全文获取类型
收费全文 | 83025篇 |
免费 | 565篇 |
国内免费 | 417篇 |
专业分类
化学 | 27816篇 |
晶体学 | 815篇 |
力学 | 6827篇 |
数学 | 32870篇 |
物理学 | 15679篇 |
出版年
2023年 | 44篇 |
2022年 | 132篇 |
2021年 | 163篇 |
2020年 | 105篇 |
2019年 | 129篇 |
2018年 | 10498篇 |
2017年 | 10304篇 |
2016年 | 6223篇 |
2015年 | 1006篇 |
2014年 | 482篇 |
2013年 | 638篇 |
2012年 | 4133篇 |
2011年 | 10869篇 |
2010年 | 5864篇 |
2009年 | 6288篇 |
2008年 | 6894篇 |
2007年 | 9056篇 |
2006年 | 537篇 |
2005年 | 1607篇 |
2004年 | 1746篇 |
2003年 | 2127篇 |
2002年 | 1175篇 |
2001年 | 315篇 |
2000年 | 344篇 |
1999年 | 204篇 |
1998年 | 230篇 |
1997年 | 188篇 |
1996年 | 265篇 |
1995年 | 153篇 |
1994年 | 112篇 |
1993年 | 129篇 |
1992年 | 97篇 |
1991年 | 95篇 |
1990年 | 69篇 |
1989年 | 86篇 |
1988年 | 82篇 |
1987年 | 78篇 |
1986年 | 82篇 |
1985年 | 81篇 |
1984年 | 68篇 |
1983年 | 58篇 |
1982年 | 70篇 |
1981年 | 55篇 |
1980年 | 66篇 |
1979年 | 59篇 |
1978年 | 45篇 |
1914年 | 45篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
981.
982.
The substrate and field dependencies of surface SPINOE enhancements using optical pumping and magic angle spinning NMR were monitored. Relaxation rates and enhancements were examined to gain an understanding of the parameters that determine the SPINOE enhancement. (13)C-labeled deuterated methanol was adsorbed on three different substrates (SnO(2), TiO(2), Ti/SiO(2)) with heats of adsorption for xenon ranging from 14.2 to 22.6 kJ/mol. The different heats of adsorption led to a range of xenon coverages and xenon relaxation rates. Using a simple model along with experimental values for the xenon surface polarization and cross- and self-relaxation rates, the (13)C signal enhancement could be predicted and compared with experimental enhancement values. Magnetic field dependence studies were also made by monitoring the (13)C enhancements via SPINOE from hyperpolarized xenon at fields of 0.075, 4.7, and 9.4 T. The pertinent parameters necessary to achieve maximum SPINOE enhancement are discussed. 相似文献
983.
984.
Danielle Giron S. Monnier M. Mutz P. Piechon T. Buser F. Stowasser K. Schulze M. Bellus 《Journal of Thermal Analysis and Calorimetry》2007,89(3):729-743
Adequate very sensitive quantification methods are needed for the development and are also now required for the monitoring
of undesirable solid form(s) as routine tests. The pre-requisite for quantitation are selectivity, sensitivity and most important
the purity of standards and their proper storage, what is a challenge for metastable forms.
Several analytical techniques are available such as X-ray diffraction, spectroscopy, thermal analysis and microcalorimetry.
The different steps of the validation of the analytical methods and problems to be solved are discussed. Examples illustrate
the different techniques and compare their possible advantages and limits. The relative standard deviation of measurements
should allow for checking the homogenization procedure of mixtures for calibration. The validation should be carried out following
ICH guidelines for validation of analytical methods. Comparison of different techniques in adequate concentration range add
confidence in the analytical results. 相似文献
985.
Andrzej Czerwiński Iwona Kiersztyn Michał Grdeń 《Journal of Solid State Electrochemistry》2003,7(6):321-326
The effect of temperature on hydrogen and deuterium electrosorption into a palladium LVE (limited-volume electrode) has been
investigated. A decrease in hydrogen capacity (H/Pd ratio) with increasing temperature has been observed. Temperature strongly
influences the plots of measured H(D)/Pd values vs. potential scan rate. In addition, hydrogen absorption was found to be
dependent on the composition of the surrounding electrolyte solution. These results have confirmed the hypothesis that two
different mechanisms of hydrogen desorption from the palladium electrode take place, namely electrochemical oxidation and
non-electrochemical recombination. Further, the ratio between the rate constants for these two processes has been found to
change with temperature.
Electronic Publication 相似文献
986.
Summary The thermal behavior of KH2PO4, NaH2PO4 and Na2HPO4 under non-isothermal conditions using TG method with different heating rates was studied. The values of the reaction rate were processed by means of Friedmans differential-isoconversional method. A dependence of the activation energy vs. conversion was observed. Therefore a procedure based on the compensation effect (suggested by Budrugeac and Segal) was applied. A less speculative data processing protocol was offered by the non-parametric kinetics method suggested by Serra, Nomen and Sempere. Three steps were observed by non-isothermal heating: a dehydration, a dimerization and a polycondensation. The differences in the intimate reaction mechanism are determined by the initial number of water molecules. 相似文献
987.
We report on a global potential energy hypersurface for the upper sheet of the lowest triplet state of H3+. The analytic representation is based on the double many-body expansion theory. The ab initio data points, calculated with a large cc-pV5Z basis, are represented with a root mean square deviation of only 5.54 cm(-1) in the energy region below the H(+)+2H(2S) dissociation threshold. The quasi-bound vibronic states supported by this surface have also been calculated. 相似文献
988.
Employing hot tungsten filament to thermal dissociate molecular hydrogen,we generated gas phase atomic hydrogen under ultra-high vacuum(UHV)conditions and investigated its interaction with Pt(111) surface.Thermal desorption spectroscopy(TDS)results demonstrate that adsorption of molecular hy- drogen on Pt(111)forms surface Had species whereas adsorption of atomic hydrogen forms not only surface Had species but also bulk Had species.Bulk Had species is more thermal-unstable than surface Had species on Pt(111),suggesting that bulk Had species is more energetic.This kind of weakly- adsorbed bulk Had species might be the active hydrogen species in the Pt-catalyzed hydrogenation reactions. 相似文献
989.
In this work we have studied the stabilizing effect of a natural antioxidant, sugar cane bagasse-lignin, on the photo-oxidation of a commercial sample of butadiene rubber. This stabilizer was investigated in various concentrations, in the pure form, and associated with a diamine or a phosphite stabilizer. Stabilization of butadiene rubber with this lignin associated with the diamine produced a material with a photo-chemical stability comparable to that of a stabilized commercial sample. 相似文献
990.
Ahmad Shaabani Abbas Rahmati Soheila Naderi 《Monatshefte für Chemie / Chemical Monthly》2007,138(6):553-557
Summary. Reaction of triphenylphosphine and an electron deficient acetylenic ester in the presence of strong N–H acid such as alkyl
and aryl sulfamides or acetamide produces phosphorous ylides at room temperature in CH2Cl2. The aryl sulfamide phosphoranes undergo a smooth transformation reaction in boiling toluene and produce iminophosphoranes. 相似文献