Papers by Author: Wilson Acchar

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Authors: Marco Antônio Schiavon, I.V.P. Yoshida, A.C. Silva, Wilson Acchar
Abstract: Ceramic matrix composites (CMC) were prepared by the active-filler-controlled polymer pyrolysis process (AFCOP) using a polysilsesquioxane resin filled with metallic niobium and alumina powders. Samples containing 60 wt% of polysilsesquioxane and 40 wt% of metallic niobium and alumina powders mixtures were homogenized, uniaxially pressed and pyrolysed in an alumina tube furnace up to 1400 °C, under argon flow. The ceramic products were characterized by X-ray diffraction (XRD), thermogravimetry (TGA), differential thermal analysis (DTA), Fourier transform infrared (FTIR) and energy-dispersive (EDS) spectroscopies. XRD analysis of the products showed the presence of crystalline phases such as NbC, Nb3Si, Nb5Si3, SiC, crystoballite and mullite. Thermogravimetry data of the composites presented low weight losses at 1000 °C. DTA curves showed an endothermic peak at 1350 °C, which was associated to the beginning of carbothermic reduction and/or the formation of silicon oxide and carbide. In addition, an exothermic peak at 1400 °C was associated to the formation of the mullite phase.
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Authors: Marco Antônio Schiavon, I.V.P. Yoshida, José Carlos Bressiani, Wilson Acchar
Abstract: In this work, ceramic matrix composites (CMC) were prepared by AFCOP process, using a polysiloxane network filled with metallic niobium and aluminum powders as active fillers. The liquid polysiloxane precursor was loaded with a suitable polymer/filler ratio in relation to stoichiometric Nb : C and Al : O molar ratios. Changing Al for a-Al2O3, which acted as an inert filler, non-stoichiometric conditions were obtained. The mixtures were blended, uniaxially warm pressed, and pyrolysed in flowing argon at 800, 1000 and 1200 °C. Thermogravimetry was used to follow the weight changes during the pyrolysis process. X-ray diffraction was used to identify the formation of new crystalline phases, such as Al2O3, NbC, Nb2C and Al3Nb in the composites. Sintered specimens were also characterized by SEM and EDS. The results indicated good potential for this system to obtain multiphasic composite material in the Al-Nb system at lower temperatures.
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Authors: Sonia Regina Homem de Mello-Castanho, Antônio Carlos da Silva, Wilson Acchar, Ana M. Segadães
Abstract: The vitrification process is an attractive route for the inertization treatment of hazardous industrial wastes. The corrosion resistance of this kind of materials is one of the most important requirements to ensure the long term retention of the toxic metals. In this work, silicate glasses with various waste concentrations were obtained using a galvanic sludge from metallurgical activities and glass forming rejects from ceramic activities. Glasses with several galvanic waste concentrations were obtained. The corrosion behaviour of the vitrified materials under various pH media was evaluated. The FTIR technique was used to investigate the glass structural modifications. Glasses containing 40 wt.% galvanic waste additions show higher resistance to corrosion media than those without waste additions.
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Authors: Uilame Umbelino Gomes, Wilson Acchar, E.C.S. Tavares, N.F. Silva
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Authors: Wilson Acchar, Marcus Diniz, Ygor Alexandre A. Fonseca, F.C.C. Costa
Abstract: By using the active filler controlled polymer pyrolysis, new and cost-effective composite materials can be obtained. In this work, ceramic matrix composites were prepared by using this precursor route, using a polysiloxane network filled with metallic niobium and aluminum powders as active fillers. The mixtures were blended, uniaxially warm pressed, and pyrolyzed in flowing argon at 1400 °C. Porous ceramic preforms were infiltrated with a LZSA glass material, in order to improve the density of a porous composite material. The properties of the pyrolyzed composite material and the effect of the LZSA infiltration on the Al2O3-NbC-SiOC ceramic composite material were investigated. The results have showed that the infiltration processes has improved the physical and mechanical properties of the composite material.
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Authors: Alan Christie Silva Dantas, Wilson Acchar
Abstract: β-Tricalcium phosphate (β-TCP) ceramics are of interest for bone requirements implants due to resoption behavior. The mechanical properties of β-TCP, however, are not yet sufficient to allow load bearing application of implants. The aim of this work was to investigate the effect of Mg2+ substitution on the strength sintered TCP. Due to promotion of a liquid phase at 1200°C, Calcium pyrophosphate (CPP-C2P2O7) was used to improve the sintering of the samples. The introduction of CPP was promoted by use of a Ca/P molar ratio of 1.45. The powders were synthesized using a mixture of Ca (OH)2 suspension and diluted H3PO4 with addition of MgO and calcined at 750 °C, 900 °C and 1050 °C. The cold isostatic pressing compacts were sintered at 1200 °C and 1300 °C, respectively.It was shown that a small Mg content (1.5 mol%) increased both compressive strength and fractional density of the TCP material sintered at 1200 °C from 132 ± 39 MPa at 92.1 % of fractional density to 193 ± 29 MPa at 94.5 % of theoretical density. Higher amounts of Mg inhibited the grain growth provoking a increase of the boundary mobility activation energy. Abnormal grain growth (AGG) was observed after sintering at 1300°C, as result CPP - liquid phase formation. Increase of Mg content promoted AGG, due to inhibition of grain growth during normal grain growth resulting in a increase of the residual elastic energy of the system.
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Authors: Yankel B.F. Silva, Wilson Acchar, Vamberto Monteiro Silva
Abstract: Zirconia ceramic stabilized with 3% yttria (3Y - TZP) is the most used ceramic dental materials. During the machining of zirconia blocks in the laboratory to prepare the various types of prostheses, much of the material is lost in powder form, which is subsequently discarded. This study investigates the possibility to recycling of zirconium oxide powder obtained during milling of dental crowns and bridges. Uniaxial and isostatic pressed samples were prepared, sintered and compared to the commercial material. The results indicated that the isostatic pressed samples shows good physical and mechanical properties, comparable to the commercial material, demonstrating the viability to use this recycle material in the same process.
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Authors: Wilson Acchar, G.C.L. Silveira, Sonia Regina Homem de Mello-Castanho, Ana M. Segadães
Abstract: The use of industrial waste materials as additives in the manufacture of ceramic products has been attracting a growing interest from researchers in recent years and is becoming common practice. The continued depletion of natural resources throws a new light on the potential use of some industrial wastes and natural sub-products as full-fledged alternative ceramic raw materials. This work describes the research carried out on the low temperature manufacturing of alumina bodies using, as additive, granite reject as-produced by an ornamental stone processing industry that saws granite stones into blocks and slabs in Rio Grande do Norte, Brazil. This reject is produced in significant amounts and is discarded in sedimentation lagoons, landfill areas or simply thrown in rivers, resulting in environmental pollution. Samples containing up to 30 wt% granite reject and 5 wt% manganese oxide (constant) were uniaxially pressed and sintered in air in an electric furnace (1150-1350 °C, for 1 hour). Sintered test pieces were characterized by X-ray diffraction, apparent density, open porosity and flexural strength. The results showed that the addition of granite reject and manganese oxide enables low temperature sintering and remarkably improves the cold mechanical properties of the alumina body.
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Authors: Wilson Acchar, Jaquelígia B. Silva, Vamberto M. Silva, Luciano Costa Góis, Ana M. Segadães
Abstract: In Brazil, the majority of construction and demolition waste materials (CDW) is sent to waste dumps or landfill sites. Having low cost applications in mind, this work has the purpose of investigating the effect of the incorporation of fired ceramic rubble reclaimed from CDW obtained directly from the building construction industry on the final properties of compressed earth blocks, which are especially interesting in low-income and marginalized communities. To this aim, clay-based mixtures containing up to 5 wt.% of ceramic rubble were prepared. Lime and cement were added as binders (6, 8, 10 and 12 wt.%). Cylindrical test pieces were produced by uniaxial compression and left to harden at ambient conditions for 7, 28 and 56 days. The hardened specimens were characterized in terms of microstructure (SEM), compressive strength, water absorption and wear resistance. The results obtained in physical and mechanical evaluation tests demonstrated that small contents of ceramic rubble from the building construction industry can easily be incorporated into compressed earth blocks without degradation of typical properties, enabling savings in cement addition.
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Authors: Wilson Acchar, Harim Revoredo de Macedo
Abstract: Cemented carbides have been intensively used as cutting tool through their high hardness, high fracture toughness and high wear resistance. A considerable amount of works has been developed in order to improve the mechanical properties of alternate cemented carbide systems. This work has the purpose to reports the first results obtained to WC-Co reinforced with 5 wt.% NbC. The mixture of powders was hot-pressed at 1250 °C in a inert atmosphere. Hardness and fracture toughness were carried out in a Vickers hardness testing machine. The results have showed that the addition of niobium carbide improves the hardness of tungsten carbide and inhibits the WCgrain growth.
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