Thermal Analysis of Pu 6 Fe Synthesized from Hydride Precursor
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Thermal Analysis of Pu6Fe Synthesized from Hydride Precursor Daniel S. Schwartz, Paul H. Tobash, and Scott Richmond Materials Science Division, MS E574, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA ABSTRACT The intermetallic Pu6Fe is a precipitate commonly found in standard-purity Pu alloys. We are interested in measuring thermophysical properties of this intermetallic, so we are developing methods for producing Pu6Fe with potential for being scaled up. We had success using a powderbased method, where finely divided PuH2 powder was mechanically mixed with Fe powder and reacted in vacuum. Differential scanning calorimetry was used to analyze the phase content of the product. We estimate ~90% yield of Pu6Fe by weight, with the remainder being Pu. Enthalpy of melting for Pu6Fe was measured to be 31.2 J/g, and the onset temperature was 411.5°C. The product was melted 3 times and appeared to become more homogeneous after each cycle. INTRODUCTION The intermetallic Pu6Fe is a common precipitate found in many different Pu alloys. It can form readily due to the low solubility limit of Fe in Pu, roughly 200 wppm at room temperature. The crystal structure was determined by x-ray diffraction to be the body centered tetragonal D2c structure [1, 2] with an x-ray density of ~17.1 g/cm3. Mardon [3] developed the Pu-Fe phase diagram using alloys produced by vacuum arc melting. Figure 1 shows the Pu-rich side (Fe concentration below 15 at. %) of their original Pu-Fe phase diagram. Pu6Fe is a line compound separating the Pu + Pu6Fe and Pu6Fe + PuFe2 phase fields. There are several unusual features in Mardon’s phase diagram, indicated with arrows in figure 1. Below 14.3 at. % Fe, a mixture of Pu + Pu6Fe exists, and the Pu6Fe component melts at ~413°C. However, below ~3 at. % Fe, this solid-liquid mixture resolidifies into δ-Pu or δ-Pu + εPu as the mixture is heated above 430°C (marked by the left-hand arrow in figure 1). This is a catatectic reaction, where a solid-liquid mixture solidifies upon heating. The right-hand arrow in figure 1 points out an unclear phase reaction. Following the pathway marked as (1) in the figure upward in temperature, we pass from a Pu6Fe-rich δ-Pu + Pu6Fe mixture into a liquid + Pu6Fe mixture. The phase diagram indicates that δ-Pu in this mixture melts at 413°C, which is unlikely. Possibly the arrowed phase region should be a continuation of the Pu-rich δ-Pu + liquid field, and the eutectic does not actually extend below ~430°C. As we continue upward in temperature, the Pu6Fe melts incongruently at 428°C, and some PuFe2 forms to give a mixture of liquid + PuFe2. If we follow the pathway marked as (2) in figure 1, the Pu6Fe melts at 428°C as we pass from the Pu6Fe + PuFe2 field into the PuFe2 + liquid field. We are faced with an illogical situation where Pu6Fe melts at 413°C when in a mixture of δ-Pu + Pu6Fe, but at 428°C when in a mixture of PuFe2+ Pu6Fe. The details of phase
reactions, particularly in the area of the right-hand arrow in figure 1, need to be confirmed if we are to understand
