Sergio Luiz Mineiro

Sintering Behavior of Tetragonal Zirconia Ceramic Stabilized with Yttria and Rare Earth Oxide (YRE) Mixtures

The sintering behavior of YRE-ZrO2 powder observed in dilatometric experiments is shown in this work. The powder analyzed was composed by ZrO2 and 6 wt% of YRE oxides mixture, which were added to stabilize the zirconia tetragonal phase. The powder mixtures obtained from controlled zirconium chloride and yttrium and rare earth chloride coprecipitation were calcined at 400°C. Isostatic pressing at 400MPa compacted the powder and the sintering was realized at constant heating rate of 10°C/min up to 1600°C. At this temperature it was realized an isothermal treatment during 1 hour. The results showed three different stages of sintering during heating, being the two initials stages exhibiting a shrinkage behavior, similar to the majority of the ceramic systems. In the third stage, above 1240°C, it was observed a small dilation, which suggests the tetragonal to monoclinic phase transformation in zirconia ceramic. Samples also were conventionally sintered at temperature range between 1200 to 1600°C. The X-ray diffraction results and SEM observations were carried out to characterize both the powder mixture and the microstructures of the ceramic bodies. These results were correlated and discussed.

Evaluation of the Performance of α-SiAlON Tool when Turning Ti–6Al–4V Alloy without Coolant

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.

Sintering Properties of ZrO2 - 3 Mol % Y2O3 Obtained from Mixture of Nano and Microcrystalline Powders

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.

Fracture Stress Analysis in Ceramic Composites of Alumina Matrix with Zirconia 3Y-TZP Nanograins for Mechanical Shielding of Satellites

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.

Comparative Analysis of Fracture Toughness Obtained by Different Techniques in Alumina Matrix - Dispersed TZP Zirconia Composites

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 (KIC). The KIC 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.

Ga2O3-Doped ZnO-Nb2O5-TiO2 Dielectric Resonators for Terrestrial and Space Telecommunications Applications

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.

ZnO-Nb2O5-TiO2 Dielectric Resonators and Porosity Influence on Dielectric Constant Values

The dielectric resonator (DR) is a ceramic component used in electronic circuits that can operate in microwave frequency range, where it plays the role of resonant element and enables the construction of high selective filters and oscillators. The dielectric properties of a ceramic resonator are influenced by their microstructure characteristics as pores amount. This work shows a study of the influence of pores amount (porosity) on the dielectric constant values of ceramics from ZnO-Nb2O5-TiO2 system. Mixtures of Nb2O5, ZnO, and TiO2 powders were compacted by uniaxial (100 MPa) and isostatic (300 MPa) pressing and sintered at 1100, 1200, and 1250°C. The experimental dielectric constant values of the sintered ceramics were corrected to eliminate the porosity influence. The results showed that the porosity, the type and quantities of crystalline chemical compounds in the ceramics influence the dielectric constant values.

Influence of TiO2 Crystalline Structure on Dielectric Resonator Properties from ZnO-Nb2O5-TiO2 Ceramic System for Use in Telecommunications

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.

Variation of Resonant Frequency with Temperature in Ceramic Resonators Based on Ba2Ti9O20 Compositions

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.

Microstructure, Crystalline Phase, and Dielectric Property Analyses of TiO2 Composition with ZrO2 Addition

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.