X-ray radiation in self-propagating high-temperature synthesis processes

Emission effects of heterogeneous combustion in the region of ionization radiation are studied. By an example of a Ti-B powder system, it is demonstrated that the processes of self-propagating high-temperature synthesis in the thermal explosion regime is accompanied by “soft” X-ray radiation with the quantum energy estimated as ≈5 keV. Key words: heterogeneous combustion, X-ray radiation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 127–129, November–December, 2008.     

Dynamics of phase-formation processes during self-propagating high-temperature synthesis in the thermal explosion regime

Specific features of formation of temperature and concentration fields during self-propagating high-temperature synthesis in the thermal explosion regime in a cylindrical reactor are studied by methods of mathematical modeling. The calculations are performed with allowance for melting of one (chemically active) component in the approximation with the high-melting component being non-soluble in the melt of the low-melting component. It is shown that the conditions of complete conversion of the original components in the volume of the reacting mixture depend on relations between the Biot criterion of the system, the ambient temperature, and the thermal effect of the reaction. After the thermal explosion, which occurs when the melting front reaches the geometric center of the reactor, a front of complete conversion is formed. This front moves from the cylinder centerline to the periphery with a gradually decreasing velocity. The diagram of the critical values of the Biot criterion at which the components burn down completely in the entire reaction volume is calculated. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 31–38, July–August, 2008.     

Experimental study of the gas phase formed in the processes of self-propagating high-temperature synthesis

We give the results of a study of impurity gas release in the processes of self-propagating high-temperature synthesis. Data on the amount and composition of the gas phase were obtained for a variety of systems by sampling from a combustion wave with further mass-spectral analysis. We also present the spectroscopic data in the visible range during combustion. The effect of reabsorption of the released gases by condensed combustion products was discovered. Institute of Structural Macrokinetics, Chernogolovka 142432, Russian Academy of Sciences. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 55–64, July–August, 1997.     

Acoustic emission during self-propagating high-temperature synthesis

It was established experimentally that during SHS, reaction systems (Ni-Al, Ti-B, Mo-B, etc.) generated acoustic oscillations in the frequency range from 5 Hz to 1.1 MHz with a pulse power of up to 17 W. It was found that the combustion of different systems is characterized by an individual set of dynamic parameters of acoustic emission in the modes of low ordered discrete pulses and highly ordered self-oscillations. It is shown that the spatial zone of acoustic emission is localized near the combustion wave. Analysis of the acoustic emission mechanisms of SHS is performed.     

Preparation of ZrC by self-propagating high-temperature synthesis

ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) based on exothermic reduction reaction of ZrO₂–C–Mg. The combustion temperature observed was 1979 K. The effects of Mg content and particle size on the combustion temperature and chemical composition of the product were investigated. The reducing agent Mg plays an important role on the purity of ZrC powder obtained by SHS process. Post-heat treatment was applied to decrease the oxygen content of the final product further.     

Obtaining aluminomagnesia spinel by self-propagating high-temperature synthesis

The conditions for forming a solid solution of aluminomagnesia spinel with magnesium oxide in the regime of self-propagating high-temperature synthesis and the properties of the resultant refractory are investigated. The main parameters of the synthesis are determined. Test batches of specimens are used to study the main physicomechanical properties.Translated from Ogneupory, No. 12, pp. 10 – 12, December, 1994.     

On the theory of self-propagating high-temperature synthesis in layered systems

The formation of a composite material during gasless combustion of a layer stack is considered. Combustion of the external donor layers causes melting of the internal inert layer, after which the melt flows into the pores of the skeleton. The flowing depth, determining the structure of the composite material depends on the pore size, surface tension, phase transition parameters, and temperature changes.     

Features of self-propagating high-temperature synthesis in a titanium-based system

The combustion temperature and rate have been measured and the product structure has been examined to define the interactions in a system designed to produce a porous permeable material having a heterogeneous framework.Barnaul. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 4, pp. 37–42, July–August, 1993.     

Features of self-propagating high-temperature synthesis of spinel pigments

The method of self-propagating high-temperature synthesis (SHS) was used to synthesize spinel ceramic pigments. Producing aluminum-nickel and aluminum-cobalt pigments in a finely dispersed state is a complex stepwise (combined) process involving a reduction stage and simultaneous natural air filtration. In the case of small-diameter samples, a flat combustion front is observed and in large samples, the front is formed of many hot spots. The dependence of combustion rate on porosity is presented. The maximum combustion rate of these systems are recorded for porosity of 50–60%. Powdered SHS pigments were obtained for the first time.     

Electroimpulsive activation of self-propagating high-temperature synthesis in powder mixtures

The effect of frequency electroimpulsive treatment on the self-propagating high-temperature synthesis reaction in a powder mixture is studied with the use of the Ni-Al system as an example. The longitudinal direction of the electric field applied to the sample and of the electric current relative to the direction of combustion-wave propagation was used. It is established that under the action of electrotreatment, the linear combustion rate increases by a factor of 1.3-1.6, the structure is transformed, and the completeness of chemical transformation increases. It is shown that the effects observed are of a nonthermal nature. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 133–136, July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 99-03-32465).     

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.     

Formation of self-propagating high-temperature synthesis products in a magnetic field

The combustion of compressed Ni+16% Al, Ni+50% Al, Co+30% S, Ti+58% FeB, Ti+20% C, and FeO+10% Fe+18% Al mixtures in a constant magnetic field is studied. The depth of conversion is found to increase and the geometric dimensions of the phase components of the reaction end products to coarsen under the influence of the field in the systems containing a ferromagnetic metal. The changes in the reaction product are related to additional mass transport behind the self-propagating high-temperature synthesis wave. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 63–66, May–June 1999.     

Application of EPR spectroscopy to the investigation of self-propagating high-temperature synthesis

Erevan. Translated from Fizika Goreniya i Vzryva, Vol. 27, No. 5, pp. 91–94, September–October, 1991.     

自蔓延高温合成镧掺杂钡铁氧体改性研究

采用白蔓延高温合成法制备了La掺杂的M型钡铁氧体BaLaxFe12-xO19（x=0,0．3,0.4,0．6）。用X射线衍射仪、振动样品磁强计和扫描电镜对粉末产品的结构与磁学性能进行了观测。系统地研究了La掺杂对M型钡铁氧体结构和磁性能的影响。实验结果表明,随着掺杂量（x）的增大,矫顽力He增大,而饱和磁化强度M6和剩余磁化强度Mr均减小。当掺杂量x=0．3时,钡铁氧体具有最佳的综合磁性能。     

Influence of constant electric field on self-propagating high-temperature synthesis wave

A model of a heterogeneous medium allowing the action of a constant electric field (CEF) on a combustion wave in the condensed phase to be taken into account is proposed. It is shown that the influence of the CEF on the combustion temperature is of threshold type: At a CEF less than the threshold, the combustion temperature is constant; above the threshold, the combustion temperature falls with increase in CEF. The combustion rate falls with increase in CEF; this is associated with slowing of the reactive diffusion in the combustion wave. On passing through the CEF threshold, there is a discontinuity in the combustion-rate curve; this is due to additional slowing of the reactive diffusion on account of the reduction in combustion temperature. Institute of Solid-State Physics and Semiconductors, Vitebsk Branch, Vitebsk 210023. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 5, pp. 58–62, September–October, 1994.     

Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis

Preparation of nano-size ZrB₂ powder by SHS has been investigated. Zr and B elemental powders were mixed with 10–50 wt.% NaCl, and prepared pellets were reacted under argon. Adiabatic temperatures were calculated by HSC software. Increasing NaCl content led to a continuous decrease in adiabatic temperatures and reaction wave velocity. Products were subjected to XRD, SEM and FESEM analyses. Average crystallite size of ZrB₂, which was 303 nm without NaCl, decreased to 32 nm with 40% NaCl addition. Distinct decrease in ZrB₂ particle size was also observed from SEM analyses. 30% NaCl addition was found to be optimum for ensuring a stable SHS reaction and providing the formation of nano-size ZrB₂ particles. It was revealed from particle size distribution measurements that ZrB₂ powder obtained by 30 wt.% NaCl addition contained particles mostly finer than 200 nm. A mechanism, similar to solution-precipitation was proposed for the particle size refining effect of NaCl.     

Conversion of methane on catalysts obtained via self-propagating high-temperature synthesis

Monolith metalloceramic catalysts for the selective oxidation of methane are prepared via self-propagating high-temperature synthesis (SHS) from NiO, ZrO₂, MgO, Al, Ni and other powders. Catalytic tests of monolith samples are performed in a flow reactor at 800°C using a methane-air mixture (methane, 29.6 vol %). SHS catalysts are shown to attain the level of platinum and platinum/rhodium catalysts through the yield of syngas (CO + H₂) and to surpass them in the case of Ni 52.9 ZrO₂ 9.5 composition. The latter is used as a catalyst to develop a pilot autothermal syngas generator with a capacity of 30 m³/h. Syngas is generated via carbon dioxide methane conversion (CDMC) on SHS platinum-modified Ni₃Al powder catalysts. The samples are tested in a flow fixed-bed reactor at a catalyst volume of 1 cm³, a grain size of 600–1000 μm, temperatures of 600–900°C, and a volumetric flow rate of 100 cm³/min (CH₄ : CO₂ : He = 20 : 20 : 60 vol %). The catalysts developed for converting natural gas into syngas are shown to be highly active and stable in a high-temperature redox medium. This work is the first step in the synthesis of dimethyl ether, which could compete successfully with diesel fuel.