全文获取类型
收费全文 | 3759篇 |
免费 | 450篇 |
国内免费 | 268篇 |
专业分类
化学 | 2539篇 |
晶体学 | 16篇 |
力学 | 179篇 |
综合类 | 25篇 |
数学 | 521篇 |
物理学 | 1197篇 |
出版年
2024年 | 13篇 |
2023年 | 72篇 |
2022年 | 82篇 |
2021年 | 110篇 |
2020年 | 110篇 |
2019年 | 105篇 |
2018年 | 83篇 |
2017年 | 69篇 |
2016年 | 141篇 |
2015年 | 116篇 |
2014年 | 164篇 |
2013年 | 250篇 |
2012年 | 331篇 |
2011年 | 320篇 |
2010年 | 228篇 |
2009年 | 184篇 |
2008年 | 250篇 |
2007年 | 195篇 |
2006年 | 190篇 |
2005年 | 176篇 |
2004年 | 146篇 |
2003年 | 155篇 |
2002年 | 125篇 |
2001年 | 98篇 |
2000年 | 75篇 |
1999年 | 68篇 |
1998年 | 56篇 |
1997年 | 61篇 |
1996年 | 66篇 |
1995年 | 50篇 |
1994年 | 45篇 |
1993年 | 40篇 |
1992年 | 28篇 |
1991年 | 22篇 |
1990年 | 27篇 |
1989年 | 21篇 |
1988年 | 16篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 20篇 |
1984年 | 24篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1981年 | 8篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1974年 | 5篇 |
排序方式: 共有4477条查询结果,搜索用时 343 毫秒
961.
Recent accurate data on F 2(x,Q) and on F 2 c (x,Q) from HERA at small-x require a more precise treatment of charm production in the global analysis of parton distributions. We improve on existing global QCD analyses by implementing the leptoproduction formalism of Aivazis et al. which represents a natural generalization of the conventional zero mass QCD parton framework to include heavy quark mass effects. We also perform analyses based on the fixed-flavor-number scheme, which is widely used in the literature, and demonstrate their uses and limitations. We discuss the implications of the improved treatment of heavy quark mass effect in practical applications of PQCD and compare our results with recent related works. 相似文献
962.
V. Fanti A. Lai L. Musa A. Nappi M.G. Setzu 《Zeitschrift fur Physik C Particles and Fields》1997,76(4):653-657
The rate of the decay of the neutral K meson into K L 0 → μμγ has been measured with the NA48 detector at the CERN SPS. A total of 59 events has been observed with an estimated background of 14 events. The overall Kaon flux was determined to be 1.2 × 109. This observation corresponds to a branching ratio of (3.4 ± 0.6stat ± 0.4sys × 10-7. Using this branching ratio the parameter αK* describing the relative strength of the two contributing amplitudes to this decay, through intermediate pseudoscalar or vectorlike mesons, was measured to be αK* = -0.04+ -0.21 +0.24 . 相似文献
963.
964.
The steroids were derivatised with aroyl chloride to yield the corresponding benzoates. Among them, their 4-methoxybenzoates were obtained in greatest yield and were analyzed by HPLC with the smallest detection limit (10 ng) by means of a UV detector (254 nm). This method of derivatization can be applied for analysis of steroids and triterpenoids from crude products with the HPLC technique. 相似文献
965.
Analytical magnetapheresis is a newly developed technique for analyzing magnetic particles. The magnetically susceptible particles form deposition patterns after flowing through a separation channel in a magnetic field. The separation channel requirements for analytical magnetapheresis are an excellent seal for the carrier flow and ease of disassembly after magnetapheresis. Previously used separation channels often exhibit variable channel leakage and unstable flow velocities. We improved the separation channel assembly to ensure stable, high flow velocities and characterized the system with various magnetically susceptible and labeled particles. Our new separation channel featured silicone sealant with embedded nylon wires and met analytical magnetapheresis requirements. Characterization of this system was performed using several magnetically susceptible particles, and we studied a variety of diamagnetic sample labels with paramagnetic ions and magnetically susceptible particles at different flow-rates and solution pH values. The minimal labeling concentration for complete deposition was determined to be approximately 2.50 x 10(10) ions per particle for test samples at a flow velocity of 0.67 mm s(-1) and a magnetic field gradient of 2.8 T mm(-1). Silicas, yeasts and blood cells were used for these studies. We determined that the minimal difference in magnetic susceptibility (delta(chi)) for successful separation was approximately 2.00 x 10(-6) [SI]. The magnetic susceptibilities of Dynabeads M-450 at several separation distances and flow-rates were determined to be 0.25 [SI], within 2% of values published by other workers. The magnetic susceptibilities of various ion-labeled yeasts and cells were determined and most varied by less than 5% at different flow-rates. The results of this study provide very important references for analytical magnetapheresis applications. 相似文献
966.
Lai‐Cai Li Ping Deng Yuan‐Qiang Zhu Dong Zha An‐Min Tian Ming‐Hou Xu Ning‐Bew Wong 《International journal of quantum chemistry》2005,104(3):367-372
In this work ab initio molecular orbital methods were employed to study the coal combustion reaction mechanisms of tin oxidized by different oxidants, including HOCl, HCl, ClO, ClO2, NO3, CO2, and O2. Eleven reaction pathways were identified. The results show that Sn can react with HCl, ClOO, CO2, O2, and NO3 to form SnO and SnCl. SnO can be oxidized into SnCl by HOCl and HCl. SnCl can be further oxidized into a soluble compound, SnCl2. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
967.
968.
Three bulky mono-dentate alkyne-bridged dicobalt-phosphine complexes [(μ-PPh2CH2PPh2)Co2(CO)4](μ,η-PhCCPPh2) 4a, [(μ-PPh2CH2PPh2)Co2(CO)4](μ,η-Me3CCCPPh2) 5a and [(μ-PPh2CH2PPh2)Co2(CO)4](μ,η-Me3SiCCPPh2) 6a were prepared from the reactions of the bis(diphenylphosphino)methylene (dppm) bridged dicobalt complex Co2(CO)6(μ-Ph2PCH2PPh2) with PhCCPPh21, Me3CCCPPh22, and Me3SiCCPPh23, respectively. These three metal-containing compounds 4a, 5a and 6a were employed as ligands, replacing conventional organic phosphines, in the Suzuki cross-coupling reactions and have been proved to be effective, authentic mono-dentate phosphine ligands. 相似文献
969.
Shyh‐Ying Chiou Gin‐Win Lai Yi‐Hon Tsai Yu‐Ru Lee Long‐Yau Lin Gia‐Lih Lin 《中国化学会会志》2005,52(4):843-848
The goal of this work is to determine enzyme kinetics and mechanisms of acetylcholinesterase and butyrylcholinesterase inhibition by five cardiovascular drugs, lovastatin, simvastatin, amlodipine besylate, nifedipine, and hydralazine hydrochloride, and two benzodiazepines, diazepam and chlordiazepoxide hydrochloride. All drugs in this study are reversible mixed‐type inhibitors of acetylcholinesterase and butyrylcholinesterase. The pKi values for acetylcholinesterase and butyrylcholinesterase inhibition by the cardiovascular drugs are linearly correlated with the molecular weights of the drugs with the slopes of 0.005 and 0.0021, respectively. Therefore, van der Waals' interactions between acetylcholinesterase and the cardiovascular drugs are stronger than those between butyrylcholinesterase and the drugs. This is probably due to a smaller active site gorge and a more significant peripheral anionic substrate binding site of acetylcholinesterase than those of butyrylcholinesterase. The fact that the pKi values for both butyrylcholinesterase and acetylcholinesterase inhibition by the cardiovascular drugs are linearly correlated with each other suggests that both enzyme inhibition reactions proceed via a common mechanism. Furthermore, amlodipine besylate may be useful in Alzheimer's disease treatment similar to donepezil. 相似文献
970.
The sol-gel processing of lead-free (Na,K) NbO3 ferroelectric films was studied. Sodium ethoxide (NaOC2H5) and potassium ethoxide (KOC2H5) were prepared by reacting solid Na and K with ethanol (99.7%) in a solvent of 2-methoxyethanol. 0.5-μm-thick (Na,K)NbO3 thin films with orthorhombic perovskite structure were obtained by pyrolyzing at 400°C and annealing at 800–900°C. The films
had relatively dense and uniform microstructure with grain size of about 50 nm, whose ferroelectricity was proved by the P-E hysteresis loop measurement. It was found that excess K was effective to reduce the annealing temperature for the crystallization
of sol-gel-derived (Na,K)NbO3 thin films. 相似文献