Synthesis of FeSi-Al2O3 Composites by Autowave Combustion with Metallothermic Reduction

Fabrication of FeSi-Al2O3 composites with a molar ratio of FeSi/Al2O3 ranging from 1.2 to 4.5 was conducted by the self-propagating high-temperature synthesis (SHS) method. The synthesis reaction involved metallothermic reduction of Fe2O3 and SiO2 by Al and the chemical interaction of Fe and Si. Two combustion systems were examined: one contained thermite reagents of 0.6Fe2O3 + 0.6SiO2 + 2Al, and the other had Fe2O3 + 2Al to mix with different amounts of Fe and Si powders. A thermodynamic analysis indicated that metallothermic reduction of oxide precursors was sufficiently exothermic to sustain the combustion reaction in a self-propagating mode. The SHS reaction carrying out co-reduction of Fe2O3 and SiO2 was less exothermic, and was applied to synthesize products with FeSi/Al2O3 = 1.2–2.5, while the reaction reducing only Fe2O3 was more energetic and was adopted for the composites with FeSi/Al2O3 = 2.5–4.5. Moreover, the former had a larger activation energy, i.e., Ea = 215.3 kJ/mol, than the latter, i.e., Ea = 180.4 kJ/mol. For both reaction systems, the combustion wave velocity and temperature decreased with increasing FeSi content. Formation of FeSi-Al2O3 in situ composites with different amounts of FeSi was achieved. Additionally, a trivial amount of aluminum silicate was detected in the products of high FeSi contents due to dissolution of Si into Al2O3 during the SHS process.

Combustion Synthesis of FeAl-based Composites from Thermitic and Intermetallic Reactions

Combustion syntheses involving intermetallic and thermitic reactions were conducted to fabricate FeAl–TiB2–Al2O3 composites. Two combustion systems consisting of Fe, Al, Ti, Fe2O3 and B2O3 were studied for formation of xFeAl–yTiB2–Al2O3 composites with x = 1.5–3.5 and y = 0.5–0.8. In the reaction series, thermitic reduction of Fe2O3 and B2O3 by Al thermally activated the reaction between Fe and Al. As a result, the combustion wave of the synthesis reaction was sufficiently exothermic to propagate in a self-sustaining manner. With an increase in TiB2 and FeAl of the composites, the decrease of reaction exothermicity resulted in a decline of the combustion wave velocity and reaction temperature. The activation energy Ea = 88.92 kJ/mol was deduced for the synergetic combustion reaction. Based on XRD analysis, a thorough phase conversion was achieved and composites composed of FeAl, TiB2, and Al2O3 with different contents were obtained. The SEM micrograph showed the FeAl-based composite with a dense and connecting morphology.

Fabrication of Ni-Al/diamond composite based on layered and gradient structures of SHS system

In this paper layered and gradient structures of Ni-Al SHS system were adopted to manufacture Ni-Al/diamond composites. The effect of the layered and the diamond mesh gradient structures of Ni-Al/diamond on the SHS process and the microstructure of the composites were investigated. It is found that with the increasing of the number of layers, the combustion wave velocity is decreased. The combustion wave velocity for diamond mesh size gradient structure of Ni-Al SHS is faster than that for the layered structure. A well bonding can be formed between diamond and the matrix in layered and gradient structure Ni-Al/diamond composites due to the melt of Ni-Cr brazing alloy.

Peculiarities of Self-Propagating High-Temperature Synthesis and Structure Formation of TiB2-Al2O3 and CrB2-Al2O3 Composites

Preparation of TiB2-Al2O3 and CrB2-Al2O3 composites with a broad range of phase composition was conducted by self-propagating high-temperature synthesis (SHS) involving reaction of different types. The formation of fibrous crystals of aluminum oxide with length of about 10-25 microns and with diameter of 200-500 nm at self-propagating high-temperature synthesis in the system 2B2O3-Cr2O3-6A1 was established. Thermite mixtures of Al-TiO2 and Al-TiO2-B2O3 were incorporated with the Ti-B combustion system to produce the composites of TiB2-Al2O3, within which the increase of the thermite mixture for a higher content of Al2O3 decreased the reaction temperature and combustion wave velocity. This implies that the thermite reaction of Al with TiO2 reduces the exothermicity of the overall SHS process. It was found that adoption of B2O3 as one of the thermite reagents improved the product formation effectively. For investigate the combustion wave in 0.75TiO2-0.25Ti-2B-Al system the «quenching» method was used. The XRD analysis shows that the final products containing diborides and aluminium oxide.

Combustion Synthesis of Si3N4 from Si/NH4Cl at Low Nitrogen Pressure

Combustion synthesis (CS) of Si3N4 was accomplished by using as-milled Si/NH4Cl as reactants at low nitrogen pressure. The additive of NH4Cl decreased the combustion temperature and promoted the Si nitridation. Full nitridation of Si was achieved by burning Si in pressurized nitrogen with 10 ~ 25 wt. % NH4Cl as additives while no Si3N4 diluent added. The maximum combustion temperature (Tc), the combustion velocity (u) together with the α-Si3N4 content and mean particle size (d50) of the powder products were found to be great dependent on the NH4Cl content added in the reactants. Fine Si3N4 powder products with α-phase content up to 85 wt. % were obtained via steady combustion mode. A mathematical approach named combustion wave velocity methods for the analysis of temperature profiles in CS was proposed and the reaction kinetics was discussed. The apparent activation energy calculated according to the temperature profile analysis method is 29.7 kJ/mol, which agrees well with the corresponding low temperature nitriding combustion of Si.

Electric Field Enhanced Synthesis of Nanostructured Tantalum Carbide

Nanocrystalline TaC was synthesized by the field-activated combustion method. The crystallite size ranged from about 30 to 55 nm, depending on the applied field. At low fields (8.54 ≤ E < 16.39 V cm−1) the average crystallite size was relatively unaffected by the field, but it showed a significant increase at fields higher than 16.39 V cm−1. From temperature measurements, this field was found to coincide with the melting of Ta. The combustion wave velocity likewise showed a significant increase when the temperature was at the melting point. The composition of the product showed a dependence on the magnitude of the applied field. At low field values (above a threshold) the product contained Ta2C. When synthesized at high fields, the product showed the presence of TaC phase only. The lattice parameter and the C/Ta ratio showed a slight dependence on the field, both increasing with an increase in the magnitude of the field.

Effect of precursors on the combustion synthesis of TiC-Al2O3 composite

In the combustion synthesis of TiC-Al2O3 composites from TiO2, Al, and C, different types of precursors, i.e., anatase and rutile TiO2, and graphite and carbon black were used. The measurements of combustion wave velocity showed that the reactivity was higher with graphite as a carbon source and anatase as a TiO2 source. This was also confirmed by analyzing the degree of conversion of reactants to products. For the precursors investigated, the wave propagation of the 3TiO2-4Al-3C system diluted by Al2O3 showed that the limit of wave propagation was more dependent on carbon source than on TiO2. The wave front of all the samples proceeded in unstable combustion mode. Temperature profiles in the reaction zone suggested that the heat-affected zone was broader in the case of graphite carbon source than carbon black. The microstructures and chemical composition of the products were also studied.