Standard molar Gibbs free energy of formation of

Standard molar Gibbs free energy of formation of Pb5CrO8(s) from EMF measurements Sulata Kumari Sahu, Rajesh Ganesan, T. Gnanasekaran Liquid Metals and Structural Chemistry Division Chemistry Group Indira Gandhi Centre for Atomic Researchs Kalpakkam – 603 102, India Abstract Equilibrium phase fields of ternary system Pb-Cr-O were established by longterm equilibrations and characterization of phases by XRD. Existence of the following phase fields was identified: (1) PbCrO4-Pb2CrO5-Cr2O3, (2) Pb2CrO5-Pb5CrO8-Cr2O3 (3) Pb 5CrO8-Cr2O3-PbO, (4) Pb-PbO-Cr2O3, (5) Pb-Cr-Cr2O3 and a partial phase diagram of Pb-Cr-O system could be established. The EMF of the following galvanic cell was measured as a function of temperature: Pt, PbO, Cr2O3, Pb5CrO8 | YSZ | O2 (0.21 atm.), Pt From the EMF measured over a temperature range of 800 to 1020 K, standard molar Gibbs free energy of formation of Pb5CrO8(s) was determined. 1. Introduction Liquid lead and lead-bismuth eutectic (LBE) alloy are the candidate coolant materials for accelerator driven systems (ADS) as they can also serve as spallation targets to produce high-energy neutrons [1]. Because of their low chemical reactivity, Pb and LBE are proposed for use as coolant in advanced fast breeder reactors [2]. These coolants are highly corrosive towards structural steels, but this corrosion can be minimized by suitable control of the oxygen potential in the coolant and thereby forming a protective oxide layer on the steels [3]. In this context, thermochemical study of components of steel with lead and bismuth in the presence of oxygen is of technological importance. In the authors’ laboratory, a systematic study of Pb-M-O and Bi-M-O (M: Fe, Cr) systems is being carried out. This paper presents the results of the phase equilibrations studies and thermochemical measurements carried out in Pb-Cr-O system. The pseudobinary phase diagram of PbO-Cr2O3 system has been reported in literature [4, 5] and three ternary compounds are known in the system, viz., PbCrO4, Pb 2CrO5 and Pb5CrO8. Among these compounds, PbCrO4 is stable from 478 K to 1030 K

while Pb2CrO5 is stable from 873 K to 1197 K; and the compound Pb5CrO8 is stable from 948 K to 1108 K [4, 5]. In the present work, the partial phase diagram of the Pb-Cr-O system was established at 973K. The standard molar Gibbs free energy of formation of Pb 5CrO8(s) was determined by measuring equilibrium oxygen pressures over the appropriate ternary phase field by employing solid oxide electrolyte based emf cell.

2. Experimental 2.1. Materials Ternary compounds of Pb-Cr-O system such as Pb 5CrO8, Pb2CrO5 and PbCrO4 were prepared by solid state reaction between appropriate ratios of Pb3O4 powder of 99% purity (M/s Aldrich Chem. Co. USA) and Cr2O3 powder of 99.999% purity (Johnson Matthey Materials Technology, U.K) in air. They were also prepared using PbO and Cr2O3 as starting materials and heating them in air. Thermo gravimetric analysis showed that the starting materials namely PbO and Cr2O3 contained 1.37 wt % and 0.3 wt % of moisture respectively. Hence, they were calcined in air at ~250˚C for 2h to remove the moisture and stored in a desiccator. Lead powder of 99.9% metal basis purity (M/s Alfa Aesar, Ward Hill, MA, US) and chromium powder of 99.8% purity (M/s Alfa Aesar, Ward Hill, MA, US) were also used for phase equilibration studies. The compounds prepared were characterised by X-ray diffraction using a Siemens D500 X-ray powder diffractometer with Cu K radiation and graphite monochromator. The XRD patterns of Pb 5CrO8, Pb2CrO5 and PbCrO4 matched with the patterns reported in JCPDS files of these compounds, viz., 49-0970, 29-0768, 73-2059 respectively.

2.2. Phase equilibration studies Mixtures of various phases (Table 1) were made by mixing and grinding the component phases in an agate mortar. The resulting powders were compacted into pellets, taken in an alumina crucible and encapsulated in quartz ampoule under vacuum. Each sample mixture was equilibrated at 973 K for a total period of ~400 h, with one intermediate grinding and recompaction. After equilibration the samples were cooled by

quenching the quartz ampoules in liquid nitrogen. The samples retrieved from the ampoules were then analyzed by X-ray diffraction to deduce the coexisting phases.

2.3. Emf studies The following galvanic cell was studied in this work.

Pt, PbO(s), Cr2O3(s), Pb 5CrO8(s) | YSZ | O2 (0.21 atm.), Pt The schematics of the experimental assembly are shown in Fig.1. One end closed 8 m/o calcia stabilised zirconia (YSZ) solid electrolyte tube with flat bottom (13 mm OD, 9 mm ID and 300 mm long) supplied by M/s Nikkato Corporation, Japan was used for constructing the galvanic cell. The reference electrode for the cell was prepared by applying platinum paste (M/s. Eletecks Corporation, India) over the inner bottom surface of the electrolyte tube and heating it at 1373 K for 4 hours in air. This resulted in a uniform and porous platinum film over the electrolyte surface. A Pt wire co-fired with the platinum paste served as the electrical lead for air reference electrode. The performance of the emf cell was tested by first measuring the null emf by maintaining the same oxygen potential in both the sides (air). For this measurement a Pt- film was made over the external bottom surface of the electrolyte tube. This arrangement of having the reference electrode at the inner side of the electrolyte tube enabled easier replacement of sample electrodes after thorough cleaning of the outer surface of the electrolyte. The sample electrode was made from a three-phase mixture of PbO + Cr2O3 + Pb5CrO8 and was in the form of pellet of dia 12.5 mm and 4 mm thickness. The sample electrode pellet was sintered at 973K for about 40 hours and taken in an alumina crucible having provision for taking out the electrical lead through its bottom. The crucible containing the sample electrode was in turn placed in a quartz crucible having provision for taking out the electrical lead. The sample electrode was spring loaded against the bottom side of the electrolyte tube to enable intimate contact. The cell was housed in a quartz tube using an O-ring seal. This arrangement had provisions for measuring the cell temperature and for flowing high purity argon and synthetic air through the sample and reference compartments, respectively. The cell assembly was placed in the constant temperature

zone of a furnace. Additionally a 100 mm long, hollow cylindrical stainless steel block was placed in the constant temperature zone to further enhance the uniformity of the temperature and the cell temperature could be controlled within 0.2 K using a PID temperature controller. The stainless steel block was electrically grounded to avoid any a.c. pickup in the emf signal. The cell temperature was measured using a K-type thermocouple. This thermocouple was calibrated prior to the actual experiments against a standard calibrated thermocouple supplied by National Physical Laboratory, India. The cell emf was measured using a high impedance electrometer (input impedance >1014, Keithley model-6514) and the temperature was measured using a multimeter (Agilent model-34970A). The emf values were acquired through an IBM PC using RS 232 interface. The readings were taken when the cell emf was very stable. The test of equilibrium was carried out by passing a small amount of current in both the directions and by shorting the two electrical leads momentarily. After completion of emf measurements, the samples were analysed by XRD to identify the coexisting phases.

Figure 1: Schematics of the EMF cell

3. Results and discussion 3.1. Phase equilibration studies The results of the phase equilibration studies are summarized in Table 1. The table lists the compounds/elements taken, together with the composition expressed in mole fractions. Results of the phase analyses after the equilibration in sealed capsules are also given in Table1. The resulting coexisting phases confirm the existence of the following phase fields at 973 K: (1) Pb + Cr + Cr2O3, (2) Pb + PbO + Cr2O3 (3) PbO + Cr2O3 + Pb5CrO8, (4) Pb 5CrO8 + Pb 2CrO5 + Cr2O3, (5) Pb 2CrO5 + PbCrO4+Cr2O3. Based on the results, the partial phase diagram of Pb-Cr-O system was constructed and is shown in Fig.2.

Table 1: Results of phase equilibration study on Pb-Cr-O system

Sl.

Phases taken before

Composition in the

No

equilibration

ternary plot

Co-existing phases after equilibration at 973K

1

Pb2CrO5+PbCrO4+Cr2O3

Pb0.043Cr0.346O0.611


2

PbO+PbCrO4+Cr2O3

Pb 0.080Cr0.30O0.620

Pb2CrO5+Cr2O3

3

Pb+PbO+PbCrO4

Pb0.340Cr0..120O0.550

Pb5CrO8+PbO+Cr2O3

4

Pb+PbO+PbCrO4

Pb0.160Cr0.260O0.580

PbO+Cr2O3+Pb5CrO8

5

Pb5CrO8+Pb 2CrO5+Cr2O3

Pb0.149Cr0.248O0.602


6

PbCrO4+PbO+Cr2O3

Pb0.150Cr0.250O0.600


7

PbCrO4+PbO+Cr

Pb0.200Cr0.376O0.424

Pb+Cr+Cr2O3

8

Pb+Cr+Cr2O3

Pb0.050Cr0.650O0.300

Pb+Cr+Cr2O3

9

PbCrO4+PbO+Cr2O3

Pb0.160Cr0.200O0.640


10

Cr+PbO+Cr2O3

Pb0.350Cr0.200O0.450

Pb+PbO+Cr2O3

Figure 2: Partial phase diagram of Pb-Cr-O system at 973K

3.2. Standard Gibbs free energy of formation of Pb5CrO8(s) To determine the standard Gibbs free energy of formation of Pb5CrO8(s), oxygen potentials were measured in the phase field PbO + Cr2O3 + Pb 5CrO8 as a function of temperature by using the emf cell-I as mentioned earlier. The variation of emf with temperature is shown in the Table 2.

Table 2: Variation of EMF with temperature

Temperature/K 925.0 974.8 875.0 949.7 820.5 899.8 974.9 724.0 824.6 875.5 799.2 772.7 723.4 950.3 855.1

EMF/mV 287.1, 273.3 300.9 280.2 316.6 294.4 273.5 361.0 315.4 300.9 324.5 333.3 399.0 279.8 306.3

Fig. 3 shows the variation of EMF as a function of temperature.

Figure 3: Variation of emf with temperature

The temperature dependence of the emf cell is given by the following expression.

E / mV (± 0.8) = 543.5 - 0.2773 T / K (800 < T < 1020K).

(1)

The Gibbs free energy of formation of Pb 5CrO8(s) can be related to the measured emf values by the following expression fGom =-3FE + 5fGom + (1/2) fGom + (3/2) RTln0.21

(2)

By substituting the Gibbs free energy of formation of the PbO and Cr2O3 from reference [6] and [7], Gibbs free energy of formation of Pb5CrO8 can be deduced as, ΔGof (kJ. mol-1) =-1808.6 + 0.4584 T / K ( 0.3) kJ.mol-1 (800 to 1020K)

(3)

Conclusion: The partial phase diagram of the ternary system Pb-Cr-O was established from the equilibration study and the Gibbs free energy of formation of the Pb5CrO8(s) was measured by emf technique.

References: [1] B. F. Gromov (Ed.-in-chief), Proceedings of Heavy Liquid Metal Coolants in Nuclear Technology (HLMC-98), Vol.1 and 2, SSC RF-IPPE, Obninsk, 1999, Ministry of Russian Federation for Atomic Energy. [2] J. Zhang, N. Li, Review of studies on fundamental issues in LBE corrosion, Los Alamos National Laboratory. LA-UR-04-0869, 2004. [3] B. F. Gromov, G. I. Toshinsky, V. V. Checkunov, Yu. I. Orlov, Yu. S. Belomytsev, I. N. Gorelov, A. G. Karabash, M. P. Leonchuk, D. V. Pankratov, Yu. G. Pahkin, in: B. F. Gromov (Ed.-in-chief), Proceedings of Heavy Liquid Metal Coolants in Nuclear Technology (HLMC-98), Vol.1, SSC RF-IPPE, Obninsk, 1999, p.14. [4] A. M. Gadalla and M. F. Abadir, Trans.J. Brit. Ceram. Soc., 76(1977), 22-26. [5] T. Negas, J. Am. Ceram. Soc 51(1968), 716-719. [6] Rajesh Ganesan, T. Gnanasekaran, R. S. Srinivasa, J. Nuclear Materials, 320(2003), 258-264. [7] L. B. Pankratz, Thermodynai properties of elements and oxides ( Bulletin ( United states. Bureau of mines): 672), vol. 1, p. 122.