全文获取类型
收费全文 | 52698篇 |
免费 | 5508篇 |
国内免费 | 11192篇 |
专业分类
化学 | 48737篇 |
晶体学 | 2102篇 |
力学 | 685篇 |
综合类 | 453篇 |
数学 | 3406篇 |
物理学 | 14015篇 |
出版年
2024年 | 108篇 |
2023年 | 960篇 |
2022年 | 933篇 |
2021年 | 1869篇 |
2020年 | 1963篇 |
2019年 | 1766篇 |
2018年 | 1485篇 |
2017年 | 1908篇 |
2016年 | 1979篇 |
2015年 | 1732篇 |
2014年 | 2273篇 |
2013年 | 4928篇 |
2012年 | 3523篇 |
2011年 | 3715篇 |
2010年 | 3127篇 |
2009年 | 3780篇 |
2008年 | 3569篇 |
2007年 | 3717篇 |
2006年 | 3502篇 |
2005年 | 3045篇 |
2004年 | 2932篇 |
2003年 | 2436篇 |
2002年 | 2099篇 |
2001年 | 1656篇 |
2000年 | 1529篇 |
1999年 | 1249篇 |
1998年 | 1067篇 |
1997年 | 917篇 |
1996年 | 825篇 |
1995年 | 805篇 |
1994年 | 701篇 |
1993年 | 565篇 |
1992年 | 525篇 |
1991年 | 402篇 |
1990年 | 279篇 |
1989年 | 234篇 |
1988年 | 207篇 |
1987年 | 136篇 |
1986年 | 113篇 |
1985年 | 123篇 |
1984年 | 98篇 |
1983年 | 43篇 |
1982年 | 73篇 |
1981年 | 107篇 |
1980年 | 72篇 |
1979年 | 83篇 |
1978年 | 52篇 |
1977年 | 55篇 |
1976年 | 37篇 |
1973年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
G. G. Danagulyan D. A. Tadevosyan F. S. Kinoyan 《Chemistry of Heterocyclic Compounds》2006,42(6):782-787
Intermediate recyclization products were obtained in a study of the Kost-Sagitullin rearrangement of a series of 1,2-dialkylpyrimidinium
iodides. The initial attack of the nucleophile leads to the formation of products of the addition of the hydroxyl group, namely,
the corresponding pseudo bases. Heating one of these intermediates in ethanol or in the presence of primary amines leads to
rearrangement to give a pyridone derivative. Upon heating in chloroform, the pseudo bases readily lose a water molecule and
are converted to anhydro bases, namely, derivatives of 1-alkyl-1,2-dihydro-2-methylidenepyrimidine.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 894–900, May, 2006. 相似文献
993.
Saw-Peng Yew 《Polymer Degradation and Stability》2006,91(8):1800-1807
This study aims to evaluate the photocatalytic activity and biodegradation of polyhydroxybutyrate (PHB) films containing titanium dioxide (TiO2). Nanosized TiO2 photocatalysts were immobilized onto PHB film to overcome the difficulty of the recovery process. PHB is a suitable base material as it is naturally biodegradable and is produced from renewable resources. The photocatalytic degradation of organic compounds, photocatalytic sterilization activity and biodegradation rate in garden soil of PHB-TiO2 composite films were investigated. After an hour under solar illumination, 96% of methylene blue solution was decolorized. The antibacterial activity against Escherichia coli (E. coli) using PHB-TiO2 composite film exhibited enhanced photocatalytic sterilization activity over time. As for the ability to biodegrade, PHB-TiO2 composite films placed on soil surface with no direct solar illumination showed slower degradation rate compared to those receiving direct solar illumination. Interestingly, the latter composite films showed faster degradation rates compared to pure PHB films indicating that the degradation is mainly due to photocatalytic activity. PHB-TiO2 composite films buried in soil generally showed slower degradation rates compared to pure PHB films and were dependent on the soil microbial activity. 相似文献
994.
锂离子电池正极材料LiMn2O4的合成与晶体结构(英) 总被引:2,自引:0,他引:2
Spinel LiMn2O4 powders were prepared using two-step synthesis method consisting of solid-state reaction method and citrate modified sol-gel method. The effects of the calcination temperature and the Li/Mn ratio of raw materials were studied on the physicochemical and electrochemical properties of the spinel LiMn2O4 powders, such as crystallinity, lattice constant and density. The title compound was characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Polycrystalline LiMn2O4 powers calcined at 750 ℃ were found to be composed of very uniformly-sized microcrystal with an average particle size of 300 nm. The improvement in electrochemical properties was mainly attributed to the process of re-grinding by absolute alcohol. 相似文献
995.
996.
Treatment of acetonide protected 4,5-dihydroxy-2-chloroglycidic ester or its rearrangement product, the acetonide protected 4,5-dihydroxy-3-chloro-2-oxo ester, with magnesium halides gave 4-halo-3-hydroxy-2-pyrone in excellent to reasonable yields in one pot. The mechanism of this novel one pot rearrangement-cyclization reaction is also proposed. 相似文献
997.
An efficient route for the regio- and stereoselective ring opening of N-tosylaziridines with zinc dihalides (ZnX2, X = Cl, Br, I) is described. Depending on the solvent and Zn(II) halide, β-halo amines or imidazolines are obtained selectively in good to excellent yields. 相似文献
998.
共沉淀浸渍法制备由合成气直接合成二甲醚的Cu-Mn催化剂 总被引:9,自引:4,他引:9
采用共沉淀浸渍法,制备了直接合成二甲醚的Cu-Mn-Zn催化剂,通过对组成成分及其配比的研究,发现Cu含量一定的条件下,n(Zn)/n(Mn)摩尔比对催化剂性能有较大的影响,当n(Zn)/n(Mn)=1/3~1/2时,催化剂对CO的转化率和对二甲醚的选择性达到最佳,分别为53.6%和63.5%;如锰添加比例过大,对催化剂催化合成二甲醚有微弱抑制;添加锌比例过大,会大大降低CO的转化率。载体Y分子筛的含量对催化剂性能也有影响,用量过大将降低催化剂的活性和对二甲醚的选择性,当其含量为33%时,催化剂上CO转化率和选择性可分别达到66%和68%,且催化剂活性随分子筛含量减少不再有明显的变化。 相似文献
999.
1000.
XU Long Ya * WANG Qing Xia LIN Li Wu XU Yi De Dalian Institute of Chemical Physics The Chinese Academy of Sciences Dalian Liaoning 《天然气化学杂志》1998,7(1):7-15
Ni/Si2催化剂具有较好的低碳烷烃与二氧化碳重整制合成气的反应性能,添加La2O3助剂和K2O助剂可提高催化剂活性和合成气收率,从而进一步改善催化剂的低碳烷烃与CO2重整制合成气的反应性能,研制的KLaNi/Si2催化剂,用于天然气与CO2重整反应制合成气可达97%的低碳烷烃转化率和95%以上的合成气收率。 相似文献