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

The combustion initiation of condensed mixture with unlimited mass by limited high-temperature spot was mathematically modelled. It was shown that in contrast to the sample critical diameter, at which the combustion dies out due to heat loss, the critical radius of the hot spot upon initiation of large-volume gas-free mixture is determined only by its physicochemical characteristics (Zel’dovich number).

In this study, we investigate the self-propagating high-temperature synthesis (SHS) and consolidation of heterophase MoSi₂–MoB ceramics alloyed with ZrB₂. A thermodynamic analysis of the combustion temperature (T_ad) and the equilibrium composition of synthesis products was performed for the Zr–Mo–Si–B system. The effect of varying Zr and B concentrations on combustion kinetics was studied in detail. The resulting heterophase SHS powders showed high structural and chemical homogeneity, though were noticeably agglomerated. We identified optimal consolidation conditions and achieved compact ceramics with a phase composition identical to the original SHS powder. The ceramic structure consists of a matrix of MoSi₂ grains with interspersed needle-like ZrB₂ grains and polyhedral inclusions of MoB. This work establishes a basis for the preparation of MoSi₂–MoB–ZrB₂ ceramics with excellent hardness, fracture toughness, thermal conductivity, and high-temperature oxidation resistance.

Experimental dependences of the combustion velocity on the size of titanium particles for powder and granular mixtures of 5Ti + 3Si, Ti + C^am, (Ti + C^am) + 20% Cu, (Ti + C^am) + 20% Ni, Ti + C^cr (with amorphous carbon in the form of soot and with crystalline carbon in the form of graphite) were compared. The results of experiments were explained by the retarding effect of impurity gases in powder mixtures when the conditions of warming up the particles before the combustion front were met. For all the studied granular mixtures, where the influence of impurity gases on the combustion velocity was leveled, analytical dependences of the combustion velocity on the size of titanium particles were in good agreement with the conclusions of the convective–conductive combustion model.

Lithium aluminate samples were obtained in reactions of solution combustion synthesis (SCS) with various types of fuel (glycine, leucine, and urea) from aluminum and lithium nitrate solutions. The simultaneous thermal analysis (STA) of precursors obtained in conditions of fuel and oxidizer stoichiometry showed the presence of impurities due to incomplete decomposition of initial salts containing carbon fragments of fuel and nitrate groups. An exception was the precursor from the dual-fuel SCS reaction, φ (glycine : urea) = 1 : 3, in which pure γ-LiAlO₂ powder was formed. Replacement of lithium nitrate with lithium carbonate was found to reduce the process temperature and the relative amount of organic fuel. As a result, the content of carbon fragments in the precursor significantly decreased after synthesis.

TiC–NiTi cermet composites were fabricated by forced SHS compaction from (Ti + C + Ni) mixtures with varying content of Ni + Ti from 28 to 90 wt %. The phase composition and microstructure were characterized by X-ray diffraction method, scanning electron microscopy, and energy dispersive spectroscopy. The microstructure of composites was shown to contain spherical TiC particles surrounded by the matrix including TiNi₃, TiNi(B2), Ti₂Ni, and Ti₃Ni₄ phases. As Ni + Ti content was increased, there was a change in the stoichiometry of TiC (from TiC₁ to TiC_0.43) and its particle size (from 6.5 to 0.2 μm). It was found that the microhardness of TiC–NiTi composites dropped markedly from 23.5 to 6.4 GPa.

The optimum composition of components in the Hf/PTFE system was determined by thermodynamic calculation. The composition 65Hf/35PTFE (in wt %) was chosen based on the maximum adiabatic combustion temperature (T_ad = 2381°C) and the fraction of condensed products (70 wt %). The study on the ignition of compositions in argon, air, and vacuum showed that in the latter case, the intensity of ignition decreases. The maximum combustion temperature and rate in argon were found to be 2250°C and 4.5 mm/s for compositions with 10 and 15 wt % Al. XRD analysis revealed the formation of a monophase HfC product in all compositions. Shock-wave loading of compositions with a steel plate at an impact velocity of 1 km/s showed the absence of exothermic reaction in the 65Hf/35PTFE composition. Increasing the impact velocity to 1.5 km/s resulted in an exothermic reaction in this composition. The maximum yield of HfC under shock-wave loading was achieved in the composition 62Hf/33PTFE/5Al, indicating its high reactivity. Thus, this composition is the most optimal for use as a reactive material.

A brief review of the history, current state, and prospects of using titanium powders to produce porous products and in SHS in Belarus, as well as in the world, was presented. New methods for obtaining materials, billets, raw materials, and products from commercially pure titanium powders were considered. Promising directions of applying methods and technologies of powder metallurgy and SHS using titanium powders toward producing new types of products were shown.

TiB₂–Ti metal–ceramic composites were produced by forced SHS compaction. The influence of superstoichiometric amount of Ti (6–20 wt %) relative to TiB₂ on the regularities of combustion and phase composition/structure formation of TiB₂–Ti composites was studied. It was shown that the phase composition of composites is inconsistent with the Ti–B phase diagram. The microhardness of composites was determined to drop from 34 to 18 GPa with increasing the binder content. Cutting properties of composites were evaluated.

Calcium phosphates are materials of wide interest in the medical and agricultural industries. Advanced in the standardization of protocols to obtain them by combustion in solution in a single step is highly desirable. This is particularly so for the alpha-tricalcium phosphate (α-TCP) phase, widely used in the development of bone cement. In this research, citrulline was used as a fuel for the first time in the one-step synthesis of calcium phosphates, and the effect of adding 0–15 g of ammonium nitrate (AN) as an extra oxidizer agent was studied, allowing the temperature of the reaction to increase from 648 to 950°C. The temperature was measured using low-cost equipment with infrared and type-K thermocouple and contrasted with the estimated theoretical values of adiabatic flame temperature. X-ray diffraction analysis showed the formation of a mixture of α-TCP and hydroxyapatite phases at the lowest combustion temperature, corresponding to 0 g of AN. When the amount of AN was increased, the XRD showed that α-TCP was obtained as a metastable phase, while the amount of hydroxyapatite decreased considerably. At higher combustion temperatures, the crystallite size estimated by Scherrer equation, in turn, increased to a value of 51.61 nm. The SEM images showed the presence of necks between particles in the powders that coincide with the estimation of high reaction temperatures.

The structure as well as the phase and granulometric compositions of the submicron-sized heterophase HfB₂–34 at % HfC powders fabricated by mechanical activation assisted self-propagating high-temperature synthesis from (Hf + B + C) mixtures were studied. It was demonstrated that HfB₂–HfC powders can be produced from (Hf + B + C) mixtures by mechanochemical synthesis in a planetary ball mill (centrifugal factor, 60 g) during more than 15 min. The SHS product with composition HfB₂–34 at % HfC consisted of a combination of highly porous agglomerates sized 5–100 µm, which can be easily broken into composite HfB₂–HfC particles sized 1–10 µm. Important that each composite particle of powder have a heterophase structure which consists of HfB₂ grains sized 0.5–2.0 µm and equiaxial HfC grains sized 0.3–1 µm. Impurity oxygen content in the SHS products did not exceed 0.29 wt %. Milling of the SHS product allowed to obtain the HfB₂–34 at % HfC powder characterized by the average particle size of 4 µm with heterophase submicron-sized microstructure and oxygen content of 0.72 wt %.