Huibin Xu

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets

The magnetostructural coupling between the structural and the magnetic transition has a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here we demonstrate the feasibility of obtaining a stable magnetostructural coupling over a broad temperature window from 350 to 70 K, in combination with tunable magnetoresponsive effects, in MnNiGe:Fe alloys. The alloy exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

Effect of Ni excess on phase transformation temperatures of NiMnGa alloys

A systematic substitution of Ni for Mn, Ga, or both Mn and Ga in the non-stoichiometric NiMnGa alloys is performed. The relationship among the composition, structure and martensitic transformation temperatures was studied in detail for the Ni excessive NiMnGa alloys. The martensitic transformation temperatures almost linearly increase with increasing Ni content in all the three series from lower than 0 °C up to 300 °C. The increases in rate of the martensitic transformation temperatures are different for the three cases. It is large for Ga substituted by Ni, slow for Mn and intermediate for both Mn and Ga. The size factor and electronic concentrations are thought to influence the martensitic transformation temperature in the NiMnGa alloys. The determined relationship will be significant for designing a suitable NiMnGa alloy with a required martensitic transformation temperature for application at a specific temperature.

A high-temperature shape-memory alloy Ni54Mn25Ga21

A high-temperature shape-memory alloy, Ni54Mn25Ga21, was developed with a shape-memory effect of 6.1% and a martensitic transformation temperature higher than 250 °C for single crystals. The measured compressive strength and strain were 845 MPa and 20.5%, respectively, with a compressive axis along the growth direction of the rods at room temperature. One thousand thermal cycles were performed on the Ni54Mn25Ga21 without obvious changes of the martensitic structure, transformation behavior, and shape-memory effect, indicating an excellent thermal stability for the present alloy.

Porous nanotubes of Co3O4: Synthesis, characterization, and magnetic properties

Stoichiometric Co3O4 porous nanotubes have been synthesized through a simple modified microemulsion method. The structural and the chemical information of the as-grown nanotubes have been investigated by means of x-ray diffraction, electron microscopy, electron energy loss spectroscopy, and dynamic force microscopy. The results reveal that the as-grown materials are formed by concentric stacking of Co3O4 (111) planes or weaved porous nanotubes with diameters ranging from tens to ∼200nm and sidewall thickness ranging from 2to∼20nm. Magnetic property of the sample demonstrates a magnetic transition temperature at 8.4K, indicating macroscopic quantum confinement effects from the sidewall thickness of the porous nanotube.

Giant magnetostrictive actuators for active vibration control

Giant magnetostrictive actuators are designed and fabricated with home-made TbDyFe magnetostrictive rods. The corresponding static and dynamic characteristics are tested. The total output displacement can be obtained up to 100 µm and the output force up to 1500 N. The dynamic responses of input and output are accordant and have a small hysteresis. Experiments on active vibration control are implemented in single-degree-of-freedom (DOF) and six-DOF platforms in a flexible space structure. The excellent damping effect, up to 30 dB, proves the good performance of the actuators, the feasibility of the control algorithms, and the reasonable design of the six-DOF platform.

Half-metallic properties for the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys: A first-principles study

The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 A. This is ...

Superhigh strains by variant reorientation in the nonmodulated ferromagnetic NiMnGa alloys

Superhigh strains of 15% for the Ni53Mn25Ga22 and 13.5% for the Ni54Mn23Ga23 with the nonmodulated martensite structure were achieved by variant reorientation. A strong magnetic anisotropy exists in the prefabricated single variant with the easy axis along the contraction [001] direction of the high-temperature cubic phase. A higher austenitic transformation temperature was found in the single variant comparing with the original multivariant sample. The same order of magnitude for the saturation magnetization Ms, magnetic anisotropy constant ku, and the twin reorientation stress σtw, namely, 57.6 A m2/kg, 1.4×105 J/m3, and approximately 20 MPa, respectively, as those in 5M martensite indicate that it is reasonable to expect a much higher magnetic field-induced-strain in the nonmodulated NiMnGa alloys.

Development of gradient thermal barrier coatings and their hot-fatigue behavior

Abstract In this paper, a new type of gradient thermal barrier coating was produced by the co-deposition of a tablet of Al–Al 2 O 3 –YSZ (Yttria Stabilized Zirconia, ZrO 2 +8 wt.% Y 2 O 3 ), or Al 2 O 3 –YSZ and YSZ onto a NiCoCrAlY bond coat by EB-PVD. The analysis of SEM with EDS showed that the composition and microstructure changed continuously across the thickness of the gradient coating. Due to interdiffusion of elements between the bond coat and the gradient zone, a thin layer of a γ′ phase (Ni 3 Al) formed at the surface of the bond coat. The coatings were subjected to a series of tests and characterization studies, including thermal cycling, isothermal oxidation, hot-corrosion and microanalyses. Results showed that the substrate temperature during deposition had great effect on the properties of the coatings. The higher the temperature of the substrate, the lower the microhardness of the gradient coating, which can be considered as the formation of microporosity in the two-phase ZrO 2 +Al 2 O 3 region in the gradient coating, and its epitaxy into the YSZ topcoat. The thermal cyclic tests of specimens were performed by exposure to air at 1323 K for 0.5 h, and then cooled to room temperature within 5 min by forced air cooling, with a lifetime of more than 500 h. In addition, compared with a conventional two-layered coating, this gradient coating exhibited a better resistance to hot-corrosion than a conventional two-layered coating.

Martensitic transformation and magnetization of Ni-Fe-Ga ferromagnetic shape memory alloys

Abstract It is shown that the martensite start temperature of Ni 2+ x Fe 1− x Ga ferromagnetic shape memory alloys increases with increasing Ni content. The thermal strain due to the reversible martensitic transformation is 0.1%. The Curie temperatures and the saturation magnetization of the alloys increase with increasing Fe content.

Co-occurrence of magnetic and structural transitions in the Heusler alloy Ni53Mn25Ga22

A co-occurrence of magnetic and structural transitions was observed in a nonstoichiometric Ni53Mn25Ga22 alloy which undergoes a reverse martensitic transformation from ferromagnetic martensite to paramagnetic austenite at a magnetic transition temperature (134 °C) higher than the Curie temperature of the stoichiometric Ni2MnGa alloy (103 °C). The effect of the magnetic field on the phase transition temperature was found to be two orders of magnitude greater in the present alloy than in Ni2MnGa due to the absence of the ferromagnetic state in austenite. This may open the possibility of utilizing NiMnGa alloys at a low magnetic field.