NSF - (US-German) Materials Collaboration: Unusual Stability of Amorphous Polymer Derived Ceramics at High Temperatures

该研究计划的重点是直接由聚合物制成的一类新型陶瓷材料的合成，结构和性能之间的关系。它们被称为PDC，或聚合物衍生陶瓷。目前，已经确定了两类PDC，碳氮化硅（SiCN）和碳氧化硅（SiCO）。PDC既不是晶体也不是玻璃;相反，它们含有尺寸约为1-5 nm的纳米畴。这些纳米畴在高达1500 ℃的高温下仍然存在;也就是说，它们的纳米畴结构在化学计量成分（Si 3 N4、SiC和SiO2）容易结晶的温度下抵抗大规模结晶。已知组合物中过量的碳会影响这种行为，但PDC的材料科学在很大程度上仍不清楚。在这个项目中，科罗拉多大学的教师和学生将与加州大学戴维斯分校的Navrotsky教授及其学生，以及来自德国三个机构的科学家和学生合作（Universities of Stuttgart，达姆施塔特和亚琛大学）来阐明（i）在受控热解过程中纳米畴结构从有机聚合物状态的结构的演变，以及（ii）SiCN和SiCO的纳米结构与它们的合金以及它们的性质（例如结晶、抗蠕变性、以及热力学和动力学行为）之间的关系。来自两国的学生将访问大西洋两岸的姐妹机构，以促进国际科学合作。该计划的主要特点之一是强调使用非传统技术来表征纳米结构和这些非传统陶瓷类的特性建模。NMR和FTIR光谱将用于识别分子的第一（和在某种程度上第二）最近邻构型，拉曼光谱用于量化碳原子的短程和长程键合，并且小角X射线散射将用于表征纳米域的尺寸和分布。从上述光谱信息和分子模型推导出的结构的热力学预测将通过高温量热法对热力学结构进行测量来检查。例如，通过分子建模预测的纳米畴的界面能将以这种方式直接测量。例如，这些信息将被构建到模型中，可以解释PDC-s对结晶的不寻常抵抗力。高温纳米技术（空间和能源是两个例子）要求材料必须具有特殊性质：它们的结构必须在原子可以移动和改变结构的温度下保持稳定。该计划的重点是一类新的陶瓷，称为聚合物衍生陶瓷或PDC的抵抗这种时间和温度依赖性的变化。PDC是一种坚固的多功能材料，它将带来突破性技术，如传感器，电子和光电技术，以及可用于极端环境的涂层。 该项目的目标是发展一个基本理解PDC的科学基础，这是不存在的，这是这些新技术向前发展所必需的。该计划涉及两所主要的美国大学，一所在科罗拉多，另一所在加州，以及三所德国大学密切合作。5至7名博士研究生和同等数量的本科生将参加美国和德国之间的这一材料世界网络合作。

This research program is focused on the relationship between synthesis, structure and properties of a new class of ceramic materials that are made directly from polymers. They are becoming known as PDC's, or polymer-derived-ceramics. Currently two families of PDC's, silicon carbonitride (SiCN) and silicon oxycarbide (SiCO), have been identified. The PDC's are neither a crystal nor a glass; instead they contain nanodomains, about 1-5 nm in size. These nanodomains persist to very high temperatures, up to 1500oC; that is, their nanodomain structure resists large-scale crystallization at temperatures where the stoichiometric constituents (Si3N4, SiC and SiO2) would readily crystallize. The excess carbon in the composition is known to influence this behavior but the materials science of PDC's remains largely un-understood. In this project the faculty and students at the University of Colorado will collaborate with Professor Navrotsky at the University of California at Davis and her students, and with scientists and students from three institutions in Germany (Universities of Stuttgart, Darmstadt and Aachen) to elucidate (i) the evolution of the nanodomain structure from the structure of the organic polymeric state during controlled pyrolysis, and (ii) the relationship between the nanostructure of SiCN and SiCO, and their alloys, and their properties, e.g. crystallization, resistance to creep, and thermodynamic and kinetic behavior. Students from both countries will visit sister institutions across the Atlantic for extended periods to foster international scientific collaboration. One of the key features of the program is to highlight the use of non-conventional techniques for characterizing the nanostructure and the modeling of properties of these non-traditional class of ceramics. NMR and FTIR spectroscopies will be used to identify the first (and to some extent the second) nearest neighbor configuration of molecules, Raman spectroscopy to quantify the short range and long-range bonding of carbon atoms, and small-angle-X-ray-scattering will be employed to characterize the size and the distribution of nanodomains. The thermodynamic predictions of the structure derived from the above spectroscopic information and molecular modeling will be checked against measurement of the thermodynamic enthalpies by high temperature calorimetry. For example, the interfacial energies of the nanodomains predicted by molecular modeling will be measured directly in this way. For example, this information will be built into models that can explain the unusual resistance of PDC-s to crystallization. Nanotechnologies for high temperatures (space and energy are two examples) require materials that must have a special property: their structure must remain stable at temperatures where atoms can move and change the structure. This program is focused on a new class of ceramics, called polymer-derived-ceramics or PDC's that resist this time and temperature dependent change. The PDC's are robust and multifunctional materials, which will usher in breakthrough technologies such as sensors, electronics and optoelectronics, and coatings that can be used in extreme environments. The objective of this project is to develop a scientific base of fundamental understanding of PDC's, which is not present, and which is essential for these new technologies to move forward. This program involves two major US universities, one in Colorado and the other in California, and three German universities in close collaboration. Five to seven doctoral graduate students and an equal number of undergraduate students will participate in this materials world network collaboration between the United States and Germany.