SiC crystal growth from transition metal silicide fluxes

SiC crystal growth in transition metal silicide melts was investigated by using spontaneous infiltration and solution methods. In the infiltration experiments, SiC powder preforms were infiltrated with Fe_xSi_y (Fe₃Si, Fe₅Si₃ and FeSi) and CoSi melts. The dissolution and precipitation of SiC led to SiC crystals growth in the infiltrated Fe₅Si₃ and CoSi melts, SiC particles coalescing in FeSi and free carbon precipitation in Fe₃Si. In the solution experiments, carbon from the graphite crucible dissolved in and reacted with FeSi₂ and Ti_2.3Si_7.7 to form SiC crystals. Scanning electron microscopy (SEM), X‐ray diffraction (XRD) and Raman scattering spectrometer were employed to investigate SiC crystals growth. Based on the investigation, the effect of solution content on the SiC crystal growth, the growth mechanisms in both methods and prototypes of the SiC crystals are also discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Results of crystal growth of bismuth-antimony alloys (Bi100–xSbx) in a microgravity environment

Results of two experiments are presented for growth of crystals from (Bi_100–xSb_x) alloys in a microgravity environment. In the growth experiments different variants of the Bridgman technique were used. It was shown that in crystal growth from the melt in closed ampoules under microgravity conditions convection can be prevented completely. Therefore it is possible to grow crystals from melts of some components under diffusion controlled conditions of mass transfer. In microgravity a reduced interaction between the melt and the confining walls was observed even if they have large contact with each other. The investigation of surface morphology corroborated the importance of surface effects for crystal growth from the melt under microgravity conditions. Measurements of electronic properties of crystals grown in microgravity showed a good quality in comparison to earth grown crystals. Because under microgravity conditions in closed ampoules the diffusion controlled mass transfer can be realized and the interaction between the melt and confining wall is reduced, homogeneous crystals with high perfection can be grown melts of some components.     

General principles of the synthesis of chalcogenides and pnictides in salt melts using a steady-state temperature gradient

The possibilities of growing crystals of metals, alloys, chalcogenides, and pnictides in halide melts using a steady-state temperature gradient are analyzed. Halides of alkali metals and aluminum can be used as transport media. The choice is determined by the melting temperature of salt mixtures. A conducting contour can also be applied to increase transport efficiency. This technique of crystal growth is similar to the electrochemical method. To eliminate interference during migration, some elements can be isolated and forced to migrate through independent channels to the crystal formation region. The technique considered here makes it possible to grow crystals of necessary quality without special equipment; the small crystal sizes are sufficient for laboratory study.     

Flux growth and characterizations of NdPO4 single crystals

Neodymium phosphate single crystals, NdPO₄, have been grown by a flux growth method using Li₂CO₃-2MoO₃ as a flux. The as-grown crystals were characterized by X-ray powder diffraction(XRPD), differential thermal analysis (DTA) and thermogravimetric analysis (TG) techniques. The results show that the as-grown crystals were well crystallized. The crystal was stable over the temperature range from 26 to 1200 °C in N₂. The specific heat of NdPO₄ crystal at room temperature was 0.41 J/g °C. The absorption and the fluorescence spectra of NdPO₄ crystal were also measured at room temperature.     

Influence of Thermal Processing Parameters and Material Properties on Velocity Configurations in Semiconductor Melts During the Vertical Bridgman Growth Technique

The paper deals with the influence of thermal boundary conditions and thermophysical material properties on velocity configurations in Bridgman arrangements. Numerical simulations are presented for (Bi_1—xSb_x)₂Te₃ melts as a representative for semiconductor melts of low Prandtl numbers. Based on two characteristic temperature profiles, results have been calculated for 2D-axisymmetric and 3D Bridgman configurations applying the FIDAP^TM FEM code using pseudo-steady-state conditions with a constant growth rate. For simulation close to real growth conditions the model used includes real geometry parameters as well as the experimentally measured temperature distribution at the outer ampoule surface and temperature depending material properties. The calculations show significant variations in the flow configuration and the resulting radial inhomogeneity of the grown crystal depending on the thermal processing parameters used.     

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.     

On the Mechanism of Crystal Growth from Undercooled Melts

The mechanism of crystal growth from undercooled melts is studied experimentally by means of a simple capillary technique. Thymol and Na₂S₂O₃ · 5 H₂O are used as model substances. Dislocation-free crystal faces of these compounds are obtained by continuous growth in the capillary or by prolonged annealing under appropriate conditions. Two-dimensional mechanism of growth was experimentally verified on such perfect crystal planes. The conditions under which the surface nuclation mechanism operates are described accounting for the supersaturation, the influence of impurities etc. Transition from two-dimensional into spiral growth of purposedly defected crystal faces is demonstrated and investigated. The mechanism of formation of perfect crystals is discussed and further experimental evidence on the possibility of two-dimensional growth from melts is given.     

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.     

Crystal growth of Mg- and Ti-substituted hexagonal ferrites from PbO-B2O3 based melts

Crystals of Mg and Ti-substituted hexaferrites have been prepared as perspective materials for the photoelectrochemical conversion of solar energy by means of anodes. The melts on the basis of PbO B₂O₃ were used to grow both bulk crystals and thin films. In the chosen system the primary crystallization field of hexaferrites was found. The crystals were grown by conventional unseeded crystal growth using (ACRT) accelerated crucible rotation technique to ensure their good quality and homogeneity. The size of the crystals varied from 10 to 23 mm in diameter. Maximum total content of Mg²⁺ and Ti⁴⁺ amounted to 2 atoms per formula unit of hexaferrite.     

Crystal Growth of Boron Sillenite Bi24B2O39

The Sillenite type Bi₂₄B₂O₃₉ is an incongruently melting compound at T_p = 650 °C. Single crystals have been grown from non-stoichiometric melts as well as from high temperature solutions by the Czochralski method and by a top seeded solution growth technique (TSSG), respectively. The main difficulty in the crystal growth of Bi₂₄B₂O₃₉ arises from the very small field of crystallization in the binary system Bi₂O₃–B₂O₃. Further problems are caused by the nearly simultaneous formation of the 2:1 compound Bi₄B²O₉ and the 12:1 compound Bi₂₄B₂O₃₉. Therefore, a precise thermal reinvestigation of the phase diagram was carried out using DTA-technique on the Bi₂O₃-rich side. Additionally, crystal growth runs have been started in the ternary system Bi₂O₃–B₂O₃–Li₂O in order to extend the crystallization field. Homogeneous melts were more difficult to prepare because of the high density difference between Bi₂O₃ (∂ = 9.3 g/cm³) and B₂O₃ (∂ = 2.46 g/cm³). The homogeneity of the melts were improved, using Bi₂O₃ and synthesized Bi₄B₂O₉ (∂ = 8.25 g/cm³) as starting materials. As a result of this procedure, small crystals of Bi₂₄B₂O₃₉ were grown from these starting materials and the lattice parameter were determined.     

Crystal growth and structure of KBi(WO4)2 single crystals

Potassium bismuth tungstate [KBi(WO₄)₂] single crystals have been grown by the top‐seeded solution growth technique. Bulk crystal with dimensions up to several centimeters is obtained for the first time. Several self‐flux systems have been used for the growth from the solution and the experiments using K₂W₂O₇ as a solvent are detailed. Powder and single crystal X‐ray diffraction of this crystal are reported. The structure refinement shows that KBi(WO₄)₂ crystallizes in the monoclinic space group C2/c, with a=10.837(3), b=10.586(3), c=7.622(2)Å, β=130.860(3)°, V=661.4(3)ų, and Z=4. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystal growth of ZrW2O8 and its optical and mechanical characterization

ZrW₂O₈ is known for its isotropic negative thermal expansion over a wide of range of temperature from ?272 to 777 °C. However, ZrW₂O₈ melts incongruently at 1257 °C and is stable only over a short temperature interval between 1105 and 1257 °C. This makes the growth of single crystals a formidable challenge. In order to study the intrinsic properties of this compound, a repeatable, viable single crystal growth strategy is required. Here we report a simple, self-seeding, self-fluxing single crystal growth process which resulted in single crystals of ZrW₂O₈ up to about 4 mm in size. Grown crystals were characterized by X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Mechanical properties of the crystals were studied using nanoindentation.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

Growth and characterization of InxBi2−xTe3 single crystals

The V–VI group narrow band gap compounds are known to have important photoconductivity and thermoelectric properties. Among these, Bi₂Te₃ is the most potential material for thermoelectric devices having a direct band gap of 0.16 eV. There has been ample study reported on crystal growth and polycrystalline thin films of both pure and indium doped Bi₂Te₃ pertaining to its basic semiconducting, optoelectronic and thermoelectric properties. It has been shown that on exceeding certain limiting concentration of indium in Bi₂Te₃, the conductivity changes from p-type to n-type. However, there is hardly any work reported in literature on crystal growth, dislocation etching and optical band gap of In_xBi_2?xTe₃ (x=0.1, 0.2, 0.5) single crystals. The authors have grown their single crystals using the zone melting method. The freezing interface temperature gradient of 70 °C/ cm^?1 has been found to yield the best quality crystals obtainable at the growth rate of 0.4 cm/h. The as-grown crystals have been observed to exhibit certain typical features on their top free surfaces. The crystals have been characterized using XRD technique. A chemical dislocation etchant has been used for estimating perfection in terms of dislocation density in the crystals. The optical absorption was measured in the wave number range 500 to 4000 cm^?1. The transitions in all the cases were observed to be allowed direct type. The detailed results are reported in the paper.     

Growth of lanthanum heptamolybdate crystals by gel technique

The growth of single crystals of lanthanum heptamolybdate is achieved by allowing controlled diffusion of La ions through silica gel using the system La(NO₃)₃ MoO₃ NH₄OH HNO₃–Na₂SiO₃. The reaction mechanism leading to the growth is reported. Mechanism of crystallization of La heptamolybdate is discussed. Formation of Liesegang rings consisting of La heptamolybdate crystals in the system is reported. It is observed that concentration programming and seeded growth enhances the size of crystals. The crystals grown in the system are single crystal platelets exhibiting squarish or octagonal facets. Some crystals exhibit a spherulitic morphology.     

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.     

Growth,characterization and fabrication of n-WSe2 PEC solar cells

Single crystals of n-WSe₂ have been grown by the chemical vapour transport (CVT) technique, using Br₂, TeCl₄, and SeCl₄ as transporters. Crystal lattice parameters have been determined for all the crystals grown above with an X-ray diffractometer. The composition of the above crystals were examined by energy dispersive analysis of X-rays (EDAX). Optical and some electrical transport measurements are carried out to judge the semiconducting nature of the as-grown crystals. Photoelectrochemical (PEC) solar cells were frabricated by using these crystals as photoelectrodes and platinum grid as counter electrode in an aqueous iodine/iodide solution. Some of the PEC cell parameters were determined. The results thus obtained have been discussed in detail.     

Specific features of the phase formation,synthesis, and growth of ZnMoO4 crystals

The ZnO-MoO₃ phase diagram in the range of ZnO compositions from 0.95 to 1.05 mol % is refined by differential scanning calorimetry. Data on crystal stoichiometry are obtained using the weighting method by reducing ZnMoO₄ in a hydrogen atmosphere. The specific features of solid-phase synthesis of ZnMoO₄ are studied, and its heat of fusion is measured. The modes of solid-phase synthesis and growth of ZnMoO₄ crystals are optimized. Some experimental data on the ZnMoO₄ crystal growth in the [001] direction by the low-thermal gradient Czochralski method are presented. Crystals with a cross section of ～50 × 50 mm², a length of 160 mm, and a weight up to 1 kg have been obtained.     

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)     

Morphology of β-BaB2O4 (BBO) Crystals in Relation to its Structure and Growth Conditions

β-BaB₂O₄ (BBO) crystals with well-defined morphology have been grown from Na₂O solutions using the top seeded solution growth (TSSG) method. The crystal morphology in relation to its structure and growth conditions has been studied in detail on the basis of crystallography and crystal chemistry. It is found that the morphological characteristics are related to the orientations of structural unit (B₃O₆)^3— anion rings in the crystal. On the other hand, the growth parameters may greatly affect the appearance of faces of the crystal, but the crystals still generally take trigonal in outline and have a diagnostic character of point group 3m. The observed morphology is in disagreement with BFDH and PBC analyses and is explained from the incorporation of the growth units on the faces and facets. Since the incorporation rates of the growth units are different on different faces, the boule habits with well-defined morphology are formed.