Ultra-rapid microwave sintering employing thermal instability and resonant absorption

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Ultra-rapid microwave sintering employing thermal instability and resonant absorption Kirill I. Rybakov1,a) , Sergei V. Egorov1, Anatoly G. Eremeev1, Vladislav V. Kholoptsev1, Ivan V. Plotnikov1, Andrei A. Sorokin1 1

Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod 603950, Russia Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. a)

Received: 29 April 2019; accepted: 25 June 2019

Ultra-rapid microwave sintering of ceramics has been recently demonstrated by the authors. In the experiments with oxide ceramic samples carried out in a 24 GHz gyrotron system for microwave processing of materials, full density was achieved in the sintering processes with a duration of the high-temperature stage of one to several minutes and zero hold at the maximum temperature. The implementation of the ultra-rapid microwave sintering processes was made possible due to fast and efficient control over the temperature of the materials and the supplied microwave power. The absorbed microwave power density was typically in the range of 10– 100 W/cm3, which is within the same order of magnitude as the power of Joule heat in the DC electric field– assisted flash sintering processes. At this power level, a thermal instability is triggered by the volumetric heating, which results in a drastic enhancement of mass transport. In addition, possibility of ultra-rapid microwave sintering of powder metals has been demonstrated within a model accounting for the effective electromagnetic properties and resonant absorption effects.

Introduction Microwave sintering of materials has been under active study for more than thirty years, with first experiments undertaken back in the 1960s [1, 2, 3, 4, 5]. Since the early studies, it has been claimed that microwave heating has a potential of enhancing the rate of the sintering process [6]. Among the proposed reasons for that were the volumetric nature of microwave absorption and the non-thermal effects associated with the interaction of the electromagnetic field with solids [7, 8, 9, 10, 11]. The power deposited volumetrically within the sample is spent on increasing its internal energy and on compensating the heat losses from the sample. The heating rate generally increases with the increase in the deposited power density. There have been many experimental studies in which relatively high heating rates, on the order of 100 °C/min, were achieved using microwave heating (e.g., [12, 13, 14]). However, in many ceramic materials, an increase in the volumetrically deposited power above a certain threshold usually causes development of thermal instability. In most cases, it is associated with the

ª Materials Research Society 2019

positive feedback between the power and the temperature occurring due to the rising dependence of the material’s effective electric conductivity (or microwave absorptivity) on temperature. Since the first studies of microwave processing of ceramics, the thermal instability has been kno

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