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.
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.
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.
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 % HfCx, 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 m0.5, 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.
The effect of the angle of inclination of a cylindrical reaction volume relative to the horizontal position on the combustion characteristics of a multicomponent thermite-type mixture FeO/TiO2/B2O3/Cr2O3/CaCrO4/Al/C was experimentally studied. It was shown that the combustion parameters, the burning velocity, material loss caused by sputtering, and the yield of target metallic phase into ingot, are unaffected by the angle of inclination. The dynamics of combustion front orientation relative to the burning velocity vector as the front propagates when the cylindrical reaction volume was tilted from 0° to 90° was revealed.
Samples of mesoporous silica gel doped with cerium and terbium and modified with silver were synthesized using the template method. The textural and morphological characteristics of the samples were studied using scanning electron microscopy, inductively coupled plasma mass spectrometry, X-ray diffraction analysis, Fourier-transform infrared (FT-IR) spectroscopy, and nitrogen adsorption-desorption analysis. Moreover, thermodynamic characteristics of adsorption (differential heats and entropies) of test organic compounds were obtained using the inverse gas chromatography method. It was established that the nature of the dopant leads to changes in the textural characteristics of the samples and the heat of adsorption for hydrocarbons and compounds prone to various types of specific interactions.
A cast oxide ceramic material consisting of Al2O3–Cr2O3 solid solution and ZrO2 inclusions uniformly distributed in it was prepared through metallothermic SHS from highly exothermic thermite-type mixtures containing molybdenum (VI) and chromium (III) oxides and Al/Zr as a combined reducing agent. The influence of green mixture mass on the synthesis parameters, composition, and microstructure of the final products was studied. The optimal conditions for high-temperature synthesis of cast composite oxide material consisting of 80 wt % Al2O3, 10 wt % Cr2O3, and 10 wt % ZrO2 were determined.
Solution combustion synthesis (SCS) is a widely used method to prepare nanomaterials tailored for specific applications, with a primary focus on understanding the influence of precursors on material properties and microstructure evolution. There is also a lack of systematic studies to understand the kinetics of SCS reactions. This work reports on the preparation of pure nickel by the combustion of Ni(NO3)2 + citric acid (C6H8O7) solution and the investigation of SCS reaction mechanism by thermal analysis technique. The lower and upper combustion limits were determined depending on the citric acid to Ni(NO3)2 ratio. The optimal composition of the initial mixture was revealed to prepare nickel powder without using an additional post–synthesis reduction of metal oxides. Ozawa’s method was employed to calculate the effective activation energy (∼101 ± 5 kJ mol–1) of the nickel formation reaction based on the TGA data. The magnetization of the nickel powder agglomerates with ∼80–200-nm nanoscale particles was measured to be 55.1 Am2 kg–1.
In our study, we aimed to synthesize the Cu2Sb phase with a tetragonal structure. We achieved this by subjecting compacts (2Cu + Sb) to electrothermal explosion (ETE) with a high current density of 500 Å. To analyze the constituent phases of the alloy composite, we employed X-ray diffraction analysis with the MAUD program, which utilizes the Rietveld method, as well as scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques. Furthermore, we investigated the mechanical properties through Vickers indentation and compression techniques.
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