Thermodiffusion and Morphological Stability in Convectionless Crystal Growth Systems from Melts and Melt-Solutions

Crystal growth experiments of multicomponent systems under microgravity require an exact analysis of the diffusion phenomena in the nutrient fluid phase. The contribution of the Soret effect to the transport and distribution of matter in a convectionless casting arrangement for shaped crystal growth and in THM melt-solution zones have been investigated. Some semiconductor systems were analysed from experimental (Bi_1-xSb_x mixed crystals) and theoretical (PbTe, InP, GaAs, CdTe compounds) point of view. The criterion for constitutional supercooling was correspondingly modified. It has been distinguished between associated (AIV BVI, AII BVI) and dissociated (AIII–BV) melt-solutions, containing different species of diffusion (molecules or atoms, respectively).     

Crystal Growth and Physico-Chemical Properties of Lead Molybdate in the Homogeneity Region

From data of both X-ray phase analysis and differential thermal analysis, as well from measurements of the crystal lattice parameters, electric conductivity and diffusion coefficients of cations in ceramics and single crystals the width of the homogeneity region of the lead molybdate phase which lies in the composition range of 49.85–50.50 mole% MoO₃, has been determined. It was also established that the compound was a daltonide phase. The results obtained explain the Pb/Mo ratio changes along the crystal.     

The Precipitation of Strontium and Lead Sulphates from Aqueous Solution. Kinetics of the Crystal Growth

The precipitation of strontium and lead sulphates from well-stirred supersaturated aqueous solutions, of initial solute concentrations C₀ = 0.001 to 0.020 M and C₀ = 0.0002 to 0.003 M respectively, was studied at 20° and 40°C by chemical analysis and optical microscopy. Nucleation occurred during induction periods and continuous regular growth then took place onto the nuclei formed during these periods. Crystallisation was complete after 4 to 48 hr. The crystal growth was rate-controlled by the rate of deposition of metal salt ions onto the growing crystal surfaces. This rate (dC/dt), at any growth time, then depended on both the overall surface area (A_t) and on the residual excess solute concentration (ΔC_t) in solution according to the relation while the growth rate (dα/dt), expressed in terms of degree of crystallisation, was . The rate constants (k_α) for the crystal growth of strontium and lead sulphates at 20°C were 22 and 4200 sec⁻¹ M⁻² respectively — that is, greatest for the salt with least cation hydration –; these constants increased 4 to 6 times for 20°C temperature rise. The rate-determining process for the metal salt deposition was probably the ion dehydration.     

The Precipitation of Barium Chromate with Slow Development of Supersaturation. Kinetics of the Crystal Growth

Slow barium chromate precipitation was studied in well-stirred solutions — final concentration 0.002 mole 1⁻¹ — at 20°C at mixing rates varying from 0.2 to 2 · 10⁶ mole 1⁻¹ sec⁻¹; the kinetics of the crystal growth process were studied by chemical analysis and by optical microscopy. The crystal growth started after induction periods; regular growth then took place on the nuclei formed during these periods. For the main growth, up to about seventy percent precipitation time, growth rates at any time depended on the (residual excess solute concentrations in solution)². Immediately after the induction periods, crystal lengths varied with growth time according to the relation, for the main growth, crystal lengths varied with growth time according to a more complex relation, where k_l is the rate constant for surface growth, R_c is the rate of development of metal salt concentration and K is a constant that depends on the number of precipitate crystals. After some growth time τ = τ^*, when l_τ = l_τ*, all fresh solute added to the solution was utilised immediately and was deposited onto the growing crystals; then, for the final thirty percent precipitation, crystal lengths varied with growth time according to the relation, .     

温度变化对草酸钙晶体生长的影响

草酸钙(CaOxa)是泌尿系结石的主要成份,其生长过程受诸多因素的影响.本文研究了温度对CaOxa晶体成核、生长、聚集、晶相、形貌和溶解度等的影响及其机理,对防治泌尿系结石具有启示作用.但高温地区尿石症发病率高的深层次原因还有待进一步研究.     

Inhibition of Calcium Oxalate Monohydrate Crystal Growth in High and Low Ionic Strength Solutions

The effect of additives on the kinetics of growth of calcium oxalate monohydrate crystals has been studied. Conductivity and potentiometry measurements have been compared. Growth rates were calculated from precipitate curves by a cubic spline method. An approach consisting on the calculation of rate constants and orders of reaction from logarithmic plots of growth rate versus supersaturation has been followed to study crystal growth kinetics. This method revealed that the presence of additives is causing not only a decrease on the rate constant but an increase on the order of reaction as well. The effect of additives (EDTA, citrate and phytate) was considerably weaker in high ionic strength media. Phytate produced a complete blockage of crystal growth in concentrations as low as 2 × 10^—6 mole/L in both methods.     

Kinetics of a Single Crystal Growth from Supersaturated Liquid Solution. An Attempt on the Analytical Description of the State of Solution

The attempt was made at formulating a mathematical model describing the state of the supersaturated liquid solution circumfluenting a single crystal, as well as the transport processes occurring in that solution. As the result of a suitable computation procedure, numerical values of velocity components, concentrations, temperatures and the normal components of the mass flux in the vicinity of the crystal surface have been obtained.     

升温重结晶过程中硫酸钙晶须的形貌控制及其生长机理

利用升温重结晶过程制备硫酸钙晶须,考察了溶液pH值、升温区间、振荡速率等操作条件对晶须生成及形貌的影响,并初步探索了十六烷基三甲基溴化铵(CTAB)、十二烷基硫酸钠(SDS)、聚乙二醇(PEG6000)和乙二胺(EDA)四种表面活性剂对晶须形貌的影响.结果表明:在升温重结晶反应体系中,各操作条件均可对晶须产物的形貌和尺寸产生影响,但pH和升温区间的影响最大;SDS会显著抑制晶核形成,质量浓度为7 g/L时生成的产物形貌为均一棒状,结晶度较高.硫酸钙晶须主要以层状形貌结构生长,少部分晶须呈现类似多晶和外延生长的形貌.     

The Slow Precipitation of Sparingly-soluble Metal Salt Powders from Aqueous Solution: Mathematical Relations for the Kinetics of the Crystal Growth Processes in Different Types of Precipitation

The following types of slow precipitation, of sparingly-soluble metal salts from aqueous solution, have been analysed: precipitations without an induction period – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites; precipitations with significant induction periods – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites. Generally, in the early stages of precipitation, growth rates depend on the rate of addition of metal salt ions to the system and on the rate constant for crystal growth; in this stage, rates increase from low values to optimum values: in the later stages of precipitation, the metal salt ions are used up as rapidly as they are added to the system and growth rates then depend only on the rate of addition and on the number of microcrystallites or seeds; in this range, rates gradually decrease to low values. Mathematical relations, for the variation of crystal size with precipitation time at different stages of growth, have been derived for each of the above types of precipitation.     

The Crystallisation of Aluminium Trihydroxide (Gibbsite) from Sodium Hydroxide Solution Onto Fine Seeds: Physico-chemical Studies on the Internal Structure and Agglomeration of Different Crystal Fractions

A batch of Bayer gibbsite crystals (particle size range 3 to 70 μm) was prepared by crystal growth onto fine seeds (1 to 3 μm) from sodium hydroxoaluminate/sodium hydroxid-solution. The internal structures and seed agglomeration of different fractions were examined by optical microscopy, infra-red spectrophotometry, thermal analysis and chemical dissolution analysis. These physico-chemical studies confirmed that crystal growth occurs with some seed agglomeration in the early stages, followed by growth onto the ‘agglomerates’. The agglomerated seeds contents (X) were as follows: for fraction DiD (l₀ = 6.3 μm), X = 0.44; for fraction Gi C (l₀ = 20 μm), X = 0.41; for fraction Gi B (l₀ = 38 μm), X = 0.31 and for fraction Gi A (l₀ = 55 μm), X = 0.24.     

Molybdenum and Tungsten Combined Oxidation by Dissociative Evaporation of Products during Refractory Oxides Crystal Growth from the Melt

The main chemical reactions and composition of gas and solid phases have been determined for the equimolar ratio Mo: W: O₂ = 1: 1: 2 at T = 2400 K in the pressure range of 1-1 x 10^-5 bar. It is established that the character of the main processes of combined oxidation depends significantly on the pressure and state of the oxidant (oxygen): at P > 7.52 x 10^-5 bar, oxidation reactions involve mainly molecular oxygen, whereas atomic oxygen dominates at lower pressures. At P ≥ 0.424 bar, the solid phase contains not only Mo but also MoO₂. At P = 1 x 10^-5 bar, the concentration of lower Mo and W oxides and elementary Mo and W in the gas phase sharply increases, which can negatively affect the main crystallization units.     

In‐Line Bulk Concentration Measurement by Method of Conductivity in Industrial KDP Crystal Growth from Aqueous Solution

The conductivity of KH₂PO₄ (KDP) aqueous solutions with varying saturation temperature from 28.5°C to 41.8°C was measured in the temperature range of 30°C to 40°C. It's proposed that the conductivity of KDP solutions can be expressed by L=L_o+b₁T+b₂T². This relation can be interpreted by Falkenhagen's (1971) conductivity model of high concentration electrolyte solution. The experimental data agrees well to the relation with an accuracy of super‐saturation within ±0.03g KDP/100g water (±0.10% relative super‐saturation).     

Growth Kinetics of GaP Layers in Liquid Phase Electroeptiaxy (LPEE) from Limited and Unlimited Source

Growth kinetics of GaP layers in two LPEE modifications has been experimentally studied. In the first modification the vapour phase is employed as a source of crystallization. In the second case the losses of the solute during crystallization are not compensated. It has been found that growth kinetics can be treated in the frame work of the model where the growth rate is governed by mass-transport in liquid phase. It's shown that electromigration gives the main contribution to mass-transport at the considered experimental conditions. The quantitative estimations of the effective mass-transport parameters for phosphorus in gallium melt in the range 800 to 1050°C have been obtained.     

单晶炉导流筒、热屏及炭毡对单晶硅生长影响的优化模拟

在CZ法生长太阳能级单晶硅中,单晶炉的导流筒、热屏和炭毡对晶体生长有很大影响.通过对上述三个部件进行改进优化,并通过数值模拟对优化前后晶体和熔体的热场、热屏外表面与石英坩埚内壁面之间的氩气流场以及晶体中的热应力进行分析,得出以下结论:石墨导流筒的引入减少了炉体上部的氩气流动涡胞,进而减少了SiO在单晶炉上部的沉积;优化后的热屏减少了加热器对晶体的烘烤,使结晶速率加快;优化后的侧壁炭毡阻止了加热器向上部的热损失.优化后在加热器功耗不变时,结晶速率至少可提高35;,而不增加宏观位错的发生概率.     

The Precipitation of Alkaline-Earth Metal Carbonate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitations of magnesium carbonate trihydrate, basic magnesium carbonate and calcium, strontium and barium carbonates was studied from equivalent solutions of concentrations from 0.0005 M to 1M, at pHs from 10 to 7, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations of the more sparingly-soluble metal carbonates — mainly studied at medium to high supersaturation — were homogeneously nucleated while the magnesium carbonate trihydrate precipitations — studied at low supersaturations at pH ≦ 7.6 — were heterogeneously nucleated. The crystal forms and numbers of the final precipitates depended on the type and numbers of nuclei (and crystallites) formed during the induction periods. Crystal numbers generally increased with increasing initial mean metal carbonate concentration according to the relation N = N₁C_MCO3^β; β was 3 for the metal carbonate precipitations and β was 4 for the basic magnesium carbonate precipitations. N₁ values increased in the order basic MgCO₃ (at pH ≧ 9), or MgCO₃ · 3H₂O (at pH ≦ 7.6) < CaCO₃ < SrCO₃ < BaCO₃. The final crystal lengths then generally decreased, from maximum values, with increasing initial concentration according to the relation l_fin = l₁/C^γ, where γ was 0.7 and 1.0. For precipitation at any concentration and pH, smaller crystal sizes were generally obtained in precipitates from solutions of the metal carbonate of lower solubility.     

The Precipitation of Alkaline-Earth Metal Oxalate Powders from Aqueous Solution. Crystal Numbers and Final Sizes

The precipitation magnesium oxalate dihydrate, calcium oxalate monohydrate, strontium oxalate monohydrate and barium oxalate hemihydrate was studied from equivalent solutions of concentrations from 0.001 M to 0.5 M, at pHs from 7 to 6, by optical microscopy and other methods. Crystal growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal form and numbers of the final precipitates depended on the type and number of the nuclei (and crystallites) formed during the induction periods. Crystal numbers at medium to high supersaturation, increased with increasing initial mean metal oxalate concentrations according to the relation, N = N₁c; β was 5 for calcium oxalate precipitations and β was 6 for the other metal oxalate precipitations. The N₁ values increased in the order MgC₂O₄ · 2 H₂O < BaC₂O₄ · 1/2 H₂O < SrC₂O₄ · H₂O < CaC₂O₄ · H₂O. The final crystal lengths, in this supersaturation range, then decreased (from maximum values) with increasing initial concentrations according to the relation, l_fin = l₁/C_MOx^β, where γ was 1.3 to 1.6. For precipitations from solution of any concentration at any pH, smaller crystals were obtained in the precipitates of the metal oxalate of lower solubility.     

The precipitation of transition metal oxalate powders from aqueous solution. Crystal numbers and final sizes

The precipitation of manganous, ferrous, cobalt, nickel and copper oxalate hydrates was studied from equivalent solutions of concentrations from 0.001 to 0.3 M at pHs 7 to 6, by optical microscopy and other methods. Crystals growth started after induction periods: the precipitations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. The crystal numbers of the final precipitates depended on the number of nuclei (and crystallites) formed during the induction periods. At medium to high supersaturations, crystal numbers increased with increasing initial metal oxalate complex ion concentrations according to the relation. N = N₁C_mox^β, where β was 5. The N values increased in the order Mn ≪ Fe < Co < < Ni < Cu. The final crystal lengths, in this range, then decreased with increasing metal oxalate complex ion concentrations according to the relation l_fin = l₁/C_mox^γ, where γ was 1.3. For precipitations from solution of any concentration, smaller crystals were generally obtained in the precipitates of the metal oxalate of lower solubility; nickel oxalate precipitations were the exception to this.     

The precipitation of alkaline-earth metal molybdate powders from aqueous solution. Crystal numbers,final morphology and sizes

The precipitation of barium, strontium and calcium molybdates was studied from neutral equivalent solutions of concentrations from 0.0004 to 0.4 M at 25 °C. Crystal growth started after induction periods; the precipitiations were heterogeneously nucleated at low supersaturations and homogeneously nucleated at medium to high supersaturations. Barium molybdate was precipitated as tetragonal bipyramids, strontium molybdate generally as prisms and calcium molybdate as platelets. Crystal numbers at medium to high supersaturations increased with increasing inital metal molybdate concentrations according to the relation, The final crystal lengths in this range than decreased from maximum values (at the critical concentrations) with increasing initial metal molybdate concentrations according to the relation. Generally, for precipitation from solutions at any concentration, larger crystals were obtained in the precipitates of the salt of higher solubility.