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Experimental and numerical analysis of the multi-recessed gate structure for microwave silicon carbide power MESFETs

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Chinese Physics B 2009年第7期18卷 3018-3023页

作者：邓小川冯震张波李肇基李亮潘宏菽 state Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China The National Key Laboratory of ASIC

This paper reports that multi-recessed gate 4H-SiC MESFETs （metal semiconductor filed effect transistors） with a gate periphery of 5-mm are fabricated and *** multi-recessed region under the gate terminal is applied to improve the gate-drain breakdown voltage and to alleviate the trapping induced instabilities by moving the current path away from the surface of the *** experimental results demonstrate that microwave output power density,power gain and power-added efficiency for multi-finger 5-mm gate periphery SiC MESFETs with multi-recessed gate structure are about 29%, 1.1dB and 7% higher than those of conventional devices fabricated in this work using the same process.

关键词： multi-recessed microwave power 4H-SiC MESFETs

Magnetization of Multifilamentary Superconductor Nb_3Sn in Perpendicular Field

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Plasma Science and Technology 2008年第6期10卷 748-753页

作者：刘方翁佩德武玉龙凤 Institute of Plasma Physics Chinese Academy of SciencesHefei 230031China

Hysteresis loss is one of the electromagnetic characteristics controlled by time evolution of magnetic field and current distribution inside the conductor. Brandt＇s method allows us to model the interaction of the conductor with an external magnetic field. Instead of the constant critical current density （Jc =CONST）, the Jc scaling law from current-voltage （I-V） measurement is used to model the magnetization loop. By comparing the calculated results with the measured data, it is shown that the Jc scaling law, i.e. the deviatoric strain model, is not useful in a very low field. To solve this problem, the Kim model about Jc as a function of applied field has been applied in the low field case. This method can be used to predict the hysteresis loss of Nb3Sn filamentary strand.

关键词： magnetization hysteresis loss Nb3Sn critical current density n-value

Gain of harmonic generation in high gain free electron laser

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Chinese Physics C 2008年第4期32卷 297-302页

作者：邓海啸戴志敏 Shanghai Institute of Applied Physics CAS

In a planar undulator employed free electron laser （FEL）, each harmonic radiation starts from linear amplification and ends with nonlinear harmonic interactions of the lower nonlinear harmonics and the fundamental radiation. In this paper, we investigate the harmonic generation based on the dispersion relation driven from the coupled Maxwell-Vlasov equations, taking into account the effects due to energy spread, emittance, betatron oscillation of electron beam as well as diffraction guiding of the radiation field. A 3D universal scaling function for gain of the linear harmonic generation and a 1D universal scaling function for gain of the nonlinear harmonic generation are presented, which promise rapid computation in FEL design and optimization. The analytical approaches have been validated by 3D simulation results in large range.

关键词： harmonic FEL gain length

Synthesis and photoluminescence properties of octahedral K_2(Ge,Si)F_6:Mn^(4+) red phosphor for white LED

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Journal of Rare Earths 2016年第12期34卷 1173-1178页

作者：金玉明刘荣辉陈观通刘元红马小乐邵冷冷王磊 National Engineering research Center for Rare Earth Materials General Research Institute for Nonferrous Metalsand Grirem Advanced Materials Co.Ltd.

A novel red-emitting K2（Ge,Si）F6：Mn^4＋ phosphor with uniform morphology was synthesized by co-precipitation method. The pure K2GeF6 phase with P63 mc space group other than P3m1 space group was affirmed just by incorporation of Si in K2GeF6 at room temperature according to XRD characterization. SEM images showed lamellar and octahedron grain morphology for K2GeF6：Mn^4＋ and K2（Ge,Si）F6：Mn^4＋ phosphors, respectively. It was also found that the photoluminescence excitation（PLE） and photoluminescence（PL） showed slight displacement in K2GeF6：Mn^4＋ and K2（Ge,Si）F6：Mn^4＋ system. And the zero-phonon line（ZPL） of the PL spectrum of K2GeF6：Mn^4＋ with Si showed a strong peak. Meanwhile crystalline field surrounding Mn^4＋ changes could affect the decay time in this fluoride system. The color gamut of the LED devices based on K2（Ge,Si）F6：Mn^4＋ and K2GeF6：Mn^4＋ reached up to 94.58% NTSC（National Television Standards Committee） and 94.386% NTSC, respectively, that was much higher than that based on nitride red phosphors. All these original characteristics in K2（Ge,Si）F6：Mn^4＋ phosphor are desirable for potential applications as a red phosphor for improving lighting and display quality of conventional white LEDs.

关键词： K2(Ge Si)F6:Mn4+ fluoride PLE and PL decay time phosphors rare earths

Decoration of activated carbon nanotubes by assembling nano-silver

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International Journal of Minerals,Metallurgy and Materials 2009年第5期16卷 598-602页

作者： Chen-sha Li Bin-song Wang Ying-jie Qiao Wei-zhe Lu Ji Liang Department of Materials Science Harbin Engineering University Harbin 150001 China Department of Mechanical Engineering Tsinghua University Beijing 100084 China School of Chemistry and Materials Science Heilongjiang University Harbin 150080 China

A facile solution processed strategy of synthesizing nano silver assembled on carbon nanotubes （CNTs） at room temperature was put forward. Activated carbon nanotubes were used as precursors for preparing silver-decorated nanotubes. The nature of the decorated nanotubes was studied using transmission electron microscopy （TEM）, scanning electron microscopy （SEM）, and energy-dispersive X-ray spectroscopy （EDX）. The inert surfaces of carbon nanotubes were activated by introducing catalytic nuclei via an oxidation-sensitization-activation approach. Activated carbon nanotubes catalyzed the metal deposition specifically onto their surfaces upon immersion in electroless plating baths. The method produced nanotubes decorated with silver. The extent of silver decoration was found to be dependent on fabrication conditions. Dense nano silver assembled on nanotube surfaces could be obtained by keeping a low reaction rate in the solution phase. The results here show that this method is an efficient and simple means of achieving carbon nanotubes being assembled by nano metal.

关键词： carbon nanotubes nano silver assembly surface activation

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Defects in CVDZnS

Rare Metals 2011年第4期30卷 387-391页

作者： FU Ligang LU Fanxiu WANG Xuewu WEI Naiguang School of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083 China Beijing Guojing Infrared Optical Technology Co. Ltd. General Research Institute for Nonferrous Metals Beijing 100088 China

Defects seriously influence the optical properties and mechanical properties of CVDZnS. The deposition technique was introduced and experiment methods such as SEM, TEM and OM were adopted to study the defects： hexagonal phase, abnormally large grains, micro-cracks, micro-pores and impurities. The microstructures of these defects were observed, and the formation mechanisms of these defects were analyzed. Hexagonal ZnS as an impurity causes anisotropy in different directions with different refraction indexes and increases the scattering of CVDZnS. Increasing the deposition temperature can inhibit the formation of the hexagonal phase. The main reason for the growth of abnormaUy large grains in CVDZnS is high deposition rate. The lower deposition temperature increases growth stress in CVDZnS, which consequently results in the bowing of CVDZnS plates, and introduces microscopic defects such as grain blending and micro-cracks in the crystal. Micro-pores and impurities are induced by high reactant concentration in the local deposition chamber, which can be avoided by improving the gas flowing so as to make the concentration more uniform.

关键词： zinc sulfide chemical vapor deposition (CVD) defects microstructure

Theoretical and numerical analysis of coherent Smith-Purcell radiation

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Chinese Physics C 2008年第11期32卷 916-923页

作者：卑华戴志敏 Shanghai Institute of Applied Physics CAS Graduate University of Chinese Academy of Sciences

Coherent enhancement of Smith-Purcell radiation has attracted people＇s attention not only in adopting a better source but also in beam diagnostics aspect. In this paper, we study the intrinsic mechanism of coherent Smith-Purcell radiation on the basis of the van den Berg model. The emitted power of Smith-Purcell radiation is determined by the bunch profile in transverse and longitudinal directions. For short bunch whose longitudinal pulse length is comparable with the radiation wavelength, it can be concluded approximately that the power is proportional to the square number of electrons per bunch.

关键词： Smith-Purcell radiation van den Berg model induced surface current model coherence

Direct electrochemical reduction of solid vanadium oxide to metal vanadium at low temperature in molten CaCl_2-NaCl

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International Journal of Minerals,Metallurgy and Materials 2012年第6期19卷 499-505页

作者： Zhuo-fei Cai Zhi-mei Zhang Zhan-cheng Guo Hui-qing Tang state Key Laboratory of Advanced Metallurgy University of Science and Technology Beijing Beijing 100083 China

V205 sintered pellets and graphite rods were employed as the cathode and the anode, respectively; a molten CaC12-NaCI salt was used as the electrolyte. Then, V205 was directly reduced to metal vanadium by the Fray-Farthing-Chen （FFC） method at 873 K to realize low-temperature electrolysis. Two typical experimental conditions, electrolysis time and voltage, were taken into account to investigate the current efficiency and remaining oxygen content in electrolyzed products. The composition and microstmcture of the products were charac- terized by X-ray diffraction （XRD） and scanning electron microscopy （SEM）. SEM observations show that a higher voltage （1.8-3.4 V） and a longer electrolysis time （2-5 h） can improve the product quality separately, that is, a lower remaining oxygen content and a more uniform microstructure. The products with an oxygen content of 0.205wt% are successfully obtained below 3.4 V for 10 h. However, the current effi- ciency is low, and further work is required.

关键词： vanadium metallurgy vanadium pentoxide direct reduction electrolysis

Microstructure evolution and phase transformation of traditional cast and spray-formed hypereutectic aluminium-silicon alloys induced by heat treatment

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International Journal of Minerals,Metallurgy and Materials 2010年第3期17卷 297-306页

作者： Long-gang Hou Yuan-hua Cai Hua Cui Ji-shan Zhang state Key Laboratory for Advanced Metals and Materials University of Science and Technology Beijing Beijing 100083 China School of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083 China

Microstructural evolution and phase transformation induced by different heat treatments of the hypereutectic aluminium-silicon alloy, Al-25Si-5Fe-3Cu （wt%, signed as 3C）, fabricated by traditional cast （TC） and spray forming （SF） processes, were investigated by differential scanning calorimetry （DSC） and scanning electron microscopy （SEM） combined with energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that A17Cu2Fe phase can be formed and transformed in TC- and SF-3C alloys between 802-813 K and 800-815 K, respectively. The transformation from β-Al5FeSi to δ-Al4FeSi2 phase via peritectic reaction can occur at around 858-870 K and 876-890 K in TC- and SF-3C alloys, respectively. The starting precipitation temperature of δ-Al4FeSi2 phase as the dominant Fe-bearing phase in the TC-3C alloy is 997 K and the exothermic peak about the peritectic transformation of δ-Al4FeSi2→β-Al5FeSi is not detected in the present DSC experiments. Also, the mechanisms of the microstructural evolution and phase transformation are discussed.

关键词： hypereutectic Al-Si alloy spray forming cast microstructure phase transformation