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

A new mode of combustion of granular mixtures Zr + 0.5C in a co-current argon flow with the formation of a finger-like front instability was discovered. The observed phenomena were explained under the assumption of a decrease in the permeability of synthesis products due to significant shrinkage of the sample in the longitudinal direction, which prevented the filtration of argon through the products. Redirection of the gas flow into the gap between the granules and the side surface of the cylindrical reactor contributed to the formation of a finger. It was shown that a decrease in the sample size in the longitudinal direction is provided by the pressure drop of argon, and a decrease in the cross-section occurs under the action of surface tension forces due to a transverse temperature gradient.

Alumina (Al₂O₃) is widely used in a variety of applications because it has superior physical and chemical properties which are high heat resistance, excellent electrical isolation, abrasion resistance, and high corrosion resistance. Generally, alumina is manufactured with a purity of 99.6–99.9% mainly by the Bayer process with bauxite as the starting material. It is used in refractory products, spark plugs, IC substrates, and so on. High-purity alumina (HPA), which has a purity of more than 99.99% and has a uniform fine particle, is widely used in translucent tubes for high-pressure sodium lamps, single crystal materials such as sapphires for watch covers, high-strength ceramic tools, abrasives for magnetic tape, and the like. In recent years, the demand for high-purity alumina has been expanding in fields that are expected to show a high growth rate e.g., display materials, energy, automobiles, semiconductors, and computers. There are several complicated processes reported in literature to produce single phase α-HPA, which consumes more energy, and power and are very costly. In this paper, we report the preparation of nano-α alumina powders with a purity of 3N (99.9%) by a simple, economical, and faster method i.e., one-step auto combustion method. To obtain single phase α-alumina, the calcination temperature required is 1200°C but, in our work, we achieved single phase α-alumina at 500°C temperature by one-step auto combustion method. The as-prepared HPA is characterized through XRD, BET surface area, SEM and ICP, TGA, and LIBS to test for purity and its application in LED fabrication.

Titanium carbide (TiC) powder was synthesized by the magnesiothermic combustion of the TiO₂-rich alteration product leucoxene and activated carbon (AC) in argon. Leucoxene and C were combined at a molar ratio of 1.0 : 1.5, and the effect of magnesium (Mg) fuel in the reaction system was studied at ratios of 1.0, 1.5, 2.0, 2.5, and 3.0. XRD analysis showed that the as-leached powder from a reactant mixture with a Mg molar ratio of 3.0 has fewer unwanted phases, and that leucoxene, C, Mg mixed at 1.0 : 1.5 : 3.0 produce TiC powder of a higher purity than the other reacted mixtures. The higher purity of the product was due to the more exothermic character of the combustion reaction, which had a higher enthalpy of reaction (ΔH) and adiabatic temperature (T_ad). SEM observation of the as-leached powder revealed agglomerated fine particles of sub-micrometer size. The TiC powder was successfully coated with nickel by an electroless plating process. SEM/EDX demonstrated that the Ni-coated TiC powder consists of Ni particles smaller than 500 nm, which are well distributed on TiC particles.

The fabrication of ternary Cu–Mn–Al alloys through self-propagating high-temperature synthesis and their thermoelectric properties, such as electrical resistivity and the Seebeck coefficient in a wide range of temperatures, were studied. XRD analysis identified the Heusler compound Cu₂MnAl in the synthesized compositions as a main precipitated phase. In addition, different secondary phases (Al₂Cu, Cu₃Al AlCu, and Mn) appear depending on an excess of aluminum or manganese in the green mixture. The magnetization (M–H) curves constructed for the studied alloys showed their weakly ferromagnetic behavior.

The initiation of combustion of a large mass of condensed mixture being in local contact with the end of the burning layer was mathematically modelled and experimentally studied. The influence of the instability of stationary combustion front in the igniting layer on the penetration of gasless combustion into large volume was showed. It was revealed that the critical conditions for initiating combustion of large volume are determined by front curvature.

The combustion of mixtures consisting of titanium powder prepared by SHS hydrogenation and dehydrogenation and black carbon/ boron was studied. It was shown that the oxygen and hydrogen content in Ti powder makes an impact on the combustion characteristics of Ti + C mixtures. It was found that an increase in the hydrogen concentration reduces the combustion temperature, meanwhile, its value is increased with increasing the oxygen content. The mixture with 0.6 wt % hydrogen in synthesized Ti powder did not burn. At 1 wt % oxygen and 0.15 wt % hydrogen, the combustion temperature was showed to reach maximum values, about 3000°C. Contrary to Ti + C mixture, Ti + B mixture components reacted in the SHS mode without additional conditions and the combustion parameters are unaffected by the oxygen and hydrogen content in Ti powder.

Bimetallic oxygen reduction reaction (ORR) catalysts based on multi-walled carbon nanotubes (MWCNTs) doped with cobalt, copper, and nickel phthalocyanines and modified with silver (MWCNT–CoPc–Ag, MWCNT–CuPc–Ag, and WMCNT–NiPc–Ag) were obtained using high-temperature synthesis. The synthesis was carried out at 900°C in a nitrogen atmosphere. Scanning electron microscopy (SEM), low-temperature nitrogen adsorption–desorption, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and powder X-ray diffraction (XRD) were used to determine the physicochemical properties of the synthesized catalysts. The effect of high-temperature synthesis on the textural and morphological properties of materials was also studied. It was shown that the physicochemical parameters of materials largely depend on the nature of the metal in the composition of original phthalocyanine. The electrocatalytic activity of the materials was studied by linear voltammetry in a three-electrode cell with a rotating disk electrode and mercury oxide reference electrode. The MWCNT–CoPc–Ag catalyst showed high electrocatalytic activity in ORR, comparable to that of a commercial platinum catalyst, as well as high corrosion resistance.

Experimental studies of the combustion of granular mixtures (100 – X)(Ti + C) + X(Ti + 2B), 0 ≤ X ≤ 100 wt %, were carried out. Granules 0.6 and 1.7 mm in size were made using an alcoholic solution of polyvinyl butyral. The combustion velocity dependence on X showed two characteristic areas with a boundary between them near X = 60 wt %. At X > 60 wt %, the combustion velocity increased significantly which allowed us to assume a convective mechanism of combustion due to the release of impurity gas. This assumption was verified by experiments in which the impurity gases were filtered through the side surface of samples to exclude the effect of a convective heat transfer. The necessary conditions for the transition to the convective combustion mode were formulated. Calculations showed that the critical conditions were met for mixture 40%(Ti + C) + 60%(Ti + 2B) with granule size of 1.7 mm. The content of impurity gas (presumably hydrogen) for mixtures burning in convective mode was estimated.

This work continues the earlier studies focusing on fabrication of heterophase micropowders and consolidated ceramics based on HfB₂–HfC–SiC ultra-high-temperature boride/carbide compositions via self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effect of NH₄Cl addition on the morphology and microstructure of the SHS powders was studied. Composite micropowders characterized by particle size of 0.2–10 μm and 40–50% content of the submicron-sized fraction were fabricated. The structure, mechanical and thermophysical properties, kinetics and mechanism of high-temperature oxidation of hot-pressed ceramic materials composed of 57–72 wt % HfB₂, 14–20 wt % HfC_x, 10–14 wt % SiC, and 8–15 wt % HfO₂ were studied. They are found to have hardness up to 18.9 GPa, crack resistance up to 9.7 MPa m^0.5, bending strength up to 400 MPa, temperature diffusivity up to 22.6 mm²/s, and thermal conductivity up to 59 W/(m K). The power law describes their oxidation kinetics. The protection mechanism against oxidation involves the formation of a multilayered heterogenous oxide film consisting of HfO₂, HfSiO₄, and borosilicate glass.