Aleksander Rečnik

Polytype induced exaggerated grain growth in ceramics

In polycrystalline materials we frequently observe exaggerated growth of some grains. One of the reasons for such growth irregularity, which we describe here, is the formation of polytypic sequences within the affected grains. We present an atomic-level study of the polytypic faults that are found in sintered perovskite ceramics with various oxide additions. Grains containing polytypic faults grow preferentially along the direction of fault planes. Results from this study indicate that both the local structure and chemistry of these faults are intimately related to the host crystal and the secondary phase that exists between the main phase and the additive.

Exaggerated Anisotropic Grain Growth in Hexagonal Barium Titanate Ceramics

BaTiO 3 ceramic sintered in a reducing atmosphere in the presence of a TiO 2 -rich eutectic exhibits exaggerated anisotropic grain growth. The growth mechanism principally involves the dissolution of cubic matrix grains, a mass transport through the liquid phase and precipitation of a hexagonal polymorph. Under these conditions hexagonal grains grow anisotropically with the highest growth rate in the direction of the prismatic planes. The kinetics of the process can be described by the Avrami equation with an exponent of 2.5. The promoted direction of anisotropic growth is correlated with the growth of parallel (111) twins in cubic BaTiO3. The presence of Ti 3+ ions triggers the formation of the Ti 2 O 9 coordination groups which considerably lowers the formation energy of the hexagonal stacking, and hence the cubic-hexagonal transition temperature. Three-dimensional Monte Carlo simulations were performed to match the experimental microstructures and to demonstrate that the surface energy anisotropy is the important impetus for anisotropic grain growth.

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.

Microstructural engineering of ZnO-based varistor ceramics

In ceramic materials, special boundaries play the key role in crystal growth. They introduce an abrupt structural and chemical anisotropy, which is readily reflected in an unusual microstructure evolution, whereas their local structure affects the physical properties of polycrystalline materials. These effects, however, can be exploited to tailor the electronic and optical properties of the materials, as demonstrated in this review. The presented topic is related to a preparatory stage of phase transformations, manifested through the evolution of chemically induced structural faults. In non-centrosymmetric structure of ZnO, inversion boundaries (IBs) are the most common type of planar faults that is triggered by the addition of the specific IB-forming dopants (Sb2O3, SnO2, TiO2). In addition to conventional TEM techniques, new methods were developed to resolve crystallography and atomic-scale chemistry of IBs. The absolute orientation of the polar c-axes on both sides of an IB was determined by micro-diffraction, providing the most reliable identification of crystal polarity in non-centrosymmetric crystals. To determine sub-monolayer quantities of dopants on the IB, we developed a special technique of analytical electron microscopy using concentric electron probe (CEP) in EDS or EELS mode, providing more accurate and precise results than any other technique. Knowing the local crystal chemistry of IBs, we were able to design experiments to identify their formation mechanism. IBs nucleate in the early stage of grain growth as a dopant-rich topotaxial 2D reaction product on Zn-terminated surfaces of ZnO grains. Soon after their nucleation, ZnO is epitaxially grown on the inherent 2D phase in an inverted orientation, which effectively starts to dictate anisotropic growth of the infected crystallite. In very short time, the grains with IBs dominate the entire microstructure via IB-induced exaggerated grain growth mechanism. This phenomenon was used to design physical properties of ZnO-based varistor ceramics, whereas the bottom-up approach demonstrated here provides the basic tool for microstructural engineering of functional materials in virtually any system that is prone to the formation of special boundaries.

Structural and morphological study of mechanochemically synthesized tin diselenide

Mechanochemical synthesis of tin diselenide, SnSe2, was performed by high-energy milling of tin and selenium powder in a planetary ball mill. The mechanosynthesized product was characterized by X-ray diffraction, 119Sn MAS NMR and 119Sn Mossbauer spectroscopy, which confirmed the presence of the hexagonal SnSe2 phase after 100 min of milling. The size and morphology of tin diselenide particles were studied by specific surface area measurements, and transmission electron microscopy. The specific surface area of powders was found to increase with increasing time of mechanochemical synthesis. Electron diffraction revealed reflections that correspond to hexagonal SnSe2 modification. TEM observations show that the mechanochemical preparation route results in the formation of nanosized thick barrel-shaped SnSe2 platelets. SnSe2 nanoparticles show good absorption in the visible region of the UV-Vis optical spectrum and they evidence direct and indirect types of transitions in the lattice.

Characterization of mechanochemically synthesized lead selenide

Mechanochemical synthesis of lead selenide PbSe nanocrystals was performed by high-energy milling of lead and selenium powders in a planetary ball mill. The structure properties of synthesized lead selenide were characterized by XRD analysis that confirmed crystalline nature of PbSe nanocrystals. Calculated average size of PbSe crystallites was 37 nm. The methods of particle size distribution analysis, specific surface area measurement, SEM and TEM were used for the characterization of surface and morphology of PbSe nanocrystals. SEM analysis revealed agglomerates of PbSe particles. However, HRTEM analysis confirmed perfect stoichiometric PbSe cubes with NaCl structure as well. UV-VIS-NIR spectrophotometry was used to confirm the blue shift of the small particles occurring in the powder product obtained by the mechanochemical synthesis.

Atomic structure and formation mechanism of (101) rutile twins from Diamantina (Brazil)

Abstract We studied the atomic structure and the chemical composition of (101)-type rutile (TiO2) twins from Diamantina in Brazil by electron microscopy methods to resolve the mechanism of their formation. The twin boundaries were studied in two perpendicular orientations to reveal their 3D structure. The presence of a precursor phase, such as Al-rich hydroxylian pseudorutile (HPR; kleberite), during the initial stages of the crystallization appears to be the necessary condition for the formation of (101) twins of rutile at this locality. The precursor with a tivanite-type structure serves as a substrate for the topotaxial crystallization of rutile. Depending on the initial crystallization pattern the rutile can grow either as a single crystal or as a twin. During the progressive crystallization of the rutile Al-rich oxyhydroxide (diaspore, α-Al2O3) clusters are concentrated at the center of the precursor where they are pinned to the twin boundary as the precursor is fully recrystallized into rutile. At the increased temperatures the remaining diaspore precipitates are converted to corundum (α-Al2O3), while the two crystal domains continue to grow in the (101) twin orientation. In addition to the primary (101) twin, series of secondary {101} twins are formed to accommodate the residual tensile stress caused by the diaspore-to-corundum transformation. Based on the observed corundum-rutile [0001]C(112̄0)C||[010]R(101)R and ilmeniterutile [0001]I(11̄00)I||[010]R(301)R crystallographic relations a unified mechanism of the genesis of the {101} and {301} reticulated sagenite twin clusters is proposed.

Quantitative microstructural and spectroscopic investigation of inversion domain boundaries in sintered zinc oxide ceramics doped with iron oxide

Abstract It is known that sintering of powders of zinc oxide (ZnO) with small additions of iron oxide results in a ceramic with grains exhibiting a characteristic inversion domain microstructure with planar inversion domain boundaries (IDBs) on two different habit planes. This study concentrates on a quantitative analysis, by a combination of different transmission electron microscopy methods, of those IDBs that are parallel to {0001} basal planes of the wurtzite structure of ZnO. Electron diffraction and dark-field imaging prove the nature of the inversion. High-resolution annular dark field scanning transmission electron microscopy allows measurement of the rigid body displacements across these IDBs and of the local lattice contraction related to the octahedral interstices that form the boundaries. Energy-selected imaging, electron energy-loss spectroscopy and energy-dispersive X-ray spectroscopy have been combined to determine the chemical composition of the IDBs quantitatively. It is thus shown unambiguously that every such fault consists of precisely one basal plane of octahedral interstices that are completely occupied by Fe3+ ions and that these FeO6 octahedra are themselves contracted along the <0001> direction. A local charge balance model explains the observations

Inversion Boundaries and Phonon Scattering in Ga:ZnO Thermoelectric Compounds

We investigated the high-temperature thermoelectric properties of Ga:ZnO bulk compounds, synthesized using a simple and scalable solid-state process. The effects of a low gallium content (x ≤ 0.04 in Zn1-xGaxO1+x/2) on the structural features and electrical/thermal properties are reviewed. Transmission electron microscopy analyses showed that 2D, nonperiodic defects had formed from a doping content as low as x = 0.01 Ga. The structural description of these nanoscale interfaces is, for the first time, carefully investigated in such low-Ga-content samples by HAADF-STEM analyses combined with structural modeling. It was found that the formation of head-to-head inversion twin (h-IT) boundaries and tail-to tail inversion boundaries (t-IB) in the bulk compounds is responsible for strong phonon scattering, while maintaining relatively good electrical conductivity and thereby enhancing the thermoelectric properties. The absolute value of the Seebeck coefficient decreases abruptly from 475 μV/K for x = 0 down to 60 μV/K for x = 0.005 at 350 K. At the same time, the electrical resistivity drops from 1 ohm cm for x = 0 to 1.7 × 10-3 ohm cm for x = 0.005. For higher Ga additions (x > 0.01), the increase in electrical resistivity is likely linked to the formation of interface defects at a larger extent in the wurtzite structure. The thermal conductivity also drops sharply with the increase in the Ga content from ∼33 W/m K for x = 0 to ∼8 for x = 0.04 at 350 K. This study is progress toward the synthesis of other thermoelectric materials where nanoscale interfaces in bulk compounds provide tremendous opportunities for further enhancing both the phonon scattering and the overall figure of merit.

Controlled synthesis of pure and doped ZnS nanoparticles in weak polyion assemblies: growth characteristics and fluorescence properties.

A polyelectrolyte multilayer (PEM) fabricated by the layer-by-layer (LbL) self-assembly of weak polyions of polyacrylic acid (PAA) and polyallylamine (PAH) was applied as a matrix for the in situ nucleation and growth of pure and Mn-doped ZnS nanocrystallites. The nucleation and growth is initiated by the adsorption and binding of the metal ions to the ionized carboxylic groups of the weak polyions within the matrix, followed by the subsequent precipitation of semiconductor nanocrystallites with Na(2)S. Transmission electron microscopy (TEM), atomic force microscopy (AFM) and UV-vis spectroscopy were employed to establish the growth characteristics of the spherical ZnS nanocrystallites in the polyion matrix. The conformational arrangement of polyion chains induced by variation in the assembly pH is the key parameter that affects the structural and morphological characteristics of ZnS nanocrystallites. Repeating the reaction cycle resulted in an increase in the volume density of ZnS nanoparticles and further growth of the initially formed particles by the Ostwald ripening mechanism. The surface passivation of the ZnS nanocrystallites within the polyion matrix enables the enhanced radiative emission of ZnS composite films in the UV range, whereas by doping the ZnS, nanocrystallites show emission characteristic of the manganese ions in the visible region.