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

本研究的重点是由聚合物直接合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。目前已鉴定出两类PDC，即碳氮化硅(SiCN)和碳氧碳化硅(SiCO)。PDC既不是晶体，也不是玻璃；相反，它们包含大约1-5纳米大小的纳米结构域。这些纳米结构域可持续到非常高的温度，最高可达1500摄氏度；也就是说，它们的纳米结构在化学计量成分(Si3N4、碳化硅和二氧化硅)容易结晶的温度下抵抗大规模结晶。众所周知，成分中的过量碳会影响这种行为，但PDC的材料科学在很大程度上仍不清楚。在这个项目中，科罗拉多大学的教职员工和学生将与加州大学戴维斯分校的纳夫罗茨基教授和她的学生，以及来自德国三所大学(斯图加特大学、达姆施塔特大学和亚琛大学)的科学家和学生合作，阐明(I)在受控热解过程中，从有机聚合态结构到纳米结构的演变，(Ii)SiCN和SiCo的纳米结构与它们的合金的性能，如结晶、抗蠕变、热力学和动力学行为之间的关系。来自两国的学生将在更长的时间内访问大西洋彼岸的姐妹机构，以促进国际科学合作。该计划的主要特点之一是突出使用非传统技术来表征纳米结构和对这些非传统类别陶瓷的性能进行建模。核磁共振和傅里叶变换红外光谱将用于识别分子的第一近邻构型(在一定程度上也将用于第二近邻构型)，拉曼光谱将用于量化碳原子的短程和长程成键，小角X射线散射将用于表征纳米结构域的大小和分布。根据上述光谱信息和分子模拟得出的结构的热力学预测将与高温量热法测量的热力学热焓进行核对。例如，通过分子建模预测的纳米结构域的界面能将通过这种方式直接测量。例如，这些信息将被内置到模型中，可以解释pdc-S对结晶的异常抵抗。用于高温的纳米技术(空间和能源是两个例子)要求材料必须具有特殊的性质：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。这个项目关注的是一类新的陶瓷，称为聚合物衍生陶瓷或PDC，它可以抵抗这种随时间和温度变化的变化。PDC是坚固耐用的多功能材料，将带来传感器、电子学和光电子学等突破性技术，以及可以在极端环境中使用的涂层。这个项目的目标是建立一个对PDC的基本了解的科学基础，这是不存在的，并且对这些新技术的发展至关重要。该项目涉及两所美国主要大学，一所在科罗拉多州，另一所在加利福尼亚州，以及三所德国大学密切合作。五到七名博士研究生和同等数量的本科生将参加这个美国和德国之间的材料世界网络合作。

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.