Growth of hollow nickel fluoride whiskers

Hollow nickel fluoride whiskers have been obtained by condensation from the vapor phase onto a platinum substrate in a flow of hydrogen fluoride. Crystals up to 5 mm in length have a square cross section with a 300 ± 30-μm side. The wall thickness is 85 ± 20 μm.     

Growth and removing of tungsten whiskers under controlled conditions

The growth kinetic and size decreasing process of tungsten rod-like crystals by heating in the iodine vapour are studied. Effects of tungsten filament temperature (2400 – 3500 K), iodine concentration (to 6 mg/cm³) and water contents (0.01 – 0.05 mg/cm³) are shown. Maximal growth rate of tungsten whiskers to 30 μm/min. Stopping of the growth and following removing by iodine reactions take place after heating to 3500 K.     

Growth rate and morphology of silicon carbide whiskers from polycarbosilane

The growth rate of silicon carbide whiskers grown from polycarbosilane was measured and the growth morphology was investigated. The plot of whisker length vs. growth time was almost linear up to a certain growth time but tended to become time-saturated, independent of growth temperature. Cessation of whisker growth was caused by a change of the growth process from vapor–liquid–solid (VLS) to vapor–solid (VS). Decrease in whisker length with growth time was observed in the higher temperature range. Arrhenius plots of growth rate were almost linear in the lower temperature range, but deviated markedly from linearity in the higher temperature range. This deviation was caused by the coexistence of the VS process and the VLS process during whisker growth.     

Growth and structure of superconducting FeSe crystals

Superconducting crystals of iron selenide FeSe are grown and their phase and elemental compositions are studied. The structural analysis of individual FeSe_{1 ? x} crystals showed the coexistence of two phases in this compound. The resistivities and magnetic susceptibilities of the crystals are investigated.     

Growth mechanism of copper whiskers by the hydrogen reduction of cuprous iodide

A small quantity of cuprous iodide (less than 30 mg) was reduced by hydrogen at 650 or 700°C for various periods of time (15 sec to 4 min) and quenched to interrupt whisker growth. This was done in order to examine the nature of the growth of copper whiskers. Scanning electron microscopic observation and X-ray microanalysis revealed that almost all the whiskers quenched after the growth for 30 sec or less had cuprous iodide droplets at their tips and also the lateral surfaces were covered by thin cuprous iodide layers. The manner in which the copper crystals grow is similar to growth by the droplet mechanism or by the VLS mechanism in that the nutrient is supplied by a liquid drop on the tip of the whisker. Whiskers thicken during and after the growth in length. The whiskers quenched after the growth for 1 min or more had well-developed lateral surfaces and did not have the droplets at the tips. Preferential condensation and reduction were found to occur at the whisker edges. Based on the observed facts, a growth mechanism is presented. Secondary whisker growth on the whisker tips was also investigated.     

Growth of tungsten whiskers oriented in <111> direction

The electron micro diffraction technique and pseudopotential ab initio calculations were implemented to study in details the formation and mechanism of growth of the tungsten whiskers during the reduction of nickel tungstate by CO gas. It has been shown that the W whiskers prefer to grow as crystals oriented in the <111> direction, and this process may be considered as the epitaxial growth on the hexagonal planes of Ni₄W particles. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of low-temperature annealings on the structural and superconducting properties of Bi 2212 and Bi 2201 whiskers

The influence of post-growth short-term low-temperature annealings (O₂, 400°C, 5–15 min) on the composition, crystal structure, and superconductivity of Bi₂Sr₂CaCu₂O_{8 + δ} and Bi₂Sr₂CuO_{6 + δ} oxycuprate whiskers freely grown in gas cavities has been investigated. The optimal conditions for growth in closed gas cavities in a flux and post-growth annealing in oxygen were found, making it possible to obtain high-quality superconducting Bi 2212 and Bi 2201 whiskers in a wide doping range (from heavily underdoped to optimally doped) with a small rocking curve half-width (～0.1^°–0.2^°) and narrow superconducting transition (ΔT = 1.5–2 K).     

Microhardness studies of GaTe whiskers

Single crystal whiskers of gallium telluride (GaTe) have been grown by the physical vapour deposition (PVD) method. Microindentation studies were carried out on the prism faces of the needles to understand their mechanical behaviour. The variation in the microhardnessof GaTe crystals with applied load has been determined at room temperature using Vickers microhardness indenter. The work‐ hardening exponent has also been computed for different load regions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sodium acetate tri-hydrate whiskers

A new kind of whisker, which has not been previously reported in the literature, was grown on membranes which were in contact with a saturated aqueous solution of sodium acetate, using several techniques. Cellulose acetate membranes were used in all the cases and growth was successful at 25 ± 2°C and 45–55% relative humidity. A dense colony of whiskers, sometimes stuck together in bundles, usually up to 5 cm in length and ranging in width from 0.3–100 μm, developed within 72 h of growth. The whiskers were identified as sodium acetate tri-hydrate by means of Debye-Scherrer X-ray powder method. Laue transmission photographs indicated the existence of twin crystals within the individual single crystal whiskers as proved by electron diffraction in TEM. Some whiskers included internal and external channels, and there were several secondary effects, such as thickening in the middle of a whisker, formation of arrow-heads or spirals at the tips and various modes of branching. The whisker morphology and the various growth phenomena may be explained as a whisker growth process from their bases, when we consider a continuous crystallization from material which is supplied through and over the membrane.     

Study of imperfections in iron whiskers

Iron whiskers grown by hydrogen reduction of ferrous chloride were studied by several X-ray and optical techniques, particularly by X-ray diffraction topography. 36 whiskers with [100], [110] and [111] axis directions, from 20 μm to 260 μm in thickness were investigated. They were of different degree of perfection with dislocations nonuniformly distributed. Three [111] whiskers showed a lattice twist around the axis. Burgers vectors of several frequent types of dislocations were found.     

Preparation and characterisation of fluorapatite whiskers

Fluorapatite (FAp) whiskers were prepared by using Molten Salt Synthesis (MSS) technique. β‐tricalcium phosphate (TCP) and various fluorine salts, along with potassium sulphate as a flux salt were used as starting materials for preparing FAp whiskers. Effects of flux to powder weight ratio, temperature and soaking time on the morphology were studied. The optimum temperature to obtain FAp whiskers was found to be 1175°C. The phase purity of the prepared FAp whiskers were confirmed by powder X‐ray diffraction and FTIR spectra, and the whiskers were further characterized by laser Raman spectra, chemical analysis and DTA/TGA thermal analysis. The morphology of the FAp whiskers were investigated by scanning electron microscope (SEM) analysis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of strontium chloroborate whiskers

Single crystalline strontium chloroborate (Sr₂B₅O₉Cl) whiskers with uniform diameter have been synthesized by a facile route based on the calcination of precursor. The precursor was prepared by the sedimentation reaction between SrCl₂ and Na₂B₄O₇ aqueous solution. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectrum (FT‐IR). An optimal synthesis temperature for preparing Sr₂B₅O₉Cl whiskers was obtained, and the possible formation process was also presented.     

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.     

Investigation of the formation of nanowires from silicon whiskers

Nanowires have been prepared by the high-temperature oxidation of Si whiskers. The dependences of the nanowire formation on the oxidation parameters have been investigated. The oxidation rate is shown to depend on the whisker diameter. Oxidation in dry oxygen at temperatures no higher than 950°C results in self-stopping; i.e., the nanowire diameter is stabilized. Stabilization is not observed at oxidation temperatures above 950°C or at oxidation in wet oxygen. Oxidation at higher temperatures made it possible to obtain nanowires ≤5 nm in diameter in relatively thick (up to 200 nm in diameter) whiskers.     

Oriented growth of whiskers of AIIIBV compounds by VLS-mechanism

The oriented growth of GaAs, GaP, InAs and GaInAs whiskers on the same (GaAs, GaP) or different (InAs/GaAs, GaInAs/GaAs) substrates was studied. A detailed morphological study of GaAs whiskers on polar A(III), B(111 ) and non-polar (001), (011) substrates was performed. The growth conditions for ordered (perpendicular to substrate) growth on the A(111) and B(111 ) faces were determined. There were found discrete spectra of whisker systems on all substrates with the preferential growth of “arsenic” B{111 } faces. The dependence of the growth rate on the whisker diameter is typical for the vapour-liquid-solid (VLS) mechanism and is used for the determination of kinetic coefficients for polar faces. There was observed a periodic instability in growth of InAs and GaInAs whiskers.     

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)     

Synthesis of nonstoichiometric zirconium carbide whiskers by chemical vapor deposition

Nonstoichiometric zirconium carbide crystals with various compositions were prepared by chemical vapor deposition. Two gaseous mixtures, zirconium tetrachloride and argon, toluene and hydrogen, were introduced to the reaction zone where a graphite substrate was heated between 1200 and 1400°C. The deposition rate was proportional to the partial pressure of toluene. The compositional ratio of n_C/n_Zr in the gaseous mixture from 2.0 to 6.0 was found to be optimum for producing needle-like crystals. Needle-like crystal with smaller size were formed when the ratio of n_C/n_Zr was smaller than 2.0, and less needle-like crystals accompanied with more carbon were also produced when the ratio of n_C/n_Zr was larger than 6.0. The temperature of the substrate suitable for the growth of needle crystals was in the range from 1250 to 1300°C. The lattice constants of the products varied as a function of the ratio of n_C/n_Zr in the gaseous mixtures.     

A study of the morphology and magnetic properties of silicon whiskers

Methods of growing Si whiskers are considered and the results of studying their magnetic susceptibility χ are reported. Silicon whiskers are grown using the method of chemical transport reactions in a closed bromide system. The crystals studied were divided into four groups: (i) crystals 0.1–0.9 μm in diameter, (ii) twinned crystals 1–2 μm in diameter, (iii) needle-like crystals from 5 to 1500 μm in diameter, and (iv) isometric crystals (the latter were not measured). With an increase in diameter, the whiskers of the first group changed their state from paramagnetic to diamagnetic, while the crystals of the third group showed a transition from a diamagnetic to a paramagnetic state. All these crystals show a nonlinear dependence of χ on the strength of an external magnetic field H; the nonlinearity increases with increasing the sample paramagnetism. The susceptibility of twinned samples is diamagnetic and close to the value of χ for bulk silicon, and the nonlinearity of the dependence χ (H) is insignificant. The measured values of χ of Si whiskers are explained by features of their crystal structure and chemical composition.     

Vapor phase growth of alumina whiskers by hydrolysis of aluminum fluoride

The vapor phase growth of alumina whiskers by the hydrolysis of aluminium flouride has been studied at an elevated temperature. The product was obtained in various forms: wool-like whiskers, needle-like whiskers, jointed needle-like crystals which consisted of linked small single crystals, tightly coagulated powder or as thin film covering the substrate. Aluminium oxi-fluoride was suggested as the intermediate gaseous species in the formation of alumina. The relation between the growth mechanism and crystal morphologies was studied.