Browsing by Author "Trindade, B."
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- Effect of particle size of starting oxide powders on the performance of doped-lanthanum oxyapatite produced by mechanical alloying followed by microwave sinteringPublication . Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Alves, Cátia; Santos, Mário João dos; Mascarenhas, João; Trindade, B.La9.33Si2Ge4O26 oxyapatite powders were synthesized at room temperature through mechanical alloying of La2O3, GeO2 and SiO2 precursor powders with different particle sizes as well as crystal structure in the case of silica powder (crystalline/amorphous). The mechanical alloyed mixtures were subsequently sintered by microwave heating at 1350 C for 1 h in order to obtain dense and homogeneous materials. All sintered materials consisted of the target apatite phase although minor amounts of secondary phases (e.g.La4GeO8) were also present only in samples obtained from micrometric SiO2 powders with a crystalline structure. The microstructure of the materials obtained from nanometric SiO2 with an amorphous structure was found to be more homogenous than the ones obtained from micrometric/crystalline silica. The mechanical behavior of the samples was slightly dependent on the particle size of the precursors and on the SiO2 crystallinity.
- Effect of the sintering conditions on the morphology of La9.33Si2Ge4O26 oxyapatite for SOFCs electrolytesPublication . Macatrão, Mafalda; Santos, Mário João dos; Alves, Cátia; Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Mascarenhas, João; Trindade, B.Solid oxide fuel cells (SOFCs) are devices that allow direct conversion of chemical to electrical energy through an electrochemical reaction in a cleaner and more efficient way than conventional processes (eg. gas turbines). They are characterized by the use of a solid oxide material as the electrolyte. Yttria-stabilised zirconia (YSZ) has traditionally been used in SOFCs electrolytes at temperatures in the range of 850-1000 °C. Recent research is being focused on the development of new materials with increased ionic conductivity at intermediate temperatures (500-800ºC) as alternative materials to YSZ. Rare earth silicates with an apatite-type structure, such as doped lanthanum oxides of general formula La10(MO4)6O2, where M = Ge, Co, Si, Al, or P, are among these materials. The major limitation associated with the manufacture of these materials is their poor sinterability, which requires high sintering temperatures (1600ºC).
- Mechanical properties of sintered La9.33Si2Ge4O26 oxyapatite materials for SOFC electrolytesPublication . Santos, Miguel; Alves, Cátia; Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Mascarenhas, João; Fernandes, J.V.; Trindade, B.Mechanical properties of La9.33Si2Ge4O26 prepared by mechanical alloying and subsequent sintering at 1300–1400 °C for 1 h were evaluated. Hardness and Young's modulus values in the range 7.3–9.6 GPa and 106–135 GPa, respectively, were obtained from nanohardness tests. The fracture toughness values derived from the Palmqvist method varied between 3.5 and 3.9 MPa m1/2 from classical microindentation test with an indentation load of 9.8 N. Yield stress (σy) was determined by inverse analysis from microhardness tests. The maximum value of σy (1829 MPa) was obtained for the sample sintered at 1400 °C showing the highest density (5.42 g/cm3).
- Novel two-step processing route combining mechanical alloying and microwave hybrid sintering to fabricate dense La9.33Si2Ge4O26 for SOFCsPublication . Santos, Mário João dos; Alves, Cátia; Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Mascarenhas, João; Cavaleiro, A.; Trindade, B.In this work, microwave hybrid sintering at 1300 and 1350 °C was carried out for densification of La9.33Si2Ge4O26 mechanically alloyed powder with apatite structure. The pellets sintered at these two temperatures present the same structure (apatite) with relative densities of 92 and 96%, respectively. Mechanical analysis performed on sintered materials revealed the following results: hardness of 7.1 and 8.0 GPa, Young's modulus of 122 and 133 GPa, yield strength of 1807 and 2073 MPa and fracture toughness of 1.5 and 1.0 MPa m1/2, respectively.
- On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCsPublication . Alves, Cátia; Marcelo, Teresa; Oliveira, Fernando Almeida Costa; Alves, L. C.; Mascarenhas, João; Trindade, B.Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La9.33(RO4)6O2 with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La9.33Si2Ge4O26 electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La2O3, SiO2 and GeO2 powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO2 used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La9.33Si2Ge4O26 integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO2 do promote densification of these electrolytes thereby improving their structural integrity.
- Synthesis and thermal behavior of La9.33Si2Ge4O26 apatite for SOFCsPublication . Serra, R.; Alves, Cátia; Oliveira, J.C.; Oliveira, Fernando Almeida Costa; Marcelo, Teresa; Mascarenhas, João; Trindade, B.Powders of La2O3, GeO2 and SiO2 were dry milled in a planetary ball mill with different rotation speeds (150–350 rpm) and increasing milling times up to 35 h in order to obtain the La9.33Si2Ge4O26 apatite phase at room temperature. The results showed that the higher the rotation speed the lower the time required for the formation of the apatite phase. No reaction between the starting powders was observed at 150 rpm. Thermal analysis of the unreacted powders milled at 150 rpm showed formation of the apatite phase around 800 °C, with enthalpies ranging from 43.5 and 48.6 kJ mol−1. An activation energy Ea of 65 kJ mol−1 was obtained applying the Kissinger equation. The mean Avrami exponent n calculated was 1.5, indicating that the apatite phase transformation occurs by a diffusion controlled process.