Materials Science Forum Vols. 727-728

Abstract: ZnO is a semiconductor that can be doped with transition metal ions, and thus becomes feasible to use in the diluted magnetic semiconductor (DMS), or semiconductor with magnetic properties. In this work we have studied the influence of doping of Ni⁺² on the structural, morphological and magnetic properties of Zn_1-xNi_xO system, to x = 0.07, 0.1 and 0.2 mol of Ni⁺² synthesized by combustion reaction. The systems were characterized by XRD, SEM and VSM. The maximum temperatures ranged from 639 K and 683 K. All systems showed a majority phase formation of ZnO, with the presence of the second phase NiO. The crystallite size for the majority phase varied between 49 and 56nm. All systems have resulted in samples with a morphology consisting of dense clusters, formed by particles pre-sintered and shaped roughly hexagonal plates. The magnetic measurements showed that the values of saturation magnetization lies between 4.6 to 28.5emu/g, remanent magnetization of 0.01 to 0.3 emu/g, coercive force values varies between 12.7 and 62.4 Oe and Curie temperature ranging from 308 to 311K.

Abstract: Oxygen-ion conductors based on strontium-and magnesium-doped lanthanum gallate have been proposed to be used as solid electrolyte in solid oxide fuel cells operating at intermediate temperatures (500-700 °C), due to their high ionic conductivity and stability over a wide range of oxygen partial pressures. In this work, the effect of attrition milling on phase composition of powder and consolidated specimens prepared by solid state synthesis has been investigated. The results show that both the attrition milling and the calcination temperature play a major role in the phase composition. Powders with negligible amount of secondary phases were obtained after two steps of calcination at high temperature followed by attrition milling.

Abstract: Ferritic stainless steels have been used to produce interconnects for intermediate temperature solid oxide fuel cells (ITSOFC) due to their appropriate properties. Ferritic stainless steel presents mechanical stability, much higher thermal and electronic conductivities; significantly lower cost, and mechanical resistance than austenitic steels. Besides, it presents a thermal expansion coefficient compatible with the other materials of the cell components. However, in the range of this device operating temperature (600 °C 800 °C) it can occur the formation of poorly conducting oxide (Cr₂O₃) reducing the fuel cell performance. The aim of this work was to obtain oxide coatings starting with La, Sr and Co nitrates applied by spray-pyrolysis technique on a stainless steel AISI 430 substrate. The coatings obtained were characterized by X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy. The oxidation resistance of the ferritic stainless steel, coated with a perovskite (La_0,6Sr_0,4CoO₃) film, was investigated by isothermal oxidation. The results showed that the coating obtained promoted the increase of the ferritic stainless steel oxidation resistance. However, after the oxidation test, it was observed a Cr enrichment and a very pronounced Sr enrichment, near to the alloy/coating interface, which can be associate to the decomposition of La_0,6Sr_0,4CoO₃ film.

Abstract: Ni_0.5Zn_0.5Fe_1.97Al_0.3O₄ ferrites were synthesized by combustion reaction method and after calcined at 700°C/1 h, with the aim to investigate how the Al³⁺ ions substitution byNi²⁺ ions can influence the magnetic properties. The results from X-ray diffraction showed the formation of a unique Ni-Zn ferrite phase with average crystallite size of 8 and 22nm, before and after calcination, respectively. The synthesize samples presented a superparamagnetic behavior, with saturation magnetization of 0.9 emu/g, and coercive field of 0.05 kOe, and after calcination the samples presented soft magnetic behavior with saturation magnetization of 40.0 emu/g and coercive field of 0.04 kOe.

Abstract: t is well known that nanostructured materials have relevant influences in properties behavior that can be achieved when compared with conventional materials. In this study is proposed an investigation of the electrical and microstructural properties of zinc oxide based varistors prepared with nanostructured zinc oxide powder obtained by a thermal evaporation process. Zinc oxide powder morphology was investigated by scanning and transmission electron microscopy (SEM and TEM, respectively) and the specific surface area evaluated by adsorption of N₂. The varistors were prepared by the mixture of typical dopants with zinc oxide powders in a ball mill. The surface area of zinc oxide powder used was 17.4 m²/g with tetra-needle like morphology. After powder mixture process it was observed by TEM micrographs that most of the tetrapod shaped zinc oxide broke into needles well mixed with dopant particles. The compressed powders were sintering at 1050, 1150 and 1250°C for 1.5 h and densification over 94% were achieved in all tested temperatures. Preliminary electrical characterization reveals that nanostructured zinc oxide compositions have interesting varistor properties.

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: The development of sensors for relative humidity monitoring, with national technology, is one of the greatest needs identified by National Institute for Space Research (INPE) to allow concerned future projects and minimize dependence on imported components and materials. Therefore, in this work, porous ceramic was manufactured from ZrO₂ and TiO₂ powders, compacted and sintered at different temperatures, for application as air humidity sensing elements. The characterization of the sintered ceramics were carried out through x-ray diffraction (crystalline phases), scanning electron microscopy, SEM (microstructure), density determination and capacitance measurements using a RLC bridge in a climatic chamber. The results evidenced that the air humidity ceramic sensing elements are very promising ones.

Abstract: In this work were investigated the systems-NiO_x-CoO_1-x-Al₂O₃ using X-ray diffraction of the powder, which identified and quantified to phase in the sample was performed through the Rietveld method. The insertion of nickel oxide in the system obeyed the proportions of molar fraction of 0,05, 0,30, 0,50, 0,70 and 0,95 (x). Analyses were realized on samples which were treated at the temperature 1773K. From the results of refinement of spinel phase structure, was analyzed the relationship between lattice parameter refinement and the substitution of Co⁺² ion by Ni⁺²ion in the structure. The electrical resistivity dc was observed as a respect to temperature and the results appears a decrease of resistivity when were simultaneously increased the temperature. It was observed that the conduction mechanism was the same in all samples and the results for activation energy, characterized like electron hopping, it due exchange setting the ion Co⁺² D Co⁺³.

Abstract: Ceramic matrix composites (CMCs) were developed to overcome the intrinsic brittleness and lack of reliability of monolithic ceramics. Their major advantages include high temperature capability, light weight, corrosion resistance and adequate damage tolerance. All-oxide Ceramic Matrix Composites (OCMCs) offer essential advantages with respect to long time stability in oxidizing atmospheres, when compared to their non-oxide counterparts. Nevertheless, there is at present almost no production concept which meets the requirements in view of cost and performance for these materials. This work aims at producing OCMCs by means of a more flexible production route. This is achieved by integrating well-known powder metallurgy routes with the prepreg technique, used at present for producing commercial high performance polymer matrix composites. The processing consists of the following steps: (a) infiltration of commercial alumina fiber fabrics (3M Nextel^TM 610) with a liquid suspension of the matrix material; (b) lamination of the pre-infiltrated fiber textiles with a paraffin-based suspension for the formation of prepregs; (c) layup of prepregs; (d) warm-pressing for the consolidation of the green body; (e) debinding and (f) reaction bonding and/or sintering for synthesis of the oxide matrix. Pure alumina or Reaction Bonded Aluminum Oxide (RBAO) can be used as matrix materials and damage tolerance is achieved by the porous, weak-matrix approach. Microstructural analysis of a pure alumina composite fabricated by this route show good infiltration of fiber bundles and proves the good adhesion of prepregs during processing. Average strength value of 199 MPa in fiber direction is in good agreement with values presented in the literature for OCMCs produced by other techniques.

Abstract: Continuous fiber reinforced glass-ceramic (GC) matrix composites are potential candidates for thermomechanical applications at moderate temperatures (up to 1000°C) due to the combination of interesting properties such as high specific strength and toughness. Crack deflection into fiber-matrix interface, as well as subsequent fiber pullout and bridging are the respective toughening mechanisms. In this paper, the compatibility between LZSA glass-ceramic matrix and commercially available oxide alumina fibers (Nextel^TM 610) is qualitatively examined. Toughening mechanisms such as crack deflection and fiber pullout are investigated by analyzing the path of Vickers-induced matrix cracks formed in the vicinity of the fibers and by investigating the crack surface of bending samples, respectively. GC matrix samples sintered and crystallized at different heat-treatment conditions have shown strong interfacial bonds between matrix and fibers, which leads to a brittle fracture without significant fiber pullout in all cases. This behavior indicates the requirement of using fiber coatings in this CMC system, to produce weak interfaces that enable toughening mechanisms to take place.