Giovanna Canu

Hydrothermal synthesis of strontium titanate: thermodynamic considerations, morphology control and crystallisation mechanisms

This article highlights the recent developments on the hydrothermal and solvothermal synthesis of strontium titanate (SrTiO3), considered as a model system, by reviewing the literature of the last 10–15 years. The most significant advantage of these solution-mediated crystallisation methods is the effective control of particle composition, size and morphology by varying some physical and chemical parameters such as temperature, concentration, pH and solvent composition, as well as using different precursors and mineralisers, and moreover, adding growth modifiers like polar organic molecules and hydrophilic polymers. Thus, the synthesis process can be designed to obtain pure and doped materials with superior functional and photocatalytic properties, including monodispersed nanoparticles, platelets, wires, porous particles, mesocrystals and heterostructures. The hydrothermal crystallisation mechanisms are critically discussed, considering both the thermodynamic and kinetic aspects. In particular, the nature and morphology of the solid titanium precursor has a significant impact on the hydrothermal crystallisation as in many cases the formation of SrTiO3 occurs on the precursor surface. The dissolution of the precursor and the nucleation and growth of the perovskite are coupled together over rather short distances, and the coupling is mediated by the solid/liquid interfaces. The morphology of strontium titanate is thus determined by the precursor/perovskite crystallographic matching, the surface density of nuclei and the rate-controlling process. Differently, crystallisation from Sr–Ti amorphous gel-like precursors occurs in the absence of crystalline surfaces and often produces mesocrystals by oriented aggregation of the primary nanocrystals. These considerations have a general validity and can be extended to many ternary and even more complex oxides.

Magnetic and dielectric properties of Ba12Fe28Ti15O84 layered ferrite ceramics

In this study we report for the first time the magnetic and dielectric properties of the quaternary layered ferrite Ba12Fe28Ti15O84. Dense ferrite ceramics were prepared by conventional sintering using powders obtained by solid-state reaction and by coprecipitation. Only the latter powder resulted in nearly single phase ceramics, whereas larger amounts of secondary phases were observed in the material obtained by the solid-state route. According to the HRTEM investigation, the ferrite lattice is originated by the intergrowth of perovskite-like and spinel-like slabs and can be considered as a natural magnetic superlattice. A ferrimagnetic order with saturation magnetization of ≈12.5 A m2 kg−1 and coercivity of ~1590 A m−1 (~20 Oe) is proposed at room temperature. The thermomagnetic data indicate a Curie temperature of ~420 K for the quaternary ferrite. An additional magnetic transition was detected at ~700 K and ascribed to a secondary magnetic phase, probably the solid solution of TiO2 in BaFe12O19. An intrinsic relative dielectric constant of the order of 23–50 at room temperature was measured at 109 Hz. At lower frequency the dielectric behaviour is dominated by extrinsic effects related to the heterogeneous electrical nature of the ceramics corresponding to semiconducting grains separated by more insulating grain boundary regions. The dielectric losses are rather high, often >1, indicating an overall semiconducting character of the material.

PVDF/BaTiO3 composites as dielectric materials: Influence of processing on properties

In this work 0-3 connectivity composites based on poly(vinylidene fluoride) (PVDF) and containing 30 vol% of BaTiO3 nanoparticles were prepared by using two different processing methods: solvent casting and melt blending. In both cases, the ensuing composite materials were compression molded to obtain sheets suitable for the subsequent characterization. In order to induce the formation of the electro-active β polymorph of PVDF, the compression molding conditions were properly adjusted. A thorough characterization was then performed to investigate the influence of processing conditions on the dielectric properties of the composites, taking into account PVDF polymorphism and particles dispersion within the polymer matrix. Experimental evidences suggest that dielectric characteristics of the composites are strongly affected by the processing procedure; in particular, composites from melt blending with high amounts of β phase have higher permittivity values compared to those obtained from solvent casting, probably due to a synergistic contribution of PVDF polymorphism and morphological features.In this work 0-3 connectivity composites based on poly(vinylidene fluoride) (PVDF) and containing 30 vol% of BaTiO3 nanoparticles were prepared by using two different processing methods: solvent casting and melt blending. In both cases, the ensuing composite materials were compression molded to obtain sheets suitable for the subsequent characterization. In order to induce the formation of the electro-active β polymorph of PVDF, the compression molding conditions were properly adjusted. A thorough characterization was then performed to investigate the influence of processing conditions on the dielectric properties of the composites, taking into account PVDF polymorphism and particles dispersion within the polymer matrix. Experimental evidences suggest that dielectric characteristics of the composites are strongly affected by the processing procedure; in particular, composites from melt blending with high amounts of β phase have higher permittivity values compared to those obtained from solvent casting, pro...

Cation Diffusion and Segregation at the Interface between Samarium-Doped Ceria and LSCF or LSFCu Cathodes Investigated with X-ray Microspectroscopy

The chemical compatibility between electrolytes and electrodes is an extremely important aspect governing the overall impedance of solid-oxide cells. Because these devices work at elevated temperatures, they are especially prone to cation interdiffusion between the cell components, possibly resulting in secondary insulating phases. In this work, we applied X-ray microspectroscopy to study the interface between a samarium-doped ceria (SDC) electrolyte and lanthanum ferrite cathodes (La0.4Sr0.6Fe0.8Cu0.2O3 (LSFCu); La0.9Sr0.1Fe0.85Co0.15O3 (LSCF)), at a submicrometric level. This technique allows to combine the information about the diffusion profiles of cations on the scale of several micrometers, together with the chemical information coming from space-resolved X-ray absorption spectroscopy. In SDC-LSCF bilayers, we find that the prolonged thermal treatments at 1150 °C bring about the segregation of samarium and iron in micrometer-sized perovskite domains. In both SDC-LSCF and SDC-LSFCu bilayers, cerium diffuses into the cathode perovskite lattice A-site as a reduced Ce3+ cation, whereas La3+ is easily incorporated in the ceria lattice, reaching 30 atom % in the ceria layer in contact with LSFCu.

Raman spectroscopic study of layered quaternary ferrite Ba12Fe28Ti15O84

Ba–Fe–Ti oxides are nowadays attracting considerable interest for the production of permanent magnets and microwave devices, due to their high dielectric constant. Among these materials, recently the quaternary ferrite Ba12Fe28Ti15O84 (BFT) was discovered to possess ferrimagnetic properties at room temperature with a main magnetic transition at about 420 K and complete disappearance of magnetisation above 700 K. In this study, we report for the first time on the Raman spectrum of BFT samples prepared with different methods. Raman spectra were recorded in dependence of temperature and a preliminary assignment of modes was attempted. Coupling the Raman results with previous magnetic studies allowed gaining more insight on the structural mechanism at play in correspondence of the main magnetic transition.