全文获取类型
收费全文 | 21294篇 |
免费 | 3646篇 |
国内免费 | 2975篇 |
专业分类
化学 | 15483篇 |
晶体学 | 272篇 |
力学 | 1257篇 |
综合类 | 194篇 |
数学 | 2596篇 |
物理学 | 8113篇 |
出版年
2024年 | 62篇 |
2023年 | 441篇 |
2022年 | 794篇 |
2021年 | 867篇 |
2020年 | 976篇 |
2019年 | 1006篇 |
2018年 | 818篇 |
2017年 | 797篇 |
2016年 | 1113篇 |
2015年 | 1184篇 |
2014年 | 1305篇 |
2013年 | 1651篇 |
2012年 | 1943篇 |
2011年 | 1925篇 |
2010年 | 1369篇 |
2009年 | 1377篇 |
2008年 | 1412篇 |
2007年 | 1189篇 |
2006年 | 1114篇 |
2005年 | 917篇 |
2004年 | 762篇 |
2003年 | 588篇 |
2002年 | 660篇 |
2001年 | 491篇 |
2000年 | 420篇 |
1999年 | 430篇 |
1998年 | 300篇 |
1997年 | 273篇 |
1996年 | 248篇 |
1995年 | 213篇 |
1994年 | 161篇 |
1993年 | 186篇 |
1992年 | 126篇 |
1991年 | 117篇 |
1990年 | 96篇 |
1989年 | 91篇 |
1988年 | 90篇 |
1987年 | 46篇 |
1986年 | 54篇 |
1985年 | 63篇 |
1984年 | 42篇 |
1983年 | 24篇 |
1982年 | 37篇 |
1981年 | 16篇 |
1980年 | 9篇 |
1979年 | 18篇 |
1978年 | 14篇 |
1977年 | 10篇 |
1976年 | 11篇 |
1975年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
901.
新型YS类高分子铑配合物催化甲醇羰基化反应制备乙酸和乙酐 总被引:3,自引:0,他引:3
以含有两种不同配位原子的线型或交联大孔球型共聚物为配体的新型铑配合物,能在温和条件下催化甲醇羰基化为乙酸和乙酐.实验结果表明,这些新型共聚物铑配合物具有鳌合型顺-二羰基铑(Ⅰ)的结构特征,并有良好的热稳定性及高的催化活性和选择性. 相似文献
902.
A semiclassical transition amplitude is derived for electronically inelastic molecular collisions in the presence of an intense electromagnetic field. The calculation of this amplitude involves the integration of classical trajectories for nuclei moving on effective potential energy surfaces. These surfaces, which exhibit an avoided crossing due to the coupling through the dipole interaction around the resonance nuclear configuration, are analytically continued to their complex intersection points, where trajectories switch surfaces. Other parts of these surfaces are similar to the field-free adiabatic surfaces, where one is shifted by ?ω. Metastable states, formed at some collision energies, are suggested in the field which are absent in the field-free system. 相似文献
903.
硫氰酸根的结构为N≡C-S-,其两端的N原子和S原子分别有一对和三对孤对电子,因此,硫氰酸根可采用多种不同的配位模式与金属离子发生配位。硫氰酸根可作为单齿配体与一个金属离子配位,形成M-SCN或M-NCS的单核配合物,也可以作为桥联配体同时与两个、三个甚至四个金属离子配位形成多核配合物犤1~3犦;另一方面,硫氰酸根是一个具有一定共轭性的偶极子,可传递磁相作用。因此,选择硫氰酸根作为桥联配体,将多个顺磁金属离子桥联形成一维、二维或三维结构的多核金属配合物分子,并研究它们的磁性已成为分子磁学的一个研究领域… 相似文献
904.
905.
The effect of antimony concentration (C(Sb)/mass%) on the surface tension of molten silicon has been determined with the sessile drop method in the temperature range from 1693 to 1773 K and in the range of the oxygen partial pressure, Po(2), in an Ar atmosphere from 10(-23) to 10(-21) MPa. The results show that the surface tension of molten silicon decreases with increasing Sb concentration in the range of C(Sb)<0.9 mass%, which indicates positive adsorption of Sb in molten silicon and can be fairly described with the Szyszkowski's equation. The maximum decrease rate of surface tension is about 65 mN m(-1) (mass% C(Sb))(-1), and the temperature coefficient of surface tension, (partial differential sigma/ partial differential T)C(Sb), increases with increasing C(Sb). The evaporation of the systems was only observed between the melting points of antimony (904 K) and silicon (1683 K), and the surface tension presents no dependence on measuring time above the melting point of silicon. 相似文献
906.
907.
Xiao-Ying Li Yong Ren Xue-Jiao Chen Da-Yong Qiao Wei-Zheng Yuan 《Journal of Radioanalytical and Nuclear Chemistry》2011,287(1):173-176
The design, fabrication, and testing of a 4H-SiC Schottky betavoltaic nuclear battery based on MEMS fabrication technology
are presented in this paper. It uses a Schottky diode with an active area of 3.14 mm2 to collect the charge from a 4 mCi/cm2
63Ni source. Some of the critical steps in process integration for fabricating silicon carbide-based Schottky diode were addressed.
A prototype of this battery was fabricated and tested under the illumination of the 63Ni source with an activity of 0.12 mCi. An open circuit voltage (V
OC) of 0.27 V and a short circuit current density (J
SC) of 25.57 nA/cm2 are measured. The maximum output power density (P
max) of 4.08 nW/cm2 and power conversion efficiency (η) of 1.01% is obtained. The performance of this battery is expected to be significantly
improved by using larger activity and optimizing the design and processing technology of the battery. By achieving comparable
performance with previously constructed p–n or p–i–n junction energy conversion structures, the Schottky barrier diode proves
to be a feasible approach to achieve practical betavoltaics. 相似文献
908.
Effect of Biosurfactants on Laccase Production and Phenol Biodegradation in Solid-State Fermentation
Zhou MF Yuan XZ Zhong H Liu ZF Li H Jiang LL Zeng GM 《Applied biochemistry and biotechnology》2011,164(1):103-114
The effects of two biosurfactants, tea saponin (TS) and rhamnolipid (RL), on the production of laccase and the degradation
of phenol by P. simplicissimum were investigated in solid-state fermentation consisting of rice straw, rice bran, and sawdust. Firstly, the effects of phenol
on the fermentation process were studied in the absence of surfactants. Then, a phenol concentration of 3 mg/g in the fermentation
was selected for detailed research with the addition of biosurfactants. The results showed that TS and RL at different concentrations
had stimulative effects on the enzyme activity of laccase. The highest laccase activities during the fermentation were enhanced
by 163.7%, 68.2%, and 23.3% by TS at concentrations of 0.02%, 0.06%, and 0.10%, respectively. As a result of the enhanced
laccase activity, the efficiency of phenol degradation was also improved by both biosurfactants. RL caused a significant increase
of fungal biomass in the early stage of the fermentation, while TS had an inhibitory effect in the whole process. These results
indicated that RL could mitigate the negative effects of phenol on fungal growth and consequently improve laccase production
and phenol degradation. TS was potentially applicable to phenol-polluted solid-state fermentation. 相似文献
909.
Hui-Lu Wu Ke Li Tao Sun Fan Kou Fei Jia Jing-Kun Yuan Bin Liu Bao-Liang Qi 《Transition Metal Chemistry》2011,36(1):21-28
Tris(N-methylbenzimidazol-2-ylmethyl)amine (Mentb) and its two complexes, [Mn(Mentb)(DMF)(H2O)](pic)2
1 and [Zn(Mentb)(pic)](pic) 2 (pic = picrate), have been synthesized and characterized by physico-chemical and spectroscopic methods. Single crystal X-ray
diffraction revealed that the two complexes have different structures. In complex 1, the coordination sphere around Mn(II) is distorted octahedral, whereas in complex 2 the coordination sphere around Zn(II) is distorted trigonal bipyramidal. The DNA-binding properties of the free ligand and
its two complexes have been investigated by electronic absorption, fluorescence, and viscosity measurements. The results suggest
that the ligand and its two complexes bind to DNA via an intercalation binding mode, and their binding affinity for DNA follows
the order 1 > 2 > ligand. 相似文献
910.
[reaction: see text] Unsaturated sulfonamides underwent direct intramolecular aziridination catalyzed by Rh(2)(OAc)(4) with PhI(OAc)(2) and Al(2)O(3) to give the corresponding aziridine products in excellent yields (up to 98%) and with good to excellent conversions. High turnovers (up to 1375) were achieved. The intermolecular rhodium-catalyzed amidation of cholesteryl acetate with PhI=NTs or PhI(OAc)(2)/NH(2)R as the nitrogen source exhibited both excellent regio- and alpha-selectivity (alpha/beta ratio up to 9:1). 相似文献