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. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from preceramic polymers.
(5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

This review highlights the synthesis, processing and properties of non-oxide silicon-based ceramic materials derived from silazanes and polysilazanes. A comprehensive summary of the preparation of precursor compounds containing Si–N–Si units, including commercially available materials, is followed by the discussion of various processing techniques. The fabrication of dense bulk ceramics in the Si/E/C/N systems is reported which involves cross-linking of the polymeric ceramic precursor followed by a polymer-to-ceramic transformation step. The cross-linked precursor can be milled, compacted and pyrolysed to form dense, additive-free, amorphous silicon carbonitride monoliths or polycrystalline composites which withstand oxidation in air at 1600°C. Furthermore, an overview is given on the fabrication of silazane derived powders and coatings involving chemical vapour deposition (CVD) methods utilising volatile precursors. Fibre spinning and fibre properties, as well as other processing techniques like infiltration of preforms, the preparation of porous ceramics and joining are briefly discussed. A state of the art of the mechanical properties of polymer derived amorphous Si/C/N and Si/B/C/N ceramics with respect to hardness as well as high-temperature creep and oxidation resistance is summarised. Finally, some important aspects of industrial applications will be considered. The review is in part based on our own work related to the polysilazane derived ceramics, but will also cover a comprehensive state of the art including the published literature in this field.

Silazane derived ceramics and related materials

Manufacturing of bulk ceramic components from materials in the system Si-Me-C-N-O (Me = Ti, Cr, V, Mo, Si, B, CrSi2, MoSi2, etc.) from preceramic organosilicon polymers - such as poly(carbosilanes), poly(silazanes), or poly(siloxanes) - has become possible by incorporating reactive filler particles into the liquid or solid polymer pre-cursor. During pyrolytic decomposition of the polymer matrix, the filler particles react with carbon from the polymer precursor or nitrogen from the reaction gas atmosphere to form new (oxy)carbide or (oxy)nitride phases embedded in a nanocrystalline Si-O-C(-N) matrix. The selective expansion encountered in the filler phase reaction can be used to compensate for the polymer shrinkage upon pyrolytic conversion. The formation of a transient pore net-work between 400° and 1000°C is governed by the polymer decomposition as well as the filler particle reaction kinetics. Thus, the properties of the oxycarbonitride composite materials can be tailored by controlling the microstructures of the polymer-derived matrix phase, the filler network, and the residual porosity. Near-net-shape forming of bulk ceramic components, even with complex geometry, is possible, making novel applications of polymer-derived bulk materials in biomedical, electrical, and mechanical fields highly interesting.

Active‐Filler‐Controlled Pyrolysis of Preceramic Polymers

This review presents the synthesis, characterization techniques, processing and potential applications of silicon-based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based polymers provides materials which are denoted as polymer-derived ceramics (PDCs). In particular, this procedure is a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO. There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs route, common preceramic polymers are poly(organosilazanes), poly(organosilylcarbodiimides) and poly(organosiloxanes). One basic advantage of the PDC route is that the materials can be easily shaped in form of fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry. The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approximately 1500°C. Recent investigations have shown that in some cases the high temperature stability in terms of decomposition and/or crystallization can be increased even up to 2000°C if the preceramic polymer contains some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely related to the variation of composition and solid state structure of these materials.

ポリマーから誘導されたケイ素系セラミックス ─合成・性質・応用に関する総説─

The principal focus of this review will be to document and discuss the recent developments in the synthesis, properties and applications of polyorganosiloxane containing block, segmented and graft copolymers and network structures. Special emphasis will be given to the synthesis and characterization of reactive, organofunctionally-terminated siloxane oligomers, which are the key starting materials or intermediates in the preparation of these well-defined copolymers. Use of these functionally terminated oligomers in the modification of network structures will be discussed. Detailed descriptions of the copolymer formation reactions and the effect of reaction variables, such as temperature and solvent, on the properties of the resultant materials will be given. Discussions of the bulk and surface morphology of siloxane containing copolymers and their blends with other polymeric systems will be provided. Potential applications of these novel materials and possible future directions in the chemistry and technology of polyorganosiloxane containing polymeric systems will also be discussed.

Polysiloxane containing copolymers: A survey of recent developments

A preceramic polymer which is a polysilazane is described. It is made by reacting an organodihalosilane with ammonia to form an ammonolysis product, and treating the ammonolysis product with a basic catalyst capable of deprotonating an NH group that is adjacent to an SiH group to form said polymer. The polymer is preferrably further treated with an electrophilic reagent. These polymers are useful for making various ceramic products.

Preceramic organosilazane polymers

A Liquid Silazane Precursor To Silicon Nitride

Aqueous precipitation has been used to prepare spherical ZnO particles. These powders have been shown to be suitable for use in the preparation of varistors. The uniform size distribution and uniform coating of these powders with the appropriate dopants allowed fabrication of varistors with improved properties over those fabricated from conventionally ball-milled and calcined powders. The improved properties were a high coefficient of nonlinearity (∼44) in the nonohmic region, a sharp change in electrical behavior from ohmic to nonohmic, and a high resistivity (5×1012 SOM°m) at low voltages.

Aqueous Precipitation of Spherical Zinc Oxide Powders for Varistor Applications

The action of basic catalysts such as KH on the ammonolysis products of CH3SiHCl2, (CH3SiHNH)n, results information of polysilazanes of intermediate molecular weight (600 to 1800). Pyrolysis of these polysilazanes gives ceramic materials, which appear to be mixtures of Si3N4, SiC, and C, in yields of 80 to 85%.

High‐Yield Synthesis of Si3N4/SiC Ceramic Materials by Pyrolysis of a Novel Polyorganosilazane

Stable, viscous polymers of silicon, nitrogen and hydrogen are formed by reacting a halosilane with ammonia in the presence of a solvent comprising an aliphatic ether, a chloromethane or mixtures thereof. After the solvent has been removed from the reaction product, the polymer can be poured into a container of desired shape and then can be sintered to form a uniform silicon nitride product.

Gary H. Wiseman

Papers

Preceramic organosilazane polymers

A Liquid Silazane Precursor To Silicon Nitride

Aqueous Precipitation of Spherical Zinc Oxide Powders for Varistor Applications

High‐Yield Synthesis of Si3N4/SiC Ceramic Materials by Pyrolysis of a Novel Polyorganosilazane

Stable liquid polymeric precursor to silicon nitride and process