Reactor irradiation PbTe, Bi 2Te 3 and ZnSb

Abstract

Comparison of pre- and post-irradiation thermoelectric properties of n- and p-type PbTe and Bi 2Te 3 has shown that the damage incurred in a nuclear reactor environment is due principally to fast neutrons. This result has been obtained by comparing changes in properties of pairs of specimens irradiated with and without cadmium shields at 60° ± 20°C to neutron doses of 1·6 × 10 −2 c m −2 fast (energies greater than 1 MeV) and 1·5 × 10 20 cm −2 thermal. Room temperature electrical resistivities and Seebeck coefficients were observed to increase in magnitude, except in the case of p-type Bi 2Te 3, where the Seebeck coefficient reversed in sign. The largest resistivity increase observed was a factor of about 25 in the case of p-type PbTe. In the case of the n-type PbTe, Hall coefficient measurements showed that the increase in Seebeck coefficient and the major part of the resistivity increase could be accounted for by a factor 4 decrease in carrier concentration, attributed to trapping by fast neutron induced defects. For all specimens for which the measurements could be made, the thermoelectric properties were essentially restored to their pre-irradiation values by annealing at temperatures ranging from 150 to 190°C. Other specimens of these materials are being irradiated to higher dose levels. Gamma ray spectroscopy was performed on a set of specimens removed from the Engineering Test Reactor after an integrated neutron dose of 6 × 10 19 cm −2 fast and 6 × 10 20 cm −2 thermal in order to search for long-lived neutron reaction products, particularly those which might result from high energy neutron cross sections whose values are unknown. The only gamma ray lines observed were from expected thermal neutron reaction products, principally iodine and tellurium isotopes. An in-pile instrumented irradiation experiment has been carried out with n-type PbTe and Bi 2Te 3 and p-type ZnSb. Electrical resistance, Seebeck e.m.f.'s and temperatures were monitored by means of thermocouples and probes welded to each sample. The changes in thermoelectric properties observed as the reactor first went to power could be accounted for, to within experimental errors, by the rise in specimen temperatures due to gamma ray heating. The accumulated neutron dose for this experiment reached an estimated 1·2 × 10 20 cm −2 fast and 6 × 10 20 cm −2 thermal before successive thermocouple failures precluded further monitoring of specimen temperatures. The Seebeck coefficients of Bi 2Te 3 and ZnSb were observed to increase approximately 15 per cent as a result of the irradiation. In addition to the smooth increases in electrical resistances as the experiment proceeded, discontinuous increases were observed following the numerous reactor “scrams” which occurred during the irradiation. Auxiliary experiments suggest that these step increases are due to effects of thermal stress in the specimens. Most of the smooth increase in electrical resistance occurred before an integrated fast neutron dose of about 5 × 10 19 cm −2. The largest increase in resistance observed was a factor of 25 in the case of the PbTe.