全文获取类型
收费全文 | 63637篇 |
免费 | 1021篇 |
国内免费 | 350篇 |
专业分类
化学 | 30925篇 |
晶体学 | 1067篇 |
力学 | 3298篇 |
综合类 | 8篇 |
数学 | 5069篇 |
物理学 | 24641篇 |
出版年
2022年 | 588篇 |
2021年 | 568篇 |
2020年 | 524篇 |
2019年 | 531篇 |
2018年 | 652篇 |
2017年 | 561篇 |
2016年 | 1059篇 |
2015年 | 785篇 |
2014年 | 1170篇 |
2013年 | 2796篇 |
2012年 | 2652篇 |
2011年 | 3378篇 |
2010年 | 2365篇 |
2009年 | 2387篇 |
2008年 | 3047篇 |
2007年 | 2838篇 |
2006年 | 2705篇 |
2005年 | 2435篇 |
2004年 | 2189篇 |
2003年 | 1944篇 |
2002年 | 1811篇 |
2001年 | 3101篇 |
2000年 | 2247篇 |
1999年 | 1601篇 |
1998年 | 1119篇 |
1997年 | 1100篇 |
1996年 | 942篇 |
1995年 | 827篇 |
1994年 | 743篇 |
1993年 | 679篇 |
1992年 | 994篇 |
1991年 | 980篇 |
1990年 | 885篇 |
1989年 | 791篇 |
1988年 | 757篇 |
1987年 | 817篇 |
1986年 | 682篇 |
1985年 | 905篇 |
1984年 | 845篇 |
1983年 | 581篇 |
1982年 | 575篇 |
1981年 | 558篇 |
1980年 | 508篇 |
1979年 | 635篇 |
1978年 | 658篇 |
1977年 | 678篇 |
1976年 | 593篇 |
1975年 | 490篇 |
1974年 | 535篇 |
1973年 | 456篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
943.
944.
945.
946.
4-Monodeuterated NAD(P)H model compounds (1,4,6,7-tetrahydro-1,6,11-trimethyl-5-oxo-5H-benzo[c]pyrido[2,3-e]az epin; 11Me-MMPAH) have been oxidized with a series of p-benzoquinone and its derivatives in the presence of Mg2+. The models have an axial chirality with respect to the orientation of carbonyl dipole, the dihedral angle of which is larger than 55 degrees out of the plane of dihydropyridine ring. Without Mg2+, the anti- (with respect to the carbonyl dipole) hydrogen is 3 to 32 times more reactive than the corresponding syn-hydrogen, whereas, when Mg2+ is present in the system, the selectivity is shifted toward the syn-preferency. Mg2+ plays the role of a Lewis acid catalyst to control the stereochemistry at the same time as it catalyzes the reaction. 相似文献
947.
Diamond-like carbon (DLC) and tetrahedral amorphous carbon (ta-C) have attracted much attention recently for biomedical and antifouling applications due to their excellent biocompatibility and inherent nonstick properties. It has been demonstrated that the solid surface free energy is a dominant factor in cellular or fouling adhesion. However, few data for the surface free energy of DLC and ta-C coatings at temperatures in the range 37-95 degrees C are available. In this study DLC and ta-C coatings on stainless steel 304 sheets were prepared using an unbalanced magnetron sputtering system and a filtered cathodic vacuum arc system, respectively. The contact angles of water, diiodomethane and ethylene glycol on the coated surfaces at temperatures in the range 20-95 degrees C were measured using a Dataphysics OCA-20 contact angle analyzer. The surface free energy of the coatings and their components (e.g., dispersion, polar or acid/base portions) were calculated using various methods. The experimental results showed that the total surface free energy and dispersive surface free energy of the ta-C coatings, DLC coatings, stainless steel 304 and titanium decreased with increasing surface temperature, while the acid-base SFE component increased with increasing temperature. 相似文献
948.
[reaction: see text] A zirconocene-mediated ring contraction of 4-vinylfuranosides generated either from d-arabinose or d-glucose is followed by sequential oxidation to the ketone and alkynyl Grignard addition. The resulting cis-cyclobutanediols are subjected in turn to thermal rearrangement and intramolecular oxymercuration-demercuration. The regiochemistry of the final ring closure is controlled by the nature of R. 相似文献
949.
Burton A Elomari S Chen CY Medrud RC Chan IY Bull LM Kibby C Harris TV Zones SI Vittoratos ES 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(23):5737-5748
The syntheses, structure solutions, and physicochemical and catalytic characterizations of the novel zeolites SSZ-53 and SSZ-59 are described. SSZ-53 and SSZ-59 were synthesized under hydrothermal conditions with the [1-(4-fluorophenyl)cyclopentylmethyl]trimethyl ammonium cation and 1-[1-(4-chlorophenyl)cyclopentylmethyl]-1-methyl azocanium cation, respectively, as structure-directing agents. The framework topology of SSZ-53 was solved with the FOCUS method, and the structure of SSZ-59 was determined by model building. Rietveld refinement of synchrotron X-ray powder diffraction data confirms each proposed model. SSZ-53 and SSZ-59 each possess a one-dimensional channel system delimited by 14-membered rings. Results from transmission electron microscopy, electron diffraction, catalytic experiments (spaciousness index and constraint index tests), and argon and hydrocarbon adsorption experiments are consistent with the proposed structures. 相似文献
950.
The equilibrium geometrical structures of small AlmFen clusters have been determined through ab initio calculation of the cluster total energy at the UB3LYP/Lanl2dz level. For dimers of iron and aluminum, the dissociation energies,
the equilibrium atomic distances, and the vibrational frequencies were calculated. The agreement between calculations and
experiments is reasonable. The ground stable geometrical structures of Fe4, FeAl3, Fe3Al and Fe2Al2 clusters favor three-dimension configurations, but Al4 tetramers are planar structures. The Al-rich tetramers are more stable than the other two composition tetramers. This is
different from that of bulk alloys. 相似文献