The high-temperature enthalpy of liquid lanthanum by levitation calorimetry

J. Chem. Thermodynamics 1975,7, 83-88

The high-temperature enthalpy of liquid lanthanum by levitation calorimetry LAWRENCE A. STRETZ and RENATO G. BAUTISTA Ames Laboratory- USAEC and Department of Chemical Engineering, Iowa State University, Ames, Iowa 5OOI0, U.S.A. (Received 14 January 1974; in revised form 29 April 1974)

A levitation calorimetry technique was used to measurethe enthalpy of liquid lanthanum from 1250 to 2420K. Lanthanum samplesranging from 0.5 to 2 g were levitated and melted in an inert atmosphereusing a 15kV A, 450kJ!Xzradio-frequency generator.The results, in the temperaturerange 1250to 2420K, were fitted by the following equation where the indicated errors were obtained from the averagedeviation of the results from the values predicted by the equation: (H(T)--H(298.15 K)}/J mole1 = (32.77&0.18)(T/K)-(2289f343). Convection and radiation heat lossesduring the fall of the sample from the levitation chamberinto the calorimeter amounted to 0.5 to 2.0 per cent and 2 to 5 per cent of the total enthalpy respectively. The levitation calorimetry method is estimated to have a maximum error of f2.5 per cent.

1. Introduction One of the main difficulties encountered in high temperature calorimetry is providing a container for the sample which will not react with or dissolve in the sample during the time that it is at high temperature. Previous work(i-3) extending into the liquid range with ordinary resistance or induction heaters points to the advantage of eliminating the sample container. The calculation of the enthalpy change from calorimetric data must include corrections for the capsules which encasethe samples and for any reactions of the sample with the capsule. One case is notedc4) where tantalum was used as a capsule material for samples of scandium, yttrium, and heavy lanthanide metals. Small amounts of tantalum dissolved into the liquid samples, necessitating a correction for the enthalpy of solution. The levitation melting technique employed in this work eliminates the need for a crucible and is adaptable to an inert atmosphere, thus eliminating sample-contamination problems. With this technique, measurementsin the range 1500 to 3000 K are feasible. Levitation melting has been successfully coupled with calorimetry in the determination of the enthalpy at high temperatures of copper and platinum by Chaudhuri et a1.“’ and for copper, platinum, niobium, and zirconium by Bonnell.@) Treverton and Margrave (‘,*) have reported utilization of the technique with samples of cobalt, palladium, iron, titanium, and vanadium. Stretz and Bautistacg9lo) have reported work on yttrium metal in the range 1800 to 2360 K using this technique.

84

L. A. STRETZ AND R. G. BAUTISTA

2. Experimental EQUIPMENT

A full description of the levitation calorimeter utilized in this work has been given by Stretz and Bautista. (‘, lo) The basic components of the apparatus include a drop calorimeter, modified to accept a sample dropped from a glass levitation chamber and a levitation melting setup employing a high-frequency levitation coil. The coil power supply was a Lapel 15 kV A, 450 kHz radio-frequency generator coupled to the coil through a matching transformer. Temperatures of the levitated sampleswere determined from brightness temperatures measured by a Leeds and Northrup automatic optical pyrometer. Calorimeter block temperature was monitored by a Hewlett-Packard Quartz Thermometer. EQUIPMENT

CALIBRATION

A measured quantity of electrical energy was dissipated to the calorimeter block through a calibration heater and the temperature response of the block was followed. An uncorrected calorimeter constant of 2025.358 J K-l was obtained, in good agreement with the predicted value of 2030.210 J K-l from the approximate mass of the calorimeter block. Corrections for the presence of material which would be removed before operation of the calorimeter and deduction of the receiving-well contribution resulted in a calorimeter-block constant of 1976.680J K-’ at 298 K. The electrical calibration of the calorimeter was checked by running samples of liquid copper over a temperature range from 1358 to 2061 K.(‘> lo) These results are in excellent agreement with the data of Kelley@‘) and of Bonnell et aLc51Q PROCEDURE

The high-purity samples of lanthanum were prepared(“) in the form of 6.35 mm diameter cylinders, approximately 6.35 mm in length, at the Ames Laboratory of the US. Atomic Energy Commission. A complete analysis of the approximately 99.98 moles per cent pure lanthanum is given in table 1. The samples were electropolished and stored under a non-reactive atmosphere to prevent oxidation. TABLE 1. Impurities in lanthanum metal samples expressed as mole fractions x or mass fractions w

Al Si Cl Ti Fe

106x t3 lo” <2 6

Ni cu Ta W Y

106x 9 1 7 t2 7

Ce Pr Nd Gd Tb Others

106 w

106 w H N 0

Initial 3.4 15 30

106X 9 6 1.6 2.7 10 <7

Final a 73 56 636

C F

LLVacuum fusion on samples subjected to run conditions.

37 13

ENTHALPY

OF LIQUID

LANTHANUM

85

For each run an individual sample was loaded into the levitation chamber and the system was sealed and flushed with inert gas to provide a non-reactive atmosphere. The sample was levitated and heated to the desired temperature and then dropped into the calorimeter receiving well which was lined with tantalum foil to prevent reaction of the sample with the calorimeter. Results from each run were reduced as previously described by Stretz and Bautista,‘gylo) including correction for loss of heat from the sample by radiation and convection during the drop. The normal spectral emissivity of lanthanum has been reported by Stretz and Bautista’13,14’ to be (0.282rtO.012) in the liquid range. The runs on lanthanum were made using a solenoid-type coil wound on a 9.525 mm former having a gap between the upper and lower sections of approximately 6.35 mm. The levitation coil has three turns in the lower section and three reverse turns in the upper section. The coil produced stable levitation of the solid lanthanum but caused rapid heating of the sample. As the sample melted the levitation became unstable, requiring an increase in the applied power to maintain levitation. By adjusting the composition of the inert atmosphere, temperatures from 1850 K to greater than 2450 K could be achieved with this coil. At temperatures above 2425 K the formation of metal smoke in the inert atmosphere made temperature measurementsuncertain. Therefore, the highest temperature used was approximately 2420 K. Attempts to obtain stable levitation of liquid lanthanum below 1850 K with this coil by varying the sample size and providing pure helium for the inert atmosphere were unsuccessful. To obtain results below 1850 K, a new solenoid-type coil was wound with three turns in the lower section and two reverse turns in the upper section. The lower section was wound on a 9.525 mm former and the top section was wound on a 19.05 mm former. The sections were separated by a 6.35 mm gap. The enlarged diameter of the upper section of the coil allowed the sample material to float higher in the gap, thus suscepting less of the intense electromagnetic field set up by the lower section of the coil. This resulted in lower sample temperatures with the new coil. One disadvantage incurred with the new coil design was the tendency for lateral drift of the sample in the field. If the sample was moving horizontally at the instant the power was cut off for the drop, the sample would not fall through the center of the drop tube. While some horizontal movement was tolerable, the number of spoiled runs was increased due to samples striking the sides of the drop tube during the fall. Careful alignment of the equipment and adjustments to the coil reduced this problem to an acceptable level (about a 25 per cent spoiled run rate). By adjusting the sample size and the inert-gas composition, temperatures from 1250 to 1850 K were obtained. This gave an overall temperature range of 1250 to 2420 K for the experimental work.

3. Results and discussion Results of the work on lanthanum are given in table 2. A linear fit to the corrected enthalpies gave, in the temperature range 1250 to 2420 K: (H(T)-H(298.15 K)} = (32.77*0.18)(T/K)-(2289+343).

86

L. A. STRETZ AND R. G. BAUTISTA

TABLE 2. Experimental results on liquid lanthanum; TB is the brightness temperature, T the true temperature, x(He) the approximate mole fraction of He in the (He + Ar) atmosphere, AH, the convection loss, AH, the radiation loss, and 10% the percentage deviation of the calculated enthalpy from the experimental enthalpy a Run

WK

41 39

1166 1214

38 37 36 34 33 32 31 30 28 27 25

1248 1295 1296 1349 1376 1410 1457

1 46 9 3 15 22 17 5

11 16 8

18 23

19 24

1502 1532 1604 1654

1696 1709 1748 1749 1763 1791 1794 1841 1843 1851 1909 1966 2031 2075 2114

T/K

lO%(He) A&/J mol-l

1253 1308 1348 1403 1404 1466

1499 1539 1595 1650 1686 1774 1835 1887

1903 1951 1952 1969 2004 2008 2068 2070 2080 2154 2227 2311 2367

2419

:: loo 90 90 80 80 70 70

60 60 60 60 90

1035 1112 968 836 790 634 743 700 764 966 664 734 776

1531 305

a:

80 80 50 50 50 50 50 30

10

1108 1104 1170 748 793 781 781 751 613 465 456 474 434

AH,/J mol-I290 344 350

395 384 420

493 559 651

901 780 967 1107 1373 1243 1387 1387 1469 1541 1601

1739 1744 1732 2030 2315 2702 2968 3192

{H(T) - H(298.15 K)}/J mol-L expt . cab. 39104 41121 41951 43784 43671 45651 46847 47920 49625 52072 52575 55406 57637

59988 59315 61981 61817 62389 63451 63260 65344 65658 66061 68072 70563 73524 75419 77464

38778 40580

41891 43694 43727 45759 46840 48151 49986

51789 52969 55853 57852 59557 60081 61654 61687 62224 63391 63522

lo26 0.83 1.32 0.14 0.21 -0.13 -0.24 0.02 -0.48 -0.73 0.54 -0.75 -0.81 -0.37 0.72

-1.29

65554 65882 68307

0.53 0.21 0.23 0.09 -0.41 -0.22 0.16 0.27 -0.35

73453 75288 76993

-0.19 0.10 0.17 0.61

65489

the drop a C, = (32.77 & 0.18) J K-l mol-I; p = 5.56 g cmW3; M(La) = 138.91 g mol-I; distance was 36 cm; the hemispherical total emissivity cht = 0.340; and normal total emissivity &,,,q= 0.282.

The heat capacity of 32.77 J K-l mole1 for lanthanum obtained in this work is in agreement with the value of 34.342 J K-l mole1 reported by Berg et CZ~.(‘,~~) Table 3 compares the results of this work with those of Berg et al. for the enthalpy in the liquid state. The comparison is also shown in figure 1 which shows their results for lanthanum from 1193 to 1373 K and the results of this work from 1253 to 2419 K. Values of the thermodynamic functions were calculated at 50 K intervals from the melting temperature (1193 K) up to 2400 K. These values are listed in table 4. The constant heat capacity observed over the entire temperature range investigated may be caused by an artificial flattening of the enthalpy curve, due to the rapid electromagnetic stirring induced in the levitated samples. The experimentally determined liquid phase heat capacity is in agreement with the “rule-of-thumb” estimate of 29 to 33 J K-l mol-I.

ENTHALPY TABLE

T,K

1193 1200 1225 1250 1275

3. Comparison

of results for liquid lanthanum deviation

Berg mole1 4 this - work (H(T) H(298.15 K)}/J

(36810) * (37040) b (37860) b 38680 39500

36957 37199 38057 38914 39772

a Reference 2. b Extrapolated below experimental

TABLE

T E 1193 1200 1250 1300 1350 E 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350

OF LIQUID

___-G J Kmol-l 32.77 32.77 32.77 32.77 32.77 32.77 32.77 32.77 32.77 32.77 32.77 32.17 32.77 32.77 32.77 32.77 32.77 32.77 32.71 32.77 32.77 32.77 32.77 32.77 32.77 32.77

T,K

-0.40 -0.42 -0.52 -0.61 -0.69

1300 1325 1350 1375

4. Thermodynamic

{s”(T) - s’(o)} J K-l mol-l 106.123 106.315 107.654 108.938 110.176 111.368 112,519 113.627 114.702 115.744 116.752 117.731 118.680 119.605 120.500 121.374 122.227 123.055 123.867 124.658 125.427 126.180 126.916 127.636 128.343 129.033

with literature

1026

temperature

{H”(T)

87

LANTHANUM

values; 1OV in the percentage

{H(T) - H(298.15 K)}/J mol-l Berg a this work 40316 41135 41955 42775

40634 41491 42349 43210

1026 -0.78 -0.86 -0.93 -1.01

range.

functions

of liquid lanthanum

- H”(298.15 K)}/T J K-l mol-l 30.854 30.866 30.942 31.013 31.080 31.138 31.197 31.247 31.297 31.343 31.389 31.427 31.465 31.502 31.536 31.569 31.603 31.632 31.657 31.686 31.711 31.732 31.757 31.778 31.799 31.820

-

{G”(T) - H”(298.15 K)j/T J K-l mol-l 75.269 75.449 76.708 77.925 79.096 80.226 81.322 82.380 83.405 84.400 85.362 86.304 87.216 88.102 88.964 89.805 90.625 90.428 92.206 92.971 93.716 94.448 95.159 95.858 96.544 97.213

Table 1 shows that the mass fraction of oxygen in the sample increased from an average of 30 x 10e6 to 636 x lo-‘. Lanthanum is very reactive and is easily oxidized in the presenceof oxygen. In view of the required handling of the sample the increase in the oxygen content is well within acceptable limits for the purpose of heat-capacity measurement.

L. A. STRETZ

AND

R. G. BAUTISTA

T/K FIGURE 1. Enthalpy of liquid lanthanum from the melting temperature (1193 K) to 2419 K; a, Berg, Spedding, and Daane;(15) 0, this work.

The measurement of the sample temperature is the largest source of error. Other sources of error include errors in the calorimeter calibration, in the measurement of the calorimeter block temperature, and those introduced by approximations and estimates in the calculations. The maximum estimated error for this technique is approximately -t 2.5 per cent. REFERENCES 1. McKeown, J. J. Ph.D. Thesis, Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50010,195S. 2. Berg, J. R. Ph.D. Thesis, Department of Chemistry, Iowa State University of Science and Technology, Ames, Iowa 50010,196l. 3. Grimley, R. T. Ph.D. Thesis, University of Wisconsin, 1958. 4. Dennison, D. H.; Gschneidner, K. A.; Daane, A. H. J. C/rem. Phys. 1966,44, 4273. 5. Chaudhuri, A. K.; Bonnell, D. W. ; Ford, L. A.; Margrave, J. L. High Temp. Sci. 1970, 2, 203. 6. Bonnell, D. W. Ph.D. Thesis, Rice University, 1972. 7. Treverton, J. A.; Margrave, J. L. J. Phys. Chem. 1971, 75, 3737. 8. Treverton, J. A.; Margrave, J. L. J. Chem. Thermodynamics 1971,3, 473. 9. Stretz, L. A. Ph.D. Thesis, Iowa State University of Science and Technology, 1973. 10. Stretz, L. A.; Bautista, R. G. Met. Trans. 1974, 5, 921. Il. Kelly, K. K. High temperature heat content, heat capacity, and entropy data for the elements and inorganic compounds. U.S. Bur. Mines Bull 584, 1960. 12. Spedding, F. H.; Beaudry, B. J.; Croat, J. J.; Palmer, P. E. Properties, preparation, and handling of pure rare earth metals. Inter-American Conference on Materials Technology Proceedings 1968, 151-171. 13. Stretz, L. A. M.S. Thesis, Iowa State University of Science and Technology, 1971. 14. Moscowitz, C. M.; Stretz, L. A.; Bautista, R. G. High Temp. Sci. 1972,4, 372. 15. Berg, J. R.; Spedding, F. H.; Daane, A. H. US. Atomic Energy Commission Report is-327 (Ames Laboratory, Iowa State University), 1961.