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Self-propagating high-temperature synthesis (SHS) was used to prepare SHS compositions based on the system Si3N4–Yb2O3 system. Synthesis was conducted in a nitrogen atmosphere at 4 MPa in a 30 L reactor. The weight of the loaded charge was 3 kg. It was ascertained that the amount of ytterbium oxide in the charge affects the combustion temperature and the phase composition of the synthesis products. As the proportion of ytterbium oxide in the charge increases, the α-Si3N4 content decreases, and the secondary phases are represented by quaternary silicon ytterbium oxynitride and ytterbium disilicate. [ABSTRACT FROM AUTHOR]

Combustion and the formation of structures and phases in a 2Co–Ti–Al system during self-propagating high-temperature synthesis in a thermal explosion are under study. It is determined that a single-phase product (a Co2TiAl Geissler compound) can be obtained in this system. Morphology, microstructure, and physical-magnetic properties of combustion products of the system are investigated. [ABSTRACT FROM AUTHOR]

A cermet material based on the system Ni–Al–SiO2 with different SiO2 weight content (0 – 50%) in the mixture was obtained by means of self-propagating high-temperature synthesis (SHS) with preliminary mechanoactivation (MA). It is shown that preliminary mechanoactivation of the powder mixture strongly affects the morphology of the final product. Depending on the composition of the initial mixture, the phases NiAl, Al2O3, Ni2Si, and NiSi2 can form in the final product of synthesis. [ABSTRACT FROM AUTHOR]

Ni–Al–Co alloy with the composition 45 wt % Ni + 41 wt % Co + 14 wt % Al has been prepared for the first time by aluminothermic reaction. The phase composition of the synthesized alloy comprises a cubic (Ni,Co)3Al solid solution of cobalt in nickel aluminide (γ'-phase) and a tetragonal (Ni,Co)3Al phase. The formation of the tetragonal structure of (Ni,Co)3Al is due to a martensitic transformation of a supersaturated initial structure in grains of the cubic (Ni,Co)3Al phase. The intragranular microhardness of the synthesized alloy is 6500 MPa. The alloy has soft magnetic properties, with the highest magnetization of 27 emu/g in a magnetic field of 10 kOe. [ABSTRACT FROM AUTHOR]

Various macrokinetic modes of interaction (self-ignition or combustion) of compact samples from nonpassivated (pyrophoric) and passivated iron nanopowders with air are under study. Experiments show that the modes of interaction with air depend on the type of the used gaseous medium (argon or air), previously containing weighting cups with samples, and on how long the bottles remain in air. The possibility of passivation of pressed samples from pyrophoric iron nanopowder is experimentally established for the first time in the case where the weighting cups with samples remain in air. Various experimental methods are used to investigate the sample heating dynamics and the effect of density inhomogeneity along the length of the sample. It is revealed that pyrophoric samples are heated nonuniformly, although heating begins simultaneously over the entire surface of the sample. [ABSTRACT FROM AUTHOR]

The use of nanopowders implies the ability to safely store them without changes in their physicochemical parameters ensured by passivation. In this work, we have determined the thermal stability range in air for compact samples of different diameters prepared from pyrophoric nickel nanopowders and then passivated. The results demonstrate that compact samples prepared from nickel nanopowder can be safely stored in air at temperatures of up to about 200°C without additional oxidation, which points to their high thermal stability. The lack of a noticeable heat release during slow heating to 200°C and the presence of only Ni in both the passivated and heat-treated samples according to X-ray diffraction data suggest that the oxide layer is very thin or that there are noncrystalline oxide phases. The observed uniform oxygen and nickel distributions over fracture surfaces of samples after heating suggests that their interaction with air during both the passivation process and heating to temperatures on the order of 200°C is a bulk process. [ABSTRACT FROM AUTHOR]

Combustion of mechanically activated mixtures in 2Co–Ti–Al and Co–2Ti–Al is investigated, and alloys based on them are obtained by self-propagating high-temperature synthesis (SHS). Microstructural, X-ray, and differential-thermal studies are performed. A Heusler phase Co2TiAl-based alloy is obtained for the first time by means of mechanical activation. Conditions for the mechanical activation of a reacting powder mixture for the formation of a single-phase material is experimentally selected. [ABSTRACT FROM AUTHOR]

It was experimentally shown for the first time that compact samples of pyrophoric nickel nanopowders can be passivated in air for 3–5 s. The passivation of the samples was confirmed by the absence of noticeable heat release during slow heating to 200°C, an insignificant oxygen content according to the data of X-ray powder diffraction analysis and scanning electron microscopy, and the retention of the chemical activity of the samples. The uniform distribution of oxygen throughout the sample suggests the volumetric nature of their interaction with air during the passivation. The critical conditions for heating of passivated samples under which ignition occurs were determined. [ABSTRACT FROM AUTHOR]

We have examined for the first time the feasibility of producing cast Nb-, Si-, and B-doped Mo-based composite materials by a spin-casting self-propagating high-temperature synthesis metallurgy method and obtained new results regarding control over their composition and structure. In synthesis, we used combinations of a highly exothermic (MoO3/Nb2O5/Al/Si/B) and a low-exothermic (Mo/Nb/Si/B) composition. We have assessed the effect of the ratio of the ratio of the highly exothermic and low-exothermic compositions (α) and acceleration (a) on the composition and structure of the cast composite materials. The mixtures have been shown to be capable of combustion at up to 80% low-exothermic composition. The combustion products have cast form in the range 0 ≤ α ≤ 65%. At α in the range from 0 to 40% and accelerations from 40g to 400g, the combustion products have the form of a two-layer ingot: the lower layer (Mo–Nb–Si–B) consists of the required product and the upper layer (Al2O3) is a slag product. The optimal parameters for the preparation of cast Mo–Nb–Si–B materials are α = 20–30% and a > 100g. [ABSTRACT FROM AUTHOR]

We have studied the combustion process and the phase composition of its products using granulated 70 wt % (Ti + C) + 30 wt % (3Cr + 2C) and 80 wt % (Ti + C) + 20 wt % (3Cr + 2C) mixtures with nickel-containing binders. The results demonstrate that the addition of a nickel binder reduces the chromium content of the (Ti,Cr)C double carbide, because nickel reacts with chromium to form Nichrome. In the case of combustion of the granulated 80 wt % (Ti + C) + 20 wt % (3Cr + 2C) mixture with the addition of Kh20N80 Nichrome, a Nichrome-binder-containing (Ti,Cr)C double carbide essentially free of impurity phases according to X-ray diffraction data has been prepared in a single step. A two-step mechanism of final product formation has been proposed. According to scanning electron microscopy data, the grain size of the combustion products is 2–5 μm, which is an order of magnitude smaller than the initial titanium and chromium particle size. In contrast to powder mixture combustion products, which are difficult to disintegrate, the agglomerate obtained via the combustion of a granulated mixture broke into separate granules, which could be ground to a desired particle size under laboratory conditions. [ABSTRACT FROM AUTHOR]

Dramatic reductions of the linewidths of well-known deep centers in 28Si reveal "isotopic fingerprints" of the constituents. The approximately 1014 meV Cu center, thought to be either a Cu pair or an isolated Cu, is shown to contain four Cu atoms, and the approximately 780 meV Ag center is shown to contain four Ag. The approximately 944 meV ;{*}Cu center, thought to be a different configuration of a Cu pair, in fact contains three Cu and one Ag, and a new two-Cu two-Ag center is found. The approximately 735 meV center, previously assigned to Fe, actually contains Au and three Cu. This suggests a family of four-atom (Cu, Ag, Au) centers.