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SOME PHYSICAL PROPERTIES OF METALLIC PLUTONIUM
Recently, some data have been published concerning the physical properties of metallic plutonium \((\mathrm{Pu}, Z = 94)\). These data, obtained by two groups of investigators 1, 2, are not yet highly accurate and cover only phase transformations, measurements of density, and electrical resistance in the temperature interval from room temperature to the melting point. However, since these are the first data of this kind relating to the transuranium elements, they are of considerable interest.
Table 1
Temperatures of phase transformations in °C
| Phase | According to dilatometric measurements 1, heating | According to dilatometric measurements 1, cooling | According to dilatometric measurements 2, heating | According to dilatometric measurements 2, cooling |
|---|---|---|---|---|
| \(\alpha \rightleftarrows \beta\) | 136 | 85 | 135 | 80 (beginning of transformation) |
| \(\beta \rightleftarrows \gamma\) | 225 | \(\sim 160\) | 225 | Transition very protracted |
| \(\gamma \rightleftarrows \delta\) | 320 | \(\sim 250\) | 315 | Transition very protracted |
| \(\delta \rightleftarrows \varepsilon\) | 480 | 480 | 450 | 480 (transformation incomplete. Density falls only to \(\sim 16.1\)) |
| Melting and freezing points | — | — | — | — |
| Phase | According to thermal analysis 2, heating | According to thermal analysis 2, cooling | According to measurements of electrical resistance 2, heating | According to measurements of electrical resistance 2, cooling |
|---|---|---|---|---|
| \(\alpha \rightleftarrows \beta\) | 140 | 80 | 135 | 90 (beginning of transformation) |
| \(\beta \rightleftarrows \gamma\) | 235 | 145 (indistinctly expressed transition) | 220 | 160 (indistinctly expressed transition) |
| \(\gamma \rightleftarrows \delta\) | 325 | 245 (indistinctly expressed transition) | 325 | 225 (indistinctly expressed transition) |
| \(\delta \rightleftarrows \varepsilon\) | 475 | 475 | 480 | 480 (transformation incomplete) |
| Melting and freezing points | 640 | 640 | — | — |
Table II
Density of plutonium (g/cm³)
| Phase | According to work¹ | According to work² |
|---|---|---|
| α | 19.0 | 19.25 |
| β | 17.4 | 17.3 |
| γ | 16.6 | 16.7 |
| δ | 15.4 | 15.7 |
| ε | 16.4 | 16.3 |
Table III
Linear coefficient of thermal expansion \((\cdot 10^{-6}/^\circ\mathrm{C})\)
| Phase | According to work¹ | According to work² |
|---|---|---|
| α, below 70° | +40 | from +50 to +65 (nonlinear) |
| α, above 70° | +65 | from +50 to +65 (nonlinear) |
| β, below 190° | −32 | +43 |
| β, above 190° | −54 | +43 |
| γ | −48 (almost linear) | +39 |
| δ | −33 | −27 (nonlinear) |
| ε | +20 (almost linear) | +20 (from ~5 to ~30) for different specimens |
Table IV
Electrical properties of plutonium according to data of work²
| Phase | Resistivity (µΩ·cm) | Temperature coefficient of resistance \((\cdot 10^{-5}/^\circ\mathrm{C})\) |
|---|---|---|
| α | 150 | from −20 to −40 |
| β | 116 | −10 |
| γ | 115 | from −4 to −20 |
| δ | 111 | +7 |
| ε | 123 | — |
In the indicated temperature interval plutonium has five allotropic modifications: $\alpha$, $\beta$, $\gamma$, $\delta$, $\varepsilon$.
Table I gives data on the temperatures of phase transformations. Upon cooling, only the transition $\varepsilon \to \delta$ takes place without hysteresis. The transition $\beta \to \alpha$ begins sharply at a temperature of about $85^\circ$ and slowly ends at room temperature. The $\delta \to \gamma$ and $\gamma \to \beta$ transitions are markedly protracted, with large differences found from one specimen to another. Therefore the transition temperature is determined only very approximately. In paper $^1$ an accuracy of $\sim 1.5^\circ\text{C}$ is indicated; in paper $^2$, which is more detailed, an accuracy of $\pm 5^\circ\text{C}$ is indicated, while individual measurements give errors much greater than this value.
The data on the density of plutonium, summarized in Table II, refer to the lower boundary of the stability range of each of the phases.
It is noted that appreciable differences occur in the density of different specimens. In addition, hysteresis occurs. The sharp increase in density in the $\delta \to \varepsilon$ transformation is noteworthy.
Table III gives data on the linear coefficient of thermal expansion. This coefficient changes sharply with temperature, and the figures given refer to the middle of the corresponding temperature range. At the lower boundary of stability of the $\delta$ phase, the linear coefficient of thermal expansion is positive, but very small. As the temperature increases it becomes negative, increasing in absolute value up to the transition point to the $\varepsilon$ phase at $480^\circ\text{C}$.
Data on electrical resistivity and on the temperature coefficient of resistivity are given in Table IV.
The resistivity data are averages for several specimens at the lower boundary of phase stability. Variations from specimen to specimen amount to $\sim \pm 3\ \mu\Omega\text{cm}$. The temperature coefficient varies strongly from specimen to specimen; the table gives the extreme values for different specimens (for the $\gamma$ phase, in one case a value of $+20 \cdot 10^{-5}/^\circ\text{C}$ was observed).
R. G.
References
- W. B. H. Lord, Nature 173, 4403, 534 (1954).
- J. G. Bell, J. A. L. Robertson, P. Marion, J. A. Lee and E. T. Adam, ibid., p. 535.