Vol 26, No 1 (2017)

Propagation of solid flame along a wire rolled in spiral was explored theoretically. Besides conductive heat transfer along the wire, a contribution from relay-race heat exchange between neighboring spiral loops was taken into account. Combustion of spiral was found to occur in a mode of pseudo-conductive or pseudo-spinning combustion. In the former case, the temperature profile of combustion was close to that in a straight wire. A contribution from relay-race heat transfer between spiral loops can be taken into account as an elevated thermal diffusivity of wire. A specific feature of pseudo-spinning combustion is the formation of a hot spot with a temperature exceeding the thermodynamic combustion temperature. Variation in a pitch and length of spiral loop can be readily used to regulate the temperature and burning velocity of spiral combustion.

Azide SHS technology with NaN₃ powder as a nitriding agent was used to fabricate TiN–BN, AlN–BN, and Si₃N₄–TiN nanopowders using Nа₂TiF₆, (NH₄)₂TiF₆, KBF₄, NH4BF₄, Nа₃AlF₆, AlF₃, Na₂SiF₆, and (NH₄)₂SiF₆ as precursors. Mixtures of the above salts with sodium azide NaN₃ readily burned in an SHS reactor to yield agglomerated nanosized or fine target powders. Phase composition of combustion products not always corresponded to expectations and often contained insoluble byproducts. Reported are optimal green compositions and procedures that afford the preparation of target TiN–BN, AlN–BN, and Si₃N₄–TiN nanopowders free of byproducts.

Solid solutions Ni_0.5–xCd_xZn_0.5Fe₂O₄ (x = 0, 0.15, 0.30) were prepared by solid-state synthesis and characterized by FTIR spectroscopy. The FTIR spectra of synthesized ferrites showed two absorption bands (ν₁ and ν₂) in the range 400–600 cm^–1 belonging to tetrahedral (A) and octahedral (B) interstitial sites in the spinel lattice. The force constants for tetrahedral (K_t) and octahedral sites (K_o) were determined, as well as Young’s modulus (E), rigidity modulus (G), bulk modulus (B), Debye temperature (Θ_D), and velocity of transverse (V_t) and longitudinal (V_l) elastic waves. The relevant interionic cation–anion, cation–cation distances and bond angles are also reported.

The SHS waves in multilayer Ni/Al nanofilms and the explosive crystallization waves in Sb films deposited onto a thin copper substrate were comparatively explored by IR imaging and SEM. Despite a markedly lower thermal effect of crystallization, the waves of chemical reaction and crystallization were found to have much in common in their temperature–time histories: warmup rate, temperature gradient in the wave front, and duration of heat release. Observed was a micro-scintillation mode of the propagation of explosive crystallization wave in Sb films.

Mineral-like Y₂(Ti_1–xZr_x)₂O₇ ceramic matrices for immobilization of actinide–Zr–RE-containing high-level nuclear wastes (HLW) were prepared by SHS method. In experiments, HLW were modeled by a mixture of CeO₂, La₂O₃, ZrO₂, MnO₂, and Fe₂O₃ powders. An increase in the HLW content of green mixture decreased the amount of Y₂(Ti_1–xZr_x)₂O₇ in combustion product and increased that of ZrO₂, LaTiO₃, and CaTiO₃; decreased the fractional substitutionality of Zr for Ti; and increased the product porosity. An increase in combustion temperature and suppression of heat sink during SHS reaction did not diminish markedly the porosity of synthesized ceramics.

Explored was the feasibility of preparing (Ti_0.8Cr_0.2)B₂–CrB–TiN ceramics by pressure-assisted SHS from reactive powder compacts containing TiN nanopowder as an inert modifier of product structure. The combustion products obtained under optimized conditions exhibited a uniform structure with a porosity of 1–3% and a Vickers hardness of 22.5 GPa.

Magnesium ferrite (MgFe₂O₄) was prepared by simple low-cost oxalate coprecipitation method and characterized by XRD and SEM. The X-ray analysis confirmed the formation of a single-phase cubic spinel structure. The product revealed a non-uniform morphology and some certain extent of agglomeration. Crystallite size, texture coefficient, dislocation density, hopping length, and mechanical properties of the product are also reported.