Magnetic properties of epitaxial Fe3_3O4_4 films with various crystal orientations and TMR effect in room temperature

Fe$_3$O$_4$ is a ferrimagnetic spinel ferrite that exhibits electric conductivity at room temperature (RT). Although the material has been predicted to be a half metal according to ab-initio calculations, magnetic tunnel junctions (MTJs) with Fe$_3$O$_4$ electrodes have demonstrated a small tunnel magnetoresistance effect. Not even the sign of the TMR ratio has been experimentally established. Here, we report on the magnetic properties of epitaxial Fe$_3$O$_4$ films with various crystal orientations. The films exhibited apparent crystal orientation dependence on hysteresis curves. In particular, Fe$_3$O$_4$(110) films exhibited in-plane uniaxial magnetic anisotropy. With respect to the squareness of hysteresis, Fe$_3$O$_4$ (111) demonstrated the largest squareness. Furthermore, we fabricated MTJs with Fe$_3$O$_4$(110) electrodes, and obtained an TMR effect of -12\% at RT. The negative TMR ratio corresponded to the negative spin polarization of Fe$_3$O$_4$ predicted from band calculations

Oxidation-induced degradation and performance fluctuation of solid oxide fuel cell Ni anodes under simulated high fuel utilization conditions

High fuel utilization (Uf) conditions in a small-scale electrolyte-supported solid oxide fuel cell (SOFC) with an Ni-ScSZ anode were approximated by adjusting the gas composition to correspond to that in the downstream region of an SOFC stack. At Uf = 80%, and with a cell voltage of 0.5 V, the ohmic resistance fluctuated slightly from the early stages of operation, and became much more significant after 80 h. High current density and large polarization were found to promote Ni agglomeration, leading to insufficient connectivity of the Ni nanoparticles. At Uf = 95%, and with a cell voltage of 0.6 V, fluctuations in the polarization were observed at a much earlier stage, which are attributed to the highly humidified fuel. In particular, significant degradation was observed when the compensated anode potential (which incorporates the anode ohmic losses) approached the Ni oxidation potential. Ohmic losses in the anode are considered to influence Ni oxidation by exposing Ni near the electrolyte to a more oxidizing atmosphere with the increase in oxygen ion transport. Stable operation is therefore possible under conditions in which the compensated anode potential does not approach the Ni oxidation potential, assuming a stable interconnected Ni network

SOFC anodes impregnated with noble metal catalyst nanoparticles for high fuel utilization

Redox-stable solid oxide fuel cell (SOFC) anodes are developed in order to improve durability at higher fuel utilization, as a possible alternative to conventional Ni-zirconia cermet anodes. Ce0.9Gd0.1O2 (GDC) is utilized as a mixed ionic and electronic conductor (MIEC), in combination with Sr0.9La0.1TiO3 (LST) as an electronic conductor. The stability of noble metals (Rh, Pt, and Pd) is analyzed via thermochemical calculation of stable phases. Noble metal catalyst nanoparticles are incorporated via co-impregnation with GDC. The electrochemical characteristics of SOFC single cells using these anode materials are investigated in highly-humidified H2 at 800 °C. Their stability at high fuel utilization is analyzed. These co-impregnated anodes with highly dispersed noble metal catalysts on the LST-GDC conducting backbones, achieve high I[sbnd]V performance comparable to conventional Ni-cermet anodes. The co-impregnated anodes also achieve considerably high catalytic mass activity. At higher oxygen partial pressure, where the Ni catalyst can be deactivated by oxidation, these noble catalysts are thermochemically stable in the metallic state, and tolerant against oxidation. This class of alternative catalyst, impregnated with low-loading of noble metals could contribute to stable operation in the downstream region of SOFC systems. A simple cost analysis indicates a tolerance of using noble metals, provided their loading is sufficiently low

Synergistic effect of sulfonation followed by precipitation of amorphous calcium phosphate on the bone-bonding strength of carbon fiber reinforced polyetheretherketone

Sulfonation and applications of amorphous calcium phosphate are known to make polyetheretherketone (PEEK) bioactive. Sulfonation followed by precipitation of amorphous calcium phosphate (AN-treatment) may provide PEEK with further bone-bonding strength. Herein, we prepared a carbon-fiber-reinforced PEEK (CPEEK) with similar tensile strength to cortical bone and a CPEEK subjected to AN-treatment (CPEEK-AN). The effect of AN-treatment on the bone-bonding strength generated at the interface between the rabbit’s tibia and a base material was investigated using a detaching test at two time-points (4 and 8 weeks). At 4 weeks, the strength of CPEEK-AN was significantly higher than that of CPEEK due to the direct bonding between the interfaces. Between 4 and 8 weeks, the different bone forming processes showed that, with CPEEK-AN, bone consolidation was achieved, thus improving bone-bonding strength. In contrast, with CPEEK, a new bone was absorbed mainly on the interface, leading to poor strength. These observations were supported by an in vitro study, which showed that pre-osteoblast on CPEEK-AN caused earlier maturation and mineralization of the extracellular matrix than on CPEEK. Consequently, AN-treatment, comprising a combination of two efficient treatments, generated a synergetic effect on the bonding strength of CPEEK

Exchange current density of reversible solid oxide cell electrodes

Reversible solid oxide cells (r-SOCs) can be operated in either solid oxide fuel cell or solid oxide electrolysis cell mode. They are expected to become important in the support of renewable energy due to their high efficiency for both power generation and hydrogen generation. The exchange current density is one of the most important parameters in the quantification of electrode performance in solid oxide cells. In this study, four different fuel electrodes and two different air electrodes are fabricated using different materials and the microstructures are compared. The temperature, fuel humidification, and oxygen concentration at the air electrode are varied to obtain the apparent exchange current density for the different electrode materials. In contrast to ruthenium-and-gadolinia-doped ceria (Rh-GDC) as well as nickel-and-gadolinia-doped ceria (Ni-GDC) electrodes, significant differences in the apparent exchange current density were observed between electrolysis and fuel cell modes for the nickel-scandia-stabilized zirconia (Ni-ScSZ) cermet. Variation of gas concentration revealed that surface adsorption sites were almost completely vacant for all these electrodes. The apparent exchange current densities obtained in this study are useful as a parameter for simulation of the internal properties of r-SOCs

Improved redox cycling durability in alternative Ni alloy-based SOFC anodes

Repeated reduction and oxidation of metallic nickel in the anodes of solid oxide fuel cell (SOFC) causes volume changes and agglomeration. This disrupts the electron conducting network, resulting in deterioration of the electrochemical performance. It is therefore desirable to develop more robust anodes with high redox stability. Here, new cermet anodes are developed, based on nickel alloyed with Co, Fe, and/or Cr. The stable phases of these different alloys are calculated for oxidizing and reducing conditions, and their electrochemical characteristics are evaluated. Whilst alloying causes a slight decrease in power generation efficiency, the Ni-alloy based anodes have significantly improved redox cycle durability. Microstructural observation reveals that alloying results in the formation of a dense oxide film on the surface of the catalyst particle (e.g. Co-oxide or a complex Fe-Ni-Cr oxide). These oxide layers help suppress oxidation of the underlying nickel catalyst particles, preventing oxidation-induced volume changes/agglomeration, and thereby preserving the electron conducting pathways. As such, the use of these alternative Ni-alloy based cermets significantly improves the redox stability of SOFC anodes

Ultrafast Control of Crystal Structure in a Topological Charge-Density-Wave Material

Optical control of crystal structures is a promising route to change physical properties including topological nature of a targeting material. Time-resolved X-ray diffraction measurements using the X-ray free-electron laser are performed to study the ultrafast lattice dynamics of VTe$_2$, which shows a unique charge-density-wave (CDW) ordering coupled to the topological surface states as a first-order phase transition. A significant oscillation of the CDW amplitude mode is observed at a superlattice reflection as well as Bragg reflections. The frequency of the oscillation is independent of the fluence of the pumping laser, which is prominent to the CDW ordering of the first-order phase transition. Furthermore, the timescale of the photoinduced 1$T^{\prime\prime}$ to 1$T$ phase transition is independent of the period of the CDW amplitude mode