Combustion of the gasless system Ti + 0.5C in a nitrogen coflow

Combustion of mixtures of titanium with carbon black of bulk density placed in a quartz tube with nitrogen purging (concurrent filtration) is examined. The nitrogen flow is provided by a fixed pressure difference not exceeding 1 atm at the ends of the mixture. The burning rate is determined as a function of the amount of titanium nitride added to the initial mixture and of the pressure at the ends of the sample. The tests show that a nitration front is formed during combustion of a Ti + 0.5C (carbon black) powder mixture in the nitrogen flow, in addition to a carbidization front. Various modes of propagation of the carbidization and nitration fronts are found and described. A classification of combustion modes of the Ti + 0.5C (carbon black) powder mixture in the nitrogen flow is proposed.     

Combustion behavior of a Ti + TiC mixture in a nitrogen coflow

Combustion of powder and granular mixtures of TiC + Ti in a quartz tube purged with nitrogen was studied. Mixtures based on fine and coarse-grained TiC were used. It is found that purging of a powder mixture of bulk density with fine titanium carbide with nitrogen coflow does not lead to the spread of the flame front, whereas granular mixtures burn at the same pressure difference. Mixtures based on coarse-grained titanium carbide powder burn in both powder and granular form. The burning rate of a granular mixture of TiC + Ti with coarse-grained titanium carbide is significantly higher than when using fine titanium carbide. It is shown that in the case of a coarse-grained TiC, granulation of the mixture of TiC + Ti significantly improves the degree of nitriding of the synthesis products compared to the powder mixture. During combustion of granular mixtures of TiC + Ti, in contrast to powder mixtures of the same composition, a singlephase product of approximate composition TiC_0.5N_0.44 is formed as a result of synthesis.     

Burning of the Ti + <Emphasis Type="Italic">x</Emphasis>C (1 &gt; <Emphasis Type="Italic">x</Emphasis> &gt; 0.5) powder and granulated mixtures

This paper describes the experiments on the burning of the Ti + 0.5C, Ti + 0.75C, and Ti + C powder and granulated mixtures. Despite the fact that there is no convective heat transfer and the contact area between the particles is small, the linear and mass burning rates of granulated compositions happened to be several times greater than in the case of powder mixtures of the same composition. The obtained experimental and computational values of the adiabatic combustion temperature were used to estimate the contribution of the radiant and conductive heat transfer in the combustion wave propagation along the granulated mixtures. The experiments with compacted samples showed that the high burning rate of the granulated mixtures is due to great velocity of the combustion wave propagation along the granule rather than the specific features of the original reagents.     

Dynamic combustion regimes of the Ti-(Ti+0.5C) layered system in a concurrent nitrogen flow

The filtration combustion of a layered porous fill consisting of alternating layers of a mixture of Ti + 0.5C a titanium powder with forced concurrent filtration of nitrogen was studied for the first time. The gas flow through the sample was provided by a vacuum pump attached to the lower end of the fill. The presence of the concurrent gas flow radically changes the character of propagation of the combustion front and the structure and composition of the products obtained. The layers consisting of carbonitride and titanium nitride make a single unit. The experiments provided scientific bases for the production of new laminated and composite ceramic materials by dynamic filtration combustion. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 44–51, November–December, 2008.     

Combustion of bulk density powder mixtures in a coflow of nitrogen gas: 3. The Ti-(Ti + 0.5C) layered system

Explored is the filtration combustion in Ti-(Ti + 0.5C) layered powder systems in a coflow of nitrogen gas. The presence/absence of nitrogen coflow through the layered system was found to drastically affect the character of wave propagation and structure/properties of product. The obtained data can be regarded as a basis for fabrication of new layered and composite materials in the mode of dynamic filtration combustion.      

Combustion of cylindrical Ti + 0.5C compacts: Influence of mechanical activation,thermovacuum degassing,and ambient pressure

We explored the influence of mechanical activation (MA), thermovacuum degassing (TVD), and ambient pressure on burning velocity and sample shrinkage/elongation for SHS reactions in cylindrical Ti + 0.5C powder compacts. Thermovacuum degassing of Ti + 0.5C mixtures was found to increase the burning velocity (2-fold) and sample shrinkage. Mechanical alloying decreased the burning velocity and gave large (3-fold) sample elongation. The results can be readily rationalized in terms of conduction-convection combustion theory.     

Deformations inside burning specimens

The dynamics of particle motion inside burning specimens producing solid combustion products was studied by flash radiography. In the experiments, we used specimens of aTi+C+20% TiC mixture, inside which marks in the form of strips of a ofZr+WO ₃ mixture or a tantalum powder were placed. The specimens were burnt in a semiclosed rigid casing with exhaust of impurity gases through slags. It is established that, just behind the combustion front, particles of combustion products begin to move in the direction opposite to the direction of motion of the combustion front. In the central zone of specimens, the particle velocity reaches values comparable with the combustion velocity of the specimens (∼20 mm/sec), whereas, on the periphery, the particle velocity is close to zero. Institute of Experimental Physics, Sarov 607190. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 4, pp. 78–83, July–August, 1997.     

Gas combustion in a narrow channel at high pressure

This paper presents experimental results on the effect of pressure on the flame propagation velocity in a tube with diameter close to the critical diameter. An important feature of the investigated combustion regime is heat transfer along the tube wall from the combustion products to the fresh mixture. Methane-air and hydrogen-air mixtures were used. The experiments show that with increasing pressure, the burning velocity of methane-air flames decreases whereas the burning velocity of hydrogen-air flames is almost unchanged. This behavior is explained by the pressure dependence of the laminar burning velocity.     

SHS and formation of structure in composite materials in three-component Ti — Si — C, Ti — Si — N, and Ti — B — N systems

Results of a study of some special features of gasless burning in ternary Ti — Si — C, Ti — Si — N, and Ti — B — N systems are presented. The optimum concentration regions for fabricating ceramics based on the Ti₃SiC₂ phase in combustion are determined. It is recommended to use nontraditional components for fabricating nitrides, borides, and silicides. A study of the dependence of the rate and regime of combustion of Ti — SiC mixtures on the density has shown new kinds of unsteadiness of the combustion front that appear due to the increase in the porosity of the reaction mixtures. In order to obtain a high porosity in the reaction mixtures and the products, an experiment has been performed under the conditions of microgravitation of the Mir orbital probe.     

Dependence of burning rate on sample size in the Ni + Al system

Burning rates were measured for samples of the starting mixture of Ni + Al and the same mixture subjected to mechanical treatment in argon and then in water. The dependences of the burning rate on the diameter of the samples of the original and mechanically activated mixtures are similar. The burning rate passes through a maximum as the diameter increases from 8 to 12 mm. It is found that the burning rate of 270–360 µm thick films obtained by rolling the starting mixture of Ni + Al and the mixture mechanically activated and then activated in water (dispersed) is 4–20 times the burning rate of cylindrical samples 8–12 mm in diameter, pressed from the same powders. The data obtained in this study were explained using a convective-conductive model of combustion-wave propagation.     

Displacement of igniter and specimen materials in combustion of gas-free systems in a rigid shell

We present results from an investigation into the burning of a gas-free Ti + C system (diluted to 20% with titanium carbide), this system contained in combination with an igniter tablet fabricated out of a Cr₂O₃ + 2Al + 4B mixture in a rigid shell. We have demonstrated that the burning is accompanied by displacement of the material through the appearance of a decompression wave (a sharp reduction in density in the vicinity of the combustion front), thus leading to a change in the burning velocity.Moscow. Translated from Fizika Goreniya i Vzryva, No. 2, pp. 94–97, March–April, 1991.     

End combustion products of mixtures of ultrafine aluminum with a zirconium—Aluminum alloy in air

The chemical composition of the products formed during combustion in air of powder mixtures of a commercial zirconium-aluminum alloy (mass concentrations of zirconium and aluminum 84 and 16%, respectively) and ultrafine Al powder produced by electric detonation of conductors in argon is studied. The capacity of the mixtures for chemical fixation of air nitrogen is analyzed. It is established that under certain conditions, the end products can contain up to 60% AlN + ZrN mixture. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 56–59, March–April, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 98-02-16321).     

Combustion of bulk density powder mixtures in a coflow of inert gas: 2. The Ti-C system

The burning velocity of bulk density Ti-C powder mixtures was found to increase with increasing flow rate of inert gas. In some cases, it was also accompanied by stabilization of planar combustion front. The obtained data were rationalized in terms of convention-conduction theory (CCT). The effect can be used as a simple and effective mean for controlling the combustion process and purification of products from impurity gases.     

Modes of gasless combustion and macrostructure of combustion Front (for the Ti-Si system as an example)

Gasless combustion of Ti+Si mixtures (taken as an example) was investigated by high-speed video recording followed by computer-assisted processing of stored still frames. The three-dimensional shape of the combustion front was reconstructed, and explanations were proposed for the combustion front propagation modes along the surface of cylindrical samples. The reconstructed front configuration was found to be nearly paraboloidal. The modes of unsteady combustion in T-rich and Si-rich mixtures turned out to be strongly different. The results were used to rationalize the available experimental data on unsteady combustion in terms of microheterogeneous theory. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 147–155, July–August, 2009.     

Impact of mechanical activation on the burning rate of pressed and bulk-density samples from a Ni + Al mixture

This paper describes a study on the burning of pressed and bulk-density samples from a Ni + Al mixture subjected to mechanical activation. It is shown that mechanical activation and dispersion have a different impact on the burning rate of the samples under study. The gas flow hardly changes the burning rate of the bulk-density mixtures. The linear burning rate of the dispersed bulk-density mixture is 1.7 times greater than that of the pressed mixture, and the mass burning rates are equal to each other. The calculations showed that the conductive heat transfer mechanism in the combustion wave of the dispersed bulk density mixture is the principal mechanism.     

Production of a complex nitride-containing mixture by oxidation of zirconium and aluminum in air

The authors investigate the chemical composition of the combustion products of mixtures of a freely poured industrially produced zirconium-aluminum alloy powder (84 wt.% zirconium) and an ultradisperse aluminum powder produced by electric detonation of conductors in argon. It is established that under certain conditions the end products can contain up to 60% AlN + ZrN mixture. Translated from Steklo i Keramika, No. 3, pp. 17–18, March, 1999.     

Combustion of bulk density powder mixtures in a coflow of inert gas: 1. The Ni-Al system

For combustion of bulk density N + Al mixtures an a coflow of argon, the burning velocity U _c was found to markedly depend on the flow rate. The effect (unexplainable in terms of classical combustion theory) has been rationalized in terms of the convective-conductive theory. Preliminary thermal treatment of green mixtures in vacuum was found to increase the initial value of U _c and to decrease the effect of pressure difference ΔP on the extent of flow-induced increase in the burning velocity.      

Reaction Synthesis of Titanium Silicides via Self-Propagating Reaction Kinetics

Self-propagating exothermic reactions in the titanium-silicon system induced by mechanical milling, shock loading, and thermal ignition of elemental powder mixtures have been investigated. After an induction period of 3 h, the 5Ti + 3Si powder mixture abruptly reacted during milling to form single-phase Ti₅Si₃ via a mechanically induced self-propagating reaction (MSR). Moreover, the formation of porous Ti₅Si₃ solid indicated that the melting of powder particles had occurred during the mechanical alloying process. The Ti + Si powder mixture also reacted via the MSR mode, but the end-product was multiphase. Shock and combustion synthesis (thermal-explosion mode) experiments essentially produced the same result.     

Combustion Synthesis of the Titanium-Aluminum-Boron System

TiB₂ and Al base composite powders, which will offer a weight-saving improvement in stiffness, were produced by combustion synthesis of Ti, Al, and B ternary powder mixtures. Finely dispersed TiB₂ was synthesized by reacting a mixture of Ti, Al, and B in the molar ratio of 1:1:4. The grain size of the TiB₂ formed was <0.5 μm, which was much smaller than that obtained from the reaction of a mixture of Ti, Al, and B in the molar ratio 1:1:2. These results are discussed in light of the reaction propagating velocity and heat removal during the combustion synthesis process.     

The numerical simulation of effects of the heterogeneities in composition and porosity on micropyretic synthesis

Micropyretic/combustion synthesis is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneities in initial composition and porosity are commonly occurred during micropyretic synthesis when powders are pressed or mixed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the initial composition and porosity are thought to result in local variations in reaction yield and such thermophysical/chemical parameters for the reactants as density, heat capacity, and thermal conductivity; further changing the average propagation velocity and the oscillatory pattern of an unstable combustion front. This study thus investigates the impact of heterogeneities in composition and porosity on the unstable micropyretic synthesis with Ti+2B by a numerical simulation. The heterogeneity maps, considering the composition and porosity heterogeneities concurrently, are also generated. The maps provide a better understanding of the heterogeneous composition and porosity effect on the average propagation velocity and oscillatory frequency during unstable micropyretic synthesis.