全文获取类型
收费全文 | 5329篇 |
免费 | 205篇 |
国内免费 | 50篇 |
专业分类
化学 | 2711篇 |
晶体学 | 31篇 |
力学 | 152篇 |
数学 | 354篇 |
物理学 | 879篇 |
无线电 | 1457篇 |
出版年
2023年 | 35篇 |
2022年 | 36篇 |
2021年 | 77篇 |
2020年 | 97篇 |
2019年 | 80篇 |
2018年 | 69篇 |
2017年 | 56篇 |
2016年 | 134篇 |
2015年 | 111篇 |
2014年 | 175篇 |
2013年 | 287篇 |
2012年 | 279篇 |
2011年 | 325篇 |
2010年 | 215篇 |
2009年 | 270篇 |
2008年 | 309篇 |
2007年 | 285篇 |
2006年 | 276篇 |
2005年 | 245篇 |
2004年 | 234篇 |
2003年 | 216篇 |
2002年 | 234篇 |
2001年 | 165篇 |
2000年 | 164篇 |
1999年 | 106篇 |
1998年 | 100篇 |
1997年 | 82篇 |
1996年 | 91篇 |
1995年 | 86篇 |
1994年 | 88篇 |
1993年 | 73篇 |
1992年 | 45篇 |
1991年 | 47篇 |
1990年 | 46篇 |
1989年 | 45篇 |
1988年 | 27篇 |
1987年 | 32篇 |
1986年 | 32篇 |
1985年 | 37篇 |
1984年 | 35篇 |
1983年 | 32篇 |
1982年 | 22篇 |
1981年 | 28篇 |
1980年 | 14篇 |
1979年 | 15篇 |
1978年 | 16篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1975年 | 18篇 |
1974年 | 11篇 |
排序方式: 共有5584条查询结果,搜索用时 15 毫秒
911.
A high-yield synthesis of steroid-type molecules under mild reaction conditions is achieved in two steps involving nucleophilic addition of alkynyl cerium reagent to an easily enolizable carbonyl compound (beta-tetralone) followed by an intramolecular Pauson-Khand reaction. 相似文献
912.
Chung JY Cvetovich RJ McLaughlin M Amato J Tsay FR Jensen M Weissman S Zewge D 《The Journal of organic chemistry》2006,71(22):8602-8609
Compound 1 is a p38 MAP kinase inhibitor potentially useful for the treatment of rheumatoid arthritis and psoriasis. A novel six-step synthesis suitable for large-scale preparation was developed in support of a drug development program at Merck Research Laboratories. The key steps include a tandem Heck-lactamization, N-oxidation, and a highly chemoselective Grignard addition of 4-(N-tert-butylpiperidinyl)magnesium chloride to a naphthyridone N-oxide. The N-oxide exerted complete chemoselectivity via chelation in directing the Grignard addition to the alpha position as opposed to 1,4-addition on the ene-lactam. The dihydropyridyl adduct was in situ aromatized with isobutylchloroformate followed by heating in pyridine. Syntheses of Grignard precursor, N-tert-butyl-4-chloro-piperidine, were accomplished via transamination with a quaternary ammonium piperidone or via addition of methylmagnesium chloride to an iminium ion. Utilizing this chemistry, multi-kilogram preparation of compound 1 was successfully demonstrated. 相似文献
913.
Hassan Hassanabadi Won Sang Chung Soroush Zare Hadi Sobhani 《The European Physical Journal C - Particles and Fields》2018,78(1):83
This paper contains a discussion of a relativistic spin-0 system in the presence of a Gödel-type background space-time. The Duffin–Kemmer–Petiau (DKP) equation in the presence of a Gödel-type background space-time is studied in detail. After a derivation of the final form of this equation in the considered framework, free spin-0 particles have been studied. 相似文献
914.
We have investigated the fluorescence lifetimes of Dy3+: 1.3 μm emission in the chalcogenide Ge–As–S glasses with different compositions but identically containing 0.5 mol% Ga and 0.5 mol% CsBr. The measured lifetimes turn out to be sensitive not only to the concentrations of Ga and CsBr but also to compositional variations in the Ge–As–S host glasses. The lifetime is enhanced conspicuously in glass of the S-sufficient compositions, relative to the stoichiometric GeS2–As2S3 composition, while this effect is not significant in the S-exact and S-deficient compositions. We employ Ga K-edge EXAFS analysis to support that the local structural environments of Ga in the modified chalcogenide glasses are closely correlated with the lifetime enhancement effect. 相似文献
915.
916.
A.S. Budiman H.-A.-S. Shin B.-J. Kim S.-H. Hwang H.-Y. Son M.-S. Suh Q.-H. Chung K.-Y. Byun N. Tamura M. Kunz Y.-C. Joo 《Microelectronics Reliability》2012,52(3):530-533
Through-silicon via (TSV) has been used for 3-dimentional integrated circuits. Mechanical stresses in Cu and Si around the TSV were measured using synchrotron X-ray microdiffraction. The hydrostatic stress in Cu TSV went from high tensile of 234 MPa in the as-fabricated state, to ?196 MPa (compressive) during thermal annealing (in situ measurement), to 167 MPa in the post-annealed state. Due to this stress, the keep-away distance in Si was determined to be about 17 μm. Our results suggest that Cu stress may lead to reliability as well as integration issues, while Si stress may lead to device performance concerns. 相似文献
917.
Byong-Deok Choi Tae-Wook Kim Mun-Kyu Lee Ki-Seok Chung Dong Kyue Kim 《Analog Integrated Circuits and Signal Processing》2011,66(3):467-474
A physical unclonable function (PUF) based on process variations on silicon wafers is a very promising technology which finds
various applications in identification and authentication, but only a few integrated circuits have been reported so far. As
those circuits are vulnerable to power supply noises, switching noises and environmental variations, they lead to a reliability
issue such as time-varying or metastable responses. To resolve this issue, this letter proposes a new integrated circuit design
for PUFs using differential amplifiers. The feasibility of the proposed circuit has been theoretically analyzed and validated
through HSPICE simulations for the previous and proposed circuits. 相似文献
918.
Zhenli Zhang Alok Chatterjee Christoph Grein Anthony J. Ciani Peter W. Chung 《Journal of Electronic Materials》2011,40(2):109-121
We simulate in three dimensions molecular beam epitaxial (MBE) growth of CdTe/ZnTe/Si using classical molecular dynamics.
Atomic interactions are simulated with Stillinger–Weber potentials, whose parameters are obtained by fitting to experimental
data or density function theory-calculated distortion energies of the component crystals. The effects of substrate temperature
and atomic species flux ratios on epilayer morphology are investigated. The agreement between simulations and experiments
suggests that this model has reasonable ability to predict the microstructures of CdTe/ZnTe/Si grown by MBE. 相似文献
919.
We report on four-point probe measurements on SiC wafers as such measurements give erratic data. Current-voltage measurements on n-type SiC wafers doped to 3 × 1018 cm−3 are non-linear and single probe I-V measurements are symmetrical for positive and negative voltages. For comparison, similar measurements of p-type Si doped to 5 × 1014 cm−3 gave linear I-V, well-defined sheet resistance and the single probe I-V curves were asymmetrical indicating typical Schottky diode behavior. We believe that the reason for the non-linearity in four-point probe measurements on SiC is the high contact resistance. Calculations predict the contact resistance of SiC to be approximately 1012 Ω which is of the order of the input resistance of the voltmeter in our four-point probe measurements. There was almost no change in two-probe I-V curves when the spacing between the probes was changed from 1 mm to 2 cm, further supporting the idea that the I-V characteristics are dominated by the contact resistance. 相似文献
920.
Pi-Ying Cheng Wen-Tse Hsiao Chien-Kai Chung Shih-Feng Tseng Ien-Chang Liao 《Optical Review》2014,21(5):732-735
To accomplish an electrode patterning in large area, we present a high speed stitching technique used in an ultraviolet laser processing system and investigate the interaction between laser beams and indium tin oxide (ITO) thin films deposited on glass substrates. After optimizing the process parameters of the laser direct imaging (LDI) for the large-area electrode patterning, the ablated lines looked like regularly fish-scale marks of about a 40 μm diameter and a 120 nm depth around the processing path. The parameters includes the laser power of 1W, the scanning speed of galvanometers of 800 mm/s, and the laser pulse repetition frequency of 50 kHz. Moreover, the resistance value of the ablated ITO thin film is larger than 200MΩ that is electrically insulated from the other regions of electrode structure. LDI technology with UV laser beam has great potential applications in patterning on wafer or sapphire substrates and patterning a conductive layer deposited on the touch panels for semiconductor and optoelectric industries, respectively. 相似文献