I came across research papers regarding microwave sintering processes on ceramics, typically hydroxyapatite, and it contributes to smaller grain size and higher fracture toughness. Referring to a conventional sintering method, it has basic sintering time, soaking time and heating rate.
On the other hand, microwave sintering uses wave, generally relates wave power and frequencies (up to my best knowledge - maybe it has more things that I am not aware of ). Some paper hinted a little bit on the temperature. But a microwave doesn't provide temperature during operation unless it is under convection mode. Is there any method that can measure the temperature, or is there any relationship between wave power and frequencies towards temperature?
MWS is a volumetic heating, so the heating rate could be very fast due to the unimportant of thermal inertia( thermal gradient) of outer layer to the core of the sample. Also, the small grain size is obtained due to the densification kinetics of MWS is greater than grain coarsening (by assisting of fast heating rate) and vice versa for conventional sintering.
MWS does not give any temperature by neither conduction nor radiation but the temperature is generated in the bulk sample body by the interaction between Electromagnetic and dielectric materials (more specifically by polarization effect).
The temperature of sample is expressed by ΔT/Δt=(ωε_0 ε_eff^'' E_rms^2)/(ρC_p ) Where ρ is bulk density of the material (kg/m3) and C_p is the heat capacity of the material at constant pressure (J/kg °C). And, where ω is angular frequency 2𝜋𝑓 (f is frequency of Microwave radiation), ε_0 is permittivity of free space (8.85x10-12 ,V/m3), ε_eff^'' is relative effective dielectric loss, E_rms^2 is internal electric field intensity (V/m), tanδ is loss tangent and ε_r^' is relative dielectric constant derived form ε^'/ε_0.
I am not convinced that MWS could be explained by the convection mechanism. AN the raising up of the temperature is expressed by the above equation and It corresponds to MW frequency.
Hope this answer could help, if you have any concern/questions please let me know.
Wirat
Great explanation Dear Wirat. For me it will take a little time for me to understand the equations. But it seems I know what you are talking about. I will find more reading regarding this. ^.^
Meanwhile, I am still finding ways for me to achieve MWS for using normal microwaves. Because certain papers will indicate the sintering time, as well as temperature. The powder is usually expressed in Watt. and the sintering time is expressed in normally minute to hours
Ng Kei Hoa,
The typical frequencies used for material processing are 915 MHz and
2.45 GHz which derived from 85% and 50% of power efficiency. And 2.45 GHz is resemble to the household MW.
Dear Ng, there are a number of papers on microwave treatment of ceramic materials. As a good starting point I recommend the following paper: Rao, K. J., B, V., Ganguli, M. & Ramakrishnan, P. A., 1999. Synthesis of Inorganic Solids Using
Microwaves. Chemistry of Materials , 3 March, 11(4), pp. 882-895. Kind Regards, Paul.
please use this website so you can find the most recent published papers on it with high quality.
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