Andrea Testino

Ferroelectric properties of dense nanocrystalline BaTiO3 ceramics

Dense BaTiO3 ceramics with 50?nm average grain size obtained by spark plasma sintering were investigated. The dielectric data show a broad ferro?para phase transition with a maximum permittivity of at 390?K and 1?kHz. The local ferroelectric switching behaviour was investigated by piezoresponse force microscopy. Typical piezoelectric hysteresis loops were recorded at different positions of the sample. The present results provide experimental evidence for polarization switching at the local scale, indicating that the critical grain size for the disappearance of ferroelectric behaviour in dense, bulk BaTiO3 nanocrystalline ceramics is below 50?nm.

Local switching properties of dense nanocrystalline BaTiO3 ceramics

The switching properties of dense BaTiO3 ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO3 ceramics retain ferroelectricity at a local scale.

Synthesis and characterization of BaSn(OH)6 and BaSnO3 acicular particles

The synthesis of BaSn(OH) 6 acicular crystals by precipitation at 100 °C from aqueous solutions and their transformation in the perovskitelike compound BaSnO 3 was investigated. Single acicular crystals 100–200 μm in length were obtained from a 0.05M solution, whereas bundlelike aggregates of 20–40 μm were precipitated from 0.2–0.6 M solutions. The x-ray diffraction pattern of barium hexahydroxostannate was indexed according to monoclinic symmetry with cell parameters a = 11.029 ± 0.002 A, b = 6.340 ± 0.001 A, c = 10.563 ± 0.001 A = 128.51 ± 0.01°, α = γ = 90°. The BaSn(OH) 6 particles decomposed to BaSnO 3 and water at approximately 270 °C and the original morphology was retained. The resulting product had specific surface area up to 30–40 m 2 /g and consisted of 10–20 nm crystallites. The larger unit cell edge in comparison to the reference value and the continuous weight loss up to 1200 °C indicate that water is not completely released during decomposition and a substantial amount of proton defects (up to 0.4 mol per mole of BaSnO 3 ) is incorporated in the perovskite lattice as OH − groups. Normal crystallographic properties of BaSnO 3 are restored only after calcination at 1300 °C.

Amphoteric behaviour of Er3+ dopants in BaTiO3: an Er–LIII edge EXAFS assessment

Er-doped BaTiO3 samples with nominal compositions BaEr0.08Ti0.92O2.96 and Ba0.96Er0.08Ti0.96O3 have been analyzed with Er–LIII edge XAFS. The first sample shows predominant Er substitution on the Ti site of the perovskite structure, the second one shows almost equal occupancy of Er on Ba and Ti sites, thus assessing the ability of the stoichiometry to control the doping mechanism. In more detail, in both cases the main doping mechanism is not exclusive. Er–O distances determined by EXAFS are in good agreement with static lattice simulations.

Influence of Irradiance, Flow Rate, Reactor Geometry, and Photopromoter Concentration in Mineralization Kinetics of Methane in Air and in Aqueous Solutions by Photocatalytic Membranes Immobilizing Titanium Dioxide

Photomineralization of methane in air (10.0–1000 ppm (mass/volume) of C) at 100% relative humidity (dioxygen as oxygen donor) was systematically studied at 318±3 K in an annular laboratory-scale reactor by photocatalytic membranes immobilizing titanium dioxide as a function of substrate concentration, absorbed power per unit length of membrane, reactor geometry, and concentration of a proprietary vanadium alkoxide as photopromoter. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed. At a fixed value of irradiance (0.30 W⋅cm−1), the mineralization experiments in gaseous phase were repeated as a function of flow rate (4–400 m3⋅h−1). Moreover, at a standard flow rate of 300 m3⋅h−1, the ratio between the overall reaction volume and the length of the membrane was varied, substantially by varying the volume of reservoir, from and to which circulation of gaseous stream took place. Photomineralization of methane in aqueous solutions was also studied, in the same annular reactor and in the same conditions, but in a concentration range of 0.8–2.0 ppm of C, and by using stoichiometric hydrogen peroxide as an oxygen donor. A kinetic model was employed, from which, by a set of differential equations, four final optimised parameters, 𝑘1 and 𝐾1, 𝑘2 and 𝐾2, were calculated, which is able to fit the whole kinetic profile adequately. The influence of irradiance on 𝑘1 and 𝑘2, as well as of flow rate on 𝐾1 and 𝐾2, is rationalized. The influence of reactor geometry on 𝑘 values is discussed in view of standardization procedures of photocatalytic experiments. Modeling of quantum yields, as a function of substrate concentration and irradiance, as well as of concentration of photopromoter, was carried out very satisfactorily. Kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading to mineralization, were considered, and it is paralleled by a second competition kinetics involving superoxide radical anion.

Nanocrystalline TiO2 with enhanced photoinduced charge separation as catalyst for the phenol degradation

Nanocrystalline TiO2 catalysts based on pure rutile (R100) and a 30% of anatase and 70% of rutile (R70) were synthesized by the sol-gel method, using Pluronic PE 6400 as templating agent. Catalysts were characterized in terms of structural and morphological properties; moreover, the formation of paramagnetic charge carriers under UV irradiation was studied and related to the activity of TiO2 in the photoinduced degradation of phenol. With respect to Degussa P25, the two sol-gel catalysts show lower surface area and a wider pore size distribution. The EPR spectra recorded under UV irradiation show enhanced charge separation in the sol-gel samples, with the O− species in higher amount than in Degussa P25. This result is in agreement with the high catalytic activity of R100 sample in the photoinduced degradation of phenol, very similar to that displayed by Degussa P25 and higher than that of R70 sample.

Continuous Polyol Synthesis of Metal and Metal Oxide Nanoparticles Using a Segmented Flow Tubular Reactor (SFTR).

Over the last years a new type of tubular plug flow reactor, the segmented flow tubular reactor (SFTR), has proven its versatility and robustness through the water-based synthesis of precipitates as varied as CaCO3, BaTiO3, Mn(1−x)NixC2O4·2H2O, YBa oxalates, copper oxalate, ZnS, ZnO, iron oxides, and TiO2 produced with a high powder quality (phase composition, particle size, and shape) and high reproducibility. The SFTR has been developed to overcome the classical problems of powder production scale-up from batch processes, which are mainly linked with mass and heat transfer. Recently, the SFTR concept has been further developed and applied for the synthesis of metals, metal oxides, and salts in form of nano- or micro-particles in organic solvents. This has been done by increasing the working temperature and modifying the particle carrying solvent. In this paper we summarize the experimental results for four materials prepared according to the polyol synthesis route combined with the SFTR. CeO2, Ni, Ag, and Ca3(PO4)2 nanoparticles (NPs) can be obtained with a production rate of about 1–10 g per h. The production was carried out for several hours with constant product quality. These findings further corroborate the reliability and versatility of the SFTR for high throughput powder production.

On the mesoscale mechanism of synthetic calcium–silicate–hydrate precipitation: a population balance modeling approach

Calcium–silicate–hydrate (C–S–H) is the most important product of cement hydration. Despite this importance, its formation mechanism is not well-understood. Here, we describe the novel application of a coupled thermodynamic-kinetic computational model based on a population balance equation in order to unravel the overall mechanism of synthetic C–S–H precipitation. The framework, embracing primary nucleation, true secondary nucleation, and molecular growth as the constituting sub-processes, is regressed to experimental Ca2+(aq) concentration vs. time data collected on a model synthetic C–S–H with Ca : Si = 2. Upon the critical appraisal of the models adjustable parameters, which turn out to adopt rational values, simulations were performed to estimate various characteristics of the aforementioned model system (e.g., the kinetic speciation during the precipitation process, or the mechanisms and activation free energies of nucleation and growth phenomena). We mechanistically account for the evolution of the C–S–H mesostructure which is made up of defective crystallites around 3–6 nm thick, nematically packing together in two dimensions giving rise to foil-like polycrystalline particles around 100 nm in breadth, close to the experimentally observed values. The computational framework is generic and can be applied to other precipitation systems and cement hydration scenarios.

Formation and transformation of calcium phosphate phases under biologically relevant conditions: Experiments and modelling

The experimental data on calcium phosphates formation were collected in dilute solution at constant pH (7.40) and temperature (37.0 °C) at different levels of ionic strength (IS). The evolution of the solid phase formation is described in detail using a thermodynamic-kinetic model. The thermodynamic model takes into account all relevant chemical species as well as Posners clusters; the kinetic model, based on the discretized population balance approach, accounts for the solid formation from solution. The experimental data are consistent with an initial formation of dicalcium phosphate dihydrate (DCPD, brushite), which dominates the nucleation rate, and its rapid transformation into octacalcium phosphate (OCP) or hydroxyapatite (HA), which dominates the growth rate. Depending on the experimental conditions and, including the influence of the IS level, OCP may be further transformed into apatite. The classical nucleation theory is able to describe the experimental results very well and the solid phase growth is limited by the diffusion of Ca2+ ions. The precipitation pathway described by a complete thermodynamic-kinetic model is expected to contribute to the understating of the in vivo osteogenesis. STATEMENT OF SIGNIFICANCE The formation mechanism of calcium phosphates under biomimetic conditions is unraveled. The formation pathway is mathematically described based on a thermodynamic-kinetic model in which (i) the nucleation stages (primary and secondary) are dominated by the formation of dicalcium phosphate dihydrate (DCPD) and (ii) the fast growth stage is limited by the diffusion of Ca2+ ions under the driving force of octacalcium phosphate (OCP), or hydroxyapatite (HA), solubility. The obtained solid phase seems correlated to the activity coefficient of phosphate ions, thus to the ionic strength and local phosphate speciation. The model, being able to highlight the details of the precipitation pathway, is expected to contribute to the understanding of the apatitic phase formation in the biomineralization-biodemineralization processes under in-vivo conditions.

In situ synchrotron diffraction study of precipitations in liquid jet

Calcium carbonate, one of the most studied biominerals, has major applications across a broad spectrum of technologies. In order to investigate the crystallization kinetics, in-situ wide-angle (WAXS) and small-angle (SAXS) synchrotron X-ray scattering experiments are being performed at the MS-X04AS Beamline of the SLS synchrotron at the PSI, Villigen, Switzerland. [1] In particular, the SAXS signal is important for detecting aggregates and their size, independently of their atomic structure, while the WAXS will enable us to distinguish between amorphous clumps and crystalline NPs. The feasibility of the WAXS data collection has been established in recent tests, while a SAXS experiment has been performed in situ on a horizontal liquid microjet. This was generated using a nozzle connected to a mixer. Four HPLC pumps were delivering solutions in order to obtain the desirable pH and saturation level of the system. The liquid was collected in a catcher where T and pH of the solution, under stirring, were monitored on line. After micro-jet optimization (pulsation damping, liquid jet diameter, solution composition, time delay between mixing point and liquid, X-ray beam focusing), measurements were carried out with stainless steel nozzles of 125 μm and 250 μm and with delay times of 0, 1, 60 s. The SAXS data (figure 1 shows a representative example of the data), collected using a Mythen II detector [2], revealed very clearly pre-nucleation spherical amorphous clusters of 20-30 nm size. Correlations between supersaturations, delay time after mixing, particle size, and concentrations are analyzed and discussed [3]. The data are corrected from air and sodium carbonate solution background.