Barbara Scherrer

Engineering disorder in precipitation-based nano-scaled metal oxide thin films

Distinctive microstructure engineering of amorphous to nanocrystalline functional metal oxide thin films for MEMS devices is of high relevance to allow for new applications, quicker response times, and higher efficiencies. Precipitation-based thin-film techniques are first choice. However, these often involve organic solvents in preparation. Their relevance on the disorder states of amorphous to fully crystalline metal oxides is unclear, especially during crystallization. In this study the impact of organic solvents on the as-deposited amorphous state and crystallization of the metal oxide, CeO(2), is reported for thin-film preparation via the precipitation-based method spray pyrolysis. The choice of organic solvent for film preparation, i.e. glycols of different chain lengths, clearly affects the structural packing and Raman bond length of amorphous states. Organic residues act as space fillers between the metal oxide molecules in amorphous films and affect strongly the thermal evolvement of microstructure, i.e. microstrain, crystallization enthalpy, crystallographic density, grain size during crystallization and grain growth. Once the material is fully crystalline, equal near- and long-range order characteristics result independent of organic solvent choice. However, the fully crystalline films still show decreased crystallographic densities, presence of microstrain, and lower Raman shifts compared to microcrystalline bulk material. The defect state of amorphous and fully crystalline thin-film microstructures can actively be modified via explicit use of organic glycols with different chain lengths for metal oxide films in MEMS.

Energetics of lanthanide cobalt perovskites: LnCoO3−δ (Ln = La, Nd, Sm, Gd)

Lanthanide cobalt perovskites LnCoO3−δ (Ln = La, Nd, Sm, and Gd) are important materials for electroceramics, catalysts, and electrodes in solid oxide fuel cells. Formation enthalpies of LnCoO3−δ compounds were measured using high temperature oxide melt solution calorimetry. The formation enthalpies of LaCoO2.992, NdCoO2.985, SmCoO2.982 and GdCoO2.968 from constituent binary oxides (Ln2O3, CoO) and O2 gas are −111.87 ± 1.36, −98.49 ± 1.33, −91.56 ± 1.46 and −88.16 ± 1.45 kJ mol−1, respectively. Thus these perovskites become energetically less stable with decrease in ionic radius of the lanthanide (from La to Gd), which corresponds to a decreasing tolerance factor and increasing oxygen deficiency. The thermodynamic stability of LaCoO2.992, NdCoO2.985, SmCoO2.982 and GdCoO2.968 was also assessed considering their oxygen partial pressures for decomposition, with good agreement between thermochemical and equilibrium data.

The Hidden Pathways in Dense Energy Materials – Oxygen at Defects in Nanocrystalline Metals

Highly abundant oxygen-rich line defects (blue) can act as fast oxygen transport paths. These defects show similar chemistry and therefore similar catalytic activity to the materials surface. These results provide the opportunity to design and produce simple scalable structures as catalysts, whose functionality derives from internal defects rather than from the materials surfaces.

Grain and Grain Boundary Conductivities in Nanocrystalline Yttria-Stabilized-Zirconia Thin Films

Nanocrystalline yttria-stabilized-zirconia thin films with grain sizes smaller than 15 nm are fabricated by spray pyrolysis. Impedance spectroscopy is performed perpendicular to the thin film between room temperature and 600 °C. For a grain size of 13 nm, the grain and grain boundary contributions of the electrical conductivity can be discerned but only between 200 °C and 400 °C. For smaller grains, and higher or lower temperatures, the grain contribution cannot be resolved.

Nucleation and Grain Growth Kinetics of Amorphous to Nanocrystalline Ceria Solid Solutions

Nanocrystalline ceria-based thin films are of potential interest for use as gas-sensing layers and electrolytes in micro-Solid Oxide Fuel Cells (micro-SOFC) used for energy supply of next generation portables. In these devices the thin films have to be operated at intermediate to high temperatures (500 - 1000 °C) to be sufficiently high electrical conductive. However, only little is known on the nucleation and grain growth kinetics of pure ceria and its solid solutions when present as nanocrystalline thin film microstructures (average grain size < 100 nm). In this study amorphous, dense and crack-free CeO2 and Ce0.8Gd0.2O1.9-x thin films have been deposited by spray pyrolysis on sapphire. These films were crystallized to biphasic amorphous-nanocrystalline and fully nanocrystalline microstructures upon annealing with respect to time, temperature, heating rate and doping. Nucleation and grain growth kinetics were studied by differential scanning calorimetry, Xray diffraction analysis with in-situ heating chamber and scanning electron microscopy.

Self-Limited to Parabolic Grain Growth Kinetics in Metal Oxide Thin Films

Distinctive microstructure engineering of amorphous to nanocrystalline electroceramic thin films is of high relevance for integration in low to high temperature operating MEMS-devices. Up to now, kinetic rules of nucleation, crystallization and grain growth of precipitation-based ceramic thin films are unknown. In this study, general rules for the crystallization and grain growth kinetics of a pure single-phase metal oxide thin film with only one kind of cation, i.e. ceria, made by spray pyrolysis from a precursor with one single organic solvent is discussed [1,. The near-and long range disorder is studied via Raman, DSC investigation of crystallization enthalpy, XRD, SEM and TEM for amorphous to fully crystalline state. These 400 nm thick-thin films were dense, crack-free and amorphous directly after deposition on a sapphire substrate. Briefly, above deposition temperature crystallization sets in with respect to temperature and persists over a broad temperature range from 400 to 950°C. In this regime, biphasic amorphous-crystallien films exist and grain growth proceeds simultaneously to crystallization. Isothermal grain growth studies showed that after short dwell times of 10-20h stable microstructures established following self-limited grain growth law [. In this state, driving force for the crystallization is the reduction of free enthalpy for phase transformation and interface diffusion prevails. A transition to classical grain curvature-driven parabolic grain growth kinetics appeared once the material reached the fully crystalline state for average grain sizes larger than 140 nm and higher annealing temperatures. Volume diffusion was then activated in addition to the interface diffusion. It was found that once crystallized the material shows independent on processing route equal XRD density and microstrain, as well as Raman characteristics. However, dependent on processing conditions i.e. choice of organic and, according, deposition temperature of the film amorphous states vary and affect strongly crystallization and grain growth history for the biphasic films.

Electrochemical Characterization of Micro-Patterned La0,6Sr0,4Co0,2Fe0,8O3 Thin Film Structures on Fused Silica

The polarization resistance of solid oxide fuel cell (SOFC) cathodes strongly depends on the structure of the cathode-electrolyte interface. Therefore measurements of the polarization resistance of cathode thin films on electrolyte thin films are required in order to predict the performance of thin film SOFCs. In this study La0,6Sr0,4Co0,2Fe0,8O3 (LSCF) thin films were deposited by pulsed laser deposition on fused silica wafers carrying pre-structured Pt/Ta electrodes and a (Y2O3)0.08(ZrO2)0.92 (YSZ) thin film. Photolithography and argon sputtering were used to fabricate micro-patterns which proved to be applicable for the simultaneous characterization of LSCF thin films with respect to electrical in-plane and cross-plane conductivity as well as polarization resistance. In the future such test platforms are destined for the characterization of different cathode thin films with respect to conduction and oxygen reduction properties.