Experimental investigation of combustion of a gasless pelletized mixture of Ti + 0.5C in argon and nitrogen coflows

Combustion of a pelletized mixture of titanium and carbon black placed in a quartz tube and exposed to a flow of argon or nitrogen is studied. The gas flow (cocurrent filtration) is provided by a fixed pressure gradient at the inlet and outlet of the tube, which did not exceed 1 atm. The possible modes of combustion of pelletized mixtures related to the presence of a more complex hierarchy of scales (micro, macro, and meso) compared to that of powder mixtures (micro, macro) are analyzed. A comparison is made of the burning rates of powder and pelletized mixtures. An increase in the burning rate when using pelletized mixtures was found experimentally. It is shown that the gas coflow through the pelletized mixture of Ti + 0.5C leads to an increase in the burning rate. It is established that the propagation of the flame front of the pelletized mixture of Ti + 0.5C in flows of nitrogen and argon is controlled by different reactions. In contrast to combustion of powder mixtures of Ti + 0.5C, in combustion of pelletized mixtures of Ti + 0.5C in a nitrogen flow, only one front is observed. It is proved that radiation plays a significant role in the propagation of the combustion front in the pelletized mixture of Ti + 0.5C.     

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.     

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.     

Microstructure of hot-pressed Ti(C,N)-based cermets

Starting from commercial nanosize TiN and carbon black powders, this study set up a synthesis of fine and pure Ti(C,N) powders. The best experimental conditions were found using a mixture TiN+10 wt.% of C processed at 1430 °C for 3 h under flowing Argon. The as-produced Ti(C,N) powder showed regularly shaped particles (100–300 nm), little agglomeration and a C/(C+N) atomic ratio ranging from 0.4 to 0.6. A mixture of Ti(C,N) +15.3 wt% of WC and 9.1 wt% of Ni+Co was prepared and hot pressed at 1620 °C for 30 min (5 MPa of applied pressure). Microstructure and some properties of the sintered ceramic–metal composite (cermet) was investigated by SEM-EDX and XRD. The material exhibited a refined microstructure, but also the presence of textured flaws of several tenths of microns, attributed to not-optimized sintering conditions. The results were compared with those obtained from a cermet manufactured with the same nominal formulation but with commercial TiC_0.5N_0.5 raw powder.     

Combustion of Ti+0.5C and Ti+C mixtures of bulk density in inert gas coflow

The combustion of Ti-C mixtures of bulk density under inert gas blowing conditions produced by evacuation of one end of the reaction cell was studied for the first time. The experiments showed that the tested mixtures in quartz cups were not ignited and did not burn without the inert gas (argon) flow. Increasing the rate of gas evacuation from the sample increased the rate of steady-state combustion of the mixture of titanium with carbon black, and for the mixture of titanium with graphite, stabilization of the flat combustion front was observed. It is shown that the presence of small pressure difference (up to 105 Pa) allows control of the combustion process and confirms the basic postulates of the convective-conductive theory of combustion for heterogeneous condensed systems. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 1, pp. 30–37, January–February, 2009.     

Filtration–Diffusion Combustion Limit of a Titanium Powder in Nitrogen upon Degassing

The propagation of the filtration—diffusion combustion front deep in a bulk of titanium powder in a nitrogen medium is studied experimentally. The dependence of the combustion–propagation depth on the height of a fill of titanium powder and the pressure of gaseous nitrogen, the effect of degassing impurities on the limit, and some specific features of combustion are revealed. A model of the process by means of which a formula for estimation of the burning–out depth of the fill is derived is proposed.     

Synthesis of niobium carbonitride by self-propagating combustion of Nb–C system in nitrogen

An experimental investigation on the preparation of niobium carbonitrides by self-propagating high-temperature synthesis (SHS) was conducted using compacted samples of niobium and carbon powders ignited in gaseous nitrogen. Effects of the carbon content, nitrogen pressure, and NbN addition on the combustion behavior and the degree of conversion were studied. Due to the nature of reaction in the combustion front, steady propagation of the combustion wave is observed in the sample of Nb + 0.3C, whereas the flame front associated with the samples of Nb + 0.5C and Nb + 0.7C travels in an oscillatory manner. Moreover, the flame-front propagation velocity was found to decrease with an increase in the carbon content of the sample. For the samples of Nb + 0.3C and Nb + 0.5C, after the passage of the combustion front the reaction continues lengthily in an afterburning stage, resulting in the nitridation percentage as high as about 80% that is nearly unaffected by the variation of nitrogen pressure. However, due to the lack of afterburning combustion the degree of nitridation in the sample of Nb + 0.7C was decreased to between 53 and 67%. The XRD analysis of final products indicates that in addition to the dominant phase of Nb(C,N), there exist small amounts of Nb and carbon left unreacted.     

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.      

Experimental estimation of impurity-gas pressure during condensed-system combustion in a cylindrical shell

A technique for determining the impurity-gas pressure in a combustion wave is proposed. Use was made of a model mixture of titanium with carbon black pressed into a cylindrical shell with a relative density of0.62. When a combustion wave passes through the point at which the force of friction of the residual matter against the shell wall becomes equal to the force caused by the action of the gas pressure at the combustion front, fracture and a shift of the unburnt part of the matter along the shell occur in the specimen. The friction force for charges of various lengths was calibrated to determine the pressure that causes the specimen to break. The dependence of the pressure on the specific gas evolution of the charge is a monotonically increasing function.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 3, pp. 53–58, May–June, 1996.     

Combustion synthesis of AlN–SiC solid solution particles

The feasibility of synthesising AlN–SiC solid solution ceramics by combustion synthesis (CS) reaction is demonstrated through igniting the mixtures of aluminium, silicon and carbon black under different nitrogen pressure values. The effects of the nitrogen pressure and the atomic ratio of (Si+C)/Al on the crystalline phases formed in the reaction product and on the characteristics of combustion behaviour were investigated. Combining thermodynamic analysis and the combustion characteristics, the reaction sequence and the formation of AlN–SiC solid solution by CS were explained.     

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.     

Effect of particle size on the formation of Ti2AlC using combustion synthesis

This paper provides an insight into the effect of particle size of elemental metal powders and carbon source on the formation mechanism of Ti₂AlC MAX-phase ceramic produced by self-propagating high-temperature synthesis (SHS). The effect of titanium, aluminium and carbon particle size on the 2Ti+Al+C→Ti₂AlC reaction, the phase evolution of the final product and the porosity in both the green body and product has been examined. The effect of the carbon source in the form of graphite, carbon black and short carbon fibres on the reaction mechanism is explained. It is found that the particle size of the titanium and aluminium reactants had little effect on the phases formed but affected the green density of the reactants and the porosity in the final product. The carbon source used in the combustion reaction had an influence on the phases formed by the SHS reaction and was influenced by the dispersion of carbon particles and the titanium–aluminium particle contact.     

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.     

SHS joining in the Ti-Si-C system: Structure of transition layer

Explored was SHS joining in sandwiches of (Ti + yC)/(Ti + xSi) powder compacts, and the transition layers were characterized by SEM, EDS, XRD, and optical microscopy. The transition layers were found to contain titanium carbides, titanium silicides, and Ti₃SiC₂. The results suggest that, in the systems under study, the weld seam is formed due to convective interpenetration of melted combustion products.     

Pressure measurements at the combustion front of gas-free pyrotechnic mixtures with low gas permeability

It is shown that the method of continuous measurement of pressure at the combustion front using an axial-force gauge is applicable to gas-free pyrotechnic systems producing solid reaction products and characterized by low gas permeability of specimens in the initial state. Small inclusions of a high-energy material producing gas combustion products are placed in the specimens to check the reliability of the result. These inclusions are located near the inlet to the main line going to the pressure gauge. The results of experiments on Ti+C+20% TiC specimens containing inclusions of pyroxylin powder are given. Institute of Experimental Physics, Sarov 607190. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 2, pp. 76–80, March–April, 1997.     

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.     

Kinetics of Combustion Synthesis in the Ti-C and Ti-C-Ni Systems

The kinetics and mechanisms of combustion reactions in the Ti-C and Ti-C-Ni systems were studied. Samples were produced by igniting compacts of elemental Ti, C, and Ni powders with a tungsten heating coil under an inert argon atmosphere. Using an elementary model of the process, the "apparent" activation energies of these highly exothermic reactions were determined by measuring combustion wave velocities and combustion temperatures. For the Ti + C ⇒ TiC reaction, two different combustion regimes were found. The first, for combustion temperatures greater than 2711 K, was postulated to be controlled by the dissolution of carbon into a titanium melt with an apparent activation energy of 124 ± 31 kJ · mol⁻¹. The second, for combustion temperatures less than 2711 K, was postulated to be controlled by the diffusion of carbon through a solid TiC layer with an apparent activation energy of 364 ± 25 kJ · mol⁻¹. For the reaction Ti + C + 25 wt% Ni ⇒ TiC + 25 wt% Ni the apparent activation energy was measured to be 133 ± 50 kJ · mol⁻¹, corresponding to the dissolution of carbon into a Ti-Ni melt. Temperature profile and microstructural information are also presented.     

Effect of heat release conditions on the phase composition of the combustion products of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe₂O₃/TiO₂/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 39–43, July–August, 2008.     

Controlling the sizes of in-situ TiC nanoparticles for high-performance TiC/Al–Cu nanocomposites

TiC/Al–Cu nanocomposites were fabricated in Al–Ti–C powder systems using carbon black, a mixture of C and carbon nanotubes (C + CNTs), and CNTs via an in-situ method involving combustion synthesis and hot pressing. As the carbon source changed from pure carbon black to the C + CNT mixture and pure CNTs, the size of the TiC nanoparticles decreased gradually. The nanocomposites synthesized based on the C + CNT mixture exhibited the most uniform dispersion of TiC nanoparticles. The 30 vol% TiC/Al–Cu nanocomposite prepared from the C + CNT mixture had the best comprehensive mechanical properties (yield strength (411 MPa), compressive strength (712 MPa), fracture strain (17.2%), hardness (206.8 HV), and wear resistance under the experimental conditions due to having the most uniformly dispersed TiC nanoparticles. The wear mechanism was a combination of plastic deformation, abrasion, and adhesion. This method may be a low-cost and convenient means to control the sizes of in-situ TiC nanoparticles and prepare high-performance TiC/Al–Cu nanocomposites.     

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.