全文获取类型
收费全文 | 1221篇 |
免费 | 58篇 |
国内免费 | 7篇 |
专业分类
化学 | 811篇 |
晶体学 | 18篇 |
力学 | 49篇 |
数学 | 153篇 |
物理学 | 255篇 |
出版年
2022年 | 11篇 |
2021年 | 17篇 |
2020年 | 27篇 |
2019年 | 10篇 |
2018年 | 9篇 |
2016年 | 26篇 |
2015年 | 30篇 |
2014年 | 38篇 |
2013年 | 100篇 |
2012年 | 41篇 |
2011年 | 45篇 |
2010年 | 44篇 |
2009年 | 41篇 |
2008年 | 51篇 |
2007年 | 55篇 |
2006年 | 38篇 |
2005年 | 60篇 |
2004年 | 46篇 |
2003年 | 42篇 |
2002年 | 29篇 |
2001年 | 23篇 |
2000年 | 19篇 |
1999年 | 24篇 |
1998年 | 24篇 |
1997年 | 15篇 |
1996年 | 14篇 |
1993年 | 19篇 |
1992年 | 14篇 |
1991年 | 18篇 |
1990年 | 11篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 15篇 |
1986年 | 13篇 |
1985年 | 15篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 18篇 |
1981年 | 18篇 |
1980年 | 24篇 |
1979年 | 11篇 |
1978年 | 13篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1975年 | 13篇 |
1974年 | 13篇 |
1973年 | 12篇 |
1972年 | 14篇 |
1971年 | 10篇 |
1969年 | 8篇 |
排序方式: 共有1286条查询结果,搜索用时 296 毫秒
91.
Photolysis of N-acyl derivatives of 2-thionothiazolidine in the presence of ethanol affords ethyl esters; a mechanism involving γ-hydrogen abstraction by sulfur is postulated. 相似文献
92.
James Hansen Lester E. Heitger Lynn McKell 《The Journal of the Operational Research Society》1978,29(8):789-802
Information systems have become increasingly complex over recent years, underscoring the challenge of designing systems which will provide the different functions in an organization with the information necessary for effective decision-making. With rapidly developing computer technology, it is natural that information systems evolve in a manner which is greatly dependent on computing hardware and software. Yet, there has been little development in the techniques of systems analysis and design to handle new and less well-understood functions which are of increasing interest.This paper demonstrates an application of structural modelling concepts to decision-support system synthesis. With the aid of the computer, structural modelling serves as an effective tool for: (1) partitioning a system into a set of subsystems; (2) specifying subsystems sufficiently to permit separate testing before total system tests; (3) eliminating redundant control information flow; and (4) facilitating decision-maker and system-analyst coordination and participation in the design process. 相似文献
93.
Aharon Loewenschuss John C. Miller Lester Andrews 《Journal of Molecular Spectroscopy》1980,80(2):351-362
Absorption spectra of ClO, BrO, and IO and the emission spectrum of IO have been observed from argon matrix samples prepared by microwave discharging the reagent mixture before condensation. Vibronic progressions were observed for each system. The spectroscopic scopic constants Te, ω′e, ωex′e, ω″e, and ωex″e were evaluated from the absorption and emission data for comparison with gas-phase constants. Very good agreement is found for ClO. The argon matrix observations dictate a revision of the gas-phase vibronic assignments for BrO. The ground-state vibrational fundamental and Te for argon matrix isolated IO are similar to the gas-phase values, but a lower excited-state spacing is found in the matrix. 相似文献
94.
95.
Liang B Andrews L Li J Bursten BE 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(19):4781-4788
Laser-ablated U atoms co-deposited with CO in excess neon produce the novel CUO molecule, which forms distinct Ng complexes (Ng=Ar, Kr, Xe) with the heavier noble gases. The CUO(Ng) complexes are identified through CO isotopic and Ng reagent substitution and comparison to results of DFT frequency calculations. The U[bond]C and U[bond]O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from neon matrix (1)Sigma(+) CUO values, which indicates a (1)A' ground state for the CUO(Ng) complexes. The CUO(Ng)(2) complexes in excess neon are likewise singlet molecules. However, the CUO(Ng)(3) and CUO(Ng)(4) complexes exhibit very different stretching frequencies and isotopic behaviors that are similar to those of CUO(Ar)(n) in a pure argon matrix, which has a (3)A" ground state based on DFT vibrational frequency calculations. This work suggests a coordination sphere model in which CUO in solid neon is initially solvated by four or more Ne atoms. Up to four heavier Ng atoms successively displace the Ne atoms leading ultimately to CUO(Ng)(4) complexes. The major changes in the CUO stretching frequencies from CUO(Ng)(2) to CUO(Ng)(3) provides evidence for the crossover from a singlet ground state to a triplet ground state. 相似文献
96.
Menon S Vaidya H Pillai S Vidya R Mitscher LA 《Combinatorial chemistry & high throughput screening》2003,6(5):471-480
A solution phase strategy for the multiple parallel synthesis of a demonstration library of indazoles is described by which regio-selectivity problems inherent to previous syntheses of this nucleus are largely overcome. Synthesis of selected components proceeded satisfactorily indicating that a fully realized library of indazole analogs could readily be produced using this methodology. Simple modifications of the basic nucleophilic aromatic substitution route unambiguously produce a range of N-1 substitutions (alkyl, aryl and aralkyl) in 50-75% yields. Next a range of substituents was introduced at the C-3 position in 50-80% yields by O-alkylation. Careful choice of reagents and reaction conditions were required to prevent by-product formation due to competing alkylation at N-2 (trace to 15% yields). When present, these contaminants were readily removed by chromofiltration. A third diversity site was sketched in at C-5 in 75-90% yield by reductive alkylation or acylation. Screening of some of the demonstration library members in vitro revealed highly active antioxidants suggesting that producing a full library would be worthwhile. 相似文献
97.
D. Korobkin Y.A. Urzhumov B. Neuner III C. Zorman Z. Zhang I.D. Mayergoyz G. Shvets 《Applied Physics A: Materials Science & Processing》2007,88(4):605-609
We theoretically and experimentally study electromagnetic properties of a novel mid-infrared metamaterial: optically thin
silicon carbide (SiC) membrane perforated by an array of sub-wavelength holes. Giant absorption and transmission is found
using Fourier transformed infrared (FTIR) microscopy and explained by introducing a frequency-dependent effective permittivity
εeff(ω) of the perforated film. The value of εeff(ω) is determined by the excitation of two distinct types of hole resonances: delocalized slow surface polaritons (SSPs) whose
frequencies are largely determined by the array period, and a localized surface polariton (LSP) corresponding to the resonance
of an isolated hole. Only SSPs are shown to modify εeff(ω) strongly enough to cause giant transmission and absorption. Because of the sub-wavelength period of the hole array, anomalous
optical properties can be directly traced to surface polaritons, and their interpretation is not obscured by diffractive effects.
Giant absorbance of this metamaterial can be utilized in designing highly efficient thermal radiation sources.
PACS 41.20.Cv; 42.70.Qs; 71.45.Gm 相似文献
98.
99.
100.