Papers by Author: Maria Helena F.V. Fernandes

Abstract: Glasses based on the 3CaO.P2O5-SiO2-MgO system present high bioactivity aiming the use as bone restorations. On the other hand, the low mechanical properties reduce the importance of this glass aiming the use as restoration bulk specimens. In this work, glass-ceramics were obtained by devitrification of this glass using different temperatures. CaCO3, SiO2, MgO and Ca(H2PO4).H2O were used as starting-powders. Powder mixture was milled/homogenized and melted at 1600°C, for 2h and annealed at 700°C for 4h with cooling rate of 3°C/min. Glass specimens of 151550mm3 were characterized by DTA and XRD analysis. Specimens were heat-treated in different temperatures between 7000C and 1050°C, for 4 hours, obtaining glass-ceramics with different crystallized phase content. Hardness and fracture toughness were determined and correlated with crystalline phase content. The results indicated that crystallization-degree increase with the temperature, and the mechanical properties are improved: Hardness values present increases lower than 20% as function of the crystallization. Fracture toughness may increase 100% as function of temperature (crystallization degree).

Abstract: This study compares the in vitro behaviour in SBF of glasses from two different systems, TiO2-CaO-P2O5 and SiO2-CaO-P2O5 with the same TiO2 and SiO2 molar content, in order to evaluate the effect of TiO2 and SiO2 on the surface reactivity of those glasses. The glass formation regions in both systems were observed for compositions with less than 40 mol% TiO2 and 40 mol% SiO2, respectively. Four glasses with similar TiO2 and SiO2 molar contents have been selected for comparison. These glasses are equimolar in CaO and P2O5 with TiO2 or SiO2 varying from 4 to 33 mol %. Powder glasses were immersed in Simulated Body Fluid (SBF) and kept at 37°C for different times, up to 14 days. Surfaces were observed by Scanning Electron Microscopy (SEM) and specimen ion leaching to SBF was studied by Inductively Coupled Plasma (ICP) spectroscopy. Preliminary spectroscopic studies by Raman were performed to identify the structure of the glasses. For glasses of the SiO2-CaO-P2O5 system a significant dissolution of all ions was observed together with the formation of phosphoric acid. In opposition, the immersion of TiO2-CaO-P2O5 glasses produced a small Ca consumption and stable Ti and P concentrations, indicating the formation of a Ca-P rich layer on these glasses. The observed differences in the dissolution behaviour are tentatively explained in terms of the glass structures obtained by spectroscopy.

Abstract: In this study, the influence of La-rich glass addition and sintering conditions on the densification and mechanical properties of 3 mol.%Y2O3-stabilized tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO2 powder stabilized with 3 mol.% Y2O3 and La2O3-Rich glass were used as starting powders. Two compositions, ZrO2 containing 5 and 10 wt.% of a La2O3-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90°C for 24 hours, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200, 1300 and 1400°C for 60min, and at 1450°C for 120min, with heating and cooling rates of 10°C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). The mechanical properties, hardness and fracture toughness, were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Samples sintered at 1300°C for 60 min presented the optimal mechanical properties with hardness of 1170 kgf.mm-2 and fracture toughness of 8.3 MPam1/2.

Abstract: This paper reports a preliminary study on the development and characterization of a porous calcium phosphate glass-ceramic for tissue engineering application. To obtain porous glass scaffolds, a mixture of 3CaO.P2O5-SiO2-MgO glass (grain size below 20 'm) and NaCl with 200- 300 'm of particle size was taken in the volume proportion 1:1. The mixture was shaped into cylindrical samples (10 mm diameter x 10 mm thick) by using unidirectional pressing. The sintering thermal cycle was selected by means of thermal analyses (DTA/TG and dilatometry) in order to attain a high enough cohesion among the glass particles. After thermal consolidation, the salt was dissolved in water, resulting in highly porous materials. The effect of the sintering thermal cycle on the structure and microstructure characteristics of the scaffolds was investigated in this paper.

Abstract: Mineralization experiments on glasses of the Si-Ca-P-Mg system were carried out for 7 days in carbonated simulated inorganic plasma (CSIP) buffered with CO2/HCO3 -. This method enables physiological buffering of the solution within the 7.3–7.4 pH interval by maintaining a HCO3 - concentration between 24 and 27 mmol.L-1, which is the normal concentration range in blood plasma. XRD, SEM/EDS and FTIR were used to characterise the glass surfaces. All glasses exhibited an apatite-like deposit whose Ca/P ratio was dependent on glass composition.

Abstract: This paper is mainly focused on the characterisation of a glass material (GM) obtained from the thermal treatment of a bottom ash (BA) produced at the Municipal Solid Waste (MSW) incineration plant of Valorsul. By melting the BA at 1400°C during 2 hours, and without using any chemical additives, a homogeneous black-coloured glass was obtained. The thermal and mechanical properties of this glass were characterised. The thermal expansion coefficient, measured by dilatometry, was 9-10 x 10-6 per °C and the modulus of rupture, determined by four-point bending test, was 75±6 MPa, which are similar values to those exhibited by commercial soda-lime-silica glasses used in structural applications. The chemical and the ecotoxicological leaching behaviour of the GM were also analysed. The GM was submitted to a leaching procedure composed of 15 sequential extraction cycles. A liquid/solid (L/S) ratio of 2 l/kg was applied in each cycle. The leachates were filtered through a membrane of PTFE (porosity: 0.45 8m). The filtered leachates were characterised for different chemical parameters and for an ecotoxicological indicator (bacterium Vibrio fischeri). The GM was also submitted to a microwave acidic digestion for the assessment of the total metal content. The crude BA was also submitted to the same experimental procedures. The GM showed levels of chemical emission and ecotoxicity for V. fischeri much lower than those determined for the crude BA. Similar characterisation studies will be pursued with the glass-ceramics produced by adequate thermal treatment of the glass, in order to investigate the effect of the crystallization on the final properties.

Abstract: Impedance spectroscopy was used to monitor the non-isothermal crystallisation of a phosphate phase in bulk samples of a glass with the molar composition 0.45SiO2–0.36MgO– 0.09K2O–0.1(3CaO⋅P2O5), intended for biomedical applications. The K+ alkali ions are probably the main charge carriers in glass of this composition [1]. Impedance spectra were obtained as a function of temperature, following non-isothermal heat treatments at 2°Cmin-1. It is shown that suitable analysis of impedance spectra can be used to evaluate the crystallization peak of a orthophosphate phase, Ca9MgK(PO4)7. Results obtained by this method are comparable to those obtained by differential thermal analysis (DTA) of the corresponding glass frits. Impedance spectroscopy is thus suitable to study the crystallization of bulk samples, and can be used both with variable temperature and under isothermal conditions.

Abstract: Impedance spectroscopy was used to measure the changes of electrical properties during the isothermal crystallisation of the phosphate phase in bulk samples of a glass with nominal molar composition (3CaO.P2O5)20(SiO2)35(MgO)38.354(K2O)6.646, developed for biomedical applications. Crystallization studies were performed in isothermal conditions at 800°C and 860°C, and were monitored by impedance spectroscopy. XRD shows that a stable orthophosphate phase, Ca9MgK(PO4)7 crystallizes at about 860°C, after formation of earlier phosphate precursors formed at lower temperatures, namely oxyapatite (Ca5(PO4)3O) and tricalcium phosphate (Ca3(PO4)2). The conductivity of bulk glass samples decreases at 800°C, as the phosphate precursors phases crystallize. The opposite trend is observed on crystallizing the orthophosphate phase, at 860°C.

Abstract: In silicate glasses the kinetics of apatite layer formation is usually rapid but the adhesion to the base glass is poor. Glass ceramics promote a stronger bonding between layer and substrate but decrease the rate of the apatite layer formation. In this work a glass of composition (wt%) 54,89%C3P-24,81%SiO2-20,30%MgO has been studied. This glass was heat treated at four temperatures (840 °C, 870 °C, 890 °C and 910 °C) to obtain glass ceramics with different contents of the same crystalline phase. A calcium magnesium phosphate phase was formed in all glass ceramics in a volume percent increasing with temperature. The apatite layer precipitated after immersion in simulated body fluid (SBF) formed faster on the glass than on the glass ceramics and a decrease in the amount of apatite formed was observed with the increase in crystallinity. It was generally concluded that heat treatment can turn a reactive glass into glass ceramics of different surface behaviors, from bioactive to quasi bio inert materials.

Abstract: Based upon the CaO-P2O5 glass system, two glass ceramics were prepared in the meta-, pyro- and orthophosphate regions. The present work describes results concerning the biological activity of MT13B (45CaO-37P2O5-5MgO-13TiO2, in mol %) and MK5B (45CaO-45P2O5-5MgO-5K2O, in mol %) using MG63 osteoblast-like cells cultured for 6 days. The influence of a 24h pretreatment with culture medium before cell culture was also evaluated (MT13Bi; MK5Bi). Both “as received” and pre-treated material samples supported cell growth, which increased with the culture time. Pre-treatment resulted in an increase of adhered cells on both materials (day 1). Throughout the culture time (days 3 and 6), MT13Bi presented higher cell numbers than that of MT13B, but no differences were found between MK5Bi and MK5B. In addition, cell growth was significantly lower in the K-containing glass ceramic in both experimental situations comparing with Ticontaining glass ceramic. SEM and confocal microscopy observation showed that cell adhesion and spreading were hampered on MK5B samples. Evident macroscopic and microscopic surface instability was observed on this material with simultaneous dissolution/precipitation processes. By contrast, MT13B presented a relatively stable surface throughout the culture time. Results suggest that the pre-incubation with culture medium improves the adhesion of the osteoblast cells to both glass ceramics and that the high degradation rate of MK5B hinders the in vitro biological performance of this material.