Low temperature properties of transuranic and other heavy metals: a study of the electrical resistivity and thermoelectric power of thorium, uranium, neptunium and plutonium

The electrical resistivity and thermoelectric power of four members of the actinide aeries of elements have been measured below room temperature. They include the artificially made, transuranic elements neptunium and plutonium, which are extremely toxic and for which special apparatus had to be designed. Neptunium is a very rare metal and only a small quantity of rather low purity material was available, but the remaining elements were able to be obtained in a high state of purity.
The importance of these metals lies in their nuclear properties, and the employment of alloys of uranium and Plutonium as nuclear fuels is now well-established. Under reactor operating conditions, fuel elements are subjected to bombardment from a variety of radiations of differing intensity, to cyclic changes of temperature and resulting thermoelastic stresses. A knowledge of the properties of the materials used in the preparation of the fuel elements, especially at elevated temperatures, is naturally of vital importance, and this accounts for the present pace of research on the subject. While low temperatures themselves are of little interest for the reactor technologist, they provide the physicist with a powerful tool in his basic research. For, with decreasing temperature, the effects of thermal vibrations are lessened and the properties being studied often yield much further useful information. Low temperature research has its own experimental problems, of course, and the techniques employed are often highly specialised.
With regard to plutonium, no work below 77°K on any of its properties had been carried out in Britain before the present research was started (1957), although since then work has also commenced at A.W.R.E., Aldermaston and at Grenoble. But at Los Alamos some low temperature research was apparently already in progress.
The problems involved in the safe handling of plutonium (and neptunium) are well-known. These metals are extremely dangerous to work with. and great care must be taken to ensure that they do not contaminate the open laboratory. Indeed, the danger level le such that the total body-burden is limited to 0.6 microgram of plutonium, while the airborne limit is set at 2.10−12 microcuries per cc. (i.e., 10−11 microgram per cc). The difficulties associated with the handling of plutonium metal arise chiefly from its readiness to oxidise at the surface into a non-adherent powder of very small particle size. This leads to a rapid build-up of &alpha-active; dust in the surrounding atmosphere, and, if a particle should become deposited in the body, severe local biological damage would result. Plutonium is therefore normally handled only in glove-boxes, and our cryostat had consequently to be designed to work in conjunction with a glove-box. The problems raised by this are discussed in this thesis, together with the effects caused by the very high self-heating of plutonium. Another method of working with radioactive specimens, in which they are sealed in cylindrical containers, is also described.
The resistances of the actinide metals investigated were found to be high, especially those of neptunium and plutonium, while the thermoelectric powers displayed complicated temperature relationships.
The temperature dependence of the resistance of α-Pu was very abnormal, being characterised by a negative temperature coefficient above 105°K and a sharp fall in resistance below this temperature. Some Pu rich δ-Pu-Al alloys also showed the same phenomenon. An explanation in terms of spin-disorder effects seems quite reasonable, and it is suggested that both α- and δ-Pu may be antiferromagnetic.
The resistance-temperature curves of uranium and neptunium are also unusual in that the temperature coefficients decrease monotonically with rising temperature. Possible reasons for this have been discussed, including that by which Mott accounted for the high temperature resistance of some transition metals. Jones has recently discussed this again more generally, and Chandrasekhar and Hulm have extended it to account for the negative temperature coefficients of b.c.c. uranium alloys. The band structure of the actinide metals is satisfactorily described in terms of a two-band model, in which a broad, low density of states conduction band is overlapped by a narrow, high density of states band. It is assumed that In uranium, neptunium and plutonium the Fermi level lies within this narrow band, but that in thorium the Fermi level has a lower energy than the bottom of this band. The properties of thorium show it to be less abnormal than the succeeding actinides, resembling the transition me tale rather than the rare earths.
Uranium was found to display anomalies In both the electrical resistivity and the thermoelectric power at about 40°K. This is close to the temperature at which Berlincourt observed anomalous changes in the Hall coefficient, and may possibly be due to a phase change. Anomalies at 155 and 273°K in one of the neptunium specimens are also discussed at length, and shown to be due to a ferromagnetic impurity.
A table has been included as an appendix which simplifies the evaluation of characteristic temperatures when comparing experimental resistance data with the Gruneisen-Bloch formula.