Wood ceramics

Ceramic based on solid wood

Wood ceramics, i.e. ceramics based on wood, are materials that are technically produced and based on the principle of silicification . The raw material used is wood, which, in contrast to many artificial raw materials, has a complex microstructure , is available in large quantities, is renewable and environmentally friendly. In addition to using solid wood, wood-based materials and composite materials can also be used as the starting material.

Manufacturing

The manufacturing process of a wood-based ceramic is divided into several stages.

1. Gentle wood drying to reduce or prevent cracking in the wood
2. Pyrolysis at up to 1000 ° C, whereby the cellulose , hemicellulose and lignin are broken down and a carbon template is created.
3. Infiltration of liquid or gaseous silicon

The different ways of making biomorphic ceramics

Wood pyrolysis

Wood consists of approx. 50% carbon, which is bound in long cellulose and hemicellulose chains . These chains are connected and held together by the lignin and form the cell structure of the wood. The carbon is separated from the wood structure through thermal decomposition of the wood.

The thermal decomposition takes place in the absence of oxygen in an inert atmosphere such as nitrogen dioxide or argon . The wood is heated up to 400–500 ° C at 1 ° C / min . At 200–260 ° C, the hemicellulose is first thermally decomposed and broken down, followed by the cellulose at 240–350 ° C. Finally, the ligning breaks down at temperatures between 280 and 500 ° C. This slow heating is necessary so that the gases produced by the degraded chemical wood components can slowly escape from the pores of the wood; on the other hand, the resulting pressure would break up the wood and make it unusable. When the 500 ° C limit is reached, the temperature is increased to approx. 800 ° C at a rate of 10–20 ° C / min. This slow process of thermal decomposition of the wood enables high carbon yields of 25–30 percent by weight of the initial weight. The carbon template created by pyrolysis , which is used as the starting material for most biomorphic ceramics, is a 1: 1 copy of the wood body, i.e. H. the microstructure and every tiny crack in the wood are reflected in the template.

During pyrolysis, there is strong shrinkage in tangential, radial and longitudinal directions, so that the end product has smaller dimensions than the starting body.

Liquid infiltration (solid SiC / Si ceramic)

The most common method is to infiltrate the silicon in liquid form; H. the silicon is heated in a vacuum to its melting point (> 1450 ° C) and placed in the carbon template. The liquid silicon penetrates the template through capillary effects , where it reacts spontaneously and exothermically , i.e. releasing energy, with the carbon atoms. In addition to the bonding of the silicon with the carbon of the carbon template, the liquid silicon accumulates in the pores, which are <50 µm in diameter, so that a solid SiC / Si ceramic is created.

The content of silicon, silicon carbide and carbon in the template, as well as the distribution of silicon in the pores, depends on the processing, the amount of silicon introduced and the shape and microstructure of the wood.

In order to maintain the pore structure of the initial body, the silicon is removed from the pore spaces using nitric acid and hydrogen fluoride.

${\displaystyle {\ce {3Si + 4HNO3 -> 3SiO2 + 4NO + 4H2O}}}$

${\displaystyle {\ce {SiO2 + 4HF -> SiF4 + 2H2O}}}$

According to this method, the porosity of the body is only similar to that of the starting body, since the pores with a diameter of <1 μm have closed due to the volume expansion during the infiltration of the liquid silicon.

Gas infiltration (porous SiC ceramic)

During gas infiltration, the silicon is brought in gaseous form into the carbon template previously produced by pyrolysis , whereby a porous ceramic is created, i.e. H. the silicon only accumulates in the free carbon atoms and the pore space remains free.

The chemical reaction can take place, among other things, through the use of silicon vapor (a), silicon monoxide (b) or trichloromethylsilane (c).

a) ${\displaystyle {\ce {C(s) + Si(g)-> SiC}}}$

b) ${\displaystyle {\ce {2C + SiO -> SiC + CO}}}$

c) ${\displaystyle {\ce {C + (1+x)CH3SiCl3 + 2H2 -> SiC + xSiC + 3(1+x)HCl + CH4}}}$

Infiltration of silicon dioxide or TEOS (porous SiC ceramic)

The third way of the production of biomorphic ceramics is due to the infiltration of a silica - Sole or tetraethylorthosilicate gels (abbreviated TEOS). A carbothermal reduction then takes place at high temperatures and in an inert atmosphere . The resulting SiC ceramic is porous, but can be distinguished from other porous SiC ceramics due to the rough surface within the pore structure.

properties

The mechanical properties vary greatly between the raw materials used and are dependent on the porosity and microstructure of the ceramics. A solid ceramic based on beech wood has a significantly higher modulus of elasticity than a porous ceramic based on red oak. The compressive and flexural strengths also depend on the porosity and the type of wood used as the starting material.

In general, wood-based ceramics have the following properties:

• High abrasion resistance
• High hardness ( Mohs hardness of 9.6)
• High chemical and thermal resistance (resistance to temperature changes> 1000 K / s)

Solid SiC / Si ceramics have the best mechanical properties at room temperature . Depending on the type of wood chosen as the starting material, the compressive strength in the axial direction is between 1100 and 1400 MPa. The compressive strengths are significantly lower perpendicular to the main axis and are between 100 and 500 MPa.

Porous ceramics, which were produced by means of steam infiltration on the basis of beech and pine wood, have compressive strengths of up to 80 MPa in the axial direction.

Uses

The various possible uses are based on the unique properties of biomorphic ceramics. The microstructure of the wood plays an important role in this.

Due to their porosity and the large internal surface, the porous ceramics are particularly suitable as catalyst carriers , especially for high-temperature reactions such as the catalytic combustion of hydrogen . These vascular properties also play an important role in medicine, where porous biomorphic ceramics could be used for implants, since on the one hand they have the necessary strength and on the other hand are not cytotoxic and biocompatible . Due to their enormous heat resistance, they can also be used in the area of microreactors , air cleaning systems for high-temperature exhaust gases or heat exchangers .

literature

1. ↑ ^{a b} Yongsoon Shin, Chongmin Wang, Gregory J. Exarhos: Synthesis of SiC Ceramics by the Caarbothermal Reduction of mineralizes Wood with Silicia . In: Advanced Materials . tape 17 , no. 1 . WILEY-VCH GmbH & Co KGaA, January 2005, p. 73-77 .
2. ↑ ^{a b c d e f g h} J. Ramirez-Rico, J. Martinez-Fernandez, M. Singh: Biomorphic ceramics from wood-derived precursors . In: International Materials Reviews . tape 62 , no. 8 , 2017, p. 465-485 .
3. ↑ ^{a b} A.R. de Arellano-López, J. Martínez-Fernández, P. González, C. Domínguez, V. Fernández-Quero, M. Singh: Biomorphic SiC: A New Engineering Ceramic Material . In: International Journal of applied ceramic technology . No. 1 , 2004, p. 56-67 .