TEM investigation on zirconate formation and chromium poisoning in LSM/YSZ cathode

Cell durability is a crucial technological issue for SOFC commercialization, and considerable progress has been made in recent years. A number of degradation pathways have been established, amongst which microstructural changes, poisoning effects and formation of less conductive phases. In this study, transmission electron microscopy was used to observe submicron-scale effects on selected cathode zones of an anode supported cell tested in SOFC stack repeat element configuration. The test has been performed with a dedicated segmented test bench, at 800 A degrees C for 1900 h, which allowed to spatially resolve degradation processes, and therefore to improve their correlation with localized post-test analysis. Evidence is presented of reaction products (mainly SrZrO(3)) at the LSM/YSZ interfaces as well as of contaminants, in particular Cr, but also Si. A polarized cell segment is compared to an unpolarized one, to assess any influence of cathode polarization.

Combinatorial High-Vacuum Chemical Vapor Deposition of Lithium Niobate Thin Films

Ali Dabirian

Lithium niobate is a key material for photonics with applications in optical modulators and light frequency converters. Present lithium niobate optical devices are realized in bulk lithium niobate single crystals while interests in higher integration and also better performance derives efforts towards high quality epitaxial lithium niobate thin films. In the past, epitaxial lithium niobate thin films obtained using chemical vapor deposition (CVD), pulsed-laser deposition and crystal-ion slicing. Among these methods CVD is the one that can provide large area growth. Optimizing films properties stays an issue in CVD because in CVD of multi-element oxides using multiple precursors the stoichiometry of the film does not reflect the precursor ratios and additionally it varies with precursors total flux values. In this thesis optimization and discovery of multi-element oxide thin films were performed in a high-vacuum CVD reactor that is capable of performing combinatorial experiments. Using this reactor epitaxial lithium niobate thin films were obtained on sapphire {001g} and lithium tantalate {001} substrates with rocking curve full-width at half-maximum (FWHM) values of 0.03° and 0.024° respectively. The epitaxial growth of lithium niobate films was confirmed from X-ray diffraction (XRD) measurements and transmission-electron microscope (TEM) observations. Niobium tetra-ethoxy-di-methy-amino-ethoxide (Nb(OEt)4(dmae)) and lithium tert-butoxide (Li(OBut)) were used as precursors. Optically induced inhomogeneities, also known as optical damage, in lithium niobate limit its usage for high laser intensity applications. Lithium niobate single crystals doped with hafnium showed improved optical damage thresholds. Therefore combinatorial HVCVD experiments were conducted and textured {001} hafnium-doped lithium niobate films, with about 6 [mol%] hafnium, were obtained on sapphire f001g. Although the crystalline quality of the film is not high (rocking curve FWHM 0.8°) its Raman spectrum is still quite close to lithium niobate single crystal and epitaxial lithium niobate films on sapphire. Hafnium tert-butoxide (Hf(OBut)4) was used as precursor for hafnium. In the HVCVD reactor, a precursor transport system was applied such that spatial control of the precursor impinging rate on the substrate can be tailored. The precursor flux distribution that was mostly used in this thesis was a linear gradient with ratio of three across a 150 mm diameter circular area. In fact combinatorial experiments were performed by producing a linear gradient for individual precursors over the substrate with a certain angle between the gradient directions. The linear gradient feature was used to perform efficiently the systematic study of single precursors providing the benefit that one deposition at certain substrate temperature gives information about deposition kinetics for a vast range of precursor flux values. A systematic study of the Nb(OEt)4(dmae) precursor was performed using this feature and chemical-reaction-limited, mass-transport-limited, and desorption-limited regimes of CVD process were identified. The possibility of screening a large range of precursor flux values led us to discovering a CVD regime in which deposition rate is decreased by raising the precursor flux. A model was proposed for interaction of gas with surfaces that can explain all the different CVD regimes observed. The precursor molecules in HVCVD reactor reach the substrate with nearly no gasphase collision. Based on this fact a lift-off process was proposed for large-area in-situ patterning of multi-element oxide thin films.

EPFL2010

Toxicity study of nanostructures

Lenke Horváth

The great achievement of nanotechnology is the controlled synthesis of a large variety of nanometer-size materials like needle-formed nanotubes, nanowires and two-dimensional graphene flakes. Due to their unique physico-chemical properties, these nanostructures are considered to be of great benefit for many applications in engineering, electronics, alternative energies and nanomedicine. Since the expectations for the improvement of our everyday life through engineered nanomaterials are high, there is rapid expansion of their manufacturing, which makes it likely that intentional and unintentional human and environmental exposure will increase in the near future. Consequently, the concern grows, related to their possible health hazards, as some of them strongly resemble asbestos. Motivated by this issue, we have investigated the in vitro acute cellular toxicity associated with four model nanomaterials: carbon nanotubes, boron nitride nanotubes, titanium dioxide nanofilaments and graphene oxide. This study focused on the toxic effect these nanomaterials had on cell types found in the respiratory system, where exposure to these materials is most prominent. These are lung epithelial cells, macrophages, and fibroblasts, but also other cell types like kidney cells were tested. The cytotoxicity was assessed by using MTT, DNA and FMCA assays, which measure different endpoints such as metabolic activity, cell proliferation and viable cell number. The cell death was determined by the Annexin V assay. We employed various microscopic techniques: light microscopy to reveal the morphological alterations associated with the nanomaterial toxicity at the cellular level; scanning electron microscopy to study the cell-nanomaterial interactions on the surface of the cell membrane; transmission electron microscopy to examine the uptake and subsequent localization of the nanomaterials within the cytosol and cell organelles. In addition, the generation of intracellular reactive oxygen species induced by graphene oxide was detected by the DCF assay. Last but not least, the pro-inflammatory potential and the biochemical perturbations in cells exposed to boron nitride nanotubes were investigated by Western blot and Synchrotron Infrared Microspectroscopy (SIRMS), respectively. All these techniques point to the adverse effects of the investigated nanostructures: i) The toxic effect of carbon nanotubes and carbon nanoparticles showed a time- and dose-dependent impairment in the metabolic activity of the cells characteristic for each cell type. Moreover, distinct morphological alterations typical for cell death were particularly apparent in macrophages. ii) The toxic potential of boron nitride nanotubes exhibited more pronounced adverse effects than carbon nanotubes. This was demonstrated by cell viability assays combined with cytopathological and biochemical analyses, showing induction of serious morphological changes, particularly in macrophages and fibroblasts, and biochemical processes characteristic for cell death. A higher acute toxicity was determined for boron nitride nanotubes when compared to crocidolite asbestos and their pro-inflammatory potential was demonstrated by the secretion of mature IL-1β cytokine in macrophages. iii) Titanium dioxide nanofilaments were also shown to impair the metabolic activity of the studied cells and induce morphological changes pointing to cell insult. iv) Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy showed that graphene oxide flakes are taken up via the endocytic pathway. In summary, our findings highlight important physico-chemical parameters, which are important in relation to the toxic effect of nanomaterials. These are: i) the chemical composition; ii) the surface modification including functionalized groups and structural defects; iii) the geometry: length, diameter and tortuosity. In addition to the identified nanomaterial characteristics, we pinpoint that the target cells ́ response depends on their type, which is likely to be linked to their physiological function.

EPFL2012

Evaluation of the toxicity of graphene derivatives on cells of the lung luminal surface

Marko Burghard, László Forró, Lenke Horváth, Klaus Kern, Arnaud Magrez, Beat Schwaller

Graphene-based nanomaterials are expected to have a profound impact on a broad range of applications. However, studies devoted to investigating the putative adverse health effects of these nanomaterials are hugely underrepresented in the current scientific literature. We have investigated the in vitro short-term cellular toxicity associated with graphene derivatives (GD): graphene oxide and reduced graphene oxide. This study focused on the toxicity of GD on two cell types.(i.e., epithelial cells and macrophages) found in the luminal aspect of the respiratory system, where the initial exposure to these materials is most prominent. Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy studies showed that graphene oxide flakes of different sizes are taken up by cells via an endocy tic pathway, both in epithelial cells and macrophages. (C) 2013 Elsevier Ltd. All rights reserved.