Characterization of amorphous silicon nitride prepared from sand and coffee husk wastes by carbothermal reduction-nitridation

Abstract

The continued coffee demand in Kenya has amplified the generation of its husk wastes causing disposal problems. This has led to serious pollution to the environment. Therefore, developing a greener and cost-effective ways to handle these wastes is necessary. The current study entailed the use of extracted silica and coffee husk biochar as novel precursor materials for the synthesis of silicon nitride (Si3 N4 ) composite in an ammonia environment. The silica was extracted from raw sand by alkali fusion route. The biochar was prepared by pyrolytic treatment of raw coffee husk biomass at 300 o

C for 5 hours followed by acid leaching. The sand, extracted silica, raw and biochar samples and silicon nitride composites were characterized using X-Ray Fluorescence (XRF), Fourier Transform Infrared (FT-IR), X-Ray Diffractometer (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX) and Thermogravimetric Analyzer (TGA). The results showed that the raw sand contain quartz, calcite, microcline and clinochlore. The results for the extracted silica showed a highly amorphous silica containing hydroxyl (-OH) and siloxane (Si-O-Si) functional groups. The alkali fusion process increased the silica content to > 92 % from 83 % with oxide impurities reduced to < 1 %. The characterization of biochar samples showed a highly amorphous, porous carbon structure with aromatic carbon bonds (C=C) and a negligible ash content. The characterization of Si3

N4 powder showed thermally stable, porous and highly amorphous material with α-Si3 N4 and - Si3 N4 phases. The FT-IR results

showed Silicon-Nitrogen-Silicon (Si-N-Si), silanol (Si- OH), Silicon-Nitrogen (Si-N) and Silicon-Silicon (Si- Si) as important functional groups present in silicon

nitride composite material. The results revealed a greener approach of Si3

N4 synthesis for application in vast

industrial fields.