Effect of inert filler addition on pore size and porosity of closed-cell silicon oxycarbide foams

Abstract

Ceramic foams are a specific class of porous materials composed of pores (usually referred to as cells), with a size ranging from a few micrometers to a few millimeters, and ceramic walls (closed-cell foams) or struts (open-cell foams). Fabrication of ceramic foams from preceramic polymers has recently attained increasing interest because of low fabrication temperatures and the unique properties of the foams, such as low density, low thermal conductivity, high permeability, high surface area, high thermal shock resistance, and high specific strength [1–4]. These characteristics make them good candidates for both structural and functional applications, such as filters, insulators, absorbents, catalyst supports, and biomedical devices [2–8]. Various processing routes have been proposed for the production of porous ceramics, including replication methods [6, 9], space holder methods [10, 11], direct foaming methods [1, 8, 12, 13], and microcellular foaming with CO2 by implementing the thermodynamic instability principle [14–16]. More recently, the generation of closed-cell ceramic foams based on the following strategy has been developed: (i) forming a compact using a mixture of preceramic polymer and expandable microspheres, (ii) forming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into a ceramic foam by pyrolysis [17]. In the present letter, the effect of inert filler addition on the pore (cell) size and porosity of closed-cell silicon oxycarbide foams was investigated using expandable microspheres as a blowing agent and SiO2 powder as an inert filler. A commercially available polysiloxane (YR3370, GE Toshiba Silicones Co., Ltd., Tokyo, Japan), an expandable microsphere (461DU40, Expancel, Sundsvall, Sweden), and SiO2 (0.8 μm, High Purity Chemicals, Saitama, Japan) were used as raw materials. Two batches of powder were mixed, each containing polysiloxane:microsphere in a weight ratio of 6:4, with one containing no silica and the other containing 10 wt% SiO2. The batch compositions are