Field-assisted sintering technology (FAST), also known as spark plasma sintering (SPS), offers rapid heating and cooling rates for pressure-assisted densification within short processing times. This method has been explored for the ability to produce conductive composites, applicable for high temperature applications, composed of fine- or coarse-grained alumina and the refractory metal niobium or tantalum. It is intended to create a conductive network with a minimum metal content. To elucidate, which structural characteristics are conducive to these networks and conductivity paths, this project employs X-ray based investigation methods like µCT. By adjusting the porosity of the composites in a targeted manner, the relationship between processing parameters, 3Dmicrostructure and the critical percolation threshold for conductive composites is investigated. Furthermore, the influence of raw materials and processing parameters on mechanical and electrical properties is discussed.