全文获取类型
收费全文 | 453071篇 |
免费 | 5839篇 |
国内免费 | 1497篇 |
专业分类
化学 | 220897篇 |
晶体学 | 6597篇 |
力学 | 15673篇 |
综合类 | 15篇 |
数学 | 41501篇 |
物理学 | 114862篇 |
无线电 | 60862篇 |
出版年
2019年 | 3664篇 |
2018年 | 4073篇 |
2017年 | 4111篇 |
2016年 | 6821篇 |
2015年 | 4638篇 |
2014年 | 7007篇 |
2013年 | 18608篇 |
2012年 | 13390篇 |
2011年 | 16495篇 |
2010年 | 11463篇 |
2009年 | 11851篇 |
2008年 | 16036篇 |
2007年 | 16586篇 |
2006年 | 15972篇 |
2005年 | 14714篇 |
2004年 | 13343篇 |
2003年 | 12213篇 |
2002年 | 11780篇 |
2001年 | 13189篇 |
2000年 | 10558篇 |
1999年 | 8618篇 |
1998年 | 7301篇 |
1997年 | 7270篇 |
1996年 | 7161篇 |
1995年 | 6644篇 |
1994年 | 6414篇 |
1993年 | 6114篇 |
1992年 | 7094篇 |
1991年 | 6897篇 |
1990年 | 6555篇 |
1989年 | 6424篇 |
1988年 | 6296篇 |
1987年 | 5697篇 |
1986年 | 5405篇 |
1985年 | 6976篇 |
1984年 | 7038篇 |
1983年 | 5744篇 |
1982年 | 5992篇 |
1981年 | 5996篇 |
1980年 | 5662篇 |
1979年 | 5903篇 |
1978年 | 6140篇 |
1977年 | 5997篇 |
1976年 | 5778篇 |
1975年 | 5364篇 |
1974年 | 5273篇 |
1973年 | 5267篇 |
1972年 | 3539篇 |
1968年 | 3603篇 |
1967年 | 3625篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
981.
The rotational barriers of the CC bonds of halocarbons are believed to be rather different than those of hydrocarbons. We wish to systematically study the effect of halogen substituents on the conformational flexibility of hydrocarbons. As our initial approach to this problem, we have synthesized the following molecules: Pyrene-(CF2)n-Pyrene (n=3, 4, and 6) and Pyrene-(CF2)n-F (n=4 and 6). We describe here the concentration and temperature dependence of the emissive behavior of these materials. 相似文献
982.
Matveeva E. D. Podrugina T. A. Grishin Yu. K. Tkachev V. V. Zhdankin V. V. Aldoshin S. M. Zefirov N. S. 《Russian Journal of Organic Chemistry》2003,39(4):536-541
A mixed phosphonium-iodonium ylide, phenyliodoniumethoxycarbonylmethylenetriphenylphosphorane borofluoride, was synthesized. Its structure was established by means of X-ray diffraction analysis. Temperature dependence of 1H, 13C, and 31P spectra of the ylide synthesized was investigated. A dynamic equilibrium between Z and E-isomers was observed. 相似文献
983.
K. Kubilius 《Acta Appl Math》2003,78(1-3):233-242
We consider the integral equation driven by a standard Brownian motion and by a fractional Brownian motion. Sufficient conditions under which the equation has a weak solution are obtained. 相似文献
984.
The form of the probability density derived from the evolution in time of a previously truncated frequency distribution of animal Liveweights is of interest in animal husbandry. Truncated frequency distributions arise when the heavier animals are sold for slaughter and the lighter animals retained. The demands of modern quality assurance schemes require that, given information on animal growth, the farmer is able to estimate the number of animals that would meet the specifications at some time in the future after truncation. Assuming that animal growth can be described by a linear stochastic differential equation, we derive an explicit expression for the probability density of animal Liveweights at any time after the truncation of an initial Gaussian density. It is shown that this probability density converges rapidly to a Gaussian density, so that after about 20 days of typical growth rates for lambs, the resulting density is practically indistinguishable from Gaussian. 相似文献
985.
Thomas H. Pate 《Linear and Multilinear Algebra》2003,51(3):263-278
If 1≤k≤n, then Cor(n,k) denotes the set of all n×n real correlation matrices of rank not exceeding k. Grone and Pierce have shown that if A∈Cor (n, n-1), then per(A)≥n/(n-1). We show that if A∈Cor(n,2), then , and that this inequality is the best possible. 相似文献
986.
987.
O. A. Alekseeva N. I. Sorokina I. A. Verin T. Yu. Losevskaya V. I. Voronkova V. K. Yanovskii V. I. Simonov 《Crystallography Reports》2003,48(2):205-211
This study is a continuation of research into the atomic structure and physical properties of niobium-doped potassium titanyl phosphate crystals, KTiOPO4 (KTP: Nb). Crystals containing 7 and 11 at. % of niobium were grown and studied. With an increase in niobium content, the number of vacancies and additional potassium positions in the structure also increase. This fact accounts for an increase in both the intensities of relaxation peaks and the conductivity of KTP: Nb crystals. 相似文献
988.
I. P. Kuranova K. M. Polyakov E. A. Smirnova W. E. Höhne V. S. Lamzin R. Meijer 《Crystallography Reports》2003,48(6):953-958
Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator. 相似文献
989.
A crystalline quartz plate of thickness 1.070 mm is calibrated between 370 and 794 nm. Throughout this spectral interval, the retardance varied by 32π and the plate introduced quarterwave retardance 16 times at different wavelengths. The birefringence (ne−no) of crystalline quartz was calculated as a single quantity and varied from 0.00971 at 370 nm to 0.00891 at 794 nm. All measurements were carried out at 23°C. 相似文献
990.
H. Yonemura S. Moribe K. Hayashi M. Noda H. Tokudome S. Yamada H. Nakamura 《Applied magnetic resonance》2003,23(3-4):289-307
Photoinduced intramolecular electron-transfer reactions in carbazole (Cz)-fullerene (C60) (Cz(8)C60) and phenothiazine (Ph)-C60 (Ph(n)C60 (n=8, 10, 12)) linked compounds have been investigated in benzene and benzonitrile by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance measurements, and by magnetic field effects on the decay rate constants of the photogenerated biradicals. In benzonitrile, photoinduced intramolecular electron transfer from Cz to the singlet excited state of C60 (1C60 *) occurred in Cz(8)C60, but not to the triplet excited state (3C60 *), while the intramolecular electron-transfer to both1C60 * and3C60 * occurred in Ph(n)C60 (n=8, 10, 12). In benzene, on the other hand, no electron transfer to both1C60 * and3C60 * took place in all linked compounds. These results were interpreted in terms of the different Gibbs free energy changes in the two solvents. 相似文献