Kallarackel T. Jacob

The potential solid electrolyte, MgZr ₄ P ₆ O ₂₄ was prepared using a modified sol-gel method. Both structural and electrical properties of the solid electrolyte were determined. DSC-TGA analysis indicated that pure dried xerogel when calcined at 900 °C converts to single phase MgZr ₄ P ₆ O ₂₄ nanopowder with good crystallinity. Pellets of 13 mm diameter and 3.8 mm thickness made by uniaxial compression were sintered at 1300 °C for 24 h. XRD and HR-TEM indicated that the crystalline phase is monoclinic having crystallite size of approximately 40 nm. The sintered pellets were stable in the temperature range from 1000 to 1300 °C, with minor extraneous peaks indicating traces of a coexistent second phase (Zr ₂ P ₂ O ₉ ) at the higher temperature. Using impedance spectroscopy, the electrical conductivity of MgZr ₄ P ₆ O ₂₄ was determined as 7.23 × 10 ^–3 Scm ⁻¹ at 725 °C. MgZr ₄ P ₆ O ₂₄ pellet was successfully used for fabrication of a solid-state Mg-sensor for measuring Mg concentration in molten Al at 700 ± 5 °C, with biphasic powder mixture of MgCr ₂ O ₄ + Cr ₂ O ₃ as ceramic reference electrode in air. A linear dependence of emf on logarithm of Mg concentration was obtained. The results indicate that the transport number of Mg ²⁺ -cation in MgZr ₄ P ₆ O ₂₄ is 0.85 ± 0.03 at 700 °C. The solid electrolyte has likely applications in high-temperature electrochemical sensors.

A solid‐state electrochemical cell with yttria‐stabilized zirconia (YSZ) as the electrolyte is used to measure accurately thermodynamic properties of PrRhO ₃ in the range of temperature from 875 to 1325 K. The standard Gibbs energy of formation of PrRhO ₃ with orthorhombically distorted perovskite structure from its binary oxides β ‐Rh ₂ O ₃ and hexagonal Pr ₂ O ₃ is given by, . Invoking the Newmann‐Kopp rule, the standard enthalpy of formation from elements and standard entropy of PrRhO ₃ at 298.15 K are evaluated: and . Phase relations in the system Pr–Rh–O at 1200 K are computed from thermodynamic data. Calorimetric data on enthalpy of formation of two intermetallic compounds are combined with the semi‐empirical model of Miedema and phase diagram to estimate Gibbs energy of formation for the intermetallics and liquid alloys. An isothermal section of ternary phase diagram, an oxygen potential‐composition diagram in 2‐D and a 3‐D chemical potential diagram for the system Pr–Rh–O at 1200 K are presented. In addition, temperature‐composition diagrams at different oxygen pressures are developed. The diagrams provide a road map for design and optimization of processing routes for catalysts based on Rh.

The solubility of oxygen in liquid Bi has been measured by two independent methods – coulometric titration using a solid electrolyte and isopiestic – in the temperature range from 800 to 1300 K. The solubility of oxygen in equilibrium with α-Bi₂O₃ (800–1002 K) can be expressed by, The solubility of oxygen in equilibrium with δ-Bi₂O₃ (1002–1078 K) can be expressed by, Variation of the activity of oxygen with concentration in Bi-O solution has also been measured using an electrochemical technique in the temperature range from 800 to 1400 K. Gibbs energy change for the oxygen dissolution reaction is given by, where the standard state for dissolved oxygen identified by square brackets is an infinitely dilute solution in which activity is equal to concentration in at.-%. Small deviations from Sievert's law were observed at all temperatures. The deviations can be quantified by the first-order interaction parameter: