Vapor Crystal Growth Research Articles

Controlled vapor crystal growth of Na4Ir3O8 : A three-dimensional quantum spin liquid candidate

We report the successful bulk single-crystal growth of the hyperkagome lattice iridate $\mathrm{N}{\mathrm{a}}_{4}\mathrm{I}{\mathrm{r}}_{3}{\mathrm{O}}_{8}$ (Na438) by vapor transport using a sealed aluminum oxide tube as a container. Crystals were characterized by magnetization, x-ray diffraction, and energy-dispersive x-ray measurements, confirming their identity and properties. Single-crystal x-ray diffraction experiments revealed superlattice peaks indexed on a propagation vector $q=(1/3,1/3,1/3)$ based on the cubic substructure with cell parameter $a=8.986(1)\phantom{\rule{0.16em}{0ex}}\AA{}$. This superlattice is three-dimensional and fully coherent. Polarization analysis rules out spin and/or orbital order as the underlying origin of the modulation and points to long-range ordering of Na ions at the notionally disordered Na sites as a plausible origin for the observed superlattice.

Effects of convection on physical vapor transport of Hg2Cl2 in the presence of Kr - Part I: under microgravity environments

Special attention in the role of convection in vapor crystal growth has been paid since some single crystals under microgravity environments less than 1 <TEX>$g_0$</TEX> exhibits a diffusive-convection mode and much uniformity in front of the crystal regions than a normal gravity acceleration of 1 <TEX>$g_0$</TEX>. The total molar fluxes show asymmetrical patterns in interfacial distribution, which indicates the occurrence of either one single or more than one convective cell. As the gravitational level decreases form 1 <TEX>$g_0$</TEX> down to <TEX>$1.0{\times}10^{-4}\;g_0$</TEX>, the intensity of convection, indicative of the maximum molar fluxes, is reduced significantly for <TEX>${\Delta}T=30K$</TEX> and 90 K. The total molar fluxes decay first order exponentially with the partial pressure of component B, PB (Torr) for 20 Torr <TEX>${\leq}PB{\leq}$</TEX> 300 Torr, and two gravity accelerations of <TEX>$g_y=1\;g_0$</TEX> and 0.1 <TEX>$g_0$</TEX>.

Importance of convection during physical vapor transport of Hg2Cl2in the presence of Kr under environments of high gravitational accelerations

Special attention in the role of convection in vapor crystal growth has been paid since some single crystals under high gravity acceleration of <TEX>$10g_0$</TEX> appear considerably larger than those under normal gravity acceleration (<TEX>$1g_0$</TEX>). With increasing the gravity acceleration from <TEX>$1g_0$</TEX> up to <TEX>$10g_0$</TEX>, the total molar flux for <TEX>${\Delta}T$</TEX> = 30 K increases by a factor of 4, while for <TEX>${\Delta}T$</TEX> = 90, by a factor of 3. The maximum molar fluxes for three different gravity levels of <TEX>$1g_0$</TEX>, <TEX>$4g_0$</TEX> and <TEX>$10g_0$</TEX>, appear approximately in the neighborhood of y = 0.5 cm, and the molar fluxes show asymmetrical patterns, which indicate the occurrence of either one single or more than one convective cell. As the gravitational level is enhanced form <TEX>$1g_0$</TEX> up to <TEX>$10g_0$</TEX>, the intensity of convection is increased significantly through the maximum molar fluxes for <TEX>${\Delta}T$</TEX> = 30 K and 90 K. At <TEX>$10g_0$</TEX>, the maximum total molar flux is nearly invariant for for <TEX>${\Delta}T$</TEX> = 30 K and 90 K. The total molar flux increases with increasing the gravity acceleration, for <TEX>$1g_0{\leq}g_y{\leq}10g_0$</TEX>, and decreases with increasing the partial pressure of component B, a noble gas called as Kr (Krypton), <TEX>$P_B$</TEX>. The <TEX>${{\mid}U{\mid}}_{max}$</TEX> is directly proportional to the gravity acceleration for 20 Torr <TEX>$P_B{\leq}300$</TEX> Torr. As the partial pressure of <TEX>$P_B$</TEX> (Torr) decreases from 300 Torr to 20 Torr, the slopes of the <TEX>${{\mid}U{\mid}}_{max}s$</TEX> versus the gravity accelerations increase from 0.29 sec to 0.54 sec, i.e. by a factor of 2. The total molar flux of <TEX>$Hg_2Cl_2$</TEX> is first order exponentially decayed with increasing the partial pressure of component B, <TEX>$P_B$</TEX> (Torr) from 20 Torr up to 300 Torr.

Vapor pressures of organic semiconductors:: α-hexathiophene and α-quaterthiophene

The sublimation vapor pressures of the thin film transistor materials, α-hexathiophene (α-6T) and α-quaterthiophene (α-4T) were measured using the Knudsen effusion method and enthalpies of sublimation were calculated. The criteria for Knudsen cell design to obtain accurate vapor pressures of large organic molecules were examined. A recording microbalance with an accuracy of ±0.1 μg was used in conjunction with a specially designed Knudsen cell. Vapor pressures from 10 −6 to 10 −2 Torr were measured. At low pressures, the mean free path was large even for oligomers with hard sphere diameters of 22 Å, so that Knudsen criteria were fulfilled. The vapor pressure results are used to discuss the observations from previously published physical vapor crystal growth experiments.

Enhancement of Mercuric Iodide Detector Performance Through Crystal Growth in Microgravity: the Roles of Lattice Order

AbstractThe hole-mobility•carrier-lifetime product of α mercuric iodide high energy radiation detectors has been enhanced through vapor crystal growth in microgravity. This improvement is closely correlated with specific characteristics of the crystal lattice, which have been identified by high resolution synchrotron x-ray diffraction imaging. These structural features and the associated performance are now being approached in terrestrial growth of α mercuric iodide.Gravity may affect the uniformity of this crystal lattice in two distinct ways: 1) directly through deformation that it imposes on the lattice during growth and 2) indirectly through convection, which mixes any extraneous material. Inclusions associated with these processes harden the lattice and facilitate lattice folding. These changes affect the electronic parameters of detectors made from the crystals. As purification procedures are optimized, the incorporation of extraneous material is curtailed, enhancing electronic properties in spite of lattice flexing through loss of precipitation hardening.These studies provide insight into the contribution of various aspects of crystalline order in α-mercuric iodide crystals to property improvement. This knowledge has led to modification of requirements for starting materials, adjustment of physical vapor growth procedures, and change in crystal handling procedures. As a result, the electronic performance of terrestrially grown radiation detectors has been improved, and we provide evidence that further enhancement is still possible.

Diffusion Control during Vapour Crystal Growth

AbstractThe crystal growth rate under diffusion control is considered. Together with nutrient diffusion, the physi‐sorption of an inert gas, which impedes the surface kinetics during vapour growth, is taken into consideration. In this way the size of the diffusion field surrounding vapour grown crystals was estimated. The relations obtained are checked with data for the growth of zinc single crystals in argon atmosphere. The appearance of shallow cavities on their basal faces is used as a morphological mark for growth under diffusion control. Besides, the microstructure of the inner edge of the macroscopically flat periphery, surrounding the shallow cavities is investigated experimentally, by means of SEM.

Recent developments in vapour crystal growth fluid dynamics

This paper deals with possible sources of convection arising from non-linear irreversible thermodynamic effects (non-Navier—Stokes fluid dynamics) and slip boundary conditions during typical processes of crystal growth from vapour phase. Attention is focused on the role played by thermal (Burnett) stresses and side-wall temperature creep in single component gases, but the extension and generalization to the more general case of binary mixtures is also indicated. A rigorous non-dimensional order of magnitude analysis is performed to compare these effects to the vapour transport mechanisms usually considered in vapour crystal growth fluid dynamics, e.g. buoyancy and Stefan-Nusselt flow; new characteristic velocities, lengths and corresponding non-dimensional numbers are introduced and discussed, and a priori conditions for the existence and characterization of all possible flow regimes are formulated. Then, a quantitative analysis of natural, thermal stress, and thermal creep convection is given by considering a simplified geometrical configuration, for which an analytical solution is derived.

Numerical simulation of non-Navier-Stokes fluid-dynamics in vapour crystal growth

In recent theoretical developments, non-linear irreversible thermodynamic effects, like Burnètt stresses and slip boundary conditions, have been recognized as playing an important role under typical conditions encountered during vapour crystal growth in microgravity, and new classes of convection and flow regimes have been identified via a rigourous non-dimensional order of magnitude analysis. In this paper we present a numerical quantitative analysis of these regimes for geometrical configurations of interest in vapour crystal growth. The results are presented for different combinations of the non-dimensional characteristic parameters, ranging from microgravity to earth environment conditions. In each case we give isotherms, stream lines and velocity profiles, and compare the results with the corresponding solution of the Navier-Stokes approximation.

Morphological instabilities of CBr4 crystals during growth from vapors.

We have studied the surface morphological instabilities of ${\mathrm{CBr}}_{4}$ crystals during growth from vapors. We have found that the crystal-growth morphologies depend not only on temperature and supersaturation, but also sensitively on the total pressure of inert gas in the vapor. With increasing inert gas pressure, at otherwise the same growth conditions, crystals with initially smooth surfaces first develop cellular patterns and then evolve into dendritic structures. Although these cellulars and dendrites bear morphological similarity with those found in melt and solution-growth experiments (i.e., the product ${\mathit{W}}^{2}$R=const, with W being either the width of a cellular or the radius of a dendritic tip, and R the growth rate at the corresponding solid front) the destabilizing mechanism appears to be more complex than that from bulk nutrient diffusion. We have found that the surface diffusion on the substrate also plays an important role in surface morphological instabilities in vapor crystal growth.

Fluid dynamic modelling of crystal growth from vapour

Navier-Stokes equations along with no-slip boundary conditions have been widely used, up till now, in modelling vapour crystal growth fluidynamics. In this paper attention is focused on new types of free convection which occur in a gas or a vapour when viscous stresses, due to velocity gradients, are of the same order of magnitude as stresses due to temperature and/or concentration gradients (thermal and/or solutal-stress convection). In this case linear phenomenological relations, based on the classical principles of non-equilibrium thermodynamics, for the diffusive fluxes of momentum, energy and species concentration (conventional Stokes, Fourier, Fick laws; Navier-Stokes fluidynamics) become inadequate and a more general theory must be formulated to account for thermal stresses convection, included in the second-order approximation for the gas-dynamic equations (Burnett equations), and side-wall gas creep, induced by the slip boundary condition in the Knudsen layer. This work deals with the above mentioned gas-kinetic phenomena, presented in the framework of non-equilibrium thermodynamics. Both phenomenological (macroscopic) and kinetic (microscopic) points of view are considered together with the question of the coexistence of these approaches. The Burnett equations are written in non-dimensional form, and a-priori criteria of the non-dimensional order of magnitude analysis lead to the identification of new characteristic velocities and corresponding non-dimensional numbers; new classes of free convection (thermal stress and thermal creep convection) are discussed. The final part of the work is devoted to the evaluation of the orders of magnitude of these new terms during typical crystal growth experiments on Earth and under microgravity, and to the quantitative analysis of thermal stress convection in a simplified geometrical configuration.

Monte-Carlo simulation of crystal growth with diffusion employing an anisotropic model

Monte-Carlo technique is used to simulate the vapour crystal growth with surface diffusion on the (100) face of a simple cubic crystal employing the anisotropic variable bond model [1]. The effects of surface diffusion on crystal growth kinetic roughening are discussed.

Mass spectroscopic characterization of the GeSe: GeI 4 vapor transport system

The GeSe: GeI 4 vapor crystal growth system was characterized mass spectroscopically. A steady-state Knudsen effusion technique was developed to simulate the equilibrium conditions at one end of a vapor transport ampoule. It was found that the previously neglected equilibrium GeSe 2(s) = GeSe(v)+ 1 2 Se 2(v) reduces the Se 2(v) concentration to an extent that sublimation/ condensation of GeSe becomes the dominant transport mechanism. At total pressure near 1 atm the concentration of an additional Ge-Se-I vapor species becomes comparable to that of GeSe(v).

ON SINGULAR BOUNDARY CONDITIONS IN MASS TRANSFER ACROSS RECTANGULAR ENCLOSURES

In modeling mass transfer across rectangular enclosures one is confronted with singular velocity boundary conditions at the geometrical corners of the enclosure. Solutions to such a problem obtained, e.g., from a finite difference scheme, may then depend on the particular way the singularities are handled. The uncertainty in the solutions is studied for a vapor crystal growth sample case, by comparing results obtained in two extreme codes, corresponding to slip and non-slip, respectively, at the corners. It is found that (within the realm of the model used) the total mass transport across the enclosure, the sublimation and the condensation fluxes at the interfaces, and the concentration profile are unaffected by the choice of codes. The velocity field, however, is affected adjacent to the singularities.

Mass Transfer in Closed Ampoule Vapor Crystal Growth

EXPANDED ABSTRACTClosed ampoule vapor transport techniques are widely employed for the preparation of single crystals [1–5]. The experimental simplicity of these techniques makes them also attractive for fundamental crystal growth transport studies in reduced gravity environments. However, the underlying transport processes are complex and difficult to quantify.