全文获取类型
收费全文 | 8846篇 |
免费 | 293篇 |
国内免费 | 207篇 |
专业分类
化学 | 5226篇 |
晶体学 | 47篇 |
力学 | 249篇 |
综合类 | 58篇 |
数学 | 1944篇 |
物理学 | 1822篇 |
出版年
2023年 | 37篇 |
2022年 | 65篇 |
2021年 | 71篇 |
2020年 | 71篇 |
2019年 | 122篇 |
2018年 | 73篇 |
2017年 | 103篇 |
2016年 | 98篇 |
2015年 | 78篇 |
2014年 | 144篇 |
2013年 | 249篇 |
2012年 | 172篇 |
2011年 | 335篇 |
2010年 | 333篇 |
2009年 | 465篇 |
2008年 | 369篇 |
2007年 | 301篇 |
2006年 | 287篇 |
2005年 | 188篇 |
2004年 | 115篇 |
2003年 | 135篇 |
2002年 | 119篇 |
2001年 | 95篇 |
2000年 | 104篇 |
1999年 | 97篇 |
1998年 | 86篇 |
1997年 | 53篇 |
1996年 | 59篇 |
1995年 | 40篇 |
1994年 | 33篇 |
1993年 | 30篇 |
1992年 | 31篇 |
1991年 | 27篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1987年 | 15篇 |
1986年 | 14篇 |
1985年 | 718篇 |
1984年 | 504篇 |
1983年 | 428篇 |
1982年 | 486篇 |
1981年 | 378篇 |
1980年 | 299篇 |
1979年 | 311篇 |
1978年 | 235篇 |
1977年 | 253篇 |
1976年 | 279篇 |
1975年 | 253篇 |
1974年 | 248篇 |
1973年 | 279篇 |
排序方式: 共有9346条查询结果,搜索用时 31 毫秒
91.
Thermal decomposition of bis(hexamethylbenzene)iron(0) in the presence of carbon monoxide yields a novel carbonyl iron complex, [C6(CH3)6]Fe(CO)2. The cyclohexadiene complex [C6(CH3)6]Fe(C6H8) is obtained from reaction of bis(hexamethylbenzene)iron(0) with either 1,3-cyclohexadiene or benzene, and the yield is much greater in the presence of hydrogen gas. Interaction of bis-(hexamethylbenzene)iron(0) with 2-butyne induces a catalytic cyclotrimerization to give more hexamethylbenzene. Kinetic and isotope distribution studies indicate that the primary step in these reactions is not a direct loss of one ring ligand, but rather an insertion of the iron center into one of the ligand methyl CH bonds, leading to a benzyl hydride complex species. Mechanisms for the subsequent reactions of this iron hydride species are proposed. 相似文献
92.
C.R. Leidner P. Denisevich K.W. Willman Royce W. Murray 《Journal of Electroanalytical Chemistry》1984,164(1):63-78
The cyclic voltammetric peaks for charging trapping and untrapping reactions between the inner and outer redox polymer films of five bilayer electrodes are compared to a theory for control of the rate of charge trapping by electron diffusion rates in the inner polymer film. The five bilayer electrodes use various different redox polymer films (electropolymerized poly-pyridine complexes of Fe, Ru, and Os, and polyvinylferrocene) arranged in different orders. The currents on the rising edge of the bilayer trapping and untrapping peaks follow the electron diffusion theory up to ca. 80% of the peak current; currents thereafter are controlled by another process(es). The analysis yields values for the electron diffusion constants in the inner bilayer polymer films, which agree with one another for different bilayers having the same inner film polymer films and which also agree with independent determinations by other methods. Two of the bilayers are made from the same two polymers, arranged in different inner-outer order. These bilayers also illustrate the occurrence of a “leak reaction”, in which charge trapped in the outer film is discharged via a thermodynamically unfavorable electron transfer reaction with the inner polymer film. 相似文献
93.
Tertiary phosphine oxides have been prepared in excellent yield from primary alkyl halides or aromatic halides and activated sodium phosphinates obtained by reaction of dialkyl phosphine oxides with complex bases (NaNH2/tBuONa). This reaction has been successfully applied to soluble and cross-linked bromopolystyrenes and to macroporous polystyrenes with bromooctyl substituents giving polymers which contain pendant phosphine oxide groups. 相似文献
94.
Horst P. Beck 《Journal of solid state chemistry》1983,47(3):328-332
A high pressure phase BaI2-II, which can be quenched and retained as a metastable form at ambient conditions, is reported. The structure has been determined by X-ray investigations using single crystals. BaI2-II crystallizes in an anti-Fe2P-type arrangement, space group Å, c = 5.173(3)Å, which is slightly denser than the PbCl2-type structure found at normal pressure. Structural features of the two phases are discussed in comparison. 相似文献
95.
U. Berg R. Isaksson J. Sandström U. Sjöstrand A. Eiglsperger A. Mannschreck 《Tetrahedron letters》1982,23(41):4237-4240
(+)- and (?)- as well as (+)- and (?)- were partially separated by liquid chromatography on triacetylcellulose; racemizations resulted in the barriers (Table 2) to rotation via planarized transition states. 相似文献
96.
Marie-Paule Pileni 《Chemical physics letters》1980,75(3):540-544
Photoinduced electron transfer from Zn tetraphenylporphyrin to acceptors solubilized either in the lipid interior or aqueous bulk of anionic oil-in-water microemulsions has been investigated by nanosecond laser photolysis. While intimate cosolubilization appears to decrease the efficiency of electron transfer in the former, enhanced charge separation of the redox products has been observed in the latter. 相似文献
97.
The intramolecular cyclization reactions of tri-1-hex-5-enyl Group III derivatives and of 1-hex-5-enyllithium have been investigated and it is proposed that the reactions may be depicted as internal addition across the terminal double bonds of the alkenyl species. The cyclization reaction proceeds more readily for the 1-hex-5-enyl derivatives than for any other chain length and is dependent on temperature, concentration and solvent. 1H NMR data and cryoscopic molecular weight data are presented for several diisobutyialkenyl- and diisobutylalkyl-aluminum derivatives. These data have been interpreted in terms of a direct intramolecular interaction between the metal center and the π-electron system of the olefinic site. The intramolecular cyclization reactions for the 1-hept-6-enyl, 1-oct-7-enyl- and 1-undec-10-enylaluminum derivatives also were investigated and it was found that cyclization occurred only for the 1-hept-6-enyl derivative. 相似文献
98.
The crystal structure of a second high-pressure copper vanadate phase, CuVO3(II), has been determined and refined by full-matrix least-squares procedures using automatic diffractometer data to a residual R = 0.042 (Rw = 0.051). The space group is rhombohedral, , with hexagonal unit cell a = 4.966(2) and c = 14.084(5) Å [aR = 5.501(2) Å and α = 53.66(3)°]. The structure is the fully ordered ilmenite-type and, on the basis of published magnetic data and the interatomic distances, the valence distribution Cu+V5+O3 is proposed. This represents a unique example of Cu+ in an octahedral environment. 相似文献
99.
NH3(MoO3)3 crystallizes with hexagonal symmetry, space group , lattice constants a = 10.568 Å, c = 3.726 Å, and Z = 2. The crystal structure has been determined by Patterson synthesis and refined assuming isotropic temperature factors to a final conventional R value of 0.085. The structure shows a three-dimensional arrangement built up of double chains of distorted MoO6 octahedra, parallel to the [001] direction. The octahedral double chains are linked among each other through common oxygen atoms. In addition to the shared oxygen atoms, each molybdenum is coordinated to one terminal oxygen. MoO distances range from 1.645 to 2.378 Å and OMoO angles from 74.3 to 114.3°. These results are consistent with the fact that molybdenum in high-valence states shows octahedral coordination with terminal oxygens. 相似文献
100.
Heat transfer from a plasma flow to a metallic or nonmetallic spherical particle is studied in this paper for the extreme case of free-molecule flow regime. Analytical expressions are derived for the heat flux due to, respectively, atoms, ions, and electrons and for the floating potential on the sphere exposed to a two-temperature plasma flow. It has been shown that the local or average heat flux density over the whole sphere is independent of the sphere radius and approximately in direct proportion to the gas pressure. The presence of a macroscopic relative velocity between the plasma and the sphere causes substantially nonuniform distributions of the local heat flux and enhances the total heat flux to the sphere. The heat flux is also enhanced by the gas ionization. Appreciable difference between metallic and nonmetallic spheres is found in the distributions along the oncoming flow direction of the floating potential and of the local heat flux densities due to ions and electrons. The total heat flux to the whole sphere is, however, almost the same for these different spheres. For a fixed value of the electron temperature, the heat flux decreases with increasing temperature ratio Te/Th. 相似文献