全文获取类型
收费全文 | 2397篇 |
免费 | 39篇 |
国内免费 | 14篇 |
专业分类
化学 | 1443篇 |
晶体学 | 31篇 |
力学 | 83篇 |
数学 | 420篇 |
物理学 | 473篇 |
出版年
2020年 | 18篇 |
2019年 | 18篇 |
2018年 | 19篇 |
2017年 | 22篇 |
2016年 | 34篇 |
2015年 | 30篇 |
2014年 | 29篇 |
2013年 | 113篇 |
2012年 | 94篇 |
2011年 | 95篇 |
2010年 | 57篇 |
2009年 | 67篇 |
2008年 | 101篇 |
2007年 | 109篇 |
2006年 | 105篇 |
2005年 | 109篇 |
2004年 | 113篇 |
2003年 | 80篇 |
2002年 | 77篇 |
2001年 | 48篇 |
2000年 | 45篇 |
1999年 | 56篇 |
1998年 | 32篇 |
1997年 | 32篇 |
1996年 | 34篇 |
1995年 | 23篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 21篇 |
1990年 | 25篇 |
1989年 | 37篇 |
1988年 | 14篇 |
1987年 | 24篇 |
1986年 | 29篇 |
1985年 | 45篇 |
1984年 | 50篇 |
1983年 | 40篇 |
1982年 | 41篇 |
1981年 | 58篇 |
1980年 | 39篇 |
1979年 | 43篇 |
1978年 | 39篇 |
1977年 | 44篇 |
1976年 | 25篇 |
1975年 | 33篇 |
1974年 | 35篇 |
1973年 | 37篇 |
1972年 | 26篇 |
1970年 | 18篇 |
排序方式: 共有2450条查询结果,搜索用时 468 毫秒
921.
J.V. Howard 《European Journal of Operational Research》1981,8(2):115-122
Every year the National Coal Board divides about £100 million capital for purchasing mining machinery between 12 mining regions. This paper discusses how the allocation should be made, and in particular what information should be provided to the senior management staff who take the final decision. 相似文献
922.
The emergence of the mechanical bond during the past 25 years is giving chemistry a fillip in more ways than one. While its arrival on the scene is already impacting materials science and molecular nanotechnology, it is providing a new lease of life to chemical synthesis where mechanical bond formation occurs as a consequence of the all-important templation orchestrated by molecular recognition and self-assembly. The way in which covalent bond formation activates noncovalent bonding interactions, switching on molecular recognition that leads to self-assembly, and the template-directed synthesis of mechanically interlocked molecules—of which the so-called catenanes and rotaxanes may be regarded as the prototypes—has introduced a level of integration into chemical synthesis that has not previously been attained jointly at the supramolecular and molecular levels. The challenge now is to carry this level of integration during molecular synthesis beyond relatively small molecules into the realms of precisely functionalized extended molecular structures and superstructures that perform functions in a collective manner as the key sources of instruction, activation, and performance in multi-component integrated circuits and devices. These forays into organic chemistry by a scientific nomad are traced through thick and thin from the Athens of the North to the Windy City by Lake Michigan with interludes on the edge of the Canadian Shield beside Lake Ontario, in the Socialist Republic of South Yorkshire, on the Plains of Cheshire beside the Wirral, in the Midlands in the Heartland of Albion, and in the City of Angels beside the Peaceful Sea. 相似文献
923.
Gutsulyak DV Kuzmina LG Howard JA Vyboishchikov SF Nikonov GI 《Journal of the American Chemical Society》2008,130(12):3732-3733
Reactions of a new borohydride complex 2 with hydrosilanes afford half-sandwich dihydride silyl complexes 3a-f. According to X-ray and DFT evidence complexes 3 have unprecedented double H...Si...H interligand interactions. 相似文献
924.
Zhang J Kumbhar A He J Das NC Yang K Wang JQ Wang H Stokes KL Fang J 《Journal of the American Chemical Society》2008,130(45):15203-15209
We report a preparation of high-quality cubic PbTe nanocrystals and their assembly into both square-array, two-dimensional patterns and three-dimensional simple cubic super crystals. The influence of oleylamine in the nanocrystal synthesis and core-shell formation through an anion-exchange mechanism was also studied. The simple cubic super crystals together with two-dimensional assembly patterns containing PbTe nanocubes and their core-shell building blocks were examined using TEM, SEM, AFM, XRD, SAXS, and FTIR. Such super crystals consisting of cubic structural building blocks may allow engineering of more complex materials from which novel properties may emerge. 相似文献
925.
Alvarez NT Kittrell C Schmidt HK Hauge RH Engel PS Tour JM 《Journal of the American Chemical Society》2008,130(43):14227-14233
Ultraviolet (UV) irradiation of single wall carbon nanotubes (SWCNTs) individually dispersed in surfactants leads to diameter and type-selective photohydroxylation of the nanotubes. Photohydroxylation of first semiconductor and then small diameter metallic SWCNTs was confirmed after 254 nm UV irradiation in acidic, neutral, and basic aqueous solutions at ambient and elevated temperatures. The increased oxygen content of the SWCNTs after UV irradiation, as detected by X-ray photoelectron spectroscopy, suggests that SWCNTs were hydroxylated by reaction with water. Attenuated total reflectance Fourier transform infrared analysis provides evidence of hydroxyl functional groups on their surface. This photochemical reaction is impeded by molecular oxygen and appears to involve a reactive intermediate generated in the vicinity of semiconducting SWCNTs. This represents a noncontaminating selective reaction in the liquid phase that uses an intrinsic property of the tubes. 相似文献
926.
Duque JG Cognet L Parra-Vasquez AN Nicholas N Schmidt HK Pasquali M 《Journal of the American Chemical Society》2008,130(8):2626-2633
Aqueous surfactant suspensions of single walled carbon nanotubes (SWNTs) are very sensitive to environmental conditions. For example, the photoluminescence of semiconducting SWNTs varies significantly with concentration, pH, or salinity. In most cases, these factors restrict the range of applicability of SWNT suspensions. Here, we report a simple strategy to obtain stable and highly luminescent individualized SWNTs at pH values ranging from 1 to 11, as well as in highly saline buffers. This strategy relies on combining SWNTs previously suspended in sodium dodecylbenzene sulfonate (SDBS) with biocompatible poly(vinyl pyrrolidone) (PVP), which can be polymerized in situ to entrap the SWNT-SDBS micelles. We present a model that accounts for the photoluminescence stability of these suspensions based on PVP morphological changes at different pH values. Moreover, we demonstrate the effectiveness of these highly stable suspensions by imaging individual luminescent SWNTs on the surface of live human embryonic kidney cells (HEK cells). 相似文献
927.
A reliable method of automatically assigning one-dimensional proton spectra is described. The method relies on the alignment of the proton spectrum with an associated heteronuclear single-quantum coherence (HSQC) spectrum, transferring the stoichiometry and couplings to the HSQC. The HSQC spectrum is then assigned using a linear assignment procedure in which a fitness function incorporating (1)H chemical shifts, (1)H couplings and (13)C shifts are employed. The method uniquely employs a sequential procedure in which only correlations of like stoichiometry are assigned at the same time. 相似文献
928.
A combination of vapor phase infrared spectroscopy and ab initio calculations has been used to show that sulfur is weaker than, but nearly equivalent to, oxygen as a hydrogen bond acceptor. Enthalpies of hydrogen bond formation were obtained for the hydrogen bonded complexes formed between methanol and either dimethyl sulfide or dimethyl ether by temperature dependence studies of the infrared spectra. 相似文献
929.
Baiget L Batsanov AS Dyer PW Fox MA Hanton MJ Howard JA Lane PK Solomon SA 《Dalton transactions (Cambridge, England : 2003)》2008,(8):1043-1054
The syntheses of the cyclic N-phosphino-amidines and -guanidines Ph2PN(Pri)C(NPri2)N(Pri) ( 1) and Ph2PN(c-Hex)C(R)N(c-Hex) [R = piperazino ( 2), morpholino ( 3), Me ( 4), and Ph ( 5)] are reported. DFT studies have identified the preferred structures for compounds 1-5 with the E-configuration being the most stable form for the N-phosphino-amidines, while the Z-conformation is preferred for the N-phosphino-guanidines something that highlights the potential of such systems to act as kappa2-P,N-chelates. The differences in donor characteristics of 2-5 have been probed through the study of their corresponding P(V) selenide derivatives ( 6-9) and their complexes with the cis-RhCl(CO) (10-12) and cis-PdCl2 (13-17) fragments. In line with the DFT studies both the amidines and guanidines are found to coordinate as kappa2-P,N-chelates, with the latter being moderately weaker donor ligands. The molecular structures of compounds 3 and 4, together with those of the Rh and Pd complexes 10 and 15, respectively, have been determined in the solid state by X-ray crystallography, the latter confirming bidentate kappa2-P,N-chelation. 相似文献
930.
The rhodium-catalyzed reductive intramolecular ring expansion of N-(ethoxycarboxymethyl)oxazolidines was carried out under an atmosphere of carbon monoxide and hydrogen to afford N-methylmorpholin-2-ones in good to excellent yields. 相似文献