Materials Science Forum Vols. 660-661

Abstract: Closed-end tubes of yttria-stabilized zirconia (YSZ) are required in oxygen sensors for automotive applications to control fuel consumption and pollutant emission. High temperature oxygen sensors were prepared by electrophoretic deposition (EPD) of submicron YSZ particles after suitably adjusting the rheological characteristics of the suspension of the ceramic particles in alcoholic medium. The ceramic tubes were analyzed by X-ray diffraction and impedance spectroscopy. The morphology of sintered specimens was studied by scanning electron microscopy. The processing by EPD of tubular ceramic solid electrolytes with electrical conductivity similar to the ones found in commercial lambda sensors has been successfully achieved.

Abstract: Many different types of ceramic materials are currently being studied as possible cathodes in Solid Oxide Fuel Cells (SOFC), in an attempt to reduce operating temperatures. Strontium-doped neodymium manganite (Nd1-xSrxMnO3±δ) was used as an intermediate temperature solid oxide fuel cell cathode. X-ray diffraction and electrical conductivity of the Nd1-xSrxMnO3±δ system with a perovskite structure were studied in function of x equal to 0.10, 0.30 and 0.50. An orthorhombic pseudo-perovskite structure was assigned to all powder compositions prepared by standard ceramic technique. Electrical conductivity was described by the small polaron hopping conductivity model, as well as, increases due to regular increments of Sr content for all compositions. Electrical conductivity was measured at 25.2, 26.4 and 37.1Scm-1 for x = 0.10, 0.30 and 0.50, respectively at 800°C.

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: 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: Applications of stabilized zirconia include uses related to its low thermal conductivity, strength and electrical properties. Rare earth oxides (REO) were used as additives to stabilize appropriate crystalline phases at zirconia compacts. In this work zirconia based solid electrolytes were sintered with 10, 15 and 20 wt % of REO as sintering aids. Samples were prepared by uniaxial cold press and sintered at 1400 °C, followed by electric properties determination via impedance spectroscopy. Complimentary characterization was made by X-Ray diffractometry and quantitative phase analysis by Rietveld refinement. The ionic conductivity is affected by the amount of REO additive and this effect is correlated to the existing tetragonal and cubic phases for each composition

Abstract: DBC is a process where copper foils are bonded to ceramic substrates for manufacturing hybrid electronic circuits and packages with high power-handling capabilities. For aluminum nitride (AlN) ceramics, a heat-treatment is required to grow an oxide layer to promote the bonding with copper. The oxidation treatment, however, must be conducted in special conditions to avoid the occurrence of severe cracking. In this work, an alternative method is proposed to form an intermediate oxide layers for DBC to AlN substrates. By this method, eutectic powder mixtures (CuO-CaO and CuO-Al2O3 systems) were applied to dense AlN substrates and then heat-treated at 1200 °C for 1 h in air. Different types of AlN ceramics sintered between 1650 and 1700 °C for 4 h in nitrogen atmosphere with additives of the system Y2O3-CaO-SrO-Li2O were investigated. The prepared oxide layers (thickness of ~25 m) presented good microstructural joining with the AlN substrates (characterized by SEM and EDS analysis), and did not affect significantly the thermal conductivity in the working temperature range of electronic devices (~100 to 50 W/m.K determined by laser flash method between 100 and 200 °C) compared to the AlN substrates.

Abstract: In this work alumina based composites ceramics with a polymeric precursor (polysiloxane) and metallic Ti with two different size particle distribution, Ti without milling (ATM) and 5h milling (ATM5), were characterized. PMS thermogravimetric curve indicates that the weight loss percentage was about 20%. The densities achieved were 3,50g/cm3 and 3,70g/cm3 for samples prepared with Ti as received and 5h milling, respectively. During decomposition of the polymer during pyrolysis and sintering, the particles of the matrix may react with carbon and Si from the polymer precursor, metallic Ti and gas atmosphere, nitrogen, to form different phases. The ATM samples presented Al2O3 and TiCN. The specimens prepared with 5h milling Ti (ATM5) is constituted by mullite besides Al2O3 and TiCN. The ATM5 samples have smaller densities than ATM but there is no significant difference in the hardness values.

Abstract: This work has for objective to obtain vitrified ceramic tiles by adding nepheline-syenite flux and talc to a kaolinitic clay from Campos dos Goytacazes. Specimens were prepared by uniaxial pressure at 30 MPa followed by firing at 1100, 1150 and 1200oC. The fired specimens were submitted to the following tests: bulk density, linear shrinkage, three point bending mechanical strength and water absorption. The microstructure of the ceramic was evaluated by SEM. The results showed that the incorporation of nepheline-syenite and talc significantly enhanced the properties of the clay making it possible to attain the standard specification corresponding to vitrified ceramic tile.

Abstract: This work has as its objective to evaluate the effect in the physical and mechanical properties of a red ceramic by the incorporation of petroleum coke from a refining plant. The characterization of the coke was done by optical microscopy and DTA/TG. Compositions were prepared with 0, 1, 2 and 4 wt.% of petroleum coke incorporated into a kaolinitic clayey body. Rectangular specimens were formed by extrusion before firing at 950°C. The determined physical and mechanical properties were: flexural rupture strength and water absorption. The microstructure of the fired ceramics was evaluated by SEM. The results showed that the incorporation of up to 4 wt.% of petroleum coke is not deleterious to the water absorption and mechanical strength of the clayey ceramic fired at 950oC.

Abstract: This work has for objective to evaluate the microstructural aspects and technological properties of a clayey ceramic incorporated with up to 30 wt.% of a waste generate during the steel-making process, denoted as steel slag. To determine the physical and mechanical properties such as linear shrinkage, water absorption and flexural strength, specimens were prepared by 18 MPa uniaxial pressure-molding and then fired in a laboratory furnace at 700, 900 and 1100oC. The microstructure of the compositions was evaluated by scanning electron microscopy and X-ray diffraction. The results showed that it is possible to recycle the finer particles of steel slag by incorporating into red ceramic as long as they are used in amounts not higher than 10 wt.% to avoid the increase in porosity and decrease of the mechanical strength.