Modeling porosity of high surface area nanopowders of the gallium nitride GaN semiconductor

Abstract

A pool of high surface area gallium nitride GaN nanopowders was prepared and suitably characterized. The powders appeared to be mostly mesoporous with increasing share of microporosity at the highest surface area end. The BET surface area values spanned the range from 6.3m2g−1 to 222m2g−1 and specific mesopore volumes reached up to 0.21cm3g−1. Powder XRD scans indicated for all samples the hexagonal polytype of GaN. Average crystallite sizes were calculated from the XRD data with Scherrer equation. Helium pycnometry was used to determine skeletal densities of the nanopowders in the wide range of 3.6–6.1gcm−3. The densities showed an evident particle size dependence that could be explained by the presence of specific crystallite agglomerates impenetrable by helium atoms. Herein, a simple model of porosity for nanocrystalline powders, specifically nano-GaN, is proposed and confronted with the experimental data. The surface area for such nanopowders was modeled with appropriately specified surfaces of close-packed spheres (rigid and non-rigid configurations) as a function of sphere diameter. Selected model variants were statistically best-fitted with experimental data points, i.e., BET specific surface areas and average crystallite sizes determined from the XRD scans, to yield very good solutions. They support the view that the BET surface area of the GaN nanocrystalline powders basically corresponds with the accessible surface area of the nanoparticles – crystallites and/or agglomerates.