Si-SiC ceramics from plant precursor

Abstract

Synthesis of materials from naturally grown plants has recently received interest [1, 2]. Plants often possess natural composite structures and exhibit high mechanical strength, low density, high stiffness, elasticity and damage tolerance. These advantages are because of their genetically built anatomy, developed and matured during different hierarchical stages of a long-term evolutionary process. There is a possibility of producing novel silicon carbide (SiC) ceramic materials nearly isomorphous to naturally grown plants, on the macroand micro scale. SiC ceramics are commonly utilized for different structural applications. Among the various methods of producing SiC ceramics, reaction bonding/reaction sintering [3] has been shown to be the cheapest and the most commercially viable. Reaction bonding/reaction sintering is based on the C-Si reaction and is conventionally restricted to synthetic preforms [3, 4]. SiC ceramics can also be made using bio-structure derived preforms [5–7]. A wide variety of plants and plant-parts can be used for making different varieties of SiC ceramics [8–10]. Biological preforms from various soft woods, hard woods and non-wood ingredients, commonly used in pulp and paper manufacturing, can be employed for producing SiC ceramics. In view of the variations in dimensions, composition and morphology of the naturally grown plant structures, the shape and composition of the bulk SiC produced will vary significantly. The formation of SiC ceramics using monocotyledonous trees as the precursor is reported here. A monocotyledonous tree from a local source was used as the precursor plant. It was transformed to a porous skeletal carbonaceous preform of rectangular shape (of approximately 41 cm2 external surface) following a pyrolysis process at around 800 ◦C without any structural damage (cracking, loss of integrity etc.) [11]; it was subsequently reacted with silicon to yield Si/SiC ceramics under vacuum at a temperature of around 1600 ◦C [12]. After the reaction the macroscopical structural integrity was totally retained. The carbon preform and the final product were subjected to: X-ray diffraction (PW 1710, Philips, Holland); microstructural examination using light microscopy (Zetopan, Reichert, Austria) and scanning electron microscopy (SE-440, Leo-cambridge, Cambridge, U.K.); determination of the volumetric phase composition of the final ceramic by the point counting method from light photomicrographs; and thermogravimetric analysis (TGA) using a thermo-balance (STA 490C, Netzsch-Geratebau GmbH, Germany) up to 1200 ◦C in flowing air (70 ml/min) at a rate of 10 ◦C/min. Density was determined by water displacement method and porosity by the boiling water method. The threepoint bending strength and Young’s modulus of Si/SiC ceramic material were determined at room temperature using an Instron Universal Testing Machine, and the deflection was monitored through a LVDT with a resolution of 0.05% of the full scale deflection. The specimens were 45 mm × 3.5 mm × 2.5 mm in size, ground and polished to 1 μm finish. Five tests were conducted and an average value was recorded.