全文获取类型
收费全文 | 540461篇 |
免费 | 4614篇 |
国内免费 | 1285篇 |
专业分类
化学 | 277626篇 |
晶体学 | 7711篇 |
力学 | 26221篇 |
综合类 | 17篇 |
数学 | 69564篇 |
物理学 | 165221篇 |
出版年
2021年 | 5138篇 |
2020年 | 5642篇 |
2019年 | 6406篇 |
2018年 | 8583篇 |
2017年 | 8746篇 |
2016年 | 12038篇 |
2015年 | 6564篇 |
2014年 | 10853篇 |
2013年 | 24283篇 |
2012年 | 19242篇 |
2011年 | 22734篇 |
2010年 | 16944篇 |
2009年 | 16689篇 |
2008年 | 21499篇 |
2007年 | 21304篇 |
2006年 | 19371篇 |
2005年 | 17319篇 |
2004年 | 16041篇 |
2003年 | 14426篇 |
2002年 | 14251篇 |
2001年 | 14869篇 |
2000年 | 11477篇 |
1999年 | 8841篇 |
1998年 | 7671篇 |
1997年 | 7540篇 |
1996年 | 7036篇 |
1995年 | 6329篇 |
1994年 | 6322篇 |
1993年 | 6094篇 |
1992年 | 6444篇 |
1991年 | 6918篇 |
1990年 | 6622篇 |
1989年 | 6524篇 |
1988年 | 6366篇 |
1987年 | 6170篇 |
1986年 | 5928篇 |
1985年 | 7492篇 |
1984年 | 7835篇 |
1983年 | 6563篇 |
1982年 | 6879篇 |
1981年 | 6386篇 |
1980年 | 6070篇 |
1979年 | 6553篇 |
1978年 | 6791篇 |
1977年 | 6668篇 |
1976年 | 6632篇 |
1975年 | 6327篇 |
1974年 | 6161篇 |
1973年 | 6450篇 |
1972年 | 4718篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
I. I. Murav'ev E. V. Chernikova A. D. Shatova A. M. Yancharina 《Russian Physics Journal》1992,35(11):1008-1011
An experimental study has been made of the characteristics of radiation from a dispersing helium plasma. From measurements of the space-time dependence of the line intensities of the helium 23P — n3D series we obtained the distribution of the populations of the n3D levels for n=3–10. The study has shown that over a wide range of conditions the high-lying levels (n=5–10) are in equilibrium with electrons while a substantial deviation from equilibrium is observed for levels n=3, 4. From the energy distribution of the populations of the upper levels we determined the space-time dependence of the electron temperature, which reflects the process of effective electron cooling during dispersal of the plasma. The gas temperature was estimated from the populations of helium singlet and triplet levels with n=5. The time distribution of the electron density in the plasma was found from the Stark broadening of the spectral lines.Deceased.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 15–19, November, 1992. 相似文献
972.
E. A. Dynin 《Theoretical and Mathematical Physics》1992,92(1):802-804
Bogolyubov's inequality is used to generalize the density functional method to take into account nuclear thermal motion. The results apply to a two-component system in which the nuclei satisfy classical statistics. Applications to a cell model describing dense heated states of matter are considered.All-Union Research Institute of Opticophysical Measurements of the USSR State Standards. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 92, No. 1, pp. 154–157, July, 1992. 相似文献
973.
G. A. Watson 《Advances in Computational Mathematics》1994,2(4):393-405
The orthogonal Procrustes problem involves finding an orthogonal matrix which transforms one given matrix into another in
the least-squares sense, and thus it requires the minimization of the Frobenius matrix norm. We consider, the solution of
this problem for a family of orthogonally invariant norms which includes the Frobenius norm as a special case. 相似文献
974.
975.
Translated from Matematicheskie Zametki, Vol. 49, No. 6, pp. 92–97, June, 1991. 相似文献
976.
P E S?rensen R A McClelland R D Gandour 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1991,45(6):558-566
The ring-chain tautomerism in aqueous solution of some aryl-substituted morpholinium salts (bromides), has been studied and equilibrium constants are reported. In the crystals the substrates exist entirely in their cyclic forms as hemiacetals, but in aqueous solution NMR measurements reveal that an equilibrium is established between the cyclic (hemiacetal) and the noncyclic (ketone) form, the degree of ring-opening being more pronounced with electron-donating aryl substituents at the carbonyl carbon. The kinetics of the ring-chain interconversion in water has been investigated spectrophotometrically by a 'pH jump' stopped-flow technique. General base catalysis is observed with a Br?nsted beta value apparently independent of substituent and equal to 0.60. The Hammett rho values for various base catalysts are close to those for very similar intermolecular reactions involving hemiacetal breakdown, leading to the suggestion of a 'normal' class n mechanism for base catalysis. For acid catalysis, however, a quite different situation is encountered, since no general acid but only (weak) catalysis by the hydronium ion can be detected. We believe this deviation from 'normal' general acid catalysis is caused by an electrostatic interaction, and we suggest that it might result from a change in the usual class e mechanism for general acid catalysis by a situation in which rate-limiting concerted proton transfer is replaced by rate-limiting preprotonation. This is supported by the observed drastic change in Hammett rho value for catalysis by the hydronium ion, compared with the 'normal' case. An interesting case is encountered for the 4-aminophenyl-substituted substrate, in which the amino group becomes protonated in acid solution, thus representing a new substituent. Despite this complication, the various equilibrium and rate constants may also be evaluated experimentally for this substrate. 相似文献
977.
Plasma B-6 vitamer and plasma and urinary 4-pyridoxic acid concentrations of 21 young white women, 21-27 years, having radiomonitored pyridoxal 5'-phosphate and coenzyme stimulation of erythrocyte alanine aminotransferase activities indicative of adequate vitamin B-6 status were determined in an effort to establish normal ranges for plasma B-6 vitamers. B-6 vitamers and 4-pyridoxic acid were quantitated using reversed phase high performance liquid chromatography with fluorometric and ultraviolet detection. Pyridoxal phosphate values obtained by radioenzymatic and chromatographic, fluorometric and ultraviolet, assays were highly correlated as were pyridoxine phosphate values determined using both detectors. The B-6 vitamer and 4-pyridoxic acid values of these subjects should be of use in the establishment of normal ranges of these congeners in women. 相似文献
978.
979.
B. Griepink E. A. Maier P. Quevauviller H. Muntau 《Fresenius' Journal of Analytical Chemistry》1991,339(9):599-603
Summary The Community Bureau of Reference (BCR) has undertaken the production of a wide range of certified reference materials (CRMs) for the improvement of accuracy and measurement harmony in various areas of concern at European Community level. This paper provides an overview of the sectors of the BCR-programme in the field of environment i.e. for water systems (water, sediment, biota), soil analysis (soil, fertiliser, waste), plant analysis (aquatic and terrestrial plants), analysis related to animals (e.g. muscle or liver) or man (e.g. industry and food). 相似文献
980.