Numerical investigation of the thermal processes associated with quenching of a material in a wave of SHS

The rate of cooling is estimated for different regimes of heat removal from the surface of an SHS sample. The extinction of the combustion front is studied numerically for the example of the system titanium carbon. The temperature dependences of the rate of cooling in different zones of the combustion wave are calculated.Minsk. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 6, pp. 64–72, November–December, 1991.     

Nitride formation during combustion of Ti-TiO2 and Ti-Al powder mixtures in air under SHS conditions

Nitride formation during SHS combustion of a micron-size titanium powder and its mixtures with additives in air was studied. It was shown that the yield of TiN Ti powder was higher for SHS combustion in air than for SHS combustion of powders of the same degree of dispersion in nitrogen. The mechanism of formation of TiN is probably determined by the reaction of the intermediate product TiO with atmospheric nitrogen. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 5, pp. 131–135, September–October, 2008.     

Effect of preheating on synthesis of tantalum nitride by self-propagating combustion

An experimental investigation of self-propagating high-temperature synthesis (SHS) of tantalum nitride (TaN) was conducted with tantalum compacts in nitrogen of 0.27–1.82 MPa. Effects of sample density, nitrogen pressure, and preheating temperature on the flame-front propagation velocity, combustion temperature, degree of conversion, and product composition were studied. Results showed that the SHS process of the tantalum/nitrogen reaction was characterized by the steady propagation of a planar combustion front, followed by a prolonged afterburning reaction. The flame-front velocity increased with nitrogen pressure, but decreased with sample density. Preheating the sample prior to ignition contributed higher combustion temperatures, thus leading to an increase in the conversion percentage. For the unpreheated samples, the conversion increased significantly with nitrogen pressure and reached around 80% at 1.82 MPa of N₂. With preheating temperatures between 150 and 300 °C, the conversion was increased by about 15% when compared with that without preheating. The nitride phase TaN was identified by XRD as the dominant composition in the combustion product.     

Time-resolved neutron diffraction study of Ti–TiC–Al2O3 composites obtained by SHS

Cermets of the titanium matrix family have potential applications in the aeronautical and automotive fields. Self-propagating high-temperature synthesis (SHS) constitutes a good method to produce them, as it implies low processing costs and energy and time efficiency. An interesting feature of this process is to determine the mechanism of reaction. A good method to do it is the use of in situ real-time diffraction techniques. The objective of this paper is to show how the use of time-resolved neutron diffraction (TRND) providing a deeper insight into the mechanism of a typical SHS reaction by monitoring the formation of a cermet of the system Ti–TiC–Al₂O₃. Neutron diffraction experiments have been conducted on Instrument D20 at the Institute Max von Laue-Paul Langevin (Grenoble, France) with a high-flux medium-resolution powder diffractometer. Results show that reaction is initiated with the melting and diffusion of aluminium through the sample, and shortly after product phases are formed. The final product shows a general structure composed of a titanium matrix with the rest of the phases located in reaction domains.     

High porosity SiC ceramics prepared via a process involving an SHS stage

The preparation of porous SiC ceramics from stoechiometric mixtures of silicon and graphite has been studied. Products with very high pore contents (≈80%) were obtained using a process which consisted of heating the reactive pellets in purified argon, at 15 °C min⁻¹, up to 1430 °C and applying a weak d.c. voltage across the sample for 20 s. The resulting electrical current was necessary for the ignition of an SHS reaction simultaneously in the whole sample. The analysis of the sample microstructure evolution all along the process has enabled the identification of the different mechanisms involved in the SiC formation. Before the SHS stage, the formation of silicon carbide, during heating from about 1325 up to 1430 °C, is associated with a large sample expansion, which mainly determined the final pore volume fraction. The pore transfer mechanisms, which occur during the SHS stage at 1430 °C, have a specific influence on the pore development. Since the final pore size distribution is strongly related to silicon grain granulometry, the porosity of the porous SiC ceramic, obtained by this process, can be easily modulated.     

Structural transformations during combustion of a gasless mixture in a continuous reactor

A mathematical model is constructed for SHS with induced filtration of an inert gas counter to the gasless combustion front. Structural transformations are analyzed for a porous charge connected with liquid-phase sintering, the force effect of the filtering gas, and the change in condensed phase volume with chemical reaction. Conditions are determined for crack formation and for obtaining a uniform structure over the length of the products. Equations are obtained for product porosity and combustion rate.Institute of the Earth Cryosphere, Siberian Branch, Russian Academy of Sciences, Tyumen. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 1, pp. 36–44, January–February, 1994.     

Thermoimpulsive synthesis of nickel aluminides

We studied the effect of explosive loading on the microstructure and phase composition of the Ni–Al system in the course of SHS interaction. X-ray diffraction, micro x-ray diffraction, and optical methods established an appreciable effect of shock-wave loading on the structural characteristics of the synthesis products. A multistage character of SHS in the system in question was noted; the nature of shock-wave action is determined by the structure which has formed by the instant of loading. It is suggested to use the method of measurement of concentration inhomogeneity for studying the effect of a shock wave on the diffusion interaction in reacting systems.RITTs PM, 614061, Perm'. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 6, pp. 83–88, November–December, 1994.     

The Influence of Green Surface Modification of Oil Palm Mesocarp Fiber by Superheated Steam on the Mechanical Properties and Dimensional Stability of Oil Palm Mesocarp Fiber/Poly(butylene succinate) Biocomposite

In this paper, superheated steam (SHS) was used as cost effective and green processing technique to modify oil palm mesocarp fiber (OPMF) for biocomposite applications. The purpose of this modification was to promote the adhesion between fiber and thermoplastic. The modification was carried out in a SHS oven at various temperature (200–230 °C) and time (30–120 min) under normal atmospheric pressure. The biocomposites from SHS-treated OPMFs and poly(butylene succinate) (PBS) at a weight ratio of 70:30 were prepared by melt blending technique. The mechanical properties and dimensional stability of the biocomposites were evaluated. This study showed that the SHS treatment increased the roughness of the fiber surface due to the removal of surface impurities and hemicellulose. The tensile, flexural and impact properties, as well as dimensional stability of the biocomposites were markedly enhanced by the presence of SHS-treated OPMF. Scanning electron microscopy analysis showed improvement of interfacial adhesion between PBS and SHS-treated OPMF. This work demonstrated that SHS could be used as an eco-friendly and sustainable processing method for modification of OPMF in biocomposite fabrication.     

Self-disseminating high-temperature synthesis: General principles

Self-disseminating high-temperature synthesis (SHS) is the solid-phase reaction of mixtures of dispersed reagents which facilitates the synthesis of various refractories and infusible compounds in a combustion cycle of a spontaneously moving zone. Complete chemical conversion is accomplished in several minutes without the application of external energy, but only on account of the inherent heat of the exothermic reaction. Using SHS it is possible to make thermal- and wear-resistant lining components for strengthening agglomeration machines and charging units of blast furnaces; volume corundum covers for the internal surfaces of metal pipes used in standard thermal-power stations etc. have been realized. The resistance of such parts has thus been considerably increased. The potential of SHS is unique, and the method can satisfy a great variety of users. The editorial board intends to publish a series of articles to familiarize readers with certain aspects of this advanced method, which may be useful for the development of new ceramics and refractories technology.Translated from Ogneupory, No. 7/8, pp. 16–17, July–August, 1992.     

Laser ignition of nickel-aluminum powder systems

A method for determining the thermokinetic constants of the SHS reaction and thermophysical properties of the initial mixture and reaction products involving the use of laser initiation of the reactive mixture has been developed. A technique for experimentally studying the ignition process of the reacting mixture is describedScientific Research Institute of Building Materials, 634003, Tomsk. Translated from Fizika Goreniya Vzryva, Vol. 30, No. 2, pp. 14–18, March–April, 1994.     

Self-Propagating High-Temperature Synthesis of Ti3SiC2: I, Ultra-High-Speed Neutron Diffraction Study of the Reaction Mechanism

In situ neutron diffraction at 0.9 s time resolution was used to reveal the reaction mechanism during the self-propagating high-temperature synthesis (SHS) of Ti₃SiC₂ from furnace-ignited stoichiometric 3Ti + SiC + C mixtures. The diffraction patterns indicate that the SHS proceeded in five stages: (i) preheating of the reactants, (ii) the α→β phase transformation in Ti, (iii) preignition reactions, (iv) the formation of a single solid intermediate phase in <0.9 s, and (v) the rapid nucleation and growth of the product phase Ti₃SiC₂. No amorphous contribution to the diffraction patterns from a liquid phase was detected and, as such, it is unlikely that a liquid phase plays a major role in this SHS reaction. The intermediate phase is believed to be a solid solution of Si in TiC such that the overall stoichiometry is ∼3Ti:1Si:2C. Lattice parameters and known thermal expansion data were used to estimate the ignition temperature at 923 ± 10°C (supported by the α→β phase transformation in Ti) and the combustion temperature at 2320 ± 50°C.     

Crystallization in a SHS wave

The crystallization of nonequilibrium amorphous SHS products increases the wave velocity. The regimes of such an acceleration are studied.Institute of Structural Makrokinetics, Russian Academy of Sciences, Chernogolovka 14232. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 3, pp. 19–21, May–June, 1995.     

Microwave Absorbing Coatings Based on a Blend of Nitrile Rubber, EPDM Rubber and Polyaniline

Summary The mechanical and microwave radiation absorbing properties of conductive ternary blends based on nitrile rubber, EPDM rubber and polyaniline, doped with dodecylbenzene sulfonic acid, have been investigated with special interest in the concentrations of nitrile rubber and conductive polymer in the blend. The ternary blends were prepared by melt blending using an internal mixer. Mechanical properties and soluble fraction analyses show that crosslinking occurs during blending, and that the crosslinking degree depends on the concentrations of doped polyaniline and nitrile rubber in the blend. The crosslinking reaction involves the doping acid (dodecylbenzene sulfonic acid) of polyaniline and the -C≡N group in the nitrile rubber. The ternary blends can be used for microwave absorption in the frequency range of 8–12 GHz. This property depends on the concentration of the conductive polymer and film thickness.     

Chemical furnace for preheating in dynamic consolidation experiments

A new method is developed for specimen preheating with subsequent loading by shock waves and conservation of the products of shock consolidation; the method is based on the use of the chemical furnace energy produced by a thermal explosion or combustion of SHS systems. The use of the thermal explosion expands the range of pre-shock heating temperatures up to 3000 °C and the range of mean pressures in the cylindrical ampoule up to 25 GPa. Two variants of preheating are considered: an inert specimen is heated by heat release in a reacting SHS system or the specimen under study itself is an SHS system.Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 1, pp. 129–135, January–February, 2005.     

Mathematical modeling of nonstationary effects at flow around a single burning particle

The behavior of the resistance and heat-exchange coefficients is examined for an evaporating spherical particle in reacting high-temperature flow on the basis of numerical solution of nonstationary Navier—Stokes equations. It is shown that during the process of flow transition to a steady state the obtaining (in the presence of chemical reaction) of the negative resistance coefficient is possible. It is noted that in the nonstationary case the appearance of an intense chemical reaction zone behind the back surface of the spherical particle excites the pressure wave, the travel of which through the particle results in substantial oscillations of the resistance coefficient and, to a lesser degree, of the heat-exchange coefficient.Barnaul. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 120–123, May–June, 1993.     

Degeneration of a steady combustion wave in self-propagating high-temperature synthesis

Degeneration of a steady wave in self-propagating high-temperature synthesis (SHS) is considered in the paper. An analytical expression for the derivative of the squared combustion rate with respect to the cutoff temperature of a thermal source is obtained. It is shown that there is a domain of combustion parameters for which the combustion wave can be roughly considered steady.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 3, pp. 65–67, May–June, 1996.     

A kinetic and electrochemical study of the zincate immersion process for aluminium

To plate aluminium, its surface is often first coated with a thin layer of zinc which is formed by immersion in an alkaline zincate solution. This paper describes a kinetic and electrochemical study of the zincate immersion reaction. Using an aluminium sample in the form of a rotating disc, the effects of varying the zinc concentration (0.01–0.5 m), disc rotation speed (66–1380 rpm), temperature (5–72°C), and sodium hydroxide concentration (1.5–9.0 m) on the kinetics were investigated. It was found that the reaction was usually first order. When the zincate concentration was 0.1 m, the reaction was chemically controlled with an activation energy of 35 ± 7 kJ mol^–1. At high zincate concentrations, high disc rotation speeds and low sodium hydroxide concentrations, a thin film of zinc metal was formed on the aluminium surface, blocking the subsequent reaction. It was found that the most compact and adherent zinc films were formed at high zincate concentrations. This finding is consistent with industrial practice. These results are explained using mixed potential measurements and Evans' diagrams.     

Effect of quenching on the polymorphic crystallization of poly(oxetane)

Summary The crystallization from the glassy state of poly(oxetane) or poly(trimethylene oxide), after quenching the molten polymer in liquid nitrogen, has been studied by differential scanning calorimetry and X-ray diffraction analysis. The results indicate that two crystalline modifications are obtained, depending on the crystallization temperature: the orthorhombic modification is produced when the temperature is higher than –30 °C, while the trigonal form is found below –45 °C, both forms coexisting at intermediate temperatures.     

Preparation and properties of zirconium refractories stabilized by cerium dioxide

Conclusions We studied the reaction of zirconium dioxide with cerium dioxide in mixtures with CeO₂ contents of 6, 8, 10, 12, 15, 18, 20, 33, and 50%. The properties of the samples of these compositions were determined.On the addition of amounts of CeO₂ from 6–12% complete stabilization of ZrO₂ was not achieved by a single sintering at 1750°C. The samples with the composition 88% ZrO₂+12% CeO₂, sintered twice at 1750°C with an intermediate grinding, stabilized almost completely as a solid solution of tetragonal structure.The thermal stress resistance of dense, completely stabilized samples with CeO₂ contents of 15–18%, prepared from finely ground raw oxides, was 3–4 thermal cyclings. It improved when the CeO₂ content was decreased, or when more monoclinic ZrO₂ was added.It was found that the onset and the inversion temperature interval depend on the CeO₂ content, the granular composition of the original oxides, the temperature, and the gaseous sintering medium. We studied the properties of synthesized compositions and their dependence on the reducing or oxidizing conditions of sintering due to a change in the valency of cerium. In order to obtain zirconium-cerium refractories with definite properties it is necessary to have strict control of the gaseous medium during sintering.Translated from Ogneupory, No. 3, pp. 37–44, March, 1969.     

The elongation of viscose cord at a load of 4.5 kgf

Conclusions 1. It is impossible to achieve an increase in the intermediate elongation by changing the parameters of the technological conditions in the formation of cord with a breaking length of 34–36 km.2. On passing to the technological conditions for obtaining fibre with a higher strength, fatigue properties improve and the intermediate elongation decreases.3. The intermediate elongation of the cord depends on the type of dynamometer and the clamps with which the determination is carried out. Consequently, it cannot characterise the fatigue properties of both raw and rubber-treated cord.4. To determine the physicomechanical characteristics of cord thread it is necessary to use the same dynamometers, clamps and materials for them.Krasnoyarsk Branch of All-Union Synthetic Fibre Research Institute (VNIIV). Translated from Khimicheskie Volokna, No. 3, pp. 65–67, May–June, 1969.