Nano-Materials from Waste

16 May 2017
Macadamia

High performance materials. In a world first, UNSW researchers have demonstrated that the tens of thousands of tonnes of macadamia nuts shells discarded annually by the Australian macadamia agricultural and food processing industries can be used to produce high value, high purity silicon carbide and silicon nitride nano-powders.

These nano-compounds are used to transform many widely used materials, such as metal alloys and plastics, into high value products by introducing superior properties, like shock, heat and wear resistance. Silicon carbide nano-powders are also of intense interest as potentially high-power semi-conductors that could revolutionise electronics by withstanding high temperatures, thereby overcoming the need for cooling. Silicon carbide and silicon nitride nano-powders are usually synthesised at high temperatures using non-renewable coke/coal, as a carbon source, and silica sand. Coal, however, is not ideal as it contains a high percentage of ash, making production technically challenging and expensive.

The UNSW team from the Centre for Sustainable Materials Research and Technologies (SMaRT Centre) have demonstrated that waste macadamia shells can be used as a superior carbon source due to its low ash content, simultaneously reducing demand for fossil fuels and productions costs in the synthesis of these high value silicon carbide and silicon nitride nano-powders.

As silicon carbide has many unique properties -- including high thermal conductivity, superior mechanical strength, corrosion resistance at elevated temperatures, high thermal stability, low thermal expansion and superior chemical stability to oxidation in extreme conditions – it is considered a promising and valuable material for in many applications including in aerospace structures, biomaterials, high temperature semi-conducting devices, high performance composite materials, membrane supports. Silicon carbide semi-conductor devices are widely used in the aerospace, nuclear power instrumentation, satellite and space exploration industries. When used at the nano-scale, silicon carbide’s favourable properties are enhanced. For example, adding SiC nano-powers to either steel or plastics creates new super strength nano-composite materials with far greater mechanical properties than the original material, for applications such as the manufacture of aircraft.

The macadamia tree is an iconic Australian native. Australia produces about 40% of the world’s supply of macadamia nuts and, after processing about 65% of the crop by weight is discarded as shells. The shells, however, have a high carbon and low ash content, making them a potentially valuable carbon source, with more attractive properties than many other renewable bio-mass sources, such waste coconut shells.

References:

  1. Ravindra, R. & Sahajwalla, V. (2016). A Cleaner, Sustainable Approach for Synthesising High Purity Silicon Carbide and Silicon Nitride Nanopowders Using Macadamia Shell Waste. Journal of Cleaner Production, 133, 1277–1282