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Preparation of the sulphides and phosphides of plutonium
J. Inorg, Nucl. Chem., 1966, Vol. 28, pp. 825 to 832. PergamonPress Ltd. Printed in Northern Ireland
P R E P A R A T I O N OF THE SULPHIDES A N D PHOSPHIDES OF P L U T O N I U M * O. L. KRUGER and J. B. MOSER Argonne National Laboratory, Argonne, Illinois
(Received 7 June 1965; in revised form 18 August 1965) Akaract--The sulphides and phosphides of plutonium were synthesized by reaction of high-purity metal with gases. A fine powder of reduced plutonium hydride was reacted with hydrogen sulphide or phosphine over the temperature range 400--600°C. The reaction products were charaO~riM by means of chemical analysis, metallographic examination and lattice constant measurements. The melting point of plutonium monosulphide was 2350°C. Plutonium monophosphide melted and decomposed at 2600°C.
PLUTONIUMsulphides were first prepared in milligram quantities by ABRAHAMe t al. ~1~ Several methods ofpreparation were tried. Plutonium dioxide was heated in a graphite crucible and allowed to react with H2S in the temperature range 1225-1300°C to form PtqOzS. Prolonged heating at temperatures from 1340-1400°C produced Pu2S3. Treatment of PuC13 with H2S at 900°C made a product believed to be Pu~Sa; however, its X-ray diffraction pattern was different from the PuzSa prepared from the dioxide. The monosulphide was obtained by reduction of PuF a with calcium metal in a barium sulphide crucible heated to 1250°C. ZACHARIASEN~2'3) identified the X-ray diffraction patterns of the above materials and determined the crystal structure of PuS as NaCl-type with a lattice constant of 5"536 + 0.001 A. Plutonium sesquisulphide was found to be cubic, isostructural with CesS4-Ce~Ss, with a lattice constant of 8-4543 4- 0.0005 A. GORUMt*~ prepared samples of plutonium monophosphide weighing 5-10 g by direct reaction of plutonium powder and phosphorus. Attempts to prepare higher phosphides of plutonium were unsuccessful. The crystal structure of PuP was identified as NaCl-type with a lattice constant of 5.644 + 0.004 A. Of the above methods of preparation none were suitable for making appreciable quantities of various Pu-S and Pu-P compositions with reproducible sulphur or phosphorus contents. Moreover, the above techniques could not be expected to produce materials with low oxygen, nitrogen and carbon contents. A method found satisfactory for the preparation of various U-S and Th-S compositions has been described by EASTMANe t al. ~5~ It involved the reaction of the metal hydrides with hydrogen * Work performed under the auspices of the U.S. Atomic Energy Commission. ¢1>B. M. AmtAnAM, N. R. DAWDSON and E. F. WESTRLrM,JR., The Transuranium Elements, Part I, National Nuclear Energy Series Voi. 14B, p. 814 (1949). t2~ W. H. ZACHARIASEN,Acta Cryst. 2, 291-296 (1949). ~s~ W. H. ZACHARIASEN,Acta Cryst. 2, 57-60 (1949). 14>A. E. GORUM,Acts Cryst. 10, 144 (1957). ~s~E. D. EASTMAN,L. BREWER,L. A. BROMLEY,P. W. GILLESand N. L. LOFGREN,J. Amer. Chem. 3oc. 72, 4019-23 (1950). o
825
826
O.L. KgUOERand J. B. MOSER
sulphide gas in the temperature range 400-500°C. This method was adapted to the synthesis of Pu-S compositions through modification o f the reaction procedure. Plutonium phosphides were prepared by substitution of phosphine for hydrogen sulphide. The apparatus and procedure for making pure Pu-S and P u - P compositions by reaction o f plutonium metal with gases is described in this paper. The reaction products were characterized by means of chemical and X-ray diffraction analysis and metallographic examination. Some of the properties of plutonium monosulphide and plutonium monophosphide are discussed. EXPERIMENTAL
Apparatus The toxicity of the plutonium, and the chemical reactivity of plutonium compounds in the powdered form, made it necessary to prepare and handle this material in glove boxes that contained an atmosphere of nitrogen gas. Impurities of oxygen and water vapour were present in the nitrogen in concentrations of about 0.005 and 0.010 wt. per cent, respectively. CALIBRATED @AS RESERVOIR
A
.
PRESSURIE
FILTER
INOICATOR
GLOVEBOX
7
II
i 1oooo7o7o
, = .
COLD TRAP
COLO T~Ap
F1o. 1.--Diagram of the apparatus used for plutonium-gas reactions. A diagram of the apparatus showing the arrangement of the piping and glove box connections is shown in Fig. 1. A molybdenum reaction chamber was placed inside a stainless steel vessel that was coated on its inside surface with a high temperature vitreous enamel. The coating was used to prevent the hot hydrogen sulphide and phosph/ne gases from contacting the stainless steel because these gases react readily with stainless steel at temperature above 400°C. The temperature of the chamber was monitored with a chromel-alumel thermocouple, which was covered by a molybdenum sheath. A cal/brated gas-reservoir and absolute pressure-indicator were used to measure the amounts of gas needed for the preparation of a particular composition. Since both H=S and PH8 gases have
Preparation of the sulphides and phosphides of plutonium
827
freezing points of --82"9 and --132.5°C, respectively, liquid nitrogen cold traps were incorporated into the system to cycle the gases through the reaction chamber by freezing and evacuating one trap while the other trap was heated. During this process any hydrogen produced from the reaction of the H,S or PHa gases with plutonium to form the sulphide or phosphide was pumped off. The amount of gas used in the reaction was then determined by heating the cold traps to room temperature and noting the pressure change of the enclosed system. Phosphine gas is highly toxic and it ignites spontaneously on exposure to air at room temperature. This is thought to be due to the P,H, component of the gas since the flash point of phosphine is about 150°C in air. Use of this gas with hydrogen in a system containing plutonium presented the hazard of a fire or explosion at the exhaust of the vacuum pump that would result in radioactive contaraination of the surrounding area. To prevent such a catastrophe, the outlet of the pump was blanketed with an inert gas. This mixture was filtered and then diluted in rapidly flowing air. Materials
High-purity plutonium metal prepared by fused salt electrolysisc6~ was used in this work so that impurities introduced into the final product could be attributed directly to the preparation process. The metal was 99.97 wt. per cent plutonium or of higher purity. Typical impurities are indicated by the analysis given in Table 1 for one of several plutonium melts used in this work. TABLE 1.--TYPICAL ANALYSIS OF IMPURITIES IN THE PLUTONIUM
Elements
ppm/wt,
ppm/at.
Chemical analyses Am C
H N
O
73 42 2.5 52 32
Si
1
U
32
72 834 592 885 477 9
32
Spectrographic analyses AI Cr Fe Li Ni
5 8 10 0.04 2
44 36 43 1"4 8
The hydrogen gas was a refined cylinder grade with a guaranteed minimum purity of 99-98 %* H2, and a typical purity of 99.99 ~o Ha. The gas, which contained major impurities of 0.005 ~o Ns, 0.001% Oi and 0.002 ~o H~O, was catalyticallypurified and then dried by passing it through activated alumina. The hydrogen sulphide gas was a CP grade with a minimum gnaranteed purity of 99.5 par cent. Mass spectrographic analysis indicated the following impurities; 0-03 VoCO~, 0~10% CS~, 0.08 % SO~, and <0"03 % N~ plus CO. Moisture was removed from the gas by passing it through an acetone-
828
O . L . KRUOER and J. B. MOSER
Argon gas with a minimum purity of 99.995 per cent was passed through activated alumina to remove the final traces of moisture. The partial pressure of water was kept low in all gas drying columns either by regeneration (heating under vacuum) or by periodic replacement of the desiccant.
Procedurefor Pu-S compositions Pieces of plutonium metal were mechanically cleaned, cut into sections about 0.63 cm in thickness, and placed in the reaction chamber. The system was purged several times with argon prior to the introduction of hydrogen. The hydride reaction usually occurred near room temperature, but the product was heated to 200°C and held at this temperature in hydrogen until a constant pressure on the manometer indicated complete conversion to PuHa. Unlike uranium, plutonium did not break down into a fine powder by alternate reaction with hydrogen and decomposition in vacuum. Therefore,
H2
cr°sh °ecom I Powder PuH3 I°se ,Vacuum,,PuH2-xI I 4°°°c -1
Pl
Pu
H2sL,°I I
Decompose~
_ H2S :500%
+ " L (Vacuum)
PuH2x-
PuS
_Decompose P+ uS + (Vacuum) Pu2S3 } Pu Pu +H2"x I PuH2+x +
Pu2S3
H2S 600%
Pu S3
Homogenize Vac. 1600% 4 hrs Fie. 2.--Synthesis of plutonium sulphide.
the hydride was removed from the apparatus, crushed to a powder of particle size smaller than 44/~ and reintroduced. Complete conversion of the plutonium to its hydride was necessary at this stage of the process to prevent contaminationof the powder through exposure of chemically reactive plutonium metal particles to the glove box atmosphere. The crushed powder was then held at 400°C until a pressure of 0.020 torr was reached. Complete decomposition of the hydride was never achieved, and X-ray diffraction powder patterns always indicated the presence of the Pull2 phase. Decomposition at temperatures above 400°C caused the powder to sinter into pieces that were too large for further reaction because of the greatly increased time needed for diffusion. The powder was partially decomposed in vacuum and subsequently reacted with H=S at temperatures of 400, 500 and 600°C. All reactions were strongly exothermic. The sequence of operations in the formation of the sulphide is shown schematically in Fig. 2. This procedure initially coated the particles with a thin surface layer of sulphide that prevented coalescence of the powder through sintering during decomposition cycles of the hydride at temperatures above 400°C. The final product consisted of a mixture of PuS, Pu=Ss and Pull=_,. This mixture was homogenized in a vacuum of 10-6 torr for four hours at a temperature of 1600°C. A tungsten crucible was used to hold the powder during the homogenization treatment. Plutonium monosulphidepowder was golden-brown in colour. Compositions near plutonium sesquisulphide were prepared by the above procedure with slight modifications so as to obtain finer powder from the hydride. After crushing, the hydride was further
Preparation of the sulphides and phosphides of plutonium
829
broken down by cycling between evacuation at 400°C and reaction with hydrogen at 200°C. This finer powder was more easily converted to the higher sulphide because it decomposed faster and was more reactive with the gases due to its increased surface area. Direct reaction of the HsS with PuHl+. powder was attempted, without success, over the temperature range 200° to 560°C. Another procedure of reacting at 400°C after decomposition of the hydride at 400, 500 and 600°C was tried but the rate of reaction was too slow at 400°C. Attempts to speed up the process by decomposing the plutonium hydride at 600°C and crushing the fused pieces did not yield pure material since the plutonium powder reacted excessively with the glove box atmosphere.
Procedurefor Pu-P compositions The same procedure was followed as for the sulphide preparation with the exception that phosphine gas was used. Since the monophosphide is the only stable high temperature compound, complete conversion of the hydride remaining at 600°C to phosphide was essential. In order to achieve this the vacuum decomposition-gas reaction cycle was repeated until no more PH8 was taken up by the powder This only left a negligibly small amount of hydride in the powder when it was vacuum annealed similarly to the sulphide but at 1400°C instead of 1600°C. Therefore, it is felt that a monophosphide very close to the stoichiometric composition was obtained. RESULTS AND DISCUSSION
Samples taken at various stages of the sulphide reaction were analysed for their hydrogen content and the results are shown in Table 2. Plutonium heated to 200°C in a hydrogen atmosphere was completely converted to plutonium hydride containing TABLE 2.--HYDKOGEN CONTENT OF POWDER AT VARIOUS
SXAO~Sor PREPA~nON OF PLtrrosroM SULPHIDE Temperature (°C) 200 400 500 600
Treatment Reaction (H~) Decomposition Reaction (HIS) Decomposition Reaction (HIS) Decomposition Reaction (HIS)
Hydrogen Content (wt. %)
H/Pu
1.25 0.79
3.00 1.89
0.88
--
0.20 0"32 0-20 0.07
-----
1.25 wt. per cent H. This hydrogen content corresponds to a composition of Pull 8. Decomposition at 400°C at a pressure of 0-020 torr reduced the hydrogen to plutonium ratio to 1.89 which appears to be consistent with the higher temperature work by other investigators on the Pu-H system. ¢7~ At 400°C, PuH~_® reacted with H~S and the presence of a sulphide layer on the surface of the particles was detected by examination of X-ray photograms. During the reaction, the hydrogen released on dissociation of H2S, combined with the powder to form a composition with an overall hydrogen content of 0.88 wt. per cent. On subsequent decomposition and reaction at 500 and 600°C the dissociated hydrogen again combined with the powder to form some hydride together with PuS and Pu~S3. Pure PuS was obtained by removal of the hydrogen under vacuum and homogenization of the remaining plutonium with the sulphide phases. All hatches of powder were analysed for oxygen and nitrogen since there was little possibility of the introduction of other impurities during preparation. Homogenized ~7~ R. N . R. MULFORD and G. E. STURDY, J. Amer. Chem. Soc. 77, 3449 (1955).
830
O.L. KRUGERand J. B. MOSIF.R
sulphide powder with as little as 0.010 wt. per cent oxygen and 0.0070 wt. per cent nitrogen was prepared with the above technique. Usually the total impurity content (oxygen and nitrogen combined) of sintered sulphide pellets did not exceed 0.05 wt. per cent. The purity of the plutonium sulphide was found to be directly related to the amount of contaminants in the H2S, particularly, CO2 and SO2. Therefore, H~S gas of lower purity than listed earlier should be avoided. Pellets that were adjudged from metallographic and X-ray examination to be near the stoicheiometric composition contained about 12wt. per cent S. The results of the sulphur analysis, although generally acceptable, could not be used to detect slight changes in composition. Plutonium monophosphide powder of a purity equal to that of the TABLE3.--LATTICECONSTANTSOF
P u S , Pu~,SaAND PuP PHASES IN VARIOUS PLUTONIUM-SULPHUR AND PLUTONIUM-PHOSPHORUS COIVIPOSITION$
Lattice constant (A) Compound
This work
Other
PuS
5"5383 ~ 0"0001 5"5393 ± 0-0001 5"5402 j: 0"0001
5"536 ~: 0"001''}
5.5408 ~ 0.0001 5.5409 ~ 0.0001
Pu2Ss PuP
8"4182 ~ 0.0001 5"6514 ~ 0"0001 5"6562 ~ 0"0001 5.6572 ~: 0.0001 5.6598 ~ 0'0001
8"4543 ± 0"0005 ~s~
5"644 ~ 0'004~4~
sulphide was generally not obtained, probably because of the lower purity of the phosphine gas compared to the hydrogen sulphide. Typical oxygen and nitrogen contents of the monophosphide were 0-05 and 0.03 wt. per cent, respectively. Phases present in the various materials were identified by use of Debye-Scherrer powder patterns taken with Cu-K~ radiation. Lattice constants of the PuS, Pu~Sa and PuP phases were determined with the aid of a computer programme. The best value of six different extrapolation techniques was used for the lattice constant. Values for the lattice constants of PuS, Pu~Sa and PuP are shown in Table 3 where they are compared with the results of earlier investigators. The maximum value of 5.5409 A for the lattice constant of PuS was obtained from a composition of this phase in equilibrium with Pu~Sa. Material that was found to be single phase PuS by metallographic examination had a lattice constant of 5-5383 A. These values are considerably greater than the ones obtained by ZACHARIAS~N(see Table 3). Further work is needed before any conclusions can be drawn regarding the compositional limits of the PuS phase. The lattice constant of Pu~Sa in equilibrium with PuS represents the minimum value for this phase. Lattice constant values for plutonium monophosphide ranged from 5.6514 to 5-6598 A (the lower value was for PuP in equilibrium with plutonium). This range of values for material of equal purity indicates that a substantial range of composition exists for the plutonium monophosphide phase field. No higher phosphides than PuP were detected in this work.
Preparation of the sulphides and phosphides of plutonium
831
The theoretical density of PuS, calculated for a stoicheiometric compound with a lattice constant of 5.540 A (average of values listed in Table 3) is 10.59 g/cms. Plutonium sesquisulphide has a theoretical density of 8.526 g/cms. This value is based on the assumption that Pu~Ss is isostructural with Ce2Ss--CeaS4as proposed by ZAC~AmASElq.m~The calculated density of PUP with a lattice constant of 5-6598 A is 9.893 g/cms. The microstmctures of plutonium monosulphide and plutonium monophosphide with less than 0.05 wt. per cent non-metallic impurities are shown in Figs. 3 and 4. These samples were taken from pellets that were sintered to a density of 90 per cent of the theoretical value by use of conventional ceramic techniques. Grain boundaries of the pure compounds were visible on etching and no impurity phases were detected in the microstructures. The characteristics of plutonium monosulphide and plutonium monophosphide are summarized in Table 4. Melting points of PUS and PuP were measured in a tungTABLE 4.--CHARACTERIffI]CSOF PLUTONIUMMONOSULPHIDEAND PLUTONIUM MONOPHOSPHIDE
Colour Homogenization temperature Melting point Lattice constant Theoretical density Oxygen content Nitrogen content
PuS
PuP
Gold-brown 1600°C 2350°C
Dark-grey 1400°C Decomposed at 2600°C (2 atm) 5-6598 A, 9.893 g/cma 0.05 wt. % (0.83 at. %) 0.03 wt. % (0.57 at. %)
5.5409 A 10.59 g/cms <0.025 wt. % (0-042 at. %) <0.025 wt. % (0.48 at. %)
sten filament furnace in flowing high-purity argon. The melting point of PuS was 2350 -4- 30°C (maximum deviation). An attempt to measure the melting point of PuP was made in argon at two atmospheres pressure. Rapid decomposition at 2600°C with melting prevented the observation of a reliable melting point. SUMMARY
Sulphides of plutonium were prepared by reaction of decomposed plutonium hydride powder with H~S gas over the temperature range 400-600°C. Each step in the decomposition of the hydride was followed by a reaction with the HsS gas to prevent the particles from sintering. Phosphide compositions were prepared by the same procedure except that phosphine gas was used for the reaction. Plutonium sulphide powder with an overall monosulphide composition was homogenized under high vacuum at 1600°C for 2 hr. The homogenization temperature of plutonium monophosphide was somewhat lower at 1400°C. Chemical analysis showed that material made by this technique was low in oxygen and nitrogen impurities. The maximum values found for the lattice constants of PuS and PuP were 5.5409 and 5.6598 A, respectively. Plutonium sesquisulphide in equilibrium with PuS had a lattice constant of 8.4182 A. Metallographic examination of 90 per cent dense plutonium monosub phide and plutonium monophosphide specimens showed only single phase material
832
O . L . KRUGER and J. B. MOSER
with no impurities present in the microstructures. The melting point of PuS was 2350°C, whereas simultaneous melting and decomposition were observed for PuP at 2600°C in argon at two atmospheres pressure.
Acknowledgements--The authors wish to express their appreciation to Messrs B. J. WRONA and J. W. THOr~SONfor their assistance in performing many of the experimental details related to this work. Thanks is also given to Messrs H. T. GOODSVEEDand J. H. MARSh,JR., of the ANL Chemistry Division for the many chemical analyses they performed.
FIG. 3.-Microstructure
of PUS showing grain boundaries and pores. (Lactic etch; 500 x).
FIG. 4.-Microstructure
of PUP showing grain boundaries and pores. (Lactic etch; 500 x).
P R E P A R A T I O N OF THE SULPHIDES A N D PHOSPHIDES OF P L U T O N I U M * O. L. KRUGER and J. B. MOSER Argonne National Laboratory, Argonne, Illinois
(Received 7 June 1965; in revised form 18 August 1965) Akaract--The sulphides and phosphides of plutonium were synthesized by reaction of high-purity metal with gases. A fine powder of reduced plutonium hydride was reacted with hydrogen sulphide or phosphine over the temperature range 400--600°C. The reaction products were charaO~riM by means of chemical analysis, metallographic examination and lattice constant measurements. The melting point of plutonium monosulphide was 2350°C. Plutonium monophosphide melted and decomposed at 2600°C.
PLUTONIUMsulphides were first prepared in milligram quantities by ABRAHAMe t al. ~1~ Several methods ofpreparation were tried. Plutonium dioxide was heated in a graphite crucible and allowed to react with H2S in the temperature range 1225-1300°C to form PtqOzS. Prolonged heating at temperatures from 1340-1400°C produced Pu2S3. Treatment of PuC13 with H2S at 900°C made a product believed to be Pu~Sa; however, its X-ray diffraction pattern was different from the PuzSa prepared from the dioxide. The monosulphide was obtained by reduction of PuF a with calcium metal in a barium sulphide crucible heated to 1250°C. ZACHARIASEN~2'3) identified the X-ray diffraction patterns of the above materials and determined the crystal structure of PuS as NaCl-type with a lattice constant of 5"536 + 0.001 A. Plutonium sesquisulphide was found to be cubic, isostructural with CesS4-Ce~Ss, with a lattice constant of 8-4543 4- 0.0005 A. GORUMt*~ prepared samples of plutonium monophosphide weighing 5-10 g by direct reaction of plutonium powder and phosphorus. Attempts to prepare higher phosphides of plutonium were unsuccessful. The crystal structure of PuP was identified as NaCl-type with a lattice constant of 5.644 + 0.004 A. Of the above methods of preparation none were suitable for making appreciable quantities of various Pu-S and Pu-P compositions with reproducible sulphur or phosphorus contents. Moreover, the above techniques could not be expected to produce materials with low oxygen, nitrogen and carbon contents. A method found satisfactory for the preparation of various U-S and Th-S compositions has been described by EASTMANe t al. ~5~ It involved the reaction of the metal hydrides with hydrogen * Work performed under the auspices of the U.S. Atomic Energy Commission. ¢1>B. M. AmtAnAM, N. R. DAWDSON and E. F. WESTRLrM,JR., The Transuranium Elements, Part I, National Nuclear Energy Series Voi. 14B, p. 814 (1949). t2~ W. H. ZACHARIASEN,Acta Cryst. 2, 291-296 (1949). ~s~ W. H. ZACHARIASEN,Acta Cryst. 2, 57-60 (1949). 14>A. E. GORUM,Acts Cryst. 10, 144 (1957). ~s~E. D. EASTMAN,L. BREWER,L. A. BROMLEY,P. W. GILLESand N. L. LOFGREN,J. Amer. Chem. 3oc. 72, 4019-23 (1950). o
825
826
O.L. KgUOERand J. B. MOSER
sulphide gas in the temperature range 400-500°C. This method was adapted to the synthesis of Pu-S compositions through modification o f the reaction procedure. Plutonium phosphides were prepared by substitution of phosphine for hydrogen sulphide. The apparatus and procedure for making pure Pu-S and P u - P compositions by reaction o f plutonium metal with gases is described in this paper. The reaction products were characterized by means of chemical and X-ray diffraction analysis and metallographic examination. Some of the properties of plutonium monosulphide and plutonium monophosphide are discussed. EXPERIMENTAL
Apparatus The toxicity of the plutonium, and the chemical reactivity of plutonium compounds in the powdered form, made it necessary to prepare and handle this material in glove boxes that contained an atmosphere of nitrogen gas. Impurities of oxygen and water vapour were present in the nitrogen in concentrations of about 0.005 and 0.010 wt. per cent, respectively. CALIBRATED @AS RESERVOIR
A
.
PRESSURIE
FILTER
INOICATOR
GLOVEBOX
7
II
i 1oooo7o7o
, = .
COLD TRAP
COLO T~Ap
F1o. 1.--Diagram of the apparatus used for plutonium-gas reactions. A diagram of the apparatus showing the arrangement of the piping and glove box connections is shown in Fig. 1. A molybdenum reaction chamber was placed inside a stainless steel vessel that was coated on its inside surface with a high temperature vitreous enamel. The coating was used to prevent the hot hydrogen sulphide and phosph/ne gases from contacting the stainless steel because these gases react readily with stainless steel at temperature above 400°C. The temperature of the chamber was monitored with a chromel-alumel thermocouple, which was covered by a molybdenum sheath. A cal/brated gas-reservoir and absolute pressure-indicator were used to measure the amounts of gas needed for the preparation of a particular composition. Since both H=S and PH8 gases have
Preparation of the sulphides and phosphides of plutonium
827
freezing points of --82"9 and --132.5°C, respectively, liquid nitrogen cold traps were incorporated into the system to cycle the gases through the reaction chamber by freezing and evacuating one trap while the other trap was heated. During this process any hydrogen produced from the reaction of the H,S or PHa gases with plutonium to form the sulphide or phosphide was pumped off. The amount of gas used in the reaction was then determined by heating the cold traps to room temperature and noting the pressure change of the enclosed system. Phosphine gas is highly toxic and it ignites spontaneously on exposure to air at room temperature. This is thought to be due to the P,H, component of the gas since the flash point of phosphine is about 150°C in air. Use of this gas with hydrogen in a system containing plutonium presented the hazard of a fire or explosion at the exhaust of the vacuum pump that would result in radioactive contaraination of the surrounding area. To prevent such a catastrophe, the outlet of the pump was blanketed with an inert gas. This mixture was filtered and then diluted in rapidly flowing air. Materials
High-purity plutonium metal prepared by fused salt electrolysisc6~ was used in this work so that impurities introduced into the final product could be attributed directly to the preparation process. The metal was 99.97 wt. per cent plutonium or of higher purity. Typical impurities are indicated by the analysis given in Table 1 for one of several plutonium melts used in this work. TABLE 1.--TYPICAL ANALYSIS OF IMPURITIES IN THE PLUTONIUM
Elements
ppm/wt,
ppm/at.
Chemical analyses Am C
H N
O
73 42 2.5 52 32
Si
1
U
32
72 834 592 885 477 9
32
Spectrographic analyses AI Cr Fe Li Ni
5 8 10 0.04 2
44 36 43 1"4 8
The hydrogen gas was a refined cylinder grade with a guaranteed minimum purity of 99-98 %* H2, and a typical purity of 99.99 ~o Ha. The gas, which contained major impurities of 0.005 ~o Ns, 0.001% Oi and 0.002 ~o H~O, was catalyticallypurified and then dried by passing it through activated alumina. The hydrogen sulphide gas was a CP grade with a minimum gnaranteed purity of 99.5 par cent. Mass spectrographic analysis indicated the following impurities; 0-03 VoCO~, 0~10% CS~, 0.08 % SO~, and <0"03 % N~ plus CO. Moisture was removed from the gas by passing it through an acetone-
828
O . L . KRUOER and J. B. MOSER
Argon gas with a minimum purity of 99.995 per cent was passed through activated alumina to remove the final traces of moisture. The partial pressure of water was kept low in all gas drying columns either by regeneration (heating under vacuum) or by periodic replacement of the desiccant.
Procedurefor Pu-S compositions Pieces of plutonium metal were mechanically cleaned, cut into sections about 0.63 cm in thickness, and placed in the reaction chamber. The system was purged several times with argon prior to the introduction of hydrogen. The hydride reaction usually occurred near room temperature, but the product was heated to 200°C and held at this temperature in hydrogen until a constant pressure on the manometer indicated complete conversion to PuHa. Unlike uranium, plutonium did not break down into a fine powder by alternate reaction with hydrogen and decomposition in vacuum. Therefore,
H2
cr°sh °ecom I Powder PuH3 I°se ,Vacuum,,PuH2-xI I 4°°°c -1
Pl
Pu
H2sL,°I I
Decompose~
_ H2S :500%
+ " L (Vacuum)
PuH2x-
PuS
_Decompose P+ uS + (Vacuum) Pu2S3 } Pu Pu +H2"x I PuH2+x +
Pu2S3
H2S 600%
Pu S3
Homogenize Vac. 1600% 4 hrs Fie. 2.--Synthesis of plutonium sulphide.
the hydride was removed from the apparatus, crushed to a powder of particle size smaller than 44/~ and reintroduced. Complete conversion of the plutonium to its hydride was necessary at this stage of the process to prevent contaminationof the powder through exposure of chemically reactive plutonium metal particles to the glove box atmosphere. The crushed powder was then held at 400°C until a pressure of 0.020 torr was reached. Complete decomposition of the hydride was never achieved, and X-ray diffraction powder patterns always indicated the presence of the Pull2 phase. Decomposition at temperatures above 400°C caused the powder to sinter into pieces that were too large for further reaction because of the greatly increased time needed for diffusion. The powder was partially decomposed in vacuum and subsequently reacted with H=S at temperatures of 400, 500 and 600°C. All reactions were strongly exothermic. The sequence of operations in the formation of the sulphide is shown schematically in Fig. 2. This procedure initially coated the particles with a thin surface layer of sulphide that prevented coalescence of the powder through sintering during decomposition cycles of the hydride at temperatures above 400°C. The final product consisted of a mixture of PuS, Pu=Ss and Pull=_,. This mixture was homogenized in a vacuum of 10-6 torr for four hours at a temperature of 1600°C. A tungsten crucible was used to hold the powder during the homogenization treatment. Plutonium monosulphidepowder was golden-brown in colour. Compositions near plutonium sesquisulphide were prepared by the above procedure with slight modifications so as to obtain finer powder from the hydride. After crushing, the hydride was further
Preparation of the sulphides and phosphides of plutonium
829
broken down by cycling between evacuation at 400°C and reaction with hydrogen at 200°C. This finer powder was more easily converted to the higher sulphide because it decomposed faster and was more reactive with the gases due to its increased surface area. Direct reaction of the HsS with PuHl+. powder was attempted, without success, over the temperature range 200° to 560°C. Another procedure of reacting at 400°C after decomposition of the hydride at 400, 500 and 600°C was tried but the rate of reaction was too slow at 400°C. Attempts to speed up the process by decomposing the plutonium hydride at 600°C and crushing the fused pieces did not yield pure material since the plutonium powder reacted excessively with the glove box atmosphere.
Procedurefor Pu-P compositions The same procedure was followed as for the sulphide preparation with the exception that phosphine gas was used. Since the monophosphide is the only stable high temperature compound, complete conversion of the hydride remaining at 600°C to phosphide was essential. In order to achieve this the vacuum decomposition-gas reaction cycle was repeated until no more PH8 was taken up by the powder This only left a negligibly small amount of hydride in the powder when it was vacuum annealed similarly to the sulphide but at 1400°C instead of 1600°C. Therefore, it is felt that a monophosphide very close to the stoichiometric composition was obtained. RESULTS AND DISCUSSION
Samples taken at various stages of the sulphide reaction were analysed for their hydrogen content and the results are shown in Table 2. Plutonium heated to 200°C in a hydrogen atmosphere was completely converted to plutonium hydride containing TABLE 2.--HYDKOGEN CONTENT OF POWDER AT VARIOUS
SXAO~Sor PREPA~nON OF PLtrrosroM SULPHIDE Temperature (°C) 200 400 500 600
Treatment Reaction (H~) Decomposition Reaction (HIS) Decomposition Reaction (HIS) Decomposition Reaction (HIS)
Hydrogen Content (wt. %)
H/Pu
1.25 0.79
3.00 1.89
0.88
--
0.20 0"32 0-20 0.07
-----
1.25 wt. per cent H. This hydrogen content corresponds to a composition of Pull 8. Decomposition at 400°C at a pressure of 0-020 torr reduced the hydrogen to plutonium ratio to 1.89 which appears to be consistent with the higher temperature work by other investigators on the Pu-H system. ¢7~ At 400°C, PuH~_® reacted with H~S and the presence of a sulphide layer on the surface of the particles was detected by examination of X-ray photograms. During the reaction, the hydrogen released on dissociation of H2S, combined with the powder to form a composition with an overall hydrogen content of 0.88 wt. per cent. On subsequent decomposition and reaction at 500 and 600°C the dissociated hydrogen again combined with the powder to form some hydride together with PuS and Pu~S3. Pure PuS was obtained by removal of the hydrogen under vacuum and homogenization of the remaining plutonium with the sulphide phases. All hatches of powder were analysed for oxygen and nitrogen since there was little possibility of the introduction of other impurities during preparation. Homogenized ~7~ R. N . R. MULFORD and G. E. STURDY, J. Amer. Chem. Soc. 77, 3449 (1955).
830
O.L. KRUGERand J. B. MOSIF.R
sulphide powder with as little as 0.010 wt. per cent oxygen and 0.0070 wt. per cent nitrogen was prepared with the above technique. Usually the total impurity content (oxygen and nitrogen combined) of sintered sulphide pellets did not exceed 0.05 wt. per cent. The purity of the plutonium sulphide was found to be directly related to the amount of contaminants in the H2S, particularly, CO2 and SO2. Therefore, H~S gas of lower purity than listed earlier should be avoided. Pellets that were adjudged from metallographic and X-ray examination to be near the stoicheiometric composition contained about 12wt. per cent S. The results of the sulphur analysis, although generally acceptable, could not be used to detect slight changes in composition. Plutonium monophosphide powder of a purity equal to that of the TABLE3.--LATTICECONSTANTSOF
P u S , Pu~,SaAND PuP PHASES IN VARIOUS PLUTONIUM-SULPHUR AND PLUTONIUM-PHOSPHORUS COIVIPOSITION$
Lattice constant (A) Compound
This work
Other
PuS
5"5383 ~ 0"0001 5"5393 ± 0-0001 5"5402 j: 0"0001
5"536 ~: 0"001''}
5.5408 ~ 0.0001 5.5409 ~ 0.0001
Pu2Ss PuP
8"4182 ~ 0.0001 5"6514 ~ 0"0001 5"6562 ~ 0"0001 5.6572 ~: 0.0001 5.6598 ~ 0'0001
8"4543 ± 0"0005 ~s~
5"644 ~ 0'004~4~
sulphide was generally not obtained, probably because of the lower purity of the phosphine gas compared to the hydrogen sulphide. Typical oxygen and nitrogen contents of the monophosphide were 0-05 and 0.03 wt. per cent, respectively. Phases present in the various materials were identified by use of Debye-Scherrer powder patterns taken with Cu-K~ radiation. Lattice constants of the PuS, Pu~Sa and PuP phases were determined with the aid of a computer programme. The best value of six different extrapolation techniques was used for the lattice constant. Values for the lattice constants of PuS, Pu~Sa and PuP are shown in Table 3 where they are compared with the results of earlier investigators. The maximum value of 5.5409 A for the lattice constant of PuS was obtained from a composition of this phase in equilibrium with Pu~Sa. Material that was found to be single phase PuS by metallographic examination had a lattice constant of 5-5383 A. These values are considerably greater than the ones obtained by ZACHARIAS~N(see Table 3). Further work is needed before any conclusions can be drawn regarding the compositional limits of the PuS phase. The lattice constant of Pu~Sa in equilibrium with PuS represents the minimum value for this phase. Lattice constant values for plutonium monophosphide ranged from 5.6514 to 5-6598 A (the lower value was for PuP in equilibrium with plutonium). This range of values for material of equal purity indicates that a substantial range of composition exists for the plutonium monophosphide phase field. No higher phosphides than PuP were detected in this work.
Preparation of the sulphides and phosphides of plutonium
831
The theoretical density of PuS, calculated for a stoicheiometric compound with a lattice constant of 5.540 A (average of values listed in Table 3) is 10.59 g/cms. Plutonium sesquisulphide has a theoretical density of 8.526 g/cms. This value is based on the assumption that Pu~Ss is isostructural with Ce2Ss--CeaS4as proposed by ZAC~AmASElq.m~The calculated density of PUP with a lattice constant of 5-6598 A is 9.893 g/cms. The microstmctures of plutonium monosulphide and plutonium monophosphide with less than 0.05 wt. per cent non-metallic impurities are shown in Figs. 3 and 4. These samples were taken from pellets that were sintered to a density of 90 per cent of the theoretical value by use of conventional ceramic techniques. Grain boundaries of the pure compounds were visible on etching and no impurity phases were detected in the microstructures. The characteristics of plutonium monosulphide and plutonium monophosphide are summarized in Table 4. Melting points of PUS and PuP were measured in a tungTABLE 4.--CHARACTERIffI]CSOF PLUTONIUMMONOSULPHIDEAND PLUTONIUM MONOPHOSPHIDE
Colour Homogenization temperature Melting point Lattice constant Theoretical density Oxygen content Nitrogen content
PuS
PuP
Gold-brown 1600°C 2350°C
Dark-grey 1400°C Decomposed at 2600°C (2 atm) 5-6598 A, 9.893 g/cma 0.05 wt. % (0.83 at. %) 0.03 wt. % (0.57 at. %)
5.5409 A 10.59 g/cms <0.025 wt. % (0-042 at. %) <0.025 wt. % (0.48 at. %)
sten filament furnace in flowing high-purity argon. The melting point of PuS was 2350 -4- 30°C (maximum deviation). An attempt to measure the melting point of PuP was made in argon at two atmospheres pressure. Rapid decomposition at 2600°C with melting prevented the observation of a reliable melting point. SUMMARY
Sulphides of plutonium were prepared by reaction of decomposed plutonium hydride powder with H~S gas over the temperature range 400-600°C. Each step in the decomposition of the hydride was followed by a reaction with the HsS gas to prevent the particles from sintering. Phosphide compositions were prepared by the same procedure except that phosphine gas was used for the reaction. Plutonium sulphide powder with an overall monosulphide composition was homogenized under high vacuum at 1600°C for 2 hr. The homogenization temperature of plutonium monophosphide was somewhat lower at 1400°C. Chemical analysis showed that material made by this technique was low in oxygen and nitrogen impurities. The maximum values found for the lattice constants of PuS and PuP were 5.5409 and 5.6598 A, respectively. Plutonium sesquisulphide in equilibrium with PuS had a lattice constant of 8.4182 A. Metallographic examination of 90 per cent dense plutonium monosub phide and plutonium monophosphide specimens showed only single phase material
832
O . L . KRUGER and J. B. MOSER
with no impurities present in the microstructures. The melting point of PuS was 2350°C, whereas simultaneous melting and decomposition were observed for PuP at 2600°C in argon at two atmospheres pressure.
Acknowledgements--The authors wish to express their appreciation to Messrs B. J. WRONA and J. W. THOr~SONfor their assistance in performing many of the experimental details related to this work. Thanks is also given to Messrs H. T. GOODSVEEDand J. H. MARSh,JR., of the ANL Chemistry Division for the many chemical analyses they performed.
FIG. 3.-Microstructure
of PUS showing grain boundaries and pores. (Lactic etch; 500 x).
FIG. 4.-Microstructure
of PUP showing grain boundaries and pores. (Lactic etch; 500 x).