March 10-15, 2019
Over the recent years, we have demonstrated ultra-rapid microwave sintering of different ceramic materials, including Al2O3, ZrO2, Y2O3, MgAl2O4, and Yb:(LaO)2O3, using a gyrotron system operating at a frequency of 24 GHz with an output microwave power of up to 6 kW [1-3]. The ceramic pellets were sintered to 98-99.5 % of the theoretical density with the duration of the high-temperature sintering stage not exceeding several minutes. We have shown that the absorbed microwave power required for the ultra-rapid sintering regimes is on the order of 10...100 W/cm3, i.e., the same order of magnitude as the power of Joule heating required for the dc current-assisted flash sintering . The ultra-rapid sintering is associated with the thermal instability that is characteristic of the volumetric heating processes regardless of the specific nature of the volumetric heat source. We have proposed a mechanism of ultra-rapid sintering which involves softening or melting of grain boundaries and propagation of a densification front from the core to the periphery of the sample [2,3].
In this paper we report the recent results that support these findings. A faster-than-Arrhenius growth of conductivity was observed in the in situ measurements of the dc conductivity in the microwave heated Yb:(LaY)2O3 samples. A low dc voltage was applied to the platinum electrodes imposed on the samples; the Joule heating of the samples by the dc current was negligible. A sharp increase in the current and a deviation from the Arrhenius dependence of the conductivity took place at a threshold temperature of about 1150 °C. Since the Arrhenius dependence of conductivity on temperature is characteristic of the dielectrics in the solid state, the deviation from this dependence observed at high levels of the absorbed microwave power suggests that the phase state of the material is changed, and a highly electrically conductive, (quasi-) liquid phase is formed in the sample.
In the experiments on the optical dilatometry of the Al2O3 samples using a recently developed system based on an infrared camera we have demonstrated that the shape of the dilatation curves obtained in the ultra-rapid microwave sintering regimes is different. Starting at the threshold temperature corresponding to the sharp increase in the effective conductivity, the dilatation rate exhibits a pronounced surge, in contrast to the steady decrease observed in the processes carried out at lower levels of microwave power.
We have also demonstrated by simulation that microwave ultra-rapid sintering can be implemented not only with ceramic materials, but also with powder metals. Although the conductivity of metals does not increase with temperature and therefore the thermal instability is not possible, the effective dielectric and magnetic properties of metal powders with poor electrical connectivity between the particles can be responsible for resonance phenomena during microwave heating . The resonances give rise to enhanced microwave absorption in the powder compact and result in rapid densification. Fast automatic regulation of intensity and/or frequency of microwave radiation is necessary for the successful implementation of rapid sintering of metal powders and metal-ceramic composite materials.
This work was supported by Russian Science Foundation, grant # 17-19-01530.
 Yu.V. Bykov et al, J. Am. Ceram. Soc. 98 (2015) 3518-3524
 Yu.V. Bykov et al, Materials. 9 (2016) 684
 Yu.V. Bykov et al, Technical Physics. 63 (2018) 391-397
 R. Raj, J. Am. Ceram. Soc. 99 (2016) 3226-3232
 K.I. Rybakov, M.N. Buyanova, Scripta Mater. 149 (2018) 108-11
Kirill I. Rybakov, Yury Bykov, Anatoly Eremeev, Sergei Egorov, Vladislav Kholoptsev, Ivan Plotnikov, and Andrei Sorokin, "Ultra-rapid microwave sintering of ceramics and powder metals" in "Electric Field Enhanced Processing of Advanced Materials II: Complexities and Opportunities", Rishi Raj, University of Colorado, USA Olivier Guillon, Forschungzentrum Jülich, Germany Hidehiro Yoshida, National Institute for Materials Science, Japan Eds, ECI Symposium Series, (2019). https://dc.engconfintl.org/efe_advancedmaterials_ii/59