Papers by Author: Mário S. Reis

Abstract: Manganites of general formula ABMnO3 (where A is a trivalent rare-earth ion and B is a divalent dopant) have interesting properties, namely colossal magnetoresistance and their applicability as materials for active magnetic regenerators. La0.70Sr0.30MnO3 (LSMO) is a ferromagnet presenting considerable magnetocaloric effect, with operating temperature TC ~ 370 K and magnetic entropy variation comparable to pure Gadolinium. The high value of TC makes LSMO unsuitable for room-temperature magnetic refrigeration applications, but by substituting La with the high-magnetic moment ion Er, TC is lowered and total magnetic entropy increases. We have found a limit of solid solubility of Er ions in LSMO, in samples prepared by either solid state or sol-gel techniques in previous works [1], in accordance with other authors [2]. We now present a more detailed study of this limit of solubility, with more samples prepared with Er substitution close to the solubility limit and SEM microscopy clearly showing the changes in microstructure caused by the formation of a secondary ErMnO3 phase, in accordance with x-ray diffraction data and TC variation along the series. The magnetocaloric properties of the series are also presented, showing the increase of Relative Cooling Power along the series, in applied magnetic fields up to 1 T.

Abstract: Bulk MgB2 samples were synthesized by hot isostatic pressing under pressures up to 200MPa at 950°C. In these conditions, full densification of samples was obtained (~98% of theoretical density). SEM, EDS and XRD analysis on final dense bodies were used to evaluate samples, and show increasingly better control over the amounts of secondary MgO (down to ~10%) and complete prevention of formation of MgB4 by using simple glass encapsulation techniques and addition of Mg(s) to the capsule. The samples display superconducting properties, including a narrow critical transition in electrical properties (Tc ~36-38K). Magnetic studies were performed, allowing the determination of the superconducting fraction and critical current density Jc of the materials. Contrary to the Tc, the Jc is quite sensitive to the processing and microstructure and values from 0.3 to 0.6x106 A/cm2 are obtained at 10K. The reduction of Jc with the applied magnetic field requires further improvements to reduce weak links.