Papers by Author: Pedro José Castro

Abstract: This paper presents an investigation of the correlation between dielectric properties and microstructure and TiO2 crystalline-phase characteristics (rutile and anatase) in ZnO-TiO2-Nb2O5 system ceramics. Such ceramics were produced from powder mixtures of zinc oxide, niobium oxide and titanium oxide. Powder mixtures were compressed by pressing (100 MPa uniaxial and 300 MPa isostatic) and sintered at 1100, 1200 and 1250 °C. The sintered ceramics were characterized on the crystalline phases, density and microstructure and microwave frequencies. As for the microwave dielectric properties, dielectric constant, quality factor and thermal coefficient in the temperature range from-20 to 50 °C were measured. The results showed that the type of titanium oxide structure (rutile or anatase) causes influence on the type and quantity of crystalline phases in relative density of sintered ceramics and, consequently in their dielectric properties measurements.

Abstract: Ceramic samples based on barium nanotitanate (Ba₂Ti₉O₂₀) have been produced for application as dielectric resonators aiming at good microwave properties as high dielectric permittivity, low dielectric loss, and high frequency stability. Two different samples were prepared: first a Ba₂Ti₉O₂₀ specimen and second a 1wt% Nb₂O₅-added Ba₂Ti₉O₂₀ composition, using BaO and TiO₂ as precursor materials. Variation of resonant frequency with temperature is commonly represented by temperature coefficient. Experimental tests were carried out to determine the dielectric permittivity (ε) and the temperature coefficient (τ) using a metal box where the specimen was placed between parallel metal surfaces. The resonant frequencies were measured as function of temperature from-20°C to +50°C in a programmable climatic chamber. As a result, the ceramics presented relatively good microwave properties: ε₁=34.6, τ₁=20.5 ppm/°C from initial frequency f₁=7.23 GHz for the former composition, and ε₂=27.5, τ₂=10.1ppm/°C from initial frequency f₂=7.80 GHz for the latter.

Abstract: Dielectric ceramics have been widely investigated and used for microwave applications such as resonators and filters. The present study deals with the influence of sintering temperature on microwave dielectric properties of TiO2 ceramics with 10, 20, and 30 wt% ZrO2. Three compositions have been developed through mixing procedures and then tested for each sintering temperature: 1500 and 1400 °C. X-ray diffraction and scanning electron microscopy are carried out aiming to explain the ceramic behavior of each sample. The dielectric constants of different ceramics for both temperatures varied from 85.4 to 62.6, while their quality factor due to dielectric losses varied from 3110 to 1630. The Q decrease is attributed to the non uniform grain growth and to the obtained crystalline phases. The best microwave parameters were obtained for the ceramics sintered at 1400 °C, which can be applied in microwave circuits as dielectric resonators.

Abstract: For microwave applications, including mobile and satellite communications, ceramic resonators should have a high dielectric constant, low dielectric losses, and high frequency stability. In this sense, TiO2-ZrO2 ceramics have been investigated as a function of sintering behavior, phase composition, and microstructure. The ceramics were densified reaching a value of about 86% of theoretical density at 1400°C sintering temperature. The ceramics are prepared by mixing raw materials with the following TiO2-ZrO2 weight % ratio: 100 to 0, 90 to 10, and 80 to 20, respectively. The measured dielectric constants are between 79 and 88 values, while the quality factor due to dielectric losses are between 2820 and 5170. These results point out the influence of Ti/Zr ratio on controlling the dielectric properties.

Abstract: This paper presents a systematic study about ceramics made of zirconium oxide and titanium oxide (ZrO2-TiO2) doped with tin oxide (SnO2), strontium oxide (SrO2) and niobium oxide (Nb2O5). These ceramics can be applied as dielectric resonators in microwave systems for telecommunications. For a good microwave performance, these ceramics must reach some parameter values as high dielectric constant and high quality factor due to the dielectric losses. The ceramics were manufactured using suitable powder mixtures of ZrO2-TiO2 with additions of 1 % of SnO2, SrO2 and Nb2O5, resulting in four different kinds of samples for analysis. The samples were compacted by an uniaxial (190 MPa) and an isostatic (300 MPa) pressing, sintered at 1200 °C for 3 hours, characterized in the chemical compound formation using X-ray Diffraction (XRD) and scanned for microstructure densification degree and grain distribution analysis in Scanning Electron Microscopy (SEM). The dielectric parameters were measured using a microwave system. The main goal of present procedure is manufacture ceramic materials for promising application as dielectric resonators.

Abstract: In this work the BaSO4 powder was synthesized from BaCO3 powder that was attacked with HCl. Then BaCl2 was precipitated in (NH4)2 SO4, containing TiO2 powder. The influences of processing and TiO2 particles presence were investigated by X-rays diffraction and SEM. It was found that the washing of BaSO4 - TiO2 precipitated by ethanol greatly benefited the formation of smaller size particles.

Abstract: The effects of different concentrations of ZrO2 on the microstructure and dielectric properties of barium nanotitanate (Ba2Ti9O20) dielectric resonators (DRs) for microwave applications are reported. High dielectric constant, small dielectric loss as well as frequency high stability are very important for use in microwave telecommunication, particularly, in a local oscillator of an INPE Communication Satellite. Three different samples of Ba2Ti9O20 with different percentages of ZrO2 were prepared: with 2.0, 2.46 and 3.0 mol% dopant content. The dielectric constant and unloaded quality factor were determined for each different sample.