查看更多>>摘要：The development of highly active and acid-stable catalysts for oxygen evolution reaction (OER) is necessary and challenging. In this work, we report a highly efficient Cu-doped RuO2 OER electrocatalyst (Cu-RuO2) synthesized via one-step calcination of amorphous RuCu sample in air atmosphere. Results show that the catalytic performances of Cu-RuO2 catalysts toward OER are strongly dependent on the calcination temperature. Among the series of Cu-RuO2 catalysts, Cu-RuO2 prepared at the calcination temperature of 300 °C (Cu-RuO2-300) exhibits the highest activity for OER. At the current density of 10 mA cm?2, Cu-RuO2-300 shows an extremely low OER overpotential of 201 mV, which is much lower than the benchmark commercial RuO2 (233 mV) and most noble metal/non-noble metal catalysts reported in literature. The Tafel slope is as low as 55 mV dec?1, which is also lower than that on RuO2 catalyst (76 mV dec?1), suggesting a faster OER kinetics. In addition, Cu-RuO2-300 can maintain excellent durability in H2SO4 electrolyte. During the chronopotentiometric (CP) test, the potential of Cu-RuO2-300 catalyst shows very small attenuation (65 mV) at the time of 24 h whereas that of the reference RuO2 catalyst exhibits a 127 mV of change only within 4 h of test. Large electrochemical surface area and abundant exposed high-index facets are responsible for the high catalytic performances of the Cu-RuO2-300 catalyst. This work is instructive for the design and preparation of highly efficient and stable noble metal-based OER catalysts in acidic media.
查看更多>>摘要：Effects of a precipitation treatment on the thermoelectric properties in a CoSi thermoelectric material are investigated by scanning and transmission electron microscopy. The Seebeck coefficient and electrical conductivity are explained based on electron-phonon scattering by considering relaxation time approximation and its electronical structure in the framework of the Boltzmann transport equation in the as-sintered (AS) sample fabricated by spark plasma sintering. A precipitation treatment at 1073 K for 30 min is applied to the AS sample. The high Seebeck coefficient and electrical conductivity are obtained, while low thermal conductivity is achieved by dispersing metallic Co2Si phases and insulating amorphous SiO2 nanoparticles in addition to small grain size of ≈ 3 μm due to enhancement in phonon scattering. The dispersion of the amorphous SiO2 is also beneficial in reducing the matrix grain size by the Zener drag.
查看更多>>摘要：Many interesting physical phenomena emerge in the vicinity of phase transition temperature (Tc) for magnetic materials. However, it remains a big challenge to elucidate the near-Tc behaviors of chiral helimagnets, owing to their diversified magnetic interactions and complicated phase transitions. Here, we have systematically studied the near-Tc properties of a chiral helimagnet Cr1/3NbS2 by magnetotransport measurements with fine tuning of temperature and magnetic field. An anomalous positive magnetoresistance (MR) appears around the critical field (Hc) in a narrow temperature range (~ 128–132 K), which can be attributed to the competing scattering effects on electrons between the chiral magnetic order and thermal fluctuation near critical point. Moreover, the negative MR continues to increase even above Hc, and this behavior becomes more pronounced near Tc. Based on the MR results, a near-Tc magnetic phase diagram of Cr1/3NbS2 is presented, with a particular emphasis on the determination of internal phase boundary of chiral soliton lattice (CSL). This work gains an insight on the near-Tc behaviors of chiral magnetic materials and sheds light on the related electronic and spintronic applications.
查看更多>>摘要：One of the obstacles in obtaining high quality and high indium-molar fraction InAl(Ga)N is the higher vapor pressure of nitrogen over group-III elements, especially indium. In this work, we used a thermodynamically motivated approach to increase the nitrogen content in vapor phase through the ammonia input partial pressure and its role on the composition of indium-rich InAl(Ga)N layers is investigated. It is shown that the increase in indium molar fraction coincides with the ammonia input partial pressure and independent of the two growth regimes: surface kinetics limited and mass transport limited. In parallel, molecular dynamics based on empirical potentials is carried out in order to investigate the strain behavior resulting from such growth kinetics. It is unveiled that at the InAl(Ga)N/GaN interface, tensile strain on Al–N and Ga–N bonds is enhanced and compressive strain in In–N bonds is relaxed. In contrast, on top of a layer, Al–N and Ga–N bonds are comparatively relaxed and In–N bonds are relatively more compressed. Clearly, this work provides a comprehensive overview of the metal-organic chemical vapor deposition (MOCVD) thermodynamics of InAl(Ga)N layers.
查看更多>>摘要：The effect of annealing temperature and Zn doping on structural, magnetic, optical, dielectric, and gas sensing properties of iron oxide nanoparticles have been investigated. For this purpose, ZnxFe3?xO4 (x = 0 and 0.5) nanoparticles have been synthesized using sol-gel auto-combustion method. The samples are annealed at 200, 400, and 500 °C temperatures for 6 h. With the increase in annealing temperature, for x = 0, the transition from Fe3O4 (magnetite, cubic) to α-Fe2O3 (hematite, rhombohedral) phase is confirmed from the XRD analysis. With Zn doping, independent of annealing temperatures, for ZnxFe3?xO4 nanoparticles, the cubic spinel structure is evidently observed. The lattice constant of iron oxide nanoparticles increases with Zn2+ doping. With increase in annealing temperature and with Zn doping in iron oxide nanoparticles, the particle size decreases from 63.89 nm to 29.25 nm. With the increase in annealing temperature and doping with Zn in iron oxide, the reduction in the band gap is observed. Dielectric properties of synthesized ZnxFe3?xO4 (x = 0 and 0.5) nanoparticles have been investigated as a function of frequency. At room temperature, the sensitivity (%) as a function of flow rate have been investigated and observed that ZnxFe3?xO4 (x = 0 and 0.5) nanoparticles annealed at 500 °C achieved the highest sensitivity towards NH3 gas. For Zn doped iron oxide nanoparticles, the enhanced sensitivity is attributed to smaller particle size and increased specific surface area as compared to that of host nanoparticles.
查看更多>>摘要：Hexagonal ferrite Ba(Fe1?xScx)12O19 is an important magnetic oxide material in both science and engineering because it exhibits helimagnetism around room temperature (300 K). In this study, the magnetic phase diagram of Ba(Fe1?xScx)12O19 consisting of ferri-, heli-, antiferro-, and paramagnetic phases has been completed through magnetization and neutron diffraction measurements. The magnetic phase transition temperature to paramagnetism decreases with the increase in x, and the temperature at which the magnetization reaches a maximum, which corresponds to the magnetic phase transition from heli- to ferrimagnetism, is determined for low x crystals. The temperatures at which helimagnetism appears are precisely determined by observing the magnetic satellite reflection peaks in neutron diffraction at various temperatures, which characterize helimagnetism. Based on these results, the magnetic phase diagram of the Ba(Fe1?xScx)12O19 system is constructed in the T-x plane. Helimagnetism appears at x ? 0.06, and magnetism with antiferromagnetic components appears as the extension phase of helimagnetism at x ? 0.19 through the coexistence region. The turn angle ?0 of the helix for each x crystal is calculated from the relationship, ?0 = 2πδ, where δ is the incommensurability. The turn angle ?0 decreases with the increase in temperature for the same x crystal, and increases with the increase in x at the same temperature. Furthermore, it is found that there are clear thresholds at which ?0 cannot take values between 0°<?0 ? 90° and 170° ? ?0< 180°.
查看更多>>摘要：In this article, the CoFe2O4/MoS2 composites have been well constructed via a facial hydrothermal method, from which the porous CoFe2O4 microspheres were embedded into MoS2 ultrathin nanosheets. As a typical two-dimensional transition metal sulfide, MoS2 with high electrical conductivity and flexible properties is expected to exhibit high dielectric loss and magnetic loss when combined with magnetic material CoFe2O4. Meanwhile, the unique hierarchical structure of as-synthesized MoS2 nanoflowers is conducive to scattering and reflection of electromagnetic waves, and endows designed composites with competitive microwave absorption properties. Notably, although the saturation magnetization of MoS2/CoFe2O4 composites is significantly decreased compared to pure CoFe2O4 microspheres, the coercivity still maintains a high value of 626.1 Oe. As expected, the results demonstrate that as-fabricated MoS2/CoFe2O4 composites do have superior microwave absorption properties in comparison with CoFe2O4 and MoS2. It is found that the complex permittivity is enhanced with increasing filling ratio from 45 wt% to 60 wt%, which is helpful for obtaining a moderate permittivity value. When the filling ratio is 50 wt%, the MoS2/CoFe2O4 composites exhibit a minimum reflection loss (RLmin) of ? 53.1 dB at 12.08 GHz when the thickness is 2.5 mm. An effective absorption bandwidth less than ? 10 dB of 6.61 GHz from 11.28 to 17.89 GHz is achieved with a thin thickness of 2.2 mm, almost covering the whole Ku-band. The improvement in microwave absorption properties can be ascribed to the synergistic effect of the magnetic CoFe2O4 and dielectric MoS2. The impedance matching characteristic and interfacial polarization are significantly optimized by inserting CoFe2O4 spheres onto the surface of MoS2 nanosheets. Considering the excellent performance of as-fabricated MoS2/CoFe2O4 composites, it is believed that the MoS2-based composites can be applied as a promising candidate of highly effective microwave absorbers with strong absorption intensity and broad absorption frequency.
查看更多>>摘要：To improve microstructure and fracture toughness of MASC alloy, the effects of element substitution of Zr for Hf on the microstructure and room-temperature fracture toughness in Nb-24.7Ti-16Si-xHf-yZr-2.0Cr-1.9Al (x + y = 8.2) alloys have been investigated in this study. The results show that the substitution of Zr for Hf in MASC alloy promotes the eutectic reaction of (Nb, X)3Si phase → Nbss/γ(Nb, X)5Si3 eutectic, and shifts the eutectic point to higher Si content, which leads the microstructure transformation from hypereutectic structure to nearly eutectic structure. The primary γ(Nb, X)5Si3 phase content decreases and Nbss phase content increases with the increase of Zr content. The room-temperature fracture toughness increases with the increase of Zr content, and the room-temperature fracture toughness values of 4Zr-4.2Hf, 6Zr-2.2Hf and 8.2Zr-0Hf alloys are 16.03–16.29 MPa m1/2. The room-temperature fracture toughness value of 16.29 MPa m1/2 is the highest value in the nondirectionally solidified Nb-Si based alloys.
查看更多>>摘要：The hot tensile tests of Ti-6Al-4V alloy sheet at different temperatures (650, 700, 750 ℃) and different strain rates (0.1 s?1, 0.01 s?1, 0.001 s?1) were carried out, five phenomenological models are established (Arrhenius with strain compensation(ARR), Johnson-Cook(JC), modified JC(m-JC), Hensel-Spittel(HS), modify HS(m-HS)). A new JC model considers the softening coefficient m as a function of strain rate and adds a temperature correction function is proposed. The electron backscatter diffraction (EBSD) test was used to observe the microstructure evolution after thermal deformation at 700 ℃-0.1 s?1. After high-temperature tensile loading, the proportion of β phase increased, the average grain size decreased by 67.69%. The macroscopic softening phenomenon is caused by dynamic recrystallization (DRX), dynamic recovery (DRV), and the formation of β phase. The prediction accuracy of the six models was discussed by comparing the AARE value and the R-value. The comparison results show that the HS model has the best prediction accuracy, and its AARE value and R-value are 5.24% and 0.98794; the JC model has the worst prediction ability, and its AARE value and R-value are 41.1% and 0.84726. The prediction accuracy of the n-JC model is higher than that of the other JC models.
查看更多>>摘要：Commercial supercapacitors are primarily assembled using carbon materials and biomass-derived carbon is one of the most promising carbon materials in the field of supercapacitors. To meet the growing market demand for energy storage devices, a dung beetle forewing-derived carbon (DBFC) material is first prepared using simple a pyrolysis method. The as-prepared DBFC possesses a hierarchical porous structure, nitrogen and oxygen self-doping, and a large surface area. To investigate its electrochemical performance, the DBFC was used to prepare a supercapacitor electrode. A DBFC electrode prepared with a KOH to biomass-derived carbon mass ratio of 3:1 at 700 °C (DBFC-3-700) exhibits a high specific capacitance of 348 F g?1 at 0.5 A g?1 in 6 M KOH liquid electrolyte. The DBFC-3-700 electrode was further assembled into a solid-state symmetric supercapacitor that displays a high specific capacitance of 260 F g?1 at 0.5 A g?1, good cycle stability, and excellent rate capability. Furthermore, three solid-state supercapacitors connected in series can successfully power red light-emitting diodes, demonstrating potential applications in the field of portable and wearable devices.
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Elsevier