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aude Paul Carrya) SIMAP, Univ. Grenoble Alpes, CNRS, Grenoble 38 000, France

Paul Bowen Powder Technology Laboratory, Material Science Institute, Swiss Federal Institute of Technology, Lausanne CH-1015, Switzerland

Zhe Zhao Department of Materials Science and Engineering, KTH Royal Institute of Technology, Ceramic Technology Division, Stockholm SE-10044, Sweden (Received 31 December 2016; accepted 20 March 2017)

In the quest for high real in-line transmittances for transparent polycrystalline alumina (PCA), we need defect free processing. One of the biggest advances in producing high density defect free ceramics over recent years has been the advent of spark plasma sintering (SPS) or pulsed electric current sintering. The production of PCA with high transmittances .60% has been demonstrated, but the mechanisms behind this fast, pressure aided sintering method are still much debated. Here, we investigate the sintering of doped a-alumina powders using traditional and pulsed electric current dilatometry. We demonstrate that at the final sintering stage, there is no major difference in the sintering mechanisms between conventional sintering and SPS sintering. High densification rates occurring in SPS are shown to be related to powder reorientation at the very early sintering stage and viscous-flow dominated densification in the intermediate sintering cycle. This paper clarifies what parameters in the processing–sintering domain have to be improved for even higher real in-line transmittances for PCA.

I. INTRODUCTION

Since its appearance a few decades ago, spark plasma sintering (SPS), nowadays more correctly known as pulsed electric current sintering (PECS), has raised increasing interest within the ceramics community due to its fast sintering cycles and the capacity to produce ultrahigh density materials. The reasons for these fast sintering cycles and successful processing of ceramics still raise questions within the community1,2 as illustrated by the fact that some researchers use PECS more like a conventional sintering method with sintering cycles up to several hours,3–6 while others focus their work on ultrashort ones of only a few minutes.7–10 Olevsky et al.2 have suggested that the most pertinent difference between PECS and conventional sintering is

Contributing Editor: Eugene Medvedovski a) Address all correspondence to this author. e-mail: paul.bowen@epfl.ch DOI: 10.1557/jmr.2017.119

the density of diffusion paths rather than changes in the mechanism itself. This follows the principle idea of Bernard–Granger et al. who suggested treating PECS as a normal hot-press sintering method.1 In this work, dilatometry studies combined with stress exponent determinations are used to compare the apparent activation energies for densification of Al2O3 by PECS with those obtained by conventional sintering or dilatometry. The combination of both provides a deeper insight into the dominant mechanisms involved during sintering. Although, the exact mechanism(s) involved in the sintering cannot be directly addressed by this method,