Shi-hai Guo

The effect of doped elements on the martensitic transformation in Ni-Mn-Ga magnetic shape memory alloy

Ni–Mn–Ga alloy is a new actuator material due to the fact that its shape memory effect can be controlled by magnetic field in addition to the conventional controls by temperature and stress. However, the alloy shows relatively low martensitic transformation and Curie temperatures. In this paper, we report the results of adding small amounts of Fe, Co and Tb to NiMnGa alloys. The effect of small additions of these doped elements on the martensitic transformation temperature is remarkable, but the Heusler structure of the alloys remains unchanged. For Ni50Mn27Ga23−xFex (x = 0,1,2) with partial replacement of Ga by Fe, the martensitic transformation temperatures increase with increase of the Fe content, and so does the Curie temperature. This phenomenon of increasing both the martensitic transformation temperatures and the Curie temperature was found for the first time. For Ni47Mn31X1Ga21 (X = Fe,Co), Fe and Co substitution for Mn, Fe increases the martensitic transformation temperature but decreases the Curie temperature, while Co has the opposite effect. For Ni48Mn33Ga18Tb1, the addition of the rare earth element Tb decreases the martensitic transformation temperature and the Curie temperature remarkably. Therefore, the transformation temperatures of the alloys can be improved by these doping methods.

Influence of rapid quenching on cyclic stability of La-Mg-Ni system (AB3-type) electrode alloys

Abstract Aiming at the improvement of the cyclic stability of La-Mg-Ni system (PuNi 3 -type) hydrogen storage alloy, Ni in the alloy was partly substituted by Fe. The electrode alloys of La 0.7 Mg 0.3 Co 0.45 Ni 2.55- x Fe x ( x =0, 0.1, 0.2, 0.3, 0.4) were prepared by casting and rapid quenching. The influence of the quenching on cyclic stability as well as structure of the alloys was investigated in detail. The results of electrochemical measurement indicated that rapid quenching significantly improved cyclic stability. When the quenching rate rose from 0 (As-cast was defined as a quenching rate of 0 m/s) to 30 m/s, the cyclic life of Fe-free alloy ( x =0) increased from 81 to 105 cycles, and for alloy containing Fe( x =0.4), it grew from 106 to 166 cycles at a current density of 600 mA/g. The results obtained by XRD, TEM and SEM revealed that the as-cast and quenched alloys had multiphase structures, including two major phases (La, Mg)Ni 3 and LaNi 5 as well as an impurity phase LaNi 2 . Rapid quenching helped the formation of an amorphous-like structure in Fe containing alloys.

Effects of Nd3+ on the microstructure and magnetic properties of Ni-Zn ferrites

Abstract Ni0.4Zn0.6Fe2−xNdxO4(x = 0-0.07) ferrites doped with different amounts of Nd2O3 were prepared using standard ceramic technique. The samples were uniaxially pressed and sintered at 1250°C for 4 h in air. The phase structure and microstructure of the samples were investigated using X-ray diffraction and scanning electron microscope, respectively. The complex permeability was measured using the impedance analyzer in the range of 1-100 MHz. The results indicate that with increasing Nd3+ content, the relative density and lattice parameter a of the sintered samples increase, whereas the real part of permeability (μ′) and the magnetic loss tangent (tan δ) decrease. The substitution of Nd3+ for Fe3+ forms a secondary phase on the grain boundary of the matrix, which strongly restrains the grain growth of the matrix.

Effects of rapid quenching on microstructures and electrochemical properties of La0.7Mg0.3Ni2.55Co0.45Bx(x=0−0.2) hydrogen storage alloy

Abstract In order to improve the electrochemical performances of La-Mg-Ni based electrode alloys with PuNi3-type structure, a trace of boron was added in La0.7Mg0.3Ni2.55Co0.45 alloy. The La0.7Mg0.3Ni2.55Co0.45(x=0, 0.05, 0.1, 0.15 and 0.2) alloys were prepared by casting and rapid quenching. The electrochemical performances and microstructures of the as-cast and quenched alloys were investigated. The effects of rapid quenching on the microstructures and electrochemical performances of the above alloys were investigated. The results show that the as-cast and quenched alloys are composed of (La, Mg)Ni3 phase, LaNi5 phase and LaNi2 phase. A trace of the Ni2B phase exists in the as-cast alloys containing boron, and the Ni2B phase in the B-contained alloys nearly disappears after rapid quenching. Rapid quenching increases the amount of the LaNi2 phase in the B-free alloy, but it decreases the amount of the LaNi2 phase in the boron-containing alloys. The effects of rapid quenching on the capacities of the boron-containing and boron-free alloys are different. The capacity of the B-free alloy monotonously decreases with increasing quenching rate, whereas the capacities of the B-contained alloys have a maximum value with the change of the quenching rate. The rapid quenching can improve the stability of La-Mg-Ni based electrode alloy but lowers the discharge plateau voltage and decreases the plateau length. The effect of rapid quenching on the activation capabilities of the alloys was complicated.

Hydrogen storage behaviours of nanocrystalline and amorphous Mg20Ni10-xCox(x=0-4) alloys prepared by melt spinning

Abstract The Mg 2 Ni-type alloys with nominal compositions of Mg 20 Ni 10- x Co x ( x =0, 1, 2, 3, 4, %, mass fraction) were prepared by melt-spinning technology. The structures of the alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption/desorption kinetics and the electrochemical performances of the alloys were measured. The results show that no amorphous phase forms in the as-spun Co-free alloy, but the as-spun alloys containing Co show a certain amount of amorphous phase. The hydrogen absorption capacities of the as-cast alloys first increase and then decrease with the incremental change of Co content. The hydrogen desorption capacities of as-cast and spun alloys rise with increasing Co content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacities and kinetics of the alloys. The substitution of Co for Ni clearly enhances the discharge capacities of the alloys and the cycle stability of the as-spun alloys.

Effects of stoichiometric ratio La/Mg on structures and electrochemical performances of as-cast and annealed La–Mg–Ni-based A2B7-type electrode alloys

Abstract In order to investigate the influences of the stoichiometric ratio of La/Mg (increasing La and decreasing Mg on the same mole ratio) on the structure and electrochemical performances of the La–Mg–Ni-based A 2 B 7 -type electrode alloy, the as-cast and the annealed ternary La 0.8+ x Mg 0.2− x Ni 3.5 ( x =0−0.05) electrode alloys were prepared. The characterization of electrode alloys by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that all the as-cast and the annealed alloys hold two major phases of (La,Mg) 2 Ni 7 and LaNi 5 as well as a residual phase of LaNi 3 . Moreover, the increase of La/Mg ratio brings on a decline of (La,Mg) 2 Ni 7 phase and a rise of LaNi 5 and LaNi 3 phases. The variation of La/Mg ratio gives rise to an evident change of the electrochemical performances of the alloys. The discharge capacities of the as-cast and the annealed alloys evidently decrease with growing the La/Mg ratio, while the cycle stabilities of the alloys visibly augment under the same condition. Furthermore, the high rate discharge ability (HRD), the electrochemical impedance spectrum (EIS), the Tafel polarization curves, and the potential step measurements all indicate that the electrochemical kinetic properties of the alloy electrodes increase with the La/Mg ratio rising.

An investigation of hydrogen storage kinetics of melt-spun nanocrystalline and amorphous Mg2Ni-type alloys

The nanocrystalline and amorphous Mg2Ni-type Mg2-xLaxNi(x=0, 0.2) hydrogen storage alloys were synthesized by melt-spinning technique. The as-spun alloy ribbons were obtained. The microstructures of the as-spun ribbons were characterized by X-ray diffraction (XRD), high resolution transmission electronic microscopy (HRTEM) and electron diffraction (ED). The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus, and their electrochemical kinetics were tested by an automatic galvanostatic system. The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys were calculated by virtue of potential-step method. The obtained results showed that no amorphous phase was detected in the as-spun La-free alloy, but the as-spun alloys substituted by La held a major amorphous phase, confirming that the substitution of La for Mg markedly intensified the glass forming ability of the Mg2Ni-type alloy. The substitution of La for Mg notably improved the electrochemical hydrogen storage kinetics of the Mg2Ni-type alloy. Furthermore, the hydrogen storage kinetics of the experimental alloys was evidently ameliorated with the spinning rate growing.

Influences of stoichiometric ratio B/A on structures and electrochemical behaviors of La0.75Mg0.25Ni3.5Mx (M=Ni, Co; x=0–0.6) hydrogen storage alloys

Abstract In order to investigate the influences of the stoichiometric ratio of B / A ( A : gross A-site elements, B : gross B-site elements) and the substitution of Co for Ni on the structures and electrochemical performances of the AB 3.5–4.1 -type electrode alloys, the La-Mg-Ni-Co system La 0.75 Mg 0.25 Ni 3.5 M x (M=Ni, Co; x = 0, 0.2, 0.4, 0.6) alloys were prepared by induction melting in a helium atmosphere. The structures and electrochemical performances of the alloys were systemically measured. The results show that the structures and electrochemical performances of the alloys are closely relevant to the B / A ratio. All the alloys exhibit a multiphase structure, including two major phases, (La, Mg) 2 Ni 7 and LaNi 5 , and a residual phase LaNi 2 , and with rising ratio B / A , the (La, Mg) 2 Ni 7 phase decreases and the LaNi 5 phase increases significantly. When ratio B / A =3.7, the alloys obtain the maximum discharge capacities. The high rate discharge(HRD) capability of the alloy (M=Ni) monotonously rises with growing B / A ratio, but that of the alloy (M=Co) first mounts up then declines. The cycle stability of the alloy (M=Co) monotonously increases with rising B / A ratio, but it first decreases slightly then increases for the alloy (M=Ni). The discharge potential of the alloy (M=Ni) declines with increasing B / A ratio ( x > 0.2), but for the alloy (M=Co), the result is contrary. The substitution of Co for Ni significantly ameliorates the electrochemical performances. For a fixed ratio B / A =3.7, the Co substitution enhances the discharge capacity from 365.7 to 401.8 mA·h/g, the capacity retention ratio ( S 100 ) after 100 charging-discharging cycles from 50.32% to 53.26% and the HRD from 88.65% to 90.69%.

Effect of cobalt content on electrochemical performance of La-Mg-Ni system (Ce2Ni7-type) electrode alloys

In order to improve the cyclic stability of La-Mg-Ni system (Ce2Ni7-type) alloy electrode, small amount of Co was added in La0.75Mg0.25Ni3.5 alloy. The effect of Co on electrochemical performance and microstructure of the alloys were investigated in detail. XRD results showed that the alloys had multiphase structure composed of (La, Mg)2Ni7, LaNi5 and small amount of LaNi2 phases. The discharge capacity of the alloys first increased and then decreased with increasing Co content. At a discharge current density of 900 mA/g, the HRD of the alloy electrodes increased from 81.3% (x=0) to 89.2 % (x=0.2), and then reduced to 87.8 % (x=0.6). After 60 charge/discharge cycles, the capacity retention rate of the alloys enhanced from 52.67% to 61.32%, and the capacity decay rate of the alloys decreased from 2.60 to 2.05 mAh/g per cycle with increasing Co content. The obtained results by XPS and XRD showed that the fundamental reasons for the capacity decay of the La-Mg-Ni system (Ce2Ni7-type) alloy electrodes were corrosion and oxidation as well as passivation of Mg and La in alkaline solution.

Electrochemical hydrogen storage characteristics of nanocrystalline and amorphous Mg2Ni-type alloys prepared by melt-spinning

The nanocrystalline and amorphous Mg2Ni-type alloys with nominal compositions of Mg2Ni1-xMnx (x=0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are obtained. The structures of the as-spun alloy ribbons were characterized by XRD and HRTEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were measured by an automatic galvanostatic system. The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation. The hydrogen diffusion coefficients (D) in the alloys were calculated by virtue of potential-step measurement. The results show that all the as-spun (x=0) alloys hold a typical nanocrystalline structure, whereas the as-spun (x=0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg2Ni-type alloy. The substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn substitution from 0 to 0.4, the discharge capacity of the as-spun (20 m/s) alloy increases from 96.5 to 265.3 mA·h/g, and its capacity retaining rate (S20) at the 20th cycle increases from 31.3% to 70.2%. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with raising the amount of Mn substitution.