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 functio n 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 t hat the transformation is controlled by nucleation and growth, the ons et temperature and the temperature range of the reaction are shown to depend markedly upon heating rate. By investigating heating rates up t o a factor of 100 lower than usual we show: (1) Earlier calorimetric m easurements report onset temperatures for the transformation which are too high. (2) The Kissinger analysis, which predicts the scanning rat e parameter to vary linearly with reciprocal temperature, does not fit the results of this study. (3) With increased scanning rate, the tran sformation occurs at higher temperatures, and the growth rate of the b eta phase at a constant fraction transformed is proportional to this t emperature increase.