Defect chemistry and transport properties of BaxCe 0 . 85 M 0 . 15 O 3 -
The site-incorporation mechanism of M dopants into ABO3 perovskites controls the overall defect chemistry and thus their transport properties. For charge-balance reasons, incorporation onto the A-site would require the creation of negatively charged point defects (such as cation vacancies), whereas incorporation onto the B-site is accompanied by the generation of positively charged defects, typically oxygen vacancies. Oxygen-vacancy content, in turn, is relevant to proton-conducting oxides in which protons are introduced via the dissolution of hydroxyl ions at vacant oxygen sites. We propose here, on the basis of x-ray powder diffraction studies, electron microscopy, chemical analysis, thermal gravimetric analysis, and alternating current impedance spectroscopy, that nominally B-site doped barium cerate can exhibit dopant partitioning as a consequence of barium evaporation at elevated temperatures. Such partitioning and the presence of significant dopant concentrations on the A-site negatively impact proton conductivity. Specific materials examined are BaxCe0.85M0.15O3(x 0.85 − 1.20; M Nd, Gd, Yb). The compositional limits for the maximum A-site incorporation are experimentally determined to be: (Ba0.919Nd0.081)(Ce0.919Nd0.081)O3, (Ba0.974Gd0.026)(Ce0.872Gd0.128)O2.875, and Ba(Ce0.85Yb0.15)O2.925. As a consequence of the greater ability of larger cations to exist on the Ba site, the H2O adsorption and proton conductivities of large-cation doped barium cerates are lower than those of small-cation doped analogs.
Atomistic Simulation of Dopant Incorporation in Barium Titanate
The results of an atomistic simulation study on the incorporation of ions of the first series of transition metals (Cr3+, Cr4+, Fe2+, Fe3+, Co2+, Co3+, Ni2+, and Ni3+), Y3+, and ions of the lanthanide series (Er3+, Gd3+, Tb3+, Pr3+, Pr4+, and La3+) in the BaTiO3 lattice are presented and discussed. The ions of the transition metals prefer to substitute at the titanium site with oxygen-vacancy compensation. For iron and cobalt, oxidation from the divalent to the trivalent state during incorporation is favored. Nickel and chromium are preferentially incorporated in the valence state 2+ and 3+, respectively. Formation of stable defect pairs with different types of lattice defects is predicted for the transition-metal impurities. For La3+ and Pr3+, substitution occurs at the barium site, whereas Y3+, Tb3+, Gd3+, and Er3+ tend to simultaneous substitution on both cation sites. Formation of dopant–titanium-vacancy pairs is predicted for the rare-earth ions and Y3+. The effect of doping on the lattice parameter of c-BaTiO3 has been studied by a mean-field calculation. Comparison with experimental data confirms the dependence of the preferred substitution site on the ionic radius of the impurity. For dopants with intermediate size (Y3+, Er3+, Tb3+, and Gd3+), the Ba/Ti ratio is important in the incorporation mechanism.
Review of Literature on Probability of Detection for Magnetic Particle Nondestructive Testing
Abstract : A critical review of the available literature concerning the minimum reliably detectable defect size aNDI for magnetic particle testing (MPT) in aerospace applications is presented. Four probability of detection (POD) studies relevant to detection of fatigue cracks in aircraft components were found over the period 1968 to 2011. As the statistical methods used in these four previous studies were either outdated or otherwise deficient, the original data were reanalysed using currently accepted techniques. A meta-analysis of the results is presented, with emphasis on statistical inferences for the defect size expected to be detected with 90% POD. It is shown that the minimum reliably detectable defect size aNDI = 2.0 mm currently specified by the Royal Australian Air Force for wet fluorescent magnetic particle inspection using the continuous method is consistent with estimates of the average performance of MPT derived from the reanalysis of the literature.