Kyle Brinkman

Enhancing grain boundary ionic conductivity in mixed ionic-electronic conductors

Mixed ionic–electronic conductors are widely used in devices for energy conversion and storage. Grain boundaries in these materials have nanoscale spatial dimensions, which can generate substantial resistance to ionic transport due to dopant segregation. Here, we report the concept of targeted phase formation in a Ce0.8Gd0.2O2−δ–CoFe2O4 composite that serves to enhance the grain boundary ionic conductivity. Using transmission electron microscopy and spectroscopy approaches, we probe the grain boundary charge distribution and chemical environments altered by the phase reaction between the two constituents. The formation of an emergent phase successfully avoids segregation of the Gd dopant and depletion of oxygen vacancies at the Ce0.8Gd0.2O2−δ–Ce0.8Gd0.2O2−δ grain boundary. This results in superior grain boundary ionic conductivity as demonstrated by the enhanced oxygen permeation flux. This work illustrates the control of mesoscale level transport properties in mixed ionic–electronic conductor composites through processing induced modifications of the grain boundary defect distribution.

Soft and central mode behaviour in PbMg1/3Nb2/3O3 relaxor ferroelectric

The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of broad-band dielectric and Fourier Transform Infrared (FTIR) transmission spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures between 20 and 900 K using PMN films on infrared transparent sapphire substrates. While thin film relaxors display reduced dielectric permittivity at low frequencies, their high frequency intrinsic or lattice response is shown to be the same as single crystal/ceramic specemins. It was observed that in contrast to the results of inelastic neutron scattering, the optic soft mode was underdamped at all temperatures. On heating, the TO1 soft phonon followed the Cochran law with an extrapolated critical temperature equal to the Burns temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode frequency leveled off and slightly increased above the Burns temperature. A central mode, describing the dynamics of polar nanoclusters appeared below the Burns temperature at frequencies near the optic soft mode and dramatically slowed down below 1 MHz on cooling below room temperature. It broadened on cooling, giving rise to frequency independent losses in microwave and lower frequency range below the freezing temperature of 200 K. In addition, a new heavily damped mode appeared in the FTIR spectra below the soft mode frequency at room temperature and below. The origin of this mode as well as the discrepancy between the soft mode damping in neutron and infrared spectra is discussed.

Far-infrared soft-mode behavior in PbSc1∕2Ta1∕2O3 thin films

Temperature dependences of the optic phonons in PbSc1∕2Ta1∕2O3 (PST) sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range of 20–900K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80cm−1 at 10K softens on heating to ≈45cm−1 following the Cochran law with extrapolated critical temperature near 700K which is 400K above the temperature of dielectric maximum, Tm. Above 600K the TO1 mode remains stabile. This mode can be assigned to the A1 component of the ferroelectric soft-mode inside polar clusters which form below the Burns temperature near 700K. In the ordered PST film another mode activates below Tm in the infrared spectra near 60cm−1, also exhibiting an anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A1 component of the F2g Raman active mode. The central mode, which app...

Acceptor Doped BiFeO3 Ceramics: A New Material for Oxygen Permeation Membranes

A new intermediate temperature mixed conducting material has been fabricated based on acceptor doping of bismuth ferrite (BiFeO3). Acceptor doping at levels of 5 mol % with either Ca or Sr was found to increase the maximum operating temperature of ceramics by more than 100 °C as well as eliminate secondary phase formation often reported in the pure BiFeO3 system. Mixed ionic and electronic conductivity was confirmed by measuring the oxygen permeation properties of doped ceramics which exhibited fluxs on the order of 0.018 µmol/(cm2 s) at 800 °C over the oxygen partial pressure range 0.21 atm (air) to 10-6 atm (He). Ceramic membranes showed an increase in oxygen flux with decreasing sample thickness indicating good surface catalysis properties of the system.

The Impact of chemical ordering on the dielectric properties of lead scandium tantalate Pb(Sc1∕2Ta1∕2)O3 thin films

The impact of chemical ordering on the dielectric properties of the thin film relaxor Pb(Sc1∕2Ta1∕2)O3 (PST) was investigated. It was found that the dielectric permittivity increased with increased B site order, directly opposite the behavior observed in ceramics. Highly ordered PST thin films on sapphire substrates were found to behave as conventional ferroelectrics with dielectric permittivities near 7000 and well developed polarization hysteresis loops below the phase transition temperature. In contrast, disordered thin films were found to exhibit relaxor behavior with the thin film permittivity reduced by an order of magnitude as compared to ceramic specimens. The direct experimental evidence of highly ordered films and ceramics possessing similar properties under similar processing conditions points to intrinsic differences in the thin film relaxor state as compared to the ceramic relaxor state. It is proposed that the low processing temperatures employed in thin film fabrication do not provide suffi...

A sinteractive Ni–BaZr0.8Y0.2O3−δ composite membrane for hydrogen separation

BaZr0.8Y0.2O3−δ (BZY) is an excellent candidate material for hydrogen permeation membranes due to its high bulk proton conductivity, mechanical robustness, and chemical stability in H2O- and CO2-containing environments. Unfortunately, the use of BZY as a separation membrane has been greatly restrained by its highly refractory nature, poor grain boundary proton conductivity, high number of grain boundaries resulting from limited grain growth during sintering, as well as low electronic conductivity. These problems can be resolved by the fabrication of a Ni–BZY composite membrane with large BZY grains, which requires the development of a sinteractive Ni–BaZr0.8Y0.2O3−δ materials system. In this work, Ni–BZY composite membranes have been fabricated by three methods: (i) a combined EDTA-citric method, (ii) a solid state reactive sintering method, and (iii) a solid state reaction method. The effects of different fabrication methods on the sintering activity, microstructure, and phase composition have been systematically investigated by dilatometry, scanning electron microscopy, and powder X-ray diffraction. After reduction, only Ni–BZY membranes prepared through the solid state reaction method were observed to be dense with large BZY grains (∼1 μm). It has been found that the densification and grain growth of Ni–BZY composite membranes were controlled by the method and sequence of NiO introduction during composite membrane processing. After process optimization, a 0.44 mm-thick Ni–BZY dense composite membrane was fabricated using the solid state reaction method which exhibited a hydrogen flux of 4.3 × 10−8 mol cm−2 s−1 in wet 40% H2 at 900 °C, significantly higher than those of non-BaCeO3-based hydrogen separation membranes.

Characterization of 3D interconnected microstructural network in mixed ionic and electronic conducting ceramic composites

The microstructure and connectivity of the ionic and electronic conductive phases in composite ceramic membranes are directly related to device performance. Transmission electron microscopy (TEM) including chemical mapping combined with X-ray nanotomography (XNT) have been used to characterize the composition and 3-D microstructure of a MIEC composite model system consisting of a Ce0.8Gd0.2O2 (GDC) oxygen ion conductive phase and a CoFe2O4 (CFO) electronic conductive phase. The microstructural data is discussed, including the composition and distribution of an emergent phase which takes the form of isolated and distinct regions. Performance implications are considered with regards to the design of new material systems which evolve under non-equilibrium operating conditions.

The Influence of Acceptor Doping on the Structure and Electrical Properties of Sol-Gel Derived BiFeO3 Thin Films

Bismuth Ferrite (BFO) films doped with strontium resulting in a nominal composition of Bi 1−x Sr x Fe 1 O 3 (x = 0–0.15) were fabricated using a sol-gel process on Pt/TiO 2 /SiO 2 /Si substrates by rapid thermal annealed in air at 700°C. A decrease in the grain size (from 1 μm to 90 nm) with increasing Sr concentration from 0 to 15% was observed, coupled with the degradation of columnar grain growth. X-ray diffraction and raman scattering showed evidence of a structural change from rhombohedral to pseudocubic with increasing Sr concentration. Leakage measurements revealed that films with Sr acceptor doping showed increased leakage compared to pure BFO, however the film leakage did not increase with increasing acceptor concentration due to microstructural modifications.

The Oxygen Permeation Properties of Nanocrystalline CeO2 Thin Films

The measurement of oxygen flux across nanocrystalline CeO{sub 2} cerium oxide thin films at intermediate temperature (650 to 800 C) is presented. Porous ceria support substrates were fabricated by sintering with carbon additions. The final dense film was deposited from an optimized sol-gel solution resulting in a mean grain size of 50 nm which displayed oxygen flux values of up to 0.014 {micro}mol/cm{sup 2}s over the oxygen partial pressure range from air to helium gas used in the measurement at 800 C. The oxygen flux characteristics confirm mixed ionic and electronic conductivity in nanocrystalline ceria films and demonstrate the role of size dependent materials properties as a design parameter in functional membranes for oxygen separation.

A-site compositional effects in Ga-doped hollandite materials of the form BaxCsyGa2x+yTi8-2x-yO16: implications for Cs immobilization in crystalline ceramic waste forms.

The hollandite structure is a promising crystalline host for Cs immobilization. A series of Ga-doped hollandite BaxCsyGa2x+yTi8−2x−yO16 (x = 0, 0.667, 1.04, 1.33; y = 1.33, 0.667, 0.24, 0) was synthesized through a solid oxide reaction method resulting in a tetragonal hollandite structure (space group I4/m). The lattice parameter associated with the tunnel dimension was found to increases as Cs substitution in the tunnel increased. A direct investigation of cation mobility in tunnels using electrochemical impedance spectroscopy was conducted to evaluate the ability of the hollandite structure to immobilize cations over a wide compositional range. Hollandite with the largest tunnel size and highest aspect ratio grain morphology resulting in rod-like microstructural features exhibited the highest ionic conductivity. The results indicate that grain size and optimized Cs stoichiometry control cation motion and by extension, the propensity for Cs release from hollandite.