VAPORPRESSURE OF THE NaF-BeF SYSTEM
April, 1958
453
VAPOR PRESSURES AND MOLECULAR COMPOSITIOY OF VAPORS OF THE SODIUM FLUORIDE-BERYLLIUM FLUORIDE SYSTEM1 BY KARLA. SENSE'"AND RICHARD W. STONE Battelle Memorial Institute, Columbus, Ohio Received December S,1967
A new study made on the vapor pressure of BeFz over the temperature interval 802 to 1025' showed good agreement w.ith previous data taken over the 802-968" range. Excessive scatter of the vapor pressure data In the lower temperature reglon melting prevented a melting point determinat,ion of BeF2 by the transpiration method alone. Thermal analysis showed On the point to be about 545". Vapor pressures of the NaF-BeF2 system were measured over the range 509 to 1061 basis of previously developed theory, it was concluded that the complex NaBeFa exist,s in the vapor phase in addition to NaF and BeF2. Resolution of the vapor phase on this basis indicated that another complex might exist in the vapor phase. A plot is presented showing the change of total pressure with composition for various temperatures.
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Introduction This work is a continuation of the physical properties of fused-salt systems which was started with the investigation of the NaF-ZrR system. Experimental The method and apparatus used have been adequately described previo~sly.2-~The only change from previous procedure was in improved oxygen removal from the argon which was used as a carrier gas. This was accomplished by passing the argon over heated uranium chips after most of the oxygen had been removed by heated copper turnings. The fused-salt mixtures were supplied by the hlound Laboratory a t Miamisburg, Ohio, and the Oak Ridge National Laboratory a t Oak Ridge, Tennessee. In most cases, the composition of the salt mixtures. changed somewhat during the runs because of the preferential vaporization of the more volatile component. Corrections were made for this effect, and the average compositions are listed in the various plots with the proper curves.
TABLE I VAPOR PRESSURES OF BeF2 Flow rate of carrier Temp.. OC.
Pressure,a mm. Calcd. Obsd.
Deviation, %
gas, cm.a/
min.
8.5 108.6 - 2.2 1025 106.2a 11.5 86.3 3.0 1010 88.9 9.0 63.1 +11.6 70.4a 989.6 11.2 49.8 +1.8 50.7b 974.8 10.0 39.4 +2.4 40.4b 960.5 14.9 30.9 $2.4 31.6a 945.9 18.9 24.3 +1.1 24.4a 932.0 21.2 24.3 -9.0 22. l b 932.0 19.9 18.8 -3.1 18.2b 917.4 20.9 18.8 -2.1 18.4a 917.4 23.3 13.93 -0.5 13.86a 900.7 23.6 11.37 -1.1 11.25a 889.7 23.0 11.17 -2.8 10.85 889.1 20.2 9.06 -5.2 8.59b 877.6 20.9 6.83 +1.6 863.0 6.94a 31.1 5.32 $3.1 5.48 850.3 31.2 4.28 -5.3 4.05a 839,5 30.8 4.15 $2.9 4.2713 838.0 32.4 3.20 +8.5 3.47 825.4 40.1 2.51 +7.4 2.70 813.9 41.1 1.95 +5.2 2.05 802.2 a, pressure based upon average of two analyses; b, pressure based upon average of three analyses. 10943 log p (mm.) = 10.466 - - -(2) T,OK. extrapolated boiling point = 1170' (3)
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Results and Discussion The Vapor Pressure of BeFz.-The vapor pressure of BeF2 had been measured some time Because the apparatus and techniques in measuring vapor pressures have been improved since publication of that paper, another set of measurements was made. For temperatures greater than about 800°, relatively little difficulty was experienced in obtaining reasonably consistent data (see Table I). The results compare favorably with those given in the previous paper.3 Figure 1 is a plot of the recent measurements and the best curve6 obtained, as well as the best curve obtained for the previous set of data. The new vapor pressure curve has a someBecause of increasing scatter of the vaporwhat flatter slope, resulting in a heat of vaporiza- pressure data below 800" the melting point of tion value of BeFz could not be redetermined. Thermal analysis showed the melting point of BeFz to be about AHvnporiEstion = 50.1 kcal./g. mole (1) 545". This is considerably lower than the melting as compared with the old value of 50.9 kcal./g. point derived from previous vapor pressure data, mole. At l/T("K.) = 7.8 X the new vapor and is in agreement with the results obtained by pressure curve yields a value about 10 per cent. Roy, Roy and Osborn.6 lower than the previous one, while at l/T(OK.) Since the scatter of the vapor pressure data of = 9.3 X i t is only about 4 per cent. lower. BeFz increased for lower temperatures an attempt The revised vapor pressure equation which holds for was made to obtain data a t lower temperatures for the temperature interval 802 to 1025" is the 26 mole yo NaF-74 mole yo BeF2 composition. ( 1 ) Work performed under AEC Contract W-7405-eng-92. The results obtained over the 509 to 977" range ( l a ) Atomics International, Canoga Park, California. are given in Table 11. When these data are plotted, (2) K . A. Sense, C. A. Alexander, R . E. Bowinan and R. B. Filbert, some scatter is evident below G35", although excesJr., THISJOURNAL, 61, 337 (1957). sive scatter starts only below 535'. A break in the (3) K . A. Sense, M. J. Snyder and J . W. C l e m ibid., 68, 223 ( 1954). curve is not evident above that temperature. (4) K. A. Sense, M. J. Snyder and R. B. Filbert, Jr., ibid., 68, 995 Why the data scatter for pure BeFz and not for (1954). (5) In the least-squares treatment, the data were weighted, since in some instances two or more analyses were made for a single run.
(6) D. M. Roy, R. Roy and E. F. Osborn, J. Am. Ceram S o e , 36, 185 (1953).
KARLA. SENSEAND RICHARD W. STONE
454
I
1050
I loo0
Reciproc 01 Temperature,& 1 TO
