全文获取类型
收费全文 | 366篇 |
免费 | 15篇 |
专业分类
化学 | 147篇 |
晶体学 | 2篇 |
力学 | 31篇 |
数学 | 16篇 |
物理学 | 43篇 |
无线电 | 142篇 |
出版年
2023年 | 3篇 |
2022年 | 1篇 |
2020年 | 10篇 |
2019年 | 5篇 |
2018年 | 2篇 |
2017年 | 4篇 |
2016年 | 9篇 |
2015年 | 10篇 |
2014年 | 8篇 |
2013年 | 17篇 |
2012年 | 15篇 |
2011年 | 35篇 |
2010年 | 13篇 |
2009年 | 23篇 |
2008年 | 24篇 |
2007年 | 21篇 |
2006年 | 16篇 |
2005年 | 12篇 |
2004年 | 15篇 |
2003年 | 17篇 |
2002年 | 14篇 |
2001年 | 11篇 |
2000年 | 13篇 |
1999年 | 9篇 |
1998年 | 12篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 1篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1981年 | 1篇 |
1975年 | 1篇 |
1969年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有381条查询结果,搜索用时 366 毫秒
291.
In this paper, we propose a new differentiated service model, referred to as Differentiated Service-EDCA (DS-EDCA), for the
Enhanced Distributed Channel Access (EDCA) of IEEE 802.11e wireless local area networks (WLANs). With DS-EDCA, both strict
priority and weighted fair service can be provided. The strict priority service is provided for high priority traffic through
carefully setting the EDCA parameter sets of lower priority traffic; the proportional fairness service is enabled by determining
the backoff intervals according to the distributed scheduling discipline (DFS). We also propose a hierarchical link sharing
model for IEEE 802.11e WLANs, in which AP and mobile stations are allocated different amounts of link resource. The performance
of DS-EDCA and EDCA is compared via ns-2 simulations. The results show that DS-EDCA outperforms the original EDCA in terms
of its support for both strict priority and weighted fair service. More importantly, DS-EDCA can be easily implemented, and
is compatible to the IEEE 802.11 Standard.
相似文献
Meng Chang ChenEmail: |
292.
Hsu IJ Shiu YJ Jeng US Chen TH Huang YS Lai YH Tsai LN Jang LY Lee JF Lin LJ Lin SH Wang Y 《The journal of physical chemistry. A》2007,111(38):9286-9290
The local and global structural changes of cytochrome c induced by urea in aqueous solution have been studied using X-ray absorption spectroscopy (XAS) and small-angle X-ray scattering (SAXS). According to the XAS result, both the native (folded) protein and the unfolded protein exhibit the same preedge features taken at Fe K-edge, indicating that the Fe(III) in the heme group of the protein maintains a six-coordinated local structure in both the folded and unfolded states. Furthermore, the discernible differences in the X-ray absorption near-edge structure (XANES) of these two states are attributed to a possible spin transition of the Fe(III) from a low-spin state to a high-spin state during the unfolding process. The perseverance of six-coordination and the spin transition of the iron are reconciled by a proposed ligand exchange, with urea and water molecules replacing the methionine-80 and histidine-18 axial ligands, respectively. The SAXS result reveals a significant morphology change of cytochrome c from a globular shape of a radius of gyration R(g) = 12.8 A of the native protein to an elongated ellipsoid shape of R(g) = 29.7 A for the unfolded protein in the presence of concentrated urea. The extended X-ray absorption fine structure (EXAFS) data unveil the coordination geometries of Fe(III) in both the folded and unfolded state of cytochrome c. An initial spin transition of Fe(III) followed by an axial ligand exchange, accompanied by the change in the global envelope, is proposed for what happened in the protein unfolding process of cytochrome c. 相似文献
293.
Jeng J Lin MF Cheng FY Yeh CS Shiea J 《Rapid communications in mass spectrometry : RCM》2007,21(18):3060-3068
We describe an innovative approach - using a high concentration of trypsin-modified magnetic nanoparticles (TMNPs) - for the rapid and efficient digestion of proteins at elevated temperature. The required digestion time could be reduced to less than 10 s. After digestion, the TMNPs were collected magnetically from the sample solution for reuse and the digested peptides were characterized using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Protein digestion was optimized when using the TMNPs (5 microg/microL) at 57 degrees C; a significantly high peptide coverage was achieved for protein identification (e.g., 98% for lysozyme). Although a high concentration of TMNPs was used for digestion, the short digestion time led to much lower amounts of trypsin peptides being produced through self-digestion. As a result, interference in the mass spectrometric detection of the peptide ions was reduced significantly. 相似文献
294.
Chih Ming Ma Young Ku Wen Wang Fu Tien Jeng 《Reaction Kinetics and Catalysis Letters》2008,94(2):199-206
The main objective of this study was to develop a simple, energy-efficient photoreactor for operating at room temperature.
In this work, the design of a new gas-phase optical fiber photoreactor (OFP) was introduced which operated under various parameters,
such as the UV light intensity and the initial concentration for the photocatalytic decomposition of acetone. Experimental
results indicated that increasing the UV light intensity or decreasing the initial concentrations of acetone by a UV/TiO2 process would result in improving the decomposition and mineralization efficiencies. The apparent quantum yield of the novel
optical fiber reactor is about 2 to 3 times greater than that of the traditional annular reactor. 相似文献
295.
Chi‐I Chang Yen‐Cheng Li Chang‐Syan Yang Che‐Yi Chao Yi‐Jen Li Hsun‐Shuo Chang Ching‐Jeng Huang Ching‐Chuan Kuo Yueh‐Hsiung Kuo 《Helvetica chimica acta》2015,98(1):123-127
Two new labdane‐type diterpene acids, (12S)‐12‐hydroxylabda‐8(17),13(16),14‐trien‐18‐oic acid ( 1 ) and (12R)‐12‐hydroxy‐15,16‐epoxylabda‐8(17),13‐dien‐18‐oic acid ( 2 ), along with a known labdane‐type diterpene, (11E)‐15,16‐bisnor‐13‐oxolabda‐8(17),11‐dien‐19‐oic acid ( 3 ), were isolated from the MeOH extract of the wood of Cunninghamia konishii. Their structures were determined by analysis of spectroscopic data and comparison with the data of known analogs. 相似文献
296.
Sergei Alexandrov Yeau-Ren Jeng 《Archive of Applied Mechanics (Ingenieur Archiv)》2013,83(12):1801-1804
The paper presents the theoretical part of a method for the identification of the modified Cockroft–Latham ductile fracture criterion at elevated temperature. Quite a general viscoplastic model is adopted to describe material behavior. The original criterion is path-dependent and involves stresses. Therefore, the identification of constitutive parameters of this criterion, as well as many other ductile fracture criteria, is rather a difficult task that usually includes experimental research and numerical simulation. The latter is impossible without a precisely specified material model and boundary conditions. It is shown in the present paper that for a wide class of material models usually used to describe the behavior of materials at elevated temperatures, the criterion is significantly simplified when the site of fracture initiation is located on traction-free surfaces. In particular, this reduced criterion solely depends on two in-surface logarithmic strains. Since there are well-established experimental procedures to measure surface strains, the result obtained can be considered as a theoretical basis for the efficient method for the identification of the modified Cockroft–Latham ductile fracture criterion at elevated temperature. 相似文献
297.
This letter presents a new analytical approximation for wave (current)-induced dynamic soil response in marine sediments. In the model, the third-order approximation for wave-current interactions is employed for the flow model, while Biot's dynamic poro-elastic model is used to simulate the porous flow in a seabed. The newly analytical solution is validated with the field observations. Based on the solution, effects of currents and wave-nonlinearity on soil response are examined and a parametric study will be carried out to examine the influence of currents on the liquefaction potential. 相似文献
298.
Ging-Ho Hsiue Rong-Ho Lee Ru-Jong Jeng Chin-Shan Chang 《Journal of Polymer Science.Polymer Physics》1996,34(3):555-563
The dielectric behavior of a series of ferroelectric side-chain liquid crystalline polysiloxanes containing 1–3 oligooxyethylene units as spacers, and 4-(S)-2-methyl-1-butyl[[[(4-hydroxy-biphenyl-4′-yl)]carbonyl]oxy] benzoate or 4-(S)-2-methyl-1-butyl[[(4-hydroxy-biphenyl-4′-yl)carbonyl]oxy]-3-fluoro benzoate side groups was studied by broadband dielectric spectroscopy. The increase of the spacer length, and incorporation of a strong dipole moment fluoro-substituent into the mesogenic group, resulted in a decrease of the relaxation activation energy and an increase in the intensity of the relaxation. Moreover, the relaxation peak of the Goldstone mode has only been observed for the FLCP with a lateral fluoro-substituent. The relationship between the thermal dynamic behavior and chemical structure is discussed in detail. Furthermore, analysis of the dielectric relaxation behavior of these FLCPs showed that the molecular relaxations could be described by the Cole-Cole equation. © 1996 John Wiley & Sons, Inc. 相似文献
299.
Ming‐Jen Cheng Bolleddula Jayaprakasam Tsutomu Ishikawa Hiroko Seki Ian‐Lih Tsai Jeh‐Jeng Wang Ih‐Sheng Chen 《Helvetica chimica acta》2002,85(7):1909-1914
Five new compounds, including a novel lactone, machilactone (=rel‐(2R,3aR,6E,6aS)‐2‐heptadecyl‐3a‐methyl‐6‐octadecylidene‐6,6a‐dihydrofuro[2,3‐d][1,3]dioxol‐5(3aH)‐one; 1 ), a new sesquiterpene, 3,4‐dihydroxy‐β‐bisabolol (=rel‐(1R,2S,4R)‐1‐[(1R)‐1,5‐dimethylhex‐4‐enyl]‐1‐methylcyclohexane‐1,2,4‐triol; 2 ), a new secobutyrolactone, methyl (2E)‐2‐(1‐hydroxy‐2‐oxopropyl)eicos‐2‐enoate ( 3 ), two new butyrolactones, machicolide A ( 4 ) and machicolide B ( 5 ) (=3E,4R,5R)‐ and (3Z,4R,5R)‐4,5‐dihydro‐4‐hydroxy‐5‐methoxy‐5‐methyl‐3‐octadecylidenefuran‐2(3H)‐one, resp.) as a mixture, together with known caryophyllene oxide (=4,12,12‐trimethyl‐9‐methylene‐5‐oxatricyclo[8.2.0.04,6]dodecane), hexacosane, tetracosanoic acid, isomahubanolide‐23 (=(3E,4R)‐4,5‐dihydro‐4‐hydroxy‐5‐methylidene‐3‐octadecylidenefuran‐2(3H)‐one), and β‐bisabolol (=(1S)‐1‐[(1S)‐1,5‐dimethylhex‐4‐enyl]‐4‐methylcyclohex‐3‐en‐1‐ol) were isolated from the stem wood of Machilus zuihoensis. The structures of these compounds were established by spectroscopic studies. The eicos‐2‐enoate ( 3 ) and β‐bisabolol exhibited marginal cytotoxicity against NUGC and HONE‐1 cancer cell lines in vitro. 相似文献
300.
A new NLO‐active polyurethane (Tg = 145°C) based on a two‐dimensional NLO chromophore has been investigated. Two ends of this lambda‐shaped chromophore can be directly bound to the main chain of polyurethane. After poling, fast relaxation of the effective second harmonic (SH) coefficient was observed at temperatures higher than 122°C. Moreover, excellent temporal stability at 100°C was obtained despite the operating temperature being very close to the fast relaxation temperature. This is due to the fact that embedding the rigid lambda‐shaped chromophores into the polymer backbone effectively restricts molecular motion at temperatures close to Tg. 相似文献