Iskanje po repozitoriju

A+ | A- | | SLO | ENG

Povzetek: Loparite is a natural perovskite (ABO3) with a complex composition, essentially a solid solution between loparite(-Ce) (Na0.5Ce0.5TiO3), lueshite (NaNbO3), and perovskite (CaTiO3), with small amounts of many other elements. The majority of reported loparite crystals are spinel-type interpenetration twins with compositional zoning. Associated with the high variability of compositions, different crystal structures of loparite were described, and the origin of twinning has not been addressed so far. In this work, we studied a loparite twin composed of two symmetrically intergrown cuboctahedra. Microprobe analyses revealed that the cubic and octahedral growth sectors have slightly different compositions. According to atomic-scale analyses, the cubic sectors with a lower Na:LREE ratio have a disordered orthorhombic structure, while the octahedral sectors with a higher Na:LREE ratio show partial ordering of Na and light rare-earth elements along A-type lattice planes in the [001] direction. The degree of ordering was evaluated by quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scale analyses of (111) and {112} twin boundary (TB) contacts have shown that all TBs contain a 1–2 nm thin layer of an Si-rich amorphous phase. Our observations suggest that the loparite twin was established in the nucleation stage of crystallisation, followed by independent crystallisation of both twin domains from the melt. The initial crystal form was cubic; octahedral sectors evolved when the crystal size was still in the nanometre range, as a result of slow crystal growth. Differences in structural ordering between adjacent growth sectors developed during slow cooling due to the compositional variations. Our results imply that compositionally zoned crystals might show different structural ordering within single domains, which should be considered when interpreting bulk diffraction data of compositionally zoned perovskites.
Ključne besede: loparite, loparite twin, growth sectors, morphology
Objavljeno v DiRROS: 02.03.2026; Ogledov: 215; Prenosov: 170
Celotno besedilo (12,71 MB)
Gradivo ima več datotek! Več...

Povzetek: The hydration kinetics, phase assemblage, microstructure and mechanical properties of belite–ye’elimite–ferrite (BYF) cement containing different secondary raw materials (red mud and waste concrete) and cured at 5, 20 or 608C were examined. Different secondary raw materials were found to lead to changes in cement hydration. The influence of these materials on the hydration processes of belite and ferrite became more noticeable at a curing temperature of 58C, but diminished with an increase in curing temperature. The BYF cement made with red mud exhibited accelerated hydration, particularly at 58C, due to higher contents of mayenite and alkali sulfate compared with cement with natural materials and waste concrete. Red mud inhibited early ettringite formation due to the presence of aphthitalite but promoted monosulfate through accelerated ye’elimite hydration. Ferrite hydration was delayed by red mud, as alkali sulfates promote belite reactivity and calcium silicate hydrate (C-S-H) formation over stra¨tlingite. Periclase increased siliceous hydrogarnet formation at higher temperatures, supporting delayed yet stable strength development. The curing temperature impacted the morphology of the C-S-H (transitioning from a fine fibrillar-like structure to a foil-like structure) and affected the chemical composition of hydrates, resulting in a lower incorporation of iron in hydrogarnet and C-S-H at higher temperatures. Increased curing temperatures promoted aluminium uptake in calcium aluminium silicate hydrate.
Ključne besede: belite–ye'elimite–ferrite (BYF) cement, cement, cement paste, cementitious materials, hydrated cement, secondary raw materials, temperature
Objavljeno v DiRROS: 28.01.2026; Ogledov: 274; Prenosov: 264
Celotno besedilo (1,78 MB)
Gradivo ima več datotek! Več...

Povzetek: Flexoelectricity – polarization induced by strain gradients – offers a route to polar functionality in centrosymmetric dielectrics, where traditional piezoelectric effects are absent. This study investigates the flexoelectric effect in epitaxial BaHfO3 (BHO) thin films, a centrosymmetric and paraelectric perovskite. While a large lattice mismatch induces defect-driven relaxation, a coherently grown BHO film undergoes elastic relaxation, forming intrinsic strain gradients exceeding 105 m−1. A 29-fold enhancement in spontaneous polarization is observed at an electric field of 4 MV cm−1 for BHO exhibiting a strain gradient compared to relaxed BHO. This enhancement is attributed to flexoelectric coupling, which is isolated from ferroelectric and piezoelectric contributions due to the centrosymmetric nature and the absence of phase transitions in BHO. The findings establish a clear link between engineered strain gradients and enhanced polarizability in oxide thin films, offering a benchmark system for deconvoluting the flexoelectric effect from other polar effects. These results provide a basis for exploiting flexoelectricity in dielectric devices and advance the fundamental understanding of strain-coupled phenomena in functional oxides.
Ključne besede: condensed matter physics, flexoelectricity, strain gradients, epitaxial oxide thin films
Objavljeno v DiRROS: 23.12.2025; Ogledov: 803; Prenosov: 190
Celotno besedilo (3,69 MB)
Gradivo ima več datotek! Več...

Povzetek: For the first time, a mixture of PbTe and Pb- and Te-oxides coated with carbon, under electron beam irradiation (EBI), was transformed into quantum dots, nanocrystals, nanoparticles and grains of PbTe with a sintered appearance. A small portion of non-stoichiometric phases was also obtained. By selecting conditions that favor the instantaneous transformation, the Gibbs free energy barrier is lowered for obtaining different PbTe structures. The driving force associated with the high-energy milling requires 4 h of processing time to reach a complete transformation, while a high-energy source kinetically affects precursor surfaces to cause an abrupt global chemical transformation instantly. Importantly, the size of the PbTe structures increases as they approach the irradiation point, implying a growth process that is affected by the local temperature reached during the EBI. Imaging after the EBI process revealed morphological variations in PbTe, which can be attractive for use in thermoelectric materials. The results of this study provide the first insights into electron-beam-induced reactions using a multiphase mixture of precursors. Therefore, it is believed that this proposal can also be applied to obtain other binary semiconductor structures, even ternary ones.
Ključne besede: semiconductor, multiphase mixture of precursors, rate of transformation
Objavljeno v DiRROS: 15.07.2025; Ogledov: 543; Prenosov: 402
Celotno besedilo (2,52 MB)
Gradivo ima več datotek! Več...

Povzetek: The influence of different curing temperatures on the hydration of belite-calcium sulfoaluminate cement was investigated at 20, 40 and 60 °C. The hydration kinetics and the hydrated phase assemblages were studied by isothermal calorimetry, X-ray powder diffraction, differential thermal analysis and thermogravimetric analysis, as well as field emission scanning electron microscopy. The compressive strength development of the cement pastes was also determined. Results showed that, at early ages, hydration was faster and early compressive strength was higher at elevated temperatures than at ambient temperature. On the other hand, at late ages in cement pastes cured at 60 °C, the amount of ettringite decreased, leading to lower compressive strength, indicating that the degree of hydration was lower at higher temperatures. Moreover, at elevated temperatures prismatic ettringite crystals became smaller due to faster hydration. Other hydration products present were aluminium hydroxide, which is formed together with ettringite from the hydration of calcium sulfoaluminate and gypsum, and C–S–H which precipitates as a main hydration product of belite. Belite hydrated in a lesser amount, especially at 60 °C, when the lowest amount of C–S–H was observed.
Ključne besede: hydration, cement, temperature, belite, calcium sulfoaluminate, open access, hidratacija, cement, temperatura, belit, kalcijev sulfoaluminat
Objavljeno v DiRROS: 04.08.2023; Ogledov: 1836; Prenosov: 1103
Celotno besedilo (7,33 MB)
Gradivo ima več datotek! Več...