M.A. Valente

Electrical properties of lithium niobium silicate glasses

Lithium niobate (LiNbO3) is a ferroelectric material in the crystalline form and considerable interest has recently been shown in the study of its properties in a glass structure. A 34SiO2–33Li2O–33Nb2O5 (mol%) glass was prepared using the conventional melt-quenching method. LiNbO3 crystals were precipitated in the glass matrix by a heattreatment (HT) process. These glasses were studied by differential thermal analysis, X-ray power diffraction (XRD), scanning electron microscopy, dc electrical conductivity (rdc), ac conductivity (rac) and dielectric measurements. LiNbO3 crystal phase was detected by XRD on the sample HT at 600 � C. The dc and ac conductivity, at 300 K, decreases with the increasing of the HT temperature. The dielectric study in function of frequency, at constant temperature, was made using the modulus (M � ) model and a distribution of relaxation times was observed. The dielectric data was fitted with the complex non-linear least squares procedure, revealing that a resistor in parallel with a constant phase element is the best equivalent circuit. The dielectric constant value, measured at 1 kHz and room temperature, decreases from 102 to 62 with the increasing of the HT temperature, suggesting a random orientation of the dipolar moments associated with the LiNbO3 nanocrystals. � 2003 Elsevier B.V. All rights reserved.

Raman and infrared spectra of KNbO3 in niobate glass-ceramics

Potassium niobophosphate glasses and glass-ceramics of the family [xNb2O5.(50-x) P2O5.50K2O]:yFe2O3 were studied by x-ray powder diffraction, infrared and Raman scattering spectroscopy. For the potassium-phosphate samples [50P2O5-50K2O] the iron oxide presents a network former behaviour, for 2 mol% of Fe2O3 doping. The precipitation of crystalline K4Nb6O17 and ferroelectric KNbO3 crystals was confirmed by x-ray powder diffraction in the samples with x = 40 and 50 mol% respectively. The infrared and Raman scattering spectroscopy results suggest that the increase of the ratio Nb2O5/P2O5 leads the niobium to sites of octahedral symmetry and consequently to the formation of ferroelectric KNbO3, as seen by x-ray diffraction analysis.

Enhanced dielectric response of GeO2-doped CaCu3Ti4O12 ceramics

CaCu3Ti4O12 ceramic samples were prepared by solid state conventional route using stoichiometric amounts of CuO, TiO2, and CaCO3. Afterward the material was doped with GeO2 with concentrations up to 6% by weight and sintered at 1050 °C for 12 h. The influence of doping on the microstructure, vibrational modes, and dielectric properties of the material was investigated by x-ray diffraction, scanning electron microscopy coupled with an energy dispersive spectrometer, and infrared and dielectric measurements between 100 Hz and 30 MHz. The materials presented huge dielectric response, which increases with doping level relative to undoped CaCu3Ti4O12. The main effect of doping on the microstructure is the segregation of Cu-rich phase in the ceramic grain boundaries. Cole–Cole modeling correlates well the effects of this segregation with the relaxation parameters obtained. The intrinsic phonon contributions for the dielectric response were obtained and discussed together with the structural evolution of the system.

Structural studies of lithium triborate (LBO–LiB3O5) in borophosphate glass-ceramics

Abstract In this paper lithium borophosphate glasses and glass-ceramics in the system 0.75| x B 2 O 3 .(100− x )P 2 O 5 |.25Li 2 O. y Fe 2 O 3 with 0≤ x ≤100 mol.% and y =4 mol.%, were studied by X-ray powder diffraction, Mossbauer and infrared spectroscopy. All the samples in the system present a glass or glass-ceramic behavior which was confirmed by X-ray diffraction. From our Mossbauer analysis, high spin Fe 2+ and Fe 3+ in a distorted octahedral coordination are present in all samples. One can conclude that both iron ions are at sites of distorted octahedral coordination, acting as network modifiers (NWM) in a very broad site distribution, which is characteristic of an amorphous structural neighborhood. Such glasses and glass-ceramics containing nonlinear optical materials formed in a controlled crystallization process would be interesting candidates for applications in new nonlinear optical devices.

Impedance and Modulus Spectroscopy Characterization of Tb modified Bi0.8A0.1Pb0.1Fe0.9Ti0.1O3 Ceramics

In this paper we present the impedance spectroscopy of ternary solid solutions of BiFeO3, TbFeO3 and PbTiO3, prepared by solid-state reaction method. The preliminary structural studies were carried out by x-ray diffraction technique, showing the formation of polycrystalline sample with ABO3 type of perovskite structure with hexagonal symmetry for Bi0.8Tb0.1Pb0.1Fe0.9Ti0.1O3system at room temperature. Dielectric and impedance study of this ceramic has been characterized in the temperature range 175 - 325 0C and frequency range 100 Hz - 1 MHz. The maximum ferroelectric transition temperature (Tc) of this system was in the range 210 - 225 0C with the dielectric constant having maximum value ~2480 at 1 kHz. The complex impedance graph exhibited one impedance semicircle arc at all reported temperatures, which indicates that the impedance response is a Cole-Cole type relaxation. Single semicircle indicate that the grain effect of the bulk in ceramic. The bulk resistance of the material decreases with increasing temperature showing negative temperature showing a typical semiconducting property, i.e. negative temperature coefficient of resistance (NTCR) behavior.

Effect of tetramethylammonium hydroxide on nucleation, surface modification and growth of magnetic nanoparticles

Nanoparticles of magnetite (Fe3O4) were obtained by reacting ferric chloride with sodium sulphite, through the reduction-precipitation method. The effects of adding tetramethylammonium hydroxide (TMAOH) during or after the precipitation of the iron oxide were studied in an attempt to obtain well-dispersed magnetite nanoparticles. Accordingly, the following experimental conditions were tested: (i) precipitation in absence of TMAOH (sample Mt), (ii) the same as (i) after peptizing with TMAOH (Mt1), (iii) TMAOH added to the reaction mixture during the precipitation of magnetite (Mt2). Analyses with transmission electron microscopy (TEM), X-ray diffraction, Mossbauer spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), zeta potential, and magnetization measurements up to 2.5 T revealed that magnetite was normally formed also in the medium containing TMAOH. The degree of particles agglomeration was monitored with laser diffraction and technique and inspection of TEM images. The relative contributions of Neel and Brownian relaxations on the magnetic heat dissipation were studied by investigating the ability of suspensions of these magnetite nanoparticles to release heat in aqueous and in hydrogel media. Based on ATR-FTIR and zeta potential data, it is suggested that the surfaces of the synthesized magnetite particles treated with TMAOH become coated with (CH3)4N+ cations.

The iron behaviour in aluminosilicate gel-derived materials

Electron spin resonance (ESR) measurements have been carried out on a 10Fe2O3 · 10Al2O3 · 80SiO2 gel heat-treated at different temperatures in air and under reducing conditions. ESR spectra were obtained at 300, 50 and 5 K. The “effective” g value (g = hν/βH), linewidth (ΔHpp) and ESR amplitude (A) depend on heat-treatment temperature of the gel-derived samples. ESR spectra exhibit different magnetic characteristics as a function of heat-treatment temperature and atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM) and a.c. susceptibility (χa.c.) analyses were used to better understand the ESR results. The results show that in the samples heat-treated in air, up to 700°C, Fe3+ ions are incorporated in the glass network in tetrahedral and/or octahedral co-ordinations. In the samples heat-treated between 250 and 700°C was not detected, by ESR, the presence of iron oxide aggregates. However, the formation of hematite particles was observed by XRD and SEM. The presence of iron oxide aggregates was detected (by ESR) in the samples heat-treated at temperatures higher than 700°C. These aggregates are formed, at 1200 and 1300°C, by hematite and magnetite particles as proved by XRD. The ESR spectra and a.c. susceptibility, of the samples heat-treated at 250°C (under reducing conditions), show a behaviour characteristic of small magnetite particles presence. The sample heat-treated at 500°C (under reducing conditios) contains magnetite particles (XRD). In the ESR spectra of the sample heat-treated at 1000°C, under reducing conditions, was not detected any resonance signal.

Effect of Fe-doping on the structure and magnetoelectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 synthesized by a chemical route

B-site Fe-doped (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 was synthesized by a facile chemical route to study the effect of doping on its physical properties. Detailed analysis of X-ray diffraction and Raman spectroscopy data revealed an increased lattice strain and thereby deviation from the morphotropic phase boundary with the progressive doping of Fe from 1 to 5 mol%. Such structural changes have resulted in the weakening of the energy band gap as well as deterioration of the ferroelectric polar nature which was evidenced by a shift of tetragonal to cubic transitions towards room temperature and hard doping effects in ferroelectric hysteresis. The doped samples exhibited room temperature ferromagnetism. Combined Mossbauer and X-ray photoelectron spectroscopic studies suggest that oxygen vacancies and defect complexes induced by Fe doping play a major role in magnetic properties. Local piezoresponse measurements illustrated imprint characteristics of ferroelectric domains in undoped and doped samples at the nanoscale. Room temperature magnetoelectric (ME) measurements revealed that 1 mol% Fe doped sample, having higher ferroelectric polarization and moderate magnetization, exhibits a strong ME response with a coefficient of 12.8 mV cm−1 Oe−1. The present study on Fe-doping effects on the structure and related ME properties of this important lead-free material is useful to tailor multiferroic applications in electronics.

Magnetoelectric studies on CoFe2O4/0.5(BaTi0.8Zr0.2O3)-0.5(Ba0.7Ca0.3TiO3) lead-free bilayer thin films derived by the chemical solution deposition

Lead-free multiferroic bilayer thin films were fabricated on (111)Pt/Si substrate via a simple sol-gel chemical solution deposition, by altering the position of piezoelectric (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 (BCTZO) and ferromagnetic CoFe2O4 (CFO). Single layer BCTZO experiences the out-of-plane compressive stress, while this layer is under tensile strain in both the bilayers. The microstructural study confirms the formation of bilayers with expected chemical composition composed of multiple well-developed crystallites having no crystallographic dependencies. Thin films of BCTZO and CFO/BCTZO exhibited saturated ferroelectric hysteresis loops at room temperature with a Pr of 7.2 and 5.6 μC/cm2. The magnetic field induced shift in phonon vibrations coupled with direct magnetoelectric (ME) measurements demonstrated a stress-mediated coupling mechanism in the bilayers. We found a superior ME coefficient (105 MV/cm Oe) and dielectric tunability (∼52%) for CFO/BCTZO bilayer compared to the BCTZO/CFO bilayer, which demonstrates that the modification of strain state in bilayers is useful for the desired ME coupling. The BCTZO having piezoelectricity on par with that of lead-based ones can be useful to tailor lead-free ME applications.

Magnetization behaviour of Gd lead borate and Fe lead borate glasses under high magnetic field (20 T)

Abstract Magnetization of Gd and Fe lead borate glasses of varying composition was measured in magnetic fields up to 20 T and temperatures up to 7 K. A convenient variable to represent the data is η = gμ B H / k B T and the data extend up to η 4. Low concentration data fit the Brillouin function, B J (η), and the value of effective magnetic moment per ion, P eff , thus deduced agrees well with that determined from the susceptibility measurement. For higher concentrations, where the magnetic interaction between ions had been deduced from susceptibility measurements, it is discussed how the data can be used, qualitatively at the moment, to infer the existence of magnetic clusters in the case of short ranged interaction of relatively closed Gd ions. In the case of Fe ions where it is believed that the interaction is stronger and is relatively long ranged, it is shown that M vs. H data at different temperatures do not collapse to single curve in M vs. η representation.