Endogenous O-17 Dynamic Nuclear Polarization of Gd-Doped CeO2 from 100 to 370 K

O-17 NMR is an invaluable tool to study the structure and dynamics of oxide materials but remains challenging to apply in many systems. Even with isotopic enrichment, studies of samples with low masses and/or concentrations of the active species, such as thin films or interfaces, are limited by low sensitivity. Here, we show how endogenous dynamic nuclear polarization (DNP) can dramatically improve the sensitivity in the oxide-ion conductor Gd-doped CeO2, with a O-17 enhancement factor of 652 at 100 K. This is the highest enhancement observed so far by endogenous DNP or Gd3+ DNP, which is explained in terms of the electron paramagnetic resonance characteristics. The DNP properties are studied as a function of Gd concentration for both enriched and natural-abundance samples, and the buildup behavior shows that spin diffusion in O-17-enriched samples improves sensitivity by relaying hyperpolarization throughout the sample. Notably, efficient hyperpolarization could still be achieved at elevated temperatures, with enhancement factors of 320 at room temperature and 150 at 370 K, paving the way for the characterization of materials under operational conditions. Finally, the application of endogenous Gd3+ DNP is illustrated with the study of interfaces in vertically aligned nanocomposite thin films composed of Gd-CeO2 nanopillars embedded in a SrTiO3 matrix, where DNP affords selective enhancement of the different phases and enables a previously infeasible two-dimensional correlation experiment to be performed, showing spin diffusion between Gd-CeO2 and the solid-solid interface.

Stretchable High Response Piezoelectric Elastomers Based on Polable Polynorbornene Fillers in a Polydimethylsiloxane Matrix

Francis Owusu

To advance the field of piezoelectrics, it is desirable to find materials that combine high piezoelectricity with high elasticity. Applications such as self-sensors, stretchable electronics, soft robots, and energy harvesting would benefit tremendously. However, this is not an easy task neither for materials based on ceramics nor for those based on polymers. While ceramics can show strong piezoelectric effects, their mechanical response tends to be weak. Polymers instead are elastic but only few have sizeable piezoelectric effects. Additionally, they cannot maintain the polarization permanently as needed for piezoelectrics. Here, an all-organic piezoelectric elastomer is synthesized by blending high glass transition temperature (T-g = 104 degrees C) polar polynorbornene nanoparticles (NPs) with a high relaxation strength (Delta epsilon ' = 22.4) into a chemically cross-linked polydimethylsiloxane matrix. After processing the blends into thin films by doctor blading, they are poled by corona discharge at elevated temperatures. Fifteen days after poling, the materials show a stable and reversible piezoelectric response d(31) = 37 pC N-1. This, to the best of the authors' knowledge, not only is the highest d(31) value reported, but the response is three times that of the well-known polyvinylidene difluoride.

WILEY-V C H VERLAG GMBH2022

Hyperpolarization of inorganic solids

Snaedis Björgvinsdóttir

Solid-state NMR can provide information about the atomic level structure and dynamics of materials. It directly probes symmetry and structure at nuclear sites, and is especially useful for investigation of disordered or amorphous solids that lack long range order. However, the application of solid-state NMR is sometimes limited by its relatively low sensitivity, caused by low concentrations and low gyromagnetic ratios of the magnetically active nuclei. Dynamic nuclear polarization (DNP) can provide significant signal enhancements in magic-angle-spinning (MAS) NMR experiments. This is usually achieved by introducing stable organic radicals to the sample of interest, and transferring their large electron spin polarization to nearby nuclear spins via microwave irradiation near to the EPR frequency. Methods to hyperpolarize the bulk of solid materials containing protons are well established, whereas NMR of proton-free bulk materials remains challenging in many cases, especially when nuclear relaxation times are long, and if isotopic enrichment or paramagnetic doping to enhance relaxation rates are not feasible. The overall objective of the work described in this thesis is to improve sensitivity in DNP enhanced solid-state NMR experiments, and to extend the application of DNP to systems that are currently difficult to access. In particular, this includes developing a strategy to hyperpolarize the bulk of proton-free inorganic materials. In the first part, the classic flip-back method to recover bulk proton magnetization is combined with DNP of 1H containing solids with characteristic build-up times spanning two orders of magnitude. Gains in sensitivity in the 13C spectra of powdered crystalline theophylline, histidine and salicylic acid are reported, on top of the enhancements already provided by relayed DNP. In the second part, a general strategy where the relayed DNP method is extended to proton-free inorganic materials is reported. The method uses a combination of impregnation DNP and slow spin diffusion between weakly magnetic nuclei such as 119Sn and 31P. Hyperpolarization is continuously generated at the surface either by direct DNP of the weakly magnetic nuclei, or by multiple bursts of cross polarization (CP) from protons in the wetting phase. Provided that bulk T1 values are long, even slow spin diffusion can then transfer the surface-generated hyperpolarization to the bulk, resulting in spectra which exceed the sensitivity of conventional solid-state NMR. As an example, multiple contact CP can provide a factor 50 gain in overall sensitivity of the 119Sn spectrum of SnO2, allowing access to materials that were previously unfeasible to study. Overall in this thesis, hyperpolarization transport by spin diffusion is confirmed between 31P nuclei in GaP and Sn2P2O7, 119Sn in SnO2, 113Cd in CdTe, 29Si in SiO2 (âº-quartz) and 6Li/7Li in lithium titanates, and shown to improve sensitivity in their bulk NMR spectra. Strategies to optimize bulk hyperpolarization are shown, as well as two-dimensional spin diffusion experiments which provide insight into the process of polarization transfer from surface to bulk. In the third part, DNP is explored at the highest field and spinning frequencies available for DNP to date. NMR signal enhancements of 200 are reported at 21.1 T, enabled by 65 kHz MAS at 100 K. The fast spinning frequencies also yield high resolution DNP enhanced 1H-detected heteronuclear correlation spectra.

EPFL2021

Advanced analytical electron microscopy applied to Solid Oxide Cell materials and their degradation

Stéphane François Jean Poitel

Electron microscopy offers a wide range of techniques to study materials. Environmental Scanning Electron Microscopy (ESEM), in particular, allows the characterisation of samples at high-temperature using a laser heating stage, and under various gaseous atmospheres leading to direct observation of the sample surface morphology dynamics in conditions that are close to those in real operating devices (i.e. at high temperature in various gases). Solid Oxide Fuel Cells (SOFC) operating around 800°C in both oxidising and reducing gases, with proven co-generation efficiencies of 90%, offer very promising solutions in the ongoing energy transition. However, production costs and materials degradation in those demanding operating conditions remain significant barriers to their large-scale deployment. Studying SOFC materials' degradation through a combination of ESEM with other advanced microscopy techniques fulfilled the double aim of demonstrating the power of an ESEM and improving the understanding of SOFC materials' degradation during operation. Two materials from the SOFC stack were selected for degradation case studies at around 800°C in oxidising/reducing gases: coated ferritic steel interconnect and Ni-YSZ anode. The objectives were to explore the parameters' influence (temperature, heating ramp, pressure, gas exposure...), set the boundaries and define the guidelines of the ESEM technique while demonstrating its capabilities. The two different materials sets were selected for their relevance both for the SOFC field and for their different specific characteristics allowing a wide range of experiments to be explored. A specific method for the study of the oxidation of coated steel was developed combining ESEM, Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM). Post-test segmentation of the in-situ observed images demonstrated the possibility to obtain quantitative data. FIB sample preparation from the exact site of ESEM observation allowed to correlate the underlying microstructure with the in-situ surface observation. Applying this method, oxidation mechanisms of several coatings were characterised and specific behaviours observed and explained such as the appearance of oxide ridges and the behaviour of a cerium barrier layer in case of cobalt-cerium coated steel. The heating ramp speed and the oxidation temperature were demonstrated to be crucial parameters to obtain the desired microstructure. The nitriding pretreatment of interconnect was investigated too and shows promising results for improving the steel oxidation resistance. Another combination of techniques was developed for the study of the Ni-YSZ anode by complementing the ESEM observation with continuous Mass Spectrometer (MS) acquisition. The first reduction of the NiO-YsZ anode was analysed under gas flows of hydrogen and hydrocarbons. Redox cycling effect on the anode microstructure was investigated and the resulting observations were in line with models from the literature for the reduction and re-oxidation kinetics. Finally, the demonstration of an active solid oxide cell in the microscope operating at more than 700°C under mixed gas paves the way to further possible studies approaching the device's observation in real operating conditions. Limitation of the ESEM techniques are discussed and recommendations for further improvements and perspectives given.