Mechanistic Studies of Combustion Synthesis

在这个项目中，将使用两种技术来研究通过SHS工艺合成材料的基本原理；第一种技术涉及在反应过程中冷却颗粒，第二种技术涉及研究衬底加热表面上的粉末颗粒。第一个程序将通过对嵌入在高导热系数块中的楔形样品进行反应来进行，从而熄灭向楔形顶端传播的穿过样品的燃烧波。淬火后，样品将被切割和抛光以进行金相研究，并将进行逐层X射线衍射分析，以定量相含量随位置的变化。扫描电子显微镜/能谱分析结合定量光学显微镜将被用来观察局部成分、颗粒大小和孔分布的变化。将研究样品制备条件，如粉末粒度和生坯密度的影响。这些实验将提供有关反应前沿传播时混合物微观结构演变的信息，从而深入了解可用于控制过程和调整产品性能的机理。在第二个步骤中，将窄尺寸的反应物粉末(较低熔点组分)分散在另一种反应物的箔上，以SHS加工中通常遇到的速度进行电加热。加热后，粉末将融化并在叶片表面铺展；在不同的加热速度和颗粒大小下，将研究铺展的动力学(在SHS加工中很重要)。在进一步加热时，会发生反应；这些反应将通过电流关闭来熄灭，并对生成的样品进行扫描电子显微镜/能谱分析，描绘反应和微观结构演变。最初的注意力将集中在Ni3Al的合成上，后来扩展到其他金属间化合物。通过燃烧反应合成先进材料是一种有吸引力的替代其他材料制备方法的方法，因为它简单、能量需求相对较低、产品纯度潜力高、以及(可能最重要的)由于快速冷却和高反应速率的组合而产生亚稳态产物的机会。在燃烧合成中，在粉末床中的高度放热的自持反应产生最终产品；在这种方法的一个变种，自蔓延高温合成(SHS)中，在粉末样品的一端开始的反应以燃烧波的形式通过它自传播，而在另一种(热爆炸模式)中，样品被均匀加热，直到整个样品开始反应。燃烧合成工艺进一步发展的一个非常重要的任务是通过显微组织控制来控制产品的性能；这种控制的实现依赖于对在传播燃烧锋面及其后面发生的化学和结构变化的基本了解。

In this program, two techniques will be used to study the fundamentals of materials synthesis via SHS processing; the first involves quenching pellets during the reaction, while the second involves study of powder particles on a heated surface of a substrate. The first procedure will be carried out by reacting a wedge-shaped sample imbedded in a high thermal conductivity block, resulting in extinguishment of a combustion wave propagating through the sample towards the apex of the wedge. After quenching, the sample will be cut and polished for metallography study and a layer-by-layer X- ray diffraction analysis will be performmed to quantitate variation of phase content as a function of position. SEM/EDXS studies combined with quantitative optical microscopy will be used to observe changes in local composition, grain size, and pore distributions. Effects of sample preparation conditions, such as powder particle sizes and green density, will be studied. These experiments will provide information on evolution of mixture micro-structure as the reaction front propagates, yielding insight into mechanisms which can be used to control the process and tailor the product properties. In the second procedure, a narrow size cut of reactant powder (lower-melting component) will be dispersed on a foil of the other reactant, which will be electrically heated at rates typically encountered in SHS processing. Upon heating, the powder will melt and spread on the foli surface; the dynamics of spreading (important in SHS processing) will be studied at various heating rates and particle sizes. Upon further heating, reactions will occur; these will be quenched by current shutoff, and the resulting samples analyzed with SEM/EDXS, delineating reaction and microstructure evolution. Initially, attention will be focussed on Ni3Al synthesis, with later extension to other intermetallics. The synthesis of advanced materials by combustion reactions is an attractive alternative to other methods of materials preparation due to simplicity, relatively low energy requirements, high product purity potential, and (probably most important) opportunity for formation of metastable products resulting from the combination of rapid cooling and high reaction rates. In combustion synthesis, highly exothermic self-sustaining reactions in a powder bed yield final products; in one variant of this procedure, self- propagating high-temperature synthesis (SHS), reaction initiated at one end of the powder sample self-propagates through it as a combustion wave, while in another (thermal explosion mode) the sample is heated uniformly until reaction is initiated throughout the sample. One very important task in further development of combustion synthesis processing is establishment of control of product properties via control of microstructure; attainment of such control depends on fundamental understanding of chemical and structural transfor- mations taking place in and behind the propagating combustion front.