C.S. Furtado

Studies on densification ofα-Fe2O3 ceramics

The densification of ceramics of α-Fe2O3 depends on the processing parameters. The separate influences of milling, sieving, isostatic pressure and sintering atmosphere were investigated. The maximum density, with a value around 96%, was obtained in a sintering atmosphere of nitrogen.

Na-β Alumina powder processing by a Na2CO3 precipitation method

A Na2CO3 precipitation method to prepare powder of Na-β-Alumina is presented. Comparisons have been made with powder of the same composition obtained by two other methods: the traditional one and a slurry-solution method. After phase characterisation by X-ray, the powders were calcined at different temperatures. The resulting products were characterised by TGA, DSC and XRD analysis. Finally, density and ionic conductivity of sintered pellets were measured.

α-Fe2O3 ceramics as negative temperature coefficient thermistors

Abstract The electrical behaviour of negative temperature coefficient (NTC) thermistors can be expressed by the relationship ϱ=ϱ 25 exp [β( 1 T − 1 298 )] . Good thermistors are thus obtained through the adequate control of the parameters β and ϱ25, which has been achieved in α-Fe2O3 ceramics by a convenient processing. Reducing in nitrogen and then reoxidizing in air has lead to thermistors with good values for β and reasonable ones for ϱ25. The samples consisted then of two phases of polycrystalline ceramics, one of hematite the other of magnetite.

Investigations on Humidity Sensing Properties of Thick Films of the TiO2:WO3 System

This paper reports the study of the humidity-sensitive electrical properties of TiO2:WO3 thick films. Prototype sensors have been prepared by depositing an emulsion of TiO2 and WO3 powders in acetone with cellulous glue onto an alumina substrate, by a spin coating technique using a low spreading speed (125 r.p.m.). Films were prepared with five different atoms proportions of Ti and W: 100:0, 100:1, 100:6, 100:18 and 100:36. The oxides thick films were successively fired at the temperatures of 600, 700, 900, 1100, and 1300 °C, during 2 hours at each of these temperatures. Measurements were obtained at a constant temperature of 25 °C and at various relative humidities (RHs) in the frequency range of 0.01 Hz – 40 MHz. It was thought that the humidity sensing of the TiO2 matrix would be influenced by the substitution of the Ti4+ ions for higher valence ions, W6+ in the present study. In the measuring frequency range, samples impedance varies of three-four orders of magnitude over the RH range of 10-100 %. It is found to be the sensitivity highly dependent on both the measuring frequency and the sintering temperature. A parameter called characteristic humidity is defined to represent the sensitive response of sensors. This parameter is used to evaluate the influence of the W6+ ions on the sensitivity behaviour of the sensors.

An Approach to the Magnetic Disaccommodation in Nd Doped Yttrium Iron Garnets

The magnetic disaccommodation on polycrystalline Nd doped YIG Y3-xNdxFe5O12 (0<x<2.5) is studied in this work. It reveals a very different behaviour with the sintering atmosphere. The results of magnetic disaccommodation for samples sintered in air show a relaxation peak with the maximum at a temperature changing from 120 K to 100 K with increasing doping rate. When the secondary perovskite phase appears with x=2.0, another relaxation process at around 300 K emerge, similar to the well-known III peak of polycrystalline magnetite. For CO2 sintered samples, the process at 120 K exhibit magnetic accommodation features (i.e. increase of apparent magnetic permeability after demagnetization), and vanishes with increasing Nd substitution, together with the appearance of the 300 K peak associated with the perovskite secondary phase which appears with x>1.2 doping rate. Introduction Yttrium iron garnets (YIG) are very interesting ferrimagnetic materials for use at high frequencies due to the low losses in this frequency range, according to their high electrical resistivity and narrow resonance linewidth [1]. These properties, together with the possibility of introducing a wide range of dopants for tailoring their properties [2], make them excellent candidates for microwave applications. It is usual the substitution of iron by metallic cations in tetrahedral or octahedral sites [3], but it is also possible the substitution of part of yttrium ions in dodecahedral sites. This latter type of substitution with a rare earth element has been studied so far [4,5], and it has been established that perovskite phase forms together with the magnetic garnet depending on the rare earth employed and on the doping rate. Recently, there is renewed interest in YIG films substituted with Nd [6,7] and Ce [8] due to the large Faraday effect and potential application in magneto-optic devices. The study of magnetic relaxations is important in order to minimize the losses. In addition, the study of this kind of processes provides information about the underlying mechanisms governing the dynamic behaviour of Bloch walls [9]. Among the different techniques available, the magnetic disaccommodation measurements are a powerful tool in the detection of magnetic relaxations in ferrites. It consists in the time variation of the mobility of domain walls after a magnetic shock, and is shown by a temporal evolution of the magnetic permeability after a demagnetization stage. This relaxation phenomenon has been observed in spinel ferrites [9], hexaferrites [10] and garnets [11]. Its origin has been attributed to either the rearrangement or the diffusion of anisotropic point defects (lattice vacancies, interstitials) within the Bloch walls, and the relaxation time, which characterizes each relaxation process, is strongly temperature-dependent. In this paper we study the effect of Nd doping on the magnetic disaccommodation in YIG. The results are compared with experimental results in undoped YIG [11] and interpreted under the theoretical background that our group has explained previously [12]. Materials Science Forum Online: 2004-05-15 ISSN: 1662-9752, Vols. 455-456, pp 143-147 doi:10.4028/www.scientific.net/MSF.455-456.143

Humidity ITO Thick Film Sensing Behaviour Investigated by Impedance Spectroscopy

With the study of composite materials based on the In2O3-SnO2 pair, we look for better sensitivity and selectivity to the gases, than those of the sensors made out of only one of those metal oxides. This would be due to the fact that some of the interstitial positions that were initially occupied by the atoms of one of the metals are now occupied by atoms of the other metal: if the single covalent/ionic adsorption is decisive in the observed changes in the materials conductivity, then the electronegativity of the occupying metal atoms may be used to regulate the sensitivity and selectivity. We will present the results obtained for a film obtained by sol-gel method and deposited using a slow spin coating process, of the In2O3-SnO2 pair, with a 2:3 mol ratio of Indium and Tin, respectively. The rather involved behaviour of our sample is understood by measuring their complex impedance subjected to an external sinusoidal varying electric field, which is being applied in the presence of different relative humidities, at various working temperatures. The main goal of this work here described is the study of the relative humidity influence on the sensing properties of the composite thick film.

Porosity Influence on SnO2 Ceramics' Sensitivity to CO

Gas detection may be addressed using one or more sensors. Solid state chemical sensing has been systematically used for the development of gas sensing devices, based on semiconductor metal oxides. Three major issues are related to chemical sensing: Selectivity, sensitivity and stability. Sensitivity may be improved by controlling the physical properties of the ceramic oxides. In this paper we describe our current work about the sensitivity issue, based on porosity control of the ceramic sensors, to CO.

A pulsed method for measuring the conductivity on solid ionic conductors

Electrode polarization is the main drawback when measuring the conductivity on solid ionic conductors. To overcome this inconvenience, a two-terminal pulsed method was used to measure, under strong electrode polarization, the dc conductivity on Na-β-alumina samples. The results agree well with those obtained with impedance spectroscopy.