En este trabajo se discute cómo influye la temperatura en la obtención de polvos cerámicos monofásicos y de tamaño nanométrico con composición K0,5Na0,5NbO3 (KNN), sintetizados por el método de combustión. Fueron usados como fuentes de cationes: K2CO3, Na2CO3, NH4[NbO(C2O4)2(H2O)] . H2O y una mezcla de úrea/glicina en relación 50/50 como combustible. La solución que contiene precursores y combustible fue colocada en un horno precalentado, donde se realizó la combustión; en este proceso fue analizado el efecto de cuatro temperaturas diferentes (500, 600, 700 y 800ºC). Las fases presentes en los polvos cerámicos, su microestructura y morfología, fueron determinadas por difracción de rayos-X (DRX), espectroscopia infrarroja con transformada de Fourier (FTIR) y microscopia electrónica de barrido (MEB). Los resultados experimentales mostraron que la fase perovskita pura fue formada a temperaturas tan bajas como 500ºC. El polvo obtenido posee simetría monoclínica y tamaño medio de partícula de ~ 27 nm. Se concluye que la homogeneidad de la solución precursora y una baja temperatura de procesamiento, juegan un papel clave en la formación de nanopartículas de KNN.

This paper discusses how temperature influences the obtention of single-phase K0.5Na0.5NbO3 (KNN) nanometric ceramic powders, synthesized by the combustion method. K2CO3, Na2CO3, and NH4[NbO(C2O4)2(H2O)] . H2O were used as cation sources and a mixture of urea / glycine in relation 50/50 was used as fuel. The solution containing precursors and fuel was placed in a preheated muffle furnace, where the combustion took place. In this process, the effect of four different temperatures (500, 600, 700 and 800°C) was analyzed. The phases present in the ceramic powders, the microstructure and the morphology were determined by X-ray diffraction (XRD), Fourier transform infrared (FTIR) Spectroscopy, and scanning electron microscopy (SEM). Experimental results show that pure perovskite phase was formed at temperatures as low as 500°C. Furthermore, the obtained powder has monoclinic symmetry and average particle size of ~ 27 nm. We conclude that the homogeneity of the precursor solution and a low processing temperature play a key role in the formation of nano-particles of KNN.

This study evaluates the structural, microstructural, electric and magnetic properties of nickel ferrite samples prepared through the solid state reaction. It was observed that an increase in the sintering temperature produces a higher cation concentration in the A site when compared to the B site. The assessment of magnetic properties showed that an increase in grain size leads to a decrease in the coercive fields verging on superparamagnetic values, while the saturation magnetization increases up to 46.5 Am².kg-1 for samples sintered at 1200 ºC. The dc electric resistivity behavior of samples was attributed to the increase in the cross-sectional area of grains as well as the different oxidation states and distribution of cations amongst the lattice sites of the spinel structure.

Multiferroic Pb(Fe2/3W1/3)O3-PbTiO3 (PFW-PT) ceramics were synthesized via a modified two-stage solid-state reaction. This method utilized Fe2WO6, prepared at a first-stage, which was subsequently reacted with a stoichiometric amount of PbO and TiO2 at the second stage. This procedure efficiently suppressed the formation of lead tungstates and leads to getting dense ceramics. Electric and dielectric properties of (1-x)Pb(Fe2/3W1/3)O3 -xPbTiO3 solid solutions were investigated as a function of the frequency and temperature. All samples present density higher than 96% of the theoretical one, low percentage of pyrochlore phase (<2%) and relatively high electrical resistivity (<10(10) Ω.m). RF dielectric measurements over the temperature showed the presence of a peak, which is related to the ferro-paraelectric phase transition and conductive contributions (in the range 200-700 K), associated to a electronic hoping mechanism. The dielectric properties of the PFW shows a typical relaxor behaviour for x = 0 and 0.10 with frequency-dependent peak temperature (Tm), while the samples with higher PT-content undergo a 'normal' para-ferroelectric transition at the Curie temperature T C.The FE phase transition of PFW is shifted to higher temperatures by PbTiO3 (PT) additions and high resistivity are obtained for PT containing samples. Different types of magnetic activity dependent on composition and temperature were found.

PbTiO3 ceramic doped with lanthanum is a promising material for piezoelectric and pyroelectric applications, however the synthesis process in order to study optical properties has not been studied in great detail. Ferroelectric ceramics (Pb1-xLax)TiO3 with 0.18 ≤ x ≤ 0.23 were fabricated by the conventional mixed-oxide and densified by hot pressing at a temperature of 1220oC for 10 h. To improve optical properties subsequent heat treatments in oxygen atmosphere were required. High-density, homogeneous microstructure and good optical characteristics were obtained. In the study of synthesis and processing of different concentrations, it was observed that transmittance of most of the ceramics were acceptable as compared with other transparent ferroelectric systems.

PbTiO3 dopado com lantânio é um material cerâmico promissor para aplicações piezoelétricas e piroelétricas, mas o processo de síntese com vistas ao estudo das propriedades ópticas, não tem sido estudado com muito detalhe. Cerâmicas ferroelétricas (Pb1-xLax)TiO3 com 0,18 ≤ x ≤ 0,23 foram fabricadas pelo método convencional de mistura de óxidos e densificadas por prensagem a quente a uma temperatura de 1220oC por 10 h. Para melhorar as propriedades ópticas foram necessários tratamentos térmicos posteriores em atmosfera de oxigênio. Alcançou-se alta densidade, microestrutura homogênea e boas características ópticas. No estudo da síntese e processamento das diferentes concentrações, observou-se que a maioria das cerâmicas apresenta transmitância aceitável enquanto comparada com outros sistemas ferroelétricos transparentes

In this work, the thermal stability of piezoelectric constants of PMN-PT ceramics in the tetragonal and rhombohedral phases were investigated in a wide range of temperatures. The results showed that the tetragonal PMN-PT presented higher thermal stability and, consequently, the temperature coefficients for the piezoelectric constants were approximately zero. This result revealed to be much better than that commonly found for PZT ceramics. Although the rhombohedral PMN-PT presented a slight lower thermal stability, the values found for the coupling factor were significantly higher than the tetragonal composition.

The texturing process and the characterization of textured Sr xBa1-xNb2O6 (x = 0.5; x = 0.6) ferroelectric ceramics were investigated. The ceramic bodies were fabricated through two stage sintering. The first and second stage consisted in conventional firing and hot forging of the ceramic bodies, respectively, both at oxygen atmosphere. The forged ceramics achieved high densification level although showing abnormal grain growth. X-ray diffraction, dielectric and pyroelectric analysis showed that is possible to obtain preferential orientation of the [00l] crystallographic planes due to grain arrangement during hot forging. Consequently, the investigated properties for this textured material were strongly dependent on the measurement direction. On the other hand, high room temperature pyroelectric coefficients, of the same order of that obtained for single crystals of same composition, were observed.