Giorgio Schileo

Ferroelectric and dielectric properties of ferrite-ferroelectric ceramic composites

Particulate composites of ferrite and ferroelectric phases with xNiFe2O4 (NF) and (1 − x)Pb0.988(Zr0.52Ti0.48)0.976Nb0.024O3 (where x = 2, 10, 20, 30, 50, 70, and 100 wt. %) were prepared in situ by sol-gel method. The presence of a diphase composition was confirmed by X-ray diffraction while the microstructure of the composites was studied by scanning electron microscopy revealing a good mixing of the two phases and a good densification of the bulk ceramics. The dielectric permittivity shows usual dielectric dispersion behavior with increasing frequency due to Maxwell-Wagner interfacial polarization. AC conductivity measurements made in frequency range 1 Hz-1 MHz suggest that the conduction process is due to mixed polaron hopping. The effect of NF phase concentration on the P-E and M-H hysteresis behavior and dielectric properties of the composites was investigated. At low NF concentration a sharp ferro-paraelectric transition peak can be observed at around 360 °C while for higher NF concentrations a tre...

Continuously controllable optical band gap in orthorhombic ferroelectric KNbO3-BiFeO3 ceramics

The optical bandgap of orthorhombic ferroelectric KNbO3 is shown to be continuously controllable via Bi and Fe co-substitution according to a K1-xBixNb1-xFexO3 doping mechanism. The room temperature X-ray diffraction data combined with Raman spectroscopy analysis show the polar orthorhombic crystal structure to persist up to x = 0.25, while the bandgap narrows monotonically by 1 eV (∼33%). In-situ Raman spectroscopy corroborates the polar nature of all compositions in the temperature range of –100 to 200 °C. The ability to control the bandgap while maintaining the spontaneous polarisation makes the K1-xBixNb1-xFexO3 system interesting for photoinduced processes in a wide temperature range.

Band gap evolution and a piezoelectric-to-electrostrictive crossover in (1 − x)KNbO3–x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 ceramics

The band gap of (1 − x)KNbO3–x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 (0 ≤ x ≤ 0.25) ceramics narrows slightly from 3.22 eV for x = 0 to 2.89 eV for x = 0.25, in broad agreement with first-principles calculations [Phys. Rev. B, 2014, 89, 235105]. In addition, an unreported piezoelectric-to-electrostrictive crossover is observed in this compositional range, which is accompanied by a continuous decrease of the maximum electric field-induced strain due to the presence of a non-ferroelectric phase. An electrostriction coefficient of 0.023 m4 C−2 is measured for x = 0.05, whilst no electromechanical response is observed for non-ferroelectric x = 0.25, even under an applied electric field of 80 kV cm−1.

Band gap narrowing in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me=Fe or Mn) ceramics

The direct optical bandgap in ferroelectric KNbO3-Bi(Yb,Me)O3 (Me = Fe or Mn) ceramics fabricated by the solid state reaction method varies from 3.2 eV for KNbO3 down to 2.2 eV for 0.95KNbO3-0.05BiYbO3, as revealed by optical spectroscopic ellipsometry. This narrowing of bandgap is accompanied by an apparent increase of the room-temperature relative permittivity from 320 for KNbO3 to 900 for 0.95KNbO3-0.05BiYbO3. All compositions studied exhibit dielectric anomalies associated with structural phase transitions, and their ferroelectric nature is corroborated by the presence of a sharp mixed mode (at ∼190 cm−1) and by a Fano-type resonant dip in their Raman spectra.

Nanoscale Mapping of Bromide Segregation on the Cross Sections of Complex Hybrid Perovskite Photovoltaic Films Using Secondary Electron Hyperspectral Imaging in a Scanning Electron Microscope

Mixed halide (I/Br) complex organic/inorganic hybrid perovskite materials have attracted much attention recently because of their excellent photovoltaic properties. Although it has been proposed that their stability is linked to the chemical inhomogeneity of I/Br, no direct proof has been offered to date. Here, we report a new method, secondary electron hyperspectral imaging (SEHI), which allows direct imaging of the local variation in Br concentration in mixed halide (I/Br) organic/inorganic hybrid perovskites on a nanometric scale. We confirm the presence of a nonuniform Br distribution with variation in concentration within the grain interiors and boundaries and demonstrate how SEHI in conjunction with low-voltage scanning electron microscopy can enhance the understanding of the fundamental physics and materials science of organic/inorganic hybrid photovoltaics, illustrating its potential for research and development in “real-world” applications.

The Influence of Rice Husk Ash Addition on the Properties of Metakaolin-Based Geopolymers

This paper investigates the replacement of metakaolin (MK) with rice husk ash (RHA) in the production of alkali-activated binders or geopolymers. The influence of the RHA addition on compressive and flexural strength, as well as water absorption and apparent porosity were determined, in terms of the percentage of RHA in the mixture and molar ratios of the mixes. Fourier Transform Infrared (FTIR) spectroscopy and Energy Dispersive spectroscopy (EDS) were carried out to assess the changes in the microstructure of the geopolymer matrices with the RHA addition. Results have shown that RHA may be a supplementary precursor for geopolymers. The composition of the geopolymer matrices containing 0-40% RHA is very similar, which indicates that the additional Si provided by RHA is not incorporated to the geopolymer matrix. In addition, geopolymers with RHA content higher than 40% present a plastic behavior, characterized by extremely low strength and high deformation, which can be attributed to the formation of silica gel in formulations containing variable Si/Al ratio.