Technical article: Induction heating reduces power cost

Advanced Battery Technology, Nov 2000 by Fireman, Jerry

The key to addressing these new applications is reducing the cost of AMTEC generators, particularly the BASE tubes. Their cost, about 250 of which are used in a single generator, is kept high by the labor-intensive nature of the process currently used to produce them. The tubes are made by sintering tubes formed from a powdered metal oxide mixture at temperatures of about 1600 degC. The primary problem is that two of the components in the tubes, sodium oxide and lithium oxide, are volatile and subject to evaporation at the high temperatures used in sintering. Normally, the tubes are sintered in a conventional industrial furnace, which means they must spend about 20 minutes at temperature, with substantial additional time for heating to the sintering temperature and cooling down. In order to avoid evaporation, the tubes must be encased in platinum or magnesium oxide. These capsules can be expensive, but the primary problem is the large amount of labor involved in applying and removing them. This constitutes the vast majority of the total cost of producing the tubes. Another problem with the conventional method is that large, expensive facilities are needed.

Shorter Heating Cycle Solves Problem

A team consisting of Dr. Johnson, post-doctoral student Forest Kaatz, and graduate student Matthew Henrichsen set out to develop a less expensive approach to creating the critical tubes. Theirs was based on an understanding that a shorter heating cycle, in the range of seconds rather than minutes, would eliminate the need for encapsulation by allowing only a minimal amount of the volatile compounds to escape. The shorter heating cycle creates no problems from a materials processing standpoint; the temperature only needs to be maintained for about 10 seconds to fully sinter the tubes. The key reason why the conventional heating cycle is so much longer is the limitations of conventional industrial heating equipment. Ultra-rapid heating is not possible, and cooling rates are slow as well. Also, extra time is required to insert and remove a batch of tubes from the furnace. Johnson and his team felt they could dramatically shorten the heating cycle by converting the process to induction coupled plasma heating. The plasma is ignited and sustained by the powerful electromagnetic field generated in the induction coil. Heat is applied directly to the tubes as they move through the plasma, which is essentially a ball of fire. They cool rapidly as they move out of the plasma.

The Northwestern researchers selected a 2.SkW vacuum tube unit with a frequency range of 3 to 8MHz from Lepel. They selected this radio frequency unit, instead of one that operates in the microwave frequency band, because it works well at atmospheric pressure, eliminating the need to contain the process within a partially evacuated vessel. The researchers developed an automated process in which unfired tubes are loaded onto a transfer system, pass through the plasma created by the induction heater, and come out finished at the other end. The plasma and cooling zone are contained within a water-cooled quartz glass tube. Manual labor is minimal - a single worker could operate several systems.