Flash Sintering of Rhenium in About 1 Minute with Electrical Current

Abstract

We show that rhenium can be sintered from powders to nearly full density (99.96 pct) by directly injecting electrical current into dogbone shaped specimens. The current was increased at a rate of 1 A s⁻¹. The specimen sintered abruptly after about 30 seconds when its temperature had risen to 900 °C. The experiments were carried out without furnace heating, within a glove box in Ar atmosphere. The following in-operando measurements are reported, (i) shrinkage strain with a rapid rate camera, (ii) resistivity measured by voltage and current, (iii) temperature measured with a pyrometer, and (iv) electroluminescence spectra measured with a spectrometer. The sintering cycle, the first, during which the sample sintered to full density, was followed by two more flash cycles with the same specimen. In the first cycle, the change in resistance exhibited a peak arising from abatement of interparticle interface resistance; the peak was absent in the second and third cycles. The rapid sintering is attributed to the generation of defects in the form of vacancy-interstitial (Frenkel) pairs. The concentration of the Frenkels was estimated from in-situ calorimetry, where the difference between the electrical input energy, and the energy lost to radiation, convection and specific heat, was attributed to an endothermic reaction for defect generation. In this way we calculated a concentration of ∼ 10 mol pct of Frenkel pairs. The resistance of the flash sintered specimens was higher than literature values, presumably due to these defects. The very low sintering temperature and the anonymously high defect concentrations mean that flash sintering of metals is a far-from-equilibrium phenomenon.