Papers by Author: Joaquim Domingues

Abstract: Hydroxyapatite (HA) is widely used as a bone repair material. The use of microwave radiation as energy source for powders densification opens new opportunities for sintering HA at lower temperatures than by conventional sintering (electric furnace). The aim of this work was to compare the densification behaviour of a commercial hydroxyapatite powder under microwave and conventional heating conditions through evaluation of the elastic properties of the resulting sintered materials. The effect of green density, sintering temperature and dwell time on the Young’s modulus E (measured by the impulse excitation of vibration method) of microwave sintered HA was evaluated by using the Taguchi method. Under the set of controlled parameters investigated, green density was the most significant factor affecting E. It was verified that the highest E values obtained with each sintering process are similar (104 and 105 GPa for microwave and conventional sintering, respectively). These values were obtained at 1250°C for 2 h in conventional sintering and at 1200°C for 10 min with microwave sintering. This illustrates the advantage of microwave processing on energy and time savings over conventional sintering.

Abstract: When applied to product engineering, the Taguchi method allows the identification of the conditions that lead to the development of similar products with a small variation around a particular target value. In the present work, the Taguchi method was used to evaluate the effect of four processing parameters on the mechanical properties of a commercial dense cordierite in order to maximize the flexural strength of the resulting bodies. Using a L9 orthogonal array, four control factors (named A, B, C and D) at three levels, namely type (AS1, AS2 and AS3) and content (18, 20 and 22 wt%) of alumina added, cooling rate (1, 3 and 5 °C⋅min-1) and average particle size of each batch (5, 6 and 7 μm) were set. Hence, with a reduced number of experiments, it was possible to identify the factors and their levels, which have most contributed to an increase in flexural strength of the resulting cordierite bodies. The optimum conditions to obtain bodies having a flexural strength higher than 100 MPa were the addition of 22 wt% of AS2 alumina, an average mixture particle size of 5 μm and a cooling rate of 1 °C⋅min-1. The type of alumina had a negligible effect on flexural strength whereas the other factors affect considerably the strength.

Abstract: A conventional method of chemical precipitation was used to produce hydroxyapatite (HAp) powders, with a molar ratio of Ca/P=1.67, for biomedical applications. The aim of this work was to study the effect of four control factors, namely pH, reaction temperature, reagent concentration and flow rate addition, at three levels, on the Ca/P ratio of the synthesized powders, using the Taguchi design of experiments method. Nine powders were synthesized using Ca(NO3)2ּ4H2O and (NH4)2HPO4 as starting reagents. The Ca/P molar ratio of each powder was determined by ICP-AES. Data obtained were used in the Taguchi’s design of experiments to optimize the Ca/P molar ratio. Results showed that the pH of the reaction is the main control parameter (74% contribution) affecting the Ca/P molar ratio of the powders. Therefore, attention should be paid to the control of the pH during the synthesis, in order to obtain HAp powders in a reproducible fashion. High pH values (9.5) and reaction temperature at 70 °C favoured the synthesis of HAp powders with a Ca/P ratio close to the target value of 1.67.