DIFFERENTIAL SCANNING CALORIMETRY STUDY OF THE EFFECTS OF HEATING RATE ON THE ALPHA-]BETA TRANSFORMATION IN UNALLOYED PLUTONIUM

The alpha --> beta transformation in plutonium is studied as a function of heating rate by differential scanning calorimetry over the range 0.01-degrees-C min-1 to 10-degrees-C min-1. Consistent with the fact that the transformation is controlled by nucleation and growth, the onset temperature and the temperature range of the reaction are shown to depend markedly upon heating rate. By investigating heating rates up to a factor of 100 lower than usual we show: (1) Earlier calorimetric measurements report onset temperatures for the transformation which are too high. (2) The Kissinger analysis, which predicts the scanning rate parameter to vary linearly with reciprocal temperature, does not fit the results of this study. (3) With increased scanning rate, the transformation occurs at higher temperatures, and the growth rate of the beta phase at a constant fraction transformed is proportional to this temperature increase.