Jinwen Ye

Facile rapid synthesis of a nanocrystalline Cu2Te multi-phase transition material and its thermoelectric performance

A highly efficient facile method was used to synthesize Cu2Te via spark plasma sintering (SPS) of high-energy ball-milled elemental powders, followed by annealing, which dramatically reduced the whole process time. The multiple phase transitions of Cu2Te over an extensive temperature range were determined using a combination of high temperature X-ray diffraction (HTXRD) and differential scanning calorimetry (DSC). This merit endows Cu2Te with the ability to trigger multiple critical scatterings from room temperature up to 900 K. Microscopic investigation showed that the Cu2Te prepared by this rapid method possesses tiny dispersive precipitates, with sizes in the range of 50–100 nm, that could serve as obstacles for the transfer of mid-wavelength phonons. As a result, a ZT value of ∼0.52 at 1000 K was obtained for the sample annealed for 10 h after SPS.

Synergistic optimization of carrier transport and thermal conductivity in Sn-doped Cu2Te

Pristine Cu2Te is a highly degenerate metallic-behaving semiconductor that exhibits poor thermoelectric performance due to its ultrahigh hole concentration derived from the intrinsically high concentration of cation vacancies. Here, through doping with Sn, the electronic transport properties of Cu2−xSnxTe are significantly promoted due to the hole concentration reduction stemming from more valence electrons being contributed by Sn; the thermal conductivity is simultaneously decreased for the same reason. Consequently, a record high zT of 1.5 was achieved in p-type Cu1.90Sn0.10Te at 1000 K, which is due to a synergistic optimization in power factor and thermal conductivity. Combined with the density functional theory (DFT) calculations, such an improvement could be mainly attributed to the increase in the density of states (DOS) in the valence band edge of Cu2−xSnxTe, which originated from the hybridization of the lone pair electrons of the Sn 5s orbital with the Cu 3d and Te 5p electrons. Additionally, transmission electron microscopy revealed the existence of a high density of twist boundaries and dislocations that may have enhanced the phonon scattering, therefore, contributing to the reduced thermal conductivity. This study explores a synergistic optimization strategy to simultaneously boost the power factor and suppress thermal conductivity, providing a feasible method to achieve high thermoelectric performance in intrinsically highly degenerate semiconductors with metallic behavior.

Electromagnetic Wave Absorption Properties of RE-Fe Nanocomposites

Recently, the number of communication devices that utilize gigahertz range microwave radiation, such as mobile phones and LAN systems, has greatly increased. However, electromagnetic interference (EMI) has become serious. One promising technique to prevent EMI is the use of microwave absorption materials. However, the applications of conventional microwave absorption materials are limited. The reasons are that Snoek’s limit of spinel-tripe ferrites is so small that the imaginary part of permeability is considerably lowered in GHz range, and metallic soft-magnet materials have high electric conductivity, which makes the high frequency permeability decreased drastically due to the eddy current loss induced by EM wave. The Nd2Fe14B/-Fe composites is composed of soft magnetic -Fe phase with high MS and hard magnetic Nd2Fe14B phase with large HA, consequently their natural resonance frequency are at a high frequency range and permeability still remains as a large value in high frequency range. Furthermore, the electric resistivity of Nd2Fe14B is higher than that of metallic soft magnetic material, which can restrain the eddy current loss. Thus, the authors have already reported that Nd2Fe14B/-Fe composites can fuction as a microwave absorber. In this present work, the electromagnetic and absorption properties of the Nd2Fe14B/-Fe nanocomposites were studied in the 0.5–18 and 26.5–40 GHz frequency ranges. Moreover, the effect of rare earth Nd content on natural resonance frequency and microwave permeability of Nd2Fe14B/Fe nanocomposites was reported in this chapter. The results show that it is possible to be a good candidate for thinner microwave absorbers in the GHz range. In order to restrain the eddy current loss of metallic soft magnetic material, Sm2O3 and SmN was introduced in Sm2O3/α-Fe and SmN/α-Fe composites as dielectric phase, and Sm2Fe17Nx with high magnetocrystalline anisotropy was introduced in SmN/αFe/Sm2Fe17Nx as hard magnetic phase. Accordingly, Sm2O3/α-Fe and SmN/αFe/Sm2Fe17Nx are possible to be another good candidate for microwave absorbers in the GHz range as the authors reported in reference. Therefore, the purpose of this study is to investigate the microwave complex permeability, resonant frequency, and microwave absorption properties of nanocrystalline rare-earth magnetic composite materials Sm2O3/αFe and SmN/α-Fe/Sm2Fe17Nx. The absorption performance and natural resonance frequency can be controlled by adjusting phase composite proportion and optimizing the microstructure.

Effect of Cr Substitution on Thermal Stability of Magnetization and Oxidation Resistance of SmFeN Compounds

We investigated the effects of Cr element substitution on thermal stability of magnetic flux and oxidation resistance of SmFeN compounds by X-ray diffraction (XRD), thermogravimetry analysis (TG), and flux aging loss measurement methods. We found that with increases in Cr substitution, the irreversible flux losses of bonded magnets made from these alloys decrease remarkably. When Cr substitution reaches 2 at%, the beneficial effect reaches a maximum; the irreversible flux loss of such bonded magnets exposed to a temperature 170degC for 2 h is only 4.97%. The oxidation resistance of SmFeN magnetic powders is also notably improved with the increased Cr substitution .