M. S. Castro

Electrical properties and thermal expansion of cordierite and cordierite-mullite materials

Abstract Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. The employed cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller size component. Mullite powder employed in composites was a 7 h-attrition milled one. The dielectric constant (e), dielectric loss tangent (tan δ), resistivity (ρ) and thermal expansion coefficient (α) were measured. The influence of the porosity, mullite and glassy phase contents and grain size in the electrical parameters was analyzed. The thermal expansion coefficient as a function of the composition was studied.

Characterization of SnO2-varistors with different additives

Abstract Some aspects of the effects of additives on the sintering process and their influence on the microstructure and electrical properties of SnO 2 varistors are still not well understood. Aiming at a better interpretation of the involved phenomena, the effects on microstructures and electrical properties of adding Co 3 O 4 , CuO, MnO 2 , Bi 2 O 3 and Sb 2 O 3 to SnO 2 have been studied. We found that Co 3 O 4 and MnO 2 enhance the densification of these ceramics by increasing the number of oxygen vacancies and, as a consequence, the sintering rate increases. Sb 2 O 3 , on the other hand, reduces the oxygen vacancy concentration and then the sintering rate is decreased. CuO and Bi 2 O 3 form a liquid phase during the sintering process enhancing the sintering rate. We found that electrical properties are improved by the addition of all the studied dopants, due to the modifications in the microstructure and in the defect concentration.

Defect profile and microstructural development in SnO2-based varistors

Abstract The microstructural development and the stabilised valence of the ions added to SnO 2 were analysed. Aiming at a better interpretation of the involved phenomena, the effects on grain growth, secondary phase formation and structure of adding Co 3 O 4 , ZnO, MnO 2 , Sb 2 O 3 , or Nb 2 O 5 have been studied. We found that cobalt, zinc and manganese stabilise as Co +2 , Zn +2 and Mn +2 in the lattice, favouring the oxygen vacancy apparition and then, the grain growth and the potential barrier formation. Sb 2 O 3 or Nb 2 O 5 reduces the total oxygen vacancy concentration and the grain growth. Sb 2 O 3 addition favours the CoSnO 3 particle formation and Nb 2 O 5 favours the formation of particles with an intermediate composition between CoSnO 3 and Co 2 SnO 4 in systems with Co 3 O 4 . These particles could also control the sintering and grain growth rates.

Synthesis of barium titanate improved by modifications in the kinetics of the solid state reaction

Abstract In this work, the synthesis of BaTiO 3 powder from BaCO 3 –TiO 2 reaction at high temperatures is studied. In order to improve the kinetics of this reaction, successive modifications on the powder processing have been implemented. The contribution of the mechanochemical activation to the BaTiO 3 formation is analysed. It was found that the use of a mechanochemical activation favours the decomposition of BaCO 3 at low temperatures and improves the barium ion diffusion through the BaTiO 3 layer. In consequence, a BaTiO 3 product free of secondary phases can be obtained at lower temperatures.

Secondary phases in Nb-doped BaTiO3 ceramics

Univ Mar del Plata, CONICET, INTEMA, Inst Invest Ciencia & Tecnol Mat, RA-7600 Mar Del Plata, Argentina

Conduction mechanism of barium titanate ceramics

Barium titanate ceramics are semiconductor devices presenting a highly non-linear resistivity vs. temperature characteristics (PTCR effect). Although it has been previously determined that this phenomenon can be described by a simple double depletion layer formed at the grain interfaces, details of some effects in these materials are not completely explained. In this work, the electrical properties of barium titanate ceramics above the Curie temperature are studied from the resistivity vs. temperature and dielectric constant vs. temperature at the grain boundaries. Experiments can be explained as the result of double Schottky barriers formed at the grain boundaries. These barriers are considered to have an exponential concentration of electrically donor active dopants.

Analysis of the microstructural evolution in hydroxyapatite ceramics by electrical characterisation

Abstract The aim of this work is to determine the effect of lithium or magnesium addition on the microstructure of hydroxyapatite ceramics sintered at 1200°C. The samples were characterised by density measurements, scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness. Dielectric properties, dc-conductivity and microhardness were related to the microstructures developed by the sintered materials. Furthermore, significant effects due to the presence of Li+ or Mg+2 in these ceramics were established.

Influence of particle size on the conductance of SnO2 thick films

Abstract The electrical response of SnO2 thick films was found to be highly dependent on the grain size. This result can be explained as the consequence of the type of intergranular potential barriers developed at intergrains. Specifically, samples with smaller particle size have overlapped potential barriers and samples with higher particle size have separated potential barriers. Also, the influence of gas transport phenomena on the resistance of the gas sensor was investigated. From an analysis of the possible mechanisms involved in the oxygen diffusion, the Knudsen diffusion is suggested as the controlling step.

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.