Cornell Researchers Develop Betavoltaic Battery with 50 Year Life and 500 Times More Power Density than Planar Designs

Cornell researchers have developed a betavoltaic cell generating 500 times more power density than current planar designs. The betavoltaic cell is low cost, long lasting, high current, energy efficient cell.  It will have a stable energy output for at least 50 years (Ni-63 half life = 100 years).  It is a safe power source that is insensitive to harsh environmental and production conditions while maintaining high thermal conductivity, good electronic mobility and low leakage currents

The increased power density is achieved through a novel device structure and the use of SiC as the substrate.  The novel device structure incorporates high aspect ratio micromachined pillars to provide a very large surface area in a relatively small volume of SiC, and provide ample space for the radioactive beta-emitting fuel material.  Silicon carbide provides an energy conversion efficiency an order of magnitude higher than silicon. 

Applications include: power supply for low accessibility sensor nodes,  on-chip power source for MEMS and standby power for cell-phones.  Nickel 63 as a β source provides a mean E of 17 keV and 100 year half life.  4H SiC is the ideal cell material with a bandgap of 3.3 eV, extreme radiation durability and good temperature stability.The expected output current density is 13 nA/cm². For a Tritium source, one would expect a current density output of  3.7μA/cm². Due to the half life of the nickel isotope, the cell is expected to last at least 50 years before the output starts to degrade substantially

 Standard fabrication techniques are used to manufacture the novel device structure with smooth pillar side walls. P or N type dopants can be added to the pillars to produce shallow junctions with increased cell efficiency.  A novel doping process utilizing borosilicate glass has also been developed that helps the device achieve its impressive power density.  The technology will find wide application in a variety of areas requiring on-chip or low-accessibility power supplies. 

Betavoltaics is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Betavoltaics are generators of electrical current, in the form of a battery, which use energy from a radioactive source emitting beta particles (electrons). The functioning of a betavoltaic device is somewhat similar to a solar panel, which converts photons (light) into electric current. This type of radioactive battery (or nuclear battery) operates on the continuous radioactive decay of certain elements. These theoretical batteries last a long time. Whether betavoltaics will replace current battery technologies remains to be seen.

Cornell Research Foundation, Inc. (Ithaca, NY) garnered U.S. Patent 7,663,288 for its improved betavoltaic cell.  Inventors by M V S Chandrashekhar Christopher, Ian Thomas  and Michael G Spencer have manufactured high aspect ratio micromachined structures in semiconductors to improve power density in Betavoltaic cells by providing large surface areas in a small volume. A radioactive beta-emitting material may be placed within gaps between the structures to provide fuel for a cell. The pillars may be formed of SiC.

SiC pillars are formed of n-type SiC. P type dopant, such as boron is obtained by annealing a borosilicate glass boron source formed on the SiC. The glass is then removed. In further embodiments, a dopant may be implanted, coated by glass, and then annealed. The doping results in shallow planar junctions in SiC.   The improved betavoltaic  battery was developed

Silicon carbide (SiC) is a wide bandgap semiconductor that has been used for high power applications in harsh conditions due to its temperature stability, high thermal conductivity, radiation hardness and good electronic mobility. The wide bandgap of the 4H hexagonal polytype (3.3 eV) provides very low leakage currents. This is advantageous for extremely low power applications. The availability of good quality substrates, along with recent advances in bulk and epitaxial growth technology, allow full exploitation of the properties of SiC.

Radioactive isotopes emitting .beta.-radiation such as Ni-63 and tritium (H-3) have been used as fuel for low power batteries. The long half-lives of these isotopes, their insensitivity to climate, and relatively benign nature make them very attractive candidates for nano-power sources.

The radiation hardness of SiC ⁴ ensures the long-term stability of a radiation cell fabricated from it. A 4H SiC p-n diode may be used as a betavoltaic radiation cell. Due to its wide bandgap, the expected open circuit voltage and thus realizable efficiency are higher than in alternative materials such as silicon.

The operation of a radiation cell is very similar to that of a solar cell. Electron-hole (e-h) pairs are generated by high-energy .beta.-particles instead of photons. These generated carriers are then collected in and around the depletion region of a diode and give rise to usable power. The dynamics of high-energy electron stopping in semiconductors are well known, with about 1/3 of the total energy of the radiation generating usable power through the creation of electron hole pairs. The remaining energy is lost through phonon interactions and X-rays. A mean "e-h pair creation energy or effective ionization parameter" in a semiconductor, takes into account all possible loss mechanisms in the bulk for an incident high-energy electron. This e-h pair creation energy is treated as independent of the incident electron energy. The effective ionization energy was calculated to be 8.4 eV for 4H SiC ⁵.

The technology is available for licensing from Cornell.  Interested parties should contact: Scott Macfarlane, Senior Technology Commercialization and Liaison Officer at ssm8@cornell.edu or
607-254-2330.