Papers by Author: Marcelo Henrique Prado da Silva

Abstract: In this paper are presented both the fill factor of 0.75 and an efficiency approaching 14.64 % of solar cell, which were achieved, despite the non-optimized process. A new Cu-based additive of pastes were applied for formation of front contact on silicon solar cells. Front contact were screen-printed using commercial silver paste containing the CuXX additive prepared at laboratory. It is the world's first copper based paste appropriate for high-temperature production processes of front contact of the solar cell.

Abstract: In this study, the following compositions were produced: Al₂O₃-YAG laser with 2, 4, 6 and 15% by weight of niobophosphate glass (30mol%P₂O₅-30mol%Nb₂O₅-20mol%CaO-20mol%CaF₂). Sintered discs were characterized by scanning electron microscopy (SEM), X-Ray diffraction (XRD) with the refinement by the Rietveld method and density. The YAG-Al₂O₃ composite sintered at 1450 °C showed densification of 90%, which indicates the effectiveness of the sintering additive.

Abstract: In this work, the adsorption of a low-concentration solution of chlorhexidine (CHX), an antimicrobial drug, in hydroxyapatite (HA) and alginate microspheres was studied. The microspheres were formed by extrusion of a 1:10 mixture of alginate and HA in two different divalent solutions: CaCl₂ and ZnCl₂. UV-Vis spectroscopy showed that the microspheres adsorbed approximately half of the chlorhexidine in solution, which was initially at 0.2%. XRD patterns obtained prior adsorption confirmed the presence of HA as the only crystalline phase. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) analysis were performed before and after CHX adsorption. The spheres produced in CaCl₂ solution did not show significant change after adsorption. However, samples obtained in ZnCl₂ solution showed a different microstructure, with the presence of crystals with a high Zn concentration. X-Ray Fluorescence (XRF) confirmed the presence of ZnO in the samples after CHX absorption.

Abstract: Bioactive glasses have widely used in biomedical applications such as bone filler due to their excellent biocompatibility, bioactivity and osteoconduction characteristics. In this work, a silicate-rich glass was synthesized by the sol-gel method with 60% SiO₂ – 30%CaO – 10% MgO composition where fumed silica acts as the silica precursor. This new method was hypothesized to reduce the gel formation time, due to the high surface area of the fumed silica. In addition, this would presumably increase the nanoporosity of the glass. For comparison purposes, we used a glass of the same composition, but with the conventional silica precursor, tetraethyl orthosilicate (TEOS), through the sol gel method. Both were heat treated at 450°C. FT-IR analysis before and after heat treatment at 450°C showed the presence of nitrate groups, especially in the TEOS samples. This is consistent with the more hygroscopic aspect of those samples. The fumed silica samples had significantly less pronounced peaks corresponding to the nitrate groups, consistent with the more porous structure and the less hygroscopic aspect. The fumed silica samples indeed had a lower gelling time and showed similar results obtained by XRD and FT-IR analyses, showing that the use of fumed silica is viable and advantageous in the glass synthesis by the sol-gel process

Abstract: In this work a two-step thermochemical deposition of calcium phosphate from ostrich eggshell on titanium surface was performed. After the deposition, the coatings were immersed into 0,1M KOH solution in order to favoring HA precipitation. The coatings were characterized by scanning electron microscope with field emission gun (FEG-SEM), and X-ray diffraction (XRD) analysis. Bioactivity assessment was performed in cell culture media, McCoy 5A medium, for 14 days. XRD patterns showed brushite/monetite as the phases present at first deposition step; in the second deposition step XRD patterns indicated HA phase peaks on titanium surface. After 14 days on McCoy 5A medium, brushite/monetite coatings revealed bone-like apatite precipitation, showing bioactivity; hydroxyapatite coatings showed a surface modification but no significant bone-like apatite precipitation, thus lower bioactivity. The use of ostrich eggshells as an alternative source to obtain bioactive calcium phosphate coatings showed to be effective.

Abstract: Silver-bioceramics have a great potential to optimize the bone grafts materials to avoid microorganism infections on patients. The present study synthesized and characterized hydroxyapatite doped with silver (Ag), by an acidic route of precipitation method, and the proposal of the Ag-HA synthesis is to promote the bioactivity and bactericidal ability with less toxicity for organism. The samples were analyzed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analysis. There was a pronounced grain growth after 1100°C HA and AgHA heat treatments for both HA and Ag-HA powders, observed by FEG-SEM analysis. XRD patterns of AgHA sintered at 1100°C showed HA as the main phase, but also a secondary whitlockite phase. However, 100% HA was observed for the Ag-HA samples heat treated at 700°C and 900°C. However, pure HA showed decomposition at 900°C. The acidic route of precipitation method showed to be effective for silver-doped hydroxyapatite production.

Abstract: The increasing interest in the use of brushite and monetite as resorbable calcium phosphate cements or graft materials is related to the fact of these phases being metastable under physiological environment, with higher solubility than hydroxyapatite phase. In this study, X-ray diffraction (XRD) and scanning electron microscopy with field emission gun (FEG-SEM) analyses were performed in order to assess the temperature influence on the production of calcium phosphate coatings by a chemical deposition method. Titanium substrates were successfully coated with brushite and monetite by a chemical deposition method and a brushite-monetite transformation was assessed with the increasing temperature. Brushite deposition was kinetically favored at low temperatures, whereas monetite was the major phase at higher temperatures.

Abstract: Nowadays, bioactive coatings or modifications on titanium surface have been tested in vitro and in vivo. In this study, two types of calcium phosphate coatings were produced by a chemical deposition method and their bioactivity assay in cell culture medium were investigated. The calcium phosphate coatings were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with field emission gun (FEG-SEM) analyses. Titanium substrate was successfully coated with brushite using chemical deposition method and, after a second step of conversion, the hydroxyapatite coating was obtained. The hydroxyapatite coating showed a bioactivity property after 14 days’ incubation in McCoy medium culture.

Abstract: Ceramic materials have particular properties when compared to other classes of materials, exhibiting poor ductility as an example. Slip casting is a widely used ceramic forming technique, and already established in the literature and in the ceramic industry. This study aims to present slip casting as a processing route for producing hydroxyapatite (HA) struts, and show the thermal phase stability. The ceramic suspension was produced and stabilized with hydroxyapatite powder, deionized water and polyethylene glycol 400. The slip was poured into gypsum mold. The green bodies were heat treated at 900 and 1100°C. Hydroxyapatite was the only phase present in all samples, before and after heat treatments. Density measurements showed that the densification was higher for the ceramic bodies sintered at 1100°C, when compared to the ones calcined at 900°C.

Abstract: The knowledge on human tissue is very important to recognize desirable properties of biomaterials. Host cells, extracellular matrix, integrated vessels and interstitial fluids create a complex and dynamic system able to regenerate and respond to environmental stimuli. Myoglobin is a protein with most of α-helices on its secondary structure, and responsible for oxygen binding and release in muscles, by the heme group. This work investigates the Mb adsorption process onto zinc-hydroxyapatite (ZnHA) surface by spectroscopic studies. To do so, ZnHA (0.05 g) was incubated with 4mL of 2mg Mb/mL on phosphate buffer solution pH 6.0 for 24h at 37°C. The FTIR analyses of ZnHA powders before and after protein adsorption provided information concerning the protein content. UV-Vis spectrocopy in the reflectance mode suggested a mixture of MbO₂ and Met Mb on lyophilized solid Mb, and the prevalence of MetMb form when Mb was adsorbed on ZnHA sample. The decrease of UV-Vis secondary bands suggests interactions through the Mb heme group and the ZnHA surfaces. Circular Dichroism (CD) spectroscopy indicated the maintenance of the Mb α-helices secondary structure after the adsorption process on ZnHA powders.