International Journal of Self-Propagating High-Temperature Synthesis最新文献

Gas emission taking place during combustion of Fe₃O₄–Al–Ni nanothermite mixtures with additives of multilayer carbon nanotubes was studied. Variation in released gas volume with pre-degassing temperature was analyzed. It was shown that the major portion of easily evaporated impurities and moisture is removed upon vacuum degassing of mixtures at 162°C. A two-stage ignition mode of mixtures was found, and their critical ignition temperatures were determined. The burning velocity varied from 10 mm/s for Ni + Al to 770 mm/s for Fe₃O₄ + Al.

M_xTiO₂ (M = Li, Na, K, and Rb; 0 < x < 1) bronzes were prepared via self-propagating high-temperature synthesis (SHS) of mechanically activated and non-activated green mixtures containing TiO₂, MI, and additive CuO + Ti as heat-generating agent. The influence of mechanical activation on the composition and structure of SHS products was studied. TEM investigations revealed the growth of the number of crystal defects after mechanical activation, in particular, the formation of edge dislocations in certain crystallographic directions, leading to an increase in the diffusion speed in the crystal. The beginning of mechanical activation was shown to be accompanied by the formation of unstable interstitial solid solutions without changing the crystal structure of TiO₂. Prolonged grinding generated stable phases with higher content of intercalated atoms and individual structure, which acted as heterogeneous crystallization nuclei during SHS. According to Rietveld refinements preliminary mechanical activation favored a higher yield of target phases with a higher content of intercalated alkali metal ions.

The thermomechanical behavior of HfB₂–SiC and (Hf_0.8Ta_0.2)B₂–SiC ceramics hot-pressed from SHS-prepared powders was studied under compression tests at 1600 and 1800°C. The high-temperature strength for ceramics was found to reach 582–606 MPa. It was showed that ceramics based on (Hf,Ta)B₂ solid solution have higher plasticity as compared to samples based on HfB₂, their deformation degree, ε = 0.027, is 2 times higher.

The thermoelectric alloy consisting of Al_(1–x)MnSi_(1+x) (Al_2.5Mn_3.0Si_3.5 and Al_0.75Mn_1.0Si_1.25), Mn₃Al, MnSi, and unreacted Si was prepared via self-propagating high-temperature synthesis. The microstructure of the alloy was shown to be porous and mainly represented by coarse Al_(1–x)MnSi_(1+x) grains (15–60 μm in size). Thermoelectric characteristics of synthesized material were studied. It was found that the Seebeck coefficient is –29 μV/K at room temperature and reaches –8 μV/K at 800 K. The maximum specific electrical resistance of 9.5 Ω cm was observed at room temperature.

The processes occurring during the synthesis of lanthanum manganites doped with alkali metal ions in combustion reactions in the nitrate–polyvinyl alcohol and nitrate–polyvinylpyrrolidone systems were investigated. The maximum combustion temperatures of compositions depending on the organic fuel content φ (ratio to stoichiometric amount) were given. It was found that the maximum temperature of the process is 531°C, which is reached when synthesizing LaMnO_{3 ± y} sample from the composition with polyvinyl alcohol (φ = 4). It was experimentally shown that the maximum combustion temperature increases with increasing organic component amount in the composition and decreasing pyrolytic impurity gas (such as carbon monoxide CO, nitrogen oxides NO and NO₂) concentrations. The processes of charge generation during combustion of nitrate–polyvinyl alcohol and nitrate–polyvinylpyrrolidone systems at different types of heating of the reactor were studied. The effect of synthesis conditions on the specific surface area of complex oxides was investigated.

The possibility was considered of using self-propagating high-temperature synthesis (SHS) in open air to obtain porous titanium carbide composites containing free graphite. The effect of graphite excess in the Ti + (1 + х)C charge (where x = 0.5, 1.0, and 1.5) and pressing pressure on the porosity of the SHS-derived TiC–C composite, as well as on the size of pores and TiC and graphite particles was shown. The direct proof of the possibility of fast graphitization of such amorphous carbon form as carbon black under conditions of SHS of porous TiC–C composites was given.

Due to its excellent engineering properties, aluminum has become an important material for processing industries. As the demand for aluminum increased, a large amount of waste aluminum dross was generated during the aluminum smelting process. The aluminum dross contained aluminum nitride and would cause odor while being disposed in landfill, making the aluminum dross disposal a tough issue. Therefore, the aluminum dross was mostly stored in the original plants. The objective of this study was to develop an economically feasible and environmentally friendly technology to recover aluminum dross. Aluminum dross ranges from 1–2% of the aluminum manufactured and comprises 30–70% aluminum by weight. 1 ton of aluminum metal production generates 10 to 20 kg of dross. Currently, in India, there is very limited organized processing of dross. The world produces approximately 65 million tons of aluminum per annum, which results in approximately 1 million tons of aluminum dross being produced every year. Furthermore, almost 100 000 of these are produced in India. Hence, the environmental impact of this waste is a global problem that needs to be addressed on priority. In this paper, a new method is developed to prepare a high pure α-alumina (99% purity) from aluminum dross by self-heat generating, one-step combustion method at 500°C furnace temperature.

The paper described the data of a preliminary experiment on laser-initiated reactions in a mixture of Ti–Al–CuO powders. A two-dimensional model of laser initiation of reactions in a powder layer located on a substrate was presented. The laser beam moved along the surface according to a given trajectory. Chemical transformations were simulated by a total scheme including the main stages (decomposition of copper oxide CuO to Cu₂O, metallothermic reactions leading to the formation of titanium and aluminum oxides, and the total reaction of matrix formation). The model was realized numerically. It was found that the process of composite synthesis can be divided into two stages. The first one consisted in partial decomposition of copper oxide; the second stage was a controlled process, when reactions took place only in the area of laser beam action. The results of the theory were found to be in qualitative agreement with the experimental data.

TiB₂ ceramics were produced by forced SHS compaction. The influence of preliminary mechanical activation of boron powder on the properties and combustion characteristics of Ti + 2B reaction mixture was studied. Mechanically activated boron powder was shown to become plastic under a pressure above 30 MPa. In mixtures with titanium, activated boron acted as a lubricant allowing free movement of coarser titanium particles relative to each other and consolidation to higher density values (0.8) without intensive growth of contact surface between the titanium particles (Ti–Ti). Mechanical activation was found to increase the contact surface between reactants (Ti–B) and consequently, the combustion temperature up to 3000°C.