Microwave Sintering of Alumina at 915 MHz: Modeling, Process Control, and Microstructure Distribution

Abstract

Microwave energy can be advantageously used for materials processing as it provides high heating rates and homogeneous temperature field distribution. These features are partly due to the large microwave penetration depth into dielectric materials which is, at room temperature, a few centimeters in most dielectric materials. However, up to now, this technology is not widely spread for high-temperature material processing applications (>1200 °C), because its reproducibly and ability to sinter large size samples (>30 cm3) still needs to be improved. In this context, this paper describes both an empirically designed 915 MHz single-mode cavity made from SiC susceptors and refractory thermal insulation, and the 3D modeling of the process in order to improve our understanding of it. Different susceptors geometries and coupling slit position were numerically tested in order to better understand how these parameters impact the field homogeneity and the process stability. It was found that positioning the largest surface of the susceptors parallel to the electrical field allows a very uniform and hybrid heating of the material, while avoiding plasma or thermal instabilities. This was correlated to the 3D modeling results. Finally, thanks to a fully-automatized system this apparatus was used to sinter large size (~30 cm3) low-loss dielectric alumina samples. The sintered materials were subsequently characterized in terms of density, grain size distribution, and homogeneity. The reproducibility was also discussed, demonstrating the process efficiency and reliability.

New process to obtain unslaked lime through microwave hybrid heating and its fluid dynamics computational modeling

“…Copper Cuo was chosen Copper oxide was chosen as a susceptor because it results in temperature ranges suitable for the calcination process. Other susceptors such as Si could also be used (Marinel et al, 2019). The upper part of the refractory ceramic has 120 mm of outer diameter, 80 mm of inner diameter and is 40 mm deep where there is the copper oxide coat which is about 1.5 mm thick.…”

Experimental and numerical thermal analysis for direct microwave heating of silicon carbide

“…The sintering process by microwave heating has been studied over the last decades, both in the context of basic research 1 and industrial applications, for various materials such as metals, [2][3][4] ceramics, [5][6][7][8][9] and composites 10,11 . Due to the penetration of the electromagnetic wave into the material, this process offers volumetric heating, selective heating, low energy consumption, and short processing time.…”

“…15 For materials with low dielectric loss (such as alumina), a hybrid configuration using a susceptor, usually in silicon carbide (SiC), should be used to initiate microwave heating. [7][8][9] Despite all benefits of microwave heating, the process is difficult to control. Indeed, conventional temperature measuring devices, such as thermocouples, cannot be used as it interacts with microwaves.…”

“…(specific heat C p , thermal conductivity κ, and complex relative permittivity ε r ), depend strongly on both temperature and density. To tackle this complexity, the numerical simulations are often associated with experiments to evaluate temperature fields 2,[6][7][8][18][19][20][21][22][23] in cylindrical or hexagonal samples.…”

“…In fact, only materials with a high dielectric loss at room temperature (silicon carbide, zinc oxide, etc) can be efficiently heated in direct configuration, that is, without using a susceptor 15 . For materials with low dielectric loss (such as alumina), a hybrid configuration using a susceptor, usually in silicon carbide (SiC), should be used to initiate microwave heating 7‐9 …”