Micropore evolution in polymer-derived ceramics

Recent global challenges in the field of energy and environment have resulted in an increased demand for materials with improved or new properties, required for the implementation of a variety of alternative industrial processes with the goal to conserve energy and reduce the environmental impact. In our research project, we explore novel ceramic materials, formed by a controlled heat treatment of silicon-based polymers, which contain microporosity in the size range below one nanometer, and which we anticipate to be applicable as materials for the separation of gases at high temperatures. The focus of our project is set on exploring the origin and collapse of microporosity in these materials, and to investigate how the pore structure affects the interaction of the materials with gases and gas mixtures. Our hypothesis is that by taking into account all process stages leading up to the final material, starting from the structure and composition of the original polymer compound up to the conditions during heat treatment, we can identify the primary factors determining the size and stability of micropores in the final material. We anticipate that the development of strategies to obtain tightly-controllable microporosity and high thermal stability leads to tailored gas transport characteristics, all of which we expect to be of major relevance towards implementing these materials in gas separation applications. To achieve our project objectives, we first clarify the impact of a wide range of process parameters on the polymer-to-ceramic transformation of the investigated materials, employing a combination of state-of-the-art and non-conventional techniques to unravel the chemical setup and composition as well as the micropore structure, some methods of which we develop and conduct in cooperation with national and international collaborators. In the second project phase, the material structure will be correlated to gas transport characteristics, after developing a methodology for the preparation of membrane structures with high thermal stability. The novelty and originality of our project lies in the consideration of the full processing chain from the preceramic polymer to the final material, aiming towards a controllability of pore structure and stability and, subsequently, gas transport characteristics. For this, we employ new techniques to monitor the pore development during the conversion process itself, instead of solely relying on well-established methods of structural investigation. With this approach, we anticipate to not only break new ground in fundamental research of polymer- derived ceramics, but also to contribute to progress in other fields involving new energy- related and environmental technologies.

In the framework of this project, novel ceramic materials derived from Silicon-containing polymers with microporosity in the range smaller than one nanometer were investigated, which were expected to exhibit promising characteristics for high temperature gas separation applications. The focus of the project was set on the investigation of the emergence and the collapse of microporosity during the preparation of these materials, as well as on the investigation of the influence of the micropore structure on the interaction with gases. Here, a holistic view on the full processing chain was of special interest, ranging from structure and composition of the preceramic starting polymer up to the conditions during the heat treatment ultimately responsible for the final materials properties. It was found that size and stability of micropores are closely related to the chemical structure of the resulting ceramics, which, in turn, can be controlled either through the cross-linking process or through process conditions during heat treatment, including gas atmosphere and temperature. With respect to the first case, it was shown that by addition of a linker molecule to the starting polymer, the chemical cross-linking mechanism taking place during the initial heat treatment stage can be changed, resulting in increased nitrogen contents and a decrease in micropore size. In the second case, the use of a reactive ammonia atmosphere during the polymer-to-ceramic conversion also results in a decrease in micropore size, accompanied by a steep increase in micropore stability from an initial limit of 600 C to 900 C, independent of the starting polymer used. As conventional methods of structure determination did not allow for a direct observation of the appearance and disappearance of microporosity during the actual conversion process, a novel method of microstructural elucidation based on small-angle X-ray scattering was developed in a collaborative effort, which for the first time facilitated the observation of the actual status of micropore structure during the conversion process itself (in-situ). Through this approach, detailed information on the effect of temperature, composition, and gas environment on the micropore evolution could be obtained, without potential interference by external factors. The results obtained over the course of this project allow for a better understanding of relevant processing conditions and fundamental mechanisms involved in the evolution of microporosity in polymer-derived ceramic materials, and can therefore be considered as a starting point for future developments in developing and implementing microporous ceramic materials for various energy- and environment-related application areas. Furthermore, fundamental insights into the tailoring of macropore structures in conventional, powder-based ceramics could be gained and new analysis techniques could be developed, with relevancy for other areas of materials development.