The effects of external fields in ceramic sintering

Abstract

Field‐assisted processing techniques can enhance the kinetics of powder synthesis, accelerate sintering processes, and drive phase transformations at significantly lower temperatures compared to conventional methods. However, the exact nature of this nonthermal interaction between field and matter remains vastly speculative. A 2‐day workshop on “Electromagnetic Effects in Materials Synthesis” was organized at Carnegie Mellon University (Pittsburgh, USA) in June 2017, jointly sponsored by the U.S. National Science Foundation and the U.S. Office of Naval Research. This workshop gathered the scientific community working on field‐assisted techniques of materials processing. Inspired by the discussions held at the workshop, this paper summarizes the advancements to date and opens scientific questions and research opportunities in the three major field‐assisted sintering techniques (laser, microwave, and flash sintering). Significant challenges remain in (a) experimental design, measurements, and computational simulations to distinguish the nonthermal effects of the externally applied fields from conventional thermal phenomena; and (b) identifying fundamental mechanisms behind low temperature, nonthermal effects that produce phase transitions and microstructural evolution in materials under externally applied fields. We also present the recent developments in multiscale characterization techniques and the theory and modeling efforts, which aim to tackle the aforementioned grand multidisciplinary challenges facing researchers.

Defect‐Mediated Anisotropic Lattice Expansion in Ceramics as Evidence for Nonthermal Coupling between Electromagnetic Fields and Matter

“…The property of a material depends on its processing parameters, such as heating schedule (or heat treatment), hold time, and processing environment. Recently, externally applied fields such as direct electric fields and electromagnetic (EM) fields have been used to reduce the time and temperature requirements for material processing . In addition to saving time and energy, applied fields can also affect material properties by inducing phase transformations or altering microstructure and morphologies of nanoparticles .…”

Synchrotron X-ray characterization of materials synthesized under microwave irradiation

“…To realize this potential, it is critical to address open questions in the community surrounding the underlying mechanisms of MWR and field-assisted synthesis methods. 10,23 Many of the synchrotron-based characterization tools presented above provide valuable insight into these mechanisms, but there is still no clear consensus regarding how EM fields promote these rapid, low-temperature processes. For example, is the rapid rate enhancement driven purely by enhanced heating rates or more efficient volumetric heating, or can applied fields influence mass transport or defect generation in ways not possible via thermal mechanisms alone?…”

“…As discussed previously, many of the proposed thermal and non-thermal mechanisms, such as rapid heating rates, localized heating, and potential fieldinduced defect generation, are consistent across both MWR-assisted synthesis and E-field-assisted sintering (e.g., flash sintering). 10,23 In particular, in situ synchrotron-based studies of flash sintering in ceramic oxides have been useful in identifying potential nonthermal effects of E-field exposure, such as field-induced texture, 39 non-thermal anisotropic lattice expansion, 53 formation of field-stabilized phases, 54 and increased disorder on the oxygen sublattice relative to the cation sublattice. 55 These observations, particularly with regard to field-mediated texture, phase formation, and oxygen defects, appear to have a high degree of similarity to the observed experimental results on MWR-assisted synthesis presented in this review.…”

“…10,16 Currently, the prevailing explanations attribute the observed effects to thermally driven rapid heating rates 17,18 and localized grain boundary heating, 19,20 while increased mass transport 6,21 and significantly enhanced defect generation 7,22 are the most commonly cited non-thermal mechanisms. For a more comprehensive discussion of the potential mechanisms underlying field-assisted techniques, the reader is directed to reviews by Raj et al 10 and Jha et al 23 To further investigate these mechanisms and understand how applied fields can drive these low-temperature transformations, it is necessary to deploy emerging, advanced characterization tools.…”

Field‐assisted heating of Gd‐doped ceria thin film

“…For example, in situ synchrotron studies 9 have observed metastable phase transformations 10,11 and abnormal lattice expansions across the sample 12,13 during flash sintering. Microstructure characterization after flash sintering revealed asymmetric grain growth, [14][15][16] texturing, 3,17 and high density of defects, 3,17,18 as a result of the generation and mobility of the charged species caused by the applied DC field.…”