全文获取类型
收费全文 | 4984篇 |
免费 | 908篇 |
国内免费 | 1008篇 |
专业分类
化学 | 3943篇 |
晶体学 | 121篇 |
力学 | 298篇 |
综合类 | 87篇 |
数学 | 549篇 |
物理学 | 1902篇 |
出版年
2024年 | 16篇 |
2023年 | 57篇 |
2022年 | 153篇 |
2021年 | 159篇 |
2020年 | 220篇 |
2019年 | 239篇 |
2018年 | 184篇 |
2017年 | 177篇 |
2016年 | 240篇 |
2015年 | 261篇 |
2014年 | 332篇 |
2013年 | 395篇 |
2012年 | 431篇 |
2011年 | 439篇 |
2010年 | 341篇 |
2009年 | 360篇 |
2008年 | 373篇 |
2007年 | 362篇 |
2006年 | 324篇 |
2005年 | 292篇 |
2004年 | 248篇 |
2003年 | 168篇 |
2002年 | 209篇 |
2001年 | 173篇 |
2000年 | 116篇 |
1999年 | 143篇 |
1998年 | 96篇 |
1997年 | 84篇 |
1996年 | 75篇 |
1995年 | 52篇 |
1994年 | 48篇 |
1993年 | 35篇 |
1992年 | 24篇 |
1991年 | 14篇 |
1990年 | 18篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1975年 | 1篇 |
1959年 | 1篇 |
1909年 | 2篇 |
排序方式: 共有6900条查询结果,搜索用时 125 毫秒
41.
IntroductionRecently ,thecompoundscontaining 1H 1,2 ,4 tria zolegrouphaveattractedmuchinterestbecausetheyex hibitsomefungicidalactivity ,plantgrowthregulatingac tivity ,1antibacterialactivityagainstPucciniareconditeandrootsgrowthregulationforcucumber .2 Also ,suchcom poundsareincreasinglybeingstudiedbecauseofthecoor dinationchemistryoftriazolesactingasligandsintransi tionmetalcompounds .Asamatteroffact ,thetriazolederivativeshavebeenextensivelyusedasterminalandbridgingligands ,andtheyleadto… 相似文献
42.
铁基超细粒子催化剂具有优良的F-T反应性能。本工作利用原位Mssbauer谱辅以XRD技术研究了含有钾助剂的F-T合成铁基超细粒子催化剂的结构与还原行为,考察了催化剂组成和第二金属组分(Mn,Zn,Mg)的影响。“纯”铁超细粒子催化剂在氢气中很容易经Fe_3O_4还原为零价铁。第二金属组分的引入,一方面阻碍了催化剂的还原,另一方面稳定了Fe~(2+)的存在而使其成为还原的主要中间相,这有利于反应条件下形成类尖晶石结构的活性相。对于Fe-Mn催化剂,上述效应随锰含量的增加而更趋显著。铁基超细粒子催化剂在氢气中的还原能力依“纯”Fe>Fe-Zn>Fe-Mn>Fe-Mg的顺序递减,这在一定程度上反映了铁与第二金属组分之间相互作用的强弱。 相似文献
43.
以Mossbauer谱为手段,研究了用于F-T合成的Fe/Zn催化剂的物相结构,并考察了组成及制备方法的影响,同时还对其还原、碳化行为进行了探讨.发现锌对铁的还原有抑制作用,它能够稳定二价铁不被进一步还原.研究还表明,我们所研制的Fe/Zn催化剂是一种高分散度的铁锌化合物,室温下具有超顺磁特性. 相似文献
44.
45.
Zou JW Jiang YJ Guo M Hu GX Zhang B Liu HC Yu QS 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(2):740-751
Ab initio calculations have been performed on a series of complexes formed between halogen-containing molecules and ammonia to gain a deeper insight into the nature of halogen bonding. It appears that the dihalogen molecules form the strongest halogen-bonded complexes with ammonia, followed by HOX; the charge-transfer-type contribution has been demonstrated to dominate the halogen bonding in these complexes. For the complexes involving carbon-bound halogen molecules, our calculations clearly indicate that electrostatic interactions are mainly responsible for their binding energies. Whereas the halogen-bond strength is significantly enhanced by progressive fluorine substitution, the substitution of a hydrogen atom by a methyl group in the CH(3)X...NH(3) complex weakened the halogen bonding. Moreover, remote substituent effects have also been noted in the complexes of halobenzenes with different para substituents. The influence of the hybridization state of the carbon atom bonded to the halogen atom has also been examined and the results reveal that halogen-bond strengths decrease in the order HC triple bond CX > H(2)C=CHX approximately O=CHX approximately C(6)H(5)X > CH(3)X. In addition, several excellent linear correlations have been established between the interaction energies and both the amount of charge transfer and the electrostatic potentials corresponding to an electron density of 0.002 au along the R-X axis; these correlations provide good models with which to evaluate the electron-accepting abilities of the covalently bonded halogen atoms. Finally, some positively charged halogen-bonded systems have been investigated and the effect of the charge has been discussed. 相似文献
46.
Lingjun Chou Yingchun Cai Bing Zhang Jianzhong Niu Shengfu Ji Shuben Li 《Reaction Kinetics and Catalysis Letters》2002,76(2):311-315
Na-Mn-W/SiO2 catalysts were prepared and their catalytic performance for oxidative coupling of methane (OCM) was evaluated in a stainless-steel microreactor at elevated pressure. The results show that a CH4 conversion of 15.1% with a C2+ selectivity of 71.8% was obtained under 750oC, 1.0×105h-1 GHSV, CH4/O2 ratio of 8 and 1.0 MPa. Moreover, 17.3% CH4 conversion with 51.6% C2 selectivity and 23.6% C3-C4 selectivity was obtained under 750oC, 2.0×105h-1 GHSV, CH4/O2 ratio of 8 and 1.0 MPa. 相似文献
47.
48.
Here we report the first example of catalytic metallogels, which are formed irreversibly in dimethylsulfoxide via the creation of cross-linked, three-dimensional coordination polymer networks by using transition-metal ions with multiple sites available for coordination and multidentate ligands. Conformational flexibility of the ligands and slow formation of the coordination polymers apparently favor the gelation. These metallogels are stable in water and most organic solvents and can catalyze the oxidation of benzyl alcohol to benzaldehyde by using their PdII moieties as the catalytic centers. The best catalytic turnover of the metallogel is twice that of [Pd(OAc)2] under similar reaction conditions. 相似文献
49.
用乙烯为原料在双功能催化剂体系的作用下通过二聚和共聚合反应,直接合成线性低密度聚乙烯(LLDPE).所用二聚反应催化剂为钛酸正丁酯(Ti(OBu)_4),共聚催化剂为TiCl_4/MgCl_2(ZM-1催化剂).研究了二聚反应动力学行为及影响双功能催化剂体系动力学过程的一些因素.结果表明利用这两种催化剂组成双功能催化剂体系能够很方便地制得密度范围在0.90-0.93,Et/1000C在10-40的LLDPE. 相似文献
50.
Yan B Collins N Wheatley J Irving M Leopold K Chan C Shornikov A Fang L Lee A Stock M Zhao J 《Journal of combinatorial chemistry》2004,6(2):255-261
We have developed a high-throughput purification system to purify combinatorial libraries at a 50-100-mg scale with a throughput of 250 samples/instrument/day. We applied an accelerated retention window method to shorten the purification time and targeted one fraction per injection to simplify data tracking, lower QC workload, and simplify the postpurification processing. First, we determined the accurate retention time and peak height for all compounds using an eight-channel parallel LC/UV/MS system, and calculated the specific preparative HPLC conditions for individual compounds. The preparative HPLC conditions include the compound-specific gradient segment for individual compounds with a fixed gradient slope and the compound-specific UV or ELSD threshold for triggering a fraction collection device. A unique solvent composition or solvent strength was programmed for each compound in the preparative HPLC in order to elute all compounds at the same target time. Considering the possible deviation of the predicted retention time, a 1-min window around the target time was set to collect peaks above a threshold based on UV or ELSD detection. Dual column preparative instruments were used to maximize throughput. We have purified more than 500 000 druglike compounds using this system in the past 3 years. We report various components of this high-throughput purification system and some of our purification results. 相似文献