High orientation degree of graphene nanoplatelets in silicon nitride composites prepared by spark plasma sintering

Abstract

The sintering process of ceramic composites is known to significantly alter the distribution of the nanoscale fillers within the matrix. Graphene nanoplatelets reinforced ceramic composites sintered under uniaxial pressure conditions (e.g., spark plasma sintering, hot pressing) display anisotropic mechanical, electrical and thermal properties. This anisotropy can be attributed to the preferential alignment of the graphene nanoplatelets perpendicular to the direction of the uniaxial pressure. Here, we provide a detailed structural characterization of the graphene nanoplatelet distribution by scanning electron microscopy and ultra-small angle neutrons scattering investigations of spark plasma sintered Si3N4/graphene nanoplatelts composites. To quantify the effect, we compare our findings for samples sintered by the spark plasma sintering with uniaxial pressure (SPS) to the same composite sintered by hot isostatic pressing employing isotropic pressure (HIP). We found that for SPS sintered composites more than 80% of the graphene nanoplatelets (GNPs) are oriented within ±15° of the dominant orientation direction, in striking contrast to the randomly orientated GNPs found in the samples sintered by HIP. These results can contribute to the quantitative understanding of the various anisotropic physical properties reported in GNP reinforced composites prepared by methods employing uniaxial pressure.