid to crystal growth, and which has opened up a new dimension in the field of macromolecular crystallization. The presence of oil is a parameter which can contribute to the accuracy, the cleanliness and to the increase in the reproducibility of the experiments. Furthermore, the oil has a role in the protection of the trials during the course of their duration and in maintaining the stability of the resulting crystals. The use of oil also applies to the crystallization of membrane proteins. The results of a wide range of experiments which exploit the presence of oil to abet macromolecular crystal growth using both vapour diffusion and microbatch are presented.     

Instability of the melt flow in VGF growth with a traveling magnetic field

The linear instability of a thermally stratified melt flow in the VGF configuration driven by a traveling magnetic field (TMF) is considered numerically and experimentally. The dependency of the instability threshold on the governing parameters is found for several cuts through the parameter space covering a wide range of possible applications. In a first approximation the linear instability occurs when the dimensionless TMF forcing parameter reaches the magnitude of the Grashof number. This is particularly true in a medium-sized crucible where the first instability is axisymmetric and sub-critical. As the Grashof number increases the flow develops self-similar boundary layers and the instability becomes three-dimensional. The instability originates in the bottom boundary layer where the convection tends to suppress the imposed temperature gradient in the central part of the melt zone. It is shown that the TMF may serve as a tool to control the phase interface shape without causing flow instationarity when the crucible diameter exceeds a certain value. This value is estimated to be around 6 cm for GaAs. The flow stays stable if the TMF is used for a reversal of the meridional flow with the aim to remove a possible dopant concentration peak on the axis.