The laws of silicon ingot formation by combustion reaction

The formation of a phase pure silicon ingot from SiO₂+Al+KClO₃ and Na₂SiO₃+Al+KClO₃ systems was investigated thermodynamically and experimentally under combustion mode, known also as self-propagating high-temperature synthesis (SHS). The regularities of combustion and phase formation versus KClO₃ concentration by a thermocouple technique were obtained. The morphology, chemical and phase composition of the silicon ingot were analyzed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and ICP-analysis. The method reported here proved effective in producing silicon ingots with a purity of 98%.     

Structural change of hydrothermal BaTiO3 powder

Formation rate of tetragonal BaTiO₃ powder by hydrothermal synthesis and its dielectric property were studied. Submicron tetragonal BaTiO₃ powders were prepared hydrothermally, using Ba(OH)₂ · 8H₂O, TiO₂ (anatase) and KOH as starting chemicals. Characterization via X-ray diffractometry, Field Emission Scanning Electron Microscopy confirmed that increasing calcination temperature (from 1100 to 1300°C) promotes the formation of tetragonal BaTiO₃. Tetragonal BaTiO₃ ceramics obtained from optimum condition (Tetragonal BaTiO₃ powders calcined at 1200°C for 3 h after hydrothermal synthesized at 200°C for 168 h) exhibited submicron size of 0.5–0.7 m, monodispersed type and high relative permittivity.     

Densification of porous materials in rigid conical and cylindrical dies

Conclusions A solution obtained by the FEM is proposed to the problem on the nonsteady-state extrusion of a cylindrical porous blank through a conical die and on the extrustion of a porous and a bimetallic blank of varying cross sections through a conical die. Zones of large and small strains are determined from the distribution of porosity. An analysis is made by the FEM of the stressed-strained state in the double-ended densification of porous blanks in rigid cylindrical dies. Cases of the densification of porous bimetallic blanks are also considered.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 22–27, May, 1982.     

Refractory metal nanopowders: Synthesis and characterization

Refractory nanometals have been the subject of interest for the past two decades in order to manufacture compact materials with dramatically improved mechanical properties for aerospace, military, chemical and metallurgical applications. The interest in nanomaterials has led to the development of many synthetic methods for their fabrication. In this review the challenges, synthesis methods and the characteristics of refractory metal nanopowders of IV–VI sub-group of the periodic table are discussed. Special attention is paid to salt-assisted combustion reaction (SACR) as a promising technique for the large-scale production of refractory metal nanopowders.The current paper will: (1) focus on the synthesis methods, morphology and physical–chemical characteristics of metal nanopowders; (2) present the specific feature of the salt-assisted combustion reaction, combustion parameters and macro-kinetic aspects of chemical reactions in the powder bed; and (3) demonstrate the perceptiveness of the fabrication route for the mass production of nanosized powders.     

Combustion synthesis of porous titanium microspheres

The synthesis of titanium porous microspheres by a combustion technique was studied under an argon atmosphere by using a TiO₂ − 2.5Mg reactive mixture. The precursor, a fine TiO₂ powder, was thermally treated in the range 600–1300 °C prior to the combustion experiments. TiO₂ microspheres whose diameters were between 10 and 50 μm were obtained from precursor particles annealed in the range 900–1100 °C. A biphase product consisting of Ti and MgO phases was obtained when the TiO₂ microspheres were reduced with Mg. The spherical morphology of the final particles was retained despite the relatively high combustion temperatures (1630–1670 °C) used in this study. Moreover, porous titanium microspheres were obtained when the MgO particles were dissolved using acid leaching. Scanning electron microscopy (SEM) images of the microspheres suggested that the spherical structure contained ∼0.5–2.0-μm-diameter porous windows. The Brunauer–Emmett–Teller (BET) surface area of the Ti microspheres was determined to be 2.8 m² g⁻¹.     

One-Dimensional Nanostructures of Polypyrrole for Shielding of Electromagnetic Interference in the Microwave Region

Polypyrrole one-dimensional nanostructures (nanotubes, nanobelts and nanofibers) were prepared using three various dyes (Methyl Orange, Methylene Blue and Eriochrome Black T). Their high electrical conductivity (from 17.1 to 60.9 S cm⁻¹), good thermal stability (in the range from 25 to 150 °C) and resistivity against ageing (half-time of electrical conductivity around 80 days and better) were used in preparation of lightweight and flexible composites with silicone for electromagnetic interference shielding in the C-band region (5.85–8.2 GHz). The nanostructures’ morphology and chemical structure were characterized by scanning electron microscopy, Brunauer–Emmett–Teller specific surface measurement and attenuated total reflection Fourier-transform infrared spectroscopy. DC electrical conductivity was measured using the Van der Pauw method. Complex permittivity and AC electrical conductivity of respective silicone composites were calculated from the measured scattering parameters. The relationships between structure, electrical properties and shielding efficiency were studied. It was found that 2 mm-thick silicone composites of polypyrrole nanotubes and nanobelts shield almost 80% of incident radiation in the C-band at very low loading of conductive filler in the silicone (5% w/w). Resulting lightweight and flexible polypyrrole composites exhibit promising properties for shielding of electromagnetic interference in sensitive biological and electronic systems.     

Exothermic reaction waves in ZrSiO4/Mg/diluent systems

An experimental study of thermal behavior in exothermic wave propagation in ZrSiO₄ + 4Mg + kSH systems (where SH is a sodium halide) was conducted using a thermocouple technique. Temperature–time profile investigations provide deeper insight into the combustion wave propagation mechanism and reveal the values of combustion parameters: ignition and combustion temperatures, wave propagation velocity, and the actual size of pre-flame and reaction zones. It was shown that the measured maximum temperature was reduced from 1900 °C for the undiluted sample to 1490 °C for samples containing 1.5 mol of sodium halide. As a consequence, pure-phase zirconium monosilicide (ZrSi) was obtained under theses temperature conditions. Scanning electron microscope (SEM) examination revealed that ZrSi particles were spherical in shape and uniform in size when NaCl was used as an inert diluent. The obtained ZrSi particles had mean diameters ranging from 0.4 to 2.5 μm based on NaCl concentration. The chemical mechanism of ZrSi formation is discussed, and a simple model for nucleation and grain growth processes is proposed.     

Thermomechanical effect in a cylindrically-hybrid nematic liquid crystal

Thermomechanical rotation of a cylindrically-hybrid nematic liquid crystal in the presence of a longitudinal temperature gradient is predicted theoretically and detected experimentally. Pis’ma Zh. Tekh. Fiz. 25, 71–73 (March 26, 1999)