Vol 52, No 3 (2016)

Experimental data are presented on the taper of silicon whiskers grown in an open chloride system. The whisker taper is shown to depend on the crystal growth rate, whisker radius and length, and process parameters. The experimental data are analyzed under the assumption that the size of the liquid droplet on the whisker tip decreases because of the heterogeneous chemical reaction responsible for the etching of the metallic component of the alloy. The influence of the crystal radius and vapor phase composition on the flux that leads to a decrease in the size of the liquid droplet. A model is proposed for whisker tapering as a result of the heterogeneous chemical reaction responsible for the etching of the metal on the surface of the liquid phase, and conditions for the growth of zero-taper whiskers are discussed.

Using scanning electron microscopy, we have studied how the conditions of preparation of granules of Sb₂Te₃–Bi₂Te₃ and Bi₂Te₃–Bi₂Se₃ solid solutions through melt solidification in a liquid influence their morphology, fractographs of fracture surfaces of samples prepared by hot-pressing the granules, and the contents of the major components in the samples. The granules are rounded (solidification in water and liquid nitrogen) or platelike (solidification in water under an excess pressure and in liquid-nitrogen-cooled ethanol) in shape. Fracture surfaces of hot-pressed samples prepared from granules comminuted in a ball mill have a uniform, fine microstructure, with faceted grains several microns in size. Characteristically, samples prepared from granules comminuted in a cutting mill have transgranular layered fractures, with layers up to hundreds of microns in thickness. The mechanical properties of the samples (ultimate strength and relative elongation) have been studied using compression tests at temperatures of 300 and 620 K. The samples experience brittle fracture. Their compression strength σ_c is 55 ± 12 MPa. With increasing temperature, σ_c varies only slightly, but at 620 K the samples become more plastic and their relative elongation ε_b increases by a factor of 2–4. The ultimate strength of hot-pressed samples prepared by uniaxial compression is 20% higher than that of samples prepared by biaxial compression.

We have studied the gamma sensitivity of single-crystal CVD diamond neutron detectors using a ²⁵²Cf neutron source placed in a moderator. It has been shown that a major contribution to the count rate of the detectors is made by the gamma rays from the source. We have compared the count rates of a detector with a ¹⁰B boron isotope-based slow-neutron converter and without it. With allowance for the theoretically calculated detection efficiency, the difference between the count rates is consistent with the fraction of slow neutrons measured using a scintillation detector.

We have analyzed interaction between reactants and substrate materials during the reduction of tantalum bromide with cadmium in the course of tantalum deposition on Cu, Fe, Mo, and Ni metallic substrates and the 25Kh3M3NBTsA, Ni₃Al, VKNA, and ZhS32 alloys. The results indicate that molybdenum and nickel substrates are the most stable to the reactants used in this study; copper and iron experience etching; and aluminum, chromium, and titanium are capable of competing with cadmium as a reducing agent. The presence of carbon in the substrate material leads to the formation of tantalum carbide in the metal–coating interfacial layer. In the absence of aluminum, oxygen impurities are likely to be removed as oxybromides. Optimal materials for the fabrication of reaction chambers for tantalum deposition are nickel-coated high-temperature steels.

Inductively coupled plasma mass spectrometry with the use of a reaction cell in collision mode has been used to determine the concentrations of 68 elemental impurities in six commercially available molybdenum( VI) oxide reagents. The results indicate that the purity of the reagents is not better than 3N (99.9 wt %). A major impurity in the MoO₃ samples is tungsten. Its concentration ranges from 5 × 10^–3 to 6 × 10^–2 wt %, depending on the manufacturer.

Codoping with iron, cerium, and copper oxides has been proposed as a means of improving the performance of scandium-containing zirconia-based solid electrolytes. We have examined three procedures for the synthesis of the (ZrO₂)_0.825(CeO₂)_0.07(Sc₂O₃)_0.07(Fe₂O₃)_0.035 solid solution through precipitation from solution. It has been shown that the highest oxygen ion conductivity is ensured by the synthesis procedure that includes the mechanochemical activation of presynthesized scandium ferrate (Sc_1.33Fe_0.67O₃) with cerium and zirconium hydroxides and a solution containing 0.5 mol % Cu.

A new technique in which the entire set of impurities in a substance is represented as a set of impurity classes has been developed for estimating the most likely and total concentrations of elemental impurities in samples of high-purity substances. Estimates of the purity of samples with the use of this technique have lower random and systematic errors in comparison with estimates without impurity classification. It has been shown that estimates of the integral characteristics of individual impurity classes in the total set of samples allow one to find the expectation of the total concentration for impurity classes which are represented in particular samples only by detection limits. The results are illustrated by the example of the rare-earth oxide samples in the Exhibition–Collection of Extrapure Substances.

Intermediate and final products of the synthesis of Cu–Al cermets have been studied by a differential dissolution (DD) stoichiographic method. Their qualitative and quantitative phase compositions have been determined. The phase composition of the intermediate products has been shown to differ from that of the final products. It has been shown that the first stage of the synthesis is the formation of a solid solution and intermetallic phase. In the final stage, the latter decomposes to give a solid solution, copper, and aluminum. The degree of decomposition depends on the mechanical activation time of the constituent metals: milling for 12 min leads to complete decomposition of the intermetallic phase. We also observe the oxidation of the forming aluminum and copper to their oxides and partial interaction between them, resulting in the formation of solid solutions of aluminum oxide in copper oxide. A comparative analysis of DD data with results obtained by physical methods— X-ray diffraction (XRD) and nuclear magnetic resonance (NMR)—demonstrates that they do not always agree. The reason for such discrepancies is that the three methods have different capabilities. We thus conclude that the use of both DD and XRD is crucial for obtaining reliable results.