Large-scale preparation of ceria/bismuth metal-matrix nano-composites with a hardness comparable to steel

Abstract

Metal based composites account for a large part of technically used alloys. Their favorable thermal and mechanical properties have been repeatedly suggested to strongly profit from the possible application of nanosized metals and ceramics as starting materials. The present work therefore explores the simultaneous production of both constituents by controlling the gas composition and reduction potential in an oxygen deficient flame spray process and choosing appropriate metal/ceramic compounds. More specifically, metallic bismuth and ceria (CeO₂) nanoparticles were prepared in a single process step at a production rate of >30 g h⁻¹ and resulted in a highly homogeneous mixture of oxide and metal particles. After characterization by X-ray diffraction, electron microscopy and energy dispersive X-ray spectroscopy, the powder was compacted at room temperature to rods of 1.2 cm diameter. These bulk samples with ceramic loadings as high as 35 vol% combined metallic (high gloss, electrical conductivity of above 100 S m⁻¹) and ceramic properties such as a Vickers hardness comparable to steel (>100 HV), while pure bismuth can be indented with a finger nail. Thermodynamic calculations illustrate how the presented reducing flame spray based process can simultaneously manufacture the metal and ceramic phase of these nano-composites. This direct route to metal/oxide nano-composites avoids the currently unsolved problem of homogeneously mixing non-wetting nano-sized ceramic and metal constituents.