Problems in the technological development of a sub-milliwatt RTG (radioisotopic thermoelectric generator)

The results of conceptual design are described regarding the radioisotopic thermoelectric power generators which is intended for cardiac pacemakers. The need for the development of thermoelectric elements with large aspect ratio (e.g. cross-section 0.08 mm phi and length 10 mm) and of high performance heat-insulating structure is indicated. From the calculation on RTG, except the examination on radiation source, the following matters are revealed. (1) The RTG efficiency of 1% is attainable, if the heat transfer rate of heat-insulating structure of 1 m W/0K and the performance index of thermoelectric elements of 2.0--2.5x10-3 (0K)-1 are obtained. (2) There is an optimum value of A/Vo (A aspect ratio, i.e. element length/element cross-section; Vo terminal voltage) for maximizing the efficiency of RTG; the value is constant, independent of the heat transfer rate of heat-insulating structure. (3) The development of thermoelectric elements should be directed so as to have large value of A, i.e. small diameter of elements. (Mori, K.)