Relationship of the structural and physicomechanical characteristics in porous SHS materials based on titanium carbide

Structural and physicomechanical characteristics of the porous SHS materials TiC_stoich, TiC_0.9, and TiC_0.8 based on titatium carbide with a porosity of 55–58% are investigated. It is shown that as the combustion temperature increases, the compressive and breaking strength decreases monotonically and a smooth coarsening of the porous stucture occurs. Introduction of nickel into the carbide improves the strength and ductility of the porous SHS material. There is an optimum initial temperature (500–600°C) ensuring maximum strength when nickel is introduced.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka. Institute for Problems of Superductility of Metals, Russian Academy of Sciences, Ufa. Translated from Inzhenerno-Fizicheskii Zhurnal. Vol. 65, No. 4, pp. 466–470, October, 1993.     

A new route to synthesize La1?xSrxMnO3

Self-Propagating High-temperature Synthesis (SHS) was used to producecomplex oxides (La_1–x Sr _x MnO₃ with x = 0, 0.1 and 0.2), which are used as the cathode in solid oxide fuel cells (SOFCs). Thermodynamic predications and experiments show that La_1–x Sr _x MnO₃ can be prepared via SHS under moderate conditions from a mixture of La₂O₃ + SrO₂ + Mn, using either gaseous oxygen or solid NaClO₄ as the oxidant. Partial melting at the high combustion temperature increased product homogeneity. The electrical conductivity of the product was 180 S·cm^–1 at 1000°C in air, matching that of sample made by other synthesis processes. SHS enables a more economical production of La_1–x Sr _x MnO₃ than existing commercial processes.     

Self-propagating solid state routes to BaSnO3; investigation of gas sensing properties

Thermal initiation of mixed powders of a tin source (Sn, SnCl₂, SnO₂) and barium peroxide in air produce a self propagating high temperature synthesis (SHS) wave of velocity 2–5 mms^–1 and temperature 950–1050°C. The product from the reaction is predominantly BaSnO₃ with some SnO₂ and Ba₂SnO₄ impurities. Annealing of the SHS powders at 1000°C for 2–72 h forms phase pure cubic BaSnO₃, a = 4.116 (3) Å. X-ray photoelectron spectroscopy showed the expected Ba: Sn: O ratios with binding energy shifts for Ba 3d_5/2 at 775.8 eV; O 1s at 526.1 eV and Sn 3d_5/2 at 482.7 eV. Scanning electron microscopy showed ca 0.5 m agglomerates of cubic crystallites of dimension 0.15 m, whilst energy dispersive analysis by X-rays showed the expected barium to tin ratios. Infra-red spectra of the annealed products were equivalent with commercial samples. The SHS prepared barium stannates were good sensors for carbon dioxide, showing n-type response.     

Kinetics of a non-catalytic gas-solid chemical reaction under SHS-like conditions

Nitration of metallic tantalum under self-propagating high-temperature synthesis (SHS) conditions (i.e. rapid heating rates 2000–3000 Ks^–1, and short heating periods 2–100 s) has been studied. Phase analysis and microstructural characterization have been performed. A two-phase mixture region of solid solution TaN_x and tantalum subnitride, namely Ta₂N, was observed in the nitrided tantalum at temperatures of 1600–2600 K and a partial nitrogen pressure of 0.1–0.8 MPa. It was also found that tantalum subnitride phase formed before the formation of Ta-N solid solution throughout the sample during nitration. A theoretical study was conducted in order to determine the intrinsic reaction kinetics of a typical combustion synthesis reaction. A new model was developed to explain all experimental observations. The computer simulation results are in good agreement with experimental data. The results indicate that at sufficiently large heating rates the product layer does not necessarily act as a diffusion barrier that prevents further uptake of nitrogen. The results are helpful in developing an understanding of the mechanism of powder-based SHS.     

Thermodynamic Feasibility of SHS of SOFC Materials

Thermodynamic analysis predicts and experiments confirm that La_1–x Sr _x CrO₃ powder, used to manufacture interconnect components in solid oxide fuel cells (SOFC), may be prepared via SHS under moderate conditions. The oxidation of Cr is the main source of heat generation, which maintains a stable reaction front. The combustion temperature, front propagating velocity and product particle size may be modified by replacing part of the metallic Cr in the reaction mixture by its oxide. Decomposition of either CrO₃, SrO₂, or NaClO₄ provides the oxygen needed for the Cr oxidation. The predicted and observed combustion temperatures are in reasonable agreement. However, experiments indicate that extensive dilution may lead to extinction under conditions for which the thermodynamic analysis predicts that SHS is feasible.     

Specific features of the behavior of synthetic diamond in a self-propagating high-temperature synthesis (SHS) combustion wave

The behavior of synthetic diamond powder in an SHS combustion wave is considered by using the model Ti-B-diamond system as an example. The results presented show that the diamond can retain its properties when exposed to the extreme thermal conditions realized in an SHS combustion wave.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 63, No. 5, pp. 577–582, November, 1992.     

Effect of the concentration of zirconium dioxide on the fracture resistance of Al2O3-ZrO2 ceramics

We analyzed structural changes and physicomechanical properties of Al₂O₃-ZrO₂ ceramics as functions of the chemical composition of highly dispersed powders (synthesized by using plasmochemical methods based on the original technology) and sintering conditions. It was established that compositions with 10–15% wt. % ZrO₂ are characterized by the best physicomechanical properties and the highest fracture resistance. Ceramics with optimal structure were obtained by sintering powders at a temperature of 1700°C for two hours.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Riga, Latvia. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 52–57, September – October, 1995.     

Effect of HCl concentration on TiB2 separation from a self-propagating high-temperature synthesis (SHS) product

This paper presents a synthesis of TiB₂ powder via self-propagating high-temperature synthesis (SHS) method using a mixture of TiO₂, B₂O₃ and Mg followed by acid leaching. In the acid leaching step, the MgO content in the SHS product was leached in different HCl concentrations. X-ray diffraction (XRD) analysis showed that when 9.3 M HCl was used, the leached SHS product was found similar to that of the commercial TiB₂ powder. However, scanning electron microscopy (SEM) and BET surface area analysis revealed that the leached product was agglomerated and exhibited very high surface area.     

Microstructure and composition of SHS products of powder multicomponent oxide mixtures with aluminum and graphite

The microstructure and composition of products of SHS combustion of two types of multicomponent oxide mixtures with aluminum and graphite powders, differing in the presence or absence of a thermite additive, are studied. The combustion product—oxide carbide ceramic based on Al₂O₃/TiC—serves for immobilization of graphite-containing radioactive wastes. It is proposed to use results of the studies of “undercombusted” samples of such composites for analysis of the initial stages of the process of their front combustion. Procedures of combustion of the composites with the presence or absence of a thermite additive are specified. The problems of distribution in the combustion products of the impurities introduced into the original system, which simulate the most characteristic composition of nuclides in nuclear-fuel degradation products and in radiation-exposed graphite, are studied     

Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.     

Effects of process parameters on tungsten boride production from WO3 by self propagating high temperature synthesis

In the present study, the production parameters of tungsten boride compounds by self-propagating high-temperature synthesis (SHS) method and following leaching process were investigated. In the SHS stage, the products consisting of tungsten borides, magnesium oxide, magnesium borate, and also minor compounds were obtained by using different initial molar ratios of WO₃, Mg and B₂O₃ as starting materials. In the leaching step, Mg containing byproducts, i.e. MgO and Mg₃B₂O₆, existed in the selected SHS product synthesized at 1:8:2.5 initial molar ratio of WO₃:Mg:B₂O₃ were leached out by using aqueous HCl solution to obtain clean tungsten boride compounds at different experimental parameters which are time, acid concentration and temperature. The acid leaching experiments of the SHS product showed that optimum leaching conditions could be achieved by using 5.8 M HCl at 1/10 S/L ratio and the temperature of 80 °C for 60 min.     

Self-propagating high-temperature synthesis of composite materials. The high-melting compounds (TiC,TiB2)-intermetallides,their structure and properties

Regularities of the SHS of four-component powder mixtures of quasibinary sections (titanium carbide or diboride-intermetallide Ni₃Al or NiTi) are studied. Combustion rates and temperatures are determined. Specific features of the combustion as a function of the composition are revealed. The character of structure formation and the properties of the synthesized materials, including those produced in SHS compaction, are considered.Siberian Physicotechnological Institute, Tomsk. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 492–495, October, 1993.     

Self-propagating high temperature synthesis of Ni0.35Zn0.65Fe2O4 ferrite powders

The stoichiometric Ni_0.35Zn_0.65Fe₂O₄ ferrite powders were synthesized by SHS method. In the process of SHS, the effects of the molar ratio Fe/Fe₂O₃ in the starting mixture, oxygen pressure, grain size and relative density of the raw materials on combustion temperature, combustion wave velocity, phase composition and microstructure of the combustion products were investigated. X-ray diffraction, scanning electron microscope, TEM, vibrating sample magnetometry were used to characterize the microstructure and magnetic properties of the products. The results showed that as the molar ratio Fe/Fe₂O₃ increases, the combustion temperature and combustion wave velocity increased. The same results can be observed when the oxygen pressure increased from 0.1 to 0.9 MPa. The increase of grain size and relative density of raw materials resulted in the decrease of combustion temperature and combustion wave velocity. Compared with other methods, SHS process leads to ferrite powders with improved magnetic properties.     

Distribution of interstitial impurities in titanium-vanadium alloys after holding in liquid lithium

The distribution of interstitial impurities between liquid lithium and solid titanium-alloyed vanadium is investigated. It is shown that titanium alloying of vandium intensifies nitrogen and carbon transport from lithium into the solid metal. At the same time, the concentration of oxygen in the surface layers of the specimens increases as a result of penetration of lithium and formation of complex oxides of the Me _x Li _y O _z type. Increase in the titanium content leads to an increase in the carbon concentration on the surface and to the formation of protective carbon films that inhibit the diffusion of interstitial impurities into deep-seated layers.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 56–60, September–October, 1993.     

Heat transfer of HTSC-ceramic cooled by liquid hydrogen

An experimental investigation of heat transfer and crises of bubble and film boiling of hydrogen on the YBa₂Cu₃O₇ HTSC-ceramic is described. Heat transfer characteristics obtained at pressures from 17 to 100 kPa and heat flux densities from 0.1 to 120 kW/m² are compared with well-known results for hydrogen boiling on metal heaters.Physicotechnical Institute of Low Temperatures, Academy of Sciences of the Ukraine, Khar'kov. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 67, Nos. 3–4, pp. 179–183, September–October, 1994.     

Mathematical modeling of SHS extrusion. 2. Rheodynamic models

A review of results of mathematical modeling of the rheodynamics of SHS hot compaction and extrusion of combustion products is presented.Institute for Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka, Russia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 1, pp. 90–94, July, 1993.     

Some characteristics of phase formation in combustion of powdered titanium-silicon and molybdenum-silicon mixtures

The phase formation processes in Ti–Si and Mo–Si systems were studied, using special markers simulating the behavior of large metal particles after they have passed the combustion front. It is shown that, depending on the combustion temperatures of the mixtures, phases can be formed by joint crystallization from the eutectic melt, or disilicides crystallize first and intermediate phases (Mo₅Si₃ and TiSi) form by a solid-phase mechanism.Institute of Chemical Physics, Academy of Sciences of Armenia, Erevan, Armenia. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 476–479, October, 1993.     

Structure and properties of a chromium diffusion coating on the inner surface of 08Kh13G14N4 steel pipes

The processes of diffusion saturation of the surfaces of 08Kh13G14N4 steel pipes with chromium from Li-Cr and Li-Cr₂O₃ melts were studied. It was established that, at high temperatures, the corrosion resistance of 08Kh13G14N4 steel with a chromium coating in liquid lithium depends mainly on the surface chromium concentration.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 41–45, September–October, 1993.     

Regularities of the change in the physicomechanical properties in the Cr2O3−Al system in accordance with SHS conditions

Based on a comprehensive study of combustion processes and variation of the phase composition and the structure of synthesized materials of the Cr₂O₃-Al system in a wide range of compositions, we established the regularities of the change in their physicomechanical properties and selected the optimal conditions for producing cermets and abrasives with high strength properties.This issue of the journal (pp. 1102–1145) continues publication of the proceedings of the First International symposium on Self-Propagating High-Temperature Synthesis held on September 23–28, 1991, in Almaty (see Inzh.-Fiz. Zh., Vol. 65, No. 4, 1993.Institute of Combustion Problems, Almaty. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No.5, pp.579–581, November, 1993.     

Heat conductivity of a beryllia-based composition at low temperatures

The thermal conductivity of beryllium oxide with additions of Y₂O₃, MgO, Fe₂O₃, TiO₂, A1₂O₃, SiO₂, ZrO₂, and glass was measured at 120–260°K. The additions were made in amounts up to 5 wt. %.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 6, pp. 980–985, December, 1980.