Papers by Author: Sergio Luiz Mineiro

Abstract: This paper presents the analysis results of fracture stress values for ceramic composites of alumina matrix mixed with nanoparticulate zirconia (3Y-TZP) ((tetragonal zirconia polycrystalline doped with 3 mol% yttria), which will be used in parts of the Brazilian satellites to act as mechanical shield for micrometeoroid and space debris impacts. The ceramic composites were obtained by mixing dispersed suspensions of alumina powders and 18.5 wt % of nanoparticulate zirconia, compacted with isostatic pressure and sintered at 1550 °C. After sintering, the ceramic composite was analyzed by SEM, X-ray diffraction and subjected to 4 points mechanical bending test. The microstructure analyses were performed and rupture stress values with Weibull distribution. The results were compared to previous works obtained after mechanical mixture of same powders. The results analyses showed that the greater homogeneity distribution of zirconia nanograins on alumina matrix contributes to increased fracture stress values.

Abstract: The fracture toughness is one of the requirements for mechanical properties of materials for use in satellites. The ceramic TZP zirconia (tetragonal zirconia polycrystals) have been investigated for applications in ballistic armor. Due to the chemical inertness and fracture toughness, this material has the potential to act as a screen against impacts of micrometeorites and space debris. The ceramic composites of alumina-zirconia 3Y-TZP (tetragonal zirconia polycrystals doped with 3 mol% ytria ) are the materials with the best benefit / cost for this application. This paper presents and discusses the results obtained from the use of two techniques for determining fracture toughness. The composite alumina - 18.5% of 3Y-TZP zirconia nanoparticles obtained from deflocculated powders have been tested for Vickers and the SEVNB penetration method (Single-Edge-Notch Beam V) to obtain the fracture toughness values (K_IC). The K_IC values obtained were analyzed due to the lower dispersion of experimental values. The SEVNB method showed better reliability in determining the toughness values in the studied ceramics.

Abstract: Dielectric ceramics find application as dielectric resonators (DRs) in communications systems operating at microwave frequencies. RDs for this application require a unique set of properties: high value of the dielectric constant, low dielectric loss and high frequency stability. This paper presents an investigation of the correlation between the dielectric properties, the characteristics of microstructure and the crystalline phases of Ga₂O₃-doped ZnO-Nb₂O₅-TiO₂ ceramic system. The ceramics sintered at 1200 °C were characterized as for density, crystalline phases, microstructure and microwave dielectric properties. The results showed that these dielectric ceramics, obtained from the TiO₂ anatase crystalline structure, present dielectric constant and quality factor (Q) values appropriate for their use as dielectric resonators in microwave circuits. According to the experiments, as the gallium doping has raised, the dielectric constant increased, the Q factor decreased and the temperature coefficient had a tendency to decrease to a certain extent.

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: Due to their high hardness and wear resistance, Si3N4 based ceramics are one of the most suitable cutting tool materials for machining cast iron, nickel alloys and hardened steels. However, their high degree of brittleness usually leads to inconsistent results and sudden catastrophic failures. This necessitates a process optimization when machining superalloys with Si3N4 based ceramic cutting tools. The tools are expected to withstand the heat and pressure developed when machining at higher cutting conditions because of their high hardness and melting point. This paper evaluates the performance of α-SiAlON tool in turning Ti–6Al–4V alloy at high cutting conditions, up to 250 m min−1, without coolant. Tool wear, failure modes and temperature were monitored to access the performance of the cutting tool. Test results showed that the performance of α-SiAlON tool, in terms of tool life, at the cutting conditions investigated is relatively poor due probably to rapid notching and excessive chipping of the cutting edge. These facts are associated with adhesion and diffusion wear rate that tends to weaken the bond strength of the cutting tool.

Abstract: The densification behavior of mixtures of nanocrystalline and conventional microcrystalline zirconia powders undergoing uniaxial pressing was investigated. The mixtures were obtained with ratios between 10 and 80 wt.% of nanoparticulate powder added to the microparticulate powder. Nitrogen and Mercury porosimetry were used to measure the pore size distributions of the compacts. The powders and the fracture surfaces of the compacts were observed via SEM. The results showed that compacts of the powder mixtures attain higher densities during pressing than the unmixed powders. The powder mixture with 80 wt. % of nanoparticles showed the better particle packing efficiency. A comparison between density values of uniaxial and isostatic pressing also was done and indicated that the best results were obtained by the isostatic method.

Abstract: The polymeric precursor route based on the Pechini process was used to prepare a mixture of nano and microcrystalline powders. The zirconia powder was stabilized with 3 mol% of yttria. Powder characteristics were evaluated by the BET technique determined the specific area and the BJH method supplied the pore size distribution. The X-ray diffraction results and SEM observations were carried out to characterize both the powder mixture and the microstructures of the ceramic bodies. The sintering behavior shown in this work was studied by dilatometric experiment, being considered the shrinkage rate and densification of the microstructure.