Some observations on synthesis and electrolytic properties of nonstoichiometric calcium zirconate

Stoichiometric CaZrO3 (CZ-50) and CaZrO3 doped with excess CaO (CZ-51) powders, whose formula (CaO)(1-x)(ZrO2)(x), where x=50 or 51 mol.% CaO was successfully synthesised by the co-precipitation calcination method with a saturated solution of (NH4)(2)C2O4 in concentrated NH3 solution as a precipitation agent. The thermal evolution of CaZrO3 dried precursor during heating them up to 1200 degrees C was monitored by thermal (DTA,TG) and X-ray diffraction analysis methods. The highest temperature (1200 degrees C) for CaZrO3 synthesis was found for stoichiometric CaZrO3, whereas introduction of excess CaO into CaZrO3 led to a decrease in the synthesis temperature to 1000 degrees C. The crystallite size d((hki)) of grounded CaZrO3 powders ranged from similar to 43 to similar to 90 nm, respectively. BET measurements indicated that in both the investigated powders, particles were agglomerated. Sintering CaZrO3-based samples at 1500 degrees C/2 h or hot-pressing process (1250 degrees C/1 h, 25MPa) was applied to obtain gas-tight CaZrO3-based ceramics. To examine the thermochemical stability of materials obtained at high temperatures, the CaZrO3-based samples were additionally heated at 1200 degrees C for 120 h or in the temperature range 1400-1600 degrees C for 24 In in air or purified argon. There were also performed and then discussed, some tests on thermal resistance of CaZrO3 against molten metals-nickel and copper. Investigations into chemical reactivity of CaZrO3 electrolyte with electrode materials involving LaCrO3 or MCr2O4 (M = Mg, Ca) in the temperature range 1000-1200 degrees C were conducted using XRD with Rietveld analysis. Electrical conductivity measurements performed by both dc and ac impedance spectroscopy method in the temperature range 200-1000 degrees C. The best oxygen ion conductivity was found for CaZrO3-doped excess CaO (CZ-51) samples sintered in air, starting from powders synthesized by co-precipitation or citrate method. The CZ-51 samples obtained via solid state reaction or hot-pressed exhibited lower values of electrical conductivity. Test results for this compound used as an electrolyte in solid oxide cells involving electrode materials MCr2O4 (M = Mg, Ca) are also reported. In this way the Gibbs free energy of formation of MgCr2O4 at 1000 degrees C was determined. The nonstoichiometric CaZrO3 seems to be a promising solid electrolyte for electrochemical oxygen probes in control of metal processing. (C) 2008 Elsevier B.V. All rights reserved.