Automated pulling of large-diameter alkali halide scintillation single crystals from the melt

Principles of a new high-productivity automated method for pulling large-diameter alkali halide crystals are described. On the stage of radial growth, the melt geometry is varied continuously by its level elevation in the conical crucible due to feeding by the melted raw material controlled by a lifting electrocontact probe. The melt level is stable when the crystal is grown in height. An automated system to control the crystal diameter has been developed using time intervals between feeding operations as the controlling parameter. This system allows control over the crystal diameter to an accuracy better than 1% over the range from 400 to 450 mm at the pulling rate from 6 to 6.5 mm/h. The method is used to produce scintillation alkali halide single crystals on an industrial scale.     

Growth of single crystals of semiconductor solid solutions by double feeding of the melt method

Crystallography Reports - The problem of component distribution in solid solution crystals grown from a melt fed by rods made of the components of the system, with allowance for the dependence of...     

Inhomogeneity of composition in near‐stoichiometric LiNbO3 single crystal grown from Li rich melt

A near‐stoichiometric LiNbO₃ single crystal has been grown by the Czochralski technique from a melt of 58.5 mol% Li₂O. Its composition homogeneity was assessed by measuring the UV absorption edge. It was found that the maximum composition difference is about 0.03 mol% in the radial direction and 0.05 mol% in the axial direction. Differential scanning calorimetry (DSC) analysis was performed on the powder from the synthesized raw material and the frozen melt after crystal growth. The analytical results indicate that, during crystal growth, the magnitude of lithium volatilization from the melt surface is more than the degree of segregation from the crystal. The volatilized lithium diffuses into the crystal to compensate for the lithium segregation in the LiNbO₃ crystal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of dislocations in a LiNbO3 single crystal grown by micro pulling down method

LiNbO₃ single crystals grown by the micro pulling down (μ-PD) method have been revealed to be as free of dislocations and subgrain boundaries up to 500 μm in diameter. On the other hand, μ-PD LiNbO₃ single crystals grown along the x-axis in diameter of 800 μm were observed to be dislocated due to the size effect of crystal. The Burgers vectors of dislocations were determined to be [22 01], [101 1], and [01 11] by X-ray topography.     

Distribution of metallic ions in a single KDP crystal grown from aqueous solution

A single KDP (potassium dihydrogen phosphate) crystal was grown in a supersaturated solution containing a metallic ion (Al³⁺, Fe³⁺, or Cr³⁺). The growth rate, morphology, and distribution of the metallic ions into the KDP crystal were measured as the ionic concentration and supersaturation in the solution changed. It was found that in the KDP crystal, Al³⁺ and Fe³⁺ were greatly concentrated, but Cr³⁺ was diluted. Complete expressions for the effect of metallic ions on all aspects of the growth of KDP crystal were suggested. The growth rates of (100) and (101) faces were well correlated by the empirical equation and resulted in good estimation of morphology. The distribution of metallic ions into KDP crystal was also correlated by the distribution model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

CFD modelling of single crystal growth of potassium dihydrogen phosphate (KDP) from binary water solution at 30 °C

Crystal growth rate depends on both diffusion and surface reaction. In industrial crystallizers, there exist conditions for diffusion-controlled growth and surface reaction-controlled growth. Using mathematical modelling and experimental information obtained from growth studies of single crystals, it is possible to separate these phenomena and study how they are affected by concentration, slip velocities of particles, temperature and finally estimate the parameters for crystal growth models.In this study, a power-law growth model using activity-based driving force is created. Computational fluid dynamics (CFD) was used to evaluate the thickness of a diffusion layer around the crystal. Parameters of the crystal growth model were estimated using a non-linear optimization package KINFIT. Experimental data on growth rate of the (1 0 1) face of a potassium dihydrogen phosphate (KDP) single crystal and simulated data on the thickness of a diffusion layer at the same crystal face were used in parameter estimation. The new surface reaction model was implemented into the CFD code. The model was used to study the effect of flow direction on growth rate of the whole crystal with various slip velocities and solute concentrations.The developed method itself is valid in general but the parameters of crystal growth model are dependent on the system. In this study, the model parameters were estimated and verified for KDP crystal growth from binary water solution.     

Growth of single crystals of monticellite CaMgSiO4: Cr from melt by floating zone technique and the study of their composition

Crystallization of monticellite CaMgSiO₄ from melt by the floating-zone technique was studied at different compositions of feed rods. The compositions of rapidly cooled melts and different regions of grown crystals were investigated by the X-ray phase and microprobe analyses. The inclusions of secondary phases were identified. The range of most promising compositions of the charge for technology of growing large high-quality crystals is established. The mathematical model of the changes in the melt composition during monticellite crystallization is constructed.     

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.     

The effect of mass transfer on the temperature gradient of a crystal growing from melt

The change in the temperature gradient on the crystal side while the rate of crystal growth from melt is varied has long been debated. Abe and Takahashi have recently reported an unambiguous experimental demonstration that the temperature gradient is a decreasing function of the growth rate, which is different from previous theories, experimental results, and widely held notion of other researchers. The present paper provides a theoretical basis for this seemingly peculiar effect of the growth rate on the temperature gradient. The essential matter is the effect of mass transfer, the role of which had been commonly disregarded in old studies. Although the rate of mass transfer is not large compared to that of heat conduction, it is proven that the temperature gradient is subjected to the mass transfer in a definite manner. Our analysis shows that the effect becomes significant when the crystal diameter is large, which is consistent with the experimental observation. Another effect of the mass transfer is the change in the shape of melt/crystal interface. In old studies, the temperature gradient was determined by Stefan's equation; however, this treatment confuses the cause and effect. The temperature gradient should be determined by the fundamental equation of heat conduction. When the gradient is determined in this way, the shape of the melt/crystal interface spontaneously adjusts to satisfy Stefan's equation.     

Mathematical modeling of the multi‐run process of crystal pulling from the melt by EFG (Stepanov) technique in dependence on the angle of inclination of the working edges of the dies

A mathematical model for the determination of melt hydrodynamics, impurity concentrations and thermal stresses in the multi‐run process of the growth of sapphire ribbons by EFG (Stepanov) technique with inclinated working surfaces of the dies is considered. The mathematical model deals with thermal conductivity equation, Navier‐Stokes equation, diffusion equation, capillary Laplace equation. This problem has been solved by the finite‐element method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the distribution of growth directions of silver and other F.C.C. single crystals from the melt

Thin single crystals of pure Ag, its dilute alloys, Ni – 10 ˜ 60 wt.% Co and Fe – 35 wt.% Ni alloys were grown by the Bridgman method. A preference of growth directions of these crystals changes from the <111>direction towards <100>with increasing growth rate or alloying effect. These results are discussed from a characteristic of nucleus interfaces.     

The influence of the melt cooling rate on the properties of YIG single crystal grown by the flux method form sintered polycrystalline maeteral

A method for the single crystal growth of Yttrium Iron Garnet (YIG) in a PbO/PbF₂/B₂O₃ flux mixture with a melt cooling rate of 1–2.5 °C/h is described. In order to keep constant the crystal stoichiometry as well as its specific structural and magnetic properties at such high rates of the cooling process, a previously sintered polycrystalline garnet has been used as raw material. The YIG single crystal quality was checked-up by metallographic analysis, X-ray diffraction, SEM, and magnetic susceptibility measurements.     

A study of the relation between crystal growth kinetics and particle size distribution of crystals produced from solution

A new relation between crystal growth kinetics and particle size distribution based on statements in the literature is derived. It can be equally used for the determination of the characteristics of crystallization caused by cooling, evaporation or precipitation. Different kinetic constants (rate constants of nuclei formation k₁, k₂, k₃, and growth K₀, K₁, K₂) can be determined, and therefrom the particle size density curve. – The effect of kinetics characteristics on particle size distribution is summarized.     

Formation of dislocation structure in shaped single crystals during their growth from the melt by Stepanov's Method

The paper is concerned with the formation of a dislocation structure due the thermal stresses in shaped single crystals with different degree of anisotropy grown by Stepanov's method. The calculation of root-mean-square tangential stresses allows to determine the regions in which the plastic deformation may occur. Analysis of thermal stresses in separate slip systems permits to explaine the slipe band distribution in shaped single crystals.     

The influence of melt stirring intensity on the axial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of modulated crucible rotation on the axial distribution of Cu, Mg and Si impurities in proustite single crystals grown by the Stockbarger method using ACRT is studied in a wide range of Taylor numbers (1.9·10⁵ < Ta < 7.12·10⁷). The impurity content in the upper part and in the tail portion of the grown crystals is measured using X‐ray‐phase analysis. Micro and macrostriations are observed in the grown crystals. The wavelengths of impurity content fluctuations have been determined. The microfluctuations of axial impurity content are caused by modulated crucible rotation. The studies have revealed that the ACRT provides an effective removal of impurities from the main part of the grown crystal at high intensity of melt stirring, and consequently, the ACRT can be applied validly to decrease the impurity content during the growing of high‐quality proustite single crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of the intensity of melt stirring on the radial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of the intensity of melt stirring on the radial impurity distribution and optical quality of proustite single crystals grown by the Stockbarger method using ACRT is studied by the example of Cu, Sb and Mg impurities. We report results obtained in a wide range of Taylor numbers (1.9×10⁵<Ta<7.12×10⁷). The studies revealed that the ACRT can be applied validly to decrease the impurity content during the growing of high-quality single crystals.     

Remarks on the separation of a solid solution transported by evaporation-condensation at a small temperature difference

The nearly congruent transport of a solid solution by incongruent solid-vapour and vapour-solid phase transitions in a closed, almost isothermal system is considered–in particular referring to the crystal growth of pseudobinary A^IVB^VI solid solutions. The fact is interpreted that the small temperature difference required for the creation of the mass driving force of the process causes composition variations smaller than often assumed.     

The growth of ZnWO4 and CdWO4 single crystals from melt by the low thermal gradient Czochralski technique

Crystallography Reports - The features of growth of single crystals of cadmium and zinc tungstates by the low thermal gradient Czochralski technique have been investigated. The effect of change in...     

Capillary shaping of crystals pulled downward from a melt by the crucibleless AHP method

A model is proposed for capillary shaping of a crystal in the crucibleless variant of the AHP (axial heat flux close to the phase interface) method when the melt in the form of a film flowing over the AHP heater is fed to a meniscus. The meniscus and the film of the melt are described by the same equation with a discontinuous right-hand side. The dependences of the crystal radius and the thickness of the melt film on the parameters of the process are numerically investigated, and the capillary stability of the pulling process is analyzed. It is demonstrated that, in this method, the thickness of the melt layer between the crystal and the heater can be considerably larger than the capillary constant.