a) SiCNW growing from a large Si-based particle, (b) representation of the morphology seen in (a).
SiCNW, silicon carbide nanowires. Credit: ACT Journal.

An article in one of ACerS’ journals reports on research by a duo at San Diego State University’s Powder Technology Lab demonstrating how natural catalysts in some forms of agricultural waste could be leveraged to convert the leftover material into a template for silicon carbide nanowires, the kind of material that some investigators are eyeing for solid state adsorption of hydrogen for storage and similar applications.

Given its abundance, there is growing interest in being creative about using Ag waste as a feedstock. Certainly, this isn’t the first time a biomass has been used to make a template for silicon carbine nanowires, nanotubes, etc. Rice husks and bean curd hulls, for example, have been used successfully in the past.

Now, William Bradbury and Eugene Olevsky,  report in a paper appearing in the Early View edition of Applied Ceramic Technology, that they have been able to use crambe abyssinica, an oil seed crop, to fabricate monolithic biomass-activated carbon templated silicon carbide-nanostructured materials with fairly high specific surface areas and, thus, may be of interest for H2 adsorption potential.

A big novel thing that Bradbury and Olevsky are doing is that they more or less invert the paradigm used, for example, with the rice hull fabrication. Instead of adding catalytic elements to the rice hulls that naturally contain silicon, Bradbury and Olvesky make use of the natural catalytic content (e.g., Fe) of the crambe but add silicon during processing.

The two, who work at San Diego State University, used a fairly conventional dip coating and thermal processes. They began with cut lengths of crambe stalks which they heated to 500°C, and held for three hours under N2 flow. They soaked the resultant carbonized products in KOH solution of ∼10M for 24 h and activated in a conventional tube furnace at 800°C in N2 for two hours. They then washed the products, and treated them with dilute hydrochloric acid solution. Next, the AC product was combined with a ethanol–silicon power slurry, and, finally, vacuum dried. After drying, they placed this AC precursor material (with SSAs of ~ 930 m2/g) in an alumina crucible in a tube furnace. The chamber was purged with argon gas and the temperature raised to 1500°C where it was held for three hours.

At the end of all this, they had template samples where portions were nearly completely covered with nanowire growth sites. The nanowires, themselves, had diverse morphologies and had SSA values that dropped to the 100 m2/g range. They believe this diversity is related to the actual chemical content of the crambe and/or growth mechanisms. Residual KOH may have also been a factor.

While the results didn’t immediately yield templates with SSAs that surpassed other ag waste precursors (rice hull templates have been made with SSAs of more than 3400 m2/g), Bradbury and Olevsky have at least demonstrated that another Ag waste product can be used and that there are still opportunities to leverage natural catalytic potentials of these materials. They think their demonstration is only a first step and believe they can ultimately tailor the SiC structures and improve the SSAs by purifying and refining the precursor materials. In addition, they are fairly confident they can use these techniques to template other systems such as ones based in TiC.

Another paper reports that Bradbury and Olevsky already have learned how to increase the amount of nanowires produced and reduce the relative amount of SSA drop off between template final product.

Adding, in regard to above paragraph, one thing I failed to mention is that researchers used a new approach, the novel Free Pressureless Spark Plasma Sintering technique, that they say looks the most promising for the development of high surface area materials. The technique is discussed in the paper, but, the benefits come because it removes pressures typical of conventional SPS processing, allowing the SSA of the material to be preserved.