Centro de Ciências Exatas e da Terra
URI Permanente desta comunidadehttps://repositorio.ufrn.br/handle/1/135
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Navegando Centro de Ciências Exatas e da Terra por Autor "Acchar, Wilson"
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Artigo Bi4Ti3O12 multilayered ceramic tapes produced by aqueous tape casting and laminating process: structural and dielectric properties(Elsevier, 2018-09) Paskocimas, Carlos Alberto; Peres, Ana Paula da Silva; Costa, Antonio Carlos Silva da; Bohn, Felipe; Correa, Marcio Assolin; Acchar, WilsonIn this work, multilayered bismuth titanate ceramic tapes engineered by combining aqueous tape casting and laminating process were produced. The structural and dielectric properties of green and sintered multilayered materials, with 1–5 layers, were explored. Diffraction peaks related to the bismuth titanate phase of structure Bi4Ti3O12 were identified in the precursor powder and the green and the sintered tapes. In addition, a Bi2Ti2O7 structure phase was indexed to the sintered tape. Modifications of the dielectric behavior with the increase in the number of layers composing the samples and with the sintering procedure were observed. Considering the green multilayered BIT ceramics, values between 10 and 14 as a function of the distinct number of layer were found at 1.0 GHz. On the other hand, for the sintered BIT multilayered, values between 2.3 and 8.5 in a wide frequency range were observed, showing a constant behavior as a function of the employed frequency. The results provided interesting insights for tailoring the structural and dielectric properties of the multilayered bismuth titanate ceramic tapes, placing them as a promising candidate fulfilling the needs in a wide variety of technological applicationsArtigo Effect of sintering parameters using the central composite design method, electronic structure and physical properties of yttria-partially stabilized ZrO2 commercial ceramics(Sciendo, 2017-02-24) Paskocimas, Carlos Alberto; Mendes, A. M.; Nascimento, Maria Carolina Burgos Costa do; Araújo, V. D.; Motta, Fabiana Villela da; Acchar, Wilson; Silva, Elson Longo da; Delmonte, Maurício Roberto Bomio; Cavalcante, L. S.In this work, the effect of sintering parameters on electronic structure and physical properties of yttria-partially stabilized ZrO2 (YPSZ) commercial ceramics has been studied using the central composite design (CCD) method. The CCD method allows using empirical modelling with better fitting, by considering the interaction between both factors. Different temperature ranges and sintering times for processing of YPSZ ceramics have been used in order to evaluate the grain growth, hardness and volumetric shrinkage by the CCD method. X-ray diffraction patterns and Rietveld refinement data indicate that non-sintered YPSZ ceramics exhibits two phases related to tetragonal and monoclinic structures, while the sintered YPSZ ceramics exhibits a single phase related to a tetragonal structure. Moreover, the monoclinic structure presents zirconium (Zr) atoms coordinated to seven oxygen (O) atoms, while in the tetragonal structure Zr atoms are coordinated to eight O atoms. Field emission scanning electron microscopy images were employed to monitor the sintering and growth process. In addition, the response surfaces obtained from calculations presented the effect of thermal and kinetic variables on the physical properties such as average grain size, volumetric shrinkage and hardness of YPSZ ceramicsArtigo Effect of Y2O3 addition on the densification and mechanical properties of alumina±niobium carbide composites(Elsevier, 2000-04-21) Acchar, Wilson; Greil, P.; Martinelli, Antonio Eduardo; Vieira, F. A.; Bressiani, A. H. A.; Bressiani, José CarlosAlumina-based composites reinforced with refractory carbides are potential cutting tool materials. They exceed the capabilities of cemented carbides with respect to hot hardness and thermal stability, resulting in faster cutting speeds. Liquid-phase sintering of Al2O3±NbC composites was investigated as an alternative to pressure-assisted processes. Al2O3 reinforced by NbC (5±40 wt.%) was sintered with 3 wt.% Y2O3. In order to assess the e ect of the formation of a liquid phase on the properties of the composites, sintering was carried out either below or above the Al2O3±Y2O3 eutectic temperature, at 1650 and 1800 C, respectively. Density, hardness, fracture toughness and wear resistance of the composite materials were evaluated. Liquid phase sintering did not affect the fracture toughness, but improved both the density and the hardness of the material, regardless of its NbC contents. Higher concentrations of NbC increased the wear resistance of the compositeArtigo Incorporation of ceramic waste into binary and ternary soil-cement formulations for the production of solid bricks(Materials Research, 2014-02-18) Silva, Vamberto Monteiro; Góis, Luciano Costa; Duarte, João Batista; Silva, Jaquelígia Brito da; Acchar, WilsonThe incorporation of ceramic waste (CW) into solid bricks made with soil-cement mixtures has been studied in recent years. The aim of this study was to evaluate the physical and mechanical properties of solid bricks made with soil-cement mixtures uniaxially pressed with the addition of construction waste, having hydrated lime and CPII F-32 Portland cement as binding agents to be used in formulations. Raw materials were characterized by particle size analysis, Atterberg limits, XRF and XRD. Solid bricks were made with soil-cement mixtures and CW, which were cured for 7, 28 and 56 days and submitted to compressive strength, water absorption and modified durability tests. The best results obtained were for percentages of 12% cement and 4% incorporated CWArtigo Reinforcing Al2O3 with W–Ti mixed carbide(Elsevier, 2000-11) Acchar, Wilson; Martinelli, Antonio Eduardo; Cairo, Carlos Alberto AlvesRecent advances in high-speed cutting materials have focused on reinforcing alumina with different carbides and nitrides in order to improve hardness and fracture toughness. However, data on mixed carbides is still scarce. The potential use of this type of material for cutting tool applications has yet to be determined. The present study reports some preliminary results obtained reinforcing Al2O3 with WTiC in the range of 5–30 wt.%. The material was hot-pressed at 1650°C for 30 min and then characterized. Vicker's microhardness (HV) and fracture toughness (KIC) were evaluated by the indentation method. The addition of WTiC did not result in any significant change in the fracture toughness of alumina, however hardness values in excess of 22 GPa were obtainedArtigo Sintering behaviour of alumina–tungsten carbide composites(Elsevier, 2000-05-31) Acchar, Wilson; Martinelli, Antonio Eduardo; Vieira, F. A.; Cairo, Carlos Alberto AlvesAlumina reinforced with tungsten carbide has been investigated as an alternative material for metalworking, combining resistance to high service temperatures and improved toughness. Pressureless sintered and hot-pressed Al2O3–WC composites were manufactured and characterised. The use of Y2O3 as a sintering additive has also been evaluated. Additions of up to 30 wt.% WC resulted in limited grain boundary pinning and corresponding high densification. Although the addition of Y2O3 improved sintering, the presence of a residual grain boundary phase (YAG) was harmful to the fracture toughness of the composites, as it affected the effectiveness of the crack deflection mechanism that takes place at the interfaces between Al2O3 and WC grains. Hot-pressing resulted in hardness ∼17.5 GPa and fracture toughness ∼7 MPa m−1/2, which is an improvement compared to alumina reinforced by other refractory carbidesArtigo Thermal decomposition of illite(Materials Research, 2004) Araújo, José Humberto de; Silva, Nagib Francisco da; Gomes, Uílame Umbelino; Acchar, WilsonThe effect of heat treatment on illite in air at temperatures ranging from 750 to 1150 °C was studied using the Mössbauer effect in 57Fe. The dependence of the Mössbauer parameters and relative percentage of the radiation absorption area was measured as a function of the firing temperature. The onset of thermal structural decomposition occurred at 800 °C. With rising temperature, the formation of hematite (Fe2O3) increased at the expense of the silicate mineral.Artigo ZrO2 tape as flexible substrate to artificially nanostructured materials(Elsevier, 2017-06-01) Corrêa, Marcio Assolin; Araújo, M. R.; Acchar, Wilson; Souza, A. L. R.; Melo, Acácio Silveira; Bohn, FelipeWe associate tape casting and magnetron sputtering techniques to engineer flexible nanostructures using ZrO2 green tape as substrate. We systematically investigate the structural, magnetic and electrical properties of NiFe/Cr/NiFe trilayer nanostructures, with variable thickness values of the NiFe and Cr layers, grown onto rigid glass and flexible ZrO2 tape substrates. We verify the mirroring of these properties in the trilayer nanostructures, irrespective on the kind of employed substrate. The fact that the trilayer nanostructures can be reproduced in distinct substrates corresponds to an important advance for their applicability. The results place the ZnO2 green tape as an attractive candidate for application flexible substrate in the development of electrical and magnetic sensor elements with high sensitivity to mechanical stress