Molar enthalpy of formation of CrB2 by high-temperature calorimetry

O-102 J. Chem. Thermodynamics 1985, 17, 109-116

Molar enthalpy high-temperature LETITIA

of formation calorimetry

of CrB,

by

TOPOR and 0. J. KLEPPA

The James Franck Institute and The Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, U.S.A. (Received 5 June 1984) The standard molar enthalpy of formation of CrB, the first time by bringing the boride to reaction with a {(l -3x)Me+ZxCr +xB)(I) alloy. The weighted experiments leading to the formation A‘Hi(298.15 K) = -(119.4+3.4) kJ.mol-r.

has been determined calorimetrically for solid Pt, Pd, or Ni at (1400+2) K to form average of the results of four types of of different liquid alloys was

1. Introduction The borides of the transition elements form a group of compounds of considerable theoretical and practical interest. It is known, for example, that a change in the number of d-electrons in the transition metal correlates with changes in the boron-to-metal structures and with changes in the physical and thermodynamic properties of the compounds. w Many of the transition-metal borides find applications in high-temperature technology. This is due to their very refractory character, chemical inertness, hardness, and high electrical and thermal conductivity. Little reliable thermochemical information is available on the transition-metal borides. This is related to the fact that combustion calorimetry of refractory borides, both in oxygen and fluorine gas, is associated with many experimental difficulties. Due to their chemical inertness, refractory borides cannot be studied by conventional solution calorimetry. During the past several years we have in this laboratory been interested in developing high-temperature calorimetric methods suitable for determining the enthalpies of formation of refractory borides. So far, we have obtained such values by means of high-temperature solution calorimetry.(2-4’ Very recently we have proposed a new calorimetric method designed for thermochemical studies of refractory borides with high boron-to-metal ratios. This method is based on bringing the solid boride to reaction with solid platinum near 1400 K to form a liquid (platinum + boron + metal) alloy. In other experiments the same amounts of the three pure elements form a liquid alloy of the same composition. The enthalpy of formation of the boride is obtained from the differences between the two enthalpies 0021-9614/85/020109+08

%02.00/O

0 1985 Academic Press Inc. (London) Limited

110

L. TOPOR AND 0. J. KLEPPA

of reaction. In the very first application of this method we determined the enthalpy of formation of LaBec5) In the present investigation we apply this method to CrB,, i.e. to a boride with a much lower boron-to-metal ratio than LaB,. Since the phase diagrams of (Pt + B), (Pd + B), and (Ni -t B) all show a significant liquid range at 1400 K, we have also carried out a few calorimetric experiments in which Pd or Ni was substituted for Pt. Although the three different metals gave rise to very different enthalpies of reaction, they all yielded the same enthalpy of formation for CrB,. This increases our confident in the signi~cance of the results.

2. Experimental APPARATUS,

ATMOSPHERE,

CALORIMETRIC

CELL

The calorimetric experiments were carried out at (1400 It 2) K using a Calvet-type twin microcalorimeter which has previously been operated successfully up to about 1500 K. The principal features of this equipment were described recently.‘s*6’ The measurements were performed in an atmosphere of argon which was purified by passing the gas through concentrated H,SO,, over activated copper at 500 K, and over titanium powder at 950 K. The gas flows through a fused-silica “liner’” which provides a complete envelope for the calorimetric cell assembly and for the final gettering system. The layout of the liner and its contents was similar to that described recently. c7)However in the present study the calorimetric cell consisted of a boron nitride crucible about ;4 mm o.d. and 19 mm high. This crucible was contained within a slightly larger alumina crucible, which in turn was inserted into a thinwalled stainless-steel tube about 20 mm in diameter and 300 mm long, with one end closed. This tube fitted snugly inside the fused-silica liner and protected the bottom of the liner against the contents of the calorimetric cell; to some extent it also contributed to the removal of traces of oxygen from the argon gas. Final gettering of the atmosphere was achieved by means of zirconium foils inside alumina tubes just above the calorimetric cell. MATERIALS

The platinum and palladium metals were of reference grade (99.99 mass per cent of platinum or palladium) purchased from Engelhard as 2 mm wire and 0.025 mm foil. The nickel metal in part was in the form of 3 mm diameter Mond-nickel pellets with 99.92 mass per cent of nickel, in part of 0.05 mm foil, purchased from Castle Metals of Franklin Park, IL. A typical analysis provided by the vendor showed about 99.9 mass per cent of nickel. Before use the plating, pa~adium, and nickel foils were annealed overnight in an inert atmosphere at 1100 K. The boron sample consisted of lumps of crystalline material of the P-rhombohedral form with 99.8 mass per cent of boron; the sample was purchased from Alfa Productsflentron. The chromium was a coarsely crystalline sample of high-purity metal cut with a spark cutter from a zone-refined rod. The CrB, was

ENTHALPY

OF FORMATION

111

OF CrB,

kindly furnished by Professor K. Spear of the Pennsylvania State University. Originally the boride had been prepared by Cerac, Inc., and was supplied in the form of a 325 mesh powder (Lot No. T-3693C), typical purity: 99 mass per cent of CrB,. A chemical analysis provided by the company showed 70.10 mass per cent of Cr (theoretical, 70.63 mass per cent); 29.37 mass per cent of B (theoretical, 29.37 mass per cent); 0.04 mass per cent of C; 0.007 mass per cent of H; 0.04 mass per cent of N; 0.25 mass per cent of 0. The boride had been arc melted in Professor Spear’s laboratory to form a small button. PROCEDURES

AND CALIBRATrONS

Before each series of experiments the silica liner was flushed for about 2 h with purified argon and then inserted into the calcrimeter where it was flushed overnight. The samples were dropped from room temperature into the calorimeter through a central thin-walled 6 mm diameter stainless-steel tube. We dropped three different kinds of samples: pure Pt, Pd, or Ni; CrB,; and (Cr + B). When CrB, or (Cr + B) were dropped, the sample consisted of a small cylindrical capsule prepared from Pt, Pd, or Ni foil; this contained the required amounts of the compound or of the elements plus some additional Pt, Pd, or Ni so as to form a liquid alloy of the desired composition. Calibration of the calorimeter was carried out at the beginning of each series of experiments by drops of pure Pt, Pd, or Ni. If only one metal sample was dropped, separate calibration experiments also were performed, based on the enthalpy of pure Pt. Small corrections were applied for the slight day-to-day variations of room tem~rature and of calorimeter temperature. We always applied a 1 per cent correction for the heat pickup of the calibrating samples during their drop into the calorimeter. Within a single series of measurements the calibration experiments were reproducible to ir: 1 per cent. The standard molar enthalpy increments (Hi(1400 K)- Wm(298.15 K)} of Pt, Pd, and Ni were taken from Hultgren et al.:@) pt, 31769 J. mol- l; Pd, 32059 J. mol- “; and Ni, 35727 J *mol- ‘. The molar masses B, 10.811 g*mol-‘; Pt, 195.09 g’mol-‘; adopted were: Cr, 51.996 g-mol-‘; Pd, 106.4 g.mol-‘; and Ni, 58.71 gemol-“.

3. Results and discussion To determine the enthalpy of formation of CrB, we performed two different types of measurements. In series 1 we initially dropped samples of the pure metals (Me = Pt, Pd, or Ni) into the calorimeter; this was followed by drops of (Me + CrB,). In this way we produced a liquid alloy of the desired com~sition. The drops of the pure metals at the beginning of each series served a dual purpose. On the one hand, they provided a calibration of the calorimeter; on the other, they made a known endothermic contribution to the overall calorimetric reaction: (0.58/0.14)Me(cr,

T,)+CrB,(cr,

Ti) = (l/O.l4)Me,,,,B,,~,Cr,.,,(1,

T).

(1) Here T1 z 298 K; T z 1400 K. Note, however, that for Me = Pt we actually carried

L. TOPOR AND 0. J. KLEPPA

112

out measurements at two slightly different liquid-alioy compositions: x(Cr) = 0.14 and x(Cr) = 0.13. In series 2 we dropped (Me + Cr -I- B) samples of the same composition into the liquid alloy already present in the calorimeter from the series 1 experiments: (O.WO.l4)Me(cr,

T,)+2B(cr,

T,)+Cr(cr,

T,) = (l/O.l4)Me,,,,B,,,,Cr,,,,(l,

T). (2)

Note that in these cases it was not possible to obtain a calibration by drops of pure Me; furthermore, by making combined drops of (Me + Cr + B) we avoided the complications arising from having to dete~ine an exothermic effect in a calorimeter calibrated for endotbe~ic values. Combining equations (1) and (2) we get Cr(cr, T1) + 2B(cr, ?;) = CrB,(cr, T,),

(3)

and also A,H;

= AH,(2)-AH,(l).

(4)

Our ex~~mental results are tabulated in tables 1 to 4. For the experiments with Pt, AH,( 1) are listed in table 1 and AH,,,(Z) in table 2. The vaiues of A, Hk are calculated TABLE 1. Enthalpy changes associated with reactions with platinum according to equation (1) Expt no. l-l

l-2

1-3

Composition - ~- _-.. ..- ..Pbd%.2sCr,.,4

n(Pt) __ mm01 ~~~~.5.1978

NCrB,) ~mmol

xm

AH,,, J

AH,(l) kJ.mol-’

1.4264

1.5989

0.1400

163.74 260.50 -..X 424.24

265.3

5.3512 1.2551

1.5946

0.1400

168.57 ---252.92 C 421.49

264.3

5.1310 4.6355 1.1049

2.6241

0.1400

161.77 146.15 410.16 .~

273.6

C 718.08 1-4

5.1074 1.1146

0.1400 1.5019

160.64 230.04 C 390.68

260.1 -_______

Average: 265.8 + 5.7

l-6

5.4092 4.8744 0.8629

2.3755

4.8703 0.4316

1.1299

0.1300

169.51 152.75 393.15 -x 715.41

301.2

0.13'8

152.62 -.-185.63 X 338.25

299.4

Average: 300.3 + 1.3

ENTHALPY

OF FORMATION

OF CrB,

113

TABLE 2. Enthalpy changes associated with reactions with platinum according to equation (21 Expt no.

Composition

2-2

Pt 0.58B0.28%14

2-3 2-4

2-5

4.8484 4.6937 4.7249 4.7176

n(Cr) + 2n(B) mm01 _--______-1.1703+2.3406 1.330+2.2660 1.1405+2.2Sl 1.1387+2.2774

0.1400 0.1400 0.1400 0.1400

187.00 159.00 165.46 176.01

159.8 140.8 145.1 154.6

4.9383

1.1920+2.3840

0.1400

166.93

140.0

3.6232 4.1794

0.8746+ 1.7491 1.0088 + 2.0176

0.1400 0.1400

126.85 156.88 Average:

145.0 155.5 148.7 k7.0

3.5660 3.6042 3.842 1

0.7600+ 1.5200 0.7681+ 1.5362 0.8188 t 1.6376

0.1300 0.1300 0.1300

126.50 137.42 155.35

166.5 178.9 189.7

3.1864 3.1087

0.6791 t 1.3810 0.6625 + 1.3250

0.1300 0.1300

120.47 177.4 109.04 164.6 Average: 1X.4& 10.2

n(w ~ mmol

Pfo.e,lR,.,,Cro.,~

2-6

x(Cr) = 0.1400, A~~~(kJ,mol-‘) x(Cr) = 0.1300, A,H~/(kJ.mol-“)

W,,G.)

A&IS

x(W

J

kJ, mol-’

= 148.7-265.8 = -117.1 k9.0. = 175.4-300.3 = - 124.9_+ 10.3.

at the bottom of table 2. The results for the Pd experiments are in table 3, and for Ni in table 4. The stated error limits in all of these tables represent the standard deviations from the calculated means. Since, for (Pd + CrB,) we carried out only a single experiment, we give no error limits for the corresponding value of ArHk. Note that series 1 experiments are designated l-l, 1-2, l-3, etc., in the three tables, while series 2 experiments are shown as 2-1, 2-2, 2-3, etc. Where more than one experiment is indicated for a given experiment number, multiple additions were made. Thus in series 2-2, table 2, there were four sequential additions of (Pt + Cr + B) to the liquid alloy, which initially was formed in series 1-2 (table 1). In table 5 we present a summary of the values of A,,Hk determined in experiments TABLE 3. Enthalpy changes associated with reactions with palladium Expt ll0.

l-l

2-l

n(P4 ~mm01

n(CrB,) mmol

n(Cr) + 2&B) mm01

x(Cr)

4.1702 0.9310

1.3762

-

3.0339 3.1353

-

0.7323 + l.4646 0.7568+ 1.5136 A,Hi/(kJ’mol-‘f

-dJi,b, J

AK,t~i ~ kJ.mol-’

0.1400

151.4 -..143.2 f: 294.6

214.1

0.1400 0.1400

47.2 70.5

= 92.5-214.1 = -121.6

AK,,C2) kJ

91.8 93.2 ___Average: 92.5 i 1.0

114

L. TOPOR AND 0. J. KLEPPA TABLE 4. Enthalpy changes associated with reactions with nickel

Expt no.

n(Ni) mm01

1-l

5.1533 4.9068

2-1

2.6444

1-2

4.2795 4.7598 2.7219

2-2

2.4156

n(CrB,)

n(Cr) -.~ + 2n(B) mm01

mmol

2.4283 -

2.8389 -

0.1400 0.6383 $1.2766 0.5831+ 1.1662

A&,* __ J

x(Cr)

0.1400

0.1400

~ WnU) kJ,mol-’

182.1 376.1 E 558.2 71.7

229.8 -

152.0 169.1 353.9 z 675.0

0.1400

70.0

AffmJ.3 kJ.mol-’

237.8 --- -

Average: 233.8 + 5.6 A,Ha(kJ.mol-‘)=

112.3

120.0 116.2 i 5.4

116.2-233.8=-117.657.8

with the three different metals and with two different alloy compositions for platinum. There is general agreement among the four values within the stated limits of error. On the basis of the (minimum) number of experiments actually averaged, we assigned a weight factor to each of the four values of A,Hi in table 5 (see tables I to 4). Applying these weight factors to our results we arrived at an overall weightedmean value of Al&, for CrB, of - 119.4 kJ ‘mol-i, with a standard deviation of the mean of +3.4 kJ*mol-‘. In table 6 we compare this new value of ArHk with one earlier experimental value, with a value given by Kubaschewski and Alcock,“) and with the recent semiempirical estimates of Miedema”‘) and Niessen and De Boer.(“) The first quantitative estimate of the enthalpy of formation of CrB, was published about 30 years ago by Brewer and Haraldsen. (“) Their value was based on a study of the equilibrium between boride, nitrogen gas, and nitride near 1800 K, In this way the authors stated that they were able to obtain “qualitative information about the relative stability of the boride phases”. Even so, Brewer and Haraldsen’s TABLE 5. Summary of standard molar enthalpies of formation. Values based on calorimetric experiments leading to the formation of four different liquid alloys at 1400 K Liquid alloy formed

A,H%/(kJ mol-‘)

%58Bo.2sCro.14

-117.1$9.0 - 124.9 + 10.3 - 121.6 - 117.6 + 7.8

%,6,Bo.26Cro.13 P~o.58~o.28C~~.14 N~o.5sBo,28%14

Weighted average: A,H~(Z98.15 K) = -(119.4+

Weight factor 4 2 1 2 3.4) kJ. mol-’

ENTHALPY

OF FORMATION

TABLE 6. A,Hi(CrB2) Source Present work Brewer and Haraldsenl ’ ‘) ’ Samsonovo4) ’ Samsonovfr3) Aronson et aLo5) Kubaschewski and Alcocktg’ Miedema”‘) Niessen and De Boer(“)

115

OF CrB,

from different sources A,Hy(kJ

mol- *)

- 119.4k3.4 3 - 126 -c--126 -126 - 126 -94 -100 - 105

u In this reference the value given in the text is as hsted; the tabulated value is given with the sign <. * This reference also quotes the values - 79.5 kJ *mol- r from R. B. Kotei’nicovim (no reference) and - 196.6 kJ.mol- r, calculated from data of Kubaschewski and Evans, Metallurigcal Ihermochemistry, 1954.

estimate, A,Hk B - 126 kJ .mol-‘, appears to be the only experimental value of the enthalpy of formation available in the literature prior to our own study; it seems to have provided the basis for most tabulations prepared during the past 30 years, i.e. the values quoted in two tabulations by S~sonov(*3,14) and in the work of Aronson et uj.(* ‘) Kubaschewski and Alcock’s value, A, H& = - 94 kJ * mol- ‘, is given without any experimental reference. Very recently Miedema’l’) and later. Niessen and De Boer,” ‘) using a Miedema-type semi-empirica! model with new parameters for boron, estimated ArHk for CrB, as - 100 kJ 4mol- ’ and - 105 kJ.mol-‘, respectively. A survey of the physical properties of the first-row transition-metal diborides shows a decrease of all parameters which characterize the strength of the metal-boron bond on passing from Group IV to Group VI (and Group VII).““* “) A similar trend should hold for the enthalpies of formation, with decreasing negative values in the sequence TiB,, VB,, CrB,, and MnB,. For these compounds we quote the following values of ArHi which are taken from the literature: -204 kJ.mol-l.(19) -303 kJ*mol-‘;(18) - 119.4 kJ. mall ’ (present work); and - 63.3 kJ mrnoll 1.(3) The compound FdB, is not stable. From SC to Cr the melting temperatures of the diborides,‘13) or the ratios of the melting tem~ratures of the boride to that of the metal, show a maximum at TiB,. If the melting tem~ratures reflect the relative stabilities of the compounds, we accordingly would have expected to find the most negative value of A,HL for TiB,. Actually, the reported values of A,Hi for this compound range from about -209 kJ.mol-’ to -324 kJ.mol- ’ .(20,“) There are no values for ScB,. To explore these problems further we are planning to determine the enthalpies of formation of VB,, TiB,, and ScB, using the method described in the present paper. We are indebted to Professor I(. Spear of the Pennsylvania State University who provided the sample of CrB,. This work was supported by the National Science Foundation under Grant CHE-8106980. It has also benefitted from the Central Facilities of the University of Chicago MRL.

116

L. TOPOR AND 0. J. KLEPPA

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

See e.g. Spear, K. E. J. Less-Common Metals 1976, 41, 195. Hong, K. C.; Kleppa, 0. J. J. Chem. Thermodynamics 1978, 10, 797 Kleppa, 0. J.; Sato, S. J. Chem. Thermodynamics 1982, 14, 133. Sato, S.; Kleppa, 0. J. Met. Trans. 1982, 13B. 251. Topor, L.; Kleppa, 0. J. J. Chem. Thermodvnamics 1984, 16,993. Watanabe, S.; Kleppa, 0. J. J. Chem. Thermodynamics 1983, 15. 633. Topor, L.; Kleppa, 0. J. Met. Trans. 1984, 15A, 203. Hultgren, R.; Desai, P. D.; Hawkins. D. T.; Gleiser, M.; Kelley, K. E.; Wagman, D. C. Selected Values of the Thermodynamic Properties a( the Elements. ASM: Metals Park, OH. 1973, pp. 353, 388, 398. 9. Kubaschewski, 0.; Alcock. C. B. Metallurgical Thermochemistry, 5th edition. Pergamon: Oxford. 1979, p. 280. 10. Miedema, A. R. J. Less-Common Metals 1976, 46, 67. 11. Niessen, A. K.; De Boer, F. R. J. Less-Common Metals 1981, 82. 75. 12. Brewer, L.; Haraldsen, H. J. Electrochem. Sot. 1955, 102. 399. 13. Samsonov, G. V. Tugoplavkie soedineniya. Metallurgizdat. Moskva, 1963. English Translation, Plenum: NY. 1964, pp. 78. 94. 14. Samsonov, G. V. Zhur. Fiz. Khim. 1956, 30, 2057. 15. Aronson, B.; Lundstrom, T.; Rundqvist, S. Borides, Silicides and Phosphides. Methuen: London. 1965, p. 36. 16. Matkovich, V. J. Boron and Refractory Borides. Springer: NY. 1977, pp. 5, 19. 17. Samsonov, G. V.; Goryachev, Yu. M.; Kovenskaya, B. A. J. Less-Common Metals 1976,47, 147. 18. Yurick. T. J.; Spear, K. E. Thermodynamics of Nuclear Materials, 1979. Vol. I, Thermodynamics of TiB, fvom Ti-B-N Studies. Int. Atomic Energy Agency: Vienna. 1980, pp. 73-90. 19. Spear, K. E.; Schafer, H.; Gilles. P. W. Thermodynamics of Vanadium Borides, High Temperature Technology. Butterworth: London. 1%9, p. 201. 20. Brotherton, R. J.; Steinberg, H. Progress in Boron Chemistry, Vol. 2. Pergamon Press: NY. 1970, p. 205. 21. Yurick, T. J. M.S. Thesis, Pennsylvania State University, 1979, p. 17.