Energy conversion device at ultrahigh efficiency using gradient structure element

In a device for converting nuclear energy to electric energy, a heat/electron conversion device and a heat/electric conversion device are connected in cascade, and the heat/electric conversion device is divided into a plurality of portions from high temperature portions to low temperature portions to provide a gradient structure using materials each having a high power generation efficiency. Then, a conversion efficiency exceeding 40% which has not been attained even by a dynamic power generation system can be attained, and heat stresses caused by temperature difference from 2000K at a heat collecting portion to 300K at heat a releasing portion can be moderated by using the material having the gradient structure. There can be obtained a highly efficient power source that can be used for a wide range including an isolated space such as a cosmic space and ocean, as well as for power source of robots, and for isolated places and islands. (N.H.)