全文获取类型
收费全文 | 4901篇 |
免费 | 74篇 |
国内免费 | 11篇 |
专业分类
化学 | 2799篇 |
晶体学 | 9篇 |
力学 | 56篇 |
综合类 | 1篇 |
数学 | 505篇 |
物理学 | 1616篇 |
出版年
2020年 | 51篇 |
2019年 | 35篇 |
2018年 | 38篇 |
2016年 | 55篇 |
2015年 | 53篇 |
2014年 | 61篇 |
2013年 | 98篇 |
2012年 | 121篇 |
2011年 | 159篇 |
2010年 | 102篇 |
2009年 | 75篇 |
2008年 | 138篇 |
2007年 | 172篇 |
2006年 | 166篇 |
2005年 | 155篇 |
2004年 | 150篇 |
2003年 | 133篇 |
2002年 | 115篇 |
2001年 | 114篇 |
2000年 | 92篇 |
1999年 | 86篇 |
1998年 | 76篇 |
1997年 | 68篇 |
1996年 | 85篇 |
1995年 | 91篇 |
1994年 | 97篇 |
1993年 | 94篇 |
1992年 | 110篇 |
1991年 | 73篇 |
1990年 | 60篇 |
1989年 | 53篇 |
1988年 | 66篇 |
1987年 | 68篇 |
1986年 | 61篇 |
1985年 | 64篇 |
1984年 | 61篇 |
1983年 | 48篇 |
1982年 | 68篇 |
1981年 | 58篇 |
1980年 | 52篇 |
1979年 | 53篇 |
1978年 | 53篇 |
1977年 | 50篇 |
1976年 | 43篇 |
1975年 | 48篇 |
1974年 | 48篇 |
1973年 | 63篇 |
1972年 | 31篇 |
1971年 | 40篇 |
1956年 | 31篇 |
排序方式: 共有4986条查询结果,搜索用时 15 毫秒
81.
82.
Srinivasachari Rajappa Kuppuswamy Nagarajan Kailasam Venkatesan Nirupa Kamath Vipanjeri Madabhusi Padmanabhan Wolfgang von Philipsborn Ban Chin Chen Raffaelo Müller 《Helvetica chimica acta》1984,67(7):1669-1680
The 1H-NMR spectra of 2-(nitromethylidene)pyrrolidine ( 7 ), 1-methyl-2-(nitromethylidene)imidazolidind ( 10 ) and 3-(nitromethylidene)tetrahydrothiazine ( 11 ) in CDCl3 and (CD3)2SO indicate that these compounds have the intramolecularly H-bonded structures (Z)- 7 , (E)- 10 and (Z)- 11 while the N-methyl derivative 8 of 7 is (E)-configurated in both solvents. 1-Benzylamino-1-(methyltio)-2-nitroehtylene ( 13 ), an acylic model, has the H-bonded configuration (E)- 13 in CDCl3 and in (CD3)2SO. 2-(Nitromethylidene)thiazolidine ( 3 ) has the (E)-configuration in CDCl3 but exists in (CD3)2SO as a mixture of (Z)- and (E)-isomers with the former predominating. Both species are detected to varying proportions in a mixture of the two solvents. 15N-NMR spectroscopy of 3 ruled out unambiguously the nitronic acid structure 6 and the nitromethyleimine structure 5 . The N-methyl derivative 4 of 3 is (Z)-configurated in (CD3)2SO. Comparison of the olefinic proton shifts of (Z)- 3 and (Z)- 4 with those of analogues and also of 1,1-bis(methylti)-2-nitroethylene ( 12 ) shows decreased conjugation of the lone pair of electrons of the ring N-atom in (Z)- 3 and (Z)- 4 . This is also supported by 13C-NMR studies. Plausible explanations for the phenomenon are offered by postulating that the ring N-atoms are pyramidal in (Z)- 3 and (Z)- 4 and planar in other cases or, alternatively, that the conjugated nitroenamine system gets twisted due to steric interaction between the NO2-group and the ring S-atom. Single-crystal X-ray studies of 3 and 8 show that the former exists in the (Z)-configuration and the latter in (E)-configuration; the ring N-atom in the former has slightly more pyramidal character than in the latter. 相似文献
83.
Bismuth Monoiodide, a Compound with Bi(O) and Bi(II) Bismuth monoiodide was synthesized in closed tubes from the elements as well as from Bi and HgI2 as a black coloured crystalline compound. With increasing temperature BiI passes two transitions. α-BiI is stable below 370 K and changes to β-BiI by a martensitic transition. γ-BiI is the stable modification above 564 K and decomposes at 585 K peritectically to BiI3 and a lower iodide. All three modification crystallize in the monoclinic space group C2/m. The structures (single crystal studies) of α-BiI and β-BiI are characterized by onedimensional infinite chains [Bi4I4] with covalent bonds but only weak interactions in between. The [Bi4I4]-chains are built up by two completely different Bi atoms. Bi(A) is only bonded to three Bi whereas Bi(B) has bonds to one Bi and four I. The average bond lengths are Bi? Bi = 304.5 pm and Bi? I = 313.7 pm respectively. The configuration of the Bi(A) atoms is typical for BiO and that one of the Bi(B) atoms is characteristic for Bi2+ with the electron configuration s2p1. Therefore, α-BiI and β-BiI are mixed valence compounds [BiOBi2+I4]. The structures are variants of the simple cubic polonium type of structure and differ in the stacking of connected units. The structures and their transitions, the possible configurations for monohalides BiX on principle as well as the energy balances of the disproportionation of Bi+ are discussed together in detail. 相似文献
84.
H. Abramowicz F. Dydak J. G. H. de Groot J. Knobloch J. May P. Palazzi F. Ranjard W. D. Schlatter J. Steinberger H. Taureg W. von Rüden H. Wahl J. Wotschack P. Buchholz F. Eisele H. P. Klasen K. Kleinknecht B. Pszola B. Renk H. J. Willutzki T. Flottmann C. Geweniger J. Krolikowski K. Tittel C. Guyot J. P. Merlo A. Para B. Peyaud J. Rander J. Rothberg J. P. Schuller R. Turlay J. T. He T. Z. Ruan W. M. Wu 《Zeitschrift fur Physik C Particles and Fields》1982,12(3):225-227
Right handed weak quark currents coupled to the usual left handed weak lepton current would be seen in inclusive antineutrino scattering on nuclei as a contribution at largey with the quark (not antiquark) structure function. We do not see such a term, and can therefore put an upper limit on the relative strengths of such right handed currents: \(\varrho ^2 = \frac{{\sigma _R }}{{\sigma _L }}< 0.009\) , 90% confidence. This measurement puts limits on the mixing angle of left-right symmetric models. In distinction to similar limits derived from muon decay or β decay, our limits are also valid if the right handed neutrino is heavy. 相似文献
85.
86.
87.
FOURIER ANALYSIS OF TEMPORAL AND SPATIAL OSCILLATIONS OF TUNNELING CURRENT IN SCANNING TUNNELING MICROSCOPY 下载免费PDF全文
Partially oxidized Si(111) surfaces and surfaces of highly oriented pyrolytic graphite (HOPG) were studied by two different ultrahigh vacuum scanning tunneling microscope (UHV-STM) systems and by an STM system working under ambient conditions, respectively. The STM current images of partially oxidized Si(111) surfaces and HOPG surfaces were analyzed by one/two-dimensional fast Fourier transformation (1D-FFT/2D-FFT). The phenomenon of temporal oscillations of tunneling current on the partially oxidized Si(111) surfaces was detected with both UHV-STM systems. Temporal as well as spatial oscillations of tunneling current appeared in highly resolved STM current images of the Si(111) surfaces simultaneously, but both kinds of oscillations could be discriminated according to their different influence on the 2D-FFT spectra of the current images, while varying the scanning range and rate. On clean HOPG surfaces only spatial oscillations of tunneling current induced by the surface structure were observed. 相似文献
88.
Experiments and computer simulations on high order harmonic generation (HOHG) from steep plasma gradients using intense femtosecond
laser pulses are presented. The importance of the plasma scale length for the efficiency of HOHG is demonstrated. By changing
the pump intensity and the target parameters the regime of HOHG can be switched from non-relativistic to relativistic. Simulations
show that attosecond pulses should already be present in our experiments in both interaction regimes. Two-color driven harmonic
generation is shown to provide a highly efficient way of attosecond pulse production. 相似文献
89.
Xilei Lin Ch. Lierse von Gostomski 《Journal of Radioanalytical and Nuclear Chemistry》2013,295(3):1921-1925
The k 0-values were determined for five high Q 0(n,γ) reactions, including 74Se(n,γ) 75Se, 109Ag(n,γ) 110mAg, 114Cd(n,γ) 115Cd–115mIn, 130Ba(n,γ) 131Ba, and 152Sm(n,γ) 153Sm. These determinations were carried out under favorable experiment conditions: the irradiations were performed in a highly thermalized neutron flux, the irradiated target samples were counted at a far distance from HPGe detector with an efficiency carefully calibrated, and the k 0-values were calculated against an internal comparator. When compared to the new values from this work, the 2003 recommended 110mAg k 0-values are confirmed. The other confirmed recommended k 0-value is that of 75Se 400.7 keV line. However, for the other 75Se γ-lines, the new k 0-values are 4–10 % higher. It is assumed that an inaccurate efficiency calibration was used when the recommended k 0-values were measured. For the other three nuclides, the new k 0-values are higher by 4 % for the 115Cd–115mIn γ-lines, lower by 6–8 % for the 131Ba γ-lines, and lower by 8.8 % for the 153Sm 103.2 keV γ-line. 相似文献
90.
Byelikov A Adachi T Fujita H Fujita K Fujita Y Hatanaka K Heger A Kalmykov Y Kawase K Langanke K Martínez-Pinedo G Nakanishi K von Neumann-Cosel P Neveling R Richter A Sakamoto N Sakemi Y Shevchenko A Shimbara Y Shimizu Y Smit FD Tameshige Y Tamii A Woosley SE Yosoi M 《Physical review letters》2007,98(8):082501
The Gamow-Teller strength distributions below the particle threshold in 138La and 180Ta, deduced from high-resolution measurements of the (3He,t) reaction at 0 degrees, allow us to evaluate the role of charged-current reactions for the production of these extremely rare nuclides in neutrino-nucleosynthesis models. The analysis suggests that essentially all 138La in the Universe can be made that way. Neutrino nucleosynthesis also contributes significantly to the abundance of 180Ta but the magnitude depends on the unknown branching ratio for population of the long-lived isomer. 相似文献