Figure 4.20 Effect of particle size on the predicted heating behavior of a tungsten compact
(reproduced from [21] with kind permission of Springer Science and Business Media)
the formulation and calculations involved. However, the exact mechanism of microwave
interaction with metals still cannot be explained effectively by existing theories because of
the complex interactions involved between different metals and microwaves. Hence, much
of the work being carried out on microwave heating of metals so far is still restricted to
experimental studies.
Perhaps there is no single theory which can be formulated to explain the complex
interactions between metal powders and microwaves, since different materials exhibit
radically different heating behavior when placed in the electric and magnetic microwave
fields. As discussed earlier, researchers have been looking back at the original equations
written by Maxwell and reanalyzing the formulation in terms of the electromagnetic
potentials proposed by Maxwell instead of the traditional approach of analyzing electromagnetic
waves in terms of the electric and magnetic fields.
4.3 Microwave Sintering of Metals
The application of microwaves for the sintering of metals is a relatively new idea and limited
work has been carried out. Table 4.4 highlights some of the publications related to the
microwave sintering of metals (excluding hardmetals) categorized according to the type of
metals that have been investigated. Publications on the applications of microwaves for the
melting, brazing or joining of metals are not included in Table 4.4 but information on these
applications will be discussed in Section 4.4 of this chapter. The list of publications is not
meant to be exhaustive and is compiled based on information freely available in journals,
conference proceedings, patents and over the Internet.
The powder metallurgy technique is used in the preparation of samples for microwave
sintering because metal couples with microwaves when present in the powder form.
Figure 4.21 shows the process flow for the preparation of samples using a typical powder
metallurgy route. First, the starting metal powders are weighed according to the required ratio/
composition and then blended or ball-milled to obtain a homogenous mixture. The blended/
ball-milled powders are subsequently pressed to obtain a green compact. Next, the green
compact is sintered. Pure microwave sintering or hybrid sintering using susceptors is
incorporated at this stage to replace conventional sintering. Finally, secondary processing
such as extrusion or machining may be carried out to prepare the samples for various
characterization tests.