350
G. M. CAMPBELL
Thermodynamic Properties of Plutonium Nitride by Galvanostatie Potential Determination1
by G.M. Campbell Los Alamos Scientific Laboratory, University of California, Los Alamos, N e w Mexico
87644
(Received J u l y 10, 1968)
A galvanostatic technique has been developed for the determination of equilibrium potentials in fused salts. The technique makes it possible to measure the emf of metal-metal ion couples without requiring the presence of the metal for long periods of time. This has obvious advantages, especially in the case of very reactive
metals where surface impurities are difficult to eliminate by the usual techniques. The technique was used to obtaiil direct measurements on the cell Pu(s, 1) PuC13, LiC1-KC11N2(g), PuN(s) over the temperature range 714-1032°K. The reaction of Pu with Ka at these temperatures precludes obtaining the emf by other methods. Over this temperature range, at 1 ntm Nz the emf is E = 7.90071 - 2.31690 x 10-2T f 2.517855 x 10-6T2 - 9.1644 x 10-9T3 (~t0.003)V. 4‘uove the melting point of Pu (913’K) the standard free energy of formation of PUS is - - G O T = 73.8 - 0.632251” kcal/rnol of PUN. From the condition X0298(PuN) X o 2 9 8 ( P u ) = S”ZOS(US) - X o 2 9 8 ( U ) the estimate -AH’zgs = 72.5 kcal/mol is made. Theoretical development of the rechiiique is included.
Introduction
a t temperatures ranging from 714 to 1032’K. Since IS2 is known to react with P u throughout this temperaThe importance of emf determination for obi aining ture range, no direct measurements are possible by the thermodynamic information has recently been reusual technique. Emf measurements have previously emphasized.2 It should be recognized, however, that bcen made in this laboratory by an indirect method4 the technical difficulties involved in ascertaining the Since the technique involved the use of a liquid junction chemical equilibrium represented by any stable emf are and a solid P u electrode, i t was necessary to restrict the stiiI in need of improvement. The difficulty of detectmeasurements to a very limited temperature range. ing side reactions which have an indeterminate effect on The galvanostafie technique has made it possib!e to the results is one of the principal uncertainties in any eliminate the liquid junction and otherwise minimize the emf determination. The galvanic potential at the technical difficulties which had existed, Rather than electrode-electrolyte interface is affected by the gradual a semipermanent reference electrode, this depend on buildup of impurities over extended periods of time, technique makes use of a W microelectrode upon which. especially if the electrode surface is easily oxidized. Pu3+ is reduced to Pu by passing a constant current Experience in this laboratory has indicated that a t between the W microelectrode and a third electrode. By temperatures above the melting point of Pu (913”1ration. Consequently, the volume containing the concentration gradient is nearly the same for planar as for cylindrical electrodes. This was verified experimentally by making galvanostatic determinations against a Pu(1) reference electrode, over the temperature range of these experiments. Agreement was always within =k0.002 V. Stripping times used were between 1 and 2.3 sec. Electrodeposition times were between 3 and 7 sec. Similar experiments on the UUs+ couple a t the same temperatures gave similar results. Experimental Section Equipment. 1. Apparatus and Materials. The cell used for emf determinations is diagrammed in Figure 2. The quartz container was surrounded by an electrically grounded nickel tube to provide shielding against stray electric fields. The quartz container, 18 in. deep and 17/8 in, in diameter, was positioned so that the lower 9 in. was inside the tube furnace. A Cu cooling coil placed around the container above the furnace kept the cell top a t a moderate temperature a t all times. The electrolyte was contained in a T a cup 2.5 in. deep and 1.5 in. in diameter. The PUN reference elec(8) D. G . Peters and J. J. Lingane, J. Electroanal. Chem., 2, 1
(1961).
THERMODYNAMIC PROPERTIES OF PUNtrode was made by clamping a doughnut-shaped sintered pellet of PUN,between W washers a t the end of a T a rod. Nitrogen content in the PUN was 0.99 atom of N to 1 atom of Pu. X-Ray powder diffraction analysis indicated that the material was single-phase PUN,with a. = 4.9055 8. Spectroscopically pure N2was maintained a t a carefully measured pressure in the cell. The W microelectrode, or working electrode, was made by suspending a 0.04-cm diameter W wire from a 3-mm i.d. quartz tube so that the wire was immersed to a depth of about 1 cm in the electrolyte. The quartz tube extended to within about 1 in. of the electrolyte and served as an electrical insulator from the other electrodes which were placed inside similar quartz tubes. The auxiliary electrode was made by melting Pu metal into a l/z in. diameter by in. deep TaC coated T a cup attached to the end of a '/s-in. T a rod. In addition to serving to carry current, this electrode helped maintain the purity of the electrolyte. Reaction with KZreduced the Pu activity on the electrode surface and prevented electronic conduction from developing in the cell. A chromel-alumel thermocouple calibrated against the melting point of KBS pure Zn and A1 metals was used in all temperature measurements. The thermocouple was inserted into a '/s-in. diameter T a tube which was closed a t one end and immersed in the molten electrolyte. Electric contacts for electrodes and thermocouples were directed through Kovm seals a t the top of the cell. This made it possible to evacuate the cell and flush with Nz to remove gaseous impurities from the cell. 2. Procedure. The initial preparation of the T,iCIKCl electrolyte was similar to that described by T,aitinen, et a1.O After treatment with HCI, the eutectic mixture was repeatedly equilibrated with Pu metal and filtered, until voltammetric analysis indicated the desired purity was obtained. All cell parts except the electrodes were assembled and outgassed overnight at 800". After cooling, the electrodes and electrolyte were added and the cell repeatedly evacuated and flushed with Nz as the temperature was slowly raised. The temperature was controlled with a Minneapolis Honeywell "Pyr-0-Vane" controller. The emf between the PUN reference electrode and the Pu auxiliary electrode was monitored continuously on a Sargent Model M R recorder. Chronopotentiograms were recorded periodically on a second Sargent Model MR recorder. A constant current source of conventional design, built by the LASL instruments group, produced direct currents ranging from about 200 to 630 PA in magnitude. The current from this source was passed through a 10-ohm resistor before and after the potential us. time trace was recorded. The chart
353 speed of the recorder during the electrolysis period was 1 in./sec. After measuring the Pu plating and stripping periods, the time during stripping a t which the Pu3+ concentration a t the microelectrode surface was equal to the bulk concentration was calculated using eq VII. By bucking the recorder circuit with the potential from a Minneapolis Honeywell potentiometer until the recorder pen was located a t the iridicated chart position, the emf was read from the potentiometer. This made it possible to malie each measurement using a standard calibrated Weston cell as a reference potential. At least three chronopotentiograms were recorded at each PUN equilibrium with the current a t a new setting each time. Regardless of the variation in plating and stripping times and in current, the measured emf usually varied less than 2 mV. In cases where the Pu plated on the microelectrode reacted to produce a significant amount of PUN,as indicated by a break in the potentialtime trace near the PUN reference potential, the stripping time for the PUN was added to the stripping time of Pu to arrive a t the total stripping time Tz. The stripping time for PUN was determined independently of that for pure Pu to avoid error due to double-layer charging. The time was measured by drawing tangents to either side of the inflections in the curve (Figure 1) and taking the intersection of the tangent lines as starting and ending times. After the emf was determined at one temperature setting, the temperature was raised or lowered by about 20'. At least 24 hr was allowed for the P U N reference electrode to reach a new equilibrium.
Results Since Nz is known to react with Pu over the range of temperatures of these experiments, the cell reaction is properly written as Pu(PuN)
+ '/2Nz +PUN
(VIII)
Ilowever, it is unlikely that the solubility of PUN in Pu is great enough to cause a significant difference in the emf a t these temperatures. For this reason the results will be treated as if the cell reaction were Pu(s, 1)
+ '/zNz(~)
--+
PuN(s)
(IX)
and the schematic representation of the cell is Pu(s, l)IPuCla, LiC1-KC1JN2(g),PuN(s) Pu(1) is of course saturated with Nz. The concentration of PuC4 in the three cells was 0.338, 0.166, and 0.112 mol %. The correction for the thermal emf of T a us. W was
E
=
-5.7240 X lo-*
+ 1.1966 X 10-'T + 2.9652 X 10+T2
(X)
(9) H.A. Laitinen, W. S. Ferguson and R. R. Osteryoung, J. Electrochem. SOC.,104, 516 (1957). Volume 78,Number 8 Februarv 1069
G. M. CAMPBELL
354 1.0
o .I ~
I
I
I
I
1
I
I
I
I
I
I
I
I
I
" L ,
I
0.5 900
920
940
960
980
TEMPERATURE
1000
1020
1040
1060
( O K )
Figure 4. Emf us. temperature above the melting point of Pu at 1 atm Nz for the cell Pu(1) IPuCls, LiCl-KC11 Nz(g), PuN(s).
where E is in volts and T is in OK. Since T a was the negative pole, the thermal emf was added to the measured galvanic emf. After correcting the Nzpressure to 1 atrn by using the equation
in the Pu-Ng reaction. This probably results because the surface of the PUN pellet is not in equilibrium with the center of the pellet at these temperatures. Using the data above the melting point of Pu -AGO,
the emf is plotted os. temperature in Figure 3. Y2 pressures measured were between 0.686 and 0.786 atm. The least-squares line is represented by the equation
E
=
7.90071 - 2.31690 X 104T
+ (XII)
with Tin "I
