Alexander J. Shapiro

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.

The Al-Cu-Fe phase diagram: 0 to 25 At. pct Fe and 50

Isothermal sections of the Al-Cu-Fe equilibrium phase diagram at temperatures from 680 °C to 800 °C were determined in the region with 50 to 75 at. pct Al and 0 to 25 at. pct Fe using scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) techniques. This re- gion includes the face-centered icosahedral phase (Ψ-Al6Cu2Fe) which has unprecedented struc- tural perfection and no apparent phason strain. The icosahedral phase has equilibrium phase fields with four distinct phases at 700 °C and 720 °C (β-Al(Fe, Cu), λ-Al13Fe4, ω-Al7Cu2Fe, and liquid) and three phases at 680 °C(β, ω, and λ) and 800 °C (β, λ, and liquid). The B2 ordered β phase has considerably greater solubility for Cu than previously reported, extending from AlFe to ∼Al50Fe5Cu45. The equilibrium range of composition for the icosahedral phase at these temperatures was determined, and a liquidus projection is proposed.

Iron magnetic moments in iron/silica gel nanocomposites

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low‐temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X‐ray diffraction, electron microscopy, Mossbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer‐sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie‐Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as‐cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After ...

Fate of nanoparticles during life cycle of polymer nanocomposites

Nanoparticles are increasingly used in consumer and structural polymeric products to enhance a variety of properties. Under the influence of environmental factors (e.g., ultraviolet, moisture, temperature) and mechanical actions (e.g., scratching, vibrations, abrasion), nanoparticles could potentially release from the products and thus have negative effects on the environment, health and safety. The fate of nanoparticles in polymer nanocomposites during their exposure to UV environment has been investigated. Epoxy polymer containing multi-walled carbon nanotubes (MWCNTs) and silica nanoparticles were studied. Specially-designed cells containing nanocomposite specimens were irradiated with UV radiation between 295 nm and 400 nm. Chemical degradation, mass loss and surface morphology of the epoxy nanocomposites, and release of nanoparticles were measured. Epoxy containing MWCNTs exposed to UV radiation degraded at a much slower rate than the unfilled epoxy or the epoxy/nanosilica composite. Photodegradation of the matrix resulted in substantial accumulation of nanoparticles on the composite surfaces. Silica nanoparticles were found to release into the environment, but MWCNTs formed a dense network on the composite surface, with no evidence of release even after prolonged exposure. Conceptual models for silica nanoparticle release and MWCNT retention on the surface during UV exposure of nanocomposites are presented.

Artifacts in ballistic magnetoresistance measurements (invited)

We have carried out an extensive search for credible evidence to support the existence of a ballistic magnetoresistance (BMR) effect in magnetic nanocontacts. We have investigated both thin-film and thin-wire geometries for both mechanically formed and electrodeposited nanocontacts. We find no systematic differences between mechanically formed and electrodeposited nanocontacts. The samples we have investigated include mechanical contacts between ferromagnetic wires, electrodeposited nanocontacts between ferromagnetic wires, ferromagnetic nanocontacts electrodeposited on Cu wires, nanocontacts electrodeposited between ferromagnetic films anchored on wafers, ferromagnetic nanocontacts electrodeposited on Cu films anchored on wafers, nanocontacts between two ferromagnetic films connected by a pinhole through an insulating film, and nanocontacts formed by focused ion-beam etching. In none of these samples did we find credible evidence for a BMR effect. However, we did find a number of artifacts due to magneto...

Compositional Control in Electrodeposition of FePt Films

Fe-Pt thin-film alloys have been grown by electrodeposition at potentials positive to that required to deposit elemental Fe. X-ray diffraction studies indicate the formation of fine grained face centered cubic alloys, while Rutherford backscattering spectrscopy and energy-dispersive X-ray spectroscopy reveal substantial incorporation of oxygen in the FePt deposits. The Fe-Pt codeposition process is driven by the negative enthalpy associated with alloy formation. The experimentally determined relationship between alloy composition and the iron group underpotential was found to be in reasonable agreement with free energy calculations for the binary alloy system, based on thermochemical data.

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...

Magneto‐optical indicator film observation of domain structure in magnetic multilayers

A new method is developed using a transparent indicator ferrimagnetic magneto‐optic film with in‐plane anisotropy for visualization and direct experimental study of dynamic magnetization processes and nondestructive characterization of the defect structure of magnetic multilayers. Some examples of its application to the investigation of peculiarities of the as‐grown magnetic structure of electrochemically produced CoNiCu/Cu multilayers with a giant magnetoresistance (GMR) effect and magnetization reversal by domain wall motion are described.

Water-soluble DNA-wrapped single-walled carbon-nanotube/quantum-dot complexes.

In recent years, carbon nanotubes (CNTs), especially singlewalled carbon nanotubes (SWCNTs), have attracted much attention due to their unique properties and potential towards broad real-world applications. The integration of SWCNTs with other unique nanoscale luminescent materials, such as quantum dots (QDs), has enabled the manufacture of many novel nanocomposite materials with enhanced structural, mechanical, optical, and chemical properties. The performance of these composite materials strongly depends upon the properties of the individual components and additives as well as the conjugation chemistry required to assemble them into composite hybrids. Therefore, a variety of new techniques have been developed to modify the optical, mechanical, chemical, and electrical properties of SWCNTs to control the properties of the final composite materials. Among the additives to SWCNT-based composites, novel nanoparticles (NPs) have been increasingly employed. Functionalized NPs can be designed to covalently bind to the functional groups expressed on the sidewalls or ends of

The palladium–zirconium phase diagram

Abstract The palladium–zirconium phase diagram has been re-investigated over its entire composition range. The revised diagram includes two intermediate phases, Pd 4 Zr 3 and Pd 11 Zr 9 , which did not appear in previous studies. The crystal structures of the new phases resemble the B2-type structure although the atoms are displaced significantly from the cubic lattice sites. The present observations are discussed and compared with similar results from the related systems Ni–Ti and Rh–Zr.