VAPORPRESSURE OF PALLADIUM
2899
The Vapor Pressure of Palladium
by P. D. Zavitsanos Missile and Space Division, General Electric Cornpang, Philadelphia, Pennsylvania
(Received March 20, 1964)
The vapor pressure and heat of vaporization of palladium were measured using a recording microbalance with the Knudsen technique. Over the temperature range of 15371841”K., the measured vapor pressure inay be presented b y : log Pat, = 5.99 - 18,898/ T . The mean heat of sublimation a t 298°K. is 89.8 i 0.9 kcal./mole and the normal boiling point was estimated to be 3150 =k 100°K.
The quantitative studies of the vaporization of pa’ ladium were rather unsatisfactory until Dreger and Ilargrave’ measured the rate of sublimation in the temperature range 1220-1640°K. using the Langmuir technique. Using the same technique in the temperature range 1294-1488’K , Hampson and Walker2 produced data that agreed quite well with Dreger and Margrave’s work. The only Knudsen effusion work on palladium was done by Haefling and This work is in wide disagreement with the Langmuir results. The vapor pressure reported by these authors was one order of magnitude higher and the second-law heat of sublimation 10 kcal. lower.
Experimental The apparatus used combines the Knudsen target technique with continuous microbalance recording. A detailed description of the technique will be given elsewhere.4 The sainplc is heated by electron bonibardment in a 2 x 2 cm. Knudsen cell while a well-defined fraction of the effusing vapor is condensed on a collector. The collector mas made from molybdenum sheet rolled to a conical shape. The weight gain of the collector was followed by a Sartorius-Electrona microbalance and plotted by a recorder. A collimating slit of 0.325-cm. radius was placed 1 . 7 cin. above the cell orifice. The total rate of effusion, m, in g . / c i n 2 see. is obtained from the observed condensation rate in’ (g./set
m
=
7n’(C2
+ R’2)/R‘2A
where R’ is the radius of collimating slit, C is the distance of the collimating slit from the cell, and A =
0.0126 c m 2 , is the area of effusion hole. (The fraction of the effusing vapor that went through the slit and condensed on the target was about 0.0353.) The vapor pressure was calculated from the Knudsen equation log Patm= log m
+
‘/2
log T -
log M - 1.647
where in is the rate of effusion in g. cm.-2 set.-' and 14 is the average molecular weight of the effusing vapor. I n our calculations the nionatomic species was considered as the only important species. The sample was obtained from Fisher Scientific Co. and was the “purified P d black” type. The Knudsen cell was made out of tungsten because tungsten metal will not react with palladium. The rate of effusion was measured mainly by continuously recording the weight of the target during the vaporization process. Several runs, however were carried out where the rate was obtained by weighing the target before and after the run with the furnace cold. The temperature of the cell was kept constant with an eniission regulator and was measured by focusing an optical pyrometer on a blackbody hole drilled on the side of the cell. The pyrometer was calibrated against an S B S lamp and all readings were adjusted for window corrections. I n the recording technique, the apparent weight of the target was influenced (reduced several micrograms) (1) L. H . Dreger and J. I,. Margrave, J . Phys. Chem., 64, 1323 (1960).
(2) R F. Hsmpson and R. F. Walker, J . Res. .VatZ. Bur. S t d , 66A, 177 (1962). (3) J. F. Haefling and A. H . D a m e , Trans. AIME, 212, 115 (1958) (4) 1’. D . Zavitsanos, Reu. Sci. Instr., 35, 1061 (1964).
Volume 68, Number 10
October, 1964
P. D. ZAVITSANOS
2900
by the momentum transfer of the vapor. This effect, however, should not affect the validity of these results since the momentum transfer exerts a constant force for constant temperature and this technique is only concerned with differences in target weight.
Results and Discussion The results of the vaporization studies on palladium are summarized in Tables I and 11. Rates of effusion were measured (1) by continuously weighing the target, Table I, whereby the corresponding weight changes were only a few micrograms and (2) by weighing the target before and after the run (with the furnace cold), Table 11, where large changes in
log
18,425 5.698 - T
Pktm =
is shown in Fig. 1 in comparison with the previous work at lower temperatures. Combination of the vapor pressures with the free energy functions from Stull and Sinkc5produced a n average third-law heat of vaporization a t 298"K., AHo2g8 = 89.8 f 0.9 kcal./mole. -4
-5
HAEFLING e DAANE
-60
R I
Table 1: Vapor Pressure Data on Palladium Metal-Continuous Weighing
a ! ? I
Rate X 108, g. min. -1
-log P ,
T,OK.
atm.
AH'zQ~
1832 1841 1587 1623 1624 1674 1721 1720 1702 1701 1649 1592
17.5 16.5 0.23 0.63 0.50 2.50 3.60 3.34 2.25 2.55 1.60 0.233
4.22 4.25 6.124 5.69 5.792 5.086 4.92 4.96 5.13 5.07 5.28 6.227
88.93 89.60 91.38 90.02 90,83 88.13 89.24 89,45 89.95 89.42 88.33 92,22
Table I1 : Vapor Pressure Data on Palladium Metal-"before and after" Weighing T,OK.
g. m h - 1
1792 1698 1649 1806 1537 1543 1570
5.35 2.08 0.861 5.83 0.242 0.183 0.456
4.74 5.16 5.55 4.61 6.12 6.24 5.84
DREGER e MARGRAVE
-0
-
.5 4
I
.56
I
I
I
I
I
I
I
-50
.60
.62
.64
.66
.60
.70
l
.72
I
l
.74
.76
.70
1000 / T OK
Figure 1. The vapor pressure of palladium.
The third-law method givcs a AH"298 = 89.8 f 0.9 kcal./mole. If one uses this value of AH"298 and the fef functions, the vapor pressure is represented by
*
and a boiling point of 3150 100°K. is calculated. was 86.7 kcal./mole. The second-law value of These results are in disagreement with the work of Haefling and Daane3 and Walker, et u Z . , ~ but in good agreement with Dreger and Margrave, where log
6.195 -
Patrn =
AH'm
91.35 89.91 90.36 90.96 88,38 89.74 88.21
AHozg8= 91
kcal./mole
log
Patm
=
0.8 kcal./mole
=
5.869 -
89.2
&
18,655 -__ T
0.8 kcal./mole
Additional confidence in these results is gained from ~
weight (several hundred micrograms) are taking place. It is apparent that the results are essentially the same. A least-squares plot of the data, described by the equation
&
19,425 T
I _
and Hampson and Walker,2where
AH02ss (av.) = 89.84 f 0.91
The Journal of Physical Chemistry
GROSS eWALKER
kcal./mole
-log P. atm.
-
HAMPSON 5 WALKER
AH02m (av.) = 89.79 f 0.90
Rate X 106,
-7
~~~
(5) D. 12.Stull and G. C . Sinke, "Thermodynamic Properties of the Ektnents," Advances in Chemistry Series, N o . 18, American Chemical Society, Washington, D. C., 1956. (6) R. F. Walker. J. Efimenko. and N. Lofgren, "Proceedings of the Conference O J ~ Physiciil Chemistry in Aerodynamics and Space Flight,'; John Wiley a n d Sons, Inc.. New York, N. Y . , 1961.
the work of Alcock and H ~ o p e r . Tjsing ~ the transpiration method, they reported the vapor pressure of palladium a t two temperatures, 1673 and 1773’K., log P,,,, = -3.34 and - 4.61, rcsprctivcly. l’roin t h r observed agrrcinent between these results aiid thosc obtained by the Langmuir and transpiration techniques, oiie can safely conclude: (a) Thc vaporization cocficicnt of Pd is close to unity. The calculated heats (89.79, 89.2, and 91.0) are in agreement within the experimental error of +0.8-0.9 kcal. With this kind of uncertainty, it is rather difficult to say whether there is a small activation energy associated with the Langmuir results. (b) The high vapor pressure values ob-
tained in the other Kiiudsen work could be due either to additional effusion through cracks or holes in the carbon cell, or to volatile impurities resulting perhaps froin the reaction between I’d aiid carbon as suggested by Dreger and l‘largrave.’ ilchnuwledgnients. The author wishes to acknomledge the assistance of Alr. G. R. Brownlee in obtaiiiing the experiniental data. This research was sponsored by the Ballistic Systems Division, USAF, Contract NO. AF’ 04(691)-222. ~
~
(7) C . B. Alcork :uid G. W. Hooper, Proc. R o y . SOC. (London), A254, 559 (1960).
Steady-State Radiolysis of Gaseous Oxygen’
by Kenji Fueki and John L. Magee Department of Chemistry and The Radiation Lnboratm uv C‘niwrsity of S o t r e Dame, h‘otre Dame, Indiana (Recriaed March SO, 196.4)
Gaseous oxygen under steady irradiation maintains a rather small amount of ozone, only a few parts per niillion, depending upon the pressure, rate of irradiation, etc. The hypothesis that the ozone concentration is limited by a negative ion-niolecule chain decomposition (eq. 4 and 5 of the text) is considered in detail and found to be consistent with the known facts. At the higher gas densities where track effects must be considered the “sharpboundary” method is employed.
1. Introduction I t has long been knownL that although the initial G value for 0 3 production in O2 is relatively high, 8-12 under various conditions, the stationary concentration ORin irradiated O2 is very low, only a few parts per million. I t has been suggrsted that a chain reaction for the drst,ruction of O3exists. The authors previously proposed3 a negative ion-molecule chain for the destruction of ozone and subsequent consideration has tended to corroborate this view. This paper presents a treatment of the oxygen system under steady irradiation in a further attempt to establish the quantitative aspects of the problem. There arr potentially two types of coniplications in
the explanation of the chemical action of high energy radiations. In the first place, a fairly coniplicated sequence of cheinical reactions iiiay be involved, and in the second place, track effects niay require explicit consideration. The authors have presented a detailed study3 of initial G values in the radiolysis of gaseous oxygen taking both coniplications into account. A digital coniputer was used t o solve the ten coupled ( 1 ) T h e Radiation Laboratory of t h e University of Notre Danre is operated under contract with the U. S. Atomic Energy Commission. ( 2 ) J. F. Kircher, J. S. M c N u l t y , .J. I,. hIcFarling, a n d A. I,evy, Radiation Res.. 13, 452 (1960). (3) K . Fueki a n d J. I,. Magee, Discussions Faradav Soc., 36, IS (1963).
Volume OR, Number 10 October, 186.4
