全文获取类型
收费全文 | 50371篇 |
免费 | 4855篇 |
国内免费 | 10452篇 |
专业分类
化学 | 45963篇 |
晶体学 | 1883篇 |
力学 | 636篇 |
综合类 | 418篇 |
数学 | 3312篇 |
物理学 | 13466篇 |
出版年
2024年 | 161篇 |
2023年 | 947篇 |
2022年 | 1782篇 |
2021年 | 1772篇 |
2020年 | 1813篇 |
2019年 | 1635篇 |
2018年 | 1351篇 |
2017年 | 1717篇 |
2016年 | 1820篇 |
2015年 | 1579篇 |
2014年 | 2099篇 |
2013年 | 4604篇 |
2012年 | 3185篇 |
2011年 | 3429篇 |
2010年 | 2953篇 |
2009年 | 3601篇 |
2008年 | 3399篇 |
2007年 | 3491篇 |
2006年 | 3290篇 |
2005年 | 2864篇 |
2004年 | 2737篇 |
2003年 | 2281篇 |
2002年 | 1921篇 |
2001年 | 1547篇 |
2000年 | 1450篇 |
1999年 | 1168篇 |
1998年 | 996篇 |
1997年 | 854篇 |
1996年 | 753篇 |
1995年 | 751篇 |
1994年 | 670篇 |
1993年 | 539篇 |
1992年 | 506篇 |
1991年 | 383篇 |
1990年 | 257篇 |
1989年 | 225篇 |
1988年 | 192篇 |
1987年 | 127篇 |
1986年 | 98篇 |
1985年 | 113篇 |
1984年 | 91篇 |
1983年 | 36篇 |
1982年 | 66篇 |
1981年 | 91篇 |
1980年 | 64篇 |
1979年 | 69篇 |
1978年 | 44篇 |
1977年 | 50篇 |
1976年 | 31篇 |
1973年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
In the present paper, we consider the problem of the optimal reconstruction of the solution of the wave equation from the approximate values of the Fourier coefficients of the function specifying the initial form of the string. For an operator defined on the weight space of vectors from l 2, we present the solution of the more general problem of reconstruction from the approximate values of the coordinates of these vectors. 相似文献
102.
103.
Aza-Michael addition of optically pure 4-phenyl-2-oxazolidinone to 3,3,3-trifluoro-1-nitropropene proceeds smoothly at low temperature with a high yield. Diastereoselectivity of the addition depends on the base used and lithiated species proved to be highly efficient affording 92% de. Optically pure 1,2-diamino-3,3,3-trifluoropropane is prepared in 58% yield from the aza-Michael addition product through a three-step procedure. 相似文献
104.
The 11 800-14 380 cm−1 frequency range has been scanned for rotationally resolved rovibronic transitions in the A2B2-X2A1 electronic band system of the symmetric (C2v) 16O14N16O and 18O14N18O isotopologues and in the corresponding electronic band system of the asymmetric (Cs) 18O14N16O isotopologue. The rotational analysis—reflecting minor differences in mass—in combination with symmetry induced spectral differences allows an identification of 68 16O14N16O vibronic levels, 26 18O14N18O vibronic levels and 51 18O14N16O vibronic levels. The bands are recorded using near infrared fluorescence spectroscopy and a piezo valve based pulsed molecular beam expansion of premixed 18O2 and 14N16O in Ar. The majority of the observed bands is rotationally assigned and can be identified as transitions starting from the vibrational ground state of one of the isotopologues. Numerous hot bands have also been identified. A comparison of the overall spectroscopic features of C2v vs. Cs symmetric species provides qualitative information on symmetry dependence of vibronic couplings. 相似文献
105.
A series of complexes formed between halogen-containing molecules and ammonia have been investigated by means of the atoms in molecules (AIM) approach to gain a deeper insight into halogen bonding. The existence of the halogen bond critical points (XBCP) and the values of the electron density (Pb) and Laplacian of electron density (V2pb) at the XBCP reveal the closed-shell interactions in these complexes. Integrated atomic properties such as charge, energy, polarization moment, volume of the halogen bond donor atoms, and the corresponding changes (△) upon complexation have been calculated. The present calculations have demonstrated that the halogen bond represents different AIM properties as compared to the well-documented hydrogen bond. Both the electron density and the Laplacian of electron density at the XBCP have been shown to correlate well with the interaction energy, which indicates that the topological parameters at the XBCP can be treated as a good measure of the halogen bond strength In addition, an excellent linear relationship between the interatomic distance d(X…N) and the logarithm of Pb has been established. 相似文献
106.
掺杂Er^3+的TiO2的发射光谱研究 总被引:1,自引:1,他引:0
用无水乙醇、冰醋酸、钛酸丁酯凝胶法制备了掺杂Er^3+的TiO2粉末,测量了其在488m激发下的Stokes发射光谱和980nm激发下的上转换发光光谱。在可见光范围内,观察到了绿光和红光,绿光从500-570nm,对应Er^3+的^2H11/2,^4S3/2→^4I15/2,红光从650~690m,对应Er^3+的^4F9/2→^4I15/2的跃迁。由ln Ivis-In Iin曲线可知,绿光和红光均为双光子过程,光强正比于泵浦功率的二次方,即Iout ∝Iin。初步研究了此材料的上转换过程。 相似文献
107.
The mechanism of the asymmetric hydrogenation of 2-acyl-1-alkylidene-1,2,3,4-tetrahydroisoquinolines, the first reported reaction with the Noyori-Takaya Ru(CH3COO)2(binap) complex, has been investigated by means of deuterium labeling, kinetics, and NMR analysis. A series of experiments has revealed that (1) a monohydride-unsaturated mechanism operates involving the initial formation of RuH followed by reaction with the enamide substrate, (2) the hydride transfer from RuH to the olefinic double bond is endothermic and reversible, and (3) the rate is determined in the hydrogenolysis step. This view is consistent with that of proposed for the BINAP-Ru catalyzed Kagan reaction. 相似文献
108.
109.
I. V. Ukrainets L. V. Sidorenko O. V. Gorokhova N. A. Jaradat 《Chemistry of Heterocyclic Compounds》2006,42(4):475-487
Various methods of synthesizing amides of 2-hydroxy-4-imino-1,4-dihydroquinoline-3-carboxylic acids have been studied. Results
of investigations on the antitubercular and antiinflammatory activity of the obtained compounds are discussed.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 542–555, April, 2006. 相似文献
110.
We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction. 相似文献