全文获取类型
收费全文 | 764061篇 |
免费 | 6921篇 |
国内免费 | 2152篇 |
专业分类
化学 | 376037篇 |
晶体学 | 10811篇 |
力学 | 40521篇 |
综合类 | 37篇 |
数学 | 121358篇 |
物理学 | 224370篇 |
出版年
2021年 | 5925篇 |
2020年 | 6616篇 |
2019年 | 7429篇 |
2018年 | 19491篇 |
2017年 | 19385篇 |
2016年 | 19760篇 |
2015年 | 8699篇 |
2014年 | 13281篇 |
2013年 | 30873篇 |
2012年 | 26814篇 |
2011年 | 37114篇 |
2010年 | 25772篇 |
2009年 | 25926篇 |
2008年 | 32183篇 |
2007年 | 33978篇 |
2006年 | 24373篇 |
2005年 | 22605篇 |
2004年 | 21189篇 |
2003年 | 19677篇 |
2002年 | 18677篇 |
2001年 | 19055篇 |
2000年 | 15043篇 |
1999年 | 11615篇 |
1998年 | 10151篇 |
1997年 | 10050篇 |
1996年 | 9427篇 |
1995年 | 8457篇 |
1994年 | 8550篇 |
1993年 | 8125篇 |
1992年 | 8687篇 |
1991年 | 9124篇 |
1990年 | 8771篇 |
1989年 | 8530篇 |
1988年 | 8460篇 |
1987年 | 8311篇 |
1986年 | 7902篇 |
1985年 | 10215篇 |
1984年 | 10761篇 |
1983年 | 9068篇 |
1982年 | 9411篇 |
1981年 | 8951篇 |
1980年 | 8640篇 |
1979年 | 9026篇 |
1978年 | 9505篇 |
1977年 | 9350篇 |
1976年 | 9353篇 |
1975年 | 8845篇 |
1974年 | 8744篇 |
1973年 | 9066篇 |
1972年 | 6581篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
M. F. Bobrov G. V. Popova V. G. Tsirelson 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2006,80(4):584-590
The restricted Hartree-Fock method was used to determine the cycle size effects on the geometric parameters of several inorganic templates, cyclophosphazenes PnNnX2n (X = H, F, Cl; n = 2, 3, 4). A topological analysis of local electronic properties at the electron density critical points of bonds allowed us to quantitatively characterize the chemical bond in cyclophosphazenes and its dependence on the cycle size and substituents at phosphorus. The calculated distributions of the electron density Laplacian and electron pair localization functions revealed the special features of the electronic structure of the nitrogen and phosphorus atoms. These results explain the nature of noncovalent interactions between the P atoms of one cyclophosphazene molecule and the N atoms of the other. 相似文献
952.
In this paper we obtain a number of Maharam-type slice integral representations, with respect to scalar measures, for positive
projections in Dedekind complete vector lattices and f-algebras.
AMS Classification: 47B65, 46A40, 06F25 相似文献
953.
Let Ψ(x,y) (resp. Ψm(x,y)) denote the number of integers not exceeding x that are y-friable, i.e. have no prime factor exceeding y (resp. and are coprime to m). Evaluating the ratio Ψm(x/d,y)/Ψ(x,y) for 1≤slantd≤slantx, m≥slant 1, x≥slant y≥slant 2, turns out to be a crucial step for estimating arithmetic sums over friable integers. Here, it is crucial to obtain formulae with a very wide range of validity. In this paper, several uniform estimates are provided for the aforementioned ratio, which supersede all previously known results. Applications are given to averages of various arithmetic functions over friable integers which in turn improve corresponding results from the literature. The technique employed rests mainly on the saddle-point method, which is an efficient and specific tool for the required design.2000 Mathematics Subject Classification: Primary—11N25; Secondary—11K65, 11N37 相似文献
954.
955.
Analysis of amlodipine in human plasma by gas chromatography 总被引:3,自引:0,他引:3
956.
957.
In this paper the performances of inhomogeneous dielectric slabs as solar light collectors for both illumination and solar energy applications are analysed. 相似文献
958.
H E Cline W E Lorensen R J Herfkens G A Johnson G H Glover 《Magnetic resonance imaging》1989,7(1):45-54
A three-dimensional examination of blood vessels is provided using MR data from seven cases. The vascular surfaces are constructed with an algorithm that automatically follows the selected artery or vein and generates a projected three-dimensional gradient shaded image. Fast 3DFT pulse sequences were optimized to enhance the time-of-flight contrast of the intravascular region. By increasing the surface threshold value in a three-dimensional head study, the flesh of a patient's face was peeled away to demonstrate the superfacial temporal artery. Gated cardiac images show the great vessels and cardiac chambers. A three-dimensional view of the aorta shows an irregular surface in the vicinity of an adrenal tumor. 3D MR exams provide a non-invasive technique for assessing vascular morphology in a clinical setting. 相似文献
959.
Molecular electroactive monolayers have been produced from vinylferrocene (VFC) via light-assisted surface anchoring to H-terminated n- and p-Si(1 0 0) wafers prepared via wet chemistry, in a controlled atmosphere. The resulting Si-C bound hybrids have been characterized by means of XPS and AFM. Their performance as semiconductor functionalized electrodes and their surface composition have been followed by combining electrochemical and XPS measurements on the same samples, before and after use in an electrochemical cell. White-light photoactivated anchoring at short (1 h) exposure times has resulted in a mild route, with a very limited impact on the initial quality of the silicon substrate. In fact, the functionalized Si surface results negligibly oxidized, and the C/Fe atomic ratio is close to the value expected for the pure molecular species. The VFC/Si hybrids can be described as (η5-C5H5)Fe2+(η5-C5H4)-CH2-CH2-Si species, on the basis of XPS results. Electrochemical methods have been applied in order to investigate the role played by a robust, covalent Si-C anchoring mode towards substrate-molecule electronic communication, a crucial issue for a perspective development of molecular electronics devices. The response found from cyclic voltammograms for p-Si(1 0 0) functionalized electrodes, run in the dark and under illumination, has shown that the electron transfer is not limited by the number of charge carriers, confirming the occurrence of electron transfer via the Si valence band. The hybrids have shown a noticeable electrochemical stability and reversibility under cyclic voltammetry (cv), and the trend in peak current intensity vs. the scan rate was linear. The molecule-Si bond is preserved even after thousands of voltammetric cycles, although the surface coverage, evaluated from cv and XPS, decreases in the same sequence. An increasingly larger surface concentration of Fe3+ at the expenses of Fe2+ redox centers has been found at increasing number of cv’s, experimentally associated with the growth of silicon oxide. Surface SiO− groups from deprotonated silanol termination, induced by the electrochemical treatments, are proposed as the associated counterions for the Fe3+ species. They could be responsible for the observed decrease in the electron transfer rate constant with electrode ageing. 相似文献
960.
Y.B. Melnichenko G.D. Wignall D.R. Cole H. Frielinghaus L.A. Bulavin 《Journal of Molecular Liquids》2005,120(1-3):7-9
Small angle neutron scattering (SANS) is a well-established technique for investigating the behavior of confined binary liquid solutions, as it can probe the correlation length and susceptibility in pores on length scales 1 – 100 nm. We applied SANS to explore the influence of confinement on critical behavior of an individual fluid carbon dioxide (CO2) in a highly porous aerogel. The results demonstrate that quenched disorder induced by aerogel significantly depresses density fluctuations. Despite the negligible volume occupied by aerogel (< 4%), the macroscopic phase separation of confined CO2 into coexisting liquid and gaseous phases is suppressed and below the critical temperature of the bulk fluid frozen methastable microdomains are formed. Experimental data show that critical adsorption is as important as the effect of confinement in defining the behavior of confined fluids. 相似文献