Vol 53, No 8 (2017)

An approach has been proposed for producing “negative” whiskers in Si wafers which is based on the thermal migration of silicon–metal (Si + M) melt droplets in the field of a transverse temperature gradient. We have obtained regular systems of negative whiskers in the form of through holes in Si {111} wafers. It has been shown that the thermal migration of Si + M melt droplets is driven by the difference between the chemical potentials of Si in the liquid and crystalline phases, which arises from the difference in curvature between the liquid/vapor and solid/liquid interfaces and leads to Si dissolution and desorption.

The Ag₂Se–GeSe₂–SnSe₂ system has been studied in the temperature range 290–430 K using differential thermal analysis, X-ray diffraction, and emf measurements concerning silver concentration cells with Ag₄RbI₅ as a solid electrolyte. We have constructed a number of vertical sections; the 300-, 800-, and 900-K isothermal sections of the phase diagram; and a projections of the liquidus surface. It has been shown that the only pseudobinary join, Ag₈GeSe₆–Ag₈SnSe₆, contains a continuous series of solid solutions between the two crystalline phases of the constituent selenides and divides the system into two independent subsystems. Ag₂Se–Ag₈GeSe₆–Ag₈SnSe₆ is a univariant, eutectic subsystem, and GeSe₂–Ag₈GeSe₆–Ag₈SnSe₆–SnSe₂ is an invariant eutectic subsystem. Experimental emf data were used to evaluate partial thermodynamic functions of the silver in the high-temperature and low-temperature Ag₈Ge_1–xSn_xSe₆ solid solutions.

We have studied reaction between ZrCl₄ and NaBH₄ at temperatures between 300 and 725°C. The results demonstrate that single-phase zirconium diboride nanoparticles are formed starting at 575°C. According to electron microscopy data, the ZrB₂ powder obtained at 575 and 725°C consists of variously shaped particles, some of which are almost spherical, ranging in diameter from ~10 to 20 and from 25 to 35 nm, respectively. These values agree with the equivalent particle diameters evaluated from the measured specific surface area of ZrB₂, ~14 and ~32 nm, respectively, and with the crystallite size extracted from X-ray diffraction data: D_hkl ~ 13 and 28 nm.

Sodium and potassium uranates with the compositions M₂[(UO₂)₆O₄(OH)₆] · 8Н₂ (M = Na, K) have been synthesized by reacting schoepite (UO₃ · 2.25H₂O) with aqueous sodium hydroxide and potassium hydroxide solutions (pH 10–12) under hydrothermal conditions at 100°C for 15 days. Chemical analysis, X-ray diffraction, IR spectroscopy, and thermogravimetry data demonstrate that these compounds are perfect compositional and crystallographic analogs.

Reactions of manganese oxides of various compositions with sulfuric acid solutions have been studied by kinetic and electrochemical methods. The surface composition of the manganese oxides has been shown to vary during the dissolution process. We present mathematical dependences of the dissolution rate on kinetic parameters of the electrolyte solution for the oxides of various compositions and interpret the behavior of the manganese oxide with various compositions during dissolution in sulfuric acid solutions of various concentrations.

We have measured photoluminescence (PL) and broadband reflection spectra of the surface of an opal photonic crystal (OPC) filled with terbium nitrate hexahydrate (TNH). The excitation sources used were a halogen lamp, lasers with emission wavelengths of 266 and 337 nm, and semiconductor light-emitting diodes (369 and 385 nm). It has been shown that resonance excitation of the TNH (Tb(NO₃)₃ · 6H₂O) filled OPC by pulsed laser light at a wavelength of 266 nm gives rise to superluminescence: an increase in the relative intensity of the 545-nm PL band, corresponding to the Tb³⁺⁵D₄–⁷F₅ transition. The efficiency of pumping the upper laser level of the OPC increases due to the increase in the density of photon states (Purcell effect) near the bandgap edge in the OPC. The lack of clear superluminescence under excitation by the other excitation sources is related to specific features of the absorption of incident light by the rare-earth ion and to the low output power of the cw light sources.

A calcium phosphate powder has been synthesized from aqueous 0.25, 0.5, and 1.0 M calcium lactate and ammonium hydrogen phosphate solutions atat a Ca/P = 1, without pH adjusting. According to X-ray diffraction data, the as-synthesized powder consisted of brushite (CaHPO₄ · 2H₂O) and octacalcium phosphate (Ca₈(HPO₄)₂(PO₄)₄ · 5H₂O). After heat treatment in the range 500–700°C, the powders were gray in color because of the destruction of the reaction by-product. The powders heat-treated in the range 500–700°C consisted largely of γ-Ca₂P₂O₇. The ceramics prepared from the synthesized powders by firing at 1100°C consisted of β-Ca₂P₂O₇ and β-Ca₃(PO₄)₂.

Using high-temperature X-ray diffraction, differential scanning calorimetry, and electron microscopy, we have studied the formation of yttrium aluminates and Nd:YAG (YAG) activated garnet nanoparticles during the thermal decomposition of a poorly crystallized carbonate precursor prepared in the NH₄Al(OH)₂CO₃–(Y,Nd)(ОН)CO₃ nanosystem and the development of the morphological structure of powders during heating to a temperature of 1350°C. The results demonstrate that heat treatment in the temperature range 850–950°C leads to the formation of metastable nonstoichiometric YAlO₃ with a garnet-like structure, which reacts with Al₂O₃ at a temperature of 1000°C to form YAG. The cubic cell parameter a and X-ray density of YAG crystals with the composition Y_2.97Nd_0.03Al₅O₁₂ synthesized at 1200°C are 1.2009 nm and 4.565 g/cm³, respectively, and the average particle size is 108 nm. Using carbonate route, we prepared transparent Nd:YAG ceramics with a relative density of 99.7%, X-ray density of 4.562 g/cm³, and crystallite size in the range 1–7 μm.