DECOMPOSITIOX PRESSURE A N D MELTING POINT OF THORIUM R~ONONITRIDE
methium. K. C. Jordan and his associates a t Rlound Laboratory perfonned a valuable service by performing the corroborative calorimetric measurements. Finally,
1223
the authors wish to thank Drs. R. L. Moore and W. C. Roesch under whose direction and encouragement this work was performed.
The Decomposition Pressure and Melting Point of Thorium Mononitridel
by W. M. Olson and R. N. R. Mulford CTniversityof California. Lo8 Alamos Scientific Laboratory, Lo8 Alamos, New Mezico
(Receiaed October 10, 1964)
+
Measurements of the decomposition pressure for the reaction ThN(s) = Th(1) l/zr\'z(g) in the temperature range 2416 to 2790' are described. ThN melts congruently a t 2790 f 30' under a nitrogen pressure somewhat less than 1 atin. The presence of T h o z as an impurity in the ThN was found to have a large effect on the melting point and decomposition pressure. The decomposition pressure -temperature relation for pure ThN is described by the equation log p (atin.) = 8.086 - 33,224/T 0.958 X 10-17T" ( T in OK.). Insofar as could be determined from X-ray lattice parameter data, ThX exists over little or no stoichiometry range.
+
Introduction As part of a continuing effort to determine the melting points and decomposition pressures of the actinide inononitrides, these properties of ThN were measured. Some experiments were also done in an attempt to ascertain if ThN exists over a range of stoichiometry, and the effect of oxygen on the melting point and decomposition pressure was examined. The decomposition pressure-temperature relation was determined by observing the apparent melting point as a function of nitrogen pressure. If T h S is heated in nitrogen, the nitride decomposes to liquid thoriuin and nitrogen gas when the temperature is such that the decomposition pressure exceeds the ambient nitrogen pressure. Since this reaction proceeds very rapidly, decomposition is easily detected by observing the apparent melting of the solid sample. Thus, measurement of the apparent melting point as a function of nitrogen pressure gives the equilibrium pressuretemperature relation for the decomposition of ThK. If the ambient nitrogen pressure is sufficiently high, ThN melts congruently.
Relatively little work on thorium mononitride has been reported in the literature. Chiottiz reports a melting point of 2630 f 50" in a helium atmosphere, but thorium metal was found in the sample after solidification, which indicates that deconiposition of the nitride occurred. Therefore his observed melting point would be expected to be lower than the coiigruent melting temperature. Chiotti also reports a lattice parameter of a = 5.195 A. for the face-centered cubic ThN before melting, and a = 5.144 -1.after melting. Street and Waters3 report a lattice paraineter of 5.159 which is in agreement with the present work.
K.,
Experimental Apparatus. The apparatus and general technique used have been described p r e v i ~ u s l y . ~Briefly, the (1) Work done under the auspices of the U. S.Atomic Energy Commission. (2) P. Chiotti, J . Am. Ceram. Soc., 35, 123 (1952). (3) R . S. Street and T. N . Waters, Document AERE M-1115; Atomic Energy Research Establishment, Harwell, England, 1962.
Volume 69, Number 4
April 1965
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ThN sample, in granular form, was heated in a 30" vee formed in the middle of a resistively heated tungsten strip within a vacuum-pressure vessel. The sample was observed and temperatures were measured with a calibrated Pyro microoptical pyrometer sighted through a prism and quartz window located a t the top of the vessel. Pyrometer readings were corrected for window and prism absorptions and also for the emissivity of the tungsten ~ e e .The ~ absolute accuracy of the temperatures reported is estimated to be f 30". The reproducibility is a t least f 10". Preparation of ThN. Thorium nitrides can be made directly from the elements or by reaction of the hydride with nitrogen or ammonia. The product of any of these reactions below 1500" is ThzN3. When ThzN3 is heated under vacuum above about 1500", ThN is formed as a golden yellow powder, which reacts readily in air to form T h o z . The ThN used in the present studies was prepared in several ways. For most of the experiments it was made by induction melting crystal bar thorium in a tungsten crucible under about 2 atm. of spectroscopically pure nitrogen. At first the temperature was increased rapidly to 2000". Then, after holding for about 15 niin. at 2000", the temperature was increased until melting occurred. The Thn' ingot thus formed was removed readily from the crucible by breaking the tungsten away from the ingot. The ingot was a golden yellow color when first removed from the furnace, but within seconds a black surface film formed. Made in this way, the ThN contained 400 p.p.m. by weight of oxygen, whereas the crystal bar thorium starting material contained 130 p.p.m. Ingots of 'ThN made by the induction melting process were stored under vacuum until needed, and then portions of the ingots were powdered in an argon atmosphere glovebox. A supply of the powder was transferred under argon to the vacuum can of the melting point furnace for storage during the runs. Argon was used to break the vacuuni after each experimental run and was allowed to flow through the melting point furnace during loading. Samples were removed from the storage cup without removing the cup from the furnace. This procedure reduced contact between air and the reactive nitride powder. In an effort to be certain that the nitride contained no oxygen, some of the samples were made in the melting point apparatus in the following manner and their melting points determined in situ without exposing the samples to air. A few thorium granules were placed in the tungsten vee and heated rapidly under nitrogen to the expected melting point of the nitride. Reaction between the metal and the nitrogen was so The Journal of Physical Chemistry
W. n4.
OLSON AND
R. N. R.
MULFORD
rapid that the metal did not melt or vaporize appreciably. Melting point results obtained with this in situ material were essentially identical with those obtained from the induction-melted nitride. Larger amounts of thorium nitride were also made in the tungsten vee, but the time of heating was usually extended to about 1 hr., and the temperature employed was between 1500 and 1800". I n this procedure the nitrogen was pumped out after reaction, and the product was held a t the reaction temperature under vacuum to decompose any Th2N3present. The use of spectroscopically pure nitrogen yielded ThN with no ThOz detectable by X-ray methods, whereas samples made with commercial "prepurified" nitrogen contained 5-10 V O ~ % . ThOz. I n a number of experiments, hot pressed ThN samples were used. These were made by hot-pressing, in a graphite die, T h N made from hydride and nitrogen, and then firing the resulting compacts a t 1800" under 3 atm. of nitrogen, after their surfaces had been machined to remove carbon contamination. The final product was found to contain 0.55 to 0.72 wt. % oxygen by fusion analysis. -Micrographic examination revealed small amounts of a dark phase presumed to be Thoz, and X-ray diffraction patterns confirmed the presence of Thoz. Thorium nitride was also prepared by arc melting thorium metal under approximately 0.8 atm. nitrogen pressure. This procedure, however, resulted in incomplete conversion; the product consisted of ThN dendrites in a thorium matrix. Procedure. The decomposition experiments were done in the following manner. Approximately 5 mg. of powdered ThN was loaded into the tungsten vee, the apparatus was evacuated, and the sample was outgassed under vacuum a t about 800" for a few minutes. Then, the desired pressure of spectroscopically pure nitrogen was admitted to the apparatus. The temperature was increased rapidly until the expected deconiposition temperature was nearly reached and then was increased more slowly until the sample melted. Pyrometer readings were taken before and after melting, and the actual melting temperature was assumed to be the average of the two values. Usually the before and after readings differed by less than 10". Pressure measurements were also made before and after melting, and the true decomposition pressure was assumed to be the average of these two values.
Results 1. Decomposition Pressure and Congruent Melting (4)
W. M. Olson and R . N. R . Mulford, J . Phys. Chem., 67, 952
(1963).
DECOMPOSITION PRESSURE AND RIELTIKG POINTOF THORIUM IONO ON IT RIDE
I
n
10-
.r
~
I
W
a
a
1
0.001 3.10
I
1
1.
3.20
3.30
3.40
3.50
3.60
3.70
IOOOOIT-~K-'
DECOMPOSITION PRESSURE OF T I N
Figure 1. Log decomposition pressure of ThN us. reciprocal of temperature: circles, piire T h N ; triangles, ThN contaminated with oxygen.
Point. In Figure 1 the decomposition pressures obtained for ThN are plotted as log p (atm.) us. 1O1000/T (7' in OK.). The solid circles represent data obtained with oxygen-free nitride niade either by the induction heating method or the in situ process. Although the data obtained from the oxygen-free nitride are considerably scattered, it is apparent that the points fall along a gentle curve which intersects the vertical line representing congruent melting. Because of this scatter and the curvature near the intersection, it is difficult to determine accurately the niinimum pressure under which congruent melting occurs, but it appears to be less than 1 atm. The congruent melting temperature of oxygen-free ThN was found to be 2790 f 30". 2. Iiffect of Oxygen. The data plotted as triangles in Figure 1 were obtained with the hot pressed ThN stock. Most of these points fall along a line about 130" lower than that passing through the data obtained with oxygen-free nitride. Since the hot pressed nitride contained 0.6 wt. yo oxygen, compared to 0.04 found in the "oxygen-free" material, it seems obvious that oxygen lowers both the congruent melting point and the decomposition temperature. In an attempt to
1225
confirm this conclusion, samples containing approximately 0.2, 0.6, 0.8, 1.2, and 2.4 wt. yo oxygen, prepared by mixing powdered ThN with ThOz, were run. Most of the data points thus obtained fell along the lower temperature line of Figure 1. Evidently, 0.2 wt. yo or less of oxygen is sufficient to lower markedly the decomposition temperature of ThN, and greater amounts of oxide have little additional effect. According to Lambertson, Mueller, and Gunzel16 the melting point of ThOz is 3220 f 50". Therefore a solid solution apparently is not formed between ThN and ThOz, as one would expect the solid solution melting point to increase with increasing proportion of T h o 2instead of decreasing as observed. Table I gives lattice parameters and other pertinent data for the ThN preparations. By comparing samples 2 and 4 it is seen that there is no large change of lattice parameter with oxygen content, and hence there is probably very small solubility of oxygen in ThN. Micrographic examination of the ThN-ThOz mixtures after melting suggested that a eutectic formed between the two compounds and that less than 0.2 wt. % oxygen was sufficient to produce the eutectic. Eutectic formation thus appears to be the most likely explanation for the observed lowering of the decomposition temperature. 3. Stoichiometry of T h N . As oxygen has little or no effect on the lattice parameter of Thh', it should be possible to determine whether ThN has a significant stoichiometry range by studying its lattice parameter as a function of nitrogen content. Samples 1 and 8 in Table I contained a thorium phase and were heat treated a t 1800 and 1000", respectively. The ThN in these samples thus had a composition on the thorium-rich boundary of the ThN single-phase region. Samples 3, 5 , and 7 were heat-treated under nitrogen and are presumed to have had compositions on the nitrogen-rich phase boundazy. There appears to be a difference of about 0.001 A. in the lattice parameters of the two sets, which we interpret as evidence that ThN exists over only a very small range of composition, if any.
Discussion It is not possible to determine the standard heat of formation of ThN from the decomposition pressure data because the liquid thorium present in equilibrium with the nitride is saturated with nitrogen and has an unknown activity less than unity. The solubility undoubtedly varies with temperature. At higher tem(5) W. A. Lambertson, Af. H . Mueller. and F. H . Gunzel, Jr.. J . Am. Ceram. SOC.,3 6 , 397 (1953).
Volume 69, Number 4
April 1965
Table I : Lattice Parameters for T h N Sample no.
T h N lattice parameter, A
Method of preparation
Condition
Induction melted Induction melted Hot pressed Hot pressed In sztv, spectro XZ In S Z ~ spectro , XZ Induction melted Arc melted in 0.7 atm. of Nz In sztu, "prepurified" N1
1800" under vacuum, T h phase present As prepared Melted in 2 atm. of NZ As prepared Melted in 1.5 atm. of Nz Melted in 0.7 atm. of Nz, held at 2000' under vacuum 1000", 4 days in 0.92 atm. of NZ 1000", 4 days under vacuum, T h phase present, As prepared
peratures more nitrogen will be dissolved in the liquid thorium, lowering its activity, and therefore the log p us. 1 / T plot would be expected to show some curvature, as indeed it does. The curvature might also be attributed to a teniperature variation of composition of the ThN phase. Although results described in the preceeding section suggest that the ThN phase does not vary in 'composition, it should be noted that a large stoichiometry range at high temperatures has heen reported for UK.6 Similar experiments to those reported here have been done on UN,4 and similarly, by the lattice parameter criterion, it was concluded that UN was stoichiometric. If it is assumed that the measured data are for a reaction in which the activity of the liquid thorium and perhaps also the Thh' activity are less than unity, and that these activities approach unity as the temperature decreases, then the lowest experimental point can be used to calculate a limit. for the standard free energy of forination of ThN. This can be done by combining an estimated standard entropy of formation with this experimental value. The lowest experimental point is P x 2 = 1.5 X 10-3atm. at 2703°K. ASrozss is taken as -20 ea1 deg.-' mole-' and AC,,' is assumed to be 1.5 cal. deg. mole-l. The conclusion from these calculations is that the standard heat of forination of T h S at 298°K. is more negative than -73 kcal. mole-'. Calorimetric nieasureiiients of the heat of formation of thorium nitride have been made by Neumann and * A direct combination of thorium and co-w~rkers.~ nitrogen a t 970" gave products that ranged in coniposition from ThN, to ThX, $ and a heat of 77.1 kral./g.-atom of N. Which thorium nitride was actually obtained is questionable, but when a sample that analyzed T h N l 32 was burned in oxygen, almost the same value was obtained (77.8 kcal./g.-atom of N); therefore it appears that the heat of formation of ThnX3was nieasured in the direct combination experi-
The Journal of Physical Chemiatry
5 5 5 5 5 5 5 5 5
1608 f 5 1597 f 7 1585 i 5 1582 f 3 1585 f 3 1583 f 3 159 f 1 159 f 1 1590 i 5
Oxygen content, wt. %
0 04 0 04 0 6 0 6 No ThOz by X-ray No ThOz by X-ray 0 04 No T h o I by X-ray 0 6-1.2 by X-ray
ment. If this is correct, then the standard heat of forniation of thorium mononitride is at least equal to and probably more negative than - 77,400 cal./mole, the average of the two determinations of Neuniann, et al. There appears to be no reliable way to estimate the difference in heats between ThN and ThzN3. In previous work with UN3 and PUS,$it was found that an empirical equation of the form log p = A B/T CT5 fitted the experimental points well. The coefficients A and B were proportional to the standard entropy and enthalpy changes at temperature, respectively, and the CT5 term allowed for deviation of the liquid nietal phase and possibly also the nitride from unit activity. The same forin of equation was fitted to the present data. A was coniputed froni the estimated entropy and heat capacity. In the present case we do not have a heat of formation value from which to compute the coefficient B , and therefore both the B and C coefficients were obtained by fitting to the experimental data instead of fitting just the C coefficient, as was done in the previous work. The resulting expression
+
+
log p (atni)
=
8.086 - 33,224/2'
+ 0.958
X 10-17T5
can probably be used with reasonable accuracy to extrapolate the data at least down to the melting point of thorium. Acknowledgment. We wish to thank F. H. Ellinger for the X-ray data reported herein and Maynard E. Smith for the fusion analyses for oxygen. (6) R. Bena and M.G. Bowman, paper presented at 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. (7) B. Neumann, C. Kroger. and H. Haebler, Z . anorg. allgem.
Chem., 207, 145 (1932).
Neumann, C. Kroger, and H. Kune, ibid., 218, 379 (1934). (9) W. AI. Olson and R. N. R. Mulford, J . Phys. Chem., 68, 1048 (1964). ( 8 ) B.
