全文获取类型
收费全文 | 1555篇 |
免费 | 24篇 |
国内免费 | 6篇 |
专业分类
化学 | 905篇 |
晶体学 | 10篇 |
力学 | 40篇 |
数学 | 174篇 |
物理学 | 456篇 |
出版年
2020年 | 13篇 |
2019年 | 11篇 |
2017年 | 12篇 |
2016年 | 21篇 |
2015年 | 13篇 |
2014年 | 25篇 |
2013年 | 49篇 |
2012年 | 56篇 |
2011年 | 71篇 |
2010年 | 47篇 |
2009年 | 34篇 |
2008年 | 72篇 |
2007年 | 61篇 |
2006年 | 64篇 |
2005年 | 58篇 |
2004年 | 39篇 |
2003年 | 51篇 |
2002年 | 58篇 |
2001年 | 46篇 |
2000年 | 39篇 |
1999年 | 32篇 |
1998年 | 25篇 |
1997年 | 30篇 |
1996年 | 27篇 |
1995年 | 30篇 |
1994年 | 31篇 |
1993年 | 27篇 |
1992年 | 28篇 |
1991年 | 16篇 |
1990年 | 23篇 |
1989年 | 18篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 24篇 |
1985年 | 27篇 |
1984年 | 17篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 18篇 |
1980年 | 31篇 |
1979年 | 16篇 |
1978年 | 19篇 |
1977年 | 17篇 |
1976年 | 15篇 |
1975年 | 17篇 |
1974年 | 21篇 |
1973年 | 15篇 |
1971年 | 9篇 |
1969年 | 9篇 |
1967年 | 8篇 |
排序方式: 共有1585条查询结果,搜索用时 453 毫秒
51.
52.
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
53.
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
54.
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
55.
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
56.
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
57.
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
58.
Alkylation of 1-dimethylamino-1-cyanomethanephosphonic acid diethyl ester (8), easily obtainable from diethyl phosphite and the 0,N-acetal 9, yields 1-alkyl derivatives 14. Elimination of HCN converts 14 into 1-phosphonoenamines 6. Carbonyl compounds react with 8 to give 1-cyanoenamines 15 which may be hydrolyzed to from the homologous carboxylic acid. Alternatively, deprotonation of 15 yields the homoenolate anions 17 which can be alkylated or hydroxyalkylated, permitting chain extension of carbonyl compounds through introduction of an α-carboxyl and a β-alkyl group. Acid catalyzed hydrolysis of 8 results in cleavage of the PC bond, leading to the corresponding α-dimethylamino alkanoic acids. A phosphonic acid 11 can be obtained from 8 by application of the silylester method. An unambigous assignment of E/Z-isomers of the cyanoenamines 15 has been derived from 13C-NMR spectroscopy. 相似文献
59.
Laser desorption—Fourier transform mass spectrometry was utilized for the determination and quantification of mixtures of ionic surfactants, both neat and from textiles. Sodium dodecyl sulfate and the perdeuterated standard were desorbed from cotton cloth and quantified over a standard: analyte ratio range of 50. A neat C10-C14 sodium alkyl benzenesulfonate homolog mixture was also assayed, and selective analyte precipitation on the copper probe from which desorption occurs was found to be a significant factor in causing component discrimination. 相似文献
60.
D. H. E. Gross R. C. Nayak L. Satpathy 《Zeitschrift für Physik A Hadrons and Nuclei》1981,299(1):63-72
The friction model for heavy ion induced fusion and deep inelastic nuclear reaction is extended to include deformation. Spheroidal deformation and relative motion of the mass centres of the two interacting nuclei are treated as dynamical variables. The radial frictional force has been assumed to be proportional to the rate of change of the separation between the two surfaces instead of the two centres of mass. The friction coefficients remain unaltered. The potential as a function of deformation and separation distance are generated by a single folding procedure. The model is applied to40Ar+232Th at 379 MeV (Lab) and136Xe+209Bi at 1,130 MeV (Lab). In case of the former, the energy loss, which has not been properly accounted for before, is now satisfactorily explained and in the case of the latter the model predicts the absence of fusion in agreement with experiment. 相似文献