On the Iso‐Diameter Growth of β ‐BaB2O4 (BBO) Crystals by Flux Pulling Method

The iso‐diameter growth of β ‐BaB₂O₄ (BBO) crystals by the flux pulling method have been studied based on the phase equilibrium diagram in the BaB₂O₄‐Na₂O pseudo‐binary system and from the interface stability. The mathematical expressions for the cooling rate in the growth of the crystals with constant diameter under stable growth conditions are derived, the experimental phenomena such as diameter contraction and difficulty to grow a lengthy crystal by the flux pulling method are explained, the prerequisite for iso‐diameter BBO crystal growth from the flux is suggested; a new continuous charging flux pulling method is introduced to grow large‐sized high quality crystals with a relative high growth rate.     

Experimental Study of Interface Inversion of Tb3ScxAl5‐xO12 Single Crystals Grown by the Czochralski Method

Thermal conditions and rotation rate were examined experimentally for obtaining a flat interface growth of high melting‐point oxide (Tb₃Sc_xAl_5‐xO₁₂ ‐ TSAG) by the Czochralski method. The critical crystal rotation rate can be significantly reduced, of about twice at low and very low temperature gradients comparing to medium temperature gradients in the melt and surroundings of the crystal. The interface shape of TSAG crystals is not very sensitive on crystal rotation rate at small rotations and becomes very sensitive at higher rotations, when the interface transition takes place. The range of crystal rotation rates during the interface transition from convex to concave decreases with a decrease of temperature gradients. At low temperature gradients interface inversion crystals takes place in very narrow range of rotation rates, which does not allow one to growth such crystals with the flat interface. Even changing crystal rotation rate during the growth process in a suitable manner did not prevent the interface inversion from convex to concave and thus did not allow to obtain and maintain the flat interface.     

Optimization of the crystallization rate in growing Bi4Ge3O12 crystals by the LTG CZ technique

The influence of the growth rate on the shaping and quality of Bi₄Ge₃O₁₂ (BGO) crystals during their growth by the low-thermal-gradient Czochralski technique has been studied. Several series of BGO crystals have been grown with the rate varying from 0.3 to 15 mm/h. The limited growing rates at which the crystal quality is not deteriorated are established. The results of the study make it possible to increase the growth rate in commercial systems by a factor of 1.5–2 and thus significantly increase the growth efficiency.     

Single crystals growth and absorption spectra of Cr-doped Al2−xInx(WO4)3 solid solutions

The growth conditions of pure and Cr³⁺-doped Al_2−xIn_x(WO₄)₃ single crystals, using top-seeded solution growth (TSSG) technique, have been studied. A series of experiments have been performed at different In concentrations, x=0.0, 0.3, 0.6 and 1.0, as well as at different concentrations of Cr³⁺ (0.0, 0.1, 0.2, 0.5 and 1.0) in at% with respect to the initial total concentration of Al and In in the starting solutions. The basic parameters of the crystal growth are varied over a wide range: seed orientation, speed of rotation, axial and radial temperature differences in the solution and the solution cooling rate. The investigated relations between the basic defects in the crystals and these parameters result in determination of the optimal conditions for growth of defect-free crystals. Distribution coefficients of Al, In and Cr have been determined, so the growth of crystals with given compositions is possible. Values of Dq/B (crystal field strength) for the various crystal compositions are calculated from the optical absorption spectra. The calculated values show that the discussed solid solutions have weak crystal field and are suitable for media with broadband emission spectra.     

Growth of several quantum crystals: CD4, 4He and 3He

Large single crystals of CD₄, ⁴He, and ³He have been grown from the melt at low temperatures. These “quantum” crystals were characterized with X-ray diffraction techniques which allow excellent determinations of crystal quality. The crystal CD₄ was grown from its melt at saturated vapor pressure. Over 100 helium crystals were grown at constant pressures between 2.9 and 13.8 MPa. It has been found that the hcp and bcc crystals grow in preferred directions, and that growth rates of about 0.3 mm/ min are appropriate for hcp ⁴He, while bcc ³He requires the slower rate of 0.06 mm/min for good quality crystals. There is some indication that it is the isotope, and not the crystal structure, which is the cause of the fact that the hcp ⁴He crystals are much more stable than the bcc ³He crystals. Comparison of several theoretical crystal growth parameters for many substances leads to the conclusion that methane is basically a normal material, while helium is abnormal, having very different growth parameters from most substances. Helium is interesting in that its crystal growth parameters can be changed by very large amounts while keeping the chemistry constant.     

Crystallization and Characterization of Orthorhombic β‐MgSO4 · 7H2O

In solution, the growth rate and the crystal habit are influenced by a number of factors such as supersaturation, temperature, pH of the solution, cooling rate, agitation, viscosity, initial state of the seed crystal and the presence of impurities. The crystallization of orthorhombic β‐MgSO₄ · 7H₂O, from low temperature aqueous solution by slow cooling process was studied. The metastable zone width, the induction periods (τ) for different supersaturations and the effect of pH on the growth rate of the crystals were investigated. The increase of pH yielded bigger crystals. The structural, optical, thermal and mechanical properties of β‐MgSO₄ · 7H₂O have been studied using FT‐IR, X‐ray diffraction, TGA‐DTG and micro hardness analyses.     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Various Growth Shapes of Na2S2O3 5H2O Crystals

The relation between the growth shapes and supersaturations and/or supercoolings was investigated in the Na₂S₂O₃ 5H₂O crystals: (i) The growth shapes changed from a hexagonal prismatic crystal, aggregate of platy crystals and to a spherulite with an increase of supersaturations and/or supercooloings. (ii) The deposition rate suddenly increased at a certain supersaturation and/or supercooling, where the growth shape changed from the hexagonal prism to the aggregate of platy crystals. (iii) The deposition rates of spherulites become higher with increasing the solute concentrations. (iv) The formation process of the spherulites were in situ observed by the Schlieren and/or Mach‐zehnder interferometer methods.     

Kinetics of growth of calcite crystals in presence of ammonium ions

The growth of calcite crystals from an aqueous supersaturated solution of initial concentration 0.15 mol m⁻³ containing 10 mol m⁻³ of (NH₄)₂CO₃ has been studied at 298 K. The progress of growth has been followed through determining the composition of the solution by chemical analysis. The crystal growth is controlled by the surface reaction mechanism and satisfies a kinetic equation of the second order with the rate constant 3.7 × 10⁻⁶ m⁴ mol⁻¹ s⁻¹ that is independent on the surface area of the present solid. NH₄⁺ ions do not change the crystal growth mechanism but decrease the growth rate of calcite crystals.     

Growth of Li2CO3 Single Crystals by Zone Melting Technique

Li₂CO₃ single crystals have been grown by zone melting technique in carbondioxide atmosphere. The diameter of the grown crystal depends on the growth rate. The quality of the crystal depends on the growth rate, temperature of the molten zone, choice of the seed and the temperature of the auxiliary furnace. The crystal shows cleavage plane. The etch studies on cleavage planes show that the etch pits are always triangular in character.     

Temperature gradient controlled growth and optical properties of Er:BaY2F8 crystals

Single crystals of Erbium (Er) doped BaY₂F₈ have been obtained by the temperature gradient technique (TGT). No‐seed‐grown crystal of Er:BaY₂F₈, with the dimensions up to several centimeters, was obtained by self‐crystallization. The optimizations of various growth parameters were systemically investigated. The results indicated that the temperature gradient of 6‐7 K/mm and the cooling velocity less than 6 K/h were suitable for the crystal growth. The XRD data and the investigations on the growth striations by a stereo polarization microscope displayed that the [001] direction is the dominating direction for the crystal growth. The crystal grown by TGT often cracks along with the (100) plane, which is caused by the excessive decrease of the temperature during the crystal growth, for there is a rapid change in the thermal expansion curve of the BaY₂F₈ crystal in the temperature range from 800 °C to 900 °C. The spectral properties of Er:BaY₂F₈ single crystals have been studied and the effects of frequency up‐conversion of the crystals are reported. Spectral data suggest that the quality of Er:BaY₂F₈ crystal obtained by TGT method is good and the crystal has the potential application in laser devices. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study on rapid growth of KH2PO4 and its characterization

Large size crystals of KH₂PO₄ (KDP) were grown by adopting rapid growth technique from point seeds in a 1500‐liter crystallizer which is used to grow KDP crystals by conventional method. The grown KDP crystal size can reach to 310 × 310 × 320 mm³ and the average growth rate was 8mm/day. The optic properties of the rapidly grown KDP crystals were characterized comparing with the KDP crystals grown by the traditional temperature reduction method. We found it that the optical quality of the KDP crystals we grown rapidly are not significantly different from those of KDP crystals grown by traditional method. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of Nd:Sr3Ga2Ge4O14 crystals by a modified Bridgman method

Bridgman growth of Nd:SGG (Sr₃Ga₂Ge₄O₁₄) crystals has been investigated for the first time. Pt crucible of ∅︁25mm×250mm with a seed well of ∅︁10mm×80 mm is used, and seed is SGG crystal of ∅︁10mm×50mm grown by Bridgman method in advance. The growth parameters are optimized as the furnace temperature is set to 1450∼1500°C, temperature gradient in the crystal‐melt interface is less than 25 K/cm and growth rate is less than 0.5mm/h. The Nd:SGG crystals with 25mm in diameter and 60mm in length are grown successfully from 1.5 to 8at% Nd³⁺ doped stoichiometric Sr₃Ga₂Ge₄O₁₄ melt. The distribution coefficient and concentration of Nd³⁺ in Nd:SGG crystals are obviously higher than those of Nd:YAG crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization kinetics of MgSO4 · 7 H2O in presence of tenside

The effect of sodium dodecyl sulfate (SDS) on crystallization kinetics and crystal habit of MgSO₄ · 7 H₂O from aqueous solutions at 25 °C was investigated in batch experiments. It highly depends on the supersaturation level. Both increasing supersaturation and rising concentration of the tenside promote the production of needle-like crystals but the influence of the driving force is much more pronounced. SDS increases the crystallization rate and the linear crystal growth rate in length direction of the crystals. To a high degree it also influences properties of the crystallizing solution such as surface tension and viscosity.     

Growth of bubble-free Ti-doped Al2O3 single crystal by the czochralski method

The behaviour of bubble entrapment in Ti-doped Al₂O₃ single crystals grown by the Czochralski method was studied from the view point of crystal growth conditions such as the crystal rotation rate and kinds of growth atmosphere. The entrapment of bubbles was not correlated with the shape of solid-liquid interface dependent on crystal rotation rate. We found that the use fo He atmosphere, instead of the conventional atmosphere such as Ar, suppresses the formation of bubbles.     

Kinetic studies on the influence of temperature and growth rate history on crystal growth

Crystallization experiments of sucrose were performed in a batch crystallizer to study the effect of temperature and growth rate history on the crystal growth kinetics. In one of the growth methods adopted, the isothermal volumetric growth rate (R_V) is determined as a function of supersaturation (S) at 35, 40 and 45 ºC. In the other, crystals are allowed to grow at constant supersaturation by automatically controlling the solution temperature as the solute concentration decreased. Using the latter method R_V is calculated as the solution is cooled. The obtained results are interpreted using empirical, engineering and fundamental perspectives of crystal growth. Firstly, the overall activation energy (E_A) is determined from the empirical growth constants obtained in the isothermal method. The concept of falsified kinetics, widely used in chemical reaction engineering, is then extended to the crystal growth of sucrose in order to estimate the true activation energy (E_T) from the diffusion‐affected constant, E_A. The differences found in the isothermal and constant supersaturation methods are explained from the viewpoint of the spiral nucleation mechanism, taking into account different crystal surface properties caused by the growth rate history in each method. Finally, the crystal growth curve obtained in the batch crystallizer at 40 ºC is compared with the one obtained in a fluidized bed crystallizer at the same temperature. Apparently divergent results are explained by the effects of crystal size, hydrodynamic conditions and growth rate history on the crystallization kinetics of sucrose. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Einfluß der Transportkinetik auf die Kristallisation von AIIBIVC-Verbindungen beim Gasphasentransport

The reported investigation intended to find tendencies in the influence of thermal and chemical conditions of chemical transport reactions on the growth of ZnSiP₂ and ZnSiAs₂ crystals. A more favourable crystallisation with less intergrowth is proved, if transport gases of the same system — as e.g. ZnCl₂ or SiCl₄ — is used instead of PbCl₂ and TeCl₄. — Depending on the concentration of the transporting medium the largest amount of crystals with lengths of more than 5 mm coincides with the point of lowest transport rate. The number of crystal defects increases with the dimensions of the crystals. — From the experimental results a hypothesis for explaining the locally different growth of crystals of ternary compounds is presented.     

Effects of ammonium citrate additive on crystal morphology of aluminum phosphate ammonium taranakite

In this communication, we demonstrated the growth of aluminum phosphate ammonium taranakite (NH₄-AlPO₄) crystals from regular hexagonal form into the disk-like form could be controlled by ammonium citrate (AMC) as the effective crystal growth modifier at 90 °C. Prepared crystals were characterized by field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD). The effects of AMC's concentration on the crystal form and morphology of NH₄-AlPO₄ were studied. The results showed that the AMC's concentration is an important parameter to control the size and morphology of NH₄-AlPO₄ crystal. The formation mechanism of the special morphology of NH₄-AlPO₄ crystals was also analyzed.     

Structural,optical and dielectric studies on K1‐x(NH4)xH2PO4 mixed crystals grown from z‐cut seeds

Mixed crystals of K_1‐x(NH₄)_xH₂PO₄(KADP) were grown from KDP (KH₂PO₄) dominated mixed solutions with varying molar proportion of ADP (NH₄H₂PO₄) addition. It was found that, as the increase of ADP molar concentration, the growth rate along z‐axis of KADP crystal decreased rapidly. The structure of KADP crystals was investigated by powder XRD and the lattice parameter was calculated. The results showed that the lattice parameter c of KADP crystal increased with the molar concentration of ADP. The optical homogeneity of grown KADP crystals was determined with a differential phase‐shifting interferometry. Frequency dependences of the dielectric constant and dielectric loss of KADP crystals were measured at room temperature (290 K). The dielectric constants of KADP crystals were almost invariant with the increase of frequency. In the region of 10²∼10⁴Hz, the values of the dielectric loss reduced with the increase of frequency. The piezo‐resonance coupling effect still exists in KADP crystals at room temperature, but shifted to low frequency band. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Edge-Defined Film-Fed (EFG) Growth of Rare-Earth Orthovanadates REVO4 (RE=Y,Gd): Approaches to Attain High-Quality Shaped Growth

In spite of their superior laser and polarizer properties rare-earth orthovanadates (REVO₄) single crystals have not been adopted yet into extensive industrial applications because of crystal growth difficulties. The main problems of CZ technique are compositional change and diameter instability. This work presents the first attempt to apply the edge-defined film-fed growth (EFG) technique by which well-shaped REVO₄crystals have been grown directly. The capillary properties of YVO₄ and GdVO₄ melt have been measured. The applicability of shaped growth for rare-earth orthovanadate family was approved by successful EFG growth of transparent rod-like macro-defect-free single crystals of YVO₄ and GdVO₄. We address two main approaches to enhance the quality of EFG crystals: (i) meniscus and crystal shape stability dependence on die top shape and (ii) the strategy of effective operating control. Concave die top was found to be the best choice for high-quality EFG growth of REVO₄ along [001] direction. The spectral analysis of weight signal from growing crystal was shown to be a useful feedforward clue to prevent crystallinity degradation at a very early stage. A reasonable stability of the EFG process was achieved using [211], [101], [001] and [100] pulling directions.