Improvement of Initial Conditions of the Crystal Growth by Dissolution and Refaceting

The influence of significant dissolution and refaceting of KDP and Rochelle salt crystals on their further growth is studied. It is shown that the crystal growth rate ‐ initial crystal size dependence is better defined for growth after refaceting. Initial conditions for growth of individual crystals after refaceting are better defined, than in the case of classical seed nucleation. By dissolution and refaceting, it is easy to provide the same initial conditions for crystal growth in each of several experiments, preformed under the same macroscopic external conditions.     

The Influence of Impurities on Crystallization Kinetics of Sodium Chloride

The influence of impurities on the crystallization kinetics of NaCl was investigated in a fluidized bed crystallizer. The growth and dissolution rates were related to the supersaturation and impurity concentrations. The effect of different impurities on the growth rate of NaCl crystals can be divided into thermodynamic effects where the impurities influence the solubility and kinetic effects where the impurities will suppress the growth rate compared to the pure NaCl. A mathematical model describing crystal growth rates from aqueous solution as a function of impurity concentration is presented. The model explains impurity concentration effects on the crystal growth rate in terms of an impurity effectiveness factor and a Langmuir adsorption isotherm for the impurity.     

Dissolution of Si in Al–Si Melts. Effect of Sb on the Crystallization Process and As-cast Structure of Al–Si Alloys

The effect of Sb on the dissolution of Si single crystals (face {111}) in Al–Si melts is studied. The dissolution isotherms at three temperatures and fixed undersaturation are obtained. The experimental data are treated according to the Bliznakov's ratio. Some conclusions are made with reference to the crystal growth process. A probable mechanism of the influence of Sb on the structure of Al–Si alloys is given.     

Exaggerated grain growth in bixbyite via fast diffusion along planar defects

Naturally occurring bixbyite single crystals from the Maynard claim at Thomas Mountain Range in Utah, USA, show pronounced grain growth, when compared to bixbyite crystals from other locations in close vicinity. These enlarged single crystals reveal characteristic linear surface features, which were initially interpreted as twin boundaries. HRTEM studies in conjunction with EDS analysis and electron diffraction, however, confirmed that these features originate from thin interlayers of braunite, Mn₇SiO₁₂, epitaxially grown within the host crystal. A model for the observed exaggerated grain growth is presented, which is based on the assumption that fast diffusion occurs along the braunite/bixbyite phase boundaries in three dimensions and, more importantly, promote nucleation at surface edges. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and dissolution of equally‐sized insulin crystals

A protocol for growing sets of nearly uniform size crystals was devised and tested experimentally. The experiments were centered on insulin because of its medical significance however the method is applicable to other substances as well (C.N. Nanev, V.D. Tonchev, F.V. Hodzhaoglu, Protocol for growing insulin crystals of uniform size, J. Cryst. Growth 375 (2013)10–15). Now, both growth and dissolution of equally‐sized crystals are described quantitatively by a common analytical model. In our model the emphasis is put on the dissolution case when crystals number and/or size are sufficiently large to secure reaching solubility, while some non‐dissolved crystalline substance is still remaining. Quantitative results are obtained for the relations between dimensionless values of crystal size, solution concentration and time elapsed, the assumption simplifying our calculations being that the crystals retain their shape during the entire dissolution process.     

Growth‐Sector Boundaries and Growth‐Rate Dispersion in Potassium Alum Crystals

Crystals of potassium alum, pure and slightly doped with Cr³⁺, were grown from aqueous solution by slow temperature lowering. In addition, short re‐dissolution periods were introduced in order to provoke growth defects and changes of growth rates. Crystal slices of about 1 mm thickness were studied by conventional LANG X‐ray diffraction topography using MoKα radiation. For Cr‐doped crystals, boundaries between {100}, {100} and {111} growth sectors appear by pronounced dynamical X‐ray topographic contrast similar to that of stacking faults. Re‐dissolution experiments provoke the formation of inclusions on {100} faces, followed by an increase of the {100} growth rate by the factor of about six, relative to the neighboured {111} faces. X‐ray topographs show that this increase is correlated with the formation of dislocations, which interestingly have pure‐edge character. During further growth these dislocations penetrate the {100}‐{111} growth sector boundary and vanish from the {100} face, which slows down and finally adopts its former growth rate before re‐dissolution.     

Investigations of the process of crystal growth from a liquid zone by Seebeck measurements

An arrangement for measuring the thermoelectric voltage (Seebeck signal) during the crystal growth from a liquid zone is described. Using the example of growing PbTe single crystals by THM it is shown that different equilibrium temperatures at both phase boundaries provide a differential Seekeck voltage depending on the crystal growth rate. Relaxation times which are needed to reach steady-state conditions with respect to the concentration difference between the growing and solving interface in the case of a start or sudden stop of the heater motion can be obtained.     

Revelation of microtwinning in hydrothermally grown synthetic quartz by growth structures

Growth structuress on prism surfaces of synthetic quartz grown by hydrothermal crystallization technique, indicative of microtwinning in them are described, illustrated and discussed. The microtwins are revealed by the misorientation of the growth structures, and it is observed that microtwinning is manifested in different forms. Twinned domains appear in the form of strictly oriented, square striated (stepped) structures with inclined surfaces, seemingly overlapping square striated structures, square and irregular regions demarcated by black line-like boundaries, patch-like structures enveloping irregularily bound domains, tongue-like terraces bordered by irregular line of discontinuity of varying height and the regions with both differently shaped as differently oriented growth structures. From the orientations of the square shaped growth structures, it is shown that crystals consist of domains which are in twin relationship to the main crystal. Twins by rotation around an axis perpendicular to (10ī0) with rotation angles of 60°, 90°, and 180° have been observed. Critical examination of some twin boundaries delineated by black line-like matter, with the application of multiple beam interferometry and FECO reveal that the black boundary is a ridge (line-elevation). The line elevation along such a twin boundary is explained to be as a result of preferential adsorption of impurity atoms along the trace of the boundary. It is further observed that discontinuity in crystalline structure along the twin boundary leads to growth forms with distorted morphology thus establishing strong dependence of growth forms on the underlying crystalline structure. Causes for generation of the microtwins (twinning on a microscale) in synthetic quartz are discussed.     

4 inch低位错密度InP单晶的VGF生长及性质研究

采用高压垂直温度梯度凝固法(VGF)生长了非掺、掺硫和掺铁的4 inch直径(100)InP单晶,获得的单晶的平均位错密度均小于5000 cm-2.对4 inch InP晶片上进行多点X-射线双晶衍射测试, 其(004)X-射线双晶衍射峰的半峰宽约为30弧秒且分布均匀.与液封直拉法(LEC)相比, VGF-InP单晶生长过程的温度梯度很低,导致其孪晶出现的几率显著增加.然而大量晶体生长结果表明VGF-InP晶锭上出现孪晶后,通常晶体的生长方向仍为(100)方向,这确保从生长的4 inchVGF-InP(100)晶锭上仍能获得相当数量的2~4 inch(100)晶片.由于铁在InP中的分凝系数很小,掺Fe-InP单晶VGF生长过程中容易出现组份过冷,导致多晶生长.通过控制生长温度梯度及掺铁量,可获得较高的掺铁InP单晶成晶率.对VGF-InP单晶的电学性质、位错密度及位错的分布特点、晶体完整性等进行了研究.     

Zur quantentheoretischen Berechnung von Aktivierungsenergien für den Stoffübergang an Phasengrenzflächen von Ionenkristallen (IV). Zur radiochemischen Bestimmung und quantentheoretischen Berechnung von Aktivierungsenergien für den Stoffübergang an den Kristallflächen des NaCl

It is shown that a direct proportionality exists between the activation energy for the mass transfer at the respective crystal faces of ionic crystals and the frequency of the phonones (longitudinal-optical), Planck's constant being found once more as a proportionality constant. Thus it could be demonstrated that the different activation energies measured at different time intervals for the mass transfer processes at phase boundaries of ionic crystals can be attributed to the specific growth of the crystal faces. Thus, NaCl crystal fractions which were mechanically stressed (pulverized and sifted) and consequently contained a great amount of {111}- and {110}-faces, respectively, experimentally yielded an activation energy wich agrees with the values determined by quantum theory when the frequency of propagation of the phonons is inserted into a derived equation. This relation was also confirmed by NaCl crystal fractions predominantly containing cubic faces. This was indicates that in mass transfer processes on phase boundaries of ionic crystals quantum mechanical laws are of importance.     

水热法KTP晶体生长与宏观缺陷研究

本文报道了水热法KTP晶体的生长工艺及晶体生长形态,系统研究了水热法KTP晶体的宏观缺陷,其宏观缺陷主要为添晶、生长脊线、裂隙和包裹体.提出了晶体生长工艺的改进措施,如提高原材料和试剂的纯度、调整籽晶的悬挂方式、减少籽晶的尺寸等,都可以减少晶体的宏观缺陷,提高晶体的质量.//(011)切向的籽晶生长的晶体质量较高,且能很好地应用于激光器件中.     

On the Formation of Thick Sector Boundaries by Appearance and Disappearance of Faces of Crystals

Wide sector boundaries revealed in KBC crystals by chemical etching have been observed and analysed. It was established that the boundaries represent sectors of small faces appearing and vanishing in the habit during crystal growth. This nature of thick boundaries has been confirmed by the analysis of values of relative growth velocities of faces with respect to the critical velocities governing the presence of faces on growing crystals.     

Etching Figures in Neodymium Gallate and Yttrium Aluminate Crystals

The dislocations existing in single crystals of neodymium gallate and yttrium aluminate grown by the Czochralski technique have been studied by means of etch pits. The data concerning their solubility in cases of different directions of a face orientation, various treatment temperatures and several enchant types are reported. The investigation of etch pits in the twinned YAlO₃ and NdGaO₃ crystals showed that twins are formed during a growth process. In the [110]‐pulled NdGaO₃ crystals the discrepancy between the twin and matrix parts of a crystal is accommodated by the dislocation congestion and the dislocation low‐angle boundaries whereas in [010]‐pulled YAlO₃ crystals the microcracks perform this function.     

Phase-field simulations of Te-precipitate morphology and evolution kinetics in Te-rich CdTe crystals

Te precipitates are one of principal defects that form during cooling of melt-grown CdTe or CZT crystals when grown Te-rich. Many factors such as the kinetic properties of intrinsic point defects (vacancy, interstitial, and antisite defects); stresses associated with the lattice mismatch between precipitate and matrix; temperature gradients and extended defects (dislocations, twin and grain boundaries); non-stoichiometric composition; thermal treatment history all affect the formation and growth/dissolution of Te precipitates in CdTe. A good understanding of these effects on Te precipitate evolution kinetics is technically important in order to optimize material processing and obtain high-quality crystals. This research develops a phase-field model capable of investigating the evolution of coherent Te precipitates in a Te-rich CdTe crystal undergoing cooling from the melt. Cd vacancies and Te interstitials are assumed to be the dominant diffusing species in the system, which is in two-phase equilibrium (matrix CdTe and liquid Te inclusion) at high temperatures and three-phase equilibrium (matrix CdTe, Te precipitate, and void) at low temperatures. Using available thermodynamic and kinetic data from experimental phase diagrams and thermodynamic calculations, the effects of Te interstitial and Cd vacancy mobility, cooling rates and stresses on Te precipitate, and void evolution kinetics are investigated.     

X‐ray Topographic Characterization of L‐Arginine Phosphate Monohydrate Crystals

L‐arginine phosphate monohydrate (LAP) organic crystals were investigated by X‐ray Lang topography. Selected topographs of nearly perfect seeded grown and self‐nucleated crystals show that typical defects inside are grown‐in and post‐growth dislocations, growth sector boundaries, and microbes, while their features are presented and discussed. No structural defects associated to the presence of step bunching on the (100) surface of the crystal have been observed by X‐ray topography.     

Studies on twin boundaries in calcite crystals

Natural single crystals of calcite have been cleaved along (100) planes and cleavage faces have been etched in 2% and 3% citric acid solutions. Etching produces twin boundaries oriented in 〈010〉 directions. The etch pits on the two sides of the twin boundary are oppositely oriented. It has been conjectured that the rows of pits might have been formed due to etching of dislocations on twin boundaries. One to one correspondence of twin boundaries has been established on matched cleavage faces. This is further confirmed by studying the induced twin regions produced on a (100) cleavage plane by indenting that plane itself. The implications are discussed.     

Study of the dissolution and growth of salt single crystals in inhomogeneous acoustic fields: II. A focused acoustic field

The growth and dissolution of the isolated (100) face of a KDP crystal at exposure of the phase boundary to focused acoustic fields have been investigated. Visualization tools have been developed and the scale and dynamics of the arising acoustic flows have been established. A quadratic dependence of the dissolution rate on the sound pressure has been obtained; such a dependence is in agreement with the theoretical concepts about the dependence of the mass exchange rate in the acoustic boundary layer on the field parameters. A significant growth response of the face under study, normally exposed to ultrasound along the acoustic axis in the prefocal and postfocal planes of a spherical concentrator (f = 1.4 MHz), has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are radically different. The results obtained show a possibility of controlling growth and dissolution of crystals by varying the parameters of an inhomogeneous acoustic field.     

Preparation of Oriented Single Crystals of Iron and Dilute Iron-Silicon Alloys by the Strain Anneal Technique

Single crystals with demanded crystallographic orientation of the crystal axis, of iron and iron alloyed with some 0.5 and 0.9 wt % silicon were prepared by the strainanneal method. The raw materials used in this study were powder carbonyl iron and pure silicon. Attempts to grow single crystals from vacuum melted iron containing about 1200 ppm oxygen were unsuccessful. The large pinning of grain boundaries by second phase particles is assumed to be the main factor influencing grain boundary mobility. From materials zone refined in hydrogen atmosphere single crystals of Fe, Fe−0.5 and 0.9 wt % Si were readily prepared and the yield of 90% of oriented single crystal was obtained. It was found that the number of twin grains on the crystal surfaces was rapidly lowered on increasing the silicon content.     

X-Ray topographic characterization of α-Fe2O3 single crystals obtained by chemical transport with TeCl4

The method of Lang is used to study the defect structure of „as-grown”︁ hematite crystals obtained by chemical transport with TeCl₄. The presence of twin boundaries, dislocations and crystallites built in the platelet matrices is established. Considerable differences in density and distribution of the crystal disturbances are observed with samples grown under the same experimental conditions. In some cases the real structure is found to be nearly perfect.     

Hydrothermal etching of flux grown phenakite (Be2SiO4) single crystals

The etching studies were made on flux grown phenakite (Be₂SiO₄) single crystals to elucidate the twinning phenomenon and the growth mechanism. The sharp and distinct etch pits and the twin boundaries were successfully obtained after the hydrothermal etching. From the etching and the X-ray precession experiments, it was conjectured that the phenakite had an inversion twin along the c axis. The twinning should be originated during the earlier stage of growth where the dendritic and acicular crystals were grown and thereafter the twinned crystals grew to the prismatic crystals during the latter stage with decreasing the temperature.