The influence of Au substitution and hydrostatic pressure on the phase transitions and magnetocaloric properties of MnCoGe alloys

In this work, the phase transitions and magnetocaloric properties of Mn 1-xAu xCoGe (0 ≤ x ≤ 0.025) alloys were studied as a function of concentration x and applied hydrostatic pressure. The increasing substitution of Au for Mn results in the decrease of the first-order martensitic transition temperature, and this first-order martensitic transition was ultimately converted to a second-order magnetic transition when the Au substitution (x) reached 0.025. The magnitudes of the maximum magnetic entropy changes increased when the magnetic and structural transitions were coupled, which occurred for 0.005 ≤ x ≤ 0.020. The largest maximum magnetic entropy change for a field change of μ 0 Δ H = 7 T was 33.1 J/kg K for the sample with x = 0.020. Similar to the effect of Au substitution, the first-order martensitic transition temperature initially decreased, and then converted to second order, when the applied hydrostatic pressure reached a large enough value. Interestingly, both Au substitution and pressure application cause a volume reduction and, in both cases, the first-order martensitic transition temperature initially reduced and then converted to second-order. These results suggest two different methods of tuning the transition temperatures in these magnetocaloric materials. One can either apply hydrostatic pressure and temporarily adjust the transition temperatures or modify the composition chemically and permanently change the transition temperatures

Effects of heat treatments on magneto-structural phase transitions in MnNiSi-FeCoGe alloys

A first-order magneto-structural transition from a ferromagnetic orthorhombic TiNiSi-type martensite phase to a paramagnetic hexagonal Ni2In-type austenite phase was observed in (MnNiSi)0.62(FeCoGe)0.38. In this work, we demonstrate that the first-order magneto-structural transition temperature for a given composition is tunable over a wide temperature range through heat treatment and hydrostatic pressure application. The first-order transition temperature decreased by over 150 K as the annealing/quenching temperature went from 700 to 1050 °C. The largest maximum magnetic-field-induced isothermal entropy change with μ0ΔH=7 T reached −36.2 J/kg-K for the sample quenched at 700 °C, and the largest effective refrigeration capacity reached 344.5 J/kg for the sample quenched at 800 °C. Similar to the influence of annealing temperatures, the first-order martensitic transition temperatures decreased as the applied hydrostatic pressure increased until they were completely converted to second order. Our results suggest that the class of MnNiSi-based alloys is a promising platform for tailoring working temperature ranges and associated magnetocaloric effects through heat treatment or application of hydrostatic pressure

Electrochemical performance of iron-doped cobalt oxide hierarchical nanostructure

In this study, hydrothermally produced Fe-doped Co3O4 nanostructured particles are investigated as electrocatalysts for the water-splitting process and electrode materials for supercapac-itor devices. The results of the experiments demonstrated that the surface area, specific capacitance, and electrochemical performance of Co3O4 are all influenced by Fe3+ content. The FexCo3-xO4 with x = 1 sample exhibits a higher BET surface (87.45 m2/g) than that of the pristine Co3O4 (59.4 m2/g). Electrochemical measurements of the electrode carried out in 3 M KOH reveal a high specific capacitance of 153 F/g at a current density of 1 A/g for x = 0.6 and 684 F/g at a 2 mV/s scan rate for x = 1.0 samples. In terms of electrocatalytic performance, the electrode (x = 1.0) displayed a low overpotential of 266 mV (at a current density of 10 mA/cm2) along with 52 mV/dec Tafel slopes in the oxygen evolution reaction. Additionally, the overpotential of 132 mV (at a current density of 10 mA/cm2) and 109 mV with 52 mV/dec Tafel slope were obtained for x = 0.6 sample towards hydrogen evolution reaction (HER). According to electrochemical impedance spectroscopy (EIS) measurements and the density functional theory (DFT) study, the addition of Fe3+ increased the conductivity at the electrode–electrolyte interface, which substantially impacted the high activity of the iron-doped cobalt oxide. The electrochemical results revealed that the mesoporous Fe-doped Co3O4 nanostructure could be used as potential electrode material in the high-performance electrochemical capacitor and water-splitting catalysts

The effects of Cu-substitution and high-pressure synthesis on phase transitions in Ni2MnGa Heusler alloys

The magnetic, structural, and thermal behaviors of the Cu-doped Heusler alloy Ni2Mn1−xCuxGa (0 ⩽ x ⩽ 0.4) were studied as a function of concentration x. As the Cu concentration increased, the structural transition temperatures increased, whereas the chemical order-disorder transitions and melting points decreased. The experimental results from temperature dependent X-ray diffraction reveal different crystal structures of the martensite phase at low temperatures for samples with different x, but all the samples ultimately crystallized in the L21 cubic crystal structure upon heating above their respective structural transitions. The experimental data were used to construct a comprehensive magnetic and structural phase diagram as a function of x from below their respective structural transition temperatures to their melting temperatures. The XRD analysis shows that the observed volume reduction is associated with the increasing structural transition temperature. Therefore, one of the samples was annealed under high pressure to permanently reduce its volume, and the correlation between the increasing structural transition temperatures and volumes was confirmed

The influence of hydrostatic pressure and annealing conditions on the magnetostructural transitions in MnCoGe

In this work, the phase transitions of stoichiometric MnCoGe alloys were studied by systematically varying the annealing conditions and applying hydrostatic pressure. First-order martensitic structural transitions from the Ni 2In-type hexagonal austenite phase to the TiNiSi-type orthorhombic martensite phase spanned a wide temperature window (\u3e - \u3e 200 K) as a result of quenching the samples at temperatures ranging from the solid phase at 700 ° C to the liquid phase at 1150 ° C. Despite the large variation in their structural transition temperatures, the changes in cell parameters across the structural transitions and the Curie-Weiss temperatures of the martensite/austenite phase were relatively small. For the sample quenched from 800 ° C, coupled magnetostructural transitions were observed, and the largest maximum magnetic entropy change was found to be - Δ S m a x = 33.6 J/kg K for a 7-T field change. The coupled magnetostructural transitions and the corresponding magnetic entropy enhancements were found to also be achievable by applying hydrostatic pressures. Meanwhile, as the quenching temperatures or hydrostatic pressures increased, the first-order martensitic structural transition shifted toward lower temperature until it was ultimately absent, in which case only the crystal structure and magnetic transition of the Ni 2In-type hexagonal austenite phase were present