Vol 28, No 4 (2019)

Metallothermic SHS in conditions of artificial gravity was numerically modelled for the 3NiO + 2Al → Al₂O₃ + 3Ni reaction taken as an example. The process was assumed to include (a) high-temperature combustion reaction yielding liquid products, (b) their gravity-assisted separation, and (c) cooling down. In our ‘throughout’ mathematical model, a three-component emulsion—gas, metal, and ceramics—with individual translational velocities and temperatures was considered. Our model may expectedly extend the range of control means for SHS reactions in extreme conditions.

Solution combustion synthesis (SCS) of zinc oxide was performed using a binary fuel, glycine and citric acid. It was established that combustion occurs due to oxidation of zinc nitrate–glycine complexes. Citric acid acts as an inhibitor of SCS reaction. An increase in relative content of organic fuel in the solution leads to a reduction in maximal combustion temperature and to formation of elemental carbon (0.2–1.6 wt %) and organic fragments (1.55–3.29 wt %) in SCS-produced zinc oxide. Carbon impurity and organic fragments were removed by annealing at 600°С. The produced wurtzite-type ZnO crystals had a size of 27–37 nm and were assembled into agglomerates. After annealing at 500°С, the specific surface of the powder was 8.44–11.09 m²/g. The photocatalytic activity of ZnO powder was evaluated from the rate of hydroquinone photodecomposition in solution.

The paper reports on the thermodynamic analysis of the CaO–Y₂O₃–ZrO₂–Ti–Fe₂O₃ system as a precursor for SHS-produced pyrochlore-based ceramics. Adiabatic combustion temperature T_ad and equilibrium concentration of pyrochlore were first calculated for the quasi-ternary system Y₂O₃–ZrO₂–(Ti + Fe₂O₃) containing no CaO. It was found that, in the presence of CaO, a best yield of Zr-enriched pyrochlore ceramic can be reached within the following domain of green compositions (wt %): CaO 2.5–5.0, Y₂O₃ 22.5–35.0, ZrO₂ 7.5–22.5, and (40% Ti + 60% Fe₂O₃) 45.0–60.0; within this domain, T_ad = 2100–2400°K. Although in control experiments T_ad was found to be within the range 1700–1970 K, nevertheless the combustion products always showed the presence of pyrochlore-based ceramic, as predicted by thermodynamic analysis.

Porous Ni–Al–CGO cermet (CGO = Сe_0.9Gd_0.1O₂) for use in solid oxide fuel cells was fabricated by thermal explosion (volume reaction) in Ni–Al–CGO powder compacts in different heat sink conditions. Temperature profiles of thermal explosion were recorded and analyzed as a function of green composition. Phase composition of resultant porous materials was found to depend on the CGO content of green mixture and temperature of vacuum annealing. Starting and final materials were characterized by XRD, SEM, and EDS. Synthesized uniform cermets with a porosity of 50–60% can be recommended for use as a support for solid oxide fuel cells with Ni/CGO anode.

One-step solution-combustion synthesis with glycine as a fuel was used to obtain ferromagnetic nickel ferrite (NiFe₂O₄) spinel. According to EDX data, the elemental composition of all synthesized samples corresponded to NiFe₂O₄, while the XRD results showed the formation of phase-pure nickel ferrite spinel. NiFe₂O₄ nanopowders had a branched porous microstructure as established by SEM analysis. Variation in the Red/Ox ratio (glycine to nitrate ratio G/N = 0.4, 0.6, 0.8, 1.0, 1.2) was found to affect the average size of nickel ferrite crystallites (D) within the range 23–37 nm. The results of vibration magnetometry showed the ferromagnetic ordering in magnetic moments of NiFe₂O₄ nanopowders. The magnetic parameters of synthesized nickel ferrite—saturation magnetization M_s = 31–59 emu/g, remanent magnetization M_r = 3–13 emu/g, and coercive force H_c = 10–95 Oe—were found to depend on crystallite size D. The fact that the values of M_s, M_r, and H_c grow with increasing D opens up a way to synthesis of NiFe₂O₄ nanopowders with controllable magnetic parameters by simply varying the G/N ratio of starting solution.