A review of absorption properties in silicon-based polymer derived ceramics

Abstract

Design and development of advanced materials for electromagnetic applications and bringing these materials into use is one of the most challenging tasks of materials engineering. Silicon-based polymer derived ceramics (PDCs) are natural candidates for these demanding applications due to their very attractive microstructure and properties. Compared with sintered technical ceramics, such as SiC, Si₃N₄, Al₂O₃ or ZrO_2, polymer derived ceramics offer the possibility of flexible plastic-technical processing, for instance by means of injection molding or extrusion without the employment of additional binder systems. The chemical synthesis permits a purposeful optimization of the polymers with respect to workability, ceramic yield and composition by the substitution of different elements in the basic structure as well as the organic side groups. In many cases the microstructure of PDCs characterized by homogeneous distribution of semiconducting or conducting nano-phases in the amorphous matrix. This can lead to the good microwave absorbing properties. Thus, those materials may not only satisfy the impedance matching but also rapidly attenuate electromagnetic waves. The absorption properties of PDCs can be easily tailored by the design of the molecular precursor, changes in morphology, and volume fraction of the filler particles. Different classes of preceramic polymers are briefly introduced and their absorption properties with adjustable phase compositions and microstructures are presented in this review.

Introduction

The need for microwave absorbers and radar-absorbing materials is ever growing in both civilian and defense-oriented applications because of the increasing use of high operating frequencies and bandwidths, especially the X-band (8.2–12.4 GHz) in electronic systems [1], [2], [3]. As an excellent radar-absorbing material, composite should satisfy the following requirements: (i) it should have a strong absorbing capability and impedance matching characteristic, which means it should have a minimal reflection coefficient (RC), (ii) it should have a wider efficient absorption bandwidth (EAB, the corresponding frequency range with which the RC is smaller than −10 dB), (iii) it is expected to have a low density and high temperature stability when it satisfies the above two requirements, (iv) good mechanical property also becomes a gradually important requirement for radar-absorbing material [4], [5], [6], [7].

A previous study indicated that ferrites and metal powders are the most commonly used absorbing materials in megahertz range because of their good electrical conductivities, magnetic and/or dielectric loss. However, their intrinsic disadvantages (Snoek’s limit and high density) restrict their widespread application in high frequency range. Polymer-based nanocomposites attract great attention as EM absorbers with low density, broad absorption band and high EM loss, however, they cannot be used at high temperatures due to their lower decomposition temperatures. Low density ceramics can offer superior absorption properties in the gigahertz frequency range, excellent chemical stability from ambient to peak operation temperatures, corrosion and radiation resistance. Recently, materials such as carbon, SiC, Mn₂O₃, and ZnO have attracted great interest as EM absorbing materials in the GHz frequency range due to their unique chemical and physical properties [8], [9], [10], [11].

Carbon-based materials, especially C nanostructures, 2D graphene and quasi-1D CNTs, are reported as the high-performance absorbing materials because of their soperior intrinsic properties, such as lower density, higher conductivity and higher specific surface area [12], [13], [14]. Silicon carbide (SiC), as a wide band gap semiconductor, possesses good microwave absorption properties [15]. The hybrid materials involving carbon (as the conductive phase) and SiC (as the semi-conductive phase) both disperses in an electrically insulating matrix, thereby offering more possibilities to achieve an promising dielectric loss, according to Lichteneker and Rother’s law [16] and previous investigations [17], [18], [19]. PDCs processing route is an effective way to attain those hybrid materials.

Compared with current technology to produce ceramics, the result of the polymer derived ceramics (PDCs) route shows better characteristics, such as excellent thermomechanical properties, stability, good usability and workability [20]. Changing the chemistry and the architecture of the preceramic precursors, suitable chemical modification of the preceramic precursors, and tuning the microstructure of the PDCs all can lead to an enormous improvement in the properties of PDCs and thereby increase their potential applications [21]. Specifically, the absorption properties of such composites can be tailored through changes in composition, morphology, and volume fraction of the filler particles. This paper offers a perspective on the experimental efforts toward the development of microwave absorbers based on polymer derived ceramics.