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.     

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 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.      

Combustion of bulk density powder mixtures in a coflow of inert gas: 4. Ti-Si and Zi-Al systems

Combustion of bulk density Ti-Si and Zi-Al mixtures an a coflow of argon was studied in the presence/absence of (1) applied pressure difference δP along the direction of wave propagation, (2) thermal treatment of green samples in vacuum, and (3) blowing agent (borax). The data on variation of the burning velocity U _c were rationalized in terms of the convection-conduction combustion theory for heterogeneous condensed systems. For Parts 1–3, see Int. J. SHS, 2008, vol. 17, no. 2.     

Combustion of Cr2O3 + Al powder mixtures in a coflow of inert gas: 5. Effect of green density

Combustion of Cr₂O₃ + Al powder mixtures in a coflow of inert gas (Ar) was investigated upon variation in green density in the presence/absence of blowing agents (borax, baking soda). The results were rationalized in terms of the convection-conduction theory for combustion in heterogeneous condensed systems. For Part 4, see Int. J. SHS, 2008, vol. 17, no. 3, pp. 199–205.     

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.     

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 of bulk density powder mixtures in a coflow of inert gas: 6*(Fe2O3 + 2Al) + 30% Al2O3 mixtures

Investigated was the combustion of bulk density (Fe₂O₃ + 2Al) + 30% Al₂O₃ mixtures in a coflow of inert gas (Ar) in the presence/absence of blowing agents (borax, baking soda) and applied pressure difference δP. The results have been rationalized in terms of the conduction-convection theory for combustion in heterogeneous systems.     

Combustion of silicon powders containing organic additives in nitrogen gas under pressure: 2. Composition of combustion products

Combustion of silicon powders containing organic dopants in nitrogen gas under pressure was found to yield a mixture of α-Si₃N₄, β-Si₃N₄, SiC, and Si₂N₂O. Relative amount of these compounds in combustion product was found to depend on the pressure of nitrogen gas, type and concentration of dopants, combustion geometry, and cooling rate. The formation of α-Si₃N₄ was found to occur in the presence of oxygen-containing dopants. The type of dopant was also found to affect the morphology of product particles.      

Combustion of silicon powders containing organic additives in nitrogen gas under pressure: 1. Effect of dopants on combustion phenomenology

Introduction of organic dopants (in amounts up to 5 wt %) to Si powders was found to intensify their combustion in nitrogen gas under pressure without significant influence on the combustion temperature [T _c = 2270 ± 50 K at P(N₂) = 70 atm]. The addition of organic compounds also suppressed the coagulation of Si particles, improved the extent of conversion, and promoted combustion of coarse Si powders. This was associated with the reaction of organic species R (formed upon thermal decomposition of dopants) with solid Si(s) giving gaseous Si(g), which intensifies the processes of mass transport involving organic species R as gas transport agents.      

The gas permeation properties of 6FDA-2, 4, 6-trimethyl-1, 3-phenylenediamine (TMPDA)/1, 3-phenylenediamine (mPDA) copolyimides

Summary The gas permeation behavior of 2, 2’-bis (3, 4’dicarboxyphenyl) hexafluoropropane dianhydride(6FDA)- 2, 4, 6-Trimethyl-1, 3-phenylenediamine (TMPDA)/1,3-phenylenediamine (mPDA) polyimides was investigated by systematically varying the diamine ratios. The physical properties of the copolyimides were characterized by IR, DSC and TGA. All the copolyimides were soluble in most of the common solvents. The gas permeabilities and diffusion coefficients decreased with increasing mPDA content; however, the permselectivity of gas pairs such as H₂/N₂, O₂/N₂, CO₂/CH₄ was enhanced with the incorporation of mPDA moiety. The permeability coefficients of H₂, O₂, N₂, CO₂ and CH₄ were found to decrease with the increasing order of kinetic diameters of the penetrant gases. Moreover, all of the copolyimides studied in this work exhibited performance near, lying on or above the existing upper bound trade-off line between permselectivity and permeability.     

Role of oxygen-vacancy in piezoelectric properties and fatigue behavior of (Bi0.5Na0.5)0.93Ba0.07Ti1+xO3 ceramics

In this reported study, (Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1+xO₃ (abbreviated as BNBT_1+x) ceramics, containing Ti-nonstoichiometry that ranged from a 2% deficiency to a 1% excess, were designed and systematically characterized. The results of the X-ray diffraction Rietveld refinement and X-ray photoelectron spectroscopy analysis of these materials revealed that the amount of Ti in the BNBT_1+x ceramics significantly affected the degree of coexistent rhombohedral/tetragonal phases and also affected the content of singly/doubly charged oxygen-vacancy (/) in the ceramics. After poling and 10⁵ fatigue cycles, the variation in Raman resonance line-width of the Ti–O bond in the BNBT_1+x ceramics was found to be strongly dependent on the amount of Ti in the ceramic. The → transformation and clustering of the defects under electrical loading were considered to be a critical factor in electric field-induced structural transition and fatigue properties of the material.     

Study of relaxor behavior in a lead-free (Na0.5Bi0.5)TiO3-SrTiO3-BaTiO3 ternary solid solution system

In this work, single phase lead-free (0.8-x)(Na_0.5Bi_0.5)TiO₃−0.2SrTiO₃-xBaTiO₃ (NBT-ST-BT) ceramics were prepared by conventional solid state reaction method. The effect of BT on the structure and on the dielectric and ferroelectric properties of NBT-ST-BT were investigated. A structural transformation from pseudo-cubic to tetragonal along with possible phase coexistence was witnessed as the BT content was increased. A diffuse phase transition with considerable frequency dispersion in the dielectric response and slim P-E loops evidenced strong relaxor behavior for the ternary system at higher compositions of BT. An analysis of the frequency dependent T_m according to V-F and Power law indicated substantial interaction between the polar nano-regions and relatively broad distribution of freezing temperatures. The study of the dielectric constant at much higher and lower temperatures than T_m in the range of Burn's temperature (T_B) to freezing temperature (T_f) to provide useful information about the growth rate of polar nano-regions and their interactions for a better understanding of the relaxor behavior exhibited by the present ternary system.     

Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 1: The mechanisms of combustion and structure formation

The self-propagating high-temperature synthesis (SHS) in Zr–Nb–B system has been thoroughly investigated with a focus on its macrokinetics and phase formation mechanisms. The activation energies were determined by analyzing the relationship between the combustion rate and temperature. For compositions NbB₂–40%ZrB₂ and NbB₂–50%ZrB₂, the increase in the initial temperature has resulted in a significant change in combustion rate's dependence on temperature, signifying a possible shift in the reaction mechanisms. The quenched combustion front technique (QCF) method in conjunction with thermodynamic and ab initio calculations were used to analyze the sequence of phase formation events, including the formation of niobium and zirconium borides through gas transport reactions involving volatile boron suboxide BO in the heating zone, followed by the emergence of a zirconium-boron melt in the combustion zone and formation of main fraction of niobium and zirconium borides. Interactions between the primary niobium and zirconium borides and the melt result in the formation of solid solutions; however, at sub-optimal combustion conditions multiple non-equilibrium solid solutions are retained in the products. To address this issue, a machine learning model was developed, attaining coefficients of determination (R²) of 89% for combustion temperature and rate predictions, thus enabling the fine-tuning of macrokinetic parameters of the SHS process in the system.     

Formation of Core/Rim Structures in Ti(C,N)-WC-Ni Cermets via a Dissolution and Precipitation Process

The core/rim structures of compounds in the Ti(C,N)- x WC-20Ni system have been investigated to determine the effect of WC and nitrogen content on the microstructure of the system. In addition, the relative dissolution rate of WC to Ti(C,N) in the system was studied by analyzing the rim compositions. Variations in the WC content had a much-lesser influence, as an additive, on the general microstructure than that of other carbides that have been used in previous studies. However, the nitrogen content in Ti(C_{1− X} N _X ) had a significant effect on its microstructure. The composition of the rim structure was determined by the ratio of solutes that were dissolved in the liquid binder, under the given processing conditions. The dissolution rate of WC was ∼2 and 5 times faster than that of Ti(C,N) in the system at temperatures of 1300° and 1510°C, respectively. The results have been interpreted in terms of phase stability and precipitation phenomena.     

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.