Sašo Šturm

CeO2 thin films obtained by sol–gel deposition and annealed in air or argon

Abstract Thin films of CeO 2 were prepared on SnO 2 /F-coated glass plates by the sol–gel dip-coating process using CeCl 3 ·7H 2 O as a precursor. The films were heat-treated in an air or argon atmosphere. The structural, electrochemical and optical properties of these films depend on the preparation conditions. Transmission electron microscopy (TEM) showed the films to be polycrystalline with randomly orientated crystallised domains of up to 10 nm in size. The degree of crystallinity of films heat-treated in argon is higher than that of those heat-treated in air, and therefore their charge capacity values (15.9 mC cm −2 after 100 cycles) and reversibility of the ion-storage process (0.99 after 100 cycles) are higher than for films heat-treated in air (10.5 mC cm −2 and 0.86 after 100 cycles, respectively). Both films are optically passive under Li + ion insertion and have high optical transmittance (>80%).

Advanced nanomechanics in the TEM: Effects of thermal annealing on FIB prepared Cu samples

The effect of focused ion beam (FIB) fabrication on the mechanical properties of miniaturized mechanical tests has recently been realized, but is not well documented. In this study, the effect of post thermal annealing on the plastic properties of FIB fabricated micro- and nanometer-sized Cu samples was studied by means of advanced analytic and in situ transmission electron microscopy. In situ heating experiments on thin films and pillars revealed a reduction of the initially high dislocation density, but never a recovery of the bulk dislocation density. Aberration-corrected atomic imaging documented the recovery of a pristine crystalline surface structure upon annealing, while electron energy-loss spectroscopy showed that the remaining contamination layer consisted of amorphous carbon. These structural observations were combined with the mechanical data from in situ tests of annealed micro- and nanometer-sized tensile and compression samples. The thermal annealing in the micron regime mainly influences the initial yield point, as it reduces the number of suited dislocation sources, while the flow behavior is mostly unaffected. For the submicron samples, the annealed material sustains significantly higher stresses throughout the deformation. This is explained by the high stresses required for surface-mediated dislocation nucleation of the annealed material at the nanoscale. In the present case, the FIB affected the surface near defects and facilitated dislocation nucleation, thereby lowering the material strength.

Stacking faults and twin boundaries in sphalerite crystals from the Trepča mines in Kosovo

Abstract The structure and chemistry of {111} twin boundaries and stacking faults in Fe-rich sphalerite crystals from the Trepča mines in Kosovo were studied using electron microscopy. The {111} twin boundaries were found to be deficient in S and enriched in O, Mn, Fe, and Cu. The deficiency in S is compensated with O, which is responsible for stabilizing the hexagonal stacking of the fault structures and the formation of {111} twin boundaries in sphalerite. Comparing the intensities of Bijvoet-related reflections we show that there is no inversion of the polar axis across the twin boundaries. In addition to twin boundaries we found two types of stacking faults with RSF1 = 1/3·[1̄12] and RSF2 = 2/3·[1̄12]. The excretion of isostructural copper from the sphalerite crystals peaks at the twin boundaries until it precipitates in the form of small chalcopyrite grains, aligned along the {111} twin boundaries.

Formation of ruddlesden-popper faults and polytype phases in SrO doped SrTiO3

The formation of so-called Ruddlesden–Popper planar faults was studied in SrO-doped SrTiO 3 for different quantities of SrO additions and sintering conditions. For small SrO additions we observed a microstructure with a uniform grain size distribution and the enrichment of SrO at the grain boundaries. Larger additions of SrO produced a microstructure of elongated grains containing random planar faults, polytypic lamellae of more or less ordered faults, and polytype loops within SrTiO 3 grains. We showed that these SrTiO 3 grains were elongated as a result of preferential growth of the polytypic lamellae. In addition, we discuss a correlation between the formation of planar faults embedded in the perovskite matrix at low firing temperatures and Ruddlesden–Popper phases that are stable at higher temperatures.

First observation of a hexagonal close packed metastable intermetallic phase between Cu and Al bilayer films

Abstract In this paper we describe the structure and formation of a new intermetallic phase in the Cu – Al system, which has not been reported before. The phase was found in Cu/Al bilayer films, which were deposited at room temperature on (0001) sapphire substrates using molecular beam epitaxy. The interfacial intermetallic phase is 8 nm thick, and possesses a hexagonal close-packed structure. The lattice parameters of the phase gradually increase from the near-Cu-side to the near-Al-side. In parallel with the described lattice expansion, the chemical composition of the interlayer also varies from 27 to 58 Al at.% from the near-Cu-side to the near-Al-side. The formation and microstructural characteristics of this new phase are explained by Hume-Rothery laws and Shockley partial dislocations. In addition, in-situ heating experiments were performed in a transmission electron microscope at ∼600 °C to investigate the thermodynamic stability of this new Cu – Al intermetallic phase. During annealing the intermetallic layer disappears and other known equilibrium intermetallic phases develop. This indicates that this new hexagonal close-packed Cu – Al intermetallic phase is metastable.

Nucleation and growth of planar faults in SrO-excess SrTiO3

Abstract A SrO-excess SrTiO 3 polycrystalline ceramic was sintered at 1450°C for different times in order to study the anisotropic growth induced by the formation of SrO-rich planar faults inside the perovskite matrix. In the early stage of sintering the microstructure consisted of anisotropic perovskite grains containing polytypic lamella and a fine perovskite, which was free of planar faults. Further treatment caused a decrease in the anisotropy: elongated grains, due to the coarsening effect, developed into equiaxed perovskite grains containing polytypic lamellae. In order to study the preference for polytypic growth we used a SrTiO 3 single crystal sintered in a SrO-rich atmosphere. We found that in the initial stage, fault-rich platelets were formed as discrete nuclei that had no crystallographic relation to the SrTiO 3 single crystal. These platelets provided nucleation sites for a fault-free perovskite, forming sandwich-like composite grains. The single crystal acted as a source of SrTiO 3 for the recrystallisation of these composite grains.

Transient dopant segregation and precipitation in yttrium-doped alumina

Abstract Ultra-pure AI2O3 ceramics doped with 2000 ppm of Y2O3 were sintered at 1500 °C or 1650 °C for 2 or 12 h. The re-sulting microstructure consisted of alumina matrix grains and second-phase precipitates. Y-doped alumina samples sintered at 1550 °C for shorter times (<2 h) contained primarily YAIO3-YAP precipitates. This is in contradiction of the binary equilibrium phase diagram for AI2O3 and Y2O3, which predicts Y3AI5O12- YAG to be the first phase to appear, as the Y-concentration exceeds the solubility limit of α-A12O3. The formation of YAG in co-existence with the YAP precipitates was observed in samples sintered at 1550°C for 12h and 1650°C for 2 h. The excess of yttrium in the grain boundary was 5.5 ± 0.9 Y-atoms/nm2 for the specimen containing primarily YAP and 4.2 ± 1.2 Y-atoms/nm2 in samples where YAP and YAG co-existed. The study demonstrates that in samples containing multiple precipitates, the grain-boundary excess concentration of Y atoms is controlled by the composition of the precipitates.

Twin-boundary formation in Japan-law twinned quartz crystals

The twin-boundary formation of Japanese twins in low-temperature quartz was studied by means of optical microscopy, electron backscatter diffraction on polished sections and transmission electron microscopy (TEM). It was found that the twin junction consists of two units, which are related to different growth stages. In the interior part the crystals exhibit an ~100 μm straight twin junction, a so-called twin boundary, which continues as an undulating boundary towards the crystal surface. Moreover, TEM results showed that this crystallographically defined straight twin boundary can only be found in the first 400 μm of the crystal, where it is attached to the bedrock. The selected-area electron-diffraction pattern, acquired at the twin boundary, showed that reflections common to both twin individuals correspond to the {1122} family of planes, which is in accordance with the Japan twin law. The combined spatially resolved EDX and EELS analyses confirmed no compositional changes across the twin boundary. It is concluded that the formation of a straight twin boundary in quartz is related to the earliest stage of crystal growth. The subsequent growth of Japanese twinned crystals is characterised by the formation of an undulating twin boundary, resulting in a random intergrowth of both twin individuals.

APPROACHES FOR A RELIABLE COMPOSITIONAL ANALYSIS OF ALKALINE-BASED LEAD-FREE PEROVSKITE CERAMICS USING MICROANALYTICAL METHODS

The article describes the approaches for a reliable, quantitative compositional analysis of lead-free perovskite ceramics in powder and bulk forms that contain volatile alkaline compounds. The combination of scanning electron microscopy (SEM) and transmission electron microcopy (TEM) with electron-probe analytical techniques, such as energy-dispersive X-ray spectroscopy (EDS), wavelength-dispersive X-ray spectroscopy (WDS) and electron-energy-loss spectroscopy (EELS) makes it possible to determine the true chemical composition, from precursor powders to synthesized ceramics or single crystals. The microscale (SEM) and nanoscale (TEM) analytical methods also give an insight into the local variations of the chemical composition.

Structure Models of Massively Transformed High Niobium Containing TiAl Alloys

Ab-initio calculations using the Vienna ab-initio simulation package (VASP) were performed for a high Nb bearing γ TiAl based alloy with a composition of Ti-46at.%Al-9at.%Nb in order to evaluate the effect of Nb on the crystal structure. The calculations revealed that upon doping with Nb the resulting structure can have Ti and Nb atoms on Al-sites, which leads to a reduction of the c/a ratio of the tetragonal γ TiAl cell to ~1.In contrast, the c/a ratio is increased, compared to the binary phase, if the Nb atoms occupy solely Ti sites and if Ti antisite defects (i.e. Ti on the Al sublattice) are formed. The relaxed structure models were used to perform high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image simulations. The results showed that the positions of the Nb atoms should be detectable by these high spatial resolution methods, although it might be easier by HAADF-STEM investigations due to the stronger dependence of the signal on the atomic number Z.