Leandro Alfredo Ramajo

Dielectric behaviour of CaCu3Ti4O12-epoxy composites

The dielectric behavior of composite materials (epoxy resin - barium titanate and epoxy - CCTO) was analysed as a function of ceramic amount. Composites were prepared by mixing the components and pouring them into suitable moulds. In some compositions, the matrix was reduced by tetrahydrofuran (THF) incorporation. Samples containing various amounts of ceramic filler were examined by TG/DTA and scanning electron microscopy analysis. Dielectric measurements were performed from 20 Hz to 1 MHz and 30 to 120 °C. It was demonstrated that the epoxy - CCTO composites possessed higher permittivity than classic epoxy - BaTiO3 composites. However, the low resin permittivity prevailed in the composite dielectric performance.

Revealing the role of cationic displacement in potassium–sodium niobate lead-free piezoceramics by adding W6+ ions

The effect of the structural modifications induced by the replacement of the B sites with W6+ ions in (K0.44Na0.52Li0.04)[(Nb0.86Ta0.10Sb0.04)1−xW5x/6]O3 lead free ceramics is investigated. Here we show the coexistence between a tetragonal symmetry (T) and an orthorhombic symmetry (O) in the perovskite structure, which is tuned by varying the doping level. In addition, this polymorphic behaviour is accompanied by the appearance of a secondary phase, which can be detected through XRD and Raman results. A correlation between the presence of both structural features and the W6+ content has been evaluated, and therefore this lead free system reveals a transition from a normal ferroelectric to a ‘relaxor-like’ ferroelectric due to the cation disorder in the perovskite-structure in doped samples. A large diffusivity value (γ) has been attained when the x value reached 0.05 owing to the involved O–T polymorphic phase, as well as due to the appearance of a secondary phase. The experimental proof makes clear that the role of the secondary phase is to capture the alkali ions of the (KNa)NbO3-based system, provoking a cation disorder in the perovskite-structure matrix. The significance of this work lies in evaluating whether such a material can benefit the understanding of (KNa)NbO3-based ceramics and the expansion of their application range.

Influence of MoO3 on electrical and microstructural properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3

Microstructure, piezoelectric properties and dielectric behavior of undoped and MoO3-doped lead-free (K0.44Na0.52Li0.04)(Nb0.86−xTa0.10−xSb0.04−x)Mo5/6xO3 were investigated. Samples were obtained by the conventional solid state reaction. X-ray diffraction (XRD) and Raman spectra revealed the formation of a perovskite phase, together with some minor secondary phase which could be assigned by XRD to K3LiNb6O17. XRD also showed that the perovskite evolved from a tetragonal to orthorhombic symmetry with the increased doping level. The temperature dependence on real permittivity revealed the presence of a diffuse phase transition. This behavior raised with MoO3 concentration. Also, dielectric relaxation processes were observed at room temperature resulting dependent on doping level and sintering time.

Evaluation and modelling of integral capacitors produced by interdigitated comb electrodes

Integral capacitors (IC) of one or two-layer printed wiring board (PWB) circuits were produced using comb electrodes fixtures and dielectric composites as the inter-electrode material. ICs were fabricated at laboratory scale, using copper comb electrodes and BaTiO3-epoxy composite materials deposited on a glass-Epoxy FR4 board. They were experimentally tested in order to obtain their electrical response. Furthermore, ICs behaviour was modelled through 2-dimensional models applying finite element method (FEM). Results showed that by this laboratory technique it was possible to obtained integral capacitors with low dielectric losses. Moreover, acceptable agreement was found between numerical and experimental capacitance results for all the different analysed ICs. In conclusion, 2D FEM models are a suitable tool to predict electric response of IC devices.

Evaluation of the performance of a lead-free piezoelectric material for energy harvesting

Vibration-based energy harvesting has been explored as an auxiliary power source, which can provide small amounts of energy to power remote sensors installed in inaccessible locations. This paper presents an experimental and analytical study of an energy harvesting device using a lead-free piezoelectric material based on -doped ()() KNL-(NTS)Mo. The harvesting model corresponds to a cantilever beam with a KNL-(NTS)Mo piezoelectric disc attached to it. We analyze the effect of electromechanical coupling and load resistance on the generated electrical power. Electromechanical frequency response functions that relate the voltage output to the translational base acceleration are shown for experimental and analytical results.

Effect of ZnO addition on the structure, microstructure and dielectric and piezoelectric properties of K0.5Na0.5NbO3 ceramics

Microstructure, structure and electrical and dielectric properties of ZnO-doped K0.5Na0.5NbO3 (KNN) ceramics were investigated. Powders were obtained by the conventional solid-state method. Samples doped with 0 to 1 mol% of ZnO were sintered at 1125 °C for 2 h. Through XRD spectra, the perovskite structure was detected, in addition to small peaks corresponding to secondary phases. It was also observed that zinc changed the microstructure and grain size of KNN ceramics. The addition of 0.5 mol% of Zn2+ produced a softening effect in the ferroelectric properties of the material, and increased its final density. Conversely, the addition of 1 mol% of Zn2+ reduced the sample densification.

BaTiO3-Filled Epoxy Resin Composites: Numerical Modelling of Dielectric Constants

The influence of BaTiO3 content on the composite dielectric properties was investigated by finite element method (FEM). This method is attractive because of its technological applications such as those in electromagnetic shields, since it allows to determine the dielectric constant of heterogeneous mediums and show or display the electric field into the material in order to understand the interfacial behaviours. The dielectric response of an anisotropic and periodic heterostructure, was evaluated and quantified by means of a quasistatic approximation using Laplaces equation which was solved by 3D FEM. The numerical calculations were confronted with experimental data. Measurements of the effective permittivity were carried out in samples prepared by dipping technique.

Mechanical properties enhancement in potassium-sodium niobate lead-free piezoceramics: the impact of chemical modifications

The development of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-based (KNL-NTS) ceramics is attracting great interest because of environmental concerns. Most of the efforts have been concentrated on improving the electrical properties. However, it is desirable to develop ceramics with good electrical response and high mechanical properties. Therefore, the evaluation of the mechanical properties is substantial and little studied. The aim of this work is to study the mechanical properties of KNL-NTS lead free ceramics doped with Zr evaluating the effects that can give rise to improved mechanical properties. Our results show that the crystalline phase, size and shape of grains in the ceramics are affected by the doping amount. We found that the microstructure does have a close relationship to the mechanical response, more specifically; a uniform and fine grain microstructure is suitable to lead high mechanical strength. Based on these results, the chemical modifications with Zr4+ ions have a significantly positive influence on hardness, strength and fracture toughness values of KNL-NTS ceramics.

Métodos de medición de espesores de películas delgadas basadas en óxidos semiconductores

Transparent films based on Ti, Sn and Zn oxides are of great importance in electronic devices such as sensors, solar cells and conductive films, then the characterization techniques are highly relevant. The aim of this work is to identify the advantages and disadvantages of direct methods, such as profilometry, and indirect methods such as ellipsometry and spectrophotometry used to quantify film thickness. In this work, films were deposited by spray-pyrolysis on glass substrates at 425±C. Thicknesses varied between 150 and 300 nm. Thicknesses calculated by means of spectrophotometry and ellipsometry, led to differences below 10% and 20 %, respectively, with respect to the value measured by profilometry.

Improvement on dielectric and microstructural properties of lead free Bi0.5Na0.5TiO3 ceramics through processing conditions

In this work, we report the influence of processing conditions on Bi0.5Na0.5TiO3-based lead–free piezoceramics properties, obtained by conventional solid-state reaction method, employing a mechanochemical activation step and comparing two types of calcination steps. All samples were characterized by X-ray diffraction, Scanning Electron Microscopy and impedance Spectroscopy. It was observed that depending on the calcination route a secondary phase is formed. Moreover, the content of the secondary phase is strongly related to the intermediate grinding in the calcined process, and has a significant effect on the final ferroelectric properties.