Face-centered Tetragonal FePt Research Articles

Overcoming poisoning issues in hydrogen fuel cells with face-centered tetragonal FePt bimetallic catalysts

Hydrogen fuel cells are expected to play a central role in the next-generation energy paradigm. However, owing to practical limitations, hydrogen is supplied in the form of refined hydrocarbons or alcohols in industrial applications. Among them, methanol is widely used as a hydrogen source, and CO is inevitably generated during its oxidation process. Even a small amount of CO (∼20 ppm) strongly binds to Pt used as a catalyst, and deactivates it. In addition to CO, surface adsorption of organic cations by binder or ionomer use in alkaline fuel cells is also one of the poisoning issues to be overcome. Herein, we propose FePt bimetallic catalysts that can resist unavoidable CO and organic cation poisoning. Our synthetic strategy, including annealing and acid treatment, allows the catalysts to possess different alloying degrees and surface structures, which in turn induce different levels of resistance to CO and organic-cation poisonings. The correlation between the surface and bulk structures of the catalysts and poisoning resistance was elucidated through X-ray photoemission spectroscopy and electrochemical analysis. The results revealed that an FePt catalyst having an ordered atomic arrangement displayed a better poisoning resistance than that having a disordered arrangement.

Nanoporous L10-FePt with high coercivity

We demonstrate the synthesis of a novel nanoporous ordered face-centered tetragonal FePt (L10-FePt) compound by selective dissolution of an Fe60Pt10B30 nanocomposite alloy, which consists of nanoscale L10-FePt and Fe2B phases. The Fe2B phase is selectively removed from the alloy by electrochemical etching in H2SO4 aqueous solution, while the preserved L10-FePt phase forms a uniform open bicontinuous interpenetrating ligament–channel structure with a ligament size of ∼15 nm. The nanoporous L10-FePt alloy possesses a high coercivity of 18.5 kOe.

Study of exchange bias effect in a patterned Fe/Pt multilayer with the thermal annealing

The present work reports the preparation of 5 μm wide and 10 μm grating periodicity patterned [Fe(2.0nm)/Pt(2.5nm)]X10 multilayer and its thermal annealing behavior. Intermixing across the Fe and Pt layers is observed with vacuum annealing at 650 K resulting in the formation of disordered face centered cubic (FCC) and ordered L10 face centered tetragonal (FCT) FePt phases. The longitudinal magneto-optical Kerr effect (L-MOKE) hysteresis loop, measured from the single 5 μm FePt stripe shows double loop behavior indicating the presence of two magnetic phases, one with the low coercivity (HC) and the second one with the higher HC values. This could be due to the presence of disordered FCC and ordered L10 FCT phases of FePt, known to be soft and hard magnetic phases. Conversion electron Mössbauer spectroscopy of the annealed sample shows the presence of FCC and FCT FePt phases corroborating the x-ray diffraction and MOKE measurements. The hysteresis loop of the soft phase shows the exchange bias effect with the remnant state of the hard magnetic phase as initial condition akin to the exchange bias as observed in conventional ferromagnetic - antiferromagnetic bilayers.

Fabrication and magnetic properties of FePt/Al2O3 composite film by atomic-layer-deposition

Self-assembled face-centered cubic FePt nanoparticles were capped by a amorphous Al2O3 capping layer with the thickness of 10nm using the atomic layer deposition (ALD) technology, and transmission electron microscopy indicates that the FePt nanocrystals are well-separated. After annealing the composite film at 700°C, well-monodispersed face-centered tetragonal FePt particles can be obtained, yielding a coercivity value Hc=5.9kOe with the magnetocrystalline anisotropy of 3.86MJ/m3. The thermal factor for the composite film is 68.5, meeting the industry requirement (KuV/kBT ≥ 50). The protection of amorphous Al2O3 matrix can effectively inhibit grain growth and particle aggregation, and preserve the ordered domains of FePt nanoparticles during the L10 ordering transition. The combination of ALD-capping layer and self-assembled FePt nanoparticles provides a new potential approach to fabricate patterned magnetic recording media with ultrahigh areal density.

One-step solid-phase synthesis of ultrasmall homogeneous face-centered tetragonal FePt nanoparticles encapsulated in thin carbon shells

Discrete, homogenous and ultrasmall (mostly 2–4 nm) L10-ordered fct FePt nanoparticles encapsulated in well-graphitized thin carbon shells have been prepared by a one-step solid-phase synthesis technique, which can be applied for the production of a number of metal alloy nanoparticles encapsulated in carbon shells. The as-synthesized FePt@C nanoparticles of size 2.1 ± 0.4 nm present superparamagnetic properties at room temperature. The 3.3 ± 0.6 nm size FePt@C nanoparticles have a high coercivity, up to 4.56 kOe at room temperature, and superior chemical stability in a high concentration HCl (10 M) solution. Furthermore, these nanoparticles functionalized non-covalently by phospholipid-poly(ethylene)glycol show biocompatibility with the tested cells (mouse macrophage and mouse L929 fibroblasts cells) in all test concentrations (0.025–0.2 mg mL−1).

Synthesis and Fabrication of a Thin Film Containing Silica‐Encapsulated Face‐Centered Tetragonal FePt Nanoparticles

Self-assembly of monodisperse, silica-encapsulated, face-centered tetragonal FePt nanoparticles forms closely packed 2D arrays (see figure). Placing monodisperse FePt nanoparticles in silica nanocapsules allows the transition from a disordered face-centered cubic phase to a ferromagnetic crystalline face-centered tetragonal structure at elevated temperature without severe sintering. These materials are potential candidates for the generation of ultrahigh-density magnetic recording media.

Synthesis of face-centered tetragonal FePt nanoparticles and granular films from Pt@Fe2O3 core-shell nanoparticles.

This paper describes a new approach for making face-centered tetragonal (fct) FePt nanoparticles with a diameter of 17 nm and granular films from Pt@Fe2O3 core-shell nanoparticle precursors. The core-shell nanoparticles were converted to fct FePt through a reduction and alloy formation process at enhanced temperatures. The Fe and Pt elemental analysis was conducted on both individual nanoparticles and granular films using energy-dispersive X-ray (EDX) spectroscopy. Our convergent evidence from selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and EDX analysis indicates that the final products are fct FePt alloys. The fct FePt films have coercivities of 8.0-9.1 kOe at 5 K and 7.0 kOe at 300 K measured by a SQUID magnetometer. These values depend on the conversion temperatures of Pt@Fe2O3 nanoparticles. Unlike the previously synthesized disordered face-centered cubic (fcc) FePt nanoparticles with diameters of 4-6 nm (Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989), the FePt nanoparticles presented in this work not only possess the preferred fct phase but also are in a size range that is expected to be ferromagnetic and have high coercivity, which is important to the practical applications in ultrahigh density data storage media and magnetic nano devices.

High energy products in rapidly annealed nanoscale Fe/Pt multilayers

Magnetic properties of nanocomposite Fe–Pt films with Fe concentration higher than 50 at % have been investigated in this study. Fe/Pt multilayers were produced by sputtering and magnetic hardening was observed after heat treatment including rapid annealing. The final nanocomposite films consisted of the hard face-centered tetragonal FePt phase and a soft face-centered-cubic phase. The maximum energy products of the optimally processed samples exceeded 40 MGOe. Evidence for exchange coupling of the hard and soft phases was found.