全文获取类型
收费全文 | 4014篇 |
免费 | 149篇 |
国内免费 | 17篇 |
专业分类
化学 | 3271篇 |
晶体学 | 26篇 |
力学 | 27篇 |
数学 | 192篇 |
物理学 | 490篇 |
无线电 | 174篇 |
出版年
2023年 | 18篇 |
2022年 | 31篇 |
2021年 | 42篇 |
2020年 | 40篇 |
2019年 | 57篇 |
2018年 | 34篇 |
2017年 | 39篇 |
2016年 | 83篇 |
2015年 | 78篇 |
2014年 | 101篇 |
2013年 | 205篇 |
2012年 | 215篇 |
2011年 | 252篇 |
2010年 | 143篇 |
2009年 | 164篇 |
2008年 | 260篇 |
2007年 | 273篇 |
2006年 | 229篇 |
2005年 | 234篇 |
2004年 | 189篇 |
2003年 | 176篇 |
2002年 | 136篇 |
2001年 | 78篇 |
2000年 | 93篇 |
1999年 | 71篇 |
1998年 | 59篇 |
1997年 | 67篇 |
1996年 | 62篇 |
1995年 | 39篇 |
1994年 | 37篇 |
1993年 | 51篇 |
1992年 | 34篇 |
1991年 | 27篇 |
1990年 | 35篇 |
1989年 | 24篇 |
1988年 | 27篇 |
1987年 | 30篇 |
1986年 | 22篇 |
1985年 | 52篇 |
1984年 | 45篇 |
1983年 | 17篇 |
1982年 | 32篇 |
1981年 | 35篇 |
1980年 | 32篇 |
1979年 | 46篇 |
1978年 | 28篇 |
1977年 | 25篇 |
1976年 | 27篇 |
1975年 | 22篇 |
1974年 | 29篇 |
排序方式: 共有4180条查询结果,搜索用时 15 毫秒
991.
This work deals with the cationic ring-opening polymerization of a cyclic thiocarbonate, 5,5-dimethyl-1,3-dioxane-2-thione (1). The polymerization was carried out with 2 mol% of trifluoromethanesulfonic acid, methyl trifluoromethanesulfonate, boron trifluoride etherate, or triethyloxonium tetrafluoroborate as an initiator to afford the polythiocarbonate with the narrow molecular weight distribution (Mn = 11200-31000, Mw/Mn = 1.04-1.15). The molecular weight of the obtained polymer could be controlled by the feed ratio of the monomer to the initiator and increased when the second monomer was added to the polymerization mixture after quantitative consumption of 1 in the first stage, supporting that the cationic ring-opening polymerization of 1 proceeded via a living process. 相似文献
992.
993.
Takeshi Sakai Takehiko Yagi Tetsuo Irifune Hirokazu Kadobayashi Naohisa Hirao Takehiro Kunimoto 《高压研究》2018,38(2):107-119
We have developed a double stage diamond anvil cell (ds-DAC) technique for reproducible pressure by precisely fabricating 2nd stage anvils using a focused ion beam system. We used 2nd stage micro-anvils made of ultra-fine (10?nm) nano-polycrystalline diamond with various shapes and dimensions synthesized from glassy carbon at high pressure and temperature. The X-ray diffraction patterns from the rhenium sample always showed very broad peaks due to large pressure gradients in the culet of the micro-anvils. Deconvolution of the broad 101 diffraction peak results in compression of rhenium to V/V0?=?0.633 for the smallest d-spacing. The calculated pressure for this minimum volume varies from 430 to 630?GPa, depending on the choice of the equation of state of rhenium. We conclude that the most likely pressure achieved for the minimum volume of rhenium is in a range of 430–460?GPa based on a calibration using the platinum pressure scale to 280?GPa and the latter value of 630?GPa is unreasonably high, suggesting that the pressures in an earlier study for the equation of state of rhenium would have been significantly overestimated. 相似文献
994.
Isoda T Yamamura I Tamai S Kumagai T Nagao Y 《Chemical & pharmaceutical bulletin》2006,54(10):1408-1411
An orally active carbapenem L-084, which exhibits high bioavailability in humans, has a 1-(1,3-thiazolin-2-yl)azetidin-3-ylthio moiety at the C-2 position of the 1beta-methylcarbapenem skeleton. We established a practical and cost-effective synthesis of 3-mercapto-1-(1,3-thiazolin-2-yl)azetidine (1) for further scale-up production of L-084. This synthesis method entails an industry-oriented reaction of azetidine ring-closure to yield N-benzyl-3-hydroxyazetidine (16), which is eventually converted to 1 via key intermediates, Bunte salts 19 and 20. 相似文献
995.
Ohshima T Xu Y Takita R Shimizu S Zhong D Shibasaki M 《Journal of the American Chemical Society》2002,124(49):14546-14547
The enantioselective total synthesis of (-)-strychnine was accomplished through the use of the highly practical catalytic asymmetric Michael reaction (0.1 mol % of (R)-ALB, more than kilogram scale, without chromatography, 91% yield and >99% ee) as well as a tandem cyclization that simultaneously constructed B- and D-rings (>77% yield). Moreover, newly developed reaction conditions for thionium ion cyclization, NaBH3CN reduction of the imine moiety in the presence of Lewis acid to prevent ring opening reaction, and chemoselective reduction of the thioether (desulfurization) in the presence of exocyclic olefin were pivotal to complete the synthesis. The described chemistry paves the way for the synthesis of more advanced Strychnos alkaloids. 相似文献
996.
Liu P Rodriguez JA Asakura T Gomes J Nakamura K 《The journal of physical chemistry. B》2005,109(10):4575-4583
X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study the interaction of thiophene, H(2)S, and S(2) with Ni(2)P(001), alpha-Mo(2)C(001), and polycrystalline MoC. In general, the reactivity of the surfaces increases following the sequence MoC < Ni(2)P(001) < alpha-Mo(2)C(001). At 300 K, thiophene does not adsorb on MoC. In contrast, Ni(2)P(001) and alpha-Mo(2)C(001) can dissociate the molecule easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C(x)H(y) fragments and S) from these surfaces. Our experimental and theoretical studies indicate that the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbed sulfur into gaseous H(2)S. Ni(2)P is a better catalyst for HDS than Mo(2)C or MoC. The P sites in the phosphide play a complex and important role. First, the formation of Ni-P bonds produces a weak "ligand effect" (minor stabilization of the Ni 3d levels and a small Ni --> P charge transfer) that allows a high activity for the dissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surface decreases due to an "ensemble effect" of P, which prevents the system from deactivation induced by high coverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding to the products of the decomposition of thiophene and the H adatoms necessary for hydrogenation. 相似文献
997.
Both the enantiomers of sclerosporin and sclerosporal were synthesized from (-)-carvone. (4R,9R,10S)-(+)-Sclerosporin and (4R,9R,10S)-(-)-sclerosporal were identified as the natural enantiomers by a comparison of their CD-spectra. An intramolecular Diels-Alder route was proved to be an efficient method of preparing sufficient amounts of (+)- for the biological study. 相似文献
998.
Masato Takenaka Yoshikazu Hashimoto Takeshi Iwasa Tetsuya Taketsugu Gediminas Seniutinas Armandas Balčytis Saulius Juodkazis Yoshiaki Nishijima 《Journal of computational chemistry》2019,40(8):925-932
First principles electrodyanmics and quantum chemical simulations are performed to gain insights into the underlying mechanisms of the surface enhanced Raman spectra of 22BPY adsorbed on pure Au and Ag as well as on Au–Ag alloy nanodiscs. Experimental SERS spectra from Au and Ag nanodiscs show similar peaks, whereas those from Au–Ag alloy reveal new spectral features. The physical enhancement factors due to surface nano-texture were considered by numerical FDTD simulations of light intensity distribution for the nano-textured Au, Ag, and Au–Ag alloy and compared with experimental results. For the chemical insights of the enhancement, the DFT calculations with the dispersion interaction were performed using Au20, Ag20, and Au10Ag10 clusters of a pyramidal structure for SERS modeling. Binding of 22BPY to the clusters was simulated by considering possible arrangements of vertex and planar physical as well as chemical adsorption models. The DFT results indicate that 22BPY prefers a coplanar adsorption on a (111) face with trans-conformation having close energy difference to cis-conformation. Binding to pure Au cluster is stronger than to pure Ag or Au–Ag alloy clusters and adsorption onto the alloy surface can deform the surface. The computed Raman spectra are compared with experimental data and assignments for pure Au and Ag models are well matching, indicating the need of dispersion interaction to reproduce strong Raman signal at around 800 cm–1. This work provides insight into 3D character of SERS on nanorough surfaces due to different binding energies and bond length of nanoalloys. © 2018 Wiley Periodicals, Inc. 相似文献
999.
Katsunobu Yamamoto Tianshu Zhou Min Zhu Takeshi Kato Litong Jin 《Analytica chimica acta》2003,480(1):109-117
In this paper, preparation of a novel pH ultramicrosensor and its physiological application has been discussed. A tungsten nanoelectrode was produced by an etching method in 0.1 mol/l NaOH solution at the potential of +0.4 V (versus Ag/AgCl reference electrode) for about 100 s and the diameters ranged from 500 to 800 nm. The pH ultramicrosensor was fabricated by producing WO3 at W nanoelectrode surface by electrooxidation in 2.0 mol/l H2SO4 solution between 1.0 and 2.0 V. At last, Nafion was coated on the surface of WO3 to protect the pH ultramicrosensor. The W/WO3 pH ultramicrosensor exhibited a good pH linear region from 2.0 to 12.0 with a super-Nernstian slope of −53.5 ± 0.5 mV/pH unit. Response times ranged from 3 s at about pH 6.0-7.0 up to 15 s at high pH. An interference of various ions to the pH measurement was also studied in this paper. We also studied the lifetime, stability and reproducibility of the W/WO3 pH ultramicrosensor. In order to testing the performance of W/WO3 ultramicrosensor, we applied it to measure the extracellular pH values and a pH variation was also given about the normal, damaged and recovery endothelial cells. 相似文献
1000.
Takeshi Higuchi Kiwamu Motoyoshi Hidekazu Sugimori Hiroshi Jinnai Hiroshi Yabu Masatsugu Shimomura 《Macromolecular rapid communications》2010,31(20):1773-1778
Block copolymer nanopaticles were prepared from the mixture solutions containing good/poor solvents by a simple evaporation process. The block copolymers formed disorder, unidirectionally stacked lamellar, and onion‐like structures in nanoparticles depending on preparation temperatures. Thermal annealing induced the disorder‐order phase transition and order‐order phase transformation in the block copolymer nanoparticles, even though the annealing temperature is lower than the of one polymer segment. The unusual thermal behaviors suggest that the glass transition temperature of the block copolymer is decreased by the effect of nanoparticle, whose surface areas are larger than their volumes.