Effect of heat release conditions on the phase composition of the combustion products of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe₂O₃/TiO₂/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 39–43, July–August, 2008.     

Combustion and structure formation in the Ti-Ta-C-Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> system

Biocompatible composites (Ti, Ta)C _x + Ca₃(PO₄)₂ for deposition of nanofilms onto load-bearing implants by ion-plasma sputtering were prepared from Ti + Ta + C + Ca₃(PO₄)₂ mixtures by forced SHS compaction. The effect of Ta + C addition to green mixtures (characterized by parameter z) on the structure/phase formation in combustion products was explored. The addition of tantalum and carbon was found to have little or no influence on the burning velocity U and combustion temperature T _c. Two thermal spikes exhibited by thermograms were associated with the occurrence of two consecutive reactions leading to formation of titanium and tantalum carbides. With increasing z, the grain size of (Ti, Ta)C was found to diminish, its relative density to decrease, while the hardness to markedly grow.      

Inexpensive synthesis of a high-performance Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> photocatalyst: Magnetic recovery and reuse

A sol-gel technique has been developed for the synthesis of a magnetite-silica-titania (Fe₃O₄-SiO₂-TiO₂) tertiary nanocomposite with improved photocatalytic properties based on the use of inexpensive titania and silica precursors. The exceptional photocatalytic activity of the resulting materials was demonstrated by using them to photocatalyze the degradation of methylene blue solution. The best formulation achieved 98% methylene blue degradation. An interesting feature of the present work was the ability to magnetically separate and reuse the catalyst. The efficiency of the catalyst remained high during two reuses. The synthesized nanomaterials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, ultra-violet-visible spectroscopy, diffuse reflectance spectroscopy, and thermogravimetric analysis. XRD analysis revealed the formation of multicrystalline systems of cubic magnetite and anatase titania crystals. SEM and TEM characterization revealed well-developed and homo-geneously dispersed particles of size less than 15 nm. FTIR spectra confirmed the chemical interaction of titania and silica. It was further noticed that the optical properties of the prepared materials were dependent on the relative contents of their constituent metal oxides.

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Combustion of metallothermic mixtures FeO-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al-C: Influence of carbon granularity and localization

Combustion of metallothermic mixtures FeO-Cr₂O₃-Al-C in air at normal conditions was studied upon variation in the size of carbon (graphite) particles and carbon localization in a green sample. Carburization conditions were optimized in terms of graphite particle size.     

Combustion of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al thermit mixtures in steel tubes

Explored was the combustion of Fe₂O₃/TiO₂/Al thermit mixtures in steel tubes upon variation in green composition and with special emphasis on the dependence of combustion temperature T _c and burning velocity U on reaction heat Q. Special attention was given to incompleteness of combustion for compositions with low Q.     

Reaction mechanism for SHS of MoSi<Subscript>2</Subscript> from mechanically activated powder mixtures

Controlling the Mechanically Activated Self-propagating High-temperature Synthesis (MASHS) process is exceedingly important for production of MoSi₂-based materials with a desired microstructure. Consequently, it seemed essential to carry out Time-Resolved X-ray Diffraction (TRXRD) experiments using an X-ray synchrotron beam (DM2, ESRF Grenoble) coupled with IR thermographic measurements to monitor in situ the structural and thermal evolutions taking place during SHS. The versatility of this technique and new possibilities offered by the design of new sample holders have already been proved. In addition, this work clearly shows that this equipment is perfectly adapted to investigating phase transitions occurring during the MASHS process in the Mo-Si system. The text was submitted by the authors in English.     

Selective Synthesis of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanowires Via a Single Precursor: A General Method for Metal Oxide Nanowires

Hematite (α-Fe₂O₃) and magnetite (Fe₃O₄) nanowires with the diameter of about 100 nm and the length of tens of micrometers have been selectively synthesized by a microemulsion-based method in combination of the calcinations under different atmosphere. The effects of the precursors, annealing temperature, and atmosphere on the morphology and the structure of the products have been investigated. Moreover, Co₃O₄ nanowires have been fabricated to confirm the versatility of the method for metal oxide nanowires.     

Structure formation during gas-detonation spraying of coatings from composite powders TiAl<Subscript>3</Subscript> and Ni<Subscript>3</Subscript>Al

This paper considers the mechanisms of structure formation during gas detonation spraying of coatings of TiAl₃ and Ni₃Al intermetallic compounds produced under equilibrium and nonequilibrium synthesis conditions. The coating sprayed from TiAl₃ has the same phase composition as the initial powder, regardless of the synthesis conditions. During spraying of Ni₃Al, the structure of the coating also does not depend on the synthesis conditions and consists of two phases — Ni₃Al and NiAl, with the crystal structure varying along the coating thickness. Comparative impact abrasion tests of the coatings showed advantages of TiAl₃ coatings over coatings based on Ni₃Al and titanium diboride. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 106–111, September–October, 2008.     

Soft Template Synthesis of Super Paramagnetic Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles a Novel Technique

The present study reports a facile technique for the synthesis of crystalline super paramagnetic nano ferrite (Fe₃O₄) particles using diethyl amine as a soft template. The spectral properties of Fe₃O₄ nanoparticles were characterized by UV–visible and Fourier Transform Infrared (FTIR) spectroscopies while the crystalline structure and particle size was estimated using X-Ray diffraction (XRD) as well as transmission electron microscopy (TEM) techniques. The super paramagnetic behavior of Fe₃O₄ nanoparticles was determined using vibrating sample magnetometer (VSM) at 300 K. The results of the studies revealed that this technique could be adopted to synthesize agglomerate free super paramagnetic Fe₃O₄ nanoparticles which may find potential application in the filed of biosensor and corrosion protective coatings.     

Synthesis of Titanates with a Ramsdellite-Type Tunnel Structure Crystallizing in the Li<Subscript>2</Subscript>O-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> System in Different Gaseous Media

The ramsdellite-type phases crystallizing in the Li₂O-Fe₂O₃-TiO₂ system in the course of synthesis in gaseous media at different oxygen partial pressures are studied. Solid solutions based on the ramsdellite structure with the composition Li₂Ti_3?xFe _x O_{7 ? δ} (0 ≤ x ≤ 0.7) are prepared in an oxidizing medium (P_O2 = 1 atm) for the first time. Analysis of the results obtained by electron paramagnetic resonance and Mossbauer spectroscopy revealed that, in these solid solutions, all iron ions are in the oxidation state Fe⁺³.     

A thermodynamic study of the Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-H<Subscript>2</Subscript>-CO system

The equilibrium composition of the Fe₂O₃-H₂-CO system depending on temperature and pressure was calculated by the methods of chemical thermodynamics at different molar ratios between the system components. It was found that the equilibrium composition of the mixtures formed in this system mainly depends on ratios between the system components. The effect of the partial pressures of H2 and CO on the equilibrium composition of products in the Fischer-Tropsch synthesis was determined.     

Structure and crystallization of ZnO-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

Structure and crystalline behavior of the ternary system ZnO-B₂O₃-P₂O₅ glasses were investigated by means of X-ray diffraction (XRD) and infrared Raman spectra. The research showed that number of the planar [BO₃] units increases with the increase of B₂O₃ content. When the B₂O₃ content is above ≥10 mol %, the relative content of planar [BO₃] units increases rapidly and causes weakening of the glass structure and decrease in the chemical stability. In the crystallized glasses the predominant crystal phase Zn₂P₂O₇ decreases with the increase of B₂O₃ content, while the crystal phase BPO₄ increases with it, which cause the declining of chemical stability and the decrease of thermal coefficients of expansion.     

Formation of La<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript> crystals from amorphous La<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> powders synthesized by the polymerized complex method

Amorphous La₂O-TiO₂ powders were synthesized by the polymerized complex (PC) method. The activation energies for crystallization and grain growth of La₂Ti₂O₇ from these precursors were determined from results of XRD and DTA and compared with those for La₂Ti₂O₇ precursors by the conventional solid-state reaction (SSR). Activation energy of grain growth of La₂Ti₂O₇ in PC-sample was determined to be 7.1 kJ/mol while that of SSR sample was 14.8 kJ/mol. The energy required for the phase transformation from amorphous PC sample to layered perovskite was 432 kJ/mol, while the SSR sample did not show this transition below 900‡C. It was clearly demonstrated that the La₂Ti₂O₇ crystals were formed at a lower temperature and they grew in size faster in the sample prepared by the PC method relative to the sample prepared by the SSR method. Mixing of elements in molecular level in PC preparation appeared responsible for these differences.     

Synthesis and thermal behavior of Fe<Subscript>3</Subscript>O<Subscript>2</Subscript>(BO<Subscript>4</Subscript>) oxoborate

Iron oxoborate Fe₃O₂(BO₄) has been first produced in solid-phase chemical reactions. Its thermal behavior in the temperature range 20–900°C is studied with the use in-situ high-temperature powder X-ray diffraction. It is shown that Fe₃O₂(BO₄) begins decomposing with the formation of Fe₂O₃ in the temperature range 660–900°C. Thermal expansion is sharply anisotropic at room temperature (α_max/α_min = 7) and becomes more isotropic with an increase in the temperature (α_max/α_min = 1.2). The degree of oxidation of Fe³⁺ has been confirmed by Mössbauer spectroscopy (at a room temperature), and two nonequivalent positions in the structure have been detected, which are occupied by iron atoms with the octahedral environment of the oxygen atoms.     

Combustion synthesis of some compounds in the Li<Subscript>2</Subscript>O-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system

At least four compounds, viz. LiAlO₂, LiAl₅O₈, Li₅AlO₄ and Li₂Al₄O₇, are known in the Li₂O-Al₂O₃ system. These compounds are important for several technological applications. Combustion synthesis of these compounds using urea as a fuel was attempted. LiAlO₂ and LiAl₅O₈ could be successfully prepared by choosing the starting materials in required stoichiometric ratios. Li₂Al₄O₇ was not obtained as a pure phase; γ-LiAlO₂ was formed as an impurity phase. Li₅AlO₄ could not be prepared by combustion process. Some phosphors based on these aluminates could also be prepared. Activation of these aluminates with Fe³⁺, Mn⁴⁺, Cu⁺, etc. was successfully achieved. Excitation and emission spectra for LiAl₅O₈: Fe³⁺, LiAl₅O₈: Mn²⁺, and Li₂Al₄O₇: Cu⁺ are reported.     

The Structure of Fusion-Cast High-Chrome Oxide Refractories in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - Al<Subscript>2</Subscript>O<Subscript>3</Subscript> System

The phase composition and structure of fusion-cast refractories composed of 57.0 – 84.2% Cr₂O₃, 4.3 – 36.1% MgO, 2.0 – 9.7% Al₂O₃, and 2.4 – 6.9% SiO₂ have been studied by petrographic and x-ray spectral microprobe analysis methods. Refractories high in MgO with modulus M = (Cr₂O₃ +Al₂O₃)/MgO = 1.64 – 3.1 are shown to consist of spinel phase Mg(Cr, Al)₂O₄ and silicate glass. Refractory materials (80.8 – 84.2% Cr₂O₃, 4.3 – 4.7% MgO, 2.0 – 9.7% Al₂O₃, and 2.7 – 6.9% SiO₂ with M = 18.7 – 20.2) are three-phase systems composed of spinel, escolaite, and glass phase. These materials, owing to their high corrosion resistance, have promising potentiality for practical applications.__________Translated from Novye Ogneupory, No. 12, pp. 69 – 74, December, 2004.     

Synthesis and properties of flux-cast refractories in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Details are given of the synthesis and testing of flux-cast refractory materials in the alumina-rich region of the Al₂O₃-MgO-B₂O₃ system; XRD and petrography indicate that the main structure-forming phases are corundum and magnesian spinel. In subordinate amounts there are the boroaluminate 9Al₂O₃·2B₂O₃ and the previously unknown compound 4Al₂O₃·MgO·2B₂O₃, whose composition has been established by microprobe analysis. Corrosion tests showed that three-component systems containing magnesium and boron oxides at levels of 5–10% do not increase the corrosion resistance of refractories in molten sodium-calcium-silicate glass and electrovacuum borosilicate glass. __________ Translated from Novye Ogneupory, No. 3, pp. 161–163, March, 2008.     

Mechanically and chemically activated SHS in the Mo-Si-C system: Synthesis of MoSi<Subscript>2</Subscript>-SiC composites

Combustion synthesis of various MoSi₂-SiC composites was carried out in conditions of combined chemical and mechanical activation upon variation in green composition, ambient gas pressure, duration of mechanical activation, amount of activating additive, etc. In the absence of additive, the SiC content of composites was found to be below 10%. But in the presence of Teflon as an additive, the SiC content of resultant composites was found to attain a value up to 90%. In case of mechanoactivation, the amount of activating additive can be reduced by a factor of four, while the resultant product becomes more finely grained and homogeneous. The text was submitted by the authors in English.     

Effects of preparation methods for V<Subscript>2</Subscript>O<Subscript>5</Subscript>-TiO<Subscript>2</Subscript> aerogel catalysts on the selective catalytic reduction of NO with NH<Subscript>3</Subscript>

A series of V₂O₅-TiO₂ aerogel catalysts were prepared by the sol-gel method with subsequent supercritical drying with CO₂. The main variables in the sol-gel method were the amounts of V₂O₅ and when the vanadium precursor was introduced. V₂O₅-TiO₂ xerogel and V₂O₅/TiO₂ (P-25) were also prepared for comparison. The V₂O₅-TiO₂ aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅-TiO₂ xerogel and impregnated V₂O₅/TiO₂ (P-25) catalysts. The catalysts were characterized by N₂ physisorption, X-ray diffraction (XRD), FT-Raman spectroscopy, temperature-programmed reduction with H₂ (H₂-TPR), and temperature-programmed desorption of ammonia (NH₃-TPD). The selective catalytic reduction of NOx with ammonia in the presence of excess O₂ was studied over these catalysts. Among various V₂O₅-TiO₂ catalysts, V₂O₅ supported on aerogel TiO₂ showed a wide temperature window exhibiting high NOx conversions. This superior catalytic activity is closely related to the large amounts of strong acidic sites as well as the surface vanadium species with characteristics such as easy reducibility and monomeric and polymeric vanadia surface species. This work was presented at the 7 ^th Korea-China Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.