M.P.F. Graça

Electric modulus-based analysis of the dielectric relaxation in carbon black loaded polymer composites

A thorough investigation of the ac electrical properties of carbon black (CB) mixed into ethylene butylacrylate copolymer has been conducted using ac impedance spectroscopy in the frequency range 10 Hz–100 kHz and over the temperature range of 150 (Tg−48 K)–319 (Tg+121 K) K. For this investigation, a series of eight samples were prepared with various filler contents above the percolation threshold. Using the electric modulus formalism it has been found that the Cole–Cole equation of dielectric relaxation expressed in the electric modulus form is capable of quantitatively describing the experimental data from which we extract the relaxation time and a parameter α which gauges the broadening of the loss spectrum. The small values of α ranging from 0.06 to 0.10 suggest a behavior close to the state of a single relaxation time. Furthermore, the relaxation time as a function of temperature is characterized by an Arrhenius behavior. While the effective activation energy is on the order of 60 meV and is insensit...

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.

Temperature-, power-, and concentration-dependent two and three photon upconversion in Er3+/Yb3+ co-doped lanthanum ortho-niobate phosphors

Lanthanum ortho-niobate matrices, LaNbO4 (LNO), doped with rare-earth ions (Er3+ and Yb3+), were prepared by a high-temperature solid-state reaction method. The LNO matrices thus prepared were confirmed by X-ray diffraction followed by Rietveld refinement. Raman spectroscopy was used to analyze the vibrational energy of the undoped and rare-earth doped LNO ceramics. The upconversion fluorescence was studied in the co-doped samples using an NIR laser (980 nm) at low and high pump power as well as the effect of sample temperature. The effect of the laser power and concentration of Yb3+ ions on the fluorescence properties were elucidated. The upconversion obtained was found to be due to two as well as three photon processes. Temperature-dependent upconversion measurements were performed to elucidate the nature of the emission processes, and it was concluded that two and three photon upconversion processes have different behaviors.

Optical and dielectric behaviour of EuNbO4 crystals

In this work, EuNbO4 samples were prepared, in the shape of fibres, by the laser floating zone (LFZ) technique. The fibres grown at pulling rates below 20 mm h−1 are transparent and monophasic, crystallizing in a monoclinic structure. At higher growth rates, non-stoichiometric europium orthoniobate phases were promoted. The physical properties analysed in this work were the electric, dielectric and photoluminescence (PL) properties. For the fibre grown at 2.5 mm h−1, the dc conductivity shows a value of 5.97 × 10−12 S m−1 and the ac conductivity, at 1 MHz and 300 K, is 3.08 × 10−4 S m−1. Under the same experimental conditions the dielectric constant is ∼39 and the loss tangent ∼0.14, also presenting a relaxation phenomenon centred at ∼120 kHz. An ionic polarization type behaviour was identified from the dielectric measurements, which justifies the temperature independent behaviour of the dielectric constant. This type of polarization was assigned to the NbO4 units. PL measurements with ultraviolet excitation reveal the intra-4f6 transitions of Eu3+ ions in the fibres, exhibiting narrow intraionic lines as expected for single crystal materials. From the PL spectra at low temperature, it was possible to identify multiple europium related centres, from different local site symmetry and/or environment. The luminescence of Eu3+ was found to be preferentially excited via a broad excitation band peaked at ∼280 nm.

Electrical conductivity of multiwalled carbon nanotubes/polyester polymer nanocomposites

Electrical conductivity of polyester filled with carbon nanotubes composites have been studied from 240 to 380 K in the frequency range from 100 Hz to 1 MHz as a function of the filler volume fraction above the percolation threshold. The frequency dependence of the electrical conductivity obeys the universal dynamic response. Positive temperature coefficient in resistivity and negative temperature coefficient in resistivity phenomena were observed at temperatures below and above the glass transition respectively. It was found that the mechanism responsible for the changes in resistivity is predominantly due to the tunneling effect. Positive temperature coefficient in resistivity intensity was also exploited which is strongly dependent on the carbon nanotube content.

Dielectric behaviour of carbon nanotubes particles-filled polyester polymer composites

This paper reports the dielectric relaxation studies of carbon nanotubes loaded in polyester polymer matrix. The study was carried out in the frequency range between 100 Hz and 1 MHz at constant temperature, T = 300 K. The frequency dependence of the electrical data was treated in the frameworks of the impedance Havriliak-Negami formalism and by using the universal Jonscher power law. The imaginary and real parts of the dielectric permittivity change with concentration of the carbon nanotubes. This work consists in studying the influence of these nanoparticles on the dielectric properties, describing the electrical relaxation and the conduction mechanisms.

Impedance Spectroscopy of Nanofluids based on Multiwall Carbon Nanotubes

This work is a contribution to the understanding of the dielectric properties of nanofluids prepared by dispersing multiwalled carbon nanotubes in transformer oil. The dielectric measurements were carried out in the frequency range 100 Hz–1 MHz at constant temperature, T = 300 K, for several volume fractions of the nanotubes in the base fluid. A relaxation phenomenon was induced in the nanofluid comparing to the base fluid. In addition, both the real and imaginary part of the dielectric permittivity changed with volume fraction of the nanotubes. These results suggest that the presence of the nanotubes greatly affect the dielectric properties of the oil as a result of polarization phenomenon induced by these nanoparticles. It was found that the measured effective dielectric permittivity follows the empirical Havriliak–Negami model. Nevertheless to take into account the electrode polarization effects, we rewrote this model, with a new term, which fits accurately the experimental data.

Study of structural, electrical, and dielectric properties of phosphate-borate glasses and glass-ceramics

In this work, phosphate-borate based glasses with molar composition 20.7P2O5–17.2Nb2O5–13.8WO3–34.5A2O–13.8B2O3, where A = Li, Na, and K, were prepared by the melt quenching technique. The as-prepared glasses were heat-treated in air at 800 °C for 4 h, which led to the formation of glass-ceramics. These high chemical and thermal stability glasses are good candidates for several applications such as fast ionic conductors, semiconductors, photonic materials, electrolytes, hermetic seals, rare-earth ion host solid lasers, and biomedical materials. The present work endorses the analysis of the electrical conductivity of the as-grown samples, and also the electrical, dielectric, and structural changes established by the heat-treatment process. The structure of the samples was analyzed using X-Ray powder Diffraction (XRD), Raman spectroscopy, and density measurements. Both XRD and Raman analysis confirmed crystals formation through the heat-treatment process. The electrical ac and dc conductivities, σac and σdc...

Highly efficient upconversion of Er 3+ in Yb 3+ codoped non-cytotoxic strontium lanthanum aluminate phosphor for low temperature sensors

Er3+ and Er3+/Yb3+ melilite-based SrLaAl3O7 (SLA) phosphors were synthesized by a facile Pechine method. The differences in emission intensities of 4I13/2 → 4I15/2 transition in NIR region when excited with Ar+ and 980 nm lasers were explained in terms of energy transfer mechanisms. Temperature and power dependence of upconversion bands in the visible region centered at 528, 548 and 660 nm pertaining to 2H11/2, 4S3/2 and 4F9/2 → 4I15/2 transitions were investigated. Fluorescence intensity ratio (FIR) technique was used to explore temperature sensing behaviour of the thermally coupled levels 2H11/2/4S3/2 of Er3+ ions in the phosphors within the temperature range 14–300 K and the results were extrapolated up to 600 K. Anomalous intensity trend observed in Er3+ doped SLA phosphor was discussed using energy level structure. Cytotoxicity of phosphors has been evaluated using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay in Bluegill sunfish cells (BF-2). The non-cytotoxic nature and high sensitivity of the present phosphors pay a way for their use in vitro studies and provide potential interest as a thermo graphic phosphor at the contact of biological products.