Michael Ramos

Alpha-plutonium's polycrystalline elastic moduli over its full temperature range.

alpha-plutoniums volume-corrected polycrystal elastic moduli were measured between 18 K and the upper limit of its occurrence, near 400 K. The two independent moduli for a polycrystal-bulk and shear-behave smoothly, indicating no phase transition. Both moduli show the same 50% increase on cooling, an order of magnitude larger than in other metals. The Debye temperature obtained from low-temperature elastic moduli, 207 K, significantly exceeds most previous estimates. The Gruneisen parameter gamma=5.3, obtained from the temperature dependence of the bulk modulus, is intermediate among previous estimates using other approaches, alpha-plutoniums Poisson ratio nu is low: 0.18, nearly temperature independent, and its small decrease on warming opposes usual behavior. The high gamma, large but equal bulk modulus and shear modulus fractional stiffening on cooling, and near-temperature-invariant nu are attributed to a single mechanism: 5-f electron localization-delocalization.

Polycrystalline gamma-plutonium’s elastic moduli versus temperature

Resonant ultrasound spectroscopy was used to measure the elastic properties of pure polycrystalline (239)Pu in the gamma-phase. Shear and longitudinal elastic moduli were measured simultaneously and the bulk modulus was computed from them. A smooth, linear, and large decrease in all elastic moduli with increasing temperature was observed. The Poisson ratio was calculated and an increase from 0.242 at 519 K to 0.252 at 571 K was found. These measurements on extremely well-characterized pure Pu are in agreement with other reported results where overlap occurs. We calculated an approximate Debye temperature Theta(D)=144 K. Determined from the temperature variation in the bulk modulus, gamma-Pu shows the same Gruneisen parameter as copper.

Temperature dependence of elastic moduli for polycrystalline Pu: Comparison of alpha and gamma phases.

The elastic properties of pure polycrystalline Pu were investigated as a function of temperature by resonant ultrasound spectroscopy. For both alpha and gamma phase, shear (G) and longitudinal (CL) elastic moduli were determined simultaneously and the bulk modulus (B) was computed from them. In alpha phase, G and B exhibit the same temperature dependence, but an unusually large change. This behavior together with a high Gruneiser parameter and an almost temperature independent Poisson ratio is attributed to the 5‐f electron localization delocalization. In the gamma phase, G and B show anomalous temperature behavior: G decreases with temperature while B is near temperature independent. The Poisson ratio increases with temperature by 4%. This normal behavior of the Poisson ratio is unusual for Pu.

Pu Has No Future: A Real‐Time Measurement of Pu Aging

Recent RUS measurements on polycrystalline samples of δ‐Pu have established that RUS can detect changes in elastic moduli of 0.3 parts per million, so that for the first time ever we accurately measured aging effects in real time for Ga‐stabilized fcc Pu. Our measurements were performed on alloys made from of nominally pure 239Pu as well as ones made with a 7.1% 238Pu fraction to accelerate aging.