Investigation of the mechanism(s) controlling microwave sintering of α-alumina: Influence of the powder parameters on the grain growth, thermodynamics and densification kinetics

Abstract

The objective of this investigation is to deepen the understanding of the mechanism(s) involved in densification and grain growth underlying microwave sintering of ␣-alumina. The densification behavior and microstructure evolution of ␣-alumina powders with different MgO doping levels as well as specific surface areas have been systematically and quantitatively studied during conventional and 2.45 GHz microwave multimode sintering. It is shown that the microwave-induced favorable effects on densification could be more important due to the existence of MgO dopant or a decrease of particle size. Combined with the thermodynamics and kinetics considerations, one assumed that grain-boundary diffusion could be significantly enhanced by microwave non-thermal effect. In addition, the grain growth retardation effect has been attributed to the fine porosity retention induced by microwave electromagnetic field, but not to the local over-heating at grain boundaries.

Direct microwave sintering of pure alumina in a single mode cavity: Grain size and phase transformation effects

“…Therefore, although densification starts earlier under microwaves, microwave heating does not allow for processing dense alumina with finer grains than conventional heating. This has also been found by Zuo et al [23,35] when they compared conventional and hybrid multimode microwave sintering of A 19 powder. Densification and grain growth are two simultaneous processes.…”

“…To try understanding our results, we considered the work by Zuo et al [23] who measured densification kinetics of the same A 19 and A 6 powders in isothermal conditions during hybrid multimode microwave sintering. They used this data to identify the diffusion mechanism involved during densification under microwaves.…”

“…However, the dominant diffusion mechanism has always been attributed to grain boundaries for alumina and a change in the specific surface of the powder should not modify the dominant diffusion mechanism [29,30]. For this reason, the value of n ¼ 3 measured for A 19 has been attributed by Zuo et al to in-series "interface-reaction (n 2)/grain boundary diffusion (n ¼ 4)" mechanisms [23]. Interface reaction as a possible mechanism controlling creep and sintering kinetics has first been introduced by Ashby [31] for fine grain materials in which grain boundaries cannot be considered as perfect sources and sinks for diffusing species.…”

“…To investigate the effect of the heating mode on grain growth, the data provided by Zuo et al [23] have been used. Zuo et al conventionally sintered a set of A 19 samples with various heating rates (between 1.6 and 25 C/min) up to different densities in order to establish the sintering trajectory in a grain size vs relative density diagram (Fig.…”

Effect of LZSA Glass-Ceramic Addition on Pressureless Sintered Alumina. Part I: Grain Growth

“…In this sense, two-step sintering has been successfully used to obtain dense alumina at low temperatures while suppressing grain growth at the end of sintering 13,14 . The suppression of grain growth of alumina can also be achieved through liquid phase sintering (LPS), which is typically performed at high temperatures (>1,600 ºC) 15 .…”