Untersuchungen zu einer neuen Methode der Gasphasenzüchtung von α-Al2O3-Whiskern und deren Realstruktur

The paper deals with the effect of the fundamental technological parameters on the mass increment of α-Al₂O₃ sapphire whiskers grown in a high-temperature resistance furnace by hydrogen reduction. Results of X-ray topographical studies of the defect structure of needle- and plate-like whiskers are presented. The directions of growth and the tensile strength of the grown whiskers are determined.     

Vapor-phase growth of CdF<Subscript>2</Subscript> whiskers in the CdF<Subscript>2</Subscript>-GaF<Subscript>3</Subscript> system

Insulating CdF₂ whiskers of helical shape with a diameter from 10 to 30 μm and a length exceeding 7 mm have been grown for the first time. The conditions of their growth suggest that they grow through the classical vapor-liquid-solid mechanism.     

Hydrothermal synthesis and formation of 4ZnO·B2O3·H2O whiskers via a new route

4ZnO·B₂O₃·H₂O whiskers were prepared from 2ZnO·3B₂O₃·3.5H₂O under hydrothermal process at 160 °C for 10 h. The synthesized product was characterized by XRD, SEM, TG‐DSC and FT‐IR. SEM results showed that the synthesized 4ZnO·B₂O₃·H₂O whiskers' length was about 3–10 μm and the diameter was 0.2–0.3 μm. Further study on the whiskers' growth process and mechanism showed that the formation of the whiskers went through three stages and the morphology of 4ZnO·B₂O₃·H₂O changed from irregular particles to rod‐like structures and finally to whiskers. The variation of the morphology of the 4ZnO·B₂O₃·H₂O whisker with the concentration of the starting material was investigated. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation of calcium sulfate whiskers using waste calcium chloride by reactive crystallization

Calcium sulfate whiskers, owing to its good thermal stability, chemical resistance and good compatibility with rubber and plastics, has a strong potential in the application of polymer reinforcing composite materials. This work deals with the preparation of calcium chloride whiskers by reactive crystallization process with sulfuric acid and calcium chloride discharged from Solvay process. Firstly, the orthogonal experiments were carried out using both CaCl₂ and H₂SO₄ in analytic grade as feedstock, and the reactive crystallization conditions were optimized, which included reaction temperature, reactant concentration, reaction time and stirring speed. Based on the optimized reactive crystallization conditions, the new process for the preparation of calcium chloride whiskers was studied using the reactive crystallization process with sulfuric acid and calcium chloride discharged from Solvay process, and the effects of impurity ions in the waste solutions on the growth of whiskers were evaluated. It was found that calcium sulfate whiskers with stable and structured fine crystals (its aspect ratio up to 190) can be prepared using waste CaCl₂ in Solvay process. The experimental results will provide the valuable information for the reasonable disposal of waste CaCl₂ solution from Solvay process. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

VLS Growth of ITO Whiskers Prepared by the Electron Shower Method

Indium tin oxide (ITO) whiskers were grown by VLS (vapour-liquid-solid) mechanism, using the electron shower method. The whiskers were grown above 200 °C, and the deposition rate was above 0.6 nm/s. The electron shower controlled the size of the whiskers, and the size was 30 nm in diameter and 600 nm in length. The whiskers grew along the substrate at t < 300 s, but grew in a direction perpendicular to the substrate at t > 300 s. When the ITO whiskers grown along the substrate were used as NO₂ gas sensor, the sensitivity was 340, and about 300 times higher than those of the whiskers grown in a direction perpendicular to the substrate and plate-like ITO crystallites.     

Bildung und Untersuchung von SixAl1−xAs-Kristallen

Si_xAl_1−xAs is formed by chemical transport reaction of AlAs using iodine as transporting agent. The experiments are performed in sealed silica tubes in the temperature range from 900 to 1000°C. The grown platelet-shaped crystals and whiskers are investigated with x-ray diffraction, light microscopy, electron microscopy and electron-probe microanalysis. The mechanism of growth is discussed.     

Dislocation and microindentation analysis of vapour grown Bi2Te3‐xSex whiskers

The structural defects and microhardness of Bi₂Te_3‐xSe_x whiskers (x = 0, 0.2 and 0.4 at% Se) grown by physical vapour deposition (PVD) method have been investigated. Concentric pairs of dislocation loops were observed on the as‐grown surfaces of short hexagonal prisms. A systematic study of dislocations in these crystals was carried out by chemical etching technique. The effects of Se doping, annealing and quenching on the mechanical properties have also been studied on the prism faces of Bi₂Te_3‐xSe_x whiskers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Lu5Ir4Si10 whiskers: Morphology,crystal structure,superconducting and charge density wave transition studies

Highly perfect single crystal whiskers of Lu₅Ir₄Si₁₀ were successfully grown out of the melt. Details of the surface and morphology of the whiskers are presented. X-ray diffraction data confirmed that the whisker structure has the same tetragonal P4/mbm space group symmetry as bulk single crystals with lattice parameters a=12.484(1) and c=4.190(2) Å. By means of field emission scanning electron microscopy, the morphology of the whiskers has been studied. Using a 4-circle X-ray diffractometer we found that whiskers grow along the c-axis direction and all side faces are oriented along the [1 1 0] direction. The mosaicity has been measured and is found to be almost perfect: below 0.15° along the c-axis. According to our transport measurements performed along the c-axis, the whiskers present a sharp superconducting transition at T_c=4.1 K and show a charge density wave (CDW) transition at 77 K. From the hysteresis of the temperature dependance of the electrical resistivity study, the CDW transition is found to be of first order.     

Über den Einfluß von Wachstumsfluktuationen auf den effektiven Verteilungskoeffizienten beim Zonenschmelzen

The influence of growth rate fluctuations on the micro and macrodistribution of Ir during zone melting of Mo is investigated. The microdistribution is principally the same as that found for W in Mo, although the equilibrium-distribution-coefficient k₀ is now smaller than 1. A mean effective distribution coefficient k_eff describes the macrodistribution. Its dependence on the fluctuations is qualitatively quite different from that of W in Mo with k₀ > 1. With the occurence of fluctuations the value of k _eff is changed continually in the direction of one. Its dependence on the zone-melting rate is found in the region of fluctuations too.     

Synthesis of bamboo‐like SiC whiskers from waste silica fume

A process for the utilization of wasted silica fume is proposed in this work. Silicon carbide (SiC) whiskers several tens of micrometers in length and with a bamboo‐like morphology have been successfully synthesized by a carbothermal reduction process using purified silica fume as the silicon source. The morphology and structure of SiC whiskers were investigated by X‐ray diffraction, Raman spectroscopy, scanning electron microscopy, and high‐resolution transmission electron microscopy. Studies found that the as‐synthesized whiskers were grown as single‐crystalline β‐SiC along the (111) growth direction. The whiskers consisted of hexagonal stems randomly decorated with larger‐diameter knots along their whole length. On the basis of the characterization results, a vapor–solid process was discussed as a possible growth mechanism of the β‐SiC whiskers.     

Über das Wachstum von Silberamalgamwhiskern

The growth of whiskers from silver amalgam is investigated; The whiskers grew in aqueous solution on mercury surface by means of one kind of internal electrolysis. They possess the shape of hexagonal capillary tubes filled with mercury. A potentiostatic method has been developed for measuring supersaturation, resp. overvoltage during the whisker growth. The same method permitted the experimentator lightly to rule the whisker growth process.     

Effect of alcohols additives on hydrothermal formation of magnesium hydroxide sulfate hydrate whiskers

Two different organic alcohols, ethylene glycol and 1, 2‐propanediol, were used in hydrothermal formation of MgSO₄·5Mg(OH)₂·2H₂O (MHSH) whiskers. Compared with the MHSH whiskers without alcohols, the crystallinity and defect of products were improved. It could also be observed that not only aspect ratio but also crystallinity and purity of the products were improved when 1, 2‐propanediol was used. A reasonable model was proposed to explain the growth process derived from the organic alcohol. In addition, the room temperature photoluminescence (PL) indicated that the spectrum intensity of MHSH whiskers used 1, 2‐propanediol was also significantly improved.     

Preparation of copper aluminum borate whiskers via flux method

Single‐crystal and uniform copper aluminum borate whiskers have been synthesized by heating a mixture of boric acid, copper sulfate and aluminum sulfate with potassium sulfate as flux at 870 °C for 4 h. The synthesized whiskers exhibit a well‐crystallized, one‐dimensional structure with diameters ranging from 100 nm to 5 μm, lengths from 5 to 100 μm. Heating temperature and flux addition affect the aspect ratio and morphology of the copper aluminum borate whiskers. A possible growth mechanism of the whiskers is proposed. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Etch pit morphology and calcium sulphate growth on {111} faces of calcium fluoride crystals etched in sulphuric acid

An investigation of the etching behaviour of cleaved octahedral faces of calcium fluoride crystals in sulphuric acid of different concentrations and at various temperatures is carried out. It is observed that the morphology of dislocation etch pits depends on the temperature and concentration of the acid. With an increase in the acid concentration, growth of whiskers, elongated platelets, hexagonal and dice-shaped elevations, spherulites and sheaves is observed. Temperature of the etchant also has an effect on their formation. By X-ray diffraction techniques, the compounds forming the whiskers and elongated platelets, and hexagonal and dice-shaped features are identified as CaSO₄ · 2 H₂O and CaSO₄, respectively. The mechanism of the growth of CaSO₄ · 2 H₂O and CaSO₄ and the change in the morphology of selective etch pit are discussed.     

Mechanism of growth of Bi2+xSr2?yCuO6+δ single crystals in gas cavities in a KCl melt

High-quality Bi_2+x Sr_2−y CuO_6+δ single crystals in a wide range of superconducting properties, from optimally doped to strongly underdoped (including insulators), have been obtained by free growth in gas cavities formed in a KCl flux. A model of crystal growth is proposed, in which the decisive parameter is the chemical transport in a cavity at a low partial oxygen pressure and feeding of the gaseous medium from the charge heated to a higher temperature. In this case, layer-by-layer growth through the vapor-solid mechanism is implemented. This growth, as the most ordered process, makes it possible to obtain faceted plates and whiskers with specular faces, without segregation of other phases. Original Russian Text ? Yu.I. Gorina, G.A. Kalyuzhnaya, V.V. Rodin, N.N. Sentyurina, V.A. Stepanov, S.G. Chernook, 2007, published in Kristallografiya, 2007, Vol. 52, No. 4, pp. 762–766.     

Growth of hollow Se–Te whisker crystals

On the basis of micro- and macro-morphological studies the mechanism responsible for the growth of Se–Te whiskers has been suggested. It has been shown that Sc–Te whiskers grow by layer growth mechanism in which screw dislocation is not the source of step but the growth proceeds by two-dimensional nucleation. This has been supported by the supersaturation data. The plausible mechanism for the growth of hollow whiskers has also been suggested.     

Fundamental aspects of VLS growth

The kinetics and the mechanism of the vapor-liquid-solid (VLS) growth are discussed. Emphasis is placed on the dependence of the growth rate on the whisker diameter. It is found that the rate decreases abruptly for submicron diameters and vanishes at some critical diameter d_c ? 0,1 μm according to the Gibbs-Thomson effect. A new method for simultaneous determination of kinetic coefficients and of supersaturations has been developed. The method can be used to measure the coefficients of some materials as well as the temperature dependence of the coefficient for silicon and the activation energy of the process. From the dependence of supersaturation on the diameter we conclude that whiskers grow by a poly-nuclear mechanism. The periodic instability of the diameter is discussed and the rate-determining step is analysed. We conclude that phenomena on the liquid-solid interface are the decisive ones. In determining the role of liquid phase in vapor growth we measured the “liquid phase effectivity coefficient” as a function of crystallization condition; the coefficient typically was about 10²?10³. It is stressed that the liquid phase reduces the activation energy both on vapor-liquid interfaces (for chemical reactions) and on liquid-solid interfaces (for nucleation). The liquid phase ensures growth rates as high as 1 cm/sec, provided there are no barriers between the interfaces. The growth mechanism of the side faces was studied, and it was observed that the faces grow mainly by a chain mechanism rather than by two-dimensional nucleation. In work on surface diffusion in the VLS whiskers growth by CVD, we found that the whiskers grow mainly by direct deposition rather than by diffusion on the side faces. It is concluded that the VLS mechanism is important also for the vapor growth of platelets, films, and bulk crystals.     

Crystallisation and performance characteristics of high‐temperature annealed electroless Ni‐W‐P coatings

Electroless deposition of Ni–P based alloys is a well‐known commercial process that has numerous applications because of their excellent anticorrosive and wear properties. However, for some special occasions, like the components for gas making furnaces in chemical fertiliser industry, the coatings must be reinforced to withstand short‐term high temperatures between 600 °C and 700 °C as well as light erosive wear. Therefore, co‐deposition of high melting point metallic element, W, has been considered as a preferred choice. In the present study, two Ni–W–P alloy coatings were deposited on mild steel panels from different alkaline baths. The microstructures of the annealed coatings were characterised by quantitative XRD, XPS and SEM/EDS analysis techniques and their microhardness, friction and wear behaviour, corrosion mechanism as well as microstrain and residual stress are discussed in comparison with the as‐plated state. The results indicate that the hardness mainly depends on the volume fraction and crystallite size of Ni₃P phase; the uniform corrosion in sulfuric acid solution is closely related to the ratio of I_Ni/I_Ni3P as well as grain size. The wear mechanism of the high‐temperature annealed coating is dominated by abrasive wear, but the wear in the early stages started from mild adhesive wear caused by adhesion between the friction couples. Electroless deposited Ni‐W‐P alloys with high phosphorus present relatively good properties, including hardness, wear and corrosion resistance when 700 °C is applied for annealing process.     

Wachstumsformen von tetragonalen Erdalkaliwolframaten in Schmelzlösungen und deren Deutung

Investigations on solubility of CaWO₄ in NaCl melts (ANIKIN ) were extended to BaWO₄ and SrWO₄. – Crystals of tungstates of Ca, Ba, and Sr from NaCl and KCl fluxes respectively were grown at temperatures from 950 to 650°C at cooling rates from 2 to 50 grd. h⁻¹. Marked differences in the forms of these tungstates did not occur. Among many whiskers bipyramidal crystals grew with dendritic, sceletal and hollow transitions forms. The theoretically calculated morphological succession (PBC model) is realised by the faces of the whiskers and platelets ([110] and [100] zones). The remaining crystals – due to face specific adsorption – show other forms similar to those in nature. – Verneuil grown crystals, however, developed – free, from adsorption phenomena – {110} and {001} forms respectively.