Inorganic Materials (v.54, #6)

Self-Propagating High-Temperature Synthesis of Titanium Carbide Powder under Pressure–Shear Conditions by A. M. Stolin; P. M. Bazhin; M. I. Alymov; M. V. Mikheev (521-527).
This paper presents results of an experimental study of the self-propagating high temperature synthesis (SHS) of titanium carbide powder under pressure in combination with high-temperature shear deformation. We have studied the effect of technologically important parameters, primarily that of the pressure and strain rate, on the synthesis process under pressure–shear conditions. The results demonstrate that a shear load applied to an incompletely formed material during synthesis may have a significant effect on the grain size and morphology, as well as on the mutual arrangement of structural constituents throughout the reaction system. Running the technological SHS process under pressure in combination with shear allows for the synthesis of refractory metals and comminution of the reaction products before cooling to room temperature in a single processing cycle.
Keywords: self-propagating high temperature synthesis; shear deformation; titanium carbide; grinding; powder

Optimization of the Carbosilicothermic Synthesis of the Ti4SiC3 MAX Phase by E. I. Istomina; P. V. Istomin; A. V. Nadutkin; V. E. Grass; A. S. Bogdanova (528-536).
We have studied the formation of the Ti4SiC3 MAX phase during the vacuum carbosilicothermic reduction of TiO2 with a combined reducing agent consisting of SiC and elemental Si and analyzed the effects of the synthesis temperature, heat treatment time, and percentage of elemental silicon in the starting mixture on the Ti4SiC3 yield. Optimal Ti4SiC3 synthesis conditions are as follows: temperature from 1550 to 1650°C, isothermal holding time of 360 min, and the starting-mixture composition TiO2 + 1.2SiC + 0.6Si. The Ti4SiC3 yield then reaches 92 wt %.
Keywords: MAX phase; Ti4SiC3 ; Ti3SiC2 ; titanium dioxide; TiC; carbothermic reduction; silicothermic reduction

Local Structure of Fe–Cr Powders Prepared by Mechanical Alloying by Yu. V. Baldokhin; V. V. Cherdyntsev (537-545).
We have studied the structure of Fe–Cr alloys prepared by mechanical alloying. According to X-ray diffraction data, all of the alloys are single-phase and consist of a BCC solid solution. The local structure of the alloys has been studied by Mössbauer spectroscopy. Analysis of the local environment of the Fe atoms leads us to assume clustering processes in the alloy containing 20 at % Cr and suggests that the alloys containing 35 and 50 at % Cr experience phase separation into regions of a homogeneous disordered chromium- enriched solid solution and an iron-enriched imperfect structure containing a considerable amount of dissolved gases.
Keywords: alloys; structure; Mössbauer spectroscopy; solid solutions; ordering; phase separation

Size Effect in the Phase Separation of Cr–W Solid Solutions by A. V. Shishulin; V. B. Fedoseev (546-549).
Size effects in phase transformations of nanoscale systems show up as significant changes in their phase diagram. Here, using a thermodynamic approach we demonstrate how the immiscibility region of Cr–W solid solutions is influenced by their particle size. For 40- and 70-nm-diameter particles, we consider two thermodynamically stable states with a core–shell configuration, differing in the composition of the core phase. It is shown that, in the nanometer range, one of these states becomes metastable and that the phase diagrams of the stable and metastable states differ significantly. Reducing the particle size leads to a decrease in the upper critical dissolution temperature (UCDP) by 300–400 K and marked changes in the mutual solubility of the components at temperatures comparable to the UCDP.
Keywords: phase transformations; size effect; core–shell structure; solubility

Oxidation Behavior of Zirconium Diboride Nanoparticles by G. V. Kalinnikov; A. A. Vinokurov; S. E. Kravchenko; N. N. Dremova; S. E. Nadkhina; S. P. Shilkin (550-557).
The products of oxidation of ZrB2 powders with average particle sizes of ~100 and ~30 nm by atmospheric oxygen under isothermal conditions and during heating have been characterized by thermal analysis, X-ray diffraction, scanning electron microscopy, IR frustrated total internal reflection spectroscopy, energy dispersive X-ray analysis, and elemental analysis. The oxidation onset has been observed at 594 and 396°C, respectively. Oxidation at temperatures of ≥800°C leads to the formation of boron oxide and monoclinic ZrO2, independent of the particle size of ZrB2. The reaction rate constants for the oxidation of ZrB2 nanoparticles ~100 and ~30 nm in size have been determined to be 0.03, 0.15, and 0.31 h–1 at 600, 650, and 700°C and 0.11, 0.35, and 0.81 h–1 at 500, 600, and 700°C, respectively. The apparent activation energies for the oxidation of the ZrB2 nanoparticles ~100 and ~30 nm in size are 161 ± 4 and 62 ± 3 kJ/mol, respectively, as evaluated from the temperature dependence of the rate constants at the above temperatures.
Keywords: zirconium diboride nanoparticles; oxidation; phase; isotherm; rate constant; apparent activation energy for the oxidation process

Contact Interaction in an M–SiO2 (M = Metal Catalyst for Nanowhisker Growth) System by V. A. Nebol’sin; A. I. Dunaev; A. S. Samofalova; V. V. Korneeva (558-563).
We have studied contact interaction effects and wettability in M–SiO2 (M = metal catalyst for nanowhisker growth) systems. The results demonstrate that the wettability of SiO2 by the metal catalysts for nanowhisker growth (Au, Ni, Cu, and Sn) improves with a decrease in the first ionization potential of their atoms and is determined by dispersion interaction with the oxide surface. The interaction of the metal atoms with the O2– anions of the oxide is responsible for the development of cohesion between the liquid metal and SiO2 on the interface.
Keywords: nanowhiskers; contact interaction; silicon dioxide; metal catalyst; contact angle

IR Scattering by Optically Inhomogeneous Nanoporous Anodic Alumina Films by E. N. Muratova; L. B. Matyushkin; V. A. Moshnikov; K. V. Chernyakova; I. A. Vrublevsky (564-567).
This paper presents results of thermographic studies of nanoporous alumina films having different geometric parameters of their porous layer (thickness and average pore diameter), which were exposed to thermal radiation. The films have been shown to shield thermal radiation. The present results suggest that nanoporous alumina membranes can be used as heat shields for reducing the thermal contrast of an object and the surrounding background in the IR spectral region.
Keywords: nanoporous films; alumina; thermal radiation; heat shield

A New Ti9O10 Nanophase Prepared by Heat-Treating Nonstoichiometric Milled TiO y Nanopowder by A. A. Valeeva; M. G. Kostenko; S. Z. Nazarova; E. Yu. Gerasimov; A. A. Rempel (568-574).
A Ti9O10 nanophase (sp. gr. Immm) has been synthesized via high-energy milling followed by prolonged high-temperature vacuum heat treatment of nonstoichiometric TiO y titanium monoxide. The atomic structure and microstructure of Ti9O10 have been investigated by X-ray powder diffraction and high-resolution transmission electron microscopy. According to quantum-chemical calculation results, macrocrystalline Ti9O10 is less energetically favorable than a disordered cubic phase of the same composition, TiO10/9, so the formation of the former phase is due to size effects.
Keywords: TiO y titanium monoxide; high-energy milling; nanocrystal; structural transformations; electronic structure

Enthalpy of Formation of MgAl2O4 · 0.793Al2O3 Nonstoichiometric Spinel at 1900 K from High-Temperature Mass Spectrometry Data by N. A. Gribchenkova; K. G. Smorchkov; A. G. Kolmakov; A. S. Alikhanyan (575-580).
The composition and partial pressures of vapor species over the pseudobinary system Al2O3–MgO have been determined by high-temperature mass spectrometry. The data obtained have been used to evaluate the standard enthalpy of formation of nonstoichiometric spinel with the composition MgAl2O4 · 0.793Al2O3 from its constituent oxides by third-law calculations. The entropy of MgAl2O4 · 0.793Al2O3 has been evaluated using a regular-solution model with allowance for cation inversion.
Keywords: Knudsen effusion mass spectrometry; Al2O3–MgO system; nonstoichiometric spinel; thermodynamic activity; regular solution; standard enthalpy

Photoelectric Fields and Band Gap in Doped Lithium Niobate Crystals by N. V. Sidorov; M. N. Palatnikov; N. A. Teplyakova; A. V. Syuy; E. O. Kile; D. S. Shtarev (581-584).
Photoinduced light scattering and optical spectroscopy have been used to study the photorefractive effect and determine the band gap in nominally undoped congruent and stoichiometric lithium niobate crystals, as well as in a series of congruent LiNbO3 crystals doped with Mg, Zn, B, Gd, Y, and Er cations and LiNbO3 single crystals codoped with Mg:Gd, Mg:Fe, Mg:Y, and Mg:Ta.
Keywords: lithium niobate single crystals; doping; photorefractive effect; photoinduced light scattering; optical spectroscopy; band gap

A CsBaGd(MoO4)3:Er3+ phosphor with a scheelite-like structure (sp. gr. P21/n) has been synthesized and its luminescence properties have been studied. The synthesized material has been characterized by X-ray diffraction, differential thermal analysis, Raman spectroscopy, and IR spectroscopy.
Keywords: phase equilibrium; vibrational spectra; Er3+ luminescence

Preparation and Thermal Expansion of Calcium Iron Zirconium Phosphates with the NaZr2(PO4)3 Structure by D. O. Savinykh; S. A. Khainakov; A. I. Orlova; S. Garcia-Granda (591-595).
Ca0.5(1 + x)Zr2–x Fe x (PO4)3 phosphates have been synthesized by a sol–gel process. The individual compounds and solid solutions obtained crystallize in the NaZr2(PO4)3 structure (trigonal symmetry, sp. gr. R $$ar 3$$ 3 ¯ ). Using high-temperature X-ray diffraction, we have determined their thermal expansion parameters in the temperature range from 25 to 800°C. With increasing x, the magnitudes of their linear thermal expansion coefficients and thermal expansion anisotropy decrease. Most of the synthesized phosphates can be rated as low-thermal-expansion compounds and can be regarded as materials capable of withstanding thermal “stress.”
Keywords: NZP; phosphate; sol–gel process; X-ray diffraction; solid solution; thermal expansion

The energies of mixing (interaction parameters) in the Sc1–x Ln x PO4 (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) systems have been calculated using Urusov’s crystal energy theory of isomorphous substitutions. The decomposition (stability) temperatures of the solid solutions have been plotted against the atomic number of the rare-earth elements at x = 0.01, 0.03, 0.05, 0.10, 0.20, and 0.50. The present results can be helpful in a search for host and activator compositions for new luminescence, laser, and other materials based on the zircon-structured rare-earth orthophosphates.
Keywords: solid solution; orthophosphate; zircon structure; energy of mixing

Electrical Conductivity of a Carbon Reinforced Alumina Resistive Composite Material Based on Synthetic Graphite and Graphene by V. M. Samoilov; E. A. Danilov; A. V. Nikolaeva; D. V. Ponomareva; I. A. Porodzinskii; E. R. Razyapov; I. A. Sharonov; N. A. Yashtulov (601-609).
We have prepared samples of carbon reinforced alumina ceramics with different volume fractions of synthetic graphite (0–20%) and graphene (0–4%) and measured their electrical conductivity. It has been shown that increasing the volume percent of the conductive component increases the electrical conductivity of the samples from 10–8 to 2 × 10–3 S/cm. The results have been analyzed in terms of percolation theory and tunneling conduction theory. The synthetic graphite-based samples show linear current–voltage behavior and their electrical conductivity increases by a factor of 1.4 to 2.8 in the temperature range 300–550 K, with a sharp rise above 550 K. The temperature dependences of their electrical conductivity are analyzed in terms of hopping transport and thermally induced tunneling conduction mechanisms. The conclusion is made that the conduction mechanism in the corundum–carbon ceramics differs significantly from that in polymer composite materials.
Keywords: carbon reinforced alumina ceramics; graphene; synthetic graphite; percolation; tunneling conduction; composite materials

Preparation, Thermal Diffusivity, and Thermal Conductivity of Phosphate Ceramics with the Tridymite Structure by V. I. Pet’kov; I. V. Korchemkin; E. A. Asabina; A. M. Kovalskii; A. A. Usenko (610-615).
We have synthesized powders of the CsMIIPO4 (MII = Mg, Mn, Co) phosphates with the β-tridymite structure. Phosphate ceramics were produced by conventional cold pressing with inorganic sintering aids, followed by programmed annealing, and by an innovative method: high-speed spark plasma sintering. The thermal diffusivity and thermal conductivity (0.3–0.4 W/(m K)) of the CsMIIPO4 ceramics indicate that they are heat insulators with a high working temperature.
Keywords: Cs-containing phosphates; β-tridymite structure; ceramics; thermal diffusivity; thermal conductivity

Synthesis of Fine-Particle Bismuth Orthogermanate in a NaCl/KCl Melt by O. M. Gaitko; V. O. Veselova; V. D. Volodin; A. V. Egorysheva (616-620).
We have developed a process for the synthesis of fine bismuth orthogermanate, Bi4Ge3O12 (BGO), powders in a NaCl/KCl melt and examined the effect of synthesis conditions on the phase composition and morphology of the powders. Our results demonstrate that the synthesized materials compare well in scintillation performance with single-crystal BGO: their scintillation efficiency is 90% relative to single-crystal BGO, with a decay time of 300 ns.
Keywords: bismuth orthogermanate; nanoparticles; molten salt synthesis; scintillators