Experimental proof of major role of magnetic field losses in microwave heating of metal and metallic composites

In 1999 we found that powdered metal samples including very complex shaped and large size (100 mm diameter, 1 kilograms) could be fully sintered in 30 min in a 2.45 GHz multi-mode microwave cavity [1]. Moreover, these samples had properties at least as good as, and usually better than, those sintered in conventional furnaces. This finding was outside the experience of a very large number of scientists whose extensive work has been covered in many reviews [2–4]. This achievement was as puzzling to us as to colleagues and efforts to explain this by skin depth absorption etc. did not work. The well known extensive theoretical treatment of microwave-material interaction by many workers (see e.g. Varadan and Varadan [5], Booske et al. [6] and others) have in common that they always treat the energy absorption mechanism as due to the dielectric loss factor. In 1994 Cherradi et al. [7] reported their preliminary work in which they showed that the magnetic field must make substantial contributions to the heating of alumina (at high temperature) and semiconductors, and metallic copper. But in their work, the experimental design of using samples of 120 mm length, where in some cases, the sample was exposed to both magnetic and electric field simultaneously, caused a complicated interplay of the different absorption. In present work, a finely tuned microwave cavity with a cross section dimension of 86 mm by 43 mm which works in TE103 single mode was used to investigate the microwave heating behaviors of various materials in different microwave fields. Fig. 1 shows the scheme of the microwave system, and the distribution of the microwave field within the cavity is sketched in Fig. 2. In the L/2 location along the length of the cavity, the maximum electric (E) field is in the center of the cross section, where the magnetic (H ) field is minimum; and the maximum magnetic field is near the wall, where the electric field is minimum. A quartz tube was introduced in this location to hold the sample and also to enable us to control the atmosphere. A 2.45 GHz, 1.2 kW microwave generator (Toshiba, Japan) with power monitor was used as microwave source. A small cylindrical sample (5 mm diameter and 3 mm thick) was placed inside at two different locations, the maximum electric field area where the magnetic field is minimum, and the maximum magnetic field area where the electric field is minimum, respectively. Sample temperatures were measured using an infrared pyrometer (Mikron Instrument Co., Model M90-BT, temperature range −50 ◦C–1000 ◦C). During the experiments, atmospheric pressure nitrogen gas was passed through the quartz tube to avoid oxidation of metal samples at high temperature. Initially, we tried to use a fixed microwave power for all samples during heating, but for some samples, the temperature increase was too fast and the highest temperature exceeded the measuring range of the pyrometer, and in some cases, discharging and arcing occurred. So we set different microwave powers for different samples to get more stable heating results. Fig. 3a shows the heating observed for a typical commercial powdered metal sample (Keystone Powdered-metal Company, Saint Marys, PA, USA. The