Jun Hu

Tsinghua University, Peping, Beijing, China

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Publications (127)186 Total impact

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    ABSTRACT: The present study examined the electrical properties of ZnO varistor ceramics co-doped with Ga2O3 and Al2O3, in particular, the current-voltage characteristics under small and intermediate currents and upturn characteristics under a large current. With increasing amounts of Ga2O3 dopant at a given Al2O3 concentration, both the threshold voltage in the small-current region and nonlinear coefficient of the varistor ceramics increased and then decreased; in contrast, the leakage current decreased and then increased. Moreover, ZnO varistor ceramics with low residual voltage ratio, high nonlinearity, and low leakage current were obtained under an optimal Ga concentration of 0.72 mol% while the Al additive content was fixed at 0.1 mol%. This novel finding will be helpful towards the manufacture of high-quality ZnO varistors.
    No preview · Article · Feb 2016 · Materials Letters
  • Bin Dang · Jinliang He · Jun Hu · Yao Zhou
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    ABSTRACT: The accumulation of space charge in high voltage direct current (HVDC) cable will bring the insulation to failure, and the addition of nanoparticles can strongly improve the space charge distribution characteristics inside the cable insulation, but particle agglomeration and cavitation lead to difficulty in controlling the properties of nanocomposites. In this paper, polypropylene (PP)/propylene-ethylene copolymer (PEC) and PP/ethylene-octene copolymer (EOC) blends were prepared by mechanical blending in order to improve both mechanical property and space charge distribution. Dynamic mechanical thermal analysis (DMTA) shows that both blends have excellent mechanical properties for recyclable power cable. Pulsed electro-acoustic (PEA) and thermally stimulated depolarization current (TSDC) tests illustrate that PP/EOC blends significantly decrease space charge accumulation and remarkably increase the trap density in the bulk than PP and PP/PEC blends. The increase of the trap density in PP/EOC blends can be explained as the result of the shallow traps introduced by the crystalline-amorphous interfaces existing in the boundaries of spherulites. The shadow traps can act as hopping sites to improve the transportation of space charges.
    No preview · Article · Jan 2016 · Polymer International
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    ABSTRACT: This paper deals with the electrical characteristics of rare-earth-doped ZnO varistor ceramics. Multiple donor dopants (Al3+, Ga3+, and Y3+) were employed to improve the comprehensive performance of ZnO varistor ceramics. The leakage current of rare-earth-doped ZnO varistor ceramics decreased noticeably with Ga2O3 dopants. The Ga3+ dopant occupies the defect sites of grain boundaries and increases the barrier potential of ZnO varistor ceramics, so the leakage current is effectively inhibited. Y2O3 is primarily located around the grains, which restrains ZnO grain growth, increasing the voltage gradient. The Al3+ goes into the lattices of ZnO grains, decreasing the grain resistance; thus, the residual voltage ratio can be controlled at low levels under a high impulse current. With the combined incorporation of Al3+, Ga3+, and Y3, excellent electrical properties of ZnO varistor ceramics can be acquired with a nonlinearity coefficient of 87, voltage gradient of 517 V/mm, leakage current of 0.96 μA/cm2, and residual voltage ratio of 1.60. These rare multiple donor dopants can aid in engineering high-quality ZnO varistors.
    No preview · Article · Jan 2016 · Journal of the American Ceramic Society
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    Bin Dang · Jinliang He · Jun Hu · Yao Zhou
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    ABSTRACT: Cross-linked polyethylene (XLPE) is a thermosetting material that cannot be recycled at the end of its lifetime. This study investigated the potential of syndiotactic polypropylene (sPP)/silica as an ecofriendly extruded insulation system for HVDC cables. We investigated the morphology, Fourier transform infrared, and thermal, thermomechanical, and electrical behaviors of sPP modified with 0.5–3% nanosilica. We found that the silica/sPP nanocomposite without cross-linking offered a suitable mechanical modulus at room temperature and sufficient intensity at high temperatures, and adding nanosilica modified by a silane coupling agent to the sPP resulted in significant DC resistivity and space charge improvement. The optimal nanosilica content in the sPP was determined by balancing the mechanical and thermomechanical characteristics and the DC resistivity. The sPP/silica nanocomposite reported here shows great potential as a candidate insulation material for future ecofriendly extruded HVDC cables.
    Full-text · Article · Dec 2015 · Journal of Nanomaterials
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    ABSTRACT: This paper studied how to improve the high-impulse current discharge capability of ZnO varistors by doping with gallium oxide. In particular, the current–voltage characteristics under small and upturn current characteristics were examined. In the small current region, the gallium dopant is available to improve the surface state density and the potential barrier, which inhibits the leakage current from increasing and leads to the improvement of the stability of ZnO varistors in operation. In the upturn region, the gallium dopant makes the I–V curve shift right, so the nonlinearity region is extended and the impulse current discharge capability of ZnO varistors is greatly improved. High-performance ZnO varistor ceramics with optimal compositions were obtained under 0.42 mol% Ga2O3; their nonlinearity coefficient and voltage gradient under 1 mA reached 80 and 432 V/mm, respectively. This finding is helpful for dramatically improving the protective effect of surge protection devices assembled with ZnO varistors as the core element in electrical systems and greatly increasing the safety of electrical systems.
    No preview · Article · Dec 2015
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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: Insulation performance of the dielectrics under extreme conditions always attracts widespread attention in electrical and electronic field. How to improve the high-temperature dielectric properties of insulation materials is one of the key issues in insulation system design of electrical devices. This paper studies the temperature-dependent corona resistance of polyimide (PI)/Al2O3 nanocomposite films under high-frequency square-wave pulse conditions. Extended corona resistant lifetime under high-temperature conditions is experimentally observed in the 2 wt% nanocomposite samples. The “thermal stabilization effect” is proposed to explain this phenomenon which attributes to a new kind of trap band caused by nanoparticles. This effect brings about superior space charge characteristics and corona resistance under high temperature with certain nano-doping concentration. The proposed theory is experimentally demonstrated by space charge analysis and thermally stimulated current (TSC) tests. This discovered effect is of profound significance on improving high-temperature dielectric properties of nanocomposites towards various applications.
    Full-text · Article · Nov 2015 · Scientific Reports
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    Lei Gao · Jinliang He · Jun Hu · Chao Wang

    Full-text · Dataset · Nov 2015
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    ABSTRACT: Advanced sensing techniques are key technologies required for the smart grid. Magnetic tunnel junction (MTJ) sensors hold great promises in the distributed linear current sensing, with small size, low price, and excellent performances. In this paper, MTJ Wheatstone bridge sensors were fabricated, attaining a typical sensitivity of 4.94 mV/V/Oe with an offset of -2.84 mV/V. A contactless current sensor based on the MTJ sensor was designed and realized, achieving a sensitivity of 7.75 mV/A and an offset of -8.68 mV. We can adjust the sensitivity and the linear range of the sensor by changing the gap of the magnetic core, and make the sensor much less susceptible to electrical wire's location and orientation. This paper provides a practical current sensing technique for smart grid applications.
    No preview · Article · Nov 2015 · IEEE Transactions on Magnetics
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    Lei Gao · Jinliang He · Jun Hu · Chao Wang
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    ABSTRACT: Microcapsule-based self-healing polymer materials are highly desirable because it can heal large-volume cracks without changing the original chemical structures of polymers. However, it's limited by processing difficulties and inhomogeneous distributions of two components. Herein, we report a one-component photo-responsive self-healing polymer composites with photo-absorbing hybrid microcapsules (PAHM), which gives the microcapsules photo-absorbing properties by introducing nano-TiO2 particles as photo-absorbing and emulsified agent in the poly(urea-formaldehyde)/TiO2 hybrid shells. Upon mechanical damages and then exposed to light, the photo-responsive healing agents in the cracks will be solidified to allow for self-healing, while the healing agents in the unbroken PAHM will be protected and remain unreacted, which endows this photo-responsive microcapsule-based self-healing composite with self-healing properties like the conventional two-component microcapsule-based systems do. Given the universality of this hybrid polymerization method, incorporation of the photo-absorbing particles to conventional polymer shells may further broaden the scope of applications of these widely used materials.
    Full-text · Article · Oct 2015 · ACS Applied Materials & Interfaces
  • Xue Fen · Jun Hu · Shan X. Wang · Jinliang He
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    ABSTRACT: In this paper, we report on the new design of an electric field sensor based on piezoelectric bending effect in a capacitive-sensing method. The bipolar electric field sensor with a range from −22 to 22 kV/cm concentrates on a bending structure to enlarge the piezoelectric effect and to control its bending orientation. It performs with a much higher sensitivity of 0.448 pF/(kV/cm) or 5.78%/(kV/cm) and has a better resolution of 7.0 V/cm at $E=0$. Factors influencing the capacitance response are also discussed, which might further optimize the sensitivity. Moreover, the analytical model and experimental results make an agreement on the reasonability of this electric field sensor, gaining an insightful guide to the potential capacitive-sensing electric field chip sensor widely distributed in sensor networks.
    No preview · Article · Sep 2015 · IEEE Transactions on Industrial Electronics
  • Yao Zhou · Jinliang He · Jun Hu · Bin Dang
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    ABSTRACT: Polypropylene (PP)/polyolefin elastomer (POE) blends and MgO/PP/POE nanocomposites were fabricated by melt blending. The morphology, mechanical, and electrical properties of the nanocomposites were investigated. Scanning electron microscopy showed that the surface-modified MgO nanoparticles were well dispersed in the polymer matrix at low loadings of less than 3 phr. X-ray diffraction demonstrated that the crystalline phases of PP in the composites were changed and that the β phase significantly increased. An examination of the electrical properties revealed that the direct-current (dc) electric breakdown strength and space-charge suppression effect were remarkably improved by the introduction of the surface-modified MgO nanoparticles. In addition, obvious enhancements in the tensile modulus and strength were obtained as a result of the synergistic toughening of the POE and MgO nanoparticles. Thus, MgO/PP/POE nanocomposites with enhanced mechanical and electrical properties have great potential to be used as recyclable insulation materials for high-voltage dc cables with large transmission capacities and high operating temperatures. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42863.
    No preview · Article · Sep 2015 · Journal of Applied Polymer Science
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    ABSTRACT: In the fabrication of linear MTJ sensors, the utilization of magnetic bias field along the easy-axis direction makes the response of MTJ nonlinear, and the Neel and magnetostatic coupling usually induces a strong interlayer coupling field along the hard-axis direction. In this paper, we investigated the influence of hard-axis and easy-axis bias field on the linearity of MTJ magnetic sensor. The prediction and optimization of linearity were done by applying an appropriate hard-axis bias field in the specific easy-axis bias field and practical TMR ratio, and the model was verified by experiments. This work can give guidance to improve the linearity and to maximize the practical TMR ratio of the MTJ sensors under a given linearity requirement.
    No preview · Article · Jul 2015 · IEEE Transactions on Magnetics
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    ABSTRACT: An advanced metering and monitoring system based on autonomous, ubiquitous and maintenance-free wireless sensor networks is of great significance to the smart grid. However, the power supply for sensor nodes (especially those installed on high-voltage side) remains one of the most challenging issues. To date, miniaturized, reliable, low-cost and flexible designs catering the massive application of self-powered sensor nodes in the smart grid are still limited. This paper presents a nonintrusive design of power supply to support the sensor network applied in the smart grid. Using a cantilever-structured magnetoelectric (ME) composite, the energy harvester is able to scavenge energy from the power-frequency (50 Hz) magnetic field distributed around the transmission line. Design considerations for this specific type of scavenger have been discussed, and optimized energy harvester prototypes have been fabricated, which are further tested on a power line platform. Experimental results show that the single-cell and double-cell energy harvesters are capable of producing 0.62 mW and 1.12 mW at 10 A, respectively, while corresponding power outputs are enhanced to 4.11 mW and 9.40 mW at 40 A. The good energy harvesting ability of this particular ME composite indicates its great potential to make a nonintrusive, miniaturized, flexible and cost-effective power supply, which possesses great application prospects in the smart grid.
    Full-text · Article · Jul 2015 · IEEE Transactions on Industrial Electronics
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    Xiao Yang · Jinliang He · Jun Hu
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    ABSTRACT: Silicone composite filled with zinc oxide microvaristors possesses excellent nonlinear conducting behavior as ZnO varistor does. For better adjusting the composite's electrical behavior to satisfy the practical field-grading requirement, this article studied the influence of ZnO filler's property on the nonlinearity of the composite. Several groups of ZnO-silicone composite samples in different filler volume fraction and filler diameter were prepared, the measured J-E characteristics show that the percolation threshold of ZnO-silicone composite is around 35%, above which the composites present reliable nonlinear behavior. The switching voltage of the composite exhibits a considerable decrease as filler's diameter increases or filler's volume fraction increases, while the nonlinear coefficient remains stable. Moreover, filler's size also has a little influence on composite's percolation limit. The conclusion above fits very well with the theory of the conducting composites and percolation process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42645.
    Full-text · Article · Jul 2015 · Journal of Applied Polymer Science
  • Fen Xue · Jun Hu · Shan X. Wang · Jinliang He
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    ABSTRACT: To improve the direct and converse magnetoelectric effects, a magnetic bias field is essential to first excite the magnetization and then determine its sensitivity to a large extent. Based on the minimum energy principle, a ferromagnetic phase model is constructed and amorphous and nanocrystalline alloys are then adopted as example materials to perform numerical calculations to optimize the magnetoelectric effect. Experimental verification of the model showed that the mechanical tensile strain determines the optimum bias in a nonlinear but bijection-type correlation. To improve the magnetoelectric effect in general terms, different ranges of induced strain require a specific optimum bias that can be precisely calculated using the model solution derived in this letter.
    No preview · Article · Jun 2015 · Applied Physics Letters
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    ABSTRACT: This experimental study reported electrical properties of linear low density polyethylene (LLDPE)/MgO nanocomposites, which were prepared by melt blending methods. The effects of surface modified MgO nanoparticles on the microstructure, space charge distribution, thermally stimulated current and DC breakdown strength of the nanocomposites were investigated. The addition of surface modified nanoparticles increases the amount of spherulites and decreases their sizes. It is found that the LDPE/MgO interface shows significant influence on electrical properties of nanocomposites. The addition of MgO nanoparticles is available to suppress the production of space charges and enhance the DC breakdown strength, depending on the loading levels of nanoparticles. Thermally stimulated currents of nanocomposites reveal strong correlation between the traps and electrical properties of nanocomposites. It is believed that this study would provide important hint to design and develop advanced polymer nanocomposites for dielectric applications, in particular the HVDC applications.
    Full-text · Article · Jun 2015 · IEEE Transactions on Dielectrics and Electrical Insulation
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    ABSTRACT: The application of giant magnetoresistance (GMR) current sensors in power grid and other industrial fields has a great prospect benefitting from their wide bands, high sensitivity, and good linearity. This paper studies the influence of mobile ions on current measurement of GMR sensor under high external electric field. The R-H curves of GMR multilayer sensor were depicted when the external electric and magnetic fields were both changed under three orthogonal electric field orientations. The experiment results indicate slightly varying resistances of GMR sensor when the external electric field was changed, and simulation analysis reveals that the resistance variation is attributed to the movement of surface ions under high external electric field. Therefore, a Faraday box is essential for GMR sensors to avoid interferences under high-strength field conditions, which is especially significant for their application as current sensors of the power grid.
    Full-text · Article · May 2015 · Journal of Applied Physics
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    ABSTRACT: Wireless sensors capable of scavenging energy from ambient environment have been increasingly attractive for their outstanding merits of self-sufficient and maintenance-free. This paper presents a specific design of magnetic energy harvester based on a piezoelectric/magnet composite and a magnetic concentrator. With the employment of concentrator, the energy harvesting properties have been greatly improved, which is theoretically analyzed and experimentally demonstrated with the 35 times power enlargement. The fabricated prototype with a 3 cm air-gap concentrator harvests 326 μW power at 10 Arms, which enables sufficient and reliable power supply for a wide range of low-power sensors.
    Full-text · Article · May 2015 · Journal of Applied Physics