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51.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献
52.
53.
Hai-Ping Cao 《Journal of fluorine chemistry》2006,127(8):1079-1086
In the presence of sodium dithionite and DMSO, the intramolecular radical cyclization of 4-chloro-1,1,2,2,3,3,4,4-octafluorobutylbenzenes is achieved to give the corresponding cyclic compounds in moderate to good yields. 相似文献
54.
Neodymium doped bismuth layer structure ferroelectrics (BLSFs) ceramics CaBi4−xNdxTi4O15 (x=0, 0.25, 0.50, 0.75) were prepared by solid-state reaction method. X-ray diffraction pattern showed that single phase was formed when x=0-0.75. The refined lattice parameters showed that a (b) axes decrease at x=0.25 and increase with more Nd3+ dopant. The effects of Nd3+ doping on the dielectric and ferroelectric properties of CaBi4Ti4O15 ceramics are studied. Nd3+ dopant decreased the Curie temperature linearly, and the dielectric loss, tan δ, as well. The remnant polarization of Nd3+ doped CaBi4Ti4O15 ceramics was increased by 80% at x=0.25, while more Nd3+ dopant decreased the remnant polarization. CaBi3.75Nd0.25Ti4O15 ceramics had the largest piezoelectric constant d33. The structure and properties of CaBi4−xNdxTi4O15 ceramics showed that Nd3+ may occupy different crystal locations when Nd3+ content x is less than 0.25 and more than 0.50. 相似文献
55.
Deposition of Hydrogen-Free Silicon Nitride Thin Films by Microwave ECR plasma Enhanced Magnetron Sputtering at Room Temperature
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Hydrogen-free silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma enhanced unbalance magnetron sputtering system. Both Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy are used to study the bonding type and the change of bonding structures of the silicon nitride films. The results indicate that the chemical structure and composition of SiNx films deposited by this technique depend strongly on the N2 flow rates, the stoichiometric SiNx film, which has the highest hardness of 22.9 GPa, could be obtained at lower N2 flow rate of 4 sccm. 相似文献
56.
室温下,通过双核配合物[Cu(dppm)(NO3)]2(dppm=双二苯基膦甲烷)与四苯基硼钠在甲醇和二氯甲烷混合溶剂中反应制备了三核铜(I)配合物[Cu3(dppm)3(NO3)(OH)](NO3),经过红外光谱、热重分析、核磁和ES-MS等现代分析手段表征了配合物的物理化学性质,并进一步研究了配合物在室温下的荧光光谱特征。 相似文献
57.
D. Liu N.Q. Ngo X.Y. Dong S.C. Tjin P. Shum 《Applied physics. B, Lasers and optics》2005,81(6):807-811
We propose and experimentally demonstrate a stable dual-wavelength erbium-doped polarization-maintaining (PM) fiber laser
with tunable wavelength spacing using an all-PM linear cavity that makes use of two reflection peaks from the PM fiber Bragg
grating (PM-FBG). Experimental results show stable dual lasing lines with a wavelength separation of ∼0.22 nm and a large
optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. By applying axial strain to the PM-FBG, the center
wavelengths of the two lasing lines can be tuned over several nanometers and the wavelength separation between the lasing
lines can also be tuned to as small as 0.05 nm, which, to our knowledge, is the smallest wavelength spacing ever obtained
from a stable room-temperature dual-wavelength fiber laser. The proposed laser configuration has the advantages of simple
structure, low loss, stable dual-wavelength operation and a very small lasing linewidth of ∼5 kHz .
PACS 42.55.Wd; 42.81.-i; 42.81.Gs 相似文献
58.
使用结构简单的单温炉设备,通过三步升温热解二茂铁、三聚氰氨混合物方法,在二氧化硅、多晶陶瓷基底上分别合成了碳纳米管阵列、碳纳米管捆束.使用扫描电子显微镜、透射电子显微镜、电子能量损失谱和x射线光电子能谱对合成样品进行了结构和成分分析.结果显示:两种基底上合成的纳米管均为多壁纯碳管;生长于光滑二氧化硅表面的碳纳米管具有高度取向性和一致的外径,长度为10—40μm.碳纳米管采取催化剂顶端生长模式并展示出类杯状形貌;生长于粗糙多晶陶瓷表面的碳纳米管捆束随机取向,碳纳米管直径为15—80nm,长度在几百微米,展示
关键词:
碳纳米管
热解法
三步升温工艺 相似文献
59.
J.J. Pan Xiangdong Qiu Kejian Guan Liang Dong FengQing Zhou Wei Wang Mingjie Zhang James Guo Jun Yang Chunmeng Wu 《光学学报》2003,23(Z1)
A low cost, coolerless 980nm diode pumped, gain flattened L band EDFA with fast transient control, high pump efficiency and gain clamping effect was realized by using FBGs as C band seed generators. 相似文献
60.
Pyroelectricity and Spontaneous Polarization in [111] Oriented 0.955 Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 Single Crystals
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We report that the measurements of the pyroelectric current of the pre-poled [111]-oriented 0.955 Pb(Zn1/3Nb2/3) O3-0.045 PbTiO3 (PZN-4.5%PT) single crystals can shed some light on the phase transition and spontaneous polarization characters of this material in a similar way to measures of remanent polarization and dielectric properties. The pyroelectric current is measured and the corresponding spontaneous polarization is calculated as a function of temperature with various poling fields added during cooling the sample from 200℃ to room temperature. Critical electric field of 0.061 k V/cm is found to be essential to induce the intermediate ferroelectric orthorhombic phase between the ferroelectric rhombohedral and tetragonal phases. Below the critical field, the polarization increases almost linearly with the increase of poling field. At the critical field, the polarization at 30OC increases abruptly from 14μC/cm^2 for a poling field of O.06kV/cm to 29.5μC/cm^2 for a poling field of 0.061 kV/cm, and afterwards, increases slowly and saturates to 31 μC/cm^2 for poling fields beyond 0.55 kV/cm. 相似文献