Growth Of Large Single Crystals Research Articles

Numerical analysis of transport phenomena in Y–Ba–Cu–O melt during growth of superconducting crystal Y123 by Czochralski method

In 1993, at the Superconductivity Research Laboratory (SRL), International Superconductivity Technology Centre (ISTEC), in Tokyo, continuous growth of large single crystals of YBa 2 Cu 3 O 7− x (Y123) was achieved by the application of a modified Czochralski method. This paper presents the numerical computations of the flow, thermal and Y concentration fields in the Ba–Cu–O melt for Y123 single crystal growth by this modified method. The finite volume method was used to calculate the fluid flow, heat transfer and yttrium distribution in the melt with staggered numerical grid. The flow in the melt was modelled as an incompressible Newtonian and Boussinesque fluid. Calculations are presented for a combined flow regime of buoyancy-driven natural convection and crystal-rotation-driven forced convection.

Growth of large single crystals of rare earth hexaborides

Single crystal growth of several rare earth hexaborides has been carried out by the floating zone technique. A high-power xenon arc lamp image furnace was used for the crystal growth. Large high-quality crystals of LaB 6, CeB 6, PrB 6 and NdB 6, about 1 cm 3 in volume have been obtained. Crystals of all these compounds have also been grown using enriched 11 B isotope for use in neutron scattering experiments.

Phonons in an entropic crystal

Hard spheres crystallize due to purely entropic forces. The underlying excluded-volume interaction is completely anharmonic and the nature of the phonon spectrum is therefore of interest. To measure the single-particle motion and the phonon spectrum by dynamic light scattering we have used a collection of novel techniques including multispeckle cross correlation on a CCD chip and the growth of large single crystals using a temperature gradient. The random hexagonal close packed crystal has a dispersion relation closer to hcp than to fcc.

Crystal-growth and spectroscopy of Er 3+ ions in calcium gallogermanate

This paper reports on the Czochralski growth of large single crystals of Er 3+ doped Ca 3Ga 2Ge 4O 14 (CGGO) in radio frequency heated induction furnaces operating at 18 kHz. Crystals were successfully grown at concentrations of 0.005, 0.05, 0.5 and 5 mol% of the ErF 3 dopant, and were of exceptionally good optical quality. Spectroscopic studies of the intra-4f shell absorption and fluorescence transitions of the Er 3+ ions in this material were performed at 10 K. Measurements of the optical linewidths show no signs of concentration broadening up to 5 mol%, a feature attributable to the intrinsic compositional disorder of the crystal. We have identified 67 crystal-field levels of the C 2 symmetry site available for occupation by a trivalent rare-earth ion and observe strong up-conversion fluorescence for excitation (both resonant and non-resonant) of the absorption transitions from the ground state to the 4F 9/2 multiplet. Prominent self-absorption dips observed in excitation spectra recorded whilst monitoring up-converted fluorescence, demonstrate the importance of radiative energy transfer at high concentrations.

00/00601 The characterisation of coals and their respective chars formed at 1300°C in a drop tube furnace

A novel crystal growth apparatus which separates the source and growth regions of a vapour transport system into separate vertical furnaces connected by a horizontal transport passage has been developed. The transport passage incorporates a flow restrictor which regulates the mass flow and decouples it from the source–sink temperature difference. Growth takes place on a seed crystal past which the growth envelope is dynamically pumped to ensure that the transport is diffusionless. This “multi-tube” arrangement offers several advantages over conventional linear vapour transport systems. In addition to control of the mass transport, source and growth regions are thermally decoupled and there is noninvasive optical access to both source and crystal growth regions and for in situ vapour pressure monitoring. In this paper, the results of in situ vapour pressure measurements made during the growth of a boule of cadmium telluride are presented and a flow model, which accounts for the significant molecular component of the transport, is developed and used to calculate the corresponding mass flows. This system offers the potential to give improved control of the nucleation and subsequent growth of large single crystals of high-vapour pressure opto-electronic materials such as the II–VI compounds.

Morphological control of particles

Abstract The objective of this review is to highlight the theoretical and practical aspects of particle morphological control. Materials with directional properties are opening new horizons in material science. Structural, optical, and electrical properties can be greatly augmented by the fabrication of composite materials with anisotropic microstructures or with anisotropic particles uniformly dispersed in an isotropic matrix. Examples include structural composites, magnetic and optical recording media, photographic film, and certain metal and ceramic alloys. The new applications and the need for model particles in scientific investigations are rapidly outdistancing the ability to synthesize anisotropic particles with specific chemistries and narrowly distributed physical characteristics (e.g. size distribution, shape, and aspect ratio). Anisotropic particles of many compositions have been produced but only a few (γ-Fe2O3 and AgI) are produced with any degree of chemical and physical control. These two examples are the result of literally decades of study. Unfortunately, the science and technology (mainly the technology) that have evolved are maintained as proprietary information. Thus, while we generally know what systems yield single crystal, anisotropic-shaped particles, we do not know how to make powders of these crystals with the desired control of shape uniformity, aspect ratio and phase composition. Particle shape control is a complex process requiring a fundamental understanding of the interactions between solid state chemistry, interfacial reactions and kinetics, and solution (or vapor) chemistry. During synthesis of other than a large single crystal the parameters controlling crystal growth must be balanced with the requirements for anisotropic powder nucleation and growth. Although there has been considerable progress in large single crystal growth and the synthesis of powders composed of monodispersed, spherical particles, these efforts have not often been transferred to the synthesis of anisotropic particles.

A novel “multi-tube” vapour growth system and its application to the growth of bulk crystals of cadmium telluride

A novel crystal growth apparatus which separates the source and growth regions of a vapour transport system into separate vertical furnaces connected by a horizontal transport passage has been developed. The transport passage incorporates a flow restrictor which regulates the mass flow and decouples it from the source–sink temperature difference. Growth takes place on a seed crystal past which the growth envelope is dynamically pumped to ensure that the transport is diffusionless. This “multi-tube” arrangement offers several advantages over conventional linear vapour transport systems. In addition to control of the mass transport, source and growth regions are thermally decoupled and there is noninvasive optical access to both source and crystal growth regions and for in situ vapour pressure monitoring. In this paper, the results of in situ vapour pressure measurements made during the growth of a boule of cadmium telluride are presented and a flow model, which accounts for the significant molecular component of the transport, is developed and used to calculate the corresponding mass flows. This system offers the potential to give improved control of the nucleation and subsequent growth of large single crystals of high-vapour pressure opto-electronic materials such as the II–VI compounds.

Facetting during directional growth of oxides from the melt: coupling between thermal fields, kinetics and melt/crystal interface shapes

Implicit in most large-scale numerical analyses of crystal growth from the melt is the assumption that the melt/crystal interface shape and position are determined by transport phenomena. Although reasonable for many materials under a variety of growth conditions, this assumption is incorrect for a number of practical systems under realistic growth conditions. Specifically, the behavior of systems (e.g. certain oxides) which tend to develop facets along the melt/crystal interface is often affected both by transport phenomena and by interfacial attachment kinetics. We present a new modeling approach which accounts for interfacial kinetic effects during melt growth of large single crystals. The isotherm condition, typically employed at the melt/crystal interface, is replaced by an equation accounting for undercooling due to interface kinetics. A finite-element algorithm, designed to accommodate its numerical mesh to the appearance of facetted interfaces, is applied to this problem. Results are presented for the simulated directional growth of oxide slabs. The interplay between evolving thermal fields and anisotropic interface kinetics is investigated. In particular, the evolution of facets and the dependence of their size on growth conditions is explored. Trends reported here are in qualitative agreement with those appearing in the literature. Discrepancies between quantitative predictions of facet sizes using a theory (see Ann. Rev. Mater. Sci. 3 (1973) 397 and references within) and those calculated in this paper can, in a number of cases, be attributed to the simplifications on which this theory is based.

Growth of large single crystals using the improved lamp-image floating-zone furnace: application to

We grew large single crystals of reproducibly over a wide doping range using an improved travelling solvent floating zone (TSFZ) method. We performed a simulation on the lamp-image focusing to find an optimum condition for a steep temperature gradient along the rod. Based on the results of simulation, we adapted smaller lamps and shielded the partial area of the mirror surfaces. The improvement of the focusing had a great effect on keeping the molten zone stable for more than 100 h. Stabilization of the molten zone improves both the reproducibility of the crystal growth and the quality of the grown crystals.

Magnetic, transport, and optical properties of monolayer copper oxides

The authors review the results of a wide variety of experiments on materials such as ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ that contain weakly coupled ${\mathrm{CuO}}_{2}$ layers. These materials are antiferromagnetic insulators with very large Heisenberg exchange energies, which become high-temperature superconductors when charge carriers are added to the ${\mathrm{CuO}}_{2}$ layers. The growth of large single crystals has made it possible to carry out neutron scattering, as well as anisotropic optical, transport, and magnetization measurements. The properties of the undoped ${\mathrm{CuO}}_{2}$ layer are reviewed, and the evolution of magnetic, optical, and transport properties with the addition of charge carriers is discussed. The emphasis is on the pure and lightly doped materials, although the magnetism in the superconductors is discussed.

Crystal growth and optical characterisation of novel 3d 2 ion laser hosts

This paper reports the Czochralski growth of large single crystals of Cr 4+ : Ca 10(PO 4) 6F 2 (CFAP), Mn 5+ : Sr 10(VO 4) 6F 2 (SVAP), and Cr 4+ : SrGdGa 3O 7 (SGGM) crystals in radio-frequency heated induction furnaces operating at temperatures in the range 1873–1923 K. Crystals having excellent optical quality were produced using growth rates of 2.8–11.2 μm/s and rotation rates between π/2 and π rad/s. The optical absorption spectra confirm that the 3d 2-configuration ions reside at distorted (VO 4), (PO 4) and (GaO 4) tetrahedra in SVAP, CFAP and SGGM, respectively, at which transition probabilities are enhanced by the odd-parity terms at the C s site symmetry in the crystal field expansion. The near-infrared luminescence is broadened by structural disorder and is strongly dependent upon both the crystal field strength and temperature. Although the luminescence in Cr 4+ : CFAP and Cr 4+ : SGGM occur as broadbands in the near infrared region, the intensity is weak at 300 K as a consequence of nonradiative decay. In contrast, the effects of nonradiative decay in Mn 5+ : SVAP are weak and strong zero-phonon lines are observed even at room temperature.

Dissolution and crystallization of KTiOPO4 in the flux system of K6P4O13

The crystallization temperatures of KTiOPO4(KTP), KTP : Nb and RbTiOPO4(RTP) in the flux systems, K6P4O13 (K6) or Rb6P4O13 (R6), were determined by thermogravimetric analysis. The systems have a large tendency to supercool, which could be greatly reduced by the top-seeding technique. Seed-testing experiments were used to study the dissolution kinetics. The diffusion equation under steady state conditions was used to derive an expression for the seed dissolution, which enables one to introduce the diffusion activation energy and the diffusion rate parameter from the experimental data. Consequently, the stable growth rate and the cooling programme for the crystal growth can be estimated. The stable growth rate decreases with temperature and is in the order of 0.1 mm day-1 in unstirred solutions. The optimal cooling rate curve, however, goes through a maximum as the temperature falls. A stable growth rate and suitable supercooling are essential for the growth of large single crystals of high quality. © 1998 Chapman & Hall

The growth and perfection of organic non-linear optical crystals: N-methyl urea (NMU) from methanol solution. III. The growth of large single crystals for optical examination

Using growth conditions defined from previous studies of the kinetics of growth of this material, large crystals of N-methyl urea (NMU) have been prepared by seeded solution growth using the temperature lowering technique. The principal constraints to good growth are that crystals should be prepared using seeds cut from good quality crystals previously prepared by this technique and that they should be developed under conditions of low supersaturation (σ < 0.5−3%). Efficient stirring conditions should be used with flow reversal to minimise the effect of volume diffusion and macrostep formation which in turn leads to the inclusion of solvent in some sectors. The role of seed orientation in the definition of the development process is discussed. Under well-defined conditions of seed orientation and development crystals of dimensions 5 × 4 × 4 cm3 have been produced of high optical and structural perfection and with efficient non-linear optical performance. This material shows a figure of merit performance which improves on urea. For the doubling of light of wavelength 871.6 nm; d14 ≈ 2pm V−1, cf. urea d14 = 1.7pm V−1.

Large single-crystal growth and characterization of the narrow-gap semiconductor

is a narrow-gap semiconductor with a layered structure, isoelectronically analogous to PbS. Large single crystals were grown by the Bridgman - Stockbarger method from the melt and characterized by x-ray diffraction. The lattice parameters were determined and the discrepancy between those already reported was removed. From IR reflectivity measurements in the plasma region, the number of carriers was calculated. Also a strong anisotropy of the electrical conductivity was detected and the narrow-gap character of the material was supported by electrical measurements in the range of 10 - 300 K.

Growth of a new highly efficient organic nonlinear optical crystal: 3-methoxy-4-hydroxy-benzaldehyde (MHBA)

The growth of large single crystals (70 × 40 × 20 mm3) of 3-methoxy-4-hydroxy-benzaldehyde (MHBA) and the best solvents for the growth process are reported. Specifically, the growth process, morphology, and defects in these MHBA crystals are discussed. A high conversion efficiency of 75% has been obtained with MHBA crystals by using a mode-locked Nd:YAG laser, and direct frequency doubling of the light from a GaAlAs laser diode was accomplished using these crystals at room temperature in order to generate the violet second harmonic.

Effect of growth conditions on crystal morphology and superconductivity of Bi-2212 oxide

The effects of growth velocity and the nominal starting composition of the feed rod on the crystal morphology and stability of the solid-liquid interface in BiSrCaCuO high- T c superconducting oxides were studied by a floating zone method. The result showed that an indispensable condition for the growth of large single crystals is that the solid-liquid interface maintains a stable planar growth front. The planar solid-liquid interface tends to break down into a cellular growth interface if the velocity is increased above 0.2 mm/h. The largest single crystals of size 20 × 5.5 × 1.5 mm 3, along the a-, b- and c-axes, respectively, were grown in 6.5 mm diameter rods. The superconducting transition temperature of the as-grown single crystals varies from 84 to 92 K under the different growth velocities and nominal compositions of a feed rod. The homogeneity of the superconducting phase of the crystals also depends on the growth conditions.

Large single-crystal growth of RBa2Cu3O7−x by the flux method (R=Y and rare earths)

In this work the thermal conditions required for single-crystal growth in RBa2Cu3O7−x systems (R=Y or rare earths apart from Ce and Tb) have been systematically revised in order to improve the size of crystals obtainable by the flux method. Large single crystals of NdBCO, SmBCO and EuBCO have been actually obtained and characterized by XRD, oxygen content and electrical resistivity.

Nucleation kinetic study of KTiOPO 4 crystallizing from high temperature solution

High temperature solution growth is an important method to grow single crystals of the NLO material KTiOPO 4. K 6P 4O 13 is amongst the best available flux for this purpose. A knowledge of the key parameters such as interfacial tension and metastable zone width will be helpful for the successful growthof large size single crystals. In the present work, the value of the interfacial tension between solid KTP and molten solution was calculated from the knowledge of the solubility data. The metastable zone width was calculated by applying the classical nucleation theory. A comparative study of the different fluxes available for the growth of KTP is made.

Growth of large single crystals of Ba 1− xCa xTiO 3 and their characterisation

Large optical quality Ba 1− x Ca x TiO 3 single crystals have been grown using a flux technique with very slow cooling rates. Growth conditions were optimized with higher concentrations of Ca (above 16 mol.%). The formation of ‘cake’ or ‘excess’ twinned particles at the bottom of the crucible was completely absent for higher calcium content. A layer growth mechanism was observed on twinned and rectangular crystals. Lattice parameters decreased with increasing calcium content, and the room temperature dielectric constant was found to decrease compared to that of pure BaTiO 3.

The growth of large single crystals of the organic nonlinear optical material 2-(α-methylbenzylamino)-5-nitropyridine

The crystal growth behaviour of the important organic nonlinear optical material 2-(a-methylbenzylamino)-5-nitropyridine (MBANP) (space group P2 1 ) has been investigated and techniques have been developed for the preparation of large (up to 7 cm x 4 cm x 3 cm) high-quality single crystals suitable for detailed optical, dielectric, ultrasonic and piezoelectric studies and for device fabrication. Observation of the crystal growth behaviour and measurement of growth kinetics revealed a strong and anomalous dependence of growth rate on supersaturation. At relative supersaturations less than 0.08, slow growing irregular faces were formed. Once the crystal was completely bounded by such faces, little or no further growth occurred. At supersaturations below 0.24 no growth was observed in either &lt;010&gt; direction. Above this, rapid, but highly imperfect, growth occurred in one &lt;010&gt; direction while the opposite end of the crystal exhibited zero growth rate at all achievable supersaturations. At relative supersaturations greater than 0.4 considerable instability in the growth along [010] was observed. The factors potentially responsible for this behaviour are discussed. A novel technique is described for the preparation of seed crystals elongated parallel to &lt;010&gt;. The use of such seeds allows the growth of large crystals of MBANP at supersaturations less than 0.24, thereby avoiding the highly imperfect and unstable growth at higher supersaturations and yielding crystals of very high quality. The nature, density and distribution of growth induced defects in large crystals of MBANP have been investigated using X-ray diffraction topography. In addition to revealing growth dislocations, growth sector boundaries, inclusions and striations, these studies provide a detailed insight into the growth history of the crystals.