On the Mechanism of Microwave Flash Sintering of Ceramics.

Yury Bykov,Sergei Egorov,Ivan Plotnikov,Kirill Rybakov,Anatoly Eremeev,Andrei Sorokin,Vladislav Kholoptsev

doi:10.3390/ma9080684

Abstract

The results of a study of ultra-rapid (flash) sintering of oxide ceramic materials under microwave heating with high absorbed power per unit volume of material (10–500 W/cm3) are presented. Ceramic samples of various compositions—Al2O3; Y2O3; MgAl2O4; and Yb(LaO)2O3—were sintered using a 24 GHz gyrotron system to a density above 0.98–0.99 of the theoretical value in 0.5–5 min without isothermal hold. An analysis of the experimental data (microwave power; heating and cooling rates) along with microstructure characterization provided an insight into the mechanism of flash sintering. Flash sintering occurs when the processing conditions—including the temperature of the sample; the properties of thermal insulation; and the intensity of microwave radiation—facilitate the development of thermal runaway due to an Arrhenius-type dependency of the material’s effective conductivity on temperature. The proper control over the thermal runaway effect is provided by fast regulation of the microwave power. The elevated concentration of defects and impurities in the boundary regions of the grains leads to localized preferential absorption of microwave radiation and results in grain boundary softening/pre-melting. The rapid densification of the granular medium with a reduced viscosity of the grain boundary phase occurs via rotation and sliding of the grains which accommodate their shape due to fast diffusion mass transport through the (quasi-)liquid phase. The same mechanism based on a thermal runaway under volumetric heating can be relevant for the effect of flash sintering of various oxide ceramics under a dc/ac voltage applied to the sample.

Highlights

In recent years considerable interest has been drawn to the processes of materials sintering making use of electric currents and/or fields
We report the results of a study of an ultra-rapid (“flash”) microwave sintering process with oxide ceramic materials Al2 O3, Y2 O3, MgAl2 O4, and Yb:(LaY)2 O3 to densities 98%–99% of the theoretical value within minutes or even fractions of a minute, without the high-temperature hold stage
On the grounds of the analysis of experimental data and microstructure characterization, we propose a mechanism of flash microwave sintering based on particle surface softening/melting

Summary

Introduction

In recent years considerable interest has been drawn to the processes of materials sintering making use of electric currents and/or fields. Enhanced sintering of various ceramic, composite and metal powder materials has been observed when using such methods as Field Assisted Sintering Techniques (FAST), Pulsed Electric Current Sintering (PECS), Spark Plasma Sintering (SPS), and Microwave. Several reviews of these methods and their applications to the sintering of a wide range of different ceramics have been published recently (see, e.g., [1,2,3]). These new techniques have attracted great attention due to their common advantage, viz. An even faster sintering method has been developed, the so called flash sintering [4,5,6,7,8]

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Results

Conclusion