In this present work, material has been developed to have thermal stability over microwave range. The SrBi4Ti4O15 ceramics are perovskite material from Aurivillius family and were produced by solid state reaction method with temperatures lower than other recent works reported. X-ray diffraction patterns have confirmed single phase and afterward the sample data were refined by using Rietveld method. V2O5 was added to the pure specimen with different amounts (5, 10 and 15 wt.%) to possibly enhance electrical and dielectric properties of SrBi4Ti4O15. An impedance analyzer and a network analyzer were used to obtain electrical measurements in the range of microwave. Thermal stability measures over a microwave range were used as well as numerical simulations of dielectric resonator antenna (DRA) using the HFSS software. The obtained results have shown an efficient synthesis, and the dielectric constants indicate a real possibility of miniaturization. It was also found that the specimen for 6 wt.% of V2O5 presented temperature coefficient ((f)) near zero at microwave region. The bandwidths measured from the specimens were classified as S-type with wide bandwidth and it can be applied for meteorological radars and satellite communications.

The complex impedance spectroscopy study of magnesium niobate Mg 4 Nb 2 O 9 (MN) ceramics with different additions of V 2 O 5 (0%, 2%, 5%) was performed in this present paper. The preparation of MN samples were carried out by using the solid-state reaction method with a high-energy milling machine. Frequency and temperature dependence of the complex impedance, complex modulus analysis, and conductivity were measured and calculated at different temperatures by using a network impedance analyzer. A non-Debye type relaxation was observed showing a decentralization of the semicircles. Cole–Cole formalism was adopted here with the help of a computer program used to fit the experimental data. A typical universal dielectric response in the frequency-dependent conductivity at different temperatures was found. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The activation energy was obtained from the Arrhenius fitting by using conductivity and electrical modules data. The results would help to understand deeply the relaxation process in these types of materials.

Variation of two and three photon upconversion band intensities in LNO:Er1%–Yb7.5% phosphor with temperature.

In the present article, optical properties and energy upconversion in Er3+/Yb3+ co-doped BiNbO4 matrix were investigated. The BiNbO4 matrix was prepared using the solid-state reaction method. X-ray diffraction of the matrix shows that the crystal structure is consistent with ICSD code 74338. The grain distribution and the behavior of doping with Er3+ and Yb3+ on the sample surface were obtained by scanning electron microscope. Raman spectral characterization was carried out to examine the behavior of the vibrational modes of the samples. Upconversion emissions in the visible region at 484.5, 522, 541.5 and 670.5nm in the matrices BiNbO4:Er,Yb and BiNbO4:Er were observed and analyzed as a function of 980nm laser excitation power and rare-earth doping concentration. The results show that BiNbO4 is a promising host material for efficient upconversion phosphors.

This article proposes a dielectric resonator in the form of a quarter of a cylinder to perform as a circularly polarized antenna using a very simple feed design on which a single probe excites two resonant modes in phase quadrature and spatial orthogonality. This study examines some electromagnetic aspects of this antenna and it also presents and discusses the results achieved from computational simulation and experimental measurements. It was verified a good agreement between simulated and experimental results showing a 3-dB axial ratio bandwidth over 3% at 2.25 GHz. (C) 2015 Wiley Periodicals, Inc.

In this paper the ceramic matrix of TiFeNbO6 (TFNO) was studied. The TFNO phase was calcined at 1,075 °C and used to prepare the samples, of 2, 4, 6, 8 and 10 wt% of the Bi2O3 and sintered at 1,125 °C. These samples were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy and dielectric microwave properties. XRD and RS were used to characterize these samples. The samples presented two new phases. The first phase is the tetragonal rutile structure with a space group of P42/mnm, equivalent at parent rutile Ti0.4Fe0.3 Nb0.3O2 (TFNO) with 040725 ICSD code, and the secondary phase belonging to the pyrochlore system Bi1.721Fe1.056Nb1.134O7 (BFNO), with a space group of Fd-3mZ (227), in a cubic structure. The dielectric properties have shown significant variation for 10 % Bi2O3-added sample, because the formation of the new phase (BFNO) contributes with the reduction of τ f from 281.12 to 77.45 ppm/°C and with increase in ε r , from 47.23 to 63.77 and an increase in the dielectric loss (tan δ), from 0.0016 to 0.0068, respectively. Even though, Bi2O3 additive deteriorates the dielectric loss of the ceramics, the permittivity has enhanced significantly, which is advantageous for reduction of the air gap between the probe and the DRA antennas that influences on the samples for future application in microwave.

We report a study of the structural and thermal characteristics of calcium titanate alloys Ca[(Li1/3Nb2/3)xTi1−x]O3−δ (abbreviated as CNLTO). CNLTO alloys were prepared using a new procedure in the solid-state reaction method. Then they were studied using x-ray diffraction, Raman scattering spectroscopy and microwave properties. The symmetry of the atomic structure of the samples belongs to the spatial group Pbnm, showing orthorhombic structure. The microwave dielectric properties of CNLTO for samples ball-milled with the ratios of 1 and 2.4 balls g−1, calcinated at 900 °C (with two different times of exposure: 3 and 5 h) and sintered at 1000 and 1100 °C (during 3 h) were investigated. For both calcination treatments (900 °C, during 3 and 5 h), the dielectric permittivity decreased with decreasing titanium concentration. It was also observed that a higher temperature of calcination contributes to increasing the εr values. Dielectric permittivity values in the range 10–50 were obtained. We noted a decrease in the temperature coefficient of resonant frequency (τf) with an increase of the x value (titanium substitution). CNLTO has excellent microwave properties with τf close to zero for concentrations between 0.925 and 0.950.