Susana Devesa

Structural, morphological and microwave dielectric properties of (Bi 1-xEu x)NbO 4 ceramics prepared by the sol-gel method

Since the microwave dielectric properties of BiNbO4 were first reported, various attempts have been undertaken to improve the properties of this ceramics, such as the addition of oxides or the substitution of bismuth by lanthanides. In this work, (Bi1-xEux)NbO4 (x = 0.00, 0.05, 0.10, 0.20, 0.50) samples were prepared using the sol-gel method and treated at different temperatures. The structure was studied by X-ray diffraction and Raman spectroscopy and the morphology by scanning electron microscopy. The measurement of the complex permittivity was made using the small perturbation theory, at resonant frequency of 2.7 GHz. In order to study the effect of the treatment temperature and x value in the dielectric properties, the two-way ANOVA method was used. It is clear that the most significant parameter is the treatment temperature. The highest Q × f value was obtained for the sample treated at 1,150°C due to the densification of the obtained ceramics.

Microwave dielectric properties of glass-reinforced polymers

Abstract The well-balanced combination of properties of some polymers makes them good materials for industrial microwave applications, such as telecommunications or microwave ovens. Particular electrical properties are usually needed and can be controlled by additives. In this work, we present the results of complex dielectric permittivity measurements, ε* = ε’ - iε’’, in the microwave frequency region (2.7 and 12.8 GHz), at constant temperature 300 K, on different glass-reinforced and pigmented plastics (poly(butylene terephthalate), polypropylene and acrylonitrile- butadiene-styrene), using the resonant cavity method. We measured the shift in the resonant frequency of the cavity, Δf, caused by the insertion of the sample, which can be related to the real part of complex permittivity, ε’, while the change in the inverse of the quality factor of the cavity, Δ(1/Q), gives the imaginary part, ε’’.

Iron Concentration Effect on the Microwave Dielectric Properties of BiNbO 4 Ceramics

Ceramic dielectrics based on bismuth are recognized as materials with low sintering temperature and have been studied for different applications in the microelectronic area. Since 1992, when Kagata reported the microwave dielectric properties of bismuth niobate (BiNbO4), various attempts have been undertaken to improve the microwave dielectric properties of this ceramic material. Besides the addition of different oxides, such as CuO, ZnO, V2O5, PbO, Bi2O3 and Fe2O3, several researchers tried to improve bismuth niobate properties by adding lanthanides. In this work, (Bi1-xFex)NbO4 (0.00≤ × ≤1.00) samples were prepared using the sol-gel method. The fine particles were pressed into cylinders and heat-treated at specific temperatures. Single phase samples of BiNbO4 (x = 0.00) and FeNbO4 (x = 1.00) were then used as precursors for (Bi1-xFex)NbO4, prepared by the solid state reaction method. The microwave dielectric characterization of the samples was performed using the small perturbation method, and related to their structure. With the sol-gel method the substitution of bismuth by iron was successful, since two non-stoichiometric phases, Bi1.34Fe0.66Nb1.34O6.35 and Bi1.721Fe1.056Nb1.134O7, were obtained. Moreover, the inclusion of iron inhibited the formation of low and high temperature triclinic bismuth niobate. With the solid state technique, the substitution of bismuth by iron was not achieved; it was observed that the dielectric constant decreases with the increase of the FeNbO4 phase and that the dielectric losses follow the opposite trend.

Photoluminescence in europium ion–substituted bismuth niobate particles

ABSTRACT In order to understand the influence of the substitution of europium ion for bismuth ion in the structural and photoluminescence properties of bismuth niobate, bismuth niobate ceramic powders, with different amount of europium ion, were prepared using sol–gel citrate route. The structural and morphological studies were made with X-ray diffraction, Raman, and scanning electron microscopy measurements. Rietveld’s analysis of X-ray diffraction proves the formation and increment of europium niobate phase in europium ion–substituted bismuth niobate samples with increase of europium ion concentration. Photoluminescence results also concur to that of X-ray diffraction and Raman with gradual swapping of europium ion emission from one phase to the other phase at higher concentration.

A 5 GHz Resonant Cavity for Complex Permittivity Measurements: Design, Test Performances and Application

The theoretical treatment of a cavity resonator consists of solving the Maxwell equations in that cavity, respecting the boundary conditions. The resonance frequencies appear as conditions in the solutions of the differential equation involved and are not significantly affected by the fact that the cavity walls have a finite conductivity. Solutions for rectangular cavities and for the lowest resonant mode, where the probability of mistaking one mode from another is slight, are readily obtained. The measurement of the complex permittivity, ε* = ε´-iε´´, can be made using the small perturbation theory. In this method, the resonance peak frequency and the quality factor of the cavity, with and without a sample, can be used to obtain the complex dielectric permittivity of the material. We measure the shift in the resonant frequency of the cavity, f, caused by the insertion of the sample, which can be related to the real part of the complex permitivitty, ε´, while the change in the inverse of the quality factor of the cavity, (1/Q), gives the imaginary part, ε´´. In this work we report the construction details, the performance tests of the cavity to confirm the possibility of the use of the small perturbation theory, and the application of the technique to measure the complex permittivity of a reinforced plastic.