Microscopy Conference 2023

Prof. Dr.

Session chairs

This session provides a forum to present and discuss recent research and development activities on amorphous as well as crystalline matter. The session will cover the wide field of microstructure-based development of materials, covering aspects of basic materials research and detailed material characterization, in particular, via electron microscopy and correlated techniques.
World experts are encouraged to present their recent research activities on ceramics and composites, aiming to stimulate interesting interdisciplinary discussions and lasting collaborations.
The anticipated topics include novel glass-based materials and glass-ceramics for various applications, nanomaterials utilized in photovoltaics, electroceramics, or multiferroics as well as ultra-high temperature ceramics (UHTCs) for the use in extreme environments.
Advances in understanding structure-property-processing correlations as well as performance of such materials, giving the possibility to tailor and control material properties via microstructural modification, are the main focus of this session.


Prof.

Studying the crystallization of inorganic materials using correlated transmission electron microscopy

MS 7: Ceramics and composites

IM 7: In situ/operando electron microscopy

Abstract text (incl. figure legends and references) Complex, electron-beam sensitive systems include organic crystals, polymers, hybrid organic-inorganic materials, some inorganic materials such as hydrates, as well as multiphase solid/liquid and solid/gas systems. Arguably they constitute the majority of systems of current interest across a wide range of scientific and engineering disciplines. We review the use of analytical transmission electron microscopy (TEM) for the characterisation of such beam-sensitive materials and complex, multiphase systems in-situ or close to their native state. Such materials are prone to damage by radiolysis which cannot be eliminated or switched off, requiring TEM analysis to be done within a dose budget so as to achieve an optimum dose-limited resolution and reliable, artefact-free results [1]. We highlight the importance of determining the damage sensitivity of a particular system in terms of characteristic changes that occur on irradiation under both an electron fluence and flux. We discuss the choice of electron beam accelerating voltage and detectors for optimizing resolution and outline the different strategies employed for low-dose microscopy in relation to the damage processes in operation. In particular, we discuss the use of scanning TEM (STEM) techniques for maximizing information content from high-resolution imaging and spectroscopy of materials and the use of cryogenic sample preservation methods. As an example, we describe studies of crystallization processes in the calcium sulfate system using a correlated approach: a combination of liquid cell TEM (LCTEM), in-situ Raman spectroscopy and cryogenic TEM (cryo-TEM). Recent studies have suggested that the calcium sulfate polymorph gypsum (CaSO4.2H2O) can form in aqueous solution via a bassanite (CaSO4.0.5H2O) intermediate. Hence, initially we studied the transformation of bassanite nanorods to gypsum in an undersaturated aqueous solution of calcium sulfate. When coupled with Raman spectroscopy, the real-time data generated by LCTEM, and structural data obtained from time resolved cryo-TEM (i.e. crystallising solution samples vitrified at various time points) showed that bassanite can transform to gypsum via multiple pathways, the predominant one being dissolution/reprecipitation as opposed to the oriented attachment of nanorods and subsequent solid state transformation. Comparisons between LCTEM and cryo-TEM also showed significantly reduced kinetics within the confined liquid cell as compared with bulk solution, however the electron beam induced radiolysis of water (so lowering the local pH) can accelerate the transformation process [2]. Further studies focused on the direct formation of gypsum from aqueous solution using a range of different sample preparation methods: drop casting of solutions and drying, vacuum filtration, chemical quenching and drop casting, time-resolved cryogenic freezing, as well as LCTEM. Here we suggest that the observed crystallization pathway can be highly dependent on the degree of supersaturation experienced during the sample preparation method, which may lead to highly erroneous observations. References: [1] M. Ilett et al., Phil. Trans. R. Soc. A 378: 20190601 (2020). doi:10.1098/rsta.2019.0601. [2] M. Ilett et al., J. Microscopy 2022, 288 : 155-168. doi: 10.1111/jmi.13102

Invited talk

Interest of TEM for the development of new out-of-equilibrium materials by crystallisation from glass

IMLB: Late breaking abstracts

Abstract text (incl. figure legends and references) Crystallization from glass or high temperature melt is an original synthesis route allowing the elaboration of new non-equilibrium crystalline phases. The (partial) control of the nucleation and growth phenomena, by means of thermal treatments and adapted compositions, thus allows the synthesis of original materials not accessible by classical solid reaction. These innovative materials frequently show (micro)structural order and disorder phenomena. The use of transmission electron microscopy, coupled to other characterization techniques such as diffraction or NMR spectroscopy allows a fine and multi-scale description, opening the way to the understanding and control of some observed properties

Structural adaptations in bronze-type niobium tungsten oxides enforced by varying stoichiometry

MS 1: Energy-related materials and catalysts

Abstract text (incl. figure legends and references) At present, niobium tungsten oxides are intensively studied as potential battery materials as their structural flexibility permits a reversible high-rate Li-ion exchange [1]. However, synthesis and characterization of phases in the pseudo-binary system Nb2O5-WO3 was a major topic in structural research already in the 1960s [2]. Remarkably, the versatile structural chemistry was explored by HRTEM investigations right from the onset of this method in the early 1970s [3]. Several phases with odd but well defined stoichiometry appear in the system Nb2O5-WO3. Their structures are based on frameworks of corner-sharing octahedra MO6 (M = Nb,W) [4]. The structure of Nb8W9O47 is closely related to that of the tetragonal tungsten bronzes (TTB) [5]: a part of the pentagonal tunnels (PTs) is filled by metal-oxygen strings in such a way that a threefold superstructure arises (Figure 1). This structure appears to be thermodynamically favored for the ratio oxygen:metal = 47:17, and any deviation from this ideal composition must somehow be compensated structurally. On the oxygen rich side of Nb8W9O47, two simultaneously occurring effects accommodate an increased O/M ratio (Figure 1) [6]: (i) formation of little ordered variants with less PTs occupied (e. g. Nb6W8O39); (ii) incorporation of single units of the neighboring phase Nb4W7O31, an intergrowth of the TTB and the ReO3 type as firstly determined by HRTEM [3]. The metal-rich side of Nb8W9O47 towards the block phases is scarcely investigated yet. The HAADF-STEM characterization of samples with composition Nb18W16O93 pointed to a denser occupation of the PTs: nano domains of units a yet unknow phase with composition Nb12W11O63 and of Nb8W9O47 appear [7]. The close relation of all these structures to the TTB-type substructure enables their intimate intergrowth as schematically shown in Figure 1 and demonstrated by the HRTEM image of a little ordered area shown in Figure 2. The results discussed here confirm that the details of such a complex real structure as realized by niobium tungsten oxides can only be figured out by a direct visualization by high-resolution TEM or STEM [8]. Figure 1: Schemes and HAADF-STEM images of Nb-W oxides that are either superstructures of the tetragonal tungsten bronze (TTB) type (blue squares) or a coherent intergrowth of TTB and ReO3 type (orange square). Figure 2: HRTEM image demonstrating the coherent intergrowth of Nb8W9O47 and Nb4W7O31 domains. The Fourier transform (inset) shows circular diffuse scattering besides the Bragg reflections of both phases. References: [1] Griffith et al., Nature 559 (2018) 556. [2] Roth, Waring, J. Res. Natl Bur. Stand. 70A (1966) 281. [3] Iijima, Allpress, Acta Crystallogr. A 30 (1974) 22. [4] Krumeich, Chem. Mater. 34 (2022) 911. [5] Sleight, Acta Chem. Scand. 20 (1966) 1102. [6] Krumeich, Acta Crystallogr. B 54 (1998) 240. [7] Krumeich, Crystals 11 (2021) 1514. [8] Electron microscopy was done at ScopeM (ETH Zurich).

Abstract talk

Crystallization of gel-derived SiO2-TiO2 amorphous nanobeads: an in situ high-temperature study

Abstract text (incl. figure legends and references) SiO2-TiO2 glasses and gels have attracted a long-standing scientific and technological interest due to their very low thermal expansion [1] and to their use as precursors for the synthesis of photocatalytic glass-ceramic materials [2]. Despite their simple binary composition, the crystallization of SiO2-TiO2 glasses and gels can be rather complex and encompass several metastable stages marked by phase transformation between different TiO2 polymorphs, such as TiO2(B), anatase and rutile [2]. In this work, the structural ordering and compositional reorganization of spray-dried amorphous nanobeads of composition 50 SiO2 – 50 TiO2 and 83 SiO2 – 17 TiO2 is studied in situ at high temperature in a JEOL ARM200F cold FEG TEM, operating at 80 kV and equipped with a Protochips heating holder. The nucleation and growth of TiO2 crystals in a single amorphous nanobead (diameter ~50 nm) were first observed in the range 30-600 ºC, continuously acquiring TEM micrographs while heating. Subsequently, the role and evolution of compositional heterogeneities were evaluated in STEM-ADF mode, performing EELS mappings at various stages of heat treatment. Combined to in situ Raman spectroscopy and XRD, the results provide an unprecedentedly complete view of the crystallization mechanism in these materials: as-prepared nanobeads already contain TiO2-enriched nanodomains, which evolve in size and composition with increasing temperature, eventually undergoing long-range ordering and yielding TiO2 crystals. More generally, our study highlights potential, challenges and limitations of high-temperature (S)TEM experiments for the elucidation of crystallization processes in oxide materials. Figure 1. Emergence and growth of anatase crystals in a nanobead of composition 50 SiO2 – 50 TiO2, observed in situ during heating in the TEM. References [1] Schultz, P. C. Binary Titania-Silica Glasses Containing 10 to 20 Wt% TiO2. Journal of the American Ceramic Society 59, 214–219 (1976) [2] Zandona, A. et al. Spray-Dried TiO2(B)-Containing Photocatalytic Glass-Ceramic Nanobeads. Advanced Functional Materials 31, 2007760 (2021).

Investigation of the morphology of TiO2-CeO2 mixed-oxide nanoparticles for photocatalytic applications

Abstract text (incl. figure legends and references) Introduction Titanium(IV)oxide (TiO2) nanoparticles are of high interest for a broad variety of applications, e.g. (photo)catalysis, self-cleaning and sensing. The photocatalytic performance of TiO2 can be improved by controlling its crystal structure [1] as well as by the addition of noble metals [2] or oxides [3]. Combination of TiO2 and other oxide especially with CeO2 is one of the promising ways for effective electron-hole separation due to easy fluctuation of oxidation states of Ce [3]. Objectives The presented research aims to find connections between the real structure of TiO2-CeO2 particles (i.e. crystal structure, morphology and composition) and their photocatalytic properties. Understanding these connections, allows to purposefully adapt the particles to the respective application and optimize their performance. Materials &amp; Methods A custom-made gas aggregation cluster source (GAS) was used to synthesize mixed-oxide nanoparticles. Ti and Ce were sputtered from a custom build Ti-Ce target, agglomerated under admixture of O2 as reactive gas and were deposited on Si wafer pieces. After deposition, the particles were heat treated at 700 °C under ambient conditions. For TEM preparation the particles were scratched off the substrate and transferred to a Cu lacey TEM grid (Plano GmbH). The particles were analyzed in a Jeol NeoArm JEM-ARM200F STEM equipped with two windowless EDX detectors. STEM-HAADF imaging in combination with EDX spectroscopy provides information on the particle morphology and the distribution of Ti and Ce. HRTEM as well as HRSTEM-HAADF images can reveal the nature of the interfaces between TiO2 and CeO2 crystals. Results First measurements on TiO2-CeO2 nanoparticles with a high concentration of Ce have shown demixing of a Ti-rich and a Ce-rich phases. Separate Ti-rich and Ce-rich particles have been observed as well as a core-shell type particle with a Ti-rich core covered by a Ce-rich shell. Ce-rich as well as Ti-rich phases where found to be crystalline. The crystal phase of CeO2 was determined to be Fm-3m by electron diffraction. Due to the low Ti concentration, the crystal phase of TiO2 was not determined by electron diffraction. Instead, the phase of TiO2 was observed to be brookite phase in FFTs of HRTEM images. The investigation of further samples with different Ti/Ce-ratios synthesized at different process parameters are expected to show a broader variation in particle morphology and give an insight into the dependence of the particle morphology on Ti/Ce-ratio as well as GAS process parameters. Conclusion At high Ce concentration, demixing into different morphologies with a Ce-rich and a Ti-rich phase can be observed in the mixed-oxide nanoparticles. Both phases are crystalline. To achieve anatase phase, which is desirable for high photocatalytic activity, the GAS process parameters need to be adjusted. Earlier studies with pure TiO2 particles have shown that the crystal phase can be varied by changing the magnetron voltage. For further investigations, the magnetron voltage window for the synthesis of anatase phase TiO2 in the mixed-oxide system will be investigated. In addition, the Ti/Ce ratio will be varied to study the influence of the particle composition on the morphology. References [1] A. Sclafani, J. M. Herrmann, J. Phys. Chem., 1996, 100, 13655–13661 [2] S. Veziroglu et al., ACS Appl. Mater. Interfaces, 2020, 12, 14983–14992 [3] S. Veziroglu et al., Nanoscale, 2019, 11, 9840-9844

Precipitate-domain wall topologies in hardened Li-doped NaNbO3

MS 2: Metals and alloys

Abstract text (incl. figure legends and references) Ferroelectric hardening is an absolute requirement for potential applications of piezoceramics in high-power devices [1,2]. The state-of-the-art researches [3,4] demonstrate that ferroelectric hardening by pinning domain walls with precipitates introduced during processing is feasible, akin to precipitation hardening in metals [5]. With the precipitation of plate-like LiNbO3 in Li-doped NaNbO3 piezoceramics, ferroelectric hardening was achieved, exhibiting an enhanced mechanical quality factor (Qm). The specific morphology and corresponding orientation of the LiNbO3 platelets, exhibiting {110}PC habit planes, were characterized by transmission electron microscopy (TEM). The experimentally revealed crystallographic structures and strain incompatibility were found to correlate well with the thermodynamic simulation of the minimum-energy aspect ratio. In particular, the topology of the precipitate-domain wall assembly was clarified, revealing a full pinning on the domain walls by the incorporated plate-like precipitates. The revealed topology provides a fundamental basis for future optimization strategies for platelet hardening in new lead-free piezoceramics for high-power applications. References: [1] K.H. Hardtl, Electrical and mechanical losses in ferroelectric ceramics, Ceram. Int. 8 (1982) 121-127. [2] K. Uchino, J.H. Zheng, Y.H. Chen, X.H. Du, J. Ryu, Y. Gao, S. Ural, S. Priya, S. Hirose, Loss mechanisms and high power piezoelectrics, J. Mater. Sci. 41 (2006) 217-228. [3] C.H. Zhao, S. Gao, T.N. Yang, M. Scherer, J. Schultheiß, D. Meier, X.L. Tan, H.J. Kleebe, L.Q. Chen, J. Koruza, J. Rödel, Precipitation hardening in ferroelectric ceramics, Adv Mater. 33 (2021) 2102421. [4] C.H. Zhao, S. Gao, H.J. Kleebe, X.L. Tan, J. Koruza, J. Rödel, Coherent precipitates with strong domain wall pinning in alkaline niobate ferroelectrics, Adv. Mater. Accepted, (2022). [5] A.J. Ardell, Precipitation hardening, Metall. Mater. Trans. A 16A (1985) 2131-2165.

MS 7
Ceramics and composites

Appointment

Session chairs

Description

Programme

MS 7Ceramics and composites

Appointment

Session chairs

Description

Programme

MS 7
Ceramics and composites