The electrical conductivity of dense and nanoporous zirconia‐based thin films is compared to results obtained on bulk yttria stabilized zirconia (YSZ) ceramics. Different thin film preparation methods are used in order to vary grain size, grain shape, and porosity of the thin films. In porous films, a rather high conductivity is found at room temperature which decreases with increasing temperature to 120 °C. This conductivity is attributed to proton conduction along physisorbed water (Grotthuss mechanism) at the inner surfaces. It is highly dependent on the humidity of the surrounding atmosphere. At temperatures above 120 °C, the conductivity is thermally activated with activation energies between 0.4 and 1.1 eV. In this temperature regime the conduction is due to oxygen ions as well as protons. Proton conduction is caused by hydroxyl groups at the inner surface of the porous films. The effect vanishes above 400 °C, and pure oxygen ion conductivity with an activation energy of 0.9 to 1.3 eV prevails. The same behavior can also be observed in nanoporous bulk ceramic YSZ. In contrast to the nanoporous YSZ, fully dense nanocrystalline thin films only show oxygen ion conductivity, even down to 70 °C with an expected activation energy of 1.0 ± 0.1 eV. No proton conductivity through grain boundaries could be detected in these nanocrystalline, but dense thin films.

No abstract

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.

The introductory part reviews the impact of thin film fabrication, precipitation versus vacuum‐based methods, on the initial defect state of the material and microstructure evolution to amorphous, biphasic amorphous‐nanocrystalline, and fully nanocrystalline metal oxides. In this study, general rules for the kinetics of nucleation, crystallization, and grain growth of a pure single‐phase metal oxide thin film made by a precipitation‐based technique from a precursor with one single organic solvent are discussed. For this a complete case study on the isothermal and non‐isothermal microstructure evolution of dense amorphous ceria thin films fabricated by spray pyrolysis is conducted. A general model is established and comparison of these thin film microstructure evolution to kinetics of classical glass‐ceramics or metallic glasses is presented. Knowledge on thermal microstructure evolution of originally amorphous precipitation‐based metal oxide thin films allows for their introduction and distinctive microstructure engineering in devices‐based on microelectromechanical (MEMS) technology such as solar cells, capacitors, sensors, micro‐solid oxide fuel cells, or oxygen separation membranes on Si‐chips.

No abstract

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr0.85Y0.15O1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.

Microstructures of yttria-stabilized zirconia (YSZ) thin fi lms deposited by spray pyrolysis at 370 ° C on sapphire are investigated. The as-deposited fi lms are predominantly amorphous and crystallize upon heating at temperatures above 370 ° C, developing grains in the range of 5 nm to several 100 nm. During post-deposition heat treatment up to 800 ° C, ∼ 50 vol% porosity develops in the center of the fi lms with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin fi lms consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 ° C. In gadoliniumdoped-ceria (CGO) thin fi lms similar microstructures and porosity are found after low-temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin fi lms is observed in both the YSZ and in CGO thin fi lms. Stagnating grain growth in the thin fi lms is rather caused by reduced grain boundary mobility, here predominately due to a "secondary phase", i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densifi cation has taken place.sapphire substrate and the obtained powder was measured in a Pt crucible up to 1300 ° C at 3 ° C min − 1 . Buoyancy and heat fl ow corrections were performed with the data from a second run using fully crystallized YSZ spray-pyrolyzed powder of the same mass.

The electrical cross-plane conductivity of 8 mol% yttria stabilized zirconia (YSZ) thin films prepared by different deposition techniques, namely aerosol assisted chemical vapor deposition, wet spray pyrolysis (SP), and pulsed laser deposition (PLD), is correlated with their microstructure. Depending on deposition technique and process conditions, microstructures ranging from amorphous to randomly oriented nanocrystalline or columnar with preferred (111) orientation are obtained. Cross-plane AC impedance measurements of these thin films show that the oxygen ion conductivity of randomly oriented nanocrystalline samples is determined by the grain boundaries, which show significantly lower transport properties than the grain interior. In columnar microstructures, the conductivity is determined by ionic transport through the grains only. The same conduction behavior is found for amorphous and randomly oriented microstructures with grain sizes between 3 nm and 9 nm, indicating that no true size effects occur in 8 mol% YSZ.Grain and grain boundary conductivity determined for nanocrystalline 8 mol% yttria stabilized zirconia thin films of different microstructures.