Virgil Provenzano

Reduction of hysteresis losses in the magnetic refrigerant Gd5Ge2Si2 by the addition of iron

The magnetocaloric effect is the change in temperature of a material as a result of the alignment of its magnetic spins that occurs on exposure to an external magnetic field. The phenomenon forms the basis for magnetic refrigeration, a concept purported to be more efficient and environmentally friendly than conventional refrigeration systems. In 1997, a ‘giant’ magnetocaloric effect, between 270 K and 300 K, was reported in Gd5Ge2Si2, demonstrating its potential as a near-room-temperature magnetic refrigerant. However, large hysteretic losses (which make magnetic refrigeration less efficient) occur in the same temperature range. Here we report the reduction (by more than 90 per cent) of these hysteretic losses by alloying the compound with a small amount of iron. This has the additional benefit of shifting the magnetic entropy change peak (a measure of the refrigerators optimal operating temperature) from 275 K to 305 K, and broadening its width. Although the addition of iron does not significantly affect the refrigerant capacity of the material, a greater net capacity is obtained for the iron-containing alloy when the hysteresis losses are accounted for. The iron-containing alloy is thus a much-improved magnetic refrigerant for near-room-temperature applications.

Self-assembled multiferroic nanostructures in the CoFe2O4-PbTiO3 system

The effect of substrate orientation on the morphologies of epitaxial self-assembled nanostructures was demonstrated using multiferroic 0.67PbTiO3-0.33CoFe2O4 thin films. The two-phase composite films were grown by pulsed laser deposition on single crystal SrTiO3 substrates having (001) and (110) orientations. The nanostructures of both orientations consisted of vertical rod- or platelet-like columns of CoFe2O4 dispersed in a PbTiO3 matrix. For the (001) orientation the platelet habits were parallel to the {110} planes, whereas for the (110) orientation the platelets were parallel to the {111} planes. The differences were explained using a thermodynamic theory of heterophase structures.

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

Enhanced magnetocaloric effects in R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0<x<1) nanocomposites

A series of R 3 (Ga 1-x Fe x ) 5 O 12 (R=Gd, Dy, Ho; 0<x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473K for 15 h, rather than at 1173K for 15h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (ΔS m ) with respect to the basic paramagnetic garnet (i.e., x = 0). In addition, with variation in x, the optimal ΔS m was measured for the x = 0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal ΔS m values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd 3 (Ga 0.5 Fe 0.5 ) 5 O 12 , despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.

The Effects of Small Metal Additions (Co,Cu,Ga,Mn,Al,Bi,Sn) on the Magnetocaloric Properties of the Gd5Ge2Si2 Alloy

The structural and magnetic properties of arc-melted and homogenized (1300°C, 1h) alloys of Gd5Ge1.9Si2X0.1(X=Cu, Co, Ga, Mn, Al, Bi, or Sn) were investigated by powder x-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and magnetometry. The addition of Cu, Ga, Mn, and Al completely eliminated the large hysteresis losses present in the undoped Gd5Ge2Si2 alloy between 270 and 330K, broadened the magnetic entropy change ΔSm peak, and shifted its peak value from 275 to 305K similar to that observed earlier for Gd5Ge1.9Si2Fe0.1. The addition of Bi or Sn had a negligible effect on either the alloy hysteresis losses or the characteristics of the ΔSm vs T peak. The microstructure of the alloy doped with Cu, Co, Ga, Mn, or Al consisted of a majority phase (depleted of silicon) and a minor intergranular phase (rich in silicon and of the corresponding metal additive). For Bi or Sn doping, the microstructure consisted of only the Gd5Ge2Si2 phase. Low temperature x-ray diffraction data o...

Direct Measurement of the Magnetocaloric Effect in Gd5Si2Ge1.9Ga0.1

Doping with some metals (Fe, Cu, Ga, Mn, Al) reduces or removes the strong thermal hysteresis in the giant magnetocaloric alloys Gd5(SixGe1−x)4. We present heat capacity and direct measurements of the isothermal entropy change, ΔST, and adiabatic temperature increment, ΔTS, of Gd5Si2Ge1.9Ga0.1. TC = 293.6 ± 0.2 K is quite higher than in the non-substituted alloys and similar to the values in the Si rich compounds (i.e. x > 0.5). The results indicate that even this small addition of Ga makes the transition of second-order type, as a usual magnetic transition in the orthorhombic phase. The magnetocaloric parameters are lower than in the non-substituted compound and comparable to those for pure Gd.

Structure and magnetocaloric properties of the Fe-doped HoTiGe alloy

The structure and magnetocaloric properties of the Fe-doped HoTiGe compound were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy, x-ray diffraction, magnetometry, and calorimetry. As with the early studies on the undoped compound, the Fe-containing alloy exhibited an antiferromagnetic-to-paramagnetic transition and a magnetocaloric effect peak at 90K. The magnetization (M) versus temperature (T) data showed peaks at 10 and 90K, while M versus field (H) curves showed the presence of a field-induced transition for all T<120K; additionally, for all T<60K, open hysteresis loops at the magnetic transitions were observed. XRD measurements between 10 and 60K under various magnetic fields up to 3184kA∕m (40kOe) showed that the hysteresis was not accompanied by any change in crystallography. The magnetization derived entropy change −ΔSm vs T plot also showed the presence of two peaks, at 20 and 90K; but below 15K, −ΔSm increased steeply with decreasing temperature. It is...

Magnetocaloric Properties and Structure of the Gd 5 Ge 1.8 Si 1.8 Sn 0.4 Compund

In this study the magnetic properties and the structure of Gd₅Ge_1.8Si_1.8Sn_0.4 alloy were investigated by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and magnetometry. The concentration of the Sn-doping in this study is four times that used in previous studies examining the magnetocaloric properties of the Gd₅Ge₂Si₂ compound doped with different metal additives. In the earlier studies it was shown the addition of about one atom percent of either Fe, Cu, Co, Ga, Mn, or Al nearly eliminated the large hysteresis losses present in the undoped Gd₅Ge₂Si₂ compound between 270 K and 300 K. Also, these metal additives affected the characteristics DeltaS_m versus T peak, resulting in a significant increase in the refrigeration capacity of the material, if the hysteresis losses are taken into account. By contrast, the same amount of either Sn or Bi had much smaller effects on both the hysteresis losses and the characteristics of the DeltaS_m versus T peak. In this study, a larger amount of Sn doping had a limited effect on the hysteresis losses and characteristics of the DeltaS_m versus T peak of Gd₅Ge₂Si₂ . But, most importantly, it resulted in a different microstructure compared to the compound with smaller Sn addition. The implications of the larger Sn doping on both the magnetocaloric properties and structure of the Gd₅Ge₂Si₂ compound are discussed.

Peak magnetocaloric effects in Al-Gd-Fe alloys

The magnetocaloric properties of several AlxGdyFez (with x+y+z=100) ternary alloys have been determined between 2 and 300 K. Three distinct peaks in the magnetic entropy change ΔSm versus T were found: a low-temperature peak (near 10 K), an intermediate temperature peak (80–160 K), and a higher temperature peak (210 to 280 K). The low-temperature peak coincides with a field-induced antiferromagnetic-to-ferromagnetic transition; the intermediate and high temperature peaks are associated with other magnetic transitions. Above 60 K, these alloys exhibited superparamagnetic behavior and possessed enhanced ΔSm values, as predicted earlier for magnetic nanocomposites.

Magnetization model for a Heusler alloy

Close to room temperature, the off-stoichiometric Ni50Mn35In15 Heusler alloy is known to undergo a first-order magnetostructural transition. This paper presents a new model that closely mimics the magnetic behavior of the virgin curve and that of the M-H loops within the temperature range where the alloy undergoes the first-order transition. The virgin curve and the M-H loops relevant to the model were measured at 280 K. Since our data show that 280 K is above the start of the transition, it implies that at this temperature the alloy is in a mixed state. The mixed state refers the presence of two distinct magnetic states. The model and mechanism we propose to explain the complex magnetic behavior of the virgin curve and of the M-H loops pertain to the action of the applied field on the transition between the two magnetic states. Both the model and the proposed mechanism provide new insight about the complex magnetic behavior displayed by the Ni50Mn35In15 alloy within the first-order transition.