自蔓延高温合成（SHS）过程的点火模型与分析

基于热力学原理，提出了一个分析自蔓延反应中点火过程的模型，并推出了一个简单，直观的点火公式。认为燃烧合成的自动蔓延过程是样品中一层一层的逐层点火过程，因此，点公式也可以用来解释自蔓高温合成过程本身。对ＮｉＡｌ和ＴｉＢ２等的点火过程实际研究表明，理论模型有一定的指导意义。     

The effect of aluminum substitution on the mechanically induced self-sustaining reaction of molybdenum-silicon powders

The effect of Al concentration on the ignition of mechanically-induced self-sustaining reaction (MSR) in Mo + (2 ? x)Si + 2xAl powder mixtures has been investigated. When MoSi₂ is prepared by SHS method, Al addition results in the formation of Si-Al eutectic and consequently lowers the ignition temperature. In the case of MSR, replacing brittle Si with ductile Al also affects the activation process. The ignition of MSR was investigated as a function of the Al content in a SPEX 8000 Mixer Mill. The ignition time decreased by 30% between x = 0 and 0.08. At the same time, the grain size increased by about 50% according to XRD linewidth measurements. Comparison of SEM images suggests that Al substitution promotes the formation of agglomerates and lamellar structures, although the differences between the binary Mo-Si and Alcontaining samples are very small. Al substitution encourages the formation of hexagonal Mo(SiAl)₂ over the tetragonal form of MoSi₂. It also promotes more complete conversion during MSR.     

容器内自蔓延高温合成固化过程的热力学数值模拟

自蔓延高温合成固化是一种利用氧化还原反应放热处理高放废物煅烧灰、污染土壤等固体放射性废物的方法,在密封容器内对放射性废物采用自蔓延高温合成固化技术可有效提高产物的致密度并控制有害气溶胶的扩散。通过分析自蔓延化学反应机理和过程,建立容器内自蔓延高温合成固化数值模拟模型,对固化过程容器内温度-压力变化特征进行分析。利用COMSOL有限元分析软件,确定模型尺寸、单元类型和边界条件,计算得到点火后7200 s的容器内温度和压力随时间变化曲线。与相同条件的容器内SHS固化实验测量结果对比分析,数值模拟与实测情况一致性较强,能够较好反映SHS固化过程点火、燃烧、熄火、保温的四阶段性特征;模拟结果可作为容器安全性设计的参考数据。     

Effect of mechanical activation on ignition and combustion of Ti-BN and Ti-SiC-C blends

Explored was the effect of mechanical activation on ignition and combustion of Ti-BN and Ti-SiC-C blends. Activated blends and combustion products were characterized by high-precision XRD and SEM. SHS in the systems under study was found to proceed in a mode of solid flame (no melted intermediates/products).     

SHS of Ti3SiC2: ignition temperature depression by mechanical activation

The influence of high-energy mechanical alloying on the self-propagating high-temperature synthesis (SHS) of titanium silicon carbide (Ti₃SiC₂) was investigated. A depression of the SHS ignition temperature as a function of milling time was observed. After 107 min of milling, a spontaneous combustion reaction (MASHS) occurred within the milling vial at 67 °C, which corresponded with an 25 °C rise in the vial temperature. Observed changes in the microstructure of the milled powders gave considerable insight into the process, which provides a means of controlling a previously difficult synthesis procedure.     

Observations on combustion front propagation in self-propagating high-temperature synthesis process producing refractory ceramics

High-temperature refractory ceramics can be produced in the combustion regime by using self-propagating, high-temperature synthesis (SHS) processes. The numerical simulation of the SHS process in a simplified diffusion-reaction system is investigated. The SHS process is simplified by the one- and two-dimensional pseudo-homoge-neous environment. The stiff equations of the SHS process are solved by using finite difference methods on two-dimensional adaptive meshes. Travelling waves with constant patterns are observed for adiabatic and nonadiabatic systems. For higher values of heat of reaction and activation energy, the combustion front starts to oscillate. Single and complex oscillating waves are detected. In oscillating combustion fronts, the temperature can overshoot the adiabatic temperature to result in the complete conversion of solid reactants. In two dimensional systems, travelling, fingering, and rotating waves are detected in the combustion synthesis process.     

The autoignition of polymers

A consistent and relatively simple method is presented for studying the unpiloted ignition of polymeric materials in contact with hot air. The ignition behavior of a particular polymer as determined by its bulk properties may be characterized by the relationship between sample mass and ignition time at constant area for a series of furnace temperatures. Extrapolation of this linear relationship at a given furnace temperature to zero mass results in an intrinsic ignition time which represents the rate of the ignition process when the time required to heat the sample to its decomposition temperature has been eliminated, i.e., the sample is brought to its decomposition temperature instantaneously. The temperature dependence of this intrinsic ignition time shows an Arrhenius relationship with an apparent activation energy of 8–10 kcal/mole for all but one material investigated. This indicates that the mechanism controlling the kinetics of such an ignition process is a physical one, most likely the diffusion of combustible gaseous products into the heated air surrounding the sample.     

High porosity SiC ceramics prepared via a process involving an SHS stage

The preparation of porous SiC ceramics from stoechiometric mixtures of silicon and graphite has been studied. Products with very high pore contents (≈80%) were obtained using a process which consisted of heating the reactive pellets in purified argon, at 15 °C min⁻¹, up to 1430 °C and applying a weak d.c. voltage across the sample for 20 s. The resulting electrical current was necessary for the ignition of an SHS reaction simultaneously in the whole sample. The analysis of the sample microstructure evolution all along the process has enabled the identification of the different mechanisms involved in the SiC formation. Before the SHS stage, the formation of silicon carbide, during heating from about 1325 up to 1430 °C, is associated with a large sample expansion, which mainly determined the final pore volume fraction. The pore transfer mechanisms, which occur during the SHS stage at 1430 °C, have a specific influence on the pore development. Since the final pore size distribution is strongly related to silicon grain granulometry, the porosity of the porous SiC ceramic, obtained by this process, can be easily modulated.     

One-step synthesis and densification of Ti-Al-C-based cermets by ETEPC

The Electro-Thermal Explosion under Pressure with Confinement (ETEPC) mode of SHS has been studied for producing possibly low-porosity Ti²AlC and Ti³AlC² from elemental powders in a very short processing time. The effect of maintaining the electric current after the ignition of exothermic reaction has also been studied. A better achievement of the desired reaction, with a total yield of MAX phases up to 73%, was obtained with maintaining the electric current for 4 s after ignition of the exothermic reaction in the Ti: Al: C = 2:1:1 composition. XRD studies revealed also composition gradients.     

自蔓延高温技术制备ZrC粉体(英文)

采用自蔓延高温合成(self-propagating high-temperature synthesis,SHS)技术,以 Zr+C 为反应体系合成了 ZrC 粉末。研究了实验参数对 SHS过程中点火电流、燃烧温度的影响。采用了 3 种碳源,研究了其对最终产物形貌及化学组成的影响。通过添加不同含量的 NaCl 作为 SHS 稀释剂,控制产物粒径及形貌。结果表明:炭黑是高温自蔓延法制备 ZrC 粉体的最佳碳源。由该体系制备的 ZrC 粉末粒径在 0.5～1 μm之间,氧含量为 0.38%。随稀释剂 NaCl 含量增加,体系燃烧温度降低,产物粒径减小。当 NaCl 含量为 30% (质量分数)时,体系燃烧温度下降至 1 810 K,产物 ZrC 粉末的粒径减小至 50 nm。     

活化温度对活性炭自燃危险性的影响

研究揭示活性炭的自燃性质及其关键影响因素,有助于避免和降低活性炭在储运过程中的自燃危险性。以杨木粉为原料,氯化锌为活化剂,在300~700℃活化温度下制备了活性炭,并考察了自制活性炭的自燃温度随活化温度的变化规律。用X射线衍射和傅里叶红外光谱研究了样品的微观结构及表面官能团特征;用STA同步热分析仪研究了活性炭的热稳定性。研究表明,活化温度对活性炭自燃性有显著影响;活化温度越低,活性炭表面官能团种类越多、含量越大,越容易自燃;随活化温度的升高,活性炭的DSC和TG曲线均向高温区移动,活性炭趋于稳定,不易自燃。适当提高活化温度,有利于降低活性炭的自燃危险性。     

Numerical simulation of the effect of multiple factors on the ignition process of a solid rocket motor

With the continuous development of manned space technology, higher requirements have been proposed for solid rocket motors. The ignition process of solid rocket motors affects the reliability, stability and safety of their operation. The ignition powder, cover opening pressure and grain length-diameter ratio are the main factors affecting the ignition process. Therefore, the influence of different factors on the ignition process of solid rocket motors is studied with numerical simulations. Based on the finite volume method, the ignition process of a solid rocket motor is modelled and experimentally verified. Then, the pressure and temperature distribution characteristics during the ignition delay, flame propagation and gas filling times are analysed. Finally, the effects of different ignition powders, cover opening pressures and length-diameter ratios on the ignition process are compared and analysed. The results show that the model has high prediction accuracy. When the ignition powder is 6 g, the maximum combustion temperature of solid rocket motor increases from 2590 K to 2620 K between 0.1 ms and 0.44 ms. Between 0.44 ms and 3.18 ms, intermittent flame propagation and pressure oscillations occur. In the gas filling time, the flow field gradually stabilizes. Increasing the ignition powder mass is beneficial to the ignition process, but the disadvantages of pressure oscillations should be considered. Increasing the cover opening pressure enhances the ignition process, while increasing the length-diameter ratio increases the ignition pressure building time. The study results provide technical support for the structural design of solid rocket motors.     

Thermal explosion in mechanoactivated 3Ni + Al mixtures

Explored was thermal explosion in mechanoactivated 3Ni + Al mixtures. Mechanoactivation was found to result in an abnormal decrease in the effective activation energy E and ignition temperature T_ign for thermal explosion. Analysis of reaction thermogram allowed us to find out the kinetic function. Mechanoactivation conditions for synthesis of Ni₃Al in thermal explosion mode have been optimized. SHS reaction in 3Ni + Al mixtures mechanoactivated for 180 s was found to obey the first-order kinetics.      

Mathematical modelling of structure formation during combustion synthesis of advanced materials

The synthesis of advanced materials by a wave of heterogeneous combustion propagating through a charge mixture, or the so-called self-propagating high-temperature synthesis (SHS) has many potential advantages over conventional techniques of synthesis. Because of high heating rates, steep temperature and concentration gradients, and fast accomplishment of reactions, the mechanisms of physical, chemical and structural transformations in the SHS wave are intricate and often not known. Further understanding of interaction mechanisms in SHS waves, relating the process parameters to structure and properties of the target material, and the application of SHS to producing final articles necessitates developing mathematical models for SHS-related phenomena.Various aspects of mathematical modelling of SHS are discussed in this paper. They include the analysis of novel factors influencing the structure formation, viz. the heating-to-reaction and mass transfer-to-reaction time ratios, autocatalysis and intrinsic stochasticity, which are unnoticed in traditional synthesis methods. The application of chemical thermodynamics and combustion theory to modelling SHS processes is outlined. Novel mathematical models are developed for SHS on condensed systems, which involve stochastic effects and autocatalysis. New models for solid-gas systems are worked out, which include the reaction kinetics and mass transfer of a gaseous reactant and permit predicting the structure formation pattern in the SHS wave. Application of mathematical modelling to producing porous final articles by means of SHS is discussed.     

SHS of shape memory CuZnAl alloys

Aiming at preparation of shape memory alloys (SMAs), we explored the SHS of Cu_{1 − x} Zn_{1 − y} Al_{1 − z} alloys (0.29 < x < 0.30, 0.74 < y < 0.75, and 0.83 < z < 0.96). The most pronounced shape memory effect was exhibited by the alloys of the following compositions (wt %): (1) Cu(70.6)Zn(25.4)Al(4.0), (2) Cu(70.1)Zn(25.9)Al(4.0), and (3) Cu(69.9)Zn(26.1)Al(4.0). The effect of process parameters on the synthesis of CuZnAl alloys was studied by XRD, optical microscopy, and scanning electron microscopy (SEM). The grain size of CuZnAl was found to depend on the relative amount of the primary CuZn and AlZn phases. Changes in the transformation temperature and heat of transformation are discussed in terms of ignition intensity and compaction. Mechanism of the process depends on the level of the temperature attained relative to the melting point of components. At the melting point of AlZn, the process is controlled by the solid-state diffusion of AlZn into a product layer. The ignition temperature for this system depends on the temperature of the austenite-martensite transformation in CuZnAl alloys. The composition and structure of the products was found to markedly depend on process parameters. The SHS technique has been successfully used to prepare a variety of SMAs.      

Laser ignition of nickel-aluminum powder systems

A method for determining the thermokinetic constants of the SHS reaction and thermophysical properties of the initial mixture and reaction products involving the use of laser initiation of the reactive mixture has been developed. A technique for experimentally studying the ignition process of the reacting mixture is describedScientific Research Institute of Building Materials, 634003, Tomsk. Translated from Fizika Goreniya Vzryva, Vol. 30, No. 2, pp. 14–18, March–April, 1994.     

Effect of preheating on synthesis of tantalum nitride by self-propagating combustion

An experimental investigation of self-propagating high-temperature synthesis (SHS) of tantalum nitride (TaN) was conducted with tantalum compacts in nitrogen of 0.27–1.82 MPa. Effects of sample density, nitrogen pressure, and preheating temperature on the flame-front propagation velocity, combustion temperature, degree of conversion, and product composition were studied. Results showed that the SHS process of the tantalum/nitrogen reaction was characterized by the steady propagation of a planar combustion front, followed by a prolonged afterburning reaction. The flame-front velocity increased with nitrogen pressure, but decreased with sample density. Preheating the sample prior to ignition contributed higher combustion temperatures, thus leading to an increase in the conversion percentage. For the unpreheated samples, the conversion increased significantly with nitrogen pressure and reached around 80% at 1.82 MPa of N₂. With preheating temperatures between 150 and 300 °C, the conversion was increased by about 15% when compared with that without preheating. The nitride phase TaN was identified by XRD as the dominant composition in the combustion product.     

气相传输SHS合成Ti5Si3的过程参数与产物结构的研究

气相传输自蔓延高温合成技术是通过引入气相传输助剂来改变ＳＨＳ过程的一种方法。ＧＴＡ的引入对ＳＨＳ过程参数，产物结构等产生显著的影响。本研究采用气相传输自蔓延高温合成法制备了Ｔｉ５Ｓｉ３，并着重研究了ＧＴＡ对热爆和自蔓延燃烧过程参数的影响，运用ＸＲＤ和ＳＥＭ对产物的组成与微观结构进行了分析。     

TiB2的自蔓延高温合成过程研究

研究了原料组成、稀释剂含量和颗粒尺寸、性质等对ＴｉＢ２的自蔓延高温合成过程的影响。随着原料颗粒尺寸的增大，燃烧温度和燃烧波速度都减小；随着稀释剂含量的增加，燃烧温度、燃烧波速度和合成样品孔隙率都呈递减趋势，最终出现不稳定燃烧波，在垂直蔓延波方向形成穿透样品的片状裂纹，以燃烧温度３０５０Ｋ为分界点，在高燃烧温度区和低燃烧温度区里，过程激活能分别为１４０ｋｊ／ｍｏｌ和３５５ＫＪ／ｍｏｎ，预示着不同的反     

陶瓷材料的SHS超快速致密化技术

自蔓延高温合成(self-propagating high-temperature synthesis,SHS)是一种在数秒～几十秒内即可完成的合成技术,但是直接产物是多孔状的.在SHS产物还处于高温软化状态时立即快速加压(quick pressing,QP),可以一步实现合成和致密化获得致密材料,这一技术被称为SHS/QP技术.传统的高温烧结的致密化过程主要靠原子扩散实现,SHS/QP过程如此之快,其致密化机理尚不明确.为此,探讨了SHS/OP技术制备金属陶瓷和复相陶瓷的致密化机理.研究了SHS/OP技术制备金属陶瓷、复相陶瓷和叠层复合材料的结构和工艺过程,并制备了密实材料.结果表明:基于塑性变形机理,SHS/QP技术能制备出密实、晶粒基本不长大的纳米陶瓷.