Metal carbides are highly useful engineering materials due to their high strength, resistance to wear, and their ability to withstand various extreme temperatures. Titanium carbide (TiC) is useful on an industrial scale due to its strength retention at high temperature with a melting point of 3065°C, relatively low density (4.93g/cm^-3), and resistance to corrosion. TiC is also relatively inexpensive due to the natural abundance of its precursor, titanium. Consequently, it is commonly used in the manufacture of cutting blades, grinding wheels, and various other machining tools.

     The most common methods for the synthesis of titanium carbide are through powder, gas phase, and sol-gel reactions at high energy. While these three procedures are effective, most commercial manufacturers use the relatively inexpensive process of carbothermal reduction of powders. In this process, titanium oxide is reduced using a carbon source and a heated environment by the following reaction, where s stands for solid and g for gas:

TiO₂(s) + 3C(s) = TiC(s) + 2CO(g)                [1]

     This carbon source is usually a derivative from petroleum, which is a nonrenewable and expensive source due to high demand from the energy market. Here we assess the feasibility of using pyrolysis of a renewable biopolymer to obtain this carbon.

     The biopolymer composite featured in this process consists of three components: carrier, filler, and titania nanoparticles. The carrier, iota-carrageenan (IC), is extracted from seaweed and is a common polymer used in the food industry as a thickener. Commercial chitin, obtained from shrimp shells, is used as the filler. Both IC and chitin are easily available and can provide sustainable, yet inexpensive precursors. For example, current prices for a gram of iota carrageenan, chitin and activated carbon are $0.62, $0.84, and $5.23 respectively.

     2.09g of titanium oxide nanoparticles were initially mixed in 30ml of ultra-pure water using a Qsonica® CL-334 sonicator for 15 minutes until a homogeneous, colloidal dispersion was obtained. A gel-like composite was subsequently created upon addition of 1g of iota-carrageenan, 4g of chitin, and another 15ml of ultra-pure water. The composite was then mixed thoroughly using a small spatula and an Omni Macro® homogenizer. The resultant biopolymer composite was loaded into a syringe and shaped into different geometries, such as a three-dimensional cube, using manual extrusion.

     These shapes were then subjected to high temperature treatment in a tube furnace. Although the reaction temperatures for TiC synthesis from solid powder mixtures is usually above 1700°C, the colloidal nature of the TiO₂ dispersion used here was expected to lower the required temperature for reaction. Consequently, the maximum temperature of the furnace was set to 1450°C for a period of 3 hours with a heating rate of 5°C/min and a vacuum environment.

     The resultant sample was analyzed using X-ray diffraction (XRD) As shown in Figure 1, the XRD results show a strong presence of TiC in the sample as well as Rutile and TiO₂ in smaller, but significant quantities. It should be noted that amorphous carbon was not clearly discernible in this sample. Furthermore, it was observed that the original extruded shapes of biopolymer gel were still preserved, despite significant shrinkage. These shapes were highly porous and brittle.

     This initial result shows the feasibility of using a colloidal dispersion of titanium oxide nanoparticles in a biopolymer matrix to manufacture titanium carbide at reduced energy requirements. This is largely due to the use of non-petroleum based precursors and lower temperatures than those used in more conventional processes. The biopolymer composite can be easily shaped through extrusion and allows for titanium carbide to be produced in complex shapes not possible through traditional methods such as powder pressing. Although isometric shrinkage occurs, the original shape is maintained.

     Ongoing work is on analyzing the impact of the temperature, heating time, heating rate, and furnace atmosphere on the properties, porosity, and shrinkage of the resultant material. The TiC yield must be increased to make the process more efficient and eliminate Rutile and unreacted TiO₂ from the product; creating a much stronger and more versatile carbide.

Figure 1: X-ray diffraction of the biopolymer composite after heat treatment of 1450°C in a vacuum environment.

References

1. Koc, R. & Folmer, J. S. “Carbothermal synthesis of titanium carbide using ultrafine titania powders”. Journal of Materials Science, 32, 3101-3111 (1996).
2. Bae, S., et al “Synthesis of titanium carbide nanoparticles with a high specific surface area from a TiO₂ core-sucrose shell precursor”. Journal of American Ceramics, 92, 2512-2516 (2009).
3. Durlu, N., “Titanium carbide based composites for high temperature applications”. Journal of the European Ceramic Society, 2415–2419 (1999).