A meeting of materials - biomorphic ceramics
Materials scientists are mimicking the hierarchical structures found in biological materials to make improved ceramics. Melanie Rutherford explains how a new technique devised by researchers at Wuhan University of Science and Technology in China is advancing the development of these so-called biomorphic ceramics.
Contrary to popular belief, composite materials are not a modern man-made invention. Since the very first trees sprouted from the ground more than 300 million years ago, wood’s complex hierarchical cellular structure has made it one of Earth’s most versatile natural composite materials. Made up of varying shapes of long, tubular cellulose fibres that are aligned with the axis of the trunk and its unique growth-ring structure, the characteristics of plant fibres have proven useful for scientists to use as bio-templates for the production of advanced ceramics. Wood-derived cellular ceramics retain the original structure of the plant fibres, which, along with their superior structures, makes the ceramics useful in a host of applications. As well as proving successful catalyst carriers and immobilisation supports for living cells, microbes or enzymes, they are of interest in applications such as high temperature-resistant exhaust-gas filters, advanced microreactor systems, waste water treatment, and acoustic and heat insulation structures.
Over the past 10 years, scientists have successfully fabricated porous carbide ceramics using wooden templates. In 2004, researchers at Xi’an Jiatong University in China used carbothermal reduction of oak wood and charcoal to produce a highly porous silicon carbide (SiC) ceramic with a woodlike microstructure. In the same year, scientists at Shanghai Jiatong University, also in China, produced morphogenetic titanium-carbide (TiC) ceramics using titanium oxide (TiO2) made from decomposed tetrabutyl titanium, subjecting it to high temperatures where it reacted in situ in the oak’s cell walls.
While these methods are nothing new, there have been fewer attempts to produce these so-called biomorphic ceramics using molten salt synthesis (MSS). The technique involves dissolving a metal powder in a molten salt solution and heating it with a carbon substrate, whereby the solution reacts with the surface of the carbon fibres to produce a composite ceramic. Perhaps the most notable effort was a collaborative study in 2008 by researchers at Wuhan University of Science and Technology, China, the University of Leeds and the University of Sheffield, both UK. After synthesising a TiC coating on carbon fibres using MSS, the scientists studied both the synthetic mechanism and the structural changes in the resulting porous ceramic at different temperatures.
More recently, in 2012 a team of researchers at the State Key Laboratory Breeding Base of Refractories and Ceramics, based at Wuhan University of Science and Technology, used this method to synthesise porous TiC/C ceramics, using phoenix wood as the carbon source.
Before wood can be converted to a porous carbon template it must be completely free of moisture, which the scientists did by drying out the material at around 110°C for 24 hours. It was then heated in an argon furnace for three hours at 650°C to complete the conversion process. The wooden templates were then covered by a molten salt mixture of potassium chloride (KCl) and potassium fluoride (KF), in which titanium metal powder had been dissolved. After being heated in argon to various temperatures (ranging from 700–1,000°C) for either three, four or five hours, the samples were left to cool to room temperature before being flushed with hot water to remove any salt solution that might remain in the pores.
Examining the evidence
The scientists then examined the various ceramic composites produced, using a combination of X-ray diffraction (XRD), SEM analysis and energy dispersive X-ray spectroscopy (EDS) to establish the chemical ratio of titanium and carbon in the coatings. Temperature, molar ratio and annealing time were all found to affect the nature of the ceramics produced:
Temperature – After examining how the titanium powder reacted with the carbon template in the KF-KCl salt at 700°C, 800°C, 900°C and 1,000°C, the scientists discovered that heating disrupted the orderly structure of some of the original components of the samples. TiC peaks started to appear at 700°C and became dominant upon increasing the temperature to 1,000°C, where carbonisation caused phase changes.
Molar ratio – After using molten salt/Ti molar ratios of 2:1, 3:1 and 4:1, the team found a ratio of 3:1 to offer the best conversion efficiency. SEM analysis of the sample heated at 900°C at a media/Ti ratio of 3:1 showed a TiC layer with a regular crystal morphology on the surface of the carbon template. The carbon pattern plays an important role in forming TiC in the molten salt system, with potassium fluoride acting as a catalyst to a certain degree. The F ions cause cubical expansion by increasing the interspace dimensions and pipeline diameters of the template. This helps the titanium to diffuse into the inner and outer regions of the carbon template, accelerating TiC generation and promoting the growth of crystals via seeding.
Annealing time – This proved an important consideration for the surface integrity and density of a ceramic produced by the MSS route. SEM images of a sample heated for five hours at 900°C revealed that some whisker-shaped crystals had been generated on the surface of the carbon template. While the large vessels remained open, either the TiC or salt had caused the smaller pores to fully or partially close. This indicates that a longer period of heat treatment is favourable to the formation and integrity of the TiC crystalline morphology.
Pores for thought
When a carbon template is soaked in a salt/metal bath, some titanium atoms or ions dissolved in the molten salt solution rapidly diffuse to the surface or open pores of the template. These react with the carbon atoms to form the corresponding carbide (depending on the solubility of the metal in the molten salt). While the salt left in the pores can be washed away with hot water, the scientists found that some potassium chloride adheres to the surface of carbon template that cannot be fully leached out. Using automatic mercury porosimetry, the researchers measured pore size distribution in the carbon template. The pore sizes of the original dried phoenix wood template mainly fell between 10–100μm, and after carbonisation the majority (about 50.9%) were microporous, with the remaining middle-micropore sized. Pores greater than 100μm were least abundant, with only 0.4% found. Carbonisation caused a large decrease in apertures greater than 100μm compared to the original wood sample – pores measuring 10–100μm decreased by more than 20% and pores less than 10μm increased by more than 20%. This demonstrates that carbonising the wooden templates produces an abundance of micropores for the titanium to diffuse into.
Finally, the scientists compared pore-size distribution in the carbonised wood template before and after heating in the molten salt/metal media. While the number of micropores measuring less than 10μm increased following the salt treatment, the number of middle micropores decreased and the number of macropores increased slightly. This, the scientists concluded, could have been due to the effect of the potassium fluoride on volume expansion, although they noted that an increase in micropores could be caused by the carbon–titanium reaction.
This successful production of a TiC coating on the surface of a wood-based carbon template using relatively low-temperature MSS is a significant step forward in biomimicry. With both molar ratio and reaction temperature found to significantly affect the density and surface properties of the TiC coating as well the distribution of micropores within both the carbonised wood and the TiC/C ceramic, the results show promise for producing ceramics with low porosity and, therefore, high mechanical strength. With potential to extend this technique to other carbides, the molten salt method offers a real solution to the growing field of wood-based ceramics.
This article is based on a paper entitled Preparation of Porous TiC/C Ceramics Using Wooden Template in Molten Salt Media by J Ding, C J Deng, W J Yuan, H X Zhu and J Li, which was published in Advances in Applied Ceramics. With thanks to Professor Michael Reece of the Department of Materials at Queen Mary, University of London and Editor of Advances in Applied Ceramics.