Polymers and Polymer-Derived Ceramics

Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs).

The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep.

In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior.

From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components.

Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities.

Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics.

The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers.

Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years.

Initially the research on PDCs was focused mainly on dense bulk materials and fibers for mechanical applications at high temperatures. Nowadays, nano powders and porous PDCs with pore sizes in the range between several microns and few nanometers for applications such as catalyst support and for liquid and (hot) gas separation processes are gaining increasingly importance.

Moreover, the polymer-to-ceramic transformation is a suitable technology to produce a broad spectrum of ceramic based composite materials with adjusted chemical, mechanical, and physical properties.

PDCs can also be processed to thin films for optoelectronic applications and to thick films, e.g. for hard coatings, environmental barrier coatings, and others. The great flexibility in terms of processing and forming of preceramic polymers into shaped-ceramic components has also enabled them to play an important role in several other applications.

To further investigate and explore the unusual microstructure and physical properties of PDCs in more detail it will be the challenge of future studies in this field, and it requires a strong interdisciplinary approach in basic research and development in chemistry, physics, and materials science and engineering.

Current Projects (Dissertations)

Sensor Test Stripe

M. Sc. Emmanuel III Ricohermoso

Supervisor: Emanuel Ionescu

Driven by the continuous demand for miniaturization of micro/nanosystems, thin-film technology has been the center of interest in recent years. Microsystem Technology (MST) allows the fabrication of Si-based piezoresistive sensors operating at a maximum temperature of about 400°C. However, several applications demanding for high-temperature (HT) sensors, e.g., pressure monitoring in gas turbines (up to 600 °C) or the gasification of coal or biomass (> 700 °C). To increase the operating temperature of strain gauges, it is, therefore, necessary to explore new classes of materials withstanding extreme conditions (high temperatures as well as harsh environments). Furthermore, the cost-efficient production of new sensors by well-established production techniques is highly desirable for competitive reasons. The present project aims to develop MST-compatible, carbon-containing C/SiCX-nanocomposite materials (X = O, N) obtained from polymer-derived ceramic (PDC) as piezoresistive sensors applicable at elevated temperatures (T > 400 °C) and corrosive atmosphere addressing both basic and application-related research. The electrical transport properties of the chosen PDC will be investigated through temperature-dependent conductivity and Hall measurements. The project is mainly focused on the investigation of the piezoresistive effect (PZR) of C/SiCX nanocomposites with 13 to 20 vol% carbon as well as the realization and characterization of high-temperature stable sensor structures on a silicon cantilever. In subsequent research, the acquired knowledge can be used for the development of future pressure sensing devices, superior to the state-of-the-art piezoresistive silicon-based sensors.

Motivation for MN/Si3N4 (M=Ta, Zr, Hf) Ceramic Nanocomposites

M. Sc. Wei Li

Supervisors: Ralf Riedel, Zhaoju Yu

The project aims to fabricate and characterize MN/Si3N4 ceramic nanocomposites by polymer-derived ceramic (PDC) approach and expand its potential applications for use under high temperature conditions in harsh environments. Appropriate single-source precursors with different molar ratios of transition metals will be synthesized and the molecular structure of the obtained single-source precursors will be characterized. The effects of the metal contents and annealing temperatures on the phase development, morphologies and mechanical properties of the resultant ceramic nanocomposites will be investigated. This work will open up a novel synthesis approach to develop silicon-metal-nitride ceramic nanocomposites with interesting functional and structural mechanical properties.

(in the frame of GRK 2561)

M.Sc. Samuel Aeneas Kredel

Supervisors: Ralf Riedel, Martin Heilmaier

Ceramics generally stand out thanks to their high thermal and chemical stability. Combined with a low thermal conductivity, these properties allow coatings for super alloys which enable their use at accelerated temperatures. The ceramic layer protects the workpiece from corrosion and lowers the temperature at the surface of the alloy. Thus, higher operation temperatures can be realized. As a result, turbines can run at higher efficiency. The aim of this project is the preparation of thermal protection coatings for applications above 1200 °C in aggressive environments.

Ultrahigh-temperature ceramic nanocomposites based on Si(M)CX will be derived via the polymer derived ceramic (PDC) route from single source precursors developed in Project 1. Syntheses conditions as well as different coating techniques, such as spin coating, dip coating and additive manufacturing, will be optimized for the preparation of thick and crack free ceramic layers on refractory (inter)metallic systems (e. g. Mo-Si-B-X). Structure and properties of the precursors and coatings will be extensively characterized. The performance of the fabricated composites will be tested at high temperatures in hostile environments. Thereby, suitable ceramic/(inter)metallic compositions with optimized structural characteristics and high-temperature capabilities will be identified.

The three doctoral projects in the corresponding phase of the RTG will emphasize on: 1) Synthesis of preceramic polymer precursors for ceramic nanocomposites and optimization of their visco-elastic properties for coating preparation purposes. 2) Fabrication and structural characterization of thick ceramic coatings (> 10 μm) on Mo-Si-B-X substrates. 3) Systematic studies of the high-temperature behaviour of optimized multi-layered ceramic/(inter)metallic systems concerning their long-term operation at temperatures above 1200 °C in hostile environments.

(in the frame of GRK 2561)

M.Sc. Jan Bernauer

Supervisors: Ralf Riedel, Hans-Joachim Kleebe

Ultra-high-temperature ceramics (UHTCs) are a class of materials with a melting point of 3000°C and beyond. Therefore, they are mainly used in applications where high temperatures (T > 2000°C) are required e.g. thermal barrier coatings. Borides, carbides and nitrides of the early transition metals (Zr, Hf, Nb, Ta) are suitable for this purpose. However, various studies revealed that single-phase bulk UHTCs have rather poor oxidation resistance under extreme and aggressive environment. Ceramic composites can be an alternative solution for applications in such environment.

The PDC (polymer derived ceramics) method is particularly suitable for the synthesis of ceramic composites. Pre-ceramic polymer precursors are thermally decomposed and thus offer simple and inexpensive access to various ceramic systems. Project 1 of GRK 2561 deals with the synthesis of ceramic composite systems for high-temperature applications by the PDC route. To produce suitable precursors, various polysilazanes [R1R2Si-NR3] are used in this project. The ternary system SiCN and modified systems with the composition Si(M, B)CN (M = Ti, Zr, Hf) are manufactured by thermal treatment of the precursors. Organometallic compounds are used to modify the polysilazanes. Spectroscopic and thermogravimetric analyzes are used for the intensive investigation of the precursors and the ceramization process.

Subsequently, various processes are used to produce monoliths of the ternary and multinary systems namely: warm pressing followed by thermal treatment, hot pressing at temperatures from 1600°C – 1800°C, as well as FAST / SPS techniques (Field Assisted Sintering Technology / Spark Plasma Sintering) at 1600°C – 1800 ° C. The monoliths produced in this way are structurally characterized using the common methods.

SEM images with sketch inlays of the synthesis process towards functionalized ceramic papers.

M. Sc. Alexander Ott

Supervisors: Ralf Riedel, Emanuel Ionescu

Funding: DFG in the frame of FIPRE – Functional Paper Research (Project A04)

Collaboration: Hans-Joachim Kleebe

Ceramic papers exhibit a typical paper-like morphology, while being composed of one or more inorganic compounds such as silica (SiO2), alumina (Al2O3) or zirconia (ZrO2). There are different methods for the synthesis of ceramic papers, like the template assisted process, where a paper is employed as a template. Also, there have been studies using preceramic papers, which are produced by mixing of organic compounds with metal or ceramic filler materials. Both methods require a heating step to transform the preceramic papers into a ceramic paper, which is done at elevated temperatures and certain atmospheres.

In this project, the pseudomorphic transformation into a functionalized ceramic paper is realized by impregnating a cellulose-based paper with suitable polymer-based single-source precursors (SSPs). Then, the impregnated paper is pyrolyzed in reactive or inert atmospheres at various temperatures (Figure 1). Utilizing this procedure, metal-modified Si(C)N or SiOC-based ceramic papers are synthesized, which yields ceramics that have undergone phase separation and thus local precipitation of metal, metal nitride or metal silicide nanoparticles within the polymer matrix. Tempering between 1100–1500° C results in the in-situ formation of Si3N4-based 1D nanostructures via the vapor–liquid–solid (VLS) process exhibiting functional metal or metal silicide tips (such as Fe, Fe3Si, Ni). This in-situ grown 1D nanostructures affect the properties of the ceramic paper such as pore structure, high temperature stability, or mechanical properties.

The aim is to get a better understanding of how the microstructure, the phase composition and the topology of the ceramic papers and the functional 1D nanofibers are obtained after the pseudomorphic transformation. Furthermore, it is paramount to investigate to what extent a mechanical flexibility of the produced ceramic papers can be achieved and if the functionalization leads to desirable (electro) catalytic properties.

M. Sc. Yongchao Chen

Supervisors: Ralf Riedel, Zhaoju Yu

Selective catalytic reduction (SCR) with NH3/urea is regarded as one of the most promising technologies for nitrogen oxide (NOx) abatement, it is of critical importance to improve the SCR performances of the catalyst. However, the performance of traditional V2O5-WO3/TiO2 catalysts is not sufficient at a wide temperature range and a drawback is the sulphur-deactivation of traditional catalyst [1]. Considering the problem of present catalysts and more and more strict rules of NOx emission, new efficient catalysts for denitrification is all along needed. Regarding to the supported transition metal oxides, comparatively little is known about the mesoporous silica supported metal oxides catalytic performance. On the one hand, mesoporous silica holds ultrahigh specific surface area [2] and nanopores of several nm in diameter even after the hydrothermal treatment [3], on the other hand, mesoporous silica is known to be more resistant to sulphur poisoning [4]. Therefore, investigation of transition metal oxide supported on mesoporous silica via single source precursor method as NH3-SCR catalysts is proposed. Single-source-precursors will be synthesized by the reaction of silica precursor with active substances as transition metal oxides source. Then, the precursor will be cross-linked and subsequently calcinate to generate active compound nanoparticles embedded homogeneously within mesoporous silica.

References:

[1] P. Spurk, M. Pfeifer, J. Gieshoff, E. Lox, A Breakthrough in SCR Technology, AutoTechnology 2 (2002) 68-70.

[2] T. Haynes, O. Bougnouch, V. Dubois, S. Hermans, Preparation of mesoporous silica nanocapsules with a high specific surface area by hard and soft dual templating approach: Application to biomass valorization catalysis, Microporous and Mesoporous Materials 306 (2020) 110400.

[3] J. Zhu, Z. Liu, L. Xu, T. Ohnishi, Y. Yanaba, M. Ogura, T. Wakihara, T. Okubo, Understanding the high hydrothermal stability and NH3-SCR activity of the fast-synthesized ERI zeolite, Journal of Catalysis 391 (2020) 346-356.

[4] K. Guo, G. Fan, D. Gu, S. Yu, K. Ma, A. Liu, W. Tan, J. Wang, X. Du, W. Zou, Pore size expansion accelerates ammonium bisulfate decomposition for improved sulfur resistance in low-temperature NH3-SCR, ACS applied materials & interfaces 11 (2019) 4900-4907.

M. Sc. Franziska Kirsch

Supervisors: Ralf Riedel, Fraunhofer-Institute LBF, Bereich Kunsstoffe, Elastomertechnologie

(in collaboration with Fraunhofer-Insitute LBF, Research area “Plastics”, Elastomer technology)

The prediction of the life time of elastomers especially in dynamic applications is a challenge. The reason is, that material failure depends on many aspects including the material selection, manufacturing and exposure conditions. This work focuses on the relation between chemical structure of the elastomeric network and the mechanical properties. The structure of sulfur-cured elastomers is described by the crosslinking density and the length of the sulfur cross-links. The different length of sulfur cross-links is generated by variating the formulation of the elastomer as well as the manufacturing conditions. Thus, the chemical network structure influences the mechanical properties of the material i.e. the tensile strength as well as the relaxation process. The three asprects formulation of the compound, chemical structure of the network, and mechanical properties of the elastomer are analyzed with regard to cyclic fatigue measurements. Cyclic fatigue is used to predict the life time of materials in dynamically loaded parts, however it is a cost- and time-consuming method. The aim of my work is, to characterize the correlation between cyclic fatigue, chemical structure and mechanical properties, which offers a new opportunity to predict the life time of elastomers.

M. Sc. Ying Zhan

Supervisors: Ralf Riedel, Emanuel Ionescu in cooperation with Merck KgaA, Darmstadt

The well-known good adhesion of polysilazane to a variety of substrate materials makes it a favorable material for the coating industry. They are promising candidates for inter-layer dielectrics, oxidation or wear protection coatings, gas barrier, marine anti-fouling coating, anti-graffiti coating for public transportation, and anti-adherent coating for demolding. Polysilazane coatings can be cured and converted into thermoset polymeric coatings by several types of treatment, such as, thermal crosslinking, UV radiation, O2 plasma treatment, and moisture curing by hydrolysis and condensation reactions. However, the curing rate of the polysilazanes has to been further improved for real-life applications.

Two approaches have been made in this PhD work.

1. Catalysts of different types, i.e., acid-base, halogens, heterogeneous catalysts, have been investigated to accelerate the moisture curing process of polysilzanes.

2. Synthesis of boron-modified polysilazane to increase its crosslinking degree, and hence, improve the mechanical properties of as obtained polysilazane-derived coatings.