全文获取类型
收费全文 | 314401篇 |
免费 | 3751篇 |
国内免费 | 1092篇 |
专业分类
化学 | 170612篇 |
晶体学 | 4517篇 |
力学 | 13030篇 |
综合类 | 7篇 |
数学 | 36267篇 |
物理学 | 94811篇 |
出版年
2020年 | 1988篇 |
2019年 | 1955篇 |
2018年 | 1965篇 |
2017年 | 1875篇 |
2016年 | 3858篇 |
2015年 | 3235篇 |
2014年 | 4423篇 |
2013年 | 13977篇 |
2012年 | 10728篇 |
2011年 | 13363篇 |
2010年 | 8270篇 |
2009年 | 8255篇 |
2008年 | 12335篇 |
2007年 | 12571篇 |
2006年 | 12212篇 |
2005年 | 11154篇 |
2004年 | 10034篇 |
2003年 | 8848篇 |
2002年 | 8700篇 |
2001年 | 10109篇 |
2000年 | 7710篇 |
1999年 | 6115篇 |
1998年 | 4894篇 |
1997年 | 4733篇 |
1996年 | 4773篇 |
1995年 | 4363篇 |
1994年 | 4114篇 |
1993年 | 3940篇 |
1992年 | 4431篇 |
1991年 | 4293篇 |
1990年 | 4011篇 |
1989年 | 3830篇 |
1988年 | 4120篇 |
1987年 | 3784篇 |
1986年 | 3677篇 |
1985年 | 5317篇 |
1984年 | 5395篇 |
1983年 | 4377篇 |
1982年 | 4753篇 |
1981年 | 4793篇 |
1980年 | 4545篇 |
1979年 | 4663篇 |
1978年 | 4665篇 |
1977年 | 4644篇 |
1976年 | 4597篇 |
1975年 | 4508篇 |
1974年 | 4356篇 |
1973年 | 4524篇 |
1972年 | 2569篇 |
1971年 | 1879篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Individual state-to-state rotational transitions have been resolved in small angle scattering of polarized CsF molecules on Ne, Ar, C2, H6, N2, CO, CO2, CHF3 at center of mass energies of about 0.1 eV. The absolute inelastic cross sections range from 5Å2 up to 600Å2. 相似文献
992.
Various techniques (thermal analysis, dilatometry, X-ray diffraction, nuclear magnetic resonance, mass spectrometry, electrical measurements) have been used to study the thermal behaviour of several “half-aromatic” polyamides and polysulphonamides. Three transition phenomena were revealed and related to morphological modification of polymers. 相似文献
993.
Gamma-butyrolactone (GBL) was shown to decompose on the clean surface of Li, Na, K, and their amalgams. Polarization curves were obtained on Pt and “Kovar” electrodes at potentials from 4 to ?4 V vs. Li?Hg in 0.5 M LiClO4 or vs. K?Hg in 0.5 M KPF6. GBL was found to reduce electrochemically at potentials more positive than 1 V. Various passivation phenomena provoked by the decomposition of the solvent were observed, hindering cathodic deposition of the alkali metals from solutions of their salts in GBL. The presence of a high-molecular surfactant in the electrolyte suppresses the decomposition of GBL and suggests a path to the deposition of Li on the electrode. 相似文献
994.
J.W. Brasch 《Journal of Molecular Structure》1975,29(2):383-384
Force constants and normal coordinates are reported for the spiropentane molecule. These data are based on the recently determined fundamental vibration frequencies of spiropentane-H8.The results have been used to predict the fundamentals of spiropentane-D8, to calculate the centrifugal distortion parameters, in particular the parameter r6, which is non-zero for molecules with a four-fold axis of rotation and to calculate thermodynamic parameters. Good agreement is observed between the calorimetric vapour heat capacity of spiropentane and the value based on our assignments for the fundamental vibrations of spiropentane.The transferability of the cyclopropane force field has been tested and shown to be valid for predicting the deformations of the methylene groups of spiropentane. 相似文献
995.
The number of products and the H2/CH4 ratio obtained from the flow pyrolyses of (CH3)3GeH and (CH3)3SiH were very different. The (CH3)3GeH decomposition is consistent with the following mechanism:
The pyrolysis of (CH3)3SiH was found to be much more complex, presumably due to the formation of silicon-carbon double bonded intermediates and the (CH3)2Si(H)CH2 radical. We also present data which supports the presence of a H atom chain sequence during this pyrolysis. 相似文献
Full-size image
996.
3-methylenebicyclo[2,2,1]heptane-2,6-carbolactone (previously incorrectly claimed) has now been prepared and converted to the 3-oxo-lactone. Some 3-hydroxy-bicyclo-[2,2,1]heptane-2,6-carbolactones have been prepared and their rates of oxidation with chromic acid determined. The results do not support a theory about the mechanism of oxidation, involving a dipolar interaction from the lactone. 相似文献
997.
J. Umemura 《Journal of Molecular Structure》1977,36(1):35-54
Temperature dependence of the IR spectra of crystalline propionic acid were examined in the temperature range 242–50 K. The intensities of most absorption bands increased on lowering the temperature, but some bands diminished until they disappeared at temperatures lower than about 120 K. Normal coordinate analyses indicate that the former bands are due to the stable cis dimer and the latter due to the less stable trans dimer which would be produced from the cis dimer by simultaneous proton transfer along two hydrogen bonds. 相似文献
998.
The vibrational spectra of compounds Ln2WO6 and Ln2MoO6 (Ln = lanthanide, including Y and Bi) are reported. Neglecting details it is possible to assign the internal vibrations of the molybdate and tungstate group. The results are in agreement with the structural data known up till now and suggest further that tungsten in the unknown Y2WO6 structure is in six-coordination, that Bi2MoO6 shows still another modification than koechlinite and that vibrational spectroscopy may yield additional structural information for compounds like those under consideration. 相似文献
999.
The crystals of Ni0,33Mo3Se4, are triclinic, space group , with two formula units in a cell: a = 6,727 (9) Å, b = 6,582 (11) Å, c = 6,751 (6) Å, α = 90.61° (10), β = 92.17° (10), γ = 90.98° (12.) The structure was solved by analogy with Mo3Se4 and refined by a full-matrix least squares program to R = 0,093 for 822 independent reflexions. The channels present in Mo3Se4 are occupied by Ni so that Ni0,33Mo3Se4 is always a metallic compound. 相似文献
1000.
L. P. Blanchard K. T. Dinh J. Moinard F. Tahiani 《Journal of polymer science. Part A, Polymer chemistry》1972,10(5):1353-1362
Anionic polymerizations of 1,2-butylene oxide were carried out in vacuum-sealed dilatometers in the range of 30–60°C. Potassium terbutoxide and dimsyl sodium were used as initiators; dimethyl sulfoxide (DMSO) and mixtures of DMSO with tetrahydrofuran were solvents. The polymer products were analyzed by gel permeation chromatography and infrared spectroscopy. The object of the investigation was to obtain information on the mechanism of the reaction and to elucidate some of its kinetic aspects. It has been shown that the polymerizations occur by two different processes, depending on the choice of experimental conditions. One of the processes involves free ions and ion-pairs, the other, ion-pairs alone. In the first case, where dimethyl sulfoxide is used as solvent, the order of the reaction with respect to the initiator concentration far exceeds unity (~1.8), while in the second case, involving mixed solvents, the order of the reaction, for all practical purposes, is one. 相似文献