Preparation of Biomorphic SiC Ceramics

This paper deals with a new method for producing non-oxide ceramic using wood as a template. SiC with a woodlike microstructure has been prepared by carbothermal reduction reactions of Tilia wood/TEOS composite at 1873K. The porous carbon preform was infiltrated with TEOS (Si(OC2H5)4), as a source of silica, without pressure at 298K. The morphology of resulting porous SiC ceramics, as well as the conversion mechanism of wood to SiC ceramics, have been investigated by scanning electron microscopy (SEM/EDS) and Xray diffraction analysis (XRD). Obtained SiC ceramics consists of β-SiC with traces of α-SiC.


Introduction
Manufacturing of ceramic materials from biological templates such as wood has become a matter of increasing interest because of the possibility of producing novel ceramic materials with a unique microstructure pseudomorphous to wood [1][2][3][4][5].
Woodceramics are promising new porous materials with large potentiality for a variety of industrial applications.
Anisotropy in wood is the result of the orientation and alignment of cells and cell walls, as well as variation in density.The highly anisotropic cellular structure of wood tissue may serve as a hierarchically structured template to generate novel cellular ceramics with micro-, meso-, and macro-structure.
Related to the anatomical structure trees can be classified as a hardwoods and as a softwoods.Wood is a natural composite with cellulose, hemicellulose, and lignin as the major constituents [6].
The morphology and the arrangement of the different cells may vary in a wide range between the different kinds of wood with large vessel cells dominanting in hardwood and tracheids dominanting in softwood.The diameter of the vessels and tracheids (named as pores) varies between 5 and 50 μm in softwood and between 1 and 300 μm in hardwood.These cells with a preferential orientation in the axial direction offer the possibility of using various infiltration techniques to transform the bioorganic wood structure into an inorganic ceramic material with tailored physical and mechanical properties.
High temperature (above 873K) pyrolyzing wood results in decomposition of the polyaromatic constituents to form carbon template with an original cellular structure.___________________________________________________________________________ 142-These carbon performs can be used for producing SiC with highly anisotropic porosity, by liquid infiltration with tetraethylorthosilicate (TEOS) and subsequent carbothermal reduction reaction at elevated temperatures.
In the present work, we demonstrated the possibility of producing porous SiC ceramics with a biomorphic microstructure by carbothermal reduction of premineralized Tilia wood with silica.

Experimental procedure
Rectangular specimens (10x5x5mm) of Tilia wood were dried at 343K for 2 days.Carbon preforms were prepared by carbonizing the dried wood at 1273K in Ar atmosphere for 2 hour with a heating rate of 5 K/min.Specimens were soaked in an ethanol solution of tetraethoxysilane [Si(OC 2 H 5 ) 4 , TEOS] (molar ratio TEOS:EtOH =1:4) and stirred for 2 hour at room temperature.The treatment procedure of impregnation, gelling, drying and 1273K treatments were repeated several times, to increase the silica content in C/SiO 2 composites.Samples after the first, second and third infiltrations were denoted as CWSI, CWSII and CWSIII.
Carbothermal reduction reaction (CRR) of the C/SiO 2 composite was carried out at 1873K in flowing Ar atmosphere in a furnace with graphitic heating element for 1 hour with a heating rate of 40 K/min to form a porous SiC ceramic.
The morphology changes and carbothermal conversion of the C/SiO 2 composite into SiC ceramic were investigated by scanning electron microscopy (SEM/EDS) and X-ray diffraction analysis (XRD).Fig. 3 shows SEM micrographs of highly porous woodlike silicon carbide ceramic obtained after CRR at 1873K and removal of free carbon at 973K (air, 2h).A preserved biomorphic cellular structure can be seen in Fig. 3a, showing channels that originate from tracheid cells that are parallel to the axis of the tree.Corresponding EDS spectra is presented ___________________________________________________________________________ 144in Fig. 4a, showing that this phase is mainly composed of Si and small amounts of O, Ca and K. Unfortunately, carbon cannot be detected due to instrument limitations.At some points in the samples, usually at the end of the channels, fibers are observed (Fig. 3b).They are about 10μm in length with diameters varying in the range from 100 to 300nm.EDS spectra (Fig. 4b) are almost identical to the previous spectra, showing only traces of O, Ca and K. Granular porous structures are presented in Fig. 3c-d, showing chain-like structure (Fig. 3c) and the pore structure i.e. open ends of channels, viewed in the axial direction (Fig. 3d).EDS spectra (Fig. 4c-d) are also identical to the previous ones, confirming homogeneity of samples, i.e. all observed phases have same chemical composition.From both XRD and EDS results, it can be concluded that all observed phases are mainly composed of SiC.Ca and K are most probably present in the form of CaO and K2O, which are the remains of corresponding water-soluble salts originally present in the raw wood material.

Conclusions
This paper demonstrated the conversion of biological cellular tissues structure into porous ceramics.This porous SiC ceramic with a wood-like microstructure was prepared by sol infiltration of carbonized Tillia wood performs using TEOS as the silica source and a carbothermal reduction reaction at 1873K.XRD and EDS analysis revealed that β-SiC is the major phase present in the cellular ceramic product.This technique provides promising future applications for designing advanced materials with low density and high porosity.

Fig. 2
Fig. 2 XRD patterns of samples treated at 1873K: a) I infiltration, b) II infiltration, c) III infiltration

Fig. 1 .Fig. 3
Fig. 1. shows the XRD pattern of CWSIII that indicates a semi-crystalline structure with characteristic (002) and (101) reflections of graphite.The XRD patterns of samples after carbothermal reduction (Fig. 2. reveal that the obtain SiC ceramics consists of β-SiC with traces of α-SiC.The amount of β-SiC increases with increasing number of infiltrations.