Fundamental Studies of Controlled Microstructure Developmentby the Self-Propagating High-Temperature Synthesis Process

对自传播高温合成（SHS）工艺制备先进材料进行了全面的研究。(SHS工艺是以粉末为基础的工艺，其中至少有一种反应物是细分化的粉末。两类主要的SHS反应是涉及两种固体的反应（S/S）和涉及固体和气体的反应（G/S）；这两种反应都是典型的放热反应。当反应物混合物的一端被“点燃”时，一个自我维持或自我传播的“燃烧波”通过堆积的物质床，在其尾流中留下凝聚的产物或产物混合物。增强和精确控制的产品微观结构是该计划的主要目标。该研究预计将确定和量化关键的微观机制过程，这些过程决定了合成材料中产物相的类型、大小和分布（以及孔隙率特征）。该计划的活动将集中在具有良好定义的微观结构的样品中的本征反应动力学。样品将由嵌入SHS粉末混合物中的金属或双金属箔组成。粉末将以通常的方式点燃，以便粉末和箔都参与SHS反应。因此，箔界面上的扩散、熔化和/或反应将在与传统SHS反应相同的温度和时间尺度上进行。因此，一个可以很容易地表征的“微观结构”将被创建，并且一个理想的大而平坦的界面将在完全基于粉末的SHS反应中受到与材料相同的温度历史。扩散系数、反应动力学参数和相分布都将得到。我们将研究固体-固体和气-固体两种体系。将建立一个相对简单的这种理想化构型的模型，以帮助提取基本动力学参数。本研究有望建立强有力的加工-微观结构-材料性能关系；加强对SHS加工过程中相互关系的基本理解，将有助于开发出具有受控/定制微结构的标准化方法来生产材料。在这个特殊的程序将重点放在实验和理论研究的内在动力学所涉及的；关于SHS处理的其他方面的额外任务，特别是用于生产“功能梯度”材料的任务，将在陆军研究办公室同时资助的平行计划中进行。需要对涉及SHS加工的详细机制进行基础研究，以实现这种生产具有独特性能的先进材料的新方法的全部潜力；如果对这些机制没有这样的理解，针对各种最终用途优化性能的产品的生产将仍然非常困难和耗时，每个新应用都必须重复一个漫长的学习过程。对SHS工艺进行微调，以生产具有良好定义（和可控）的成分梯度和/或孔隙结构的材料，将对催化剂支撑材料、热机部件、复合材料制造的多孔预制件、盘式制动器、高温过滤介质、高表面积吸附材料、压电器件和形状选择分离介质等广泛应用做出重大贡献。

A comprehensive investigation of the preparation of advanced materials by the Self-Propagating High-Temperature Synthesis (SHS) Process will be conducted. (The SHS process is a powder- based process, where at least one of the reactant materials is a finely-divided powder. Two primary classes of SHS reactions are those involving two solids (S/S) and those involving a solid and a gas (G/S); the reactions in both cases are characteristically exothermic. When the reactant mixture is "ignited" at one end, a self-sustaining or self-propagating "combustion wave" proceeds through the packed bed of material, leaving a condensed product or product mixture in its wake.) Enhanced and precise control of the product microstructure is a major goal of this program. The study is expected to identify and quantify key micromechanistic processes which determine the type, size, and distribution of the product phases (as well as the porosity characteristics) in the synthesized material. Activity in this program will focus on the intrinsic reaction kinetics in samples with a well-defined micro-structure. The samples will consist of a metallic or bimetallic foil embedded in an SHS powder mixture. The powder will be ignited in the usual fashion so that both the powder and the foil participate in the SHS reaction. The diffusion, melting, and/or reaction at the foil interface will thus be driven on the same temperature and time scales as a conventional SHS reaction. Thus, a "microstructure" which can be readily characterized will be created, and an idealized large, flat interface will be subjected to the same temperature history as material in a totally powder-based SHS reaction. Diffusion coefficients, reaction kinetic parameters, and resulting phase distributions will all be obtained. Both a solid-solid and a gas-solid system will be studied. A relatively simple model of this idealized configuration will be developed to aid in the extraction of the fundamental kinetic parameters. This study is expected to establish a strong processing-microstructure-material properties relationship; the enhanced fundamental understanding of the interrelationships involved in SHS processing will lead to development of standardized approaches for producing materials with a controlled/tailored microarchitecture. Emphasis in this particular program will be placed on experimental and theoretical studies of the intrinsic kinetics involved; additional tasks regarding other aspects of SHS processing, particularly as applied to production of "functionally gradient" materials will be carried out on a parallel program being funded concurrently by the Army Research Office. Fundamental studies of the detailed mechanisms involved in SHS processing are needed for attainment of the full potential of this novel method for production of advanced materials with unique properties; without such understanding of these mechanisms, production of products with properties optimized for various end uses will remain very difficult and time consuming, with a long learning process having to be repeated for each new application. Fine-tuning the SHS process to produce materials with well-defined (and controlled) composition gradients and/or pore structures will make major contributions to such widely varied applications as catalyst support materials, heat-engine components, porous preforms for composite materials fabrication, disk brakes, high-temperature filter media, high-surface area adsorption materials, piezoelectric devices, and shape-selective separation media.