US-German Materials Collaboration: Unusual Stability of Amorphous

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。已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，Navrotsky教授和她的加州大学Davis分校的研究小组将与科罗拉多大学的教职员工和学生以及来自德国三个机构(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)纳米结构在受控热解过程中从有机聚合物状态的结构演变而来，(Ii)SiCN和SiCo的纳米结构与它们的合金之间的关系，以及它们的性能，如结晶、抗蠕变、热力学和动力学行为。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热力学，并对这些非传统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热力学预测将与高温量热法测量的生成和结晶热进行核对。用这种方法可以直接测量分子模型预测的纳米结构域的界面能。这些量热研究是戴维斯团队的主要和独特的贡献。例如，这些信息将被内置到模型中，这些模型可以解释PDC对结晶的异常抵抗。该项目将在跨学科和国际研究方面教育年轻人。来自两国的学生将在更长的时间内访问大西洋彼岸的姊妹机构，以促进国际科学合作。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是坚固的多功能材料，将带来突破性技术，如传感器、电子学和光电子学，以及可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。该计划通过与德国科学家的国际合作获得了巨大的影响力，并涉及两所主要大学，一所在科罗拉多州，另一所在加利福尼亚州，参与这一重要努力。该项目将在跨学科和国际研究方面教育年轻人。来自美国和德国的5至7名博士后研究生和/或博士后研究员将参与这一合作努力。来自科罗拉多州和加州大学戴维斯分校的本科生将在这个项目中长期担任研究实习生。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。

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 PDCs remains largely un-understood. In this project Professor Navrotsky and her research group at the University of California Davis will collaborate with the faculty and students at the University of Colorado 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. One of the key features of the program is to highlight the use of non-conventional techniques for characterizing the nanostructure and thermodynamics and the modeling of properties of these non-traditional 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 enthalpies of formation and crystallization by high temperature calorimetry. The interfacial energies of the nanodomains predicted by molecular modeling will be measured directly in this way. These calorimetric studies are the major and unique contribution of the group at Davis. This information will be built into models that can explain the unusual resistance of PDC's to crystallization, for example. This project will educate young people in interdisciplinary and international research. Students from both countries will visit sister institutions across the Atlantic for extended periods to foster international scientific collaboration.Nanotechnologies for high temperatures (space and energy are two examples) require materials which 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. The program derives great leverage through international collaboration with German scientists, and involves two major universities, one in Colorado and the other in California in this important endeavor. This project will educate young people in interdisciplinary and international research. Five to seven doctoral graduate students and/or post-doctoral fellows from the US and Germany will participate in this collaborative effort. Undergraduate students from Colorado and UC Davis will serve as research interns in this program for long terms. Students from both countries will visit sister institutions across the Atlantic for extended periods to foster international scientific collaboration.