Universität Stuttgart
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Item Open Access Charge carrier formation, mobility and microstructure of sulfonated polyelectrolytes for electrochemical applications(2015) Wohlfarth, Andreas; Maier, Joachim (Prof. Dr.)Polyelectrolytes are materials consisting of a polymer backbone with covalently attached positively or negatively charge groups including their counterions. Sulfonated polyelectrolytes are a specific class, which is especially interesting for electrochemical application as they can be used to separate the electrodes and mediate the electrochemical reactions taking place at anode and cathode by conducting a specific ion; this ion may be H+ in the case of PEM-fuel cells or Li+ and Na+ in various battery systems. The key challenges for the development of these electrolytes is the combination of good mechanical properties and high ion transport as well as high electrochemical stability. High ionic conductivity of polyelectrolytes depends on the presence of small polar solvents to ensure efficient dissociation and mobility of the counterions. These processes cannot be understood by merely considering electrostatics (i.e. Deby-Hückel approach) such as in Manning counterion condensation theory. Specific interactions between solvent-ion-polymer and the molecular conformations have to be taken into account as well. This is one of the results of the present work using sulfonated polyelectrolytes, different cations and solvents as model systems with a combined approach of experimental techniques and simulations. The dissociation behavior of sulfonated polysulfones was investigated by a combined electrophoretic (E) NMR, pulsed magnetic field gradient (PFG) NMR and conductivity approach. Since the results from the NMR experiments, especially from E-NMR which is by far no standard measurement, are crucial for the key conclusions drawn in this thesis, some critical issues of this technique are studied and discussed in detail. E-NMR is essentially a PFG-NMR experiment with an applied electric field; the applied voltage can reach up to 300 V. Therefore, it was necessary to determine a measurement window in which no decomposition or other interfering effects appeared. In addition, polymers gererally exhibit some polydispersity with the low molecular weight fraction showing a higher diffusion coefficients and drift velocities, which had to be taken into account. By concentrating ionic groups on the polymer, specific polyelectrolyte effects show up. Dissociation is no longer complete, the interaction between ionic charges and the solvent is heavily modified and correlations of ionic motion start to appear. According to a MD-simulation, this very much depends on the polymer conformation and position of the ionic groups as well as the chemical nature of the solvent. Once the density of ionic groups (-SO3H) of polysulfones reaches a point where their average separation is of the order of the Bjerrum length of water, the degree of counterion condensation is shown to depend on details of the molecular structure and the accessible conformations of the polymer chain. In this regime, well-defined ionic aggregates occur, i.e. triple-ions form. The conformational details depend on the degrees of freedom and specific interactions between ions and solvent. When it comes to ion conducting membranes, increasing the ion exchange capacity (decreasing the average separation of ionic groups) is a common measure to increase ionic conductivity. However, the results on dissociation and conductivity of synthesized polysulfones containing octasulfonated units (currently the material with highest known IEC) clearly reveal the limit of this approach. The short separation of ionic charges in such systems at high concentrations additionally leads to electrostatic interactions between neighboring polymer strands. This is the driving force for a nanoscale ordering in polyelectrolyte membranes. Different kinds of solvents, ions and ion exchange capacities directly affect the microstructure formation. Finally, the effects of acid-base interactions between sulfonic acid-based polyelectrolytes and weakly basic modified polymers were investigated as blending of both is a way to form stable membranes for electrochemical applications. Here, the membrane formation process and the resulting properties, in particular proton conductivity, microstructure and mechanical strength have been studied. The developed polymer blends are the first example in which an improvement of mechanical properties not goes along with a significant decrease of proton conductivity. Key to success was to use a hydrophilic polymer with a high IEC and a hydrophobic polymer with a low number of basic groups. In summary, this thesis provides insides into the charge carrier formation process, the transport and microstructure of sulfonated polyelectrolytes by identifying the relevant molecular interactions. Together with the superior mechanical properties of the developed blend membranes, this work significantly contributes to solve the key challenges for electrolytes in electrochemical application.Item Open Access A multiscale study of transport in model systems for proton conducting polybenzimidazole phosphoric acid fuel cell membranes(2015) Melchior, Jan-Patrick; Maier, Joachim (Prof. Dr.)Proton conducting membranes are a key component in modern low (T < 100 °C) and intermediate (T < 180 °C) temperature fuel cells for mobile energy conversion and stationary combined heat and power generators. Finding and evaluating suitable membrane materials with high protonic conductivity in the intermediate temperature range presents a veritable challenge in material science. In this temperature regime neat nominally water free phosphoric acid (H3PO4) was early on identified as conductor with the highest intrinsic proton conductivity. Its high conductivity is due to structure diffusion involving proton transfers between diverse phosphate species. This process contributes up to 97 % to the observed conductivity and makes phosphoric acid an ideal electrolyte for fuel cell applications, which is employed, for example, in the form of phosphoric acid imbibed polybenzimidazole membranes (PBI-PA membranes) in fuel cell technology since the 1990s. However, the membrane’s proton conductivity is decreased in comparison to neat phosphoric acid’s conductivity and the reasons for this stark decrease are not yet fully understood. Indeed, even for neat phosphoric acid only recently a comprehensive rationale of its structure diffusion mechanism on a molecular level was proposed based on ab initio molecular dynamics simulations. This rationale emphasizes the role of frustration in phosphoric acid’s hydrogen bond network caused by the imbalance in the number of proton donors and acceptors. The present thesis provides, for the first time, quantitative experimental insights into how proton conductivity and the underlying conduction mechanisms are affected through changes of hydrogen bond network frustration. Such insights are enabled through the separation of hydrodynamic and structure diffusion in model systems. Model systems with defined composition, furthermore, have been chosen in a way as to discriminate between effects of water and benzimidazole on proton transport in phosphoric acid. In the actual membranes such effects can hardly be separated as the benzimidazole-phosphoric acid ratio is usually insufficiently defined and as the water content under fuel cell operation varies as a function of relative humidity (RH) and—as shown in this work—of the aforementioned benzimidazole content. Hydration isotherms recorded in this study, which encompass the operational temperature and RH range of fuel cells, allow to relate the results obtained from model systems to situations occurring in a running fuel cell. Different experimental techniques are combined to probe proton dynamics on multiple time- and length-scales. These complementary techniques assess dynamic ranges from the subnanosecond regime of quasielastic neutron scattering (QNS) to the millisecond regime of pulsed field gradient nuclear magnetic resonance (PFG-NMR). PFG-NMR is used to measure diffusion coefficients of different nuclei distinguished by their chemical surroundings. Through the analysis of coalescing NMR spectra lifetimes for exchange of nuclei between such chemical surroundings are evaluated also on the millisecond scale. On the nanosecond scale, 1H NMR relaxation measurements are sensitive towards fluctuations involved in hydrodynamic diffusion, while 17O NMR relaxation measurements and QNS are sensitive to the underlying dynamics of proton dislocation in vicinity of the oxygen atoms or transfers inside the hydrogen bond, respectively. In aqueous mixtures four principal transport regimes are identified, i.e., water contents (given as P2O · λH2O) where certain proton diffusion and conduction mechanisms prevail: At high water contents (λ > 14), in the acidic aqueous regime proton transport is essentially that of an acidic aqueous solution. Towards lower water contents (14 > λ > 6), in the viscosity controlled regime, the decrease of conductivity and diffusion is associated with increasing viscosity. With further decreasing water content (λ < 6), in the transition regime, phosphate species progressively aggregate, forming hydrogen bonded structures. The predominant proton transport mechanism changes from hydrodynamic diffusion to structure diffusion. In the decoupling regime, for water contents λ < 3, the molar fractions of condensation products (H4P2O7, etc.) are severely increased and proton transport is further decoupled from the reduced hydrodynamic diffusion. It is found that addition of the same molar amount of either benzimidazole or imidazole to nominally dry phosphoric acid reduces the structure diffusion coefficients by the same degree. That is, the structure diffusion coefficient is associated with the number of additional proton acceptor sites provided by benzimidazole or imidazole. On a related note, this investigation furthermore disproves that structure diffusion rates were also influenced by exchange of protons between benzimidazole and H3PO4 as stated in the literature. The proton exchange in question is determined as to be on the millisecond scale by analysis of coalescing 1H NMR spectra, which is 9 orders of magnitude slower than fast proton exchange between H3PO4 molecules. Proton exchange between benzimidazole and H3PO4, however, affects the decay of the echo intensity in 1H PFG-NMR experiments through which diffusion coefficients are measured. This effect was overlooked in previous literature PFG-NMR measurements on PBI-PA membranes, resulting in deviations between the measured apparent and the actual proton diffusion coefficients. In this work, therefore, a model for intermediate exchange rates is used to obtain 1H PFG-NMR diffusion coefficients of H3PO4; proton exchange rates between (benz)imidazol and phosphoric acid and the benzimidazole diffusion coefficient serve as input parameters for evaluating the echo decay. In order to confirm that the changes in proton dynamics observed in phosphoric acid - benzimidazole mixtures on the millisecond scale are caused by reduced dynamics in the frustrated hydrogen bond network, proton dynamics is also probed on the nanosecond scale. Spatial information is obtained through fitting of the Q-dependence of the quasielastic linebroadening, as obtained from backscattering QNS, to a jump diffusion model. Through this model the diffusion coefficient, lifetime, and “jump length” of a proton involved in structure diffusion are attained. It is found that the activation energies from 17O relaxation rates, backscattering QNS diffusion coefficients, and the PFG-NMR structure diffusion coefficients are virtually identical. Proton transfer inside hydrogen bonds is identified as the rate limiting step of structure diffusion. Addition of benzimidazole neither changes the activation energy of this proton transfer, nor affects proton transport on any scale between the nanosecond and millisecond regime; all influences of the additive on proton transport must occur on lower timescales. Hydration isotherms of phosphoric acid and the phosphoric acid mixtures demonstrate that water uptake is modified by the additive. Benzimidazole reduces the water uptake at fixed RH and the low nominal water content λ of phosphoric acid under fuel cell operational conditions, i.e., without additional humidification, prevails up to higher relative humidity in phosphoric acid - (benz)imidazole mixtures. It is one of the surprising insights of this thesis that the reduced ionic conductivity associated with low water contents is not detrimental to fuel cell performance. Conductivity is still high enough to avoid unacceptable ohmic losses and proton conductivity is mainly occurring through structure diffusion, i.e., the diffusion of protons is well decoupled from the hydrodynamic background. With increasing water content, however, mainly the increasing hydrodynamic background diffusion increases ionic conductivity. This also means an increase in contribution from rapid H3O+ conductance which is associated with electro-osmotic transport of water to the cathode side. Such accumulation of water will eventually increase leaching of phosphoric acid from the membrane and decreases the conductivity on the water deprived side through enhanced condensation. The low contribution of H3O+ at fuel cell operational conditions and the relatively high contribution of structure diffusion are presumptively the reasons why PBI-PA membrane fuel cells are performing well.Item Open Access Multi-phase La-Sr-Co-O fuel cell cathode materials and influence of phase boundaries on oxygen exchange activity(2016) Stämmler, Sebastian; Maier, Joachim (Prof. Dr. rer. nat.)The present work deals with the investigation of dense pulsed laser deposited (PLD) La-Sr-Co-O thin films which contain one or more perovskite related phases with catalytic activity for the oxygen reduction reaction (ORR). For the performance of solid oxide fuel cells (SOFC), a high ORR activity of the cathode material is decisive, in particular when attempting to lower the operation temperature. There are literature reports that enhanced ORR activities can be reached at hetero-interfaces of two different electronically and ionically conducting cathode materials, as demonstrated for (La,Sr)CoO3-d perovskite phase / (La,Sr)2CoO4+d Ruddlesden-Popper phase ("TPB effect", Kawada and Sase et al. 2006 and Crumlin et al. 2010). So far, no systematic experiments were performed to quantify the strength of the TPB effect (i.e. contribution of TPBs to ORR activity per TPB length). The present work therefore concentrated on the fundamental investigation of the TPB effect, a proper chemical and morphological characterization of the self-assembled La-Sr-Co-O composite cathode films containing perovskite and Ruddlesden-Popper phase and the quantification of the strength of the TPB effect.Item Open Access Structural and electronic properties of nickelate heterostructures(2016) Wrobel, Friederike; Keimer, Bernhard (Prof. Dr.)The fabrication of thin films and multilayers has led to the discovery of novel functional properties which are widely used in electronic devices nowadays. The limit of such a material design is atomic layer-by-layer deposition which was made possible through shuttered molecular beam epitaxy (MBE) growth. In the course of this thesis project a newly developed oxide MBE system was used to grow two different types of nickelate heterostructures, namely superlattices (SLs) consisting of metallic and paramagnetic LaNiO3 sandwiched between a large band-gap insulator and a combination of lanthanum nickelate and cuprate layers into a single hybrid structure. The former type was intensively studied in the last years and a transition to a weakly insulating, antiferromagnetically ordered state was observed in samples where the LaNiO3 thickness had been reduced to only two unit cells. So far little was known about the influence of the growth method on the defects in the samples and consequently on their physical properties. The use of oxide MBE enabled us to improve the overall sample quality of nickelate SLs and to design a novel material. We first optimized the growth of LaNiO3 and thoroughly analyzed the heterostructures by synchrotron-based x-ray diffraction, transmission electron microscopy, and temperature-dependent electrical resistivity. Furthermore we conducted in-depth studies, including x-ray absorption and magneto-transport measurements. The knowledge gained thereby was used grow new, layered nickelate-cuprate hybrid structures with novel electronic and magnetic properties.Item Open Access Oxidation kinetics of metal films and diffusion in NiO for data storage(2016) Unutulmazsoy, Yeliz; Maier, Joachim (Prof. Dr.)The growth of pore-free oxide layers on metals is determined by formation and migration of point defects and thus is an issue of fundamental importance. So far, mainly oxidation kinetics of thick metal films, crystals, and bulk samples were investigated in the literature. The oxidation of thin metal films on insulating substrates can easily be monitored by measuring the conductivity of the remaining metal. This dissertation focuses on understanding the processes that determine the oxide growth rate (e.g., surface reaction or chemical diffusion) in thin films, and also attempts to increase the reaction rate constant for potential applications in irreversible data storage systems. In this work, the resistance changes upon oxidation were measured by electrical impedance spectroscopy during the oxidation of metal films on Al2O3 substrates, and the oxide thicknesses were calculated. The oxide growth follows the parabolic rate law of oxidation for Cr, Al, Ti, V, Zn, Ni and Co films with a thickness typically ranging from 10 to 150 nm. Thus, in spite of their small thicknesses, the rate determining process of the oxidation of these metal films is found to be diffusion through the oxide layer according to the Wagner theory. Ni and Co exhibit a higher oxide growth rate than Cr, Al, Ti, V, and Zn. The oxidation rate constant of Ni is not significantly changed by applying different conditions such as different pO2, UV illumination, ozone exposure, and varying metal film thickness. This confirms the validity of the parabolic rate law of oxidation for the samples with 10-150 nm thickness in the temperature range of 250-500 °C. A comparison of Ni tracer diffusion coefficients between single- and polycrystalline NiO from literature studies and the present work with polycrystalline films (grain size: 10-30 nm) shows that a decreased grain size increases the effective diffusion coefficient by orders of magnitude pointing towards fast Ni diffusion along the grain boundaries in NiO. NiO was chosen as an example for a more detailed investigation of oxidation kinetics and diffusion. Ni diffusion in NiO is mainly controlled by Ni vacancies, and the vacancies are compensated by electron holes in undoped NiO. Thus, donor doping of NiO (e.g., with Al3+, Cr3+, etc.) is expected to increase the reaction rate constant by increasing the Ni vacancy concentration. However, the oxidation rate constant of Cr-doped Ni films with different thicknesses and different Cr concentrations (0.1 and 1 %) showed no significant change compared to undoped NiO. Depth profiles of Cr concentration in Cr-doped Ni films before and after oxidation were obtained by X-ray photoelectron spectroscopy (XPS). The results indicated that Cr was not homogeneously distributed in the films. While there is a Cr-rich surface layer in the metal form and almost homogenous distribution of Cr over the rest of the metal film, in the oxide form of the sample most of the Cr remained at the interface of the oxide and the substrate. Therefore the inhomogeneous Cr distribution in the growing oxide film prevented a faster oxidation. Since it is difficult to precisely control the grain size and to obtain a homogenous dopant distribution in the growing oxide films, the transport properties of undoped and donor-doped NiO ceramic samples were studied by conductivity relaxation measurements. NiO powders were synthesized by nitrate-glycine synthesis method and compacted to dense ceramic samples by spark plasma sintering (SPS). The grain size of the ceramic samples was controlled by additional annealing. Conductivity (σ) and chemical diffusion coefficient (Dδ) values were obtained as a function of oxygen partial pressure and temperature. These values were found to depend in a nontrivial way on Cr content (0.1, 0.3 and 1 %), grain size and thermal history of the samples. The comparison of σ and Dδ values of as-prepared (SPS, 5 min at 1000 °C) and annealed (8 h at 1500 °C) samples showed that the Cr dopants seem to be electrochemically inactive for the as-prepared ceramics. For annealed samples, 0.1 % Cr-doped NiO showed a significant decrease in σ and increase in Dδ compared to undoped NiO, indicating that the dopants are activated. However, a further increase in Cr content caused a decrease in chemical diffusion coefficient for annealed samples. The reason for this unexpected behavior is an inhomogeneous Cr distribution in the samples. The inhomogeneous Cr distribution and formation of undesired NiCr2O4 spinel is detected by transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM/EDX) at grain boundaries for almost all samples, and for high Cr concentrations even in the grains. The obtained conductivities and chemical diffusion coefficients indicate a much lower solubility limit for Cr than reported in the literature. The maximum achievable increase of the chemical diffusion coefficient by donor doping is more than one order of magnitude at 700 °C. In-situ oxidation kinetics measurements of metals revealed that Co films show the highest reaction rate constants within the studied metals; extrapolation yields a millisecond oxidation time at a temperature of 540 °C. The thin films in this study also showed much higher oxidation rate constant than the literature values because of their very small grain size. However, the oxidation rate constant is still not in the desired range for a routine long-term data storage application, which requires microsecond oxidation time. Nonetheless, for special purposes, Co thin films could be candidates for a long-time data archiving system owing to the very low oxidation rate constants at room temperature.Item Open Access In situ characterization of phase evolution in LiFePO4(2015) Ohmer, Nils; Maier, Joachim (Prof. Dr.)Among the candidates for electrodes in future Li-based batteries, LiFePO4 (LFP) is one of the most important and most frequently studied materials, undergoing a phase transformation upon delithiation to FePO4 (FP). In spite of the great scientific and practical interest in this material, there is still an extensive debate on the mechanism of this phase transformation and the underlying factors of influence. Within the framework of this thesis, first studies are carried out ex situ on multi-particle, full electrode LFP materials, being electrochemically cycled and analyzed at various states of charge by a combination of highly spatially resolved methods (high-resolution transmission electron microscopy and electron energy loss spectroscopy (HRTEM, EELS)) and integral measurement techniques (analyzing the X-ray diffraction and X-ray absorption near edge structure (XRD, XANES)). This combination of characterization techniques allows one to distinguish between the cycling behaviour of differently sized crystallites within the same electrode. It is found that for electrodes with hydrothermally grown LFP as active material, a particle size dependent cycling behaviour exists, with nanosized particles apparently not participating in the charging process at all. A turbostratic stacking of layers in these nanosized particles is found and identified to be responsible for sluggish lithium insertion and extraction. These higher dimensional defects prevent the small particles from participating in the charging process, most likely by disturbing the lithium diffusion along the 1-dimensional channels, as well as impair the transport along the other directions in the LFP host structure and thus blocking the lithium transport, resulting in a comparibly lower practical capacity during electrochemical cycling. To study the lithium exchange mechanism upon charging a LFP thin film cathode, an all-solid-state thin film battery cell with a lateral design concept is developed and realized by pulsed laser deposition (PLD) and thermal evaporation techniques. Using PLD and shadow masks LFP cathode, Li2O-V2O5-SiO2 (LVSO) electrolyte and LiAl anode thin films are deposited sequentially in a way that the Li transport pathway in the resulting battery is along the X-ray transparent commercial Si3N4 membrane substrate. This enables the usability of synchrotron-based energy resolved scanning transmission X-ray microscopy (STXM) with its high chemical and spatial resolution to perform in situ absorption measurements at the Fe L3 edge. Upon delithiation, a shift in the main absorption feature from 708 to 710 eV is used to fingerprint the change in the local state of charge, identifying areas containing Fe2+ (lithiated) and Fe3+ (delithiated), respectively. The initial lithiation process of a LFP thin film cathode material has been followed by in situ STXM, with a lateral resolution of 30 nm, during electrochemical charging of the thin film battery. The observed initial lithiation process does not follow the classical particle by particle mechanism, typical for multi-particle LFP cathodes, but instead a rather simultaneous, although inhomogeneous, lithiation is observed. The reason for this change in mechanism, compared to multi-particle powder electrodes, is found in mechanical interactions within the thin film upon lithiation, i.e. in the corresponding volume expansion and formation of high energy surfaces, changing the shape of the single-particle chemical potential to a monotone form upon lithiation. This has far-reaching consequences: not only the many-particle mechanism is changed to a concurrent lithiation, but also the single-particle mechanism is changed from a two-phase to a single-phase mechanism upon lithiation. Furthermore, a vanishing hysteresis loop and the disappearing of the memory effect is predicted. These findings are rather general and applicable to all kind of thin films of phase separating intercalation materials, undergoing a volume change upon lithium exchange. To fill the gap in literature on in situ observations of the (L)FP phase evolution on a single-particle level with appreciable space and time resolution, a micrometer-sized all-solid-state thin film battery is built with a defect-chemically well characterized LFP single crystal as cathode material with dimensions of 16x1x0.2 micrometer. Using STXM, the phase evolution along the fast (010) orientation is followed during in situ electrochemical (de)lithiation on a micro-meter scale with a lateral resolution of 30 nm and with minutes of time resolution. Furthermore, the STXM measurements performed on this sample are one of the few experiments ever taken on LFP materials with a well defined defect chemistry, even though fundamentally necessary for an overall understanding of the materials behaviour. This combination discloses not only the mechanism of LFP transformation on a single-particle level, but also the significance of elastic effects on the (de)lithiation process. Using a defect chemical analysis, the position of phase formation is found to be determined by the defect chemical situation, while the growth pattern of both LFP and FP is found to be dominated by elastic effects.Item Open Access Mass storage at heterojunctions: theory, experimental evidence, and electrochemical applications(2016) Chen, Chia-Chin; Maier, Joachim (Prof. Dr.)Item Open Access Heterogeneous doping and superconductivity in La2CuO4-based heterostructures(2015) Baiutti, Federico; Maier, Joachim (Prof. Dr.)Effects arising at interfaces in ionic materials represent an exciting opportunity towards the realization of nanosized systems having improved or novel functionalities. For this reason, they have been the object of a number of studies in recent years. In particular heterogeneous doping, relying on the introduction of interfaces in order to locally tune the chemical and functional properties, has been extensively studied in the context of ionic and mixed ionic-electronic conductors, and has proven its potential in comparison with the more "conventional" homogeneous doping, which is based instead on the introduction of randomly placed dopant ions in the host material. In a similar way, in the field of oxide electronics, a number of investigations highlighted the possibility of engineering interfacial properties by coupling materials in a crystallographic ordered fashion, i.e. forming epitaxial interfaces. In the present study, we investigate the possibility of obtaining high-temperature superconductivity in epitaxial systems based on lanthanum cuprate (La2CuO4) by means of heterogeneous doping. La2CuO4, which undergoes an insulator-to-superconductor transition upon hole doping, represents a suitable model system due to his well-known physical and chemical properties and to the relatively simple structure. In addition, the study of cuprates in relation to high-temperature superconductivity represents a highly fascinating topic owing to the still open questions related to such an effect and to its great potential for electronic applications. Two type of structures are investigated: the first one consists of heterostructures of La2CuO4 and La2-xSrxNiO4 (lanthanum strontium nickelate), the second one of two-dimensionally doped La2CuO4. In order to ensure the best sample quality, epitaxial thin films are realized by means of atomic-layer-by-layer oxide molecular-beam-epitaxy (ALL-Oxide MBE). The feasibility of the synthesis of the structures described above is first proved by successfully implementing ALL-oxide MBE for the growth of the single phase constituents, i.e. La2CuO4, La2NiO4 and La2-xSrxNiO4. Phase stability limits and synthesis critical points are put into the fore. In the case of La2CuO4 / La2-xSrxNiO4 heterostructures, bi- and multilayered (superlattice) structures are studied in order to unveil possible interface effects arising from the migration of positive charge carriers, namely electron holes and oxygen vacancies, from La2-xSrxNiO4 into La2CuO4. In such a situation, in order to maintain thermodynamical equilibrium, a region of hole accumulation is expected to form at the La2CuO4 side of the interface (space charge effect), having as a final consequence the occurrence of high-temperature superconductivity due to local p-type doping. Notably, although none of the constituting phases is superconducting if taken singularly, the resulting samples indeed exhibit high-temperature superconductivity with critical temperature Tc up to 35 K. Complementary investigations indicate that effects of charge redistribution at the interface, in which also the cationic stoichiometry plays a role, can be accounted for the findings. In the second case i.e. two dimensionally doped lanthanum cuprate, superlattices are obtained by substituting LaO atomic layers in the La2CuO4 crystal structure with MO layers, in which M is an acceptor dopant (M=Sr, Ba, Ca). A number of independent experiments demonstrate that, in proximity of the two-dimensionally doped layer, a sharp hole accumulation occurs giving rise to high-temperature superconductivity (Tc up to 35 K). Such an effect is ascribed to the occurrence of both heterogeneous and homogeneous doping mode, which are simultaneously active at the opposite sides of the doped layer. Remarkably, they both independently contribute to superconductivity. Such a situation allows for a direct comparison between the two doping modes and for pointing out the potentialities of heterogeneous doping over the homogeneous situation. The findings presented in this work demonstrate the feasibility of inducing high-temperature superconductivity in La2CuO4-based heterostructures by heterogeneous doping. Moreover, they highlight the complex interplay among (i) charge redistribution based on space-charge effects, (ii) cationic intermixing and (iii) structural aspects, which contribute to the definition of the fascinating properties of epitaxial oxide interfaces.Item Open Access Interface effects in Y2Zr2O7 thin films(2016) Gilardi, Elisa; Maier, Joachim (Prof.Dr.)The study of thin films and heterostructures has been extensively used in recent years to investigate the relevance of grain boundaries and interface effects (film/film and film/substrate) on the ionic transport properties of several oxides. It is worth noting that thanks to their configuration such systems allow for the direct investigation of aspects (e.g. interface effects in epitaxial films) that otherwise are not easily accessible in ceramics. In this way, it is possible to study for example the redistribution of charge carriers due to space charge effects (heterogeneous doping) or the enhancement of the ionic migration owing to strain effects. This thesis focuses on the study of selected ionically conducting thin films and multilayers, with the aim of investigating the role of interface effects (both space charge and strain effects) on the overall ionic transport properties. In order to address these aspects, the defective fluorite Y2Zr2O7 has been taken as a model system, because of its high chemical stability and substantial ionic conductivity (via oxygen vacancies). Thanks to these properties, this oxide has been considered also as possible candidate for applications as electrolyte in solid oxide fuel cells (SOFCs). While for others A2B2O7 compositions many contributions can be found in the literature, there is a substantial lack of information about Y2Zr2O7. The first part of this work deals with the investigation of the electrical properties of Y2Zr2O7 both ceramics pellet and thin films with different microstructures (epitaxial vs. textured). As epitaxial layers can be considered structurally similar to single crystals, the comparison between textured and epitaxial films allowed for obtaining important information on the role of grain boundaries on the overall ionic transport. In particular, textured thin films of 100 nm thickness have been grown via pulsed laser deposition (PLD) technique on Al2O3 (0 0 0 1) and Al2O3 (1 -1 0 2) while it was possible to obtain epitaxial thin films on MgO (1 1 0). The microstructure of the films was characterized by X - ray diffraction (XRD), pole figures, high resolution transmission electron microscopy (HR - TEM) and atomic force microscopy (AFM). XRD characterization indicated a preferential alignment along the (1 1 1) direction on both Al2O3 (0 0 0 1) and Al2O3 (1 -1 0 2) with a minor (2 2 0) orientation. Pole figures on Al2O3 (0 0 0 1) confirmed the textured structure of the films. Samples grown on MgO (1 1 0) instead were epitaxial, oriented along the (1 1 0) direction. Pole figures and HR-TEM analysis confirmed the single orientation of the crystallographic planes and the absence of grain boundaries. AFM images indicated a uniform surface with low roughness (RMS on the order of one unit cell) on all Y2Zr2O7 thin films. The electrical properties were analyzed via electrochemical impedance spectroscopy (EIS) at different temperatures and different oxygen partial pressures. Remarkably, the comparison of the Arrhenius plots obtained from ceramic pellet, textured and epitaxial thin films pointed out a minor blocking effect of the grain boundaries on the ionic conductivity. Only a small difference in conductivity and activation energy was detected between these samples. A further detailed study of the electrical properties of Y2Zr2O7 thin films was carried out by analyzing the conductances of epitaxial layers of different thickness (from 70 nm up to 300 nm). The EIS measurement revealed a significant enhancement of the conductance at the interface between the Y2Zr2O7 layer and the MgO substrate. After annealing at high temperature (700 ºC) for several hours there was a clear decrease of the interfacial conductance. Along with this, HR-TEM analysis showed a high density of misfit dislocations at the interface, which is structurally required to release the compressive strain stemming from the large mismatch between film and substrate. Interestingly, the misfit dislocation density also decreased after annealing. These results are complemented by electron energy-loss spectroscopy (EELS) measurements performedat the O-K-edge (1s to 2p electronic transition). The analysis of the spectra revealed an increase of the oxygen first peak intensity (stemming from the transitions to the d-states of Zr hybridized with oxygen 2p states) near the Y2Zr2O7/MgO interface in the samples not exposed to the heat-treatment at 700 °C, indicating a different local environment surrounding the oxygen ions. This enhancement does not appear in the samples after annealing. The interpretation of the enhanced interfacial conductance takes into account both space-charge and mobility effects. The segregation of negatively charged defects at the interface yielding the accumulation of in the space charge zone can only partially explain the experimental results. Indeed, an additional increase in the charge carriers’ mobility also emerges from the analysis of the conductance as function of the film thickness. The interface structural modifications after annealing (lower misfit dislocation density) together with the strain relaxation (decreased mobility) can explain the variation in conductance. For the study of the effect of the strain on the ionic conductivity, multilayers of Y2Zr2O7 and 8 mol% yttria stabilized zirconia with different number of interfaces but same overall thickness have been deposited (from 1 up to 9 interfaces) and characterized via EIS and XRD. For comparison, also single layers have been included in the measurement. XRD characterization indicated the epitaxial growth of multilayers along the (1 1 0) orientation and high quality of the interfaces. Remarkably, the EIS characterization pointed out a negligible role of the strain on the ionic conductivity despite the substantial lattice mismatch of 1.6%.