Melting point of Cm2O3

Notes

241

Here CHNO8is the total molar acid concentration in water, a the degree of dissociation of nitric acid and y± the average activity coefficient of the H ÷ and the NOz- ions. Values were taken from the table by Davis and De Bruin[4].

Acknowledgements-The first named author is indebted to the Government of the United Arab Republic and grateful for financial support to the Ministry of Higher Education (Cairo), Department of Mission. He also wishes to thank the authorities of the Atomic Energy Establishment (Cairo) for giving him the opportunity to carry out his research. This work is part of the research program of the Institute for Nuclear Physics Research (IKO), made possible by financial support from the Foundation for Fundamental Research on Matter (FOM) and the Netherlands Organization for Pure Scientific Research (ZWO).

lnstituut voor Kernphysisch Onderzoek Ooster Ringd(ik 18 Amsterdam-O Netherlands

M. K. K. SHEHATAT A. H. W. ATEN, Jr.

*This work has been described in greater detail in: M.K.K. Shehata, "Solvent extraction study of traces of plutonium and hafnium in the tetravalent state from nitric acid solution". Thesis, University of Amsterdam 1968. t Present address: Atomic Energy Establishment, Chemistry Department, Anshas, Egypt. 4. W. Davis Jr. and H. J. De Bruin, J. inorg, nucl. Chem. 26, 1069 (1964).

J. inorg,nucl.Chem..1969.Vol. 3I, pp. 241 to 245. PergamonPress. Printedin Great Britain

Melting point of Cm~03 (Received 10 May 1968) CURIUM-244 oxide is being considered as a fuel material for thermionic isotopic power sources operating at temperatures near 1750°C. A high-melting oxide is required to prevent melting on temperature excursions and to allow a greater margin for safety in the operation of the power source. McHenry has reported the melting point of a curium oxide, believe to be Cm~O3, as 1950°C[1]. This melting temperature appears low for several reasons. (l) Plots of log P vs. 1/ T from measurements of vaporization rates of CmzO3 show no effect of the heat of fusion up to 2335°C [2]. (2) Cm203 exhibits structures and bond energies quite similar to those of the lighter rare-earth sesquioxides[2, 3], which generally melt between 2200-24000C; the heat and entropy of fusion of CmzOz should be about the same, and would fix the melting temperature in the same range as for rareearth sesquioxides. (3) Since the molecular weight of ~Cm203 is so large (536 g/mole), aggregate light cation impurity levels of the order of 1 wt per cent would amount to 10 mole per cent, which could lower the melting temperature of the oxide by 200-300°C.

1. R. E. McHenry, Trans.Am. nucl. Soc. 8, 75 (1965). 2. P. K. Smith and D. E. Peterson, High Temperature Evaporation and Thermodynamic Properties of Cm203. Abstracts of Papers, 155th Am. Chem. Soc. Meet. San Francisco, April 1-5. No. 0-031 (1968). 3. W. C. Mosley, B-Type ~Cm203: A Candidate Isotopic Power Fuel. Abstracts of Papers, 155th Am. Chem. Soc. Meet. San Francisco, April 1-5, No. 0-173 (1968).

242

Notes

(4) I n M c H e n r y ' s measurements, curium oxide of unknown purity was melted oniridium filaments, which has subsequently been shown to react with Cm2Oa at high temperatures, even in reducing atmospheres [4]. A redetermination of the melting points of Cm20~ is reported in this work. EXPERIMENTAL Apparatus and techniques. Melting points were measured in a filament furnace similar to that described by Newkirk and Bates [5]. Tungsten filaments with Mendenhail open-V wedges were used [6]: Expanded length Width Thickness Apex angle

3.00 in. 0.50 in. 0.005 in. 15°.

The furnace was contained in a glove box to control radioactivity from z44Cm. Power was supplied from a H a n s o n - V a n Winkle-Munning, 500 A, d.c. power supply equipped with a continuous manual control. Temperatures were measured with a Micro-optical Pyrometer*, which was sighted down into the bottom of the open V. Glass absorbance and any deviation from black body conditions were corrected for by sightings through the optics onto a calibrated pyrometer lamp, and by measuring the melting points of high-purity A1203 and Tm2Oa. Curium oxide was reduced to Cm2Oa by heating the V at 500-7000(2 for 5 min before melting. Melting of 1-mg samples was detected by sighting into the end of the V with a Berger 22X telescope; melting of individual particles and bulk powders was easily detected. All measurements were made in flowing 4 per cent H2-He. The precision of the observation was within 10°C, and the accuracy within 20°C. Source o f Curium-244. Curium oxide from several sources was melted. (1) High purity curium oxide was obtained by air oxidizing a curium metal residue from a tungsten cell used to measure the vapor pressure of curium metal. More than half the metal and most of the impurities had been vaporized from the cell. Emission spectrographic analyses before the metal vaporization experiment and after vaporization and oxidation of the residue are shown in Table 1. Except for about 8 mole per cent of plutonium, americium, and zirconium oxides the total of the light metal oxide impurities, primarily stainless steel and silicon impurities, were reduced from 5 mole per cent to 0.5 mole per cent by the vaporization. (2) Samples of "process" curium oxide, typical of the product in the campaign to produce 5.5 kg of 24~-Cmat Savannah River except for 5"6 mole per cent of ~°Pu decay product, were also melted [7]. The analysis of these samples prior to melting is shown in Table 1. (3) One curium oxide sample was obtained by collecting vapor deposits produced in an earlier experiment to measure vaporization rates of Cm~O3 from tungsten in vacuum. The deposit was dissolved from quartz with nitric acid, precipitated as oxalate, and calcined to the oxide. This sample contained the same level of impurities as the "process" oxide. (4) All curium samples except one described in the last column of Table 1 were known to contain about 5-6 mole per cent of ~4°Pu which was formed from the alpha decay of 2~Cm over 2 yr. One sample of processed oxide was repurified by anion exchange followed by two successive oxalate precipitations to remove plutonium; this treatment reduced the plutonium content to 0.51 mole per cent. RESULTS AND DISCUSSION The melting data are listed in Table 2. The Cm2Oa samples made from oxidized metal melted * Product of the Pyrometer Instrument Company. 4. W. C. Mosley, Unpublished information. Savannah River Laboratory, E. I. du Pont de Nemours, Aiken, S. C. (1967). 5. H. W. Newkirk, Jr. andJ. L. Bates, Rev. scient, lnstrum. 30, 645 (1959). 6. C. E. Mendenhall,Astrophys.J. 33, 91 (191 l). 7. G. A. Burney, Nucl. Appl. 4, 217 (1968).

243

Notes

Table 1. Emission spectrographic analyses of impurities in curium samples used for melting point determinations Curium metal before vaporization

Oxidized curium metal after vaporization

Mole% oxide in CmzO3

ppm

Mole % oxide in CmzO3

5.6 0-15-0.35 <0.008 0-1 <0.007

56,000 1500-3500 <10 200-500 <10

5.6 0.15-0.35 <0.008 0.2-0.5 <0.007

5100" 8500* 150 150 <10

0.51 * 0.85* <0.10 0.15 <0.007

50 < 10 100 <10 < 10

0.08 <0.002 0.08 <0.007 <0.004

50 25 50-150 50-100 < 10

0.08 0.005 0.04-0.12 0.035-0.07 <0.004

100 300 300 100 < 100

0.16 0-06 0.24 0-07 <0-04

<50 <10 250 <10 150

<0.035 <0.001 0-14 <0.003 0-05

<50 <10 <25-750 <10 <50

<10 <50 <500 35 <10

<0-007 <0.02 <0.35 0.015 <0.005

<10 <50 <500
0.02 <0-09

50 <50 <100 <10

0.04 <0.09

70-150 < 150 <25 <25 < 10 <5 <5 <5 <3

0.04-0.08 <0.008 <0-004 <0-01 <0.003 <0.001 <0-001 <0-001 <0.003

ppm

24°pu Am B Si Mn

56,000 1500-3500 50-100 100-1000 25

56,000 1500-3500 <10 100 <10

Mg Pb Fe Ni Cr

50-150 50-250 2500-5000 150-1500 1500-2000

Co Bi AI Mo Sn

<50 <10 100 <10 <50-375

Cu Cd Zn Ag Ca

50-150 <50 <500-1000 50-375 25--100

Na Li Ta Zr W K Ce La

150-500 <50 5000

"'Process" oxide low plutonium

ppm

Impurity

ppm

25 <50 5000 high

Mole % oxide in Cm203

*'Process" oxide and deposits

2 high

P Ba Cs Nb Sb Ti

* Determined by alpha counting and mass spectrometry.

<0.035 <0-001 <0-015-0,4 <0.003 <0.015

<50 <10 500 < 10 <50

<0.035 <0.001 0.28 <0-003 <0.015

<0.007 <10 <0.02 <50 <0.35 <500 <0-005 <10 0.012-0-12 100

<0.007 < 0-02 <0.35 <0.005 0-05

<0-004

200 <50 <100 <400

0-16 <0.09 <0.01 <0.16

244

Notes Table 2. Melting points of Cn%Oa, Ai~O3 and Trn~O3

Oxide

244Cm2Oa(Process) 2~Cm2Oa (Vapor deposit) mCm~Os (Oxidized metal) 2~Cm2Os (Process, low Pu) Al~Oal" [ F o r calibration of the Tm203~ I.m.p. equipment

Observed melting point, °C 25°C/see 2.5*C/see

Reported melting point, °C

2186* 3186 2237 2255 2053, 2036

2173" 2180 2253 2277 2047

2049 ± 5 [8]

2385

2370

2375 ± 25 [9]

*Evidence of some premelting or softening. t 99.87% AhOa, 0-06% Na~O,0.02% SiO2,0.04% CaO. ,99.8% Tm~Oa.

sharply and completely at 2245 ± 10°C; however, the sample contained about 6 mole per cent of plutonium and americium and 2 mole per cent of zirconium. The "process" oxide and oxide vapor deposits, containing 6 mole per cent of plutonium and americium, but no zirconium, melted about 90"C lower, probably because of higher silicon impurities. Some samples of the "process" oxide softened (both as individual particles and bulk powder) at 100*C below the temperature of complete melting. Such softening might have arisen from partial melting at the lower temperature. The melting point of oxide containing only 1.3mole per cent of plutonium and americium and <0.16 mole per cent of zirconium was 2265 ± 12°C. Since the melting temperature of the oxidized metal sample varied very little from that of the low plutonium oxide, the effect of plutonium or zirconium content on melting temperature is small. High temperature X-ray diffraction experiments and thermo-gravimetric analyses[3] have established: the decomposition of CmO2; the composition of Cm203; its high tenlperature behavior in reducing, inert, and oxidizing environments; and its congruent melting behavior. Cm203 melted without interaction with tungsten. The absence of interaction between tungsten and Cm208 was established in previous Cm2Oz vaporization experiments[2]. Duplicate samples melted at the same temperature regardless of heating rate. Individual particles in the specimen retained their spherical form until melting, at which temperature wetting of the tungsten was sharp and complete, again implying little transport of tungsten into the oxide or vice versa. Solidification on cooling could not be detected because the liquid sample wetted the tungsten filament and could not be optically distinguished in the telescope. Compounds of the Type 3 Cm20~'WO3 are thought to exist by analogy with similar rare-earth compounds [ 10]. Since the chemistry of curium is similar to that of samarium, the melting temperature of such a compound would be about 2250°C. However, such a compound requires excess oxygen and is thought unlikely to form in the reducing environment of these experiments. Radiation damage is not thought to affect the melting temperature significantly since other transition temperatures, structures, and stabilities are not seriously affected. The melting point of Cm~O3 derived from this work is 2265 ± 20°C. This melting point for Cm2Os is entirely consistent with the comparable rare-earth sesquioxides[2, 3]. However, the melting point is 70° lower than that inferred from Cm203 vaporization rate measurements [2]; this discrepancy is attributed to the limited precision of the vaporization rate measurements.

8. E. M. Levin, C. R. Robbins and H. F. McMurdie, Phase Diagram for Ceramists, p. 569. The American Ceramic Society, Columbus, Ohio (1964). 9. P. K. Smith, J. R. Keski and C. L. Angerman, Properties of Thulium Metal and Oxide, USAEC Rep. No. DP-I 114. Savannah River Laboratory, E. I. du Pont de Nemours, Aiken, S. C. (1967). 10. Marc Foex, Bull. Soc. chirn. Ft. 10, 3696 (1967).

Notes

245

A c k n o w l e d g e m e n t s - T h e information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy Commission. The author is indebted to Dr. G. A. Burney for the curium oxide purification, to Dr. M. C. Thompson for curium metal preparation, and to R. H. Gaddy for the analyses.

Savannah River Laboratory E. I. du Pont de Nemours and Co. A iken, South Carolina 29801

J. inorg, nucl. Chem., 1969, Vol. 31, pp. 245 to 246.

P. K. S M I T H

Pergamon Press.

Printed in Great Britain

The vapour pressures of some phosphonitrilic halides-- II (Received2 May 1968) INTRODUCTION VAPOUR pressures for crystalline (NPCI2)o and (NPBr2)4 have been measured within the range 59-100°C by a Knudsen effusion technique using a Sartorius Electrono torsion balance microbalance. The experimental arrangement and procedure are described in a previous paper[l]. Latent heats of sublimation for the compounds have been calculated from the vapour pressure curves.

EXPERIMENTAL (NPCI2)6 was supplied by Albright and Wilson Ltd., and was purified by recrystallisation from petrol ether 60-80 and dried in vacuo at 60°C. The crystalline product had a m.p. 93.5-94,5°C and a P - N stretching frequency at 1345 cm -1. (NPBr2)4 was prepared by the method of John and Moeller[2], recrystallised from petrol ether 60-80 and dried in vacuo at 60°C. The crystalline product had a m.p. 200-201°C and a P-N stretching frequency at 1275 cm -1.

RESULTS The values of the vapour pressures obtained are given in Table 1. Vapour pressure/temperature equations (Table 2) were derived from these values by the method of least squares. From the equations lattice energies were calculated (also listed in Table 2) using the integrated Clausius-Clapeyron equation.

Department o f Chemistry and Chemical Technology Borough Polytechnic London S.E. 1

S. COTSON K. A. H O D D *

*Present address: School of Materials Science and Technology, Brunel University, London W.3. t. S. Cotson and K. A. Hodd, J. inorg, nucl. Chem. 27,335 (1965). 2. K. John and T. Moeller, J. inorg, nucl. Chem. 22, 199 ( 1961).