2976
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
A.
Differential Thermal Analysis of the System NaF-Na,CO,
F. Koster
Van Groos
to 10 kbar
A. F. Koster Van Groos" Centre De Synthese et Chimie des Mineraux, 45045 Orleans Cedex, France (Received September 11, 1978; Revlsed Manuscript Received July 9, 1979)
The system Nal-Na2C03 was investigated between 600 and 1000 "C at 10 kbar pressure by DTA. The reaction NaF + Na2C03 L has a dT/dP = 10.3 f 0.6 "C/kbar. The eutectic composition shifts toward NaF, dX/dP = 0.45 f 0.1 mol %/kbar. -+
Introduction Much is known about the phase relationships in simple salt systems under atmospheric conditions. The effect of pressure on these systems, however, is rarely investigated. This is unfortunate, because information obtained at higher pressures may provide additional insight in various systems. In this paper a differential thermal analysis technique (DTA) is presented which may help to remedy this. The study of a salt system a t pressures up to 10 kbar was considered to be simple and straightforward. The system investigated, NaF-Na2C03, was selected because it appears to be close to ideal a t 1 bar pressure, and because it is of some geological interest, especially in relation to alkaline rocks and carbonatites. Technique The use of the internally heated pressure vessel (IHPV) for the study of relatively fast reactions by dynamic thermal methods has been sporadic. This is partially due to the small number of this type of apparatus in operation. Also, the experimental difficulties are severe. Usually the techniques are cumbersome, necessitating a great amount of labor for the accumulation of a few ~Iata.l-~This is unfortunate because the IHPV is eminently suitable for thermal studies up to 10 kbar pressure. For reactions in which only solid phases are present a fast and simple method for DTA under pressure was developed5 but this method could not be used in systems involving fluid phases. Therefore, the method was modified to allow DTA in closed systems under pressure. The DTA cell is shown in Figure 1. As in previously developed methods6 a thermocouple is located in a reentry well of a capsule. In the present method the DTA cell and the capsules have a highly reproducible geometry. This is achieved by using a DTA cell which can accommodate exchangeable capsules. The capsules were made by relatively simple extrusion and spinning techniques from gold, and recently also from platinum foil. The capsules are 10 mm long. The outside diameter is 3.2 mm, and the wall thickness is 0.1 mm. The reentry well is 2.5 mm deep, with a slight conical shape. It is made to fit standard ceramic double-bore tubing with an outside diameter of 1.5 mm. While these capsules easily hold 50 mg of sample (density 2.5), usually 25 f 0.02 mg samples were used. The capsules .were closed by using a drill chuck, and welded shut. Total length of the capsule is then about 5 mm. With no special effort the weights of the capsules were within 5% of each other. The hot junction of the thermocouple was made so that the bead was spherical with a diameter of 0.5 f 0.1 mm. Good thermal contact with the capsule was assured *On leave from the Department of Geological Sciences, University of Illinois, Chicago, IL 60680. 0022-3654/79/2083-2976$0 1 .OO/O
during loading of the DTA cell by inserting the thermocouple until electrical contact with the capsule was present. The DTA cell was made of copper in order to minimize temperature differences between sample and reference capsules. In the current setup two sample capsules and one reference capsule could be accommodated. The capsule wells of the cell were coated with TiOz before inserting the capsules, thus isolating capsules with respect to temperature. This is done to allow the temperature of the capsule to deviate slightly. After addition of the capsule the remainder of the well was packed tightly with a silica wool filler, and a cap was mounted on the cell. This is in order to prevent irregular temperature variations caused by convection of the argon pressure medium. The sensitivity of the cell is illustrated by the high-low quartz transition (AH= 0.290 kcal'). A 25-mg sample at a heating rate of 10 "C/min gave a 0.2 "C signal for the transition, which is better than 20 times the detection limit. The transition temperature was 572.5 "C, which agrees well with earlier work.5 Reproducibility of the signal area was within 10%. This indicates that quantitative DTA is very well possible. While the method provided excellent results, the system needs further perfection. Sometimes large differences in temperature occurred between the reference and sample junction, and no data could be obtained. This was probably caused by loss of thermal contact between the thermocouple and the capsules. Also, the system was rather sensitive to electrical noise, for it occasionally began to oscillate slightly. Furthermore, there is a problem of leaks developing during the run, especially along the reentry well of the capsule, where the wall is probably very thin. Therefore, only about 10% of the runs were considered successful. It is expected that the success rate will increase with experience and with further refinement of the equipment. A typical but idealized signal is shown in Figure 2 for a run with the composition 0.3 mol of NaF 0.7 mol of Na2C0, a t 6.2 kbar. It differs from a real signal because the baseline often has a steady drift with temperature. The heating and cooling rates were approximately 10 "C/min. The effect of the approaching solidus on heating is shown by the deviation of the base line at A. The temperature at A is usually considered meaningless. The peak is often fairly broad with respect to temperature, and a tangent can be drawn to the main part of the signal, generating point B. The temperature at this point is usually used as the reaction temperature. The curve shows a maximum, which when corrected for the temperature deviation of the signal generates point C. The baseline is slightly shifted past the peak, indicating both a change in the C, of the phases and the effect of continuously dissolving crystals. A t higher temperatures the baseline shifts again over a limited temperature range. The second inflection point,
+
0 1979 American Chemical Society
DifferentialThermal Analysis of NaF-Na,C03
The Journal of Physical ChemistIy, Vol. 83, No. 23, 1979 2977
TABLE I: Results from the Svstem NaF-Na.COP
TM,"C X
N
~ 1 bar
0.90
687.2 687.2 686.6 688.7 692.4
0.80 0.75 0.60 0.50
TO,"C
10khar
dT/dP
SD
n
1 bar
10 kbar
dT/dP
SD
n
790.3 789.2 790.7 789.3 7R9.1
10.31 10.20 10.41 10.06 9.7.5
1.6 0.6 1.3 2.0 2.6
4 6 5 5 I
945 898 883.2 804.2 742.7
1074 1023 1004 908.1 824.7
13 12.5 12.1 10.39 8.2
C C
2 2 2 4 7
817 858h
979.6 1024
15.2 16.4 16.6
0.12 0.00
d 5.1 2.2
g
1.,53 10'
1 4 6
The temperatures for the solidus, TM. and the liquidus, TD, at 1 bar and 10 kbar are calculated as the intercept of a linear regression of the data a t different pressures (n is the number of pressures investigated). The slope, dT/dP, in "C/khar, and the standard deviation, SD,is given also. Pistorius.s Poor D value, only good points a t 0.6 and 1 kbar. Mediocre D value, only good points at 0.1 and 6.9 kbar. e Poor D value, one poor point at 0.35 kbar, slope from E values. f Poor D value, one good point at 7.75 kbar, slope from E values. Poor D value, one good point at 5.75 kbar, slope from E values. Janz et al.,' Koster van Groos and Wyllie." Good point at 7.9 kbar.
'
sample
ins u I a tio n
thermocouple IO -re
\
filler
mm
1. Details of the copper DTA cell. Sample is preseni
goid capsules.
in sealed
D, probably indicates when all crystals are molten. Thus, D represents the liquidus. The total shift of the baseline represents ACJmixture). On cooling the reverse takes place. Usually considerable undercooling occurs before crystallization commences. Therefore, when crystallization does occur, at point E, a fairly large heat effect is detected. The break at E is always sharp. Undercooling of 30 "C is not unusual. A t point F crystallization of both phases takes place. Also, here the break in signal is sharp, indicating a degree of undercooling. The temperature of the eutectic reaction in the runs was determined as follows. It differs from TB,which is often considered as the reaction temperature. The change was necessary because the temperature at F, TF,was often higher than TBindicating that a t least for this setup TB cannot be the reaction temperature. Therefore, the reaction is assumed to occur arbitrarily at the temperature T, where T , = (Tc + TB)/2 or (Tc + TF)/2, whichever is larger. The difference between Tp and TBwas within 1 "C, however. The temperature measurements are considered to have an accuracy within 1OC; the pressure measurements are within 1% ?
Results In order t o determine the phase relations in this system compositions containing 90,80,75,60, 50,45,40,35,30,
Fhure 2. DTA curve in the system NaF-Na&O, at 6.2 kbar (A', = 0.30). On the heating cycle, A represents initiation of the eutectic reaclim: 8. the onset 01 reacibn: C. the peak temperature corrected lor An and D, the liquidus. On the cooling cycle, E represents the beginning of crystallization. note undercooling with respect to D and F, the eutectic reaction.
20, and 12 mol % NaF were investigated. The compositions were heated in the capsule to above the melting temperature of the sample in order to remove HzO, after which they were sealed. Temperatures in excess of 1000 OC were avoided after a copper DTA cell melted unintentionally, ruining the platinum furnace. For each composition, with the exception of the NazC08-12NaFcomposition, DTA data were collected of both the solidus and liquidus a t pressures up to 10 kbar. Each determination was repeated three times after which the average was taken. Reproducibility of the solidus temperature was within 1 "C. For the D values reproducibility was within 2.5 "C. The P-T curve in the pressure range investigated appear to be linear within the precision of the measurements. Therefore, a linear regression was made of the data. The intercepts at 1bar and 10 kbar were calculated as was the slope, dT/dP, and the standard deviation, SD. These values are presented in Table I. Using the SD as the uncertainty, we plotted the same values in Figure 3. Clearly, the solidus could be determined with good precision. The liquidus determination was more difficult. In several runs only one reliable D value has been determined. In such a case the d T / d P slope of the E values was used; in these cases an uncertainty of *15 "C was assumed. When only two D values were obtained, an uncertainty of
2978
The Journal of Physical Chemistry, Vol. 83, No. 23, 1979
A. F. Koster Van Groos
composition was determined by extrapolating the liquidus 1 boundaries to the eutectic temperature. The precision of the liquidus temperature determinations is not as good as those of the solidus. However, because both the NaF liquidus and the Na2C03liquidus must intersect at the solidus, the eutectic composition can be determined with reasonable accuracy. The eutectic liquid at 1bar contains 42 f 1mol % NaF; at 10 kbar it contains 46.5 f 1mol %. The value at 1 bar compares well with a value of 40 mol % determined by visual observation methods.ll These data indicate that the eutectic composition shifts with pressure toward the NaF composition with d X / d T = 0.45 f 0.1 mol %/kbar. This shift probably reflects the fact that the melting temperature of NaF increases less with pressure than the melting temperature of Na2C03. The thermodynamic implications of the results will be presented in a forthcoming paper. NaF
2o
40
6o
* O Na,CO,
Figure 3. The melting relations of the system NaF-Na,CO, at 1 bar (solid line) and 10 kbar (dashed line). The brackets show the standard deviation, SD, see text. The data indicated with dashed brackets indicate that they were extrapolated from data obtained at 0.6 and 1.0 kbar.
10 "C was assumed. Melting of NaF at 1 bar and 10 kbar occurs at 992 f 1 and 1121 f 3 "C, respectively, with no pressure correction applied; the P-T curve is not linear, d T / d P = 15.5 "C/ kbar at 1 bar and 11.5 "C/kbar at 10 kbar.8 Melting of Na2C03at 1 bar occurs at 858 0C.9 Between 1 bar and 1 kbar, d T / d P = 16 f 1 "C/kbar.lo At 10 kbar I found it to melt at 1024 f 10 "C. The latter value is not very accurate because, for some unknown reason, the data were poor, except at 7.9 kbar. Thus, over the 10-kbar pressure range, d T / d P = 16.6 f 1 "C/kbar. The eutectic temperature at 1 bar is the average of the intercept temperatures of the reaction at 1 bar (Table I). It lies at 688.1 f 3 "C; this compares well with a previous value of 690 "C.ll At 10 kbar it occurs at 791.0 f 3 "C. The results show that the P-T slope of the eutectic reaction is linear within the precision at the measurements; the value for d T / d P is 10.3 "C/kbar. When corrected for pressure effects on the thermocouple" the value d T / d P = 10.9 "C/kbar. Figure 3 shows the phase diagram at 1 bar and 10 kbar. The system is of a simple eutectic type. The eutectic
Conclusions The data presented here show that with a limited number of successful runs the phase relations in a salt system at elevated pressures can be easily resolved. Currently, the success ratio is low, but can be improved significantly with more experience.
Acknowledgment. This research has been supported by a grant from the National Science Foundation, NSF 7602853. References and Notes (1) H. S. Yoder, J. Geol., 59, 364-374 (1951). (2) R. I. Harker, Am. Mineral., 49, 1741-1747 (1964). (3) M. Rosenhauer, Carnegle Inst. Washington, Yearb., 75, 648-650 (1976). (4) D. H. Eggler and M. Rosenhauer, Am. J . Sci., 278, 64-94 (1978). (5) A. F. Koster van Groos and J. P. ter Heege, J . Geol., 81, 717-724 (1973). (6) S.P. Clark, J . Chem. Phys., 31, 1526-1531 (1959). (7) R. A. Robie and D. R. Waldbaum, Geol. Surv. Bull., 1259 (1968). (8) C. W. F. T. Pistorius, J. Chem. Phys., 45, 3513-3519 (1966). (9) G. J. Janz, E. Neuenschwander, and F. J. Kelly, Trans. Faraky Soc., 59, 841-845 (1963). (IO) A. F. Koster van Groos and P. J. Wyllie, Am. J . Sci., 264, 234-255 (1966). (11) A. G. Bergmann and V. V. Rubleva, "In Phase Diagrams for Ceramists", E. M. Levin, C. R. Robbins, and H. F. McMurdle, American Ceramic Society, Columbus, Ohio, 1957, p 509. (12) I. C. Getting and G. C. Kennedy, J . Appl. Phys., 41, 4552-4562 (1970).
