A study of palladium thin-films for radioisotope storage in betavoltaic power sources designs

Thomas E Adams, Purdue University

Abstract

A betavoltaic cell is a type of radioisotope power source where the energy of beta radiation is converted into electricity. The goal of the research is to design a novel thin-film material to store radioisotopes that exhibits consistent and optimal radiation emission efficiencies. To achieve the main goal, three tasks were performed: review the radioisotope power technology, evaluate the methods of absorbing tritium, and determine the emission flux leaving the surface of the thin-film material. A design was proposed using a semiconductor p-n junction made from silicon with a thin film of palladium deposited on the silicon to hold tritium. Palladium was chosen as the source material for its affinity to store hydrogen at room temperature. The evaluation of the radioisotope source involved experiments and modeling for various tritium loading techniques and for the beta emission efficiency versus palladium thickness. Instead of using tritium in the experiments, hydrogen was used since hydrogen and tritium behave the same chemically. Three hydrogen loading techniques were evaluated: diffusion, ion implantation and electro-absorption. Diffusion and electro-absorption were evaluated with experiments while ion implantation was evaluated with the MC-SET simulation tool. For the experiments, test samples were made by depositing palladium in thicknesses of 50 nm, 150 nm, and 250 nm, on silicon wafers. A 5 nm layer of titanium was needed between the Pd and Si layer for adhesion. The best loading method appears to be electroabsorption; it is least expensive and provides consistent concentrations. However, the ion implantation method appears to be capable of implanting the highest concentration by applying the ion dose for longer periods of time. Beta emission efficiency was determined using MC-SET simulation software. The results indicated that the beta emission remained constant at film thicknesses greater than 500 nm, but is more efficient at lower thicknesses. The palladium-tritium system appears to be a suitable source for betavoltaic power sources. Alternative tritium loading processes can be used instead of traditional diffusion methods to make tritiated films with consistent concentrations. The thickness of the palladium film should not be greater than 500 nm to provide more efficient use of the tritium’s beta energy.

Degree

M.S.

Advisors

Fentiman, Purdue University.

Subject Area

Nuclear engineering

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