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Dy3+-activated β/α′-Sr2SiO4 phosphors were successfully prepared by solid-state reaction method with ammonium chloride (NH4Cl) as the flux. The influences of calcination temperatures, amounts of NH4Cl and the concentrations of Dy3+ on phase composition, morphology and the photoluminescent properties of as-prepared powders were investigated in detail. The β and α′ phases of Sr2SiO4 were obtained with 1 wt% and 2-5 wt% NH4Cl, respectively, as the sintered condition was at 1000 °C for 4 h. With increasing the amount of NH4Cl, the morphology of phosphors changed from needlelike to regular polyhedron shape and the colors of the Sr2SiO4:Dy3+ phosphors changed from blue-green to white. The luminescence intensity of 4F9/2 → 6H15/2 transition was slightly higher than that of 4F9/2 → 6H13/2 (ΔL = 2, ΔJ = 2) transition owing to the low-symmetry around Dy3+ ions. The optimum concentration of Dy3+ was 2.0 mol% and the concentration quenching were caused by the d-d interaction and a cross relaxation. The yellow-to-blue intensity ratio (Y/B) of Dy3+ emission did not to change with varying the Dy3+ concentration using Li+ ions for charge compensation. These indicate that this phosphor can be used as a potential candidate for the phosphor-converted white LEDs with a UV chip. 相似文献
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The IEEE 802.16 standard specifies the air interface of wireless metropolitan area network (WMAN), and aims to provide wireless broadband access for integrated voice and video services. This paper presents the efficient design and implementation of fast Frouier transform (FFT) and inverse fast Frouier transform (IFFT) for the application in IEEE 802.16d orthogonal frequency division multiplexing (OFDM) system. In this design, a novel pipeline structure for the branch of butterfly unit (BU) is proposed, which can improve the processing symbol rate by adding the number of branch flexibly. The symmetrical ping-pang structure of random access memory (RAM) is performed to increase the system throughput. Simulation results reveal that only with 1 branch of BU, the proposed FFT/IFFT design can almost achieve the maximum bandwidth requirement of IEEE 802.16d OFDM system. And this design has been verified by FPGA and successfully implemented in the prototype of WiMAX transceiver. 相似文献
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Chu-Li Fu Yuan-Xiang ZhangHao Cheng Yun-Jie Ma 《Engineering Analysis with Boundary Elements》2012,36(4):493-504
The problem of numerical analytic continuation of an analytic function with complex variable, in general, is ill-posed and frequently encountered in many practical applications. However, there are few results for this problem until now. In the present paper, this problem with a single complex variable is considered on bounded domains. A very simple meshless algorithm based on the method of fundamental solution (MFS) combining with numerical differentiation is established. Some numerical examples verify the feasibility and effectiveness of this method. 相似文献
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A water-quenching technique was adopted to evaluate the thermal shock behavior of as-sintered and pre-oxidation Zr2Al4C5–20 vol.%SiC composites in an air atmosphere. The strength retention of the two kinds of composites was measured after varying temperature (ΔTc) up to 800 °C. As-sintered Zr2Al4C5–20 vol.%SiC showed rapidly drop in flexural strength above ΔT of 400 °C. However, the pre-oxidation Zr2Al4C5–20 vol.%SiC composite showed a higher retained strength up to 800 °C, and the critical thermal shock temperature difference (ΔTc) is as high as 660 °C. The main reasons for a great improvement of thermal shock resistance (TSR) were also discussed and analyzed. 相似文献
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Shixue Zhou Xiaoli ZhangTao Li Naifei WangHaipeng Chen Tonghuan ZhangHao Yu Haili NiuDi Liu 《International Journal of Hydrogen Energy》2014
The structure and properties of magnesium nanoparticles for hydrogen storage from reactive milling under hydrogen atmosphere with the carbon from anthracite coal carbonization as milling aid were investigated. Experiment showed that after 3 h of milling under 1 MPa of hydrogen with 30 wt.% of carbon additive, the magnesium particles were milled to 20–60 nm and hydrided into β-MgH2 with a crystallite size of 29.7 nm. For the hydrogen desorption of the material, the onset temperature was determined to be 270 °C. In 270–390 °C, the enthalpy and entropy changes were calculated to be 44.5 kJ/mol and 83.8 J/(mol K), respectively, and the activation energy as pseudo first-order reaction was 127.1 kJ/mol. The carbon still played a role of nano-confinement for magnesium to prevent particles from coalescing in the process of repeatedly heating for hydrogen storage. 相似文献
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采用粉末冶金法制备了Mo掺杂的Al_2O_3基陶瓷/金属复合材料(Al_2O_3/Mo),利用X射线衍射、扫描电镜等测试分析方法研究了Mo掺杂对复合材料微观形貌及介电性能的影响。结果表明:Al_2O_3/Mo复合材料主要由Al_2O_3相和Mo相组成,未出现新相。Mo主要分散在Al_2O_3晶界,并且随着Mo掺杂量的增加,Al_2O_3晶粒尺寸逐渐减小、材料气孔率逐渐增加。当Mo掺杂量小于20%(质量分数)时,复合材料电阻率(10~(12)?·cm)与相对介电常数(8~9)没有明显变化;而当Mo掺杂量大于20%,Mo由弥散相转变为连续相,复合材料的体电阻率急剧下降到10~(10)?·cm;当Mo掺杂量达到40%时,由于复合材料中Mo已经形成连续的贯穿网络,体电阻率下降趋势减缓,稳定在10?·cm左右。因此,通过调控Al_2O_3基体中的Mo掺杂量及相分布,可以制备出具有不同电阻率的陶瓷/金属复合材料。 相似文献
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Flow channel in bipolar plate for PEMFC has the function of transporting the fuel and oxygen and water. Stamping process for flow channel on metallic bipolar plates is an economic and convenient method for mass production. It is extremely important to predict and prevent the tearing, wrinkling, thinning, skid mark and spring back in stamping process. In this study, finite element method is used to predict the imperfection and analyze thickness variation during the stamping process. Dynaform software is employed to predict the performance of a stamping process. Forming Limit Diagrams are used to determine the safe limit of the metallic bipolar plates. In simulation, the dimension of flow channels, punch speed, radius of punch and die and draft angle were selected as variable process parameters. Experiments were conducted to compare with the simulation results. It is demonstrated that the simulation results are in good agreement with the experimental ones for ss304 metal sheets. So simulation model could be employed as a predictive tool to provide optimal parameters for better performance of the stamping process. 相似文献
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YongCai ZhangHao Wang Bo WangHui Yan Anwar AhniyazMasahiro Yoshimura 《Materials Research Bulletin》2002,37(8):1411-1417
A mild hydrothermal method using Li-birnessite (LixMnO2·nH2O) ultrafine fiber as the precursor has been adopted to prepare Li4Mn5O12, which is of interest as an electrode material for 3 V rechargeable lithium ion batteries. X-ray diffraction data reveal that the obtained powders have a pure spinel structure with a lattice constant of 8.135 Å. The scanning electron microscopy image of the obtained powders shows the particles are cubic-shaped whose average size is about 40-50 nm. The results from inductively coupled plasma-atomic emission spectroscope and wet chemical analysis indicate that a Li/Mn ratio of 0.796, and an average valence of 3.96 of Mn ion have been achieved in the as-prepared products. The thermogravimetric and differential thermal analysis data also agree with the previous reports on Li4Mn5O12, suggesting that near stoichiometry of Li4Mn5O12 has been synthesized by this procedure at the rather low temperature 110°C. 相似文献
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