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

Light turquoise pigment was synthesized by solution combustion of a mixture containing Al(OH)₃, solutions of boric, citric, and phosphoric acids, and copper nitrate and subsequent annealing at temperatures up to 900°C. To increase the thermal stability of pigments, the synthesis was carried out using an aluminophosphate binder. The synthesized pigments were found to contain amorphous borates, boron phosphate, phosphates and pyrophosphates of aluminum and copper. The copper phosphide CuP₂ impurity formed during the synthesis process and subsequent annealing at 700°C gives the pigment a grayish tint but increases its anticorrosion properties.

MgAlON were prepared by self-propagation high temperature synthesis (SHS) using powder and granular mixtures of aluminum, aluminum oxide, magnesium oxide, and magnesium perchlorate. The influence of granulation of starting particles of Al + Al₂O₃ + MgO + Mg(ClO₄)₂ powder mixtures on the microstructure and phase composition of combustion products was studied. It was revealed that the granulation of mixtures reduces the combustion temperature and burning velocity. It was found that the combustion products derived from granular mixtures consists of up to four phases (MgAlON, aluminum oxide, aluminum nitride, and unreacted aluminum), while the products of powder mixtures are represented by single MgAlON phase.

Using scanning electron microscopy, X-ray phase analysis, and hardness measurement we investigated the structure, phase composition, and mechanical properties of Ni₃Al–TiC composite (TiC content varied in the interval from 0 to 30 vol %) fabricated by self-propagating high-temperature synthesis in the thermal explosion mode from a powder mixture of nickel, aluminum, and titanium carbide. It was found that the synthesis of Ni₃Al intermetallic compound occurred almost completely when TiC content in the green powder mixture was up to 15 vol %. TiC particles were arranged in clusters and individually. Each particle, including in the clusters, was surrounded with the matrix material. The hardness of the composite essentially increased with an increase in the TiC content in the green powder mixture up to 10 vol %. Then the hardness gain was slow. The matrix of the composite contained Ni₃Al and NiAl intermetallic phases as well as unreacted nickel when the fraction of TiC in the green powder mixture increased to 30 vol %. TiC particles were adjacent to each other in the clusters and there was a free volume between them. Thus, it was concluded that the synthesis of Ni₃Al–TiC composite under thermal explosion condition from the mixture of nickel, aluminum, and titanium carbide powders satisfactorily took place when the fraction of titanium carbide in the green powder mixture was 15 vol % and less.

A study was made of the combustion of powder and granular mixtures (1 – X)(Ti + C) + X(5Ti + 3Si), where 0 ≤ X ≤ 1, on the base of titanium powder with the characteristic size of Ti particles d(Ti) = 120 μm. The values of the burning velocity of powder mixtures were shown to depend on the free volume above the charge in the reactor. The velocity dependences on X had a maximum at about X = 0.4 in contrast to minimum in the earlier study of the same mixtures with d(Ti) = 20 μm. The results were explained using the convective–conductive model of combustion. It was shown that the velocity of the combustion front of the powder mixture depends on the location of the predominant release of impurity gas in the charge: in front of or behind the melt layer. The maximum content of the liquid phase in the reaction mixture at 0.4 < X < 0.6 ensured the high filtration resistance of the melt layer and its maximum (at d(Ti) = 120 μm) or minimum (at d(Ti) = 20 μm) propagation velocity. Changing the structure of mixtures by granulation ensured the leveling of the effect of impurity gases and a decrease in the burning velocity corresponding to the decrease in the content of desorbing components, titanium and soot.

An inhomogeneous model of gas-free combustion of a mixture consisting of activated and non-activated cells was proposed. The structure of the combustion wave was studied to suit the scale of heterogeneity, reaction cell size. The influence of gas dispersed layers separating the reaction cells on the burning velocity of a combined mixture was analyzed. The burning velocity dependences at varying effective porosity, effective thermal conductivity, and external heat transfer parameters were constructed.

Decagonal quasicrystals in Al–Co–Ni and Al–Co–Cu systems were first prepared by SHS method. XRD analysis showed that the synthesis of Al–Co–Cu system yields Al₇₀Co₁₅Ni₁₅ quasicrystalline phase; meanwhile, in the Al–Co–Ni system, the synthesized product contains Al₇₀Co₁₅Ni₁₅ quasicrystals as a basis with minor addition of cubic Al₅₈Co₇₆Ni₆₆ phase. Both synthesized materials have weak magnetic properties with maximum magnetization of 0.145–0.730 emu/g for the applied magnetic field of 10 kOe.

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.