Mechanistic Studies of Combustion Synthesis

在这个项目中，将使用两种技术来研究 SHS工艺材料合成的基本原理 首先是在反应过程中淬灭颗粒， 第二个涉及粉末颗粒在加热的 衬底的表面。 第一个程序将进行 通过将楔形样品嵌入高浓度的 导热块，导致 燃烧波通过样品向 楔形的顶点。 淬火后，将切割样品 并抛光用于金相学研究和逐层X- 将进行射线衍射分析以定量 作为位置的函数的相含量的变化。 SEM/EDXS研究结合定量光学显微镜 将被用来观察当地成分的变化， 尺寸和孔分布。 样品制备的影响 条件，如粉末粒度和绿色密度， 将被研究。 这些实验将提供信息 作为反应前沿的混合物微观结构的演化 传播，从而深入了解可以使用的机制， 以控制过程并定制产品特性。 在 第二个步骤，反应物粉末窄尺寸切割 （较低熔点组分）将分散在该组合物的箔上。 其它反应物，其将以一定速率电加热 通常在SHS处理中遇到。 加热后， 粉末将熔化并在叶表面上扩散;动力学 扩散（在SHS处理中很重要）将在 不同的加热速率和颗粒尺寸。 经过进一步 加热，反应将发生;这些将被淬灭 电流关断，并使用 SEM/EDXS，描绘反应和微观结构演变。 首先，注意力将集中在Ni 3Al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， 一个有吸引力的替代其他方法的材料 由于制备简单，能量相对较低， 要求，高产品纯度潜力，以及（可能 最重要的）形成亚稳态的机会 快速冷却和 高反应速率。 在燃烧合成中，高度 在粉末床中的放热自持反应产生 最终产品;在该过程的一个变型中，自 蔓延高温合成，反应 在粉末样品的一端开始自传播 通过它作为燃烧波，而在另一个（热 爆炸模式），将样品均匀加热直至反应 在整个样品中引发。 一个非常重要的任务 在燃烧合成工艺的进一步发展中， 通过控制产品特性建立控制 微观结构;这种控制的实现取决于 对化学和结构转换的基本理解， 发生在传播燃烧中和传播燃烧后的 前面

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.