of formation of ternary oxides Pb5CrO8(s), Pb2CrO5(s) and PbCrO4(s) were determined by measuring equilibrium oxygen partial pressures over relevant phase ...
Some phase equilibria and thermochemical studies on Pb-Cr-O and Pb-Fe-O system Lead and lead-bismuth eutectic (LBE) alloy are considered as a coolant in Accelerator Driven Sub-critical Systems (ADSS) [1]. Owing to the corrosive nature of these coolants towards the structural steels, studies on Pb-M-O (M: steel components) are important for understanding the material compatibility issues. The present work focuses on the thermochemical behavior of Pb-Cr-O and Pb-Fe-O system. Equilibrium phase fields of the Pb-Cr-O ternary system were identified 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 and (5) Pb-Cr-Cr2O3. Based on these results, a partial phase diagram of Pb-Cr-O system has been reported. Standard molar Gibbs energy of formation of ternary oxides Pb5CrO8(s), Pb 2CrO5(s) and PbCrO4(s) were determined by measuring equilibrium oxygen partial pressures over relevant phase fields using manometry and solid oxide electrolyte based emf methods and are given by: Δf Gom Pb 5CrO8(s) ± 0.55/(kJ.mol-1) = -1809.4 + 0.6845 (T / K) (837 ≤ T/K ≤1008) o -1 Δf G m Pb 2CrO5(s) ± 0.30/(kJ.mol ) = -1161.3 + 0.4059 (T/K) (859≤ T/K ≤1021) Δf Gom PbCrO4(s) ± 0.17 / (kJ.mol-1) = -909.8 + 0.3111 (T/K) (863 ≤ T/K ≤ 1093) The standard molar enthalpies of formation, Δ f H om (298.15 K) of Pb2CrO5(s) and Pb 5CrO8(s) were determined using an isoperibol calorimeter. The enthalpies of solution of pure compounds, viz., Pb 2CrO5(s), and Pb 5CrO8(s) as well as mixtures viz., {PbCrO4(s) + PbO(s)} and {PbCrO4(s)+ 4PbO(s)} in HNO3 (aq, 8 mol.dm-3) at 298.15 K were measured. Using these values, Δ f H om (298.15 K) of Pb 2CrO5(s) and Pb 5CrO8(s) were determined as -(1171.5 1.7) kJmol-1 and -(1850.6 8.7) kJmol-1, respectively. Heat capacities of PbCrO4(s), Pb2CrO5(s) and Pb5CrO8(s) were measured using differential scanning calorimetry in the temperature range 300-750 K, 300-700 K and 300-600 K respectively. From the measured heat capacity data, thermodynamic functions such as enthalpy increments, entropies and Gibbs energy functions were derived. Partial phase diagram of Pb-Fe-O ternary system at three different temperatures viz. 848 K, 1048 K and 1133 K has been determined. The following equilibrium phase fields were identified at 1048 & 1133 K, by long-term equilibrations and characterization of phases by XRD: (1) Pb-Fe-FeO, (2) Pb-FeO-Fe3O4, (3) Fe 3O4-PbFe12O19-PbFe5O8.5, (4) Pb-Fe3O4-PbFe5O8.5, (5) Pb-PbFe5O8.5-Pb2Fe2O5 and (6) Pb-PbO-Pb2Fe2O52. Equilibration studies carried out at 848 K shows the following phase fields: (1) Pb-FeFeO, (2) Pb-FeO-Fe3O4, (3) Pb-PbO-Fe3O4 and (4) PbO- Fe3O4-Fe2O3. Standard molar Gibbs energy of formation of ternary oxides PbFe5O8.5 and Pb 2Fe2O5 were determined by measuring equilibrium oxygen partial pressures over relevant phase fields using solid oxide electrolyte based emf method and are given by: Δf Gom PbFe5O8.5 ± 1.02 /(kJ.mol-1) = -2208.55+0.6680 T (K) (948≤ T/K≤1133) Δf GomPb2Fe2O5 ± 0.31/(kJ.mol-1) = -1178.39 + 0.3620 T (K) (1048≤T/K≤1130)
The standard molar enthalpies of formation, Δ f H om (298.15 K) of Pb2Fe2O5(s) and PbFe5O8.5(s) have been determined using an isoperibol calorimeter. The enthalpies of solution of the compound as well as mixtures, Δ sol H om of Pb2Fe2O5(s), PbFe5O8.5(s), Fe2O3 and Pb3O4 in HCl (aq, 6 mol.dm-3) at 298.15 K were measured. Using these values, Δ f H om of Pb2Fe2O5(s) and PbFe5O8.5(s) were determined as -(1324.2±11.1) kJmol-1 and (2347.8±10.7) kJmol-1, respectively.
References 1. T. Obera, T. Miura, H. Sekimoto, Prog. Nucl. Energy 47, (1-4) 577 (2005).
