Sergio Gama

Thermodynamic optimisation of the Nb-Cr system

Abstract In order to optimise the Nb-Cr system all available thermodynamic and phase diagram data were evaluated. Inconsistent values were not used in the final calculation. The liquid, Nb-bcc and Cr-bcc phases were described by the Redlich-Kister model, while the NbCr 2 (r) and NbCr 2 (h) phases were described by the Wagner-Schottky model using the sublattice formalism. A Nb-Cr phase diagram calculated from the thermodynamically optimised description is presented.

The magnetic and magnetocaloric properties of Gd5Ge2Si2 compound under hydrostatic pressure

The Gd5Ge2Si2 compound presents a giant magnetocaloric effect with transition temperature at around 276 K and is a very good candidate for application as an active regenerator material in room temperature magnetic refrigerators. Recently it has been shown that pressure induces a colossal magnetocaloric effect in MnAs, a material that presents a giant magnetocaloric effect and a strong magnetoelastic coupling, as also happens with the Gd5Ge2Si2 compound. This motivated a search of the colossal effect in the Gd5Ge2Si2 compound. This work reports our measurements on the magnetic properties and the magnetocaloric effect of Gd5Ge2Si2 under hydrostatic pressures up to 9.2 kbar and as a function of temperature. Contrary to what happens with MnAs, pressure increases the Curie temperature of the compound, does not affect the saturation magnetization and decreases markedly its magnetocaloric effect.

Experimental study of the AlCr equilibrium diagram

Abstract The phase diagram for the AlCr system has been experimentally revised. The main features of the diagram in the literature have been confirmed. The γ (Al1,Cr2) phase has not been found in as-cast samples, heat-treated samples or samples subjected to differential thermal analysis. A diffusion couple treated at 650°C has not shown this phase either. The reaction involving the aluminium phase has been found to be of a eutectic nature. A new version of the AlCr phase diagram is proposed, taking account of these findings and literature data.

Influence of the growth rate on the microstructure of a Nb–Al–Ni ternary eutectic

Abstract Solidification of a eutectic alloy from the melt produces composite materials, composed of two or more solid phases. Processing by directional solidification presents interesting and different properties when compared to the properties of its constituent phases. One of the ternary systems that has been little studied and exhibits eutectic reaction resulting in three intermetallic phases is the Nb–Al–Ni ternary system, which involves the NbAl3, Nb2Al and AlNbNi phases. Eutectic alloys from this system were processed in an arc melting furnace and in a Bridgman equipment for directional growth. The objective of the experiments was to determine the influence of growth parameters on the eutectic microstructure. The mechanical behavior of the composite was studied through microhardness testing and fracture toughness using Vickers indentation technique. Microstructural characterization and chemical compositions of the samples were investigated by using optical and scanning electron microscopy with energy dispersive spectroscopy.

Structural and magnetic study of the MnAs magnetocaloric compound

The temperature induced phase transition in MnAs is analyzed in this work using X ray Rietveld refinement. The results show the presence of the hexagonal phase (P63/mmc) at room temperature and the first-order structural-magnetic transition to the orthorhombic phase (Pnma) around 318 K was followed in detail. The MnAs magnetic characterization allowed to obtain the transition temperature and a maximum value of 47 J/(kg.K) for the measured magnetocaloric effect for a magnetic field variation of 5 T.

Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure

The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room temperature to which a reasonably high magnetocaloric effect is associated, turning this material of interest for application in magnetic refrigeration. We synthesized this compound in fiber single crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by x-ray diffraction and magnetic measurements as a single phase and with the required magnetic properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases. This is in close agreement with the literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that the peak of the effect follows the trend of the Cur...

Enhanced grain growth in YBa2(Cu1−xMnx)3O7-δ compounds

Abstract We have replaced Cu by Mn in the high- T c superconductor YBa 2 Cu 3 O 7−δ . Samples of the YBa 2 (Cu 1− x Mn x ) 3 O 7−δ compounds were prepared by liquid-phase sintering for x =0.0, 0.025, 0.05, 0.075, 0.10, 0.15 and 0.20. Electrical resistivity and magnetic susceptibility measurements showed no significant changes in the critical temperature as function of the concentration of Mn. X-ray diffraction analysis demonstrates the existence of additional semiconducting phases, but the basic orthorhombic structure is preserved for all samples. The amounts of semiconducting extra phases increase with x and are the orthorhombic Y 2 BaCuO 5 and the tetragonal Ba 2 Mn 3 O 8 . This result is also consistent with the observed reduction in the diamagnetic signal with the increase of the Mn content. Microstructural analysis reveals more densely packed plate-shaped grains, larger than the ones produced by the solid diffusion method. At the sample with 2.5 at% Mn substitution a maximum grain size enhancement is observed.

Directional growth and characterization of Fe–Al–Nb eutectic alloys

The manufacturing of components for operation at high temperatures requires the use of metallic materials which can keep satisfactory mechanical and chemical properties, even at temperatures beyond 1000°C. An interesting alternative to solve such a problem is the use of directionally solidified eutectic alloys. A potentially promising system for the manufacture of structural materials, and so far not totally studied, is the eutectic based on the Fe–Al–Nb system, which involves the (FeAl)2Nb phase and the FeAl solid solution. Eutectic samples from this system were directionally solidified in a vertical Bridgman crystal growth unit. The objective of the experiments was to determine the influence of the growth rate on the eutectic microstructure. The ingots obtained were investigated by using optical and electron scanning microscopy. At low growth rate, the eutectic microstructure remained regular, even though it showed several types of microstructure defects. As the growth rate was increased, a transition from lamellar to fibrous morphology was observed.

History dependence of directly observed magnetocaloric effects in (Mn, Fe)As

We use a calorimetric technique operating in sweeping magnetic field to study the thermomagnetic history-dependence of the magnetocaloric effect (MCE) in Mn0.985Fe0.015As. We study the magnetization history for which a “colossal” MCE has been reported when inferred indirectly via a Maxwell relation. We observe no colossal effect in the direct calorimetric measurement. We further examine the impact of mixed-phase state on the MCE and show that the first order contribution scales linearly with the phase fraction. This validates various phase-fraction based methods developed to remove the colossal peak anomaly from Maxwell-based estimates.

Diffusion patterns of formation of the phase Fe17Nd2Nx at 400°C

Abstract Samples of Fe 17 Nd 2 N x have been prepared for x =0.0, 0.5, 1.0, 1.5, 2.0 and 2.1 nitrogen atoms per formula unit, at 400°C. They were analysed by metallography, thermomagnetic analysis, electron microprobe and X-ray diffraction. The measurements show that all the samples contained the pure Fe 17 Nd 2 phase alongside the saturated Fe 17 Nd 2 N 2.3 phase. The measurements also show that the diffusion mainly proceeds along extended defects as grain and phase boundaries, dislocations and dislocations arrays. The expected shell-core diffusion model is not observed. Heavily nitrided samples shows the presence of striae depicting fast diffusion paths from the exterior to the interior of the powder particles.