The thermodynamics of vaporization of neptunium and plutonium

J. Chem. Thermodynamics 1975,7,211-218

The thermodynamics of vaporization of neptunium and plutonium ’ R. J. ACKERMANN

and E. G. RAUH

Chemistry Division, Argonne Illinois 60439, U.S.A. (Received I7 April

National

Laboratory,

Argonne,

1974)

The vapor pressure of liquid neptunium was determined over the temperature range 1540 to 2140 K by a combination of mass-effusion and mass-spectrometric measurements. The measured pressures are given by loglO{po(Np)/atm} = -(22370 f 95)K/T + (5.196 4 0.056) and the resulting enthalpy and entropy of vaporization and the standard deviations therein are AH,“(Np, 1800 K) = (102.4 f 0.4) kcal,, mol-1 and AS.?(Np, 1800 K = (23.8 •t 0.3) calth K-l mol-I. The enthalpy of sublimation at 298 K is estimated as AHHs”(Np, 298 K) = (110 f 2) kcal,, mol-I. The vapor pressure of plutonium was redetermined over the temperature range 1210 to 1620 K. The results yield log{p”(Pu)/atm} = -(17120 f 114)K/T + (4.592 & 0.082), AHt(Pu, 1400 K) = (78.3 f 0.5) kcal,, mol-I, and ASG(Pu, 1400 K) = (21.0 f 0.4) caltb K-l molll. The measured pressures are 20 to 30 per cent less than those of two previously reported absolute measurements. Based on a recent assessment of the thermal functions for solid and liquid plutonium, the results of the present study show the best second-third law agreement, i.e., AH,“(II, Pu, 298.15 K) = (82.8 f 0.5) kcalth mol-’ and AH,“(III, Pu, 298.15 K) = (83.3 * 0.4) kcal,, mol-I.

1. Introduction The vapor pressure of neptunium has been measured on only one previous occasion by Eick and Mulford.“’ Their reported enthalpy of vaporization at 1800 K, (94 _+6) kcal,, mol-‘, has been called into question by Nugent, Burnett, and Morss(‘) in their attempts to correlate the thermodynamic properties of the actinides in aqueous solution with those in the metallic and vapor phase.? These correlations suggest that the measured value may be 15 kcal,,, mol -I too small. Eick and Mulford”) had no direct knowledge of the constitution of the vapor, and since NpO,(s) was detected in one of the residues, they recognized the possible role of the gaseous monoxide because in many cases the volatilization of reactive metals is thus increased by the presence of oxygen. (3) Because of these uncertainties and because accurate vapor pressures will be needed for the subsequent determination of the standard Gibbs energy of formation of NpO(g), the vaporization of neptunium was re-examined by a combination of mass-spectrometric and mass-effusion measurements with special attention to the composition of the liquid metal and gas phase. There exist no reliable thermal data for neptunium above 500 K and since the accuracy of the measured a Work performed under the auspices of the U.S. Atomic Energy Commission. i Throughout this paper cahh = 4.184 J; eV z 1.6021 x lo-l8 J; atm = 101.325 kPa.

212

R. J. ACKERMANN

AND E. G. RAUH

vapor pressure will depend solely on second-law determinations, the enthalpy of vaporization of lanthanum and the vapor pressure of plutonium, third-law data being available for both, were remeasured to establish the reliability of the massspectrometric and mass-effusion measurements, respectively.

2. Experimental methods and results The mass-spectrometric measurements were obtained with a Bendix model 12-101 time-of-flight mass spectrometer equipped with an effusion cell heated by electron bombardment. Details of the cell assembly,‘4) power regulator,(5) and measurement of temperature (6) have been reported elsewhere. All temperatures are based on IPTS68. A molybdenum cell with a channel orifice and an inner cup of single-crystal tungsten was used. The enthalpy of vaporization of lanthanum was redetermined as a test of the present state of the equipment for this type of measurement. A 100 mg sample of lanthanum metal was heated for several hours at 1900 K until the LaO+ ion intensity had been reduced to a few per cent of the La+, after which the La+ intensity was measured at several random temperatures in the range 1475 to 1925 K. The results, Iog{l(La+)T} expressed as a linear function of K/T, were treated by the method of least squares and the enthalpy of vaporization calculated from the slope. The results of three determinations, AH,“(La, 1700 K) = (97.7 +0.4), (98.8 f0.4), and (98.OLO.3) kcal,, mol-‘, were in good agreement with the third-law value AH”,(La, 1700 K) = 99.1 kcal,, mol-‘,(7,8) and established the absence of systematic errors and significant temperature gradients in the cell. NEPTUNIUM Neptunium metal of high purity was obtained from Los Alamos Scientific Laboratory. The surface of the sample was bright and appeared to be free of oxide. The major impurity was 450 p.p.m. by mass of 23*Pu all other metallic elements reported were below 100 p.p.m. by mass. No analysis foi carbon or oxygen was given. An 80 mg sample was added to the thoroughly outgassed cell and heated stepwise to 1850 K. Pu+ was not observed. Initially, the NpOf ion intensity at this temperature was about 2.5 times that of the Np+ (ionizing electron energy: 12 eV) but decreased to somewhat less than that of the Npf in 2 h. The Npf intensity was measured as a function of the cell temperature in the range 1625 to 1930 K. Four additional series of measurements were made, each after 20 to 50 min at 1900 K to further deoxygenate the metal. During the fourth series I(NpO+) had decreased to about 1 per cent of I(Np+). The results of the five determinations of the enthalpy of evaporation from least-squares analyses of the measurements are given in table 1. Mass-effusion measurements were carried out by condensation of a known fraction of the effusate on platinum targets and subsequent determination of neptunium by alpha counting. The collection-type apparatus and procedures were essentially the same as those described in earlier studies. (‘) The pressure p of neptunium is related to the mass m of neptunium on the target by the equation: p = (m/at)(2?cRT/M)“‘{(r’

+d’)/r’),

(1)

VAPORIZATION TABLE

OF NEPTUNIUM

213

AND PLUTONIUM

1. Results of mass-spectrometric and mass-effusion determinations of enthalpy of vaporization AH, of Np in the temperature range T1 to Tz (Cal,,, = 4.184 J) AKWP,

0 =

Determination

TJK

G/K

kcal,, mol - i

MS-Np-1 MS-Np-2 MS-Np-3 MS-Np-4 MS-Np-5 MS-Series 111

1625 1570 1530 1520 1535 1618

1930 1930 1930 1930 2180 2074

104.8 100.7 101.3 102.4 102.2 100.4

f f & i & &

1.5 1.0 0.8 0.7 0.6 1.2

(1The errors are standard deviations.

a is the area of the orifice, t the time of collection, d the orifice-to-collimator distance, and r the radius of the collimator. Because of the high specific activity of 238Pu which was always present to some extent in the effusate, the mass of Np on the targets was determined by a combination of total alpha count from a 5 1.7 per cent geometry proportional counter plus an alpha energy analysis using a surface barrier silicon detector. The half-life of neptunium used was 2.14 x 106a.(“’ The same sample used in the mass-spectrometric observations was employed for the masseffusion measurements. In order to establish that the partial pressure of NpO(g), resulting from possible oxidation of the sample during the transfer from the mass spectrometer, was insignificant and that the plutonium on the targets was sufficiently low to ensure a reliable alpha energy analysis, six preliminary exposures of 10 min each at 1900 K were made. Each exposure was preceded by heating for 10 min at 2100 K. The neptunium count decreased by a factor of 2 for the first two exposures but was constant for the last four indicating that the sample had been adequately de-oxygenated. The ratio of the plutonium and neptunium counts decreased from approximately 10 for the first exposure to about 2 for the sixth. A further heating for 1 h at 2160 K decreased the Pu count to l/7 of that of the Np at which concentration the background of the Np counts due to the attenuation “tail” of the 238Pu was insignificant. Eight exposures were made at random temperatures in the range 1618 to 2074 K. The temperatures, measured mass effusion rates W(Np) = (m/at){(r2+d2)/r2), and calculated pressures are given in section A of table 2 and are shown in figure 1 as log,,(p/atm} plotted against K/T. Also given in section A of table 2 is the leastsquares analysis of the results, equation (2a), which yields an enthalpy of vaporization : AH”(Np, 1800 K) = (100.4+ 1.2) kcal,, mol-‘, included in table 1. The six enthalpies given in table 1 were weighted inversely to the squares of the standard deviations and the average was normalized to equation (2a) at 1800 K resulting in the following expression for the partial pressure of neptunium saturated with tungsten, in the temperature range 1600 to 2100 K: where

log,,(p(Np,

I)/atm)

= -(22270+95)R/T+

(5.128+0.056).

(2)

R, J. ACKERMANN

214

AND E. G. RAUH

TABLE 2. Mass effusion results (atm = 101.325 kPa)

T

ii 2008 1924 2074 1825

Section 1. Neptunium T P(NP) atm x

lOa B’(Np) g cmwa s-l 1670 575 3290 127 loglo{p(Np)/atm}

1.10 3.70 2.20 7.95 = -(21930

x x x x f

lo8 W(NP) gcm-2s-1

1o-6 1618 lo-’ 1692 lo-@ 1762 10-a 1890 260) K/T + (4.947 i 0.140)

4.26 16.2 47.5 288 (1618 to 2074 K)

PWP) atm 2.51 x 9.77 x 2.92 x 1.84 x (2, a)

1O-9 10-Q lo-* 1O-7

Section B. Plutonium T E

1504 1504 1315 1287 1396 1209 1343 1448 1449

10VV(Pu) g crnea s-l

atm

K

109W(Pll) g cmWa s-l

x x x x x x x x x

1563 1379 1424 1266 1517 1314 1478 1619

7710 291 689 20.5 3360 70.2 2030 17900

T

Pow

2920 2920 69.4 36.5 356 5.79 135 992 1010

1.65 1.65 3.67 1.91 1.94 1.94 7.24 5.51 5.58

lo+ 1O-7 1O-9 1o-e 10-e 10-10 10-O 10-e 10-a

T/K 1600

2000 7800 -5-“‘I

-10

”

““‘1”~“““““““““““““” 5.0 5.5 6.0

’

6.5

1400 I

7.0

I

7.5

Pm.0

atm 4.45 1.58 3.79 1.07 1.91 3.72 1.14 1.05

x x x x x x x x

10-T 10-e 10-e 10-o 10-1 1O-9 lo-’ 1o-6

1200 L

8.0

104K/T FIGURE 1. Plot of loglo{p/atm)} against 104K/T for neptunium and plutonium. Curve a: partial pressure of Np saturated with tungsten, this study, equation (2); 0, mass effusion results; 0, normalized mass-spectrometric results from MS-Np-5 ; +, mass-effusion results from deliberately oxidized Np. Curve b: results of Eick and Mulford.“) Curve c: vapor pressure of plutonium, this study, equation (4) and those of Kent. (W Curve d: results of Mulfordu6) and Phipps et ai.“@

VAPORIZATION

OF NEPTUNIUM

AND PLUTONIUM

215

Equation (2) is shown as curve a in figure 1. Also shown are the normalized massspectrometric results of the series MS-Np-5, and, for comparison, the results of Eick and Mulford,“) curve b.t The solubility of tungsten in liquid neptunium is not known and could not be measured because of the limited quantity of neptunium metal available. However, the solubility can be estimated from a consideration of the solubility parameters, {AH,(298.15 K)/V} ‘/’ , for neptunium and uranium. (I ‘) For neptunium the solubility parameter can be calculated from the enthalpy of sublimation, AHi(298.15 K) =(110_+2) kcal,, mol-’ (see discussion section) and the molar volume V = 11.6 cm3 mo1-1,(12) and has a value of 98 ca1,,1’2 cm-3/2; that for uranium is 100 cal,h1’2 cm -3/2 . (11) The near identity of the two values indicates that the solubility of tungsten in neptunium is equal to that in uranium. In a previous study(“) the solubility of tungsten in liquid uranium was determined in the temperature range of the present measurements (mole fraction of W: 0.01 to 0.04) and these values were applied as ideal-solution corrections to the partial pressures given by equation (2). The corrections are comparable with the statistical errors in the measurements but they provide a systematic increase in the pressure with increasing temperature, and hence may be justified, The vapor pressure of pure neptunium is thus given by log,,(p”(Np,

I)jatm)

= -(22370+95)KjT+(5.196+0.056),

(3)

in the temperature range 1540 to 2150 K, from which the enthalpy and entropy of vaporization at 1800 K are, respectively, (102.4f0.4) kcal,, mol- ’ and (23.78 kO.26) Cal,, K-’ mol- I. The errors quoted are standard deviations and hence measure only precision; however, a systematic error of 110 per cent in the pressures is certainly possible. Since the mass-spectrometric observations showed an initially high partial pressure of NpO(g), the sample was oxidized in a controlled manner until the surface darkened slightly and the total volatility of neptunium-bearing species was measured by a series of four exposures without preliminary heating. The results of these four determinations are shown as the + points in figure 1 and indicate that even a minor amount of oxygen contamination which may not represent saturation can result in total pressures equivalent to those reported by Eick and Mulford.“’ PLUTONIUM

The vapor pressure of plutonium was remeasured by the mass-effusion method under conditions similar to those existing during the neptunium measurements. A 300 mg sample of high-purity plutonium metal was used. Chemical and spectroscopic analysis showed less than 500 p.p.m. by mass total of C, H, 0, N, and Si, and less t It was found that the least-squares equation given by Eick and Mulford did not agree with the neptunium pressures given in their table of results. The results as given yield the equation: log&(Np)/atm} = -(21470 f 9OO)K/T + (5.573 f 0.500). A closer examination revealed a 25 per cent error in the calculated neptunium pressures in Series L. The least-squares equation resulting from all points in Series I, J, and 0 and the corrected values for Series L is log,,{p(Np)/atm} = -(21880 & 9OO)K/T + (5.827 f 0.495); it is this equation which is plotted as curve b in figure 1.

216

R. 3. ACKERMANN

AND

E. G. RAUH

than 100 p.p.m. by mass total of metallic elements. The specific activity was determined from the isotopic analysis: 239Pu, 95.1; 240Pu, 4.5; 241Pu, 0.4; 242Pu, ~0.02 moles per cent, and the pertinent half-lives(‘3’ and the targets were assayed by total alpha count after pulse-height analysis had shown no alpha activity other than that from Pu. The results of measurements at random temperatures in the range 1210 to 1620 K are given in section B of table 2 and are shown in figure 1. No corrections were made for the solubility of tungsten in liquid plutonium since an extrapolation of the results of Bowersox and Leary (14) indicated that such corrections (about 1 per cent) would be far less than the uncertainties in the measurements. Least-squares analysis of the results yielded the equation for the vapor pressure of liquid plutonium: log,,{p”(Pu,

l)/atm} = -(17120f114)K/T+(4.592+0.082),

(4) in the temperature range 1210 to 1670 K, which is shown as curve c in figure 1. For comparison the results of Mulford”‘) are shown as curve d but those of Phipps et a1.‘t6) are not shown since on the scale of figure 1 they are indistinguishable from those of Mulford. The mass-spectrometric measurements reported by Kent(“) based on the vapor pressure of gold have been corrected for the most recent assessment thereof by Hultgren et al. (‘1 The results are nearly identical with those of the present study, and hence are closely approximated by curve c in figure 1. Each of the pressures given in section B of table 2 was used with the thermodynamic functions of Hultgren et aZ.(*) for gaseous and liquid Pu, the latter corrected for the recently established value of S”(Pu, 298.15 K) = 13.4 caJh K-’ mol-‘,(1s*19) to calculate AH,“(Pu, 298.15 K). The average value of (83275&40) Cal,, mol-’ so obtained showed no trend with temperature beyond the uncertainty in the average and yielded a thirdlaw enthalpy of vaporization, AH”,(Pu, III, 1400 K) = 78770 Cal,, mol-‘. This is in good agreement with the directly measured second-law value, AHt(Pu, II, 1400 K) = 78340 Cal,, mol- ‘, from equation (4).

3. Discussion As seen in figure 1, the vapor pressures of plutonium reported by Mulford,(“) Phipps et uZ.,(i@ and Kent,(“) and the results of the present study agree within 30 per cent. A second-third law comparison for each of the three investigations is given in table 3. As before, the thermodynamic functions of Hultgren et a1.‘*’ corrected by the more recent value of s”(Pu, 298.15 K) reported by Lee et a1.(‘*) and by Sandenaw and Gibney” 9, were used for the calculations. Ostensibly the present results show the best second-third law consistency but this close agreement may be in part fortuitous since the heat capacity of liquid Pu is largely estimated. The present measurements are the most consistent with a heat capacity of liquid Pu near 10.0 Cal,, K-’ mol-’ which Hultgren et al.@) selected from the few measurements of Loasby,(20) and which is generally supported by values for the heavier transition and lanthanide metals.(21) The measurements of Eick and Mulford”) on neptunium and those of Mulford(15) on plutonium were obtained with the same apparatus under nearly the same conditions. In view of the near agreement of the present measurements on plutonium

VAPORIZATION TABLE

3. Comparison

OF NEPTUNIUM

of second- and third-law values of enthalpy of sublimation (call,, = 4.184 J)

-~ Phipps et c11.(‘~’ Mulfordu5) Kent(17) This study

AH.“(II, 298.15 K) a kcal,, mol - 1 .__-.. 85.0 g 0.5 84.2 xk 0.5 84.2 f 0.8 82.8 f 0.5

217

AND PLUTONIUM

of plutonium

AH:(III, 298.15 .__- K) b kcal,, mol-’ 82.7 82.6 83.1 83.3

a The errors are standard deviations in the measured temperature-dependences of the vapor pressure. b Estimated errors of f0.4 kcal,, mol-1 based on h15 per cent uncertainty in the measured vapor pressures.

with those of Mulford, apparatus error could not account for the factor of 7 between the Np pressures of the present study, curve a in figure 1, and those of Eick and Mulford,“’ curve b. The mass-spectrometric observations and the total pressures of neptunium-bearing species measured over the oxidized metal strongly suggest that the higher pressures reported by Eick and Mulford”) can indeed be explained by the presence of NpO in the vapor phase. Since no thermal data for liquid neptunium above 500 K are available, only a second-law value of the enthalpy of vaporization, equation (3), can be reported. However, the measured value seems to be reliable since the enthalpy of vaporization of lanthanum, determined mass-spectrometrically under similar experimental conditions, agreed with the previously reported third-law value and since the masseffusion measurements on plutonium yielded a temperature dependence also in good agreement with auxiliary thermal data. The enthalpy of vaporization of neptunium at 298.15 K, AH,“(Np, 298.15 K) = (llO_f2) kcal,, mol-‘, is estimated. The lower limit was obtained by applying to the measured enthalpy at 1800 K, AHt(Np, 1800 K) = (102.4+0.4) kcal,, mol- ’ from equation (3), the difference in {H”(T)--H”(298.15 K)}/T intermediate between those for Pu and U.@) In the present study the entropy of vaporization of neptunium at 1800 K is 23.8 Cal,,, K-’ mol- i from equation (3), a value which lies between those of uranium and plutonium. (W However this value combined with the standard entropy of gaseous neptuniumCz2’ yields thk standard entropy of liquid neptunium at 1800 K, S”(Np, 1, 1800 K) z 34 Cal,, K-i mol-‘, which value is larger than those for liquid Pu and U, 33.5 and 31.3 Cal,, K-’ mol-‘, respectively. @) Hence, the ostensibly larger entropy suggests a larger {H”(T)-H”(298.15 K)}/T, and therefore, a value of AHi(Np, 298.15 K) greater than 108 kcal,, mol-‘. The value proposed is a substantial but not a complete improvement for neptunium in the thermodynamic correlation of the actinide metals developed by Nugent et a/.(‘) A value of 99 kcal,, mol-’ had been derived from the measurements of Eick and Mulford(” whereas a value near 117 kcal,, mol-’ had been predicted from the correlations.

218

R. J. ACKERMANN

AND E. G. RAUH

The authors are indebted to W. C. Bentley and H. Diamond for the alpha counting and pulse height analyses, to Dr Daniel Lam for his help in acquiring the neptunium sample, and to R. L. Faircloth, Chemistry Division, U.K.A.E.A., Harwell, England, for some of the preliminary plutonium measurements. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22,

Eick, H. A.; Mulford, R. N. R. J. Chem. Phys. 1964,41, 1475. Nugent, L. J.; Burnett, J. L.; Moms, L. R. J. Chem. Thermodynamics 1973, 5, 665. See for example, Ackermann, R. J.; Rauh, E. G. J. Chem. Thermodynamics 1972,4, 521. Rauh, E. G.; Sadler, R. C.; Thorn, R. J. Argonne National Laboratory Report ANL-6536, April 1962. Cremer, H. H. 2. Znstr. 1964, 7, 72. See for a description of the equipment and procedure, Thorn, R. J.; Winslow, G. H. Amer. Sot. Mech. Eng. 1963, Paper No. 63-WA-244. Ackermann, R. J.; Rauh, E. G. J. Chem. Thermodynamics 1971, 3, 445. Hultgren, R.; Desai, P. D.; Hawkins, D. T.; Gleiser, M.; Kelley, K. K.; Wagman, D. D. Selected values of the Thermodynamic Properties of the Elements. Amer. Sot. Metals, 1973. See, for example, Ackermann, R. J.; Gilles, P. W.; Thorn, R. J. J. Chem. Phys. 1956, 25, 1089. Brauer, F. P.; Stromatt, R. W.; Ludwick, J. D.; Roberts, F. P.; Lyon, W. L. .Z.Znorg. Nucl. Chem. 1960,12,234. Ackermann, R. J.; Rauh, E. G. Htgh Temperature Science 1972,4,496. Asprey, L. B.; Penneman, R. A. Chem. Eng. News 1%7,45, 75. See for example, Jordan, K. C. MoundLaboratory Report MLM-1443, March 1968, pp. 9-30. Bowersox, D. F.; Leary, J. A. J. Nuclear Mat. 1%7, 21, 219. Mulford, R. N. R. Thermodynamics, Vol. I. International Atomic Energy Agency, Vienna, 1966, p. 231. Phipps, T. E.; Sears, G. W.; Siefert, R. L.; Simpson, 0. C. U.N. Znt. Conf Peaceful Uses of Atomic Energy 1956, 7, 382. Kent, R. A. High Temp. Sci. 1969, 1, 169. Lee, J. A.; Mendelssohn, K.; Sutcliffe, P. W. Proc. Roy. Sot. Lond. 1970, A317, 303. Sandenaw, T. A.; Gibney, R. B. J. Chem. Thermodynamics 1971,3,85. Loasby, R. G. In Plutonium 1960, Grison, E.; Lord, A. E.; Fowler, G. A., Editors. CleaverHume Press Ltd: London. 1961. Margrave, J. L. Etude Des Transformations Crystalfines A Haute Temperature Au-dessus De 2OOOK. Editions Du Centre National De La Recherche Scientifique: Paris. 1972, p. 71. Feber, R. C.; Herrick, C. C. Las Alamos Scientific Report, LA-3184. February, 1965.