Figure 4.7 Arrhenius plot for the shrinkage parameter of an iron compact (reproduced from [17] by
permission of Elsevier)
microwave heating. The Arrhenius plot for the iron compact from the study is reproduced in
Figure 4.7.
It was proposed by Saitou that the free energy of the high temperature phase is
lowered by microwave radiation, leading to a reduction of the transformation temperature
and the formation of a more stable grain boundary structure after the transformation
under microwave radiation. Additional work is continuing in this area to validate the
hypothesis.
It was also reported that microwave radiation does not change the activation energy and
therefore does not change the sintering mechanisms. The activation energies, Qa derived
from the experiments and the sintering mechanisms, are reproduced in Table 4.3.
4.2.4 Proposed Microwave Sintering Model by Luo et al.
While many attempts have been made to document the response of metal particles to
microwaves under different conditions, few attempts have been made so far to explain the
mechanism involved in the interaction of microwaves with metal particles.
Recently, two separate theoretical models have been proposed to explain the absorption
of electromagnetic waves by powdered metals [6, 18–20]. First, an attempt was made by
Luo et al. to develop a model based on electromagnetic induction heating to explain the
interaction between microwaves and powdered metals in a multimode cavity [6, 18]. Based
on classical electromagnetic theory, the magnetic induction in a metal at microwave
frequencies is larger than the electric field and therefore the heating of powdered metals can
be attributed to the result of eddy currents or magnetic induction heating [6, 18]. A model
assuming the shape of the oven as a perfect metal circular cylinder and the powdered metal