全文获取类型
收费全文 | 5153篇 |
免费 | 101篇 |
国内免费 | 28篇 |
专业分类
化学 | 3382篇 |
晶体学 | 40篇 |
力学 | 110篇 |
数学 | 761篇 |
物理学 | 989篇 |
出版年
2021年 | 33篇 |
2020年 | 53篇 |
2019年 | 63篇 |
2018年 | 46篇 |
2017年 | 42篇 |
2016年 | 89篇 |
2015年 | 74篇 |
2014年 | 122篇 |
2013年 | 240篇 |
2012年 | 208篇 |
2011年 | 250篇 |
2010年 | 166篇 |
2009年 | 148篇 |
2008年 | 236篇 |
2007年 | 215篇 |
2006年 | 216篇 |
2005年 | 203篇 |
2004年 | 157篇 |
2003年 | 158篇 |
2002年 | 168篇 |
2001年 | 107篇 |
2000年 | 111篇 |
1999年 | 77篇 |
1998年 | 82篇 |
1997年 | 81篇 |
1996年 | 86篇 |
1995年 | 88篇 |
1994年 | 79篇 |
1993年 | 79篇 |
1992年 | 80篇 |
1991年 | 80篇 |
1990年 | 62篇 |
1989年 | 67篇 |
1988年 | 79篇 |
1987年 | 61篇 |
1986年 | 80篇 |
1985年 | 82篇 |
1984年 | 78篇 |
1983年 | 61篇 |
1982年 | 62篇 |
1981年 | 63篇 |
1980年 | 84篇 |
1979年 | 55篇 |
1978年 | 58篇 |
1977年 | 53篇 |
1976年 | 46篇 |
1975年 | 44篇 |
1974年 | 44篇 |
1973年 | 37篇 |
1971年 | 26篇 |
排序方式: 共有5282条查询结果,搜索用时 15 毫秒
71.
72.
Lapeña AM Gross AF Tolbert SH 《Langmuir : the ACS journal of surfaces and colloids》2005,21(1):470-480
In this work, we examine the role of curvature and surfactant packing in controlling the structure of periodic silica/surfactant composites by driving such materials through a transformation from a hexagonal to a lamellar phase. We focus on how the interplay of desired packing and volume constraints dictates the resulting structures. In general, surfactants expand in a complex way upon heating, and this can cause a change in the optimal packing geometry. However, the presence of a rigid silica framework may prevent surfactants from reaching this preferred volume and/or curvature. Real-time in situ X-ray diffraction is used to monitor the structural evolution of these materials heated under hydrothermal treatments. Because the thermal-driven disorder of the surfactant tails drives the phase transition, we examine four types of composites with varying tail density. Ordinarily, composites consist of surfactants with one 20-carbon tail and one positively charged ammonium headgroup. Tail density is varied by replacing a small amount (0-16%) of these single-tail, single-head surfactants with single-tail, double-head 'gemini' surfactants. A greater head--tail ratio indeed produces different results, causing the phase transition to occur at higher temperatures. Using simple geometric models to gain better understanding of our experimental results, we find that, while both unfavorable curvature and limited volume may exist for the surfactants in these composites, the constrained curvature appears to be the dominant effect in driving structural rearrangement. 相似文献
73.
Boullant E Cano J Journaux Y Decurtins S Gross M Pilkington M 《Inorganic chemistry》2001,40(16):3900-3904
The numerical simulations of the magnetic properties of extended three-dimensional networks containing M(II) ions with an S = 5/2 ground-state spin have been carried out within the framework of the isotropic Heisenberg model. Analytical expressions fitting the numerical simulations for the primitive cubic, diamond, together with (10-3) cubic networks have all been derived. With these empirical formulas in hands, we can now extract the interaction between the magnetic ions from the experimental data for these networks. In the case of the primitive cubic network, these expressions are directly compared with those from the high-temperature expansions of the partition function. A fit of the experimental data for three complexes, namely [(N(CH(3))(4)][Mn(N(3))] 1, [Mn(CN(4))](n)() 2, and [Fe(II)(bipy)(3)][Mn(II)(2)(ox)(3)] 3, has been carried out. The best fits were those obtained using the following parameters, J = -3.5 cm(-)(1), g = 2.01 (1); J = -8.3 cm(-)(1), g = 1.95 (2); and J = -2.0 cm(-)(1), g = 1.95 (3). 相似文献
74.
An ion chromatographic method was developed for the determination of nitrate and nitrite in vegetable and fruit baby foods. The introduction of nitrate or nitrite to food may be natural or artificial as a preservative. Because of the higher pH found in babies' stomachs, nitrate can act as a reservoir for the production of nitrite by nitrate-reducing bacteria that can be harbored in the intestinal tract. This problem does not exist in adults because of the lower pH of the adult stomach. Exposure to nitrite by infants can result in methemoglobinemia (blue baby syndrome). There are also indications that carcinogenic nitrosamines can be formed from nitrates at the higher pH. These gastric conditions disappear at approximately 6 months of age. In this method, nitrate and nitrite were separated on a hydroxide-selective anion exchange column using online electrolytically generated high-purity hydroxide eluant and detected using suppressed conductivity detection. Average recoveries of spiked nitrite residue ranged from 91 to 104% and spiked nitrate residue ranged from 87 to 104%. This method and the AOAC Official Method yield comparable results for samples containing incurred nitrate residue. In addition, this method eliminates the hazardous waste associated with the use of cadmium found in the AOAC Official Method. 相似文献
75.
Time-dependent density functional theory (TDDFT) is presently enjoying enormous popularity in quantum chemistry, as a useful tool for extracting electronic excited state energies. This article discusses how TDDFT is much broader in scope, and yields predictions for many more properties. We discuss some of the challenges involved in making accurate predictions for these properties. 相似文献
76.
α-Halogeno ethers, sulfides, and amines are reactive compounds which can be used for the formation of new C? C bonds, either as nucleophilic or as electrophilic reagents, or by α-elimination. The use of these compounds in the synthesis of many classes of organic compounds is reviewed. 相似文献
77.
The addition of C6H5? to the NN' ligand (NN' = Schiff base of pyridine carbaldehyde-(2) and (S)-(—)-α-phenylethylamine) in [C5H5 Mo(CO)2 NN'] PF6 occurs stereospecific from the side opposite to the C5H5-ring. 相似文献
78.
79.
The reaction of lead tetraacetate with unsaturated carboxylic acids (or salts) to from bislactones (γ or δ) can be controlled to produce efficiently addition of two carboxylic oxygens to the double bond, in consonance with an initial plumbolactonization step followed by SN2 displacement of lead. 相似文献
80.
NMR spectroscopy plays an important part in the determination of the structures of organic compounds. The parameters of importance here are the chemical shifts of the 1H and 13C nuclei and the spin-spin interactions both between 1H nuclei and between 1H and 13C nuclei. Couplings between 13C nuclei were almost completely neglected until a few years ago, since they were extremely difficult to observe because of the low natural abundance of 13C. However, it is these couplings which afford information directly on the carbon-carbon connectivities in the molecule. It is now possible to use a special NMR pulse sequence to make these couplings more readily visible: the result of using this sequence is a 13C-NMR spectrum from which the carbon skeleton concerned can be directly read off. Two-dimensional spectra in particular are very easy to evaluate. The pulse sequence involved, which bears the somewhat puzzling name INADEQUATE, produces double-quantum coherences from which the NMR signals of the coupled carbon nuclei can be obtained. In this article the principle of double-quantum coherence is described and a number of examples for the application of the INADEQUATE pulse sequences to problems in synthetic organic chemistry, biosynthesis and natural products chemistry are presented; in addition, the possibility of applying the INADEQUATE method to other nuclei is considered. 相似文献