A numerical study of the heterogeneous porosity effect on self-propagating high-temperature synthesis (SHS)

Self-propagating high-temperature synthesis (SHS), or the so-called micropyretic/combustion synthesis, is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneous distributions of porosities are common during self-propagating high-temperature synthesis when powders are pressed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the porosity are thought to result in local variations of such thermophysical/chemical parameters for the reactants as density and thermal conductivity further changing the combustion temperature, the propagation velocity, and the propagation pattern of a combustion front. This study investigates the impact of porosity variations during self-propagating high-temperature synthesis with Ti + 2B. In addition, the simulations for the propagation of combustion fronts across a non-uniform compact where the porosity is monotonically decreased or increased along the specimens due to die wall friction are also carried out.     

Numerical analysis for micropyretic synthesis of NiAl intermetallic compound

A numerical investigation of the micropyretic synthesis response parameters of the Ni-Al stoichiometric compound was undertaken. The influence of the enthalpy of the combustion reaction,Q, activation ienergy,E, amount of diluent, pre-exponential factor,K ₀, and initial temperatureT ₀, on the combustion velocity, temperature, and mode was studied. The porosity of the unreacted compact, which is related to the initial compaction pressure, was considered in the calculation. It was found that the change in porosity significantly affects the thermal conductivity and the length of the pre-heat zone as also do the temperature patterns and propagation velocities. The combustion front was noted to be extinguished if the temperature in the reaction zone became lower than the melting point of the aluminium phase. This result was obtained simply by considering the changes in the thermal conductivity after the melting of aluminium without having to invoke any changes in the rate of reaction after the melting. A comparison of the numerical data with the experimental and analytical results was also made.     

The energy required to ignite micropyretic synthesis. Part I: stable Ni + Al reaction

The progression of chemical reactions is determined by both thermodynamics and kinetics factors. Micropyretic/combustion reaction is a cascade of many chain chemical reactions and thermodynamics and kinetics of the ignition reaction are expected to greatly affect the overall reaction outcome. Furthermore, the stability of the sequential reaction and its progression are correspondingly changed once micropyretic parameters are changed. Improper ignition of micropyretic reaction provides either excessive or insufficient external energy, thus causes over-heating or extinguishing of the combustion front during propagation and therefore the heterogeneous structures. To achieve the homogeneous micropyretic reaction, it is thought possible to control ignition energy. A numerical study on the correlation of thermodynamics and kinetics factors of ignition on the stable Ni + Al reaction and the required ignition energy is reported in this study. The influences of activation energy (E), enthalpy of the micropyretic reaction (Q), pre-exponential factor (K _o), thermal conductivity (K), heat capacity (C _p ), and thermal activity of the reactants and product, on the temperature/heat loss at the ignition spot and the length of pre-heating zone are respectively studied. It is found that the activation energy and heat capacity have the most significant effects on the ignition energy. The required ignition energy is increased by 44.0% and 23.9%, respectively, when the activation energy and the heat capacity are both increased by 40.0%     

Centrifugal effects on combustion synthesis of (Ti-B-C) compound system

A centrifugal force plays an important role on the control of combustion synthesis. In the present work, the data of reaction propagation rates obtained by changing the direction of reaction propagation and centrifugal force are evaluated in order to make clear the effect of centrifugal force on reaction propagations and product formation. As a result, the reaction propagation rate in the case of the direction of centrifugal force inverse to reaction propagation is larger than that in the case of the same direction, and product grains become smaller in size. It is confirmed that the centrifugal effect is much larger for the present combustion synthesis process in the case that the reaction propagates inversely to the direction of centrifugal force. Since molten titanium near combustion front tends to coalesce into larger drops in that case, reactants of boron and carbon would diffuse more sufficiently into titanium.     

Influence of multi-dimensional oscillating combustion fronts on thermal profiles

A multi-dimensional numerical model for micropyretic/combustion synthesis was developed and then applied to a special configuration. The configuration was chosen to illustrate the differences between one-dimensional and two-dimensional combustion. The features of the model include the melting of each constituent of the reactants and the products, and considerations of porosity for both the reactants and the products. Application of this model to the oscillatory combustion synthesis of TiB₂ has been carried out, for the first time, to study two-dimensional-combustion-front movement. The model predicts higher hot-spot temperatures in a two-dimensional situation than those obtained in a one-dimensional experiment. Additionally, hot spots are noted to traverse along orthogonal directions. Some processing implications of such results are examined.     

Self-propagating high-temperature synthesis of molybdenum disilicide

Molybdenum disilicide was synthesized from elemental reactants in argon and vacuum atmospheres by utilizing the exothermicity of the reaction using self-propagating high temperature synthesis. Experiments were carried out using powdered reactants and compacts with varying densities. The reaction front propagated at a finite velocity depending upon the atmosphere, the diameter of the pellet and the particle sizes of the reactants. The exothermicity of the reaction between molybdenum and silicon raised the temperature of the product to 1886 K, which is close to the theoretical adiabatic combustion temperature, 1900 K. X-ray diffraction analysis of the product confirmed the product to be a single phase MoSi₂ crystallizing in a tetragonal structure. Microstructural examination revealed melting of Si and its capillary flow, and chemical analysis indicated that the product is much purer than the reactants.     

Dimensional changes during micropyretic synthesis

This article reports on the influence of the processing and material parameters, namely initial temperature, compaction pressure, particle size, diluent, and chemical composition on the dimensional changes of the product during micropyretic synthesis. The relationship between the porosity change and elongation is established. From a knowledge of this relationship it is possible to tailor the dimensions of a synthesized product for proper design in an engineering application. Conversely, from the noted dimensional changes it is possible to obtain the final porosity values and thus the mechanical properties.     

场激活燃烧合成碳化钨(Ⅰ)——场激活燃烧合成

研究了场激活下燃烧合成碳化钨,研究结果显示,只有当施加的场强超过临界值(1V·cm^-1),燃烧波才能蔓延下去.燃烧产物的特性与场强有关,当场强增加时,X射线衍射圈中WC相的衍射峰强度增强,表明碳在钨中的扩散随场强的增强而增大.钨颗粒的粒径大小和样品初始相对密度对燃烧温度和燃烧波蔓延速率的影响研究表明,随着钨颗粒的缩小,燃烧温度和燃烧波蔓延速率变大,而燃烧温度和燃烧波蔓延速率的最大值出现在一个合适的相对密度处.     

An investigation of the ignition manner effects on combustion synthesis

The multi-point ignition of combustion synthesizing NiAl compound created by computational means has been analyzed in this article. Since the combustion reaction of Ni and Al is a low exothermic reaction, it has been found that the combustion front hardly propagates in order to complete the reaction. In this study, the reaction is subsequently ignited at different points or it is simultaneously ignited at several points to help the combustion front to propagate completely. The different positions of ignition are found to influence the temperature profiles and an increase in the number of ignition points is noted to increase the propagation velocity. In addition, the effect of a second ignition of the extinguished combustion reaction is also studied. It is noted that the position and time of the second ignition has dramatically influenced on the propagation velocity and combustion temperature, thus resulting in different grades of reactions. The extent of reacting for each double-ignition condition is calculated in order to generate the reacting maps. From the reacting maps generated in this study, the appropriate double-ignition condition can be chosen to synthesize homogeneous products.     

Numerical modeling of field-activated combustion synthesis process of the B4C system

A field-activation combustion synthesis process of the 4B + C reactive system was numerically simulated to investigate the effect of external field and porosity on the combustion reaction by an implicit difference method and a Gauss-Seidel iteration procedure. The new features of the model include a consideration of the melting of each constituent of the reactants and product and the inclusion of considerations involving porosity and dilution. The results show that the self-sustaining reactions are not possible until field-activated temperature is more than 1100 K, which agree with the theoretic calculation. As the reactant porosity values are decreased from 60% to 20%, the combustion velocity first increases because of an increase in the thermal conductivity. The combustion velocity, after reaching a maximum, decreases with a further decrease in the porosity because of the high value of the thermal conductivity of the reactants.     

Structure of the combustion wave in the combustion synthesis of titanium carbides

From the recorded images of combustion wave propagation during the combustion synthesis of titanium carbide, real time combustion velocities have been determined with increase of density of the compact. Results demonstrate that the combustion velocity exhibits a maximum with increase of density of the compact, and the wave propagates in a steady-state manner. Real time temperature profiles suggest that adiabatic conditions exist during the steady-state propagation of the combustion wave. Analysis of the images for the propagation of the combustion wave suggest that density plays a major role in the nature of propagation and whether the combustion wave propagates or not. Pore structural analysis of the carbides indicate collapse of the original porosity of the carbons, and the collapse of porosity is attributable to an exothermic diffusional reaction occurring between liquid titanium and carbon forming a titanium carbide product layer.     

Cost effective combustion synthesis of silicon nitride

The feasibility of mechanical activation (MA)-assisted combustion synthesis (CS) of Si₃N₄ was demonstrated by using Si/NH₄Cl as reactants under a nitrogen pressure of 2 MPa. MA treatment significantly enhances the reactivity of Si powders, which effectively promotes the nitridation of silicon. The NH₄Cl had the same effect as Si₃N₄ diluent in preventing extensive melting of Si within the combustion wave zone. Full nitridation of Si was achieved at diluent levels as low as 5 wt%. Si₃N₄ powders with -phase contents of up to 90.6 wt% were obtained. The temperature gradient induced by heat release through radiation was responsible for discrepancies in the phase composition and morphologies of the as-synthesized product at different locations.     

高压氮气中自蔓延燃烧合成氮化钛

利用钛粉在高压氮气中的自蔓延燃烧合成（ＳＨＳ），制备了含氮量较高的ＴｉＮ，研究了反应物的松装密度、氮气压的改变与稀释剂的加入对燃烧波蔓延速率和产物转化率的影响，还观察到燃烧方式的改变。     

高压氮气中自蔓延燃烧合成氮化铝

高压氮气下，自蔓延燃烧合成（SHS）氨化铝实验中，研究了稀释剂含量、添加剂含量、氮气压力、反应物的相对密度、反应物厚度对燃烧波最高温度、燃烧波蔓延速率的影响，并制备了含氮量较高（33.4Wt％）的氨化铝.     

高压氮气中自蔓延燃烧合成氮化铝

高压氮气下，自蔓延燃烧合成（ＳＨＳ）氧化铝实验中，研究了稀释剂含量、添加剂含量、氮气压力、反应物的相对密度、反应物厚度对燃烧波最高温度、燃烧波蔓延速率的影响，并制备了含氮量较高的（３３．４ｗｔ％）的氮化铝。     

Role of mechanical activation in SHS synthesis of TiC

The effect of the mechanical activation of the reactants on the self-propagating high-temperature synthesis (SHS) of titanium carbide was investigated. The SHS experiments were performed on two compositions, Ti₅₀C₅₀ and Ti₇₀C₃₀, which define the homogeneity range of the TiC equilibrium carbide. Milling times were progressively increased up to the time at which a combustion-like process ignites spontaneously under milling. The combustion peak temperature, wave velocity, and ignition temperature were markedly influenced by the degree of mechanical activation of the reactants. In particular the ignition temperature was observed to decrease from a temperature corresponding to the melting point of Ti to 500°C. The apparent activation energy for propagation of the combustion wave was also determined (～100 kJ·mol^?1) and was found to be independent of both the degree of mechanical activation and the composition of the starting mixture.     

In-situ combustion synthesis and densification of TiC–xNi cermets

The combustion synthesis reaction was combined with quasi-isostatic pressing (QIP) technique to fabricate full density TiC–xNi composites in a single processing operation. Combustion wave velocity and temperature of Ti–C–Ni were measured and the microstructure of the product was characterized by X-ray diffraction and scanning electron microscopy. With increasing Ni content in TiC–xNi, both the combustion wave velocity and temperature decrease. The Ni additive, mainly as a diluent and the binder of TiC grains in a matrix, formed a quasi-continuous phase enveloping spheroidal TiC particles and brought about a grain size decrease from 9 to 1 μm. TiC-20 wt% Ni cermet produced by the combustion synthesis/quasi-isostatic pressing process under 160 MPa for 20 s show near full density, high hardness and transverse rupture strength (1024.2 MPa).     

Propagation of a combustion front over the surface of metal powders with diluents

An experimental study of the effect of foreign inclusions of both chemically active materials, capable of burning in an air atmosphere and inert oxide materials on the propagation of a combustion front over the surface of titanium powders was carried out. It has been established how the character and the rate of surface combustion V_c of titanium powders change upon their dilution, depending on diluent concentration, melting temperature, disperse composition, and chemical activity.Institute of Structural Macrokinetics, Russian Academy of Sciences, Chernogolovka. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 4, pp. 394–402, October, 1993.     

铝粉在高压氮气中自蔓延燃烧合成氮化铝

利用铝粉在高压氮气中的自蔓延燃烧合成（SHS）方法,制备了氮含量较高（33．5wt％）的AIN,研究了稀释剂含量、添加剂含量、氮气压力、反应物的相对密度、反应物厚度对燃烧产物氮含量的影响,并对后燃烧现象进行了分析,产物中发现氮化铝晶须和棒晶．     

The energy required to ignite micropyretic synthesis. Part II: unstable Ti + 2B reaction

In the previous study, the influences of micropyretic parameters on the ignition energy for lower exothermic heat and lower activation energy of stable Ni–Al reaction have been studied. In this study, we studied the effect of ignition energy on the unstable Ti + 2B micropyretic reaction, which has higher exothermic heat and higher activation energy. Three-dimensional maps are generated to illustrate the influences of micropyretic synthesis parameters on the required ignition energy. In addition, the comparisons in the ignition energy for the micropyretic reactions with Ni + Al and Ti + 2B are studied. The numerical calculation indicates that the changes in the ignition energy caused by the thermal conductivity for the micropyretic reaction with Ti + 2B is smaller as compared with for the micropyretic reaction with Ni + Al. However, the required ignition power is found to significantly change with the thermal conductivity for the NiAl micropyretic reaction when a higher pre-exponential factor is taken in the calculation. In addition, the difference in the required ignition energy caused by the thermal conductivity is noted to be larger for the Ni–Al micropyretic reaction with a lower pre-exponential factor.