Available online at www.sciencedirect.com Journal of the European Ceramic Society 32 (2012) 3675–3683 Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of an externally applied field Troy B. Holland a,∗ , Tien B. Tran a , Dat V. Quach a , Umberto Anselmi-Tamburini b , Joanna R. Groza a , Amiya K. Mukherjee a a Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, United States b Department of Physical Chemistry, University of Pavia, V.le Taramelli 16, 27100 Pavia, Italy Received 13 December 2011; received in revised form 15 May 2012; accepted 16 May 2012 Available online 9 June 2012 Abstract In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 ◦ C/min up to 800 ◦ C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielectric ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielectric ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate. © 2012 Elsevier Ltd. All rights reserved. Keywords: Sintering; ZnO; Apatite; FAST/SPS 1. Introduction The field-assisted sintering technique (FAST), also known as spark plasma sintering (SPS), has demonstrated excellent con- solidation behaviors in a wide variety of powder systems. 1,2 Benefits from an applied field are reported to include rapid consolidation, reduced sintering temperatures, lower sintering pressure requirements, and small final grain sizes. 2–9 The mech- anisms by which applied fields impart these benefits are now being heavily investigated and have been usefully separated into their contributory aspects: thermal and athermal. 3 The thermal mechanisms of FAST/SPS relate heating rate effects alone, while the athermal mechanisms are related to contributions from the applied fields. Typically, discussions of the thermal components of sintering are considered in terms of the distinct stages of sintering. However, this has largely not been the case for the athermal components of sintering in any formal sense. It is useful to apply current knowledge of the individual stages of sintering ∗ Corresponding author. Tel.: +1 530 752 6290. E-mail address: tbholland@ucdavis.edu (T.B. Holland). as a foundation upon which to appropriately characterize the athermal contributions. The efficacy of FAST/SPS processing is often ascribed to the role of applied fields on the movement of defects that makes sintering possible in nonconductive materials. In literature, the effects are often described as occurring ubiquitously throughout sintering with little mention of the possible mechanistic differ- ences that fields can introduce during the three distinct stages of sintering: initial, intermediate, and final. Theoretical mod- els on the contribution of heating rates 3 to the densification of coarse-grained powders suggest that accelerated densification rates are possible using the heating rates typically applied dur- ing FAST/SPS processing. Direct experimental comparisons to FAST/SPS processing at these heating rates, without the compli- cating contributions of an applied field, have not been performed to the best of our knowledge, though Chen and Mayo have shown that heating rates of up to 200 C/min do not appear to aid the densification behavior of ZrO 2 –3 mol% Y 2 O 3 . 10 Using the techniques described here, we are able to determine the rel- ative contributions of athermal mechanisms to the sinterability of dielectric ceramic systems by comparing densification when FAST/SPS-relevant rates heating rates are used with and without 0955-2219/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2012.05.027