C.F. Conde

A Finemet-type alloy as a low-cost candidate for high-temperature magnetic refrigeration.

The refrigerant capacity (RC) of Fe68.5Mo5Si13.5B9Cu1Nb3 alloy is studied. For the amorphous sample, RC=63Jkg−1 for an optimal reversible cycle with cold and hot ends at 328K and 520K, respectively, for a maximum applied field H=15kOe. Nanocrystallization diminishes both the peak entropy change and RC of the material. Although the measured RC is smaller than for Gd5Ge1.9Si2Fe0.1 (240Jkg−1 for H=50kOe), the Mo-Finemet alloy is more than 20 times cheaper, the applied field employed is smaller, and the temperature span of the optimal cycle is increased. This makes this alloy a promising material for high-temperature refrigeration.

A constant magnetocaloric response in FeMoCuB amorphous alloys with different Fe∕B ratios

The magnetocaloric effect of Fe91−xMo8Cu1Bx (x=15,17,20) amorphous alloys has been studied. The temperature of the peak of magnetic entropy change can be tuned by altering the Fe∕B ratio in the alloy, without changing its magnitude, ∣ΔSMpk∣. The average contribution of the Fe atoms to ∣ΔSMpk∣ increases with increasing B content. This is correlated with the increase in the low temperature mean magnetic moment of Fe. A recently proposed master curve behavior for the magnetic entropy change is also followed by these alloys and is common for all of them.

Enhanced magnetocaloric response in Cr∕Mo containing Nanoperm-type amorphous alloys

The magnetocaloric effect of Fe76Cr8−xMoxCu1B15 (x=0,4) alloys is studied. Although the combined addition of Cr and Mo is more efficient in tuning the Curie temperature of the alloy, the Mo-free alloy presents a higher magnetocaloric response. The refrigerant capacity (RC) for the Mo-containing alloy is comparable to that of Gd5Ge1.9Si2Fe0.1 (for a field of 50kOe, RC=273Jkg−1 for the Mo alloy vs 240Jkg−1 for the Gd-based one), with a larger temperature span of the optimal refrigeration cycle (250K vs 90K, respectively). The restriction of the temperature span to 90K gives RC=187Jkg−1 for the Mo alloy. A master curve behavior for the magnetic entropy change is also evidenced.

The magnetocaloric effect in soft magnetic amorphous alloys

The influence of different compositional modifications on the magnetic entropy change and refrigerant capacity of Finemet, Nanoperm, HiTperm, and bulk amorphous alloys is presented. For all the studied alloys, the field dependence of the magnetic entropy change exhibits a quadratic dependence in the paramagnetic regime, a linear dependence in the ferromagnetic temperature range, and a potential law with a field exponent ∼0.75 at the Curie temperature. This exponent can be explained using the critical exponents of the Curie transition. It is shown that for alloys of similar compositional series, the magnetic entropy change follows a master curve behavior.

Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300 K range. The results are found to be consistent with a spin-glasslike state of the oxide layer inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled hysteresis loops at temperatures below the freezing at approximately 50 K.

Refrigerant capacity of FeCrMoCuGaPCB amorphous alloys

The magnetocaloric effect of the FeCrMoCuGaPCB alloy series, suitable for being prepared as bulk amorphous alloys, has been studied. Optimal refrigeration cycles have a cold reservoir close to room temperature. The refrigerant capacity of these alloys is comparable to that of a Mo-containing Finemet-type alloy and is ∼40% bigger than that of other bulk amorphous alloys with comparable working temperatures. Analysis of the field dependence of the magnetic entropy change evidences a power law for all the magnetic regimes.

Crystallisation process in (FeCo)78Nb6(BCu)16 alloys

Crystallisations of Fe 78-x Co x Nb 6 B 16-y Cu y (x = 18, 39, 60; y = 0, 1) alloys at different heating rates have been studied by differential scanning calorimetry (DSC), thermomagnetic gravimetry (TMG), and X-ray diffraction techniques. α-Fe,Co nanometer sized crystals dispersed in a residual amorphous matrix are obtained in a first stage and the crystallisation is completed after a second stage with the formation of various boride phases. Co alloying decreases the temperature for the crystallisation onset and increases the Curie temperature of the amorphous phase. The volume fraction and the lattice parameters of the α-Fe,Co crystals decrease as the Co content in the alloy increases. The Avrami exponents for the first crystallisation stage are approximately 1.

Dependence of exchange anisotropy and coercivity on the Fe–oxide structure in oxygen-passivated Fe nanoparticles

Ultrafine Fe particles have been prepared by the inert gas condensation method and subsequently oxygen passivated. The as-obtained particles consist in an Fe core surrounded by an amorphous Fe-oxide surface layer. The antiferromagnetic character of the Fe-oxide surface induces an exchange anisotropy in the ferromagnetic Fe core when the system is field cooled. Samples have been heat treated in vacuum at different temperatures. Structural changes of the Fe–O layer have been monitored by x-ray diffraction and transmission electron microscopy. Magnetic properties as coercivity, hysteresis loop shift, and evolution of magnetization with temperature have been analyzed for different oxide crystallization stages. A decrease of the exchange anisotropy strength is reported as the structural disorder of the surface oxide layer is decreased with thermal treatment.

The melting behavior of passivated nanocrystalline aluminum

Abstract A nanocomposite Al 2 O 3 Al material has been prepared by consolidation of nanocrystalline aluminum particles which have been passivated with an alumina overlayer prior to the compaction step. Nanostructured Al has been prepared by the gas phase condensation method. The material has been characterized by XPS (X-ray photoelectron spectroscopy). DSC (differential scanning calorimetry), TEM (transmission electron microscopy) and AFM (atomic force microscopy). This study allowed us to demonstrate that the consolidated Al 2 O 3 Al material, although showing a metallic shine and an ohmic electrical resistivity, presents a microstructure constituted by Al grains interconnected by a very thin alumina network, which prevents the material from falling apart when heated at temperatures above the melting point of aluminum.

Thermomagnetic detection of recrystallization in FeCoNbBCu nanocrystalline alloys

The recrystallization process in FeCoNbBCu nanocrystalline alloys is evidenced from thermomagnetic results as a significant decrease in magnetization at the second crystallization stage. The lowering in the volume fraction of α-FeCo crystals indicates that some of these crystals contribute to the boride phases formed. Electron microscopy images reveal that the final microstructure consists of large crystals (∼500 nm) of a fcc (FeCo)23B6 phase and small crystals (∼20 nm) of bcc α-FeCo and of some boride phases as such (FeCo)2B.