Liquid Semiconductor Could Yield Better Battery

A miniature, super-efficient nuclear power plant that’s simple to build and doesn’t get much hotter than a kitchen oven could be the result of an innovation being developed by scientists at Global Technologies (GTI) in Idaho Falls, Idaho. GTI’s president, Francis Tsang, and colleagues are working on a nuclear voltaic cell consisting of a semiconductor and an amount of radioactive material. The semiconductor sits between two conductors to form a Shottky diode, and is bombarded by particles from uranium, plutonium, or less dangerous radioactive material.

Radiation has the same effect on a semiconductor as light has on a typical solar cell. In a solar cell, the impact of a photon with the semiconductor crystal creates an electron and a positively charged particle called a hole. Because the cell’s semiconductor has been doped with chemical impurities, it has a natural polarization that draws the electron to one electrode and the hole toward the other, thereby producing current.

Early atomic battery concepts from the 1950s sought to harvest semiconductor energy from alpha radiation (positively charged helium nuclei) or beta radiation (electrons). Although they offered the hope of efficient batteries that would last for decades, they were limited by problems associated with their radiation sources.

Tsang, a former U.S. Energy Department researcher, was well aware of the beta cell’s problems. “Shoot a bullet into a block of ice, and the ice will shatter and can’t go back into its original form,” says Tsang. “But if you shoot a bullet into water, the water repairs itself.” So he began experiments replacing solid semiconductors with molten selenium and molten sulfur, both of which become semiconductors in their liquid state and melt at less than 300C. Because liquids don’t suffer any structural damage, Tsang’s nuclear battery could run on much more powerful radiation than a beta cell and generate more electricity.

A liquid nuclear diode could catch energetic alpha and beta particles, gamma rays, and even the new atoms left over from the fission of larger atoms. Fissle fragments could be a particularly good source of energy. In the fission of U-235, for example, the fragments are heavy, as they plow through the semiconductor they “make a shower of electron-hole pairs along the path,” Tsang says.

The U.S. Defense Advanced Research Projects Agency (DARPA) has provided $1.4 million to GTI to prove the concept by producing current in a test cell, with a provision that could allow for additional funding up to $26.6 million for more than four and a half years.