Thermal Stability of Ceramic Materials in Microwave Heating

Abstract

Thermal stability was examined theoretically for various commercial grades of alumina heated by microwave energy. The critical temperature for thermal runaway was evaluated as a function of Biot number and ambient temperature, employing various functional forms of the dependence of dielectric loss with temperature. Field‐temperature relationships were derived for the limit of small temperature gradients in the specimen. Finite difference methods were used under conditions where significant temperature gradients were sustained in the specimen. The critical temperature approached limiting asymptotes for the extremes in Biot number, with smooth variation in the intermediate regime. Stability diagrams were constructed which denote regions of stability and their dependence on important system parameters. The effects of ambient temperature on critical temperature were determined analytically and compared with experimental observations on heating alumina fiber bundles by a hybrid configuration utilizing an external suseptor. The results have implications for temperature control near the critical temperature and the design of hybrid heating systems.

Microwave versus conventional sintering: A review of fundamentals, advantages and applications

“…Firstly, most of the research on material processing by microwaves is based on conventional low-frequency (2.45 GHz) microwave applicators; however, such applicators do not couple microwave power efficiently to many materials at room temperature and poor microwave absorption characteristics make initial heating difficult [21]. Secondly, thermal instabilities may occur, which can lead to the phenomenon of temperature runaway [21]; i.e., the specimen overheats catastrophically. This occurs for a wide vari- Fig.…”

“…During sintering materials by direct microwave heating, various fundamental problems are usually encountered [21]. Firstly, most of the research on material processing by microwaves is based on conventional low-frequency (2.45 GHz) microwave applicators; however, such applicators do not couple microwave power efficiently to many materials at room temperature and poor microwave absorption characteristics make initial heating difficult [21].…”

“…ety of ceramic materials including Al 2 O 3 , SiO 2 , Fe 3 O 4 , ␤-alumina, ZrO 2 , etc. [10,21]. The inherent temperature gradients present during volumetric heating can lead to severe temperature nonuniformities, which at high heat-up rates may cause non-uniform properties and cracking [19,22].…”

A review on microwave synthesis of electrode materials for lithium-ion batteries

“…Many researchers have also demonstrated carbon as preheating source [20]. Some authors like Spotz et al have also reported LaCrO 3 and mixture of Al 2 O 3 +LaCrO 3 as suitable susceptor materials for certain applications [21]. P.D.…”

Microwave Fast Sintering of Ceramic Materials

“…), and was a serious problem in the processing of materials whose dielectric properties are abruptly altered with rising temperature. Studies (Spotz et al, 1995) demonstrated that the insulation normally used to contain heat inside the cavity can exacerbate the problem of thermal runaway. However, the data also suggested that thermal runaway may be minimized or controlled by hybrid heating, thus allowing low-loss ceramic materials to be processed using rapid firing.…”