Optimizing compositional and atomic-level information of oxides in atom probe tomography

Atom probe tomography (APT) is a 3D analysis technique that offers unique chemical accuracy and sensitivity with sub-nanometer spatial resolution. Recently, there is an increasing interest in the application of APT to complex oxides materials, giving new insight into the relation between local variations in chemical composition and emergent physical properties. However, in contrast to the field of metallurgy, where APT is routinely applied to study materials at the atomic level, complex oxides and their specific field evaporation mechanisms are much less explored. Here, we perform APT measurements on the hexagonal manganite ErMnO3 and systematically study the effect of different experimental parameters on the measured composition and atomic structure. We demonstrate that both the mass resolving power (MRP) and compositional accuracy can be improved by increasing the charge-state ratio (CSR) working at low laser energy (< 5 pJ). Furthermore, we observe a substantial preferential retention of Er atoms, which is suppressed at higher CSRs. We explain our findings based on fundamental field evaporation concepts, expanding the knowledge about the impact of key experimental parameters and the field evaporation process in complex oxides in general

Transmission electron microscopy of ferroic materials

Materials science has the last few decades been advancing by an increasing insight in structure and local chemical composition at the sub-nanometre scale. In this context, the transmission electron microscope (TEM) has proved to be an indispensable tool due to its spatial resolving powers. Using electrons, rather than X-rays, to study ferroelectric materials also has the advantage of the strong interaction between the electrons and matter, which can reveal the presence of a non-centrosymmetric crystal structure, a prerequisite for ferroelectric properties. In this thesis work, three structurally complex ferroelectric materials were studied by TEM, taking advantage of the possibility to perform imaging, diffraction, and spectroscopy within the same instrument. The three materials systems investigated were ferroelectric BaTiO3 films, tetragonal tungsten bronzes (TTB) with various chemical compositions, and improper ferroelectric Gd2(MoO4)3. The BaTiO3 films were deposited on three differently oriented SrTiO3 substrates and demonstrated to be of high quality considering the chemical solution deposition method which was applied to synthesize them. Importantly, all the films showed an epitaxial orientation relation to the substrate, where misfit dislocations were introduced at the substrate-film interface due to the lattice mismatch between the two materials. A thorough investigation of the misfit dislocations was performed, demonstrating that strain in the films were completely relaxed by the dislocations. Among the TTBs K4Bi2Nb10O30 and Rb4Bi2Nb10O30 were investigated by convergent-beam electron diffraction (CBED) to determine the crystal symmetry. They were revealed to belong to the P4/mbm space group. Moreover, a systematic study of cation intermixing at different lattice sites in the TTB structure were investigated in Ba4A2Nb10O30 (A=Na, K, Rb) by scanning TEM (STEM) combined with energy-dispersive X-ray spectroscopy (EDS). Combined with X-ray diffraction (XRD), the intermixing was determined to rely on the Shannon ionic radii of the constituent elements. Lastly, high quality STEM images were obtained for the beam sensitive improper ferroelectric Gd2(MoO4)3, where the goal was to obtain the polarization orientation from atomically resolved images. However, because of the minor structural differences between the two polarization states in Gd2(MoO4)3, this turned out to be a challenge. This motivated a thorough study of the improper phase transition of Gd2(MoO4)3 by temperature dependent XRD. This study revealed different critical behavior for different cations across the phase transition, which may lead to a better understanding of the emergence of polarization in Gd2(MoO4)3 and improper ferroelectric materials in general. The work presented in this thesis has demonstrated that the TEM is a very useful tool in the study of ferroic materials. Local variation in the structure and chemistry at an atomic scale were investigated in oxide films as well as bulk materials, which would have been difficult to obtain by other experimental techniques. Determination of the centrosymmetric crystal symmetry of two compounds, previously not determined, was also demonstrated by CBED. However, when the structural variations between two domains became miniature, on the order of picometers, or when specimen damage occurred by the incident high-energy electron beam, alternative characterization techniques, such as XRD provided useful information, demonstrating an important feature of modern materials science, that a combination of experimental techniques is essential in order to advance the field

On the Viability of Lithium Bis(fluorosulfonyl)imide as Electrolyte Salt for Use in Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) represent promising high-power energy storage devices, most commonly composed of a lithium-ion intercalation anode (e. g., graphite or hard carbon), a supercapacitor activated carbon (AC) cathode, and an electrolyte with 1 M LiPF6 in carbonate solvents. LiPF6 is susceptible to hydrolysis, forming HF, which leads to challenges for disassembly and recycling, risks during hazardous events, and extensive energy consumption during production. Here, we report on the feasibility of replacing LiPF6 with the non-hydrolysing salt LiFSI for use with AC electrodes. Based on voltage hold measurements in a half-cell setup, good long-term stability is achieved with an upper cut-off voltage of 3.95 V vs. Li/Li+, potentially enabling cell voltages of ~3.8 V when combined with graphite or silicon-based anodes (operating at ~0.1 V vs. Li/Li+) in LIC full cells. The lower cut-off voltage was determined to be 2.15 V vs. Li/Li+. The systematic comparison of CV, leakage current analysis and capacity retention upon voltage hold highlights the importance of the latter method to provide a realistic assessment of the electrochemical stability window (ESW) of LiFSI on a commercial AC electrode. The morphological and surface-chemical post-mortem analysis of AC electrodes used with LiFSI revealed that the oxidation of the FSI anion, as evidenced by the presence of new S 2p and N 1s features in the XPS spectra, and an increasing number of oxygenated species on the AC were the main processes causing capacity fade at positive polarization.publishedVersio

Epitaxial (100), (110), and (111) BaTiO3 films on SrTiO3 substrates — A transmission electron microscopy study

Chemical solution deposition (CSD) is a versatile method to fabricate oxide films. Here, the structure and local variations in the chemical composition of BaTiO3 (BTO) films prepared by CSD on (100), (110), and (111) SrTiO3 (STO) substrates were examined by transmission electron microscopy. The films were shown to be epitaxial and the relaxation of the films occurred by the formation of edge dislocations at the substrate–film interfaces. The Burgers vectors of the dislocations were determined to be a⟨010⟩, a[11¯0] and a[001], and a⟨110⟩ for the (100), (110), and (111) films, respectively. Due to the difference in thermal expansion between STO and BTO, the films are demonstrated to be under tensile strain. Furthermore, the boundaries between each deposited layer in the BTO films were found to be Ba-deficient in all cases. In the case of the (111) oriented film, defects like an anti-phase boundary or a thin layer with a twinned crystal structure were identified at the boundary between each deposited layer. Moreover, a larger grain was observed at the film surface with a twinned crystal structure. The interdiffusion length of A-cations at the STO–BTO interface, studied by electron energy-loss spectroscopy, was found to be 3.4, 5.3, and 5.3 nm for the (100), (110), and (111) oriented films, respectively. Interdiffusion of cations across the STO–BTO interface was discussed in relation to cation diffusion in bulk BTO and STO. Despite the presence of imperfections demonstrated in this work, the films possess excellent ferroelectric properties, meaning that none of the imperfections are detrimental to the ferroelectric properties

The Structure, Morphology, and Complex Permittivity of Epoxy Nanodielectrics with in Situ Synthesized Surface-Functionalized SiO2

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Insights on microstructural evolution and capacity fade on diatom SiO2\hbox {SiO}_2 SiO 2 anodes for lithium-ion batteries

Abstract $$\hbox {SiO}_2$$ SiO 2 is a promising material for developing high-capacity anodes for lithium-ion batteries (LIBs). However, microstructural changes of $$\hbox {SiO}_2$$ SiO 2 anodes at the particle and electrode level upon prolonged cycling remains unclear. In this work, the causes leading to capacity fade on $$\hbox {SiO}_2$$ SiO 2 anodes were investigated and simple strategies to attenuate anode degradation were explored. Nanostructured $$\hbox {SiO}_2$$ SiO 2 from diatomaceous earth was integrated into anodes containing different quantities of conductive carbon in the form of either a conductive additive or a nanometric coating layer. Galvanostatic cycling was conducted for 200 cycles and distinctive trends on capacity fade were identified. A thorough analysis of the anodes at selected cycle numbers was performed using a toolset of characterization techniques, including electrochemical impedance spectroscopy, FIB-SEM cross-sectional analysis and TEM inspections. Significant fragmentation of $$\hbox {SiO}_2$$ SiO 2 particles surface and formation of filigree structures upon cycling are reported for the first time. Morphological changes are accompanied by an increase in impedance and a loss of electroactive surface area. Carbon-coating is found to restrict particle fracture and to increase capacity retention to 66%, compared to 47% for uncoated samples after 200 cycles. Results provide valuable insights to improve cycling stability of $$\hbox {SiO}_2$$ SiO 2 anodes for next-generation LIBs

Tailoring preferential orientation in BaTiO3-based thin films from aqueous chemical solution deposition

Ferroelectric properties of thin films can be enhanced by crystallographic texture. In this work, we report on how heat treatment of films can be designed to tailor the degree of preferential orientation in BaTiO3-based thin films from aqueous chemical solution deposition. In situ synchrotron X-ray diffraction in combination with Rietveld refinements was used to study the crystallization process of films from a single deposition and to give an in depth characterization of the crystallographic texture of the films. Transmission electron microscopy was employed to evaluate the microstructure and degree of preferred orientation in thicker films from multiple depositions. Texture was induced in the multilayer films by a repeated annealing process. Cube-on-cube growth was demonstrated to occur in both single and multi-layered films provided the heating program was designed to give limited nucleation and growth below the threshold for where cube-on-cube growth is favoured, resulting in a very high degree of preferred orientation. The cube-on-cube grown films were relaxed with respect to the lattice unit cell mismatch between the film and the substrate, where strain and relaxation depend on the film thickness. Texture and cube-on-cube growth were demonstrated on several types of single crystal substrates. Calcium and zirconium substitution did not alter the crystallization process, but zirconium decreased the texture formation kinetics. The ferroelectric response was strongest in the films with a high degree of preferred orientation

Anisotropic in-plane dielectric and ferroelectric properties of tensile-strained BaTiO3 films with three different crystallographic orientations

Ferroelectric properties of films can be tailored by strain engineering, but a wider space for property engineering can be opened by including crystal anisotropy. Here, we demonstrate a huge anisotropy in the dielectric and ferroelectric properties of BaTiO3 films. Epitaxial BaTiO3 films deposited on (100), (110), and (111) SrTiO3 substrates were fabricated by chemical solution deposition. The films were tensile-strained due to thermal strain confirmed by the enhanced Curie temperature. A massive anisotropy in the dielectric constant, dielectric tunability, and ferroelectric hysteresis loops was observed depending on the in-plane direction probed and the orientation of the films. The anisotropy was low for (111) BaTiO3, while the anisotropy was particularly strong for (110) BaTiO3, reflecting the low in-plane rotational symmetry. The anisotropy also manifested at the level of the ferroelectric domain patterns in the films, providing a microscopic explanation for the macroscopic response. This study demonstrates that the properties of ferroelectric films can be tailored not only by strain but also by crystal orientation. This is particularly interesting for multilayer stacks where the strain state is defined by the boundary conditions. We propose that other materials can be engineered in a similar manner by utilizing crystal anisotropy

The Structure, Morphology, and Complex Permittivity of Epoxy Nanodielectrics with in Situ Synthesized Surface-Functionalized SiO2

Epoxy nanocomposites have demonstrated promising properties for high-voltage insulation applications. An in situ approach to the synthesis of epoxy-SiO2 nanocomposites was employed, where surface-functionalized SiO2 (up to 5 wt.%) is synthesized directly in the epoxy. The dispersion of SiO2 was found to be affected by both the pH and the coupling agent used in the synthesis. Hierarchical clusters of SiO2 (10–60 nm) formed with free-space lengths of 53–105 nm (increasing with pH or SiO2 content), exhibiting both mass and surface-fractal structures. Reducing the amount of coupling agent resulted in an increase in the cluster size (~110 nm) and the free-space length (205 nm). At room temperature, nanocomposites prepared at pH 7 exhibited up to a 4% increase in the real relative permittivity with increasing SiO2 content, whereas those prepared at pH 11 showed up to a 5% decrease with increasing SiO2 content. Above the glass transition, all the materials exhibited low-frequency dispersion effect resulting in electrode polarization, which was amplified in the nanocomposites. Improvements in the dielectric properties were found to be not only dependent on the state of dispersion, but also the structure and morphology of the inorganic nanoparticles

The Structure, Morphology, and Complex Permittivity of Epoxy Nanodielectrics with in Situ Synthesized Surface-Functionalized SiO2

Epoxy nanocomposites have demonstrated promising properties for high-voltage insulation applications. An in situ approach to the synthesis of epoxy-SiO2 nanocomposites was employed, where surface-functionalized SiO2 (up to 5 wt.%) is synthesized directly in the epoxy. The dispersion of SiO2 was found to be affected by both the pH and the coupling agent used in the synthesis. Hierarchical clusters of SiO2 (10–60 nm) formed with free-space lengths of 53–105 nm (increasing with pH or SiO2 content), exhibiting both mass and surface-fractal structures. Reducing the amount of coupling agent resulted in an increase in the cluster size (~110 nm) and the free-space length (205 nm). At room temperature, nanocomposites prepared at pH 7 exhibited up to a 4% increase in the real relative permittivity with increasing SiO2 content, whereas those prepared at pH 11 showed up to a 5% decrease with increasing SiO2 content. Above the glass transition, all the materials exhibited low-frequency dispersion effect resulting in electrode polarization, which was amplified in the nanocomposites. Improvements in the dielectric properties were found to be not only dependent on the state of dispersion, but also the structure and morphology of the inorganic nanoparticles