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Interface engineering of ionic conductor multilayer and cathode nanocomposite thin film oxides

Final Report Summary - OXIDE INTERFACES (Interface engineering of ionic conductor multilayer and cathode nanocomposite thin film oxides)

Introduction

In the last strategic plan, the European community has stressed the importance of reducing greenhouse effect and developing renewable energy sources. The EU’s goal is to achieve a 20% reduction in greenhouse gas emissions by 2020, mainly by boosting the use of renewable energy and cutting energy consumption. Among the different candidate technologies to substitute the present dependence on fossil energy sources, solid oxide fuel cells (SOFCs), that can use different fuels as an energy source (hydrogen, methane etc.), appears to be one of the most promising. Reducing the SOFC’s cells cost to make them more attractive to the industrial market, by lowering their operating temperature down to an intermediate level of 500-600ºC, is a fundamental goal. To low the operating SOFC temperature maintaining the energy conversion performances it is imperative to develop new materials and in particular the ones employed for the fuel cell cathode.
The fundamental aim of this project was the investigation of the physical and chemical properties of nanostructured thin films and their interfaces related to the possibility of designing a new generation of materials for electrochemical devices such as oxygen membranes, SOFC cathodes, electrolyte and gas sensors. Pulsed laser deposition technique has been used to deposit ultra thin film heterostructures with a thickness control in the nanometric range. It also allowed the fabrication of films with a controlled defect density and films with different level of residual strain to study how these elements affect the ionic conductivity.

Results

The main results obtained are:

• During the study of the YSZ epitaxial thin film growth by PLD on different single crystal substrates, we found an interesting growth behaviour. When the mismatch between film and substrate is larger than a certain value, YSZ on MgO(100) single crystal for example, the semi coherent film growth is less probable and the surface energy component of the nucleation free-energy takes over favouring the (111) YSZ film orientation. We discovered as the substrate surface roughness plays a fundamental role in releasing the theoretical large heterostructure strain stabilizing the semi-coherent (100) YSZ film orientation trough the formation of a large number of dislocations.

• Controversial YSZ enhancement ionic conductivity obtained by epitaxial strained thin films has been reported in the literature. Inconsistent conductivity values ranging from an enhancement of up to three orders of magnitude [1, 2] to a reduction by a factor of around four [3] have been observed by AC impedance spectroscopy .
Our work has carefully separated the ionic and electronic contribution by combining the AC-impedance spectroscopy characterization and the isotope oxygen exchange diffusion experiments. Lattice strain, dislocation density and dopant segregation, possible causes of electronic or ionic enhancement effect, have been cautiously studied by Transmission Electron Microscopy (TEM).

• We succeed on preparing dense bulk target of La0.8Sr0.2Ga0.8Mg0.2O3 (LSGMO) by sol gel technique and growing epitaxial thin films by PLD with it. The film strain has been modulated by using different single crystal substrates with different crystallographic cell parameters. The isotope oxygen diffusion experiments and the D coefficients obtained by SIMS analysis have clearly shown the effect of strain on oxygen diffusivity.

• The influence of the gas atmosphere on the thermal annealing process needed to obtain a fully A-site NGO(110) terminated surface has been investigated. To study the uppermost NGO surface atomic composition, LEIS (Low Energy Ion Scattering) which has a unique sensitivity to the outermost atomic layer has been employed. Our results clearly demonstrate for the first time, the role of the atmosphere in the control of the NGO single crystal surface termination during thermal annealing.


Fig.1: TEM analysis of a lamella prepared by focus ion beam FIB from an NGO(110)sc as- received from the supplier. The first 20-23 nm contain Nd and Ga but in an amorphous state. The LEIS depth analysis has been superimposed for comparison.


• The cubic delta-phase Bi2O3 ionic conductor, naturally stable over 730°C and which holds the ionic conductivity material record, has been successfully stabilized at room temperature by PLD epitaxial thin film growth. The isotope oxygen diffusion together with AC-measures and TEM analysis have completed the film characterization.

• LEIS has also been employed to characterize the chemical composition of the STO/LAO interface (two-dimensional electron gas). This work provided the missing understanding of the non-conducting behaviour observed when LAO is grown on h-STO and sets the basis for future works on superlattices where conducting interfaces have also to be formed with STO thin films.


FIG. 2. (a) LEIS depth profile across the interface of the Sr/Ti ratio for different thicknesses of h-STO films grown with the 'standard' target and a bare TiO2-terminated STO substrate. (b) h-STO thickness dependency of the area below the Sr/Ti ratio curve and extent of the Sr-rich region. (c) h-STO thick-ness dependency of the surface Sr/Ti ratio of the same samples


• Oxygen 18 isotope exchange and diffusion TOF-SIMS measures on the epitaxial LCNO thin films have been performed. The results suggested a surprising interface effect. This effect seems related to a change in the chemical composition of the single crystal substrate (NGO) during the PLD deposition process. If confirmed by the TEM analysis it will be the first time that an increase of protonic conductivity due to interface effect is observed. New samples have been prepared to corroborate this finding by TEM analysis. (Analysis underway)

• A pioneering new concept of nano-composite cathodes has been developed. We demonstrated the feasibility of the sphere nanolithography method to product new cathodes nano-composites. Fig.3 (a,b) show, by secondary electron microscopy analysis (SEM), the polystyrene ordered nano-spheres before and after the silver deposition and sphere removal. The final YSZ-Ag nano-composite cathode obtained by the polystyrene nanolithography method has been analysed by TEM in fig.3c.


Fig.3a on the left, is a FE-SEM surface image of a silicon single crystal recovered by a single layer of self-assembled polystyrene nanoparticles. In Fig.3b instead the silver matrix obtained after the RIE sphere diameter reduction, the silver sputtering deposition and the final nanoparticles removal has been shown (SEM). The final YSZ-Ag cathode nanocomposite has been analyzed by TEM in fig.3c.

The innovating methodology proposed in this project to create a new class of ionic and MIEC materials is expected to become an important tool for a better comprehension of the interface effect involved in the cathode oxygen reduction. The pioneering use of the LEIS technique to study the chemical change in material surface has demonstrated its primary importance in understanding interface and surface unexpected phenomena.

[1] Sillassen, M., et al., Low-Temperature Superionic Conductivity in Strained Yttria-Stabilized Zirconia. Advanced Functional Materials, 20(13):2071-2076, 2010.
[2] Kosacki, I., et al., Nanoscale effects on the ionic conductivity in highly textured YSZ thin films. Solid State Ionics, 176(13-14):1319-1326, 2005.
[3] Guo, X., et al., Ionic conduction in zirconia films of nanometer thickness. Acta Materialia, 53(19):5161-5166, 2005