D. Yu. Kovalev

Effect of mechanical activation on thermal explosion in Ni-Al mixtures

The effect of mechanical activation (MA) on thermal explosion in equimolar Ni-Al mixtures was studied by time-resolved XRD. MA was also found to increase the burning velocity and decrease the ignition temperature. Thermal explosion in non-activated Ni-Al mixtures was found to proceed via formation of liquid (melted) intermediate products, while that in the activated mixtures gave no liquid intermediates.

Thermal decomposition of TiH2: A TRXRD study

Thermal decomposition of SHS-produced TiH2 powder in vacuum at temperatures below 570°C was explored by time-resolved XRD. The process got started with transformation of starting TiH2 into solid solution of hydrogen in β-Ti (β-Ti[H]) and then followed by the polymorphic β-Ti[H] → α-Ti[H] transition and subsequent hydrogen elimination to yield α-Ti. Marked changes in the phase composition of starting TiH2 were found to get started around 450°C.

Dynamics of Phase Formation during Combustion of Zr and Hf in Air

The processes of phase formation taking place during combustion of Zr and Hf in air have been explored by time-resolved XRD (TRXRD) at a time resolution of 0.1 s. In case of Zr combustion, the final product was found to form in two stages: first the tetragonal high-temperature modification of ZrO2 is formed (in the combustion wave) and then the latter undergoes polymorphic transition (behind the combustion wave) into its monoclinic modification. In combustion of Hf, product formation proceeds in three stages: the low-temperature monoclinic modification of α-HfO2 is formed in the combustion wave, which is followed by its transformation into the tetragonal β-HfO2 and backward conversion of the latter into α-HfO2 behind the combustion wave. The obtained data were comparatively analyzed with those reported for combustion of Ti in air.

Solution Combustion Synthesis: Dynamics of Phase Formation for Highly Porous Nickel

The combustion of reaction media produced bydissolving the initially solidphase chemical substances in liquid media—the socalled solution combustion synthesis (SCS)—is a new method for production of nanomaterials [1]. The composition of thesolutions is determined according to the main principle of selfpropagating hightemperature synthesis(SHS), namely, the rule that the heat released in theprocess should be sufficient for the selfsustainingpropagation of a chemical reaction [2]. The main difference between SCS and the classical SHS is in themicrostructure of the initial reaction medium. ForSHS, mixtures of solid powders are typically used andthe scale of the heterogeneity of a reaction mixture isdetermined by the solidphase particle size (usually 1–100 μm), whereas reactants in solutions are mixed at avirtually molecular level. The latter simplifies the synthesis of nanomaterials at high temperatures of thecombustion wave. Although a lot of compounds(mainly oxides) have already been produced by SCS,the combustion mechanism, reaction wave structure,and phase formation dynamics in this process havebeen poorly studied.The general equation of a chemical reaction withmetal nitrate as an oxidant and glycine as a fuel can berepresented as(1)where M is a metal, ν is the metal valence, ϕ is the fuel(glycine)tooxidant (metal nitrate) weight ratio, andthe function

Silicon carbide ceramics SHS-produced from mechanoactivated Si–C–B mixtures

We investigated the influence of mechanical activation (MA) and initial temperature T0 on combustion temperature Tc and burning velocity Uc for 90% (Si + C)–10% (4B + C) mixtures as well as on product morphology. Mechanical activation was found to strongly change the size and morphology of Si and graphite particles (by a factor of 6–8), lead to accumulation of micro strains, and increase the reactivity of green mixture. An increase in T0 was found to proportionally increase the values of Tc and Uc but produce little or no influence on the phase composition of product. The SiC powder consisting of 50–100 nm crystallites was obtained by diminution of combustion product and then used to prepare, by hot pressing, a ceramic PVD target for magnetron sputtering.

SHS of TiC-TiNi composites: Effect of initial temperature and nanosized refractory additives

For SHS reactions in Ti-Ni-C powder blends yielding TiC-TiNi composites, investigated was the effect of initial temperature (T0), green composition, and nanosized refractory additive (alloying agent) on combustion parameters and structure formation in combustion products. An increase in T0 elevated combustion temperature T and burning velocity U, while the addition of micro- and nano-sized ZrO2 particles diminished the above parameters due to partial blocking of reactive Ti-Ni contacts by ZrO2 agglomerates. At the same time, the addition of alloying agents enlarged the number of crystallization centers for the growth of primary TiC grains in the melt.

Deposition of composite metallic coating onto Al through mechanical impregnation followed by thermal treatment

Investigated were the processes taking place in thermally treated Al plates mechanically impregnated with a mixture of metallic particles. Vibratory treatment of an Al surface in a powder mixture of metals was found to result in destruction of the surface Al2O3 film and formation of extensive physical contacts between the Al matrix and the metallic particles mechanically impregnated into the near-surface layer of Al. Subsequent thermal treatment was then used to launch chemical reaction yielding intermetallides within the impregnated layer. Performing SHS reactions yielding melted intermetallides in the coatings (or pellets) deposited (or placed) onto the surface of thus treated Al plates, one can obtain strong weld joining between solidified SHS products and Al. This approach can also be used for deposition of coatings with a desired composition onto Al substrates.

SHS of MAX compounds in the Ti-Si-C system: Influence of mechanical activation

Mechanical activation of Ti-Si-C green mixtures was found to negatively affect the composition, microstructure, and properties of synthesized Ti3SiC2.

Formation of nanolaminate structures in the Ti-Si-C system: A crystallochemical study

Combustion synthesis of laminate Ti3SiC2 structures from the elements was explored by time-resolved XRD. Crystallochemical modeling and quantum-chemical calculations suggest that the laminate structure of Ti3SiC2 arises due to regular accumulation of difference between the bond lengths of Ti3SiC2 and TiC and population of thus formed vacancies with Si atoms in the TiC lattice.

SHS hydrogenation of titanium: Some structural and kinetic features

Hydrogenation of Ti sponge was performed in conditions of hydrogen deficiency (a) and excess (b). The extent of conversion was low and non-uniform in case (a) and close to unity in conditions (b). The combustion products were characterized by XRD, SEM, time-of-flight mass spectrometry (TOF MS), and time-resolved XRD.