全文获取类型
收费全文 | 7008篇 |
免费 | 369篇 |
国内免费 | 804篇 |
专业分类
化学 | 3523篇 |
晶体学 | 249篇 |
力学 | 677篇 |
综合类 | 102篇 |
数学 | 2094篇 |
物理学 | 1536篇 |
出版年
2024年 | 19篇 |
2023年 | 73篇 |
2022年 | 119篇 |
2021年 | 142篇 |
2020年 | 149篇 |
2019年 | 158篇 |
2018年 | 130篇 |
2017年 | 163篇 |
2016年 | 181篇 |
2015年 | 187篇 |
2014年 | 214篇 |
2013年 | 500篇 |
2012年 | 283篇 |
2011年 | 516篇 |
2010年 | 462篇 |
2009年 | 493篇 |
2008年 | 487篇 |
2007年 | 465篇 |
2006年 | 413篇 |
2005年 | 360篇 |
2004年 | 317篇 |
2003年 | 268篇 |
2002年 | 411篇 |
2001年 | 187篇 |
2000年 | 176篇 |
1999年 | 165篇 |
1998年 | 146篇 |
1997年 | 106篇 |
1996年 | 129篇 |
1995年 | 102篇 |
1994年 | 83篇 |
1993年 | 56篇 |
1992年 | 60篇 |
1991年 | 35篇 |
1990年 | 26篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1985年 | 40篇 |
1984年 | 21篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 44篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1978年 | 25篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1975年 | 20篇 |
1974年 | 24篇 |
1973年 | 20篇 |
排序方式: 共有8181条查询结果,搜索用时 15 毫秒
991.
In this paper we prove the best possible Lp estimates for the ?-b -equation on the boundaries of real ellipsoids in Cn, and provide examples to show why they cannot be improved. 相似文献
992.
For a large class of groups G a precise congruence subgroup of the group generated by the bicyclic units of the integral group ring ZG is determined. As an application an upper bound is calculated for the index in the unit group of ZG for the group generated by the Bass cyclic units and the bicyclic units. 相似文献
993.
Jeremy Bernstein 《Physics in Perspective (PIP)》2008,10(3):269-286
I present a biographical profile of John S. Bell based upon extensive interviews I had with him. I present Bell‘s views on
the quantum theory along with a simple explanation of his identity.
Jeremy Bernstein is Professor Emeritus of Physics at the Stevens Institute of Technology
and a former staff writer for The New Yorker. 相似文献
994.
Synthesis and Characterization of Bis(methyl-2-pyridylmethylidenedrazinecarbodithioate)manganese(II)
Mao-Sheng Liu Qiong-Yan Yu Zheng-Yuan Zhou Qing-Guang Zhan He-Ping Zeng Yue-Peng Cai 《Journal of chemical crystallography》2008,38(11):845-849
Abstract Brown crystals of title compound, bis(methyl-2-pyridylmethylidene-drazinecarbodithioate)manganese(II), was formed by reaction
of methyl-2-pyridylmethylidenehydrazinecarbodithioate(HNNS) with manganese perchlorate at 323 K temperature and recrystallized
from ethanol at room temperature, crystallizing in the monoclinic system, space group P2(1)/n, with a = 11.631(2), b = 14.010(3), c = 13.128(3) ?, β = 105.791(11)°; V = 2058.6(7) ?3, D
c
= 1.533 g/cm3, Z = 4, C16H16N6MnS4, M
r
= 475.53, μ(M0 K
α
) = 1.060 mm−1, F (000) = 972. The structure was refined to R = 0.0441, wR = 0.1196 for 3,682 (I > 2σ(I)) reflections and S = 1.110. In title complex the coordination geometry about manganese is distorted octahedral and the two
ligands in the mer configuration (S and tertiary N atom cis to each other and the iminic N atoms trans). Furthermore, the neutral molecule units MnII(NNS)2 are connected by hydrogen bonds C–H···N and C–H···S and form a three dimensional ordered network structure.
Graphical Abstract The title compound, Mn(NNS)2, exists as discrete molecules in which the central MnII
atom is coordinated by two S atoms and four N atoms from two ligands NNS- molecules
in a mer-octahedral configuration.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
995.
Hansen MR Brorson M Bildsøe H Skibsted J Jakobsen HJ 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,190(2):316-326
The WURST (wideband uniform rate smooth truncation) and hyperbolic secant (HS) pulse elements have each been employed as pairs of inversion pulses to induce population transfer (PT) between the four energy levels in natural abundance solid-state (33)S (spin I=3/2) MAS NMR, thereby leading to a significant gain in intensity for the central transition (CT). The pair of inversion pulses are applied to the satellite transitions for a series of inorganic sulfates, the sulfate ions in the two cementitious materials ettringite and thaumasite, and the two tetrathiometallates (NH(4))(2)WS(4) and (NH(4))(2)MoS(4). These materials all exhibit (33)S quadrupole coupling constants (C(Q)) in the range 0.1-1.0 MHz, with precise C(Q) values being determined from analysis of the PT enhanced (33)S MAS NMR spectra. The enhancement factors for the WURST and HS elements are quite similar and are all in the range 1.74-2.25 for the studied samples, in excellent agreement with earlier reports on HS enhancement factors (1.6-2.4) observed for other spin I=3/2 nuclei with similar C(Q) values (0.3-1.2 MHz). Thus, a time saving in instrument time by a factor up to five has been achieved in natural abundance (33)S MAS NMR, a time saving which is extremely welcome for this important low-gamma nucleus. 相似文献
996.
In this paper, a biocatalytic route is described wherein PPL, lipase from porcine pancreas, in conjunction with water on reaction with different thiophenols and styrene oxides undergo thiolysis with C-S bond formation without the use of any metal catalysts, oxidants, bases, additives or organic solvents towards formation of β-hydroxysulfides in good to excellent yields with high regioselectivity at room temperature. Furthermore, PPL also facilitates thiophenols to undergo hydrothiolation with styrenes or phenylacetylenes in sole water and thus forming linear thioethers or vinylsulfides respectively via C–S bond formation. In addition to the straightforward and atom-efficient protocol, a gram-scale synthesis of β-hydroxysulfide and recyclability for three consecutive cycles without decrease in efficiency of PPL make our biocatalytic protocol for constructing C–S bond highly valuable from both environmental and economic viewpoints than traditional chemical practices. 相似文献
997.
A novel and efficient route to asymmetric synthesis of Maraviroc by using (S)-tert-butanesulfinamide as chiral auxiliary is described. Two interesting impurities of the process are isolated and identified. The synthesis was concise, mild, and easy to operate. The overall yield and stereoselectivity were excellent. 相似文献
998.
Nikita A. Kazin Nadezhda S. Demina Roman A. Irgashev Ekaterina F. Zhilina Gennady L. Rusinov 《Tetrahedron》2019,75(33):4686-4696
The effective approaches for regioselective double formylation and acetylation of 5,12-dialkyl-6,7-diaryl-substituted 5,12-dihydroindolo[3,2-a]carbazoles by their treatment with dichloromethyl methyl ether in the presence of SnCl4 or with acetyl chloride in the presence of AlCl3 to afford the 2,9-diformyl or 2,9-diacetyl derivatives, respectively, were developed. Furthermore, new 2,9-bis(2,2-dicyanovinyl) derivatives were synthesized by the Knoevenagel condensation of diformyl-containing substrates with malononitrile, when new 2,9-bis(quinoxaline-2-yl)- and 2,9-bis(benzo[g]quinoxaline-2-yl) derivatives were formed via microwave-promoted oxidation of diacetyl-containing substrates with SeO2 to the corresponding diglyoxals, followed by the reaction of these intermediates with o-phenylendiamine or 2,3-diaminonaphthalene, respectively. The basic optical and electrochemical properties of some 5,12-dihydroindolo[3,2-a]carbazoles were investigated. 相似文献
999.
The new transition metal oxo‐thiostannate {[Ni(cyclen)]6[Sn6S12O2(OH)6]} · 2(ClO4) · 19H2O ( 1 ) was prepared under hydrothermal conditions using Na4SnS4 · 14H2O and [Ni(cyclen)](ClO4)2 as reactants. In the crystal structure the rare [Sn6S12O2(OH)6]10– anion is observed, which is composed of SnS2O(OH)3 and SnS4O2 octahedra, and SnS4 tetrahedra sharing edges and corners. The anion is expanded by six Ni2+ centered complexes via Ni–S and Ni–OH bonds. The photocatalytic properties for the visible light driven hydrogen evolution reaction shows that 26.6 mmol · g–1 H2 were evolved after 3 h. 相似文献
1000.
Junpeng Xie Xiaodan Li Haojie Lai Zhijuan Zhao Jinliang Li Wanggang Zhang Weiguang Xie Yiming Liu Wenjie Mai 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(41):14882-14889
Metal–organic framework‐derived NiCo2.5S4 microrods wrapped in reduced graphene oxide (NCS@RGO) were synthesized for potassium‐ion storage. Upon coordination with organic potassium salts, NCS@RGO exhibits an ultrahigh initial reversible specific capacity (602 mAh g?1 at 50 mA g?1) and ultralong cycle life (a reversible specific capacity of 495 mAh g?1 at 200 mA g?1 after 1 900 cycles over 314 days). Furthermore, the battery demonstrates a high initial Coulombic efficiency of 78 %, outperforming most sulfides reported previously. Advanced ex situ characterization techniques, including atomic force microscopy, were used for evaluation and the results indicate that the organic potassium salt‐containing electrolyte helps to form thin and robust solid electrolyte interphase layers, which reduce the formation of byproducts during the potassiation–depotassiation process and enhance the mechanical stability of electrodes. The excellent conductivity of the RGO in the composites, and the robust interface between the electrodes and electrolytes, imbue the electrode with useful properties; including, ultrafast potassium‐ion storage with a reversible specific capacity of 402 mAh g?1 even at 2 A g?1. 相似文献