Huaizhou Zhao

Study on the preparation techniques and properties of bonded magnetostrictive materials

The preparation and measurement of the magnetoelastic properties of bonded Terfenol-D and Pr0.15Tb0.3Dy0.55Fe2 composites are reported. The experiments were carried out in various conditions, including compaction pressure and binder proportion in order to optimize the preparation method. The typical conditions are with the compaction pressure between 48 and 80 MPa and a weight ratio of binder to powder of 4:100. Large magnetostrictions of 1186×10−6 at 450 KA/m for the Terfenol-D composite with an applied compressive stress of 10 MPa and 900×10−6 at 900 KA/m for the Pr0.15Tb0.3Dy0.55Fe2 composite have been obtained with the following conditions: particle size of 165 μm, weight ratio of binder to powder of 4:100, and a compaction pressure of 80 MPa.

Synthesis and thermoelectric properties of half-Heusler alloy YNiBi

The half-Heusler (HH) alloy, YNiBi, was synthesized through a reaction between Bi and the intermediate YNi phase. The thermoelectric properties of HH YNiBi were measured most thoroughly. A moderate power factor of 13.3u2009μWcm−1K−2 was achieved at 485u2009K, and rather low lattice thermal conductivity was identified, consistent with the theoretical expectation for YNiBi. A significant bipolar contribution to thermal conductivity was observed in YNiBi, which induces a relatively low thermoelectric dimensionless figure-of-merit zT in this material. Enhancement of zT for YNiBi could be realized through appropriate doping to suppress the bipolar effect.

Enhancement of the thermoelectric properties of MnSb2Se4 through Cu resonant doping

MnSb2Se4 is a narrow band semiconductor having large Seebeck coefficients and intrinsically low thermal conductivities, but modest thermoelectric zT values due to having low carrier concentrations and high electrical resistivity. Here, we report that Cu substituted Mn1−xCuxSb2Se4 (0 ≤ x ≤ 0.35) materials display resonant doping behavior, leading to significantly enhanced power factors (PFs) and overall thermoelectric zT values in the measured temperature range. For the optimized composition Mn0.75Cu0.25Sb2Se4, the PF reaches 0.26 mW m−1 K−2 at 773 K, coupled with low thermal conductivities of 0.61 W K−1 m−1 to 0.32 W K−1 m−1 over the measured temperature range. A peak zT of 0.64 at 773 K is achieved, which is a 100% increase in comparison to undoped MnSb2Se4. Such a high zT has rarely been seen in thermoelectric materials with a low symmetry of crystallization, implying that Cu-doped MnSb2Se4 could be considered as a new platform in thermoelectric research for intermediate temperature power generation.

Growth-induced uniaxial magnetic anisotropy in Co/Cu(100)

Growth-induced uniaxial magnetic anisotropy of Co/Cu(100) films were investigated using surface magneto-optic Kerr effect and scanning tunneling microscopy (STM). We found that the Co films off-grown at 230 K show an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the growth incident plane. STM measurements show that the low temperature grown Co film consists of smaller islands without obvious anisotropic roughness or elongated islands. This result implies that the dipolar interaction does not dominate the uniaxial magnetic anisotropy. The CO absorption experiment further suggests that the uniaxial magnetic anisotropy originates from the magnetocrystalline step anisotropy of the Co film surface.

Thermoelectric Properties of Heavily Doped n-type Pb1−xYxTe Compounds

Structure, transport and thermoelectric properties of Y-doped PbTe samples are reported. The combined analysis of powder x-ray diffraction patterns and scanning electron microscopy images indicates that the Pb1−xYxTe samples with xxa0=xa00.015, 0.02, 0.04, and 0.06 are single phase with the NaCl-type structure. Hall effect measurements reveal that all the samples are heavily doped n-type thermoelectric materials. The substitution of trivalent Y for bivalent Pb provides additional electrons to the PbTe matrix, leading to an increase in carrier concentration at room temperature ranging from 4.36xa0×xa01019xa0cm−3 for the sample with xxa0=xa00.015 to 2.50xa0×xa01020xa0cm−3 for the sample with xxa0=xa00.06. Both electrical resistivity and the Seebeck coefficient decrease with the increase of Y content. Meanwhile, the total thermal conductivity presents a notable and unsurprising decline with the increase of Y content, which can be ascribed to the mass fluctuation effect. An optimized figure of merit, ZT, of 1.0 is achieved at 831xa0K for the sample with xxa0=xa00.02.

Improved thermoelectric performance in p-type Bi0.48Sb1.52Te3 bulk material by adding MnSb2Se4 *

Bismuth telluride (BiTe based alloys, such as p-type Bi Sb Te , have been leading candidates for near room temperature thermoelectric applications. In this study, Bi Sb Te bulk materials with MnSb Se were prepared using high-energy ball milling and spark plasma sintering (SPS) process. The addition of MnSb Se to Bi Sb Te increased the hole concentration while slightly decreasing the Seebeck coefficient, thus optimising the electrical transport properties of the bulk material. In addition, the second phases of MnSb Se and Bi Sb Te were observed in the Bi Sb Te matrix. The nanoparticles in the semi-coherent second phase of MnSb Se behaved as scattering centres for phonons, yielding a reduction in the lattice thermal conductivity. Substantial enhancement of the figure of merit, ZT, has been achieved for Bi Sb Te by adding an Mn Cu Sb Se (2 mol%) sample, for a wide range of temperatures, with a peak value of 1.43 at 375 K, corresponding to improvement over its Bi Sb Te counterpart. Such enhancement of the thermoelectric (TE) performance of p-type Bi Te based materials is believed to be advantageous for practical applications.

Enhancement of Thermoelectric Properties of Molybdenum Diselenide Through Combined Mg Intercalation and Nb Doping

Thermoelectric properties of MoSe2.1 were enhanced through a combination of Mg intercalation and Nb doping. Magnesium intercalation simultaneously enhances the Seebeck coefficient and electrical conductivity, owing to a favorable modification of band structure upon Mg intercalation. And Nb substitution on the Mo site increases carrier concentration by two orders of magnitude, in addition to reducing the thermal conductivity of the lattice. With systematic study of the anisotropic thermal and electrical transport properties, an optimized ZT of 0.2 was achieved at 888xa0K for a Nb0.03 Mo0.97Se2.1Mg0.2 sample along its out-of-plane direction, far exceeding the ∼0.01 value for intrinsic MoSe2.1. While 2 dimensional (2D) transitional-metal dichalcogenides with layered structure have been extensively studied for the fields of ion batteries, optical and electronic devices, and so on, enhancement of thermoelectric properties for these intrinsic semiconductors has rarely been investigated.

Perpendicular anisotropy in the amorphous TbCo/Si multilayers

The TbCo/Si multilayers prepared by the rf magnetron sputtering system with various Si thickness have been investigated. X-ray diffraction, magnetic measurement and Kerr rotation have been performed. No antiferromagnetic coupling was found for the system. With the thickness of Si layer tSi increasing, the perpendicular anisotropy constant Ku, and the saturation magnetization Ms decreased rapidly. It was assumed that Co2Si and Tb had been formed in the interfacial zone between TbCo and Si layers due to the interlayer diffusion. The decreasing of Ms is attributed to the decreasing of the effective thickness of magnetic layer.

In situ conductivity study of the phase transition in Sb-doped C60

C60/Sb bilayers were prepared on the substrate of mica, and their electrical properties were investigated by in situ dc conductivity measurements. The results indicate that the Sb doping in C60 significantly affects the critical temperature (Tc) for the orientational order–disorder transition of C60. The Tc of Sb-doped C60 increases to about 278 K, ∼18 K higher than that of the pristine C60. This transition is sensitive to Sb content and disappears upon annealing. A possible mechanism of such a phase transition is discussed.

New method for preparing graphene by peeling graphite and facile fabrication of bulk Bi0.45Sb1.55Te3.02/graphene composites with dense texture and high ZT

We report a new method for peeling graphite to graphene, with which we develop a facile procedure for the fabrication of bulk Bi0.45Sb1.55Te3.02/graphene by pushing thin graphite foils into pressed Bi0.45Sb1.55Te3.02 powders and then foliated into graphene under pressure and high direct current (up to 1000 A). The pushing force results from the huge repulsive Coulomb force between the layers in the graphite foil. The Coulomb force arises from electron agglomeration as a result of the Lorentz force that the large direct-current-produced magnetic field applies on the moving electrons in the graphite foil. The incorporated graphene sheets act as growth templates for the Bi0.45Sb1.55Te3.02 grains. Consequently, fast grain growth and a densely textured microstructure with laminates were observed in the Bi0.45Sb1.55Te3.02/graphene bulk. By combining direct current and applied pressure, anisotropic textures were obtained, with the laminates oriented mostly along the axial direction. Since the laminates can filter low-energy carriers and scatter long-distance phonons, an increased Seebeck coefficient, decreased thermal conductivity, and a consequential 25% enhancement in the figure of merit, ZT (1.40 at 90.9 °C), were observed in the direction along which pressure was applied. This work suggests that graphene can be utilized as a template to rapidly grow single crystals of materials with similar crystal structures, as well as to adjust the textures of materials. This facile method is expected to be applicable in the fabrication of bulk semiconductor or graphene/alloy composites.