EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Pure tungsten carbide nano-powder can be densified in less than 10 s by Electrical Resistance Sintering (ERS) technology under high current and low voltage which activate an ultra-fast Joule heating. The triggering of the flash-like phenomenon in the metal-like conductor is shown to be concurrent with a non-linear reduction of the green compact resistivity. This work investigates the relationship between the electrical properties of the conductive ceramic and the possibility to achieve the condition of “flash sintering”, wherein complete densification can be achieved in a couple of seconds. In particular the nature of the superior resistivity of conductive particles in contact was investigated.
ERS experiments were carried out at various applied pressure to analyze its effect on the electro-thermal runaway phenomenon. The constriction effects at particle-particle nanojunctions and surface chemistry as characterized by XRD and XPS were shown to be fundamental in determining the very high initial resistivity. A higher pressure increases the particles contact radius and the initial conductivity of the system. A larger contact radius shortens the time-scale and the magnitude of the resistivity drop, thus decreasing the onset time for the occurrence of the flash-like power evolution. The attainable final density is therefore limited by the applied pressure, decreasing from 94% at 4 MPa down to 70% at 1000 MPa. This effect is more evident for lower sintering times (10 s), when the material densifies under the power surge with respect to 60 s duration where the sintering progresses also under constant current.
The obtained results reveal the potential to reduce the energy consumption in the processing of conductive ceramics by more than 1000% if one operates on the ratio of the electrical energy absorbed during the power surge with that corresponding to the entire process.