全文获取类型
收费全文 | 5211篇 |
免费 | 551篇 |
国内免费 | 386篇 |
专业分类
化学 | 2592篇 |
晶体学 | 27篇 |
力学 | 197篇 |
综合类 | 20篇 |
数学 | 488篇 |
物理学 | 1388篇 |
无线电 | 1436篇 |
出版年
2024年 | 16篇 |
2023年 | 79篇 |
2022年 | 133篇 |
2021年 | 143篇 |
2020年 | 157篇 |
2019年 | 131篇 |
2018年 | 123篇 |
2017年 | 113篇 |
2016年 | 160篇 |
2015年 | 171篇 |
2014年 | 223篇 |
2013年 | 283篇 |
2012年 | 390篇 |
2011年 | 351篇 |
2010年 | 258篇 |
2009年 | 264篇 |
2008年 | 338篇 |
2007年 | 263篇 |
2006年 | 240篇 |
2005年 | 223篇 |
2004年 | 178篇 |
2003年 | 157篇 |
2002年 | 137篇 |
2001年 | 132篇 |
2000年 | 136篇 |
1999年 | 109篇 |
1998年 | 97篇 |
1997年 | 103篇 |
1996年 | 110篇 |
1995年 | 74篇 |
1994年 | 86篇 |
1993年 | 68篇 |
1992年 | 76篇 |
1991年 | 64篇 |
1990年 | 46篇 |
1989年 | 54篇 |
1988年 | 48篇 |
1987年 | 37篇 |
1986年 | 45篇 |
1985年 | 34篇 |
1984年 | 46篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1977年 | 18篇 |
1976年 | 22篇 |
1975年 | 17篇 |
排序方式: 共有6148条查询结果,搜索用时 0 毫秒
121.
122.
Laser cooling below a photon recoil with three-level atoms 总被引:1,自引:0,他引:1
123.
Leslie M. Werbel Edward F. Elslager Vera P. Chu 《Journal of heterocyclic chemistry》1973,10(4):631-635
3,4-Dichlorophenylisothioeyanate ( 10 ) was allowed to react with 2-methy1-2-thiopseudourea to give methyl 4-(3,4-dichlorophenyl)(dithioaltophanimidate ( 11 ) (41%), which upon treatment with hydrazine afforded 3-amino-5-(3,4-dichloroanilino)-s-triazole ( 12 ) (54-91%). Ring-closure with ethyl acetoacetale in acetic acid afforded 2-(3,4-dichloroanilino)-5-methyl-s-triazolo[ 1,5-α ]-pyrimidin-7-ol ( 13 ) (81%). Chlorination with phosphorus oxychloride gave 7-chloro-2-(3,4-dichloroanilino)-5-methyl-s-triazolo[1,5-α ]pyrimidine ( 14 ) (98%), which was condensed with various amines to yield the desired 2-(3,4-diehloroanilino)-7-¶[(dialkylamino)alkyl]arnino¶-5-methyl-s-triazolo[ 1,5-α]pyrimidines ( 6 a-d). The structures of the s-triazolo[ 1,5-α ]pyrimidines were based on nmr spectroscopy and ring stability considerations. Several of the amino-s-triazolo[ 1,5-α ]pyrimidines possessed antimalarial activity against P. berghei in mice. 相似文献
124.
Vijay M. Krishnamurthy Dr. Brooks R. Bohall Chu‐Young Kim Dr. Demetri T. Moustakas Dr. David W. Christianson Prof. Dr. George M. Whitesides Prof. Dr. 《化学:亚洲杂志》2007,2(1):94-105
This paper describes a calorimetric study of the association of a series of seven fluorinated benzenesulfonamide ligands (C6HnF5?nSO2NH2) with bovine carbonic anhydrase II (BCA). Quantitative structure–activity relationships between the free energy, enthalpy, and entropy of binding and pKa and log P of the ligands allowed the evaluation of the thermodynamic parameters in terms of the two independent effects of fluorination on the ligand: its electrostatic potential and its hydrophobicity. The parameters were partitioned to the three different structural interactions between the ligand and BCA: the ZnII cofactor–sulfonamide bond (≈65 % of the free energy of binding), the hydrogen bonds between the ligand and BCA (≈10 %), and the contacts between the phenyl ring of the ligand and BCA (≈25 %). Calorimetry revealed that all of the ligands studied bind in a 1:1 stoichiometry with BCA; this result was confirmed by 19F NMR spectroscopy and X‐ray crystallography (for complexes with human carbonic anhydrase II). 相似文献
125.
Chu I Liu F Soares A Kumari P Nomeir AA 《Rapid communications in mass spectrometry : RCM》2002,16(15):1501-1505
Rapid, generic gradient liquid chromatography/tandem mass spectrometry (LC/MS/MS) assays, designed to accelerate sample analyses, have been developed to keep pace with the productivity of advanced synthetic procedures. In this study, LC/MS/MS was combined with an in vitro, cell-based, blood-brain barrier (BBB) model to evaluate the potential of new chemical entities (NCEs) to cross the BBB. This in vitro assay provides the permeability of discovery compounds across a monolayer of a primary culture of bovine brain microvessel endothelial cells in a fraction of the time that is required for in vivo studies (brain/plasma concentrations), using only 2 mg of the compound. The results are consistent with in vivo brain/plasma concentration ratio data. 相似文献
126.
Hydrogen peroxide (HOOH) in ice and snow is an important chemical tracer for the oxidative capacities of past atmospheres. However, photolysis in ice and snow will destroy HOOH and form the hydroxyl radical (*OH), which can react with snowpack trace species. Reactions of *OH in snow and ice will affect the composition of both the overlying atmosphere (e.g., by the release of volatile species such as formaldehyde to the boundary layer) and the snow and ice (e.g., by the *OH-mediated destruction of trace organics). To help understand these impacts, we have measured the quantum yield of *OH from the photolysis of HOOH on ice. Our measured quantum yields (Phi(HOOH --> *OH)) are independent of ionic strength, pH, and wavelength, but are dependent upon temperature. This temperature dependence for both solution and ice data is best described by the relationship ln(Phi(HOOH --> *OH)) = -(684 +/- 17)(1/T) + (2.27 +/- 0.064) (where errors represent 1 standard error). The corresponding activation energy (Ea) for HOOH (5.7 kJ mol(-1)) is much smaller than that for nitrate photolysis, indicating that the photochemistry of HOOH is less affected by changes in temperature. Using our measured quantum yields, we calculate that the photolytic lifetimes of HOOH in surface snow grains under midday, summer solstice sunlight are approximately 140 h at representative sites on the Greenland and Antarctic ice sheets. In addition, our calculations reveal that the majority of *OH radicals formed on polar snow grains are from HOOH photolysis, while nitrate photolysis is only a minor contributor. Similarly, HOOH appears to be much more important than nitrate as a photochemical source of *OH on cirrus ice clouds, where reactions of the photochemically formed hydroxyl radical could lead to the release of oxygenated volatile organic compounds to the upper troposphere. 相似文献
127.
Poly(N,N-dimethylacrylamide) (PDMA) with a molecular mass of 5.2 x 10(6) g/mol has been synthesized and used in DNA sequencing analysis by capillary electrophoresis (CE). A systematic investigation is presented on the effects of different separation conditions, such as injection amount, capillary inner diameter, polymer concentration, effective separation length, electric field and temperature, on the resolution. DNA sequencing up to 800 bases with a resolution (R) limit of 0.5 (and 1,000 bases with a resolution limit of 0.3) and a migration time of 96 min was achieved by using 2.5% w/v polymer, 150 V/cm separation electric field, and 60 cm effective separation length at room temperature on a DNA sample prepared with FAM-labeled--21M13 forward primer on pGEM3Zf(+) and terminated with ddCTP. Ultrafast and fast DNA sequencing up to 420 and 590 bases (R > or = 0.5) were also achieved by using 3% w/v polymer and 40 cm effective separation length with a separation electric field of 525 and 300 V/cm, and a migration time of 12.5 and 31.5 min, respectively. PDMA has low viscosity, long shelf life and dynamic coating ability to the glass surface. The unique properties of PDMA make it a very good candidate as a separation medium for large-scale DNA sequencing by capillary array electrophoresis (CAE). 相似文献
128.
Kun-Mo Chu Jung-Hwan Choi Jung-Sub Lee Han Seo Cho Seong-Ook Park Hyo-Hoon Park Duk Young Jeon 《Advanced Packaging, IEEE Transactions on》2006,29(3):409-414
This paper describes low-temperature flip-chip bonding for both optical interconnect and microwave applications. Vertical-cavity surface-emitting laser (VCSEL) arrays were flip-chip bonded onto a fused silica substrate to investigate the optoelectronic characteristics. To achieve low-temperature flip-chip bonding, indium solder bumps were used, which had a low melting temperature of 156.7/spl deg/C. The current-voltage (I-V) and light-current (L-I) characteristics of the flip-chip bonded VCSEL arrays were improved by Ag coating on the indium bump. The I-V and L-I curves indicate that optical and electrical performances of Ag-coated indium bumps are superior to those of uncoated indium solder bumps. The microwave characteristics of the solder bumps were investigated by using a flip-chip-bonded coplanar waveguide (CPW) structure and by measuring the scattering parameter with an on-wafer probe station for the frequency range up to 40 GHz. The indium solder bumps, either with or without the Ag coating, provided good microwave characteristics and retained the original characteristic of the CPW signal lines without degradation of the insertion and return losses by the solder bumps. 相似文献
129.
Tunable Optical Notch Filter Realized by Shifting the Photonic Bandgap in a Silicon Photonic Crystal Line-Defect Waveguide 总被引:1,自引:0,他引:1
Chu T. Yamada H. Gomyo A. Ushida J. Ishida S. Arakawa Y. 《Photonics Technology Letters, IEEE》2006,18(24):2614-2616
A tunable optical notch filter was realized by thermally shifting the TM-like (the light's electric field perpendicular to the substrate) bandgap of a silicon photonic crystal slab W1 line-defect waveguide with silica cladding. This device is compact-its footprint is 340times16 mum2, excluding the electrode pads. The 3-dB bandwidth of the device was about 5 nm, and the extinction ratio at the center wavelength was as high as 40 dB. A maximum center wavelength shift of 17.9 nm was attained at a heating power of 0.7W, with a tuning efficiency of 25.5 nm/W. The tuning response time was less than 100 mus 相似文献
130.
C. Ji N. Chubun R.G. Broeke J. Cao Y. Du S.J.B. Yoo K.Y. Liou J.R. Lothian S. Vatanapradit S.N.G. Chu B. Patel W.S. Hobson W.T. Tsang 《Photonics Technology Letters, IEEE》2006,18(4):625-627
We report a 10-GHz colliding pulse mode-locked laser fabricated with integrated active-passive waveguides. The laser fabrication adopted a deep reactive ion etching and single-step metal-organic vapor phase epitaxy regrowth process for forming the buried heterostructure waveguide. Clean output pulses resulted from laterally tilting the active-passive interface and effectively suppressing residual back-reflections at the interface. Hybrid mode-locking resulted in a synchronized transform-limited sech/sup 2/optical waveform. Pulsewidth, chirp, timing jitter, and frequency-locking range were investigated through systematic device biasing condition optimization. 相似文献