全文获取类型
收费全文 | 54898篇 |
免费 | 6026篇 |
国内免费 | 11358篇 |
专业分类
化学 | 48140篇 |
晶体学 | 1898篇 |
力学 | 1098篇 |
综合类 | 489篇 |
数学 | 4305篇 |
物理学 | 16352篇 |
出版年
2024年 | 113篇 |
2023年 | 971篇 |
2022年 | 1345篇 |
2021年 | 1925篇 |
2020年 | 2027篇 |
2019年 | 1836篇 |
2018年 | 1504篇 |
2017年 | 1896篇 |
2016年 | 1994篇 |
2015年 | 1761篇 |
2014年 | 2367篇 |
2013年 | 5100篇 |
2012年 | 3537篇 |
2011年 | 3771篇 |
2010年 | 3221篇 |
2009年 | 3906篇 |
2008年 | 3771篇 |
2007年 | 3843篇 |
2006年 | 3659篇 |
2005年 | 3195篇 |
2004年 | 3040篇 |
2003年 | 2552篇 |
2002年 | 2132篇 |
2001年 | 1734篇 |
2000年 | 1625篇 |
1999年 | 1327篇 |
1998年 | 1154篇 |
1997年 | 993篇 |
1996年 | 841篇 |
1995年 | 843篇 |
1994年 | 742篇 |
1993年 | 599篇 |
1992年 | 576篇 |
1991年 | 438篇 |
1990年 | 302篇 |
1989年 | 259篇 |
1988年 | 225篇 |
1987年 | 157篇 |
1986年 | 125篇 |
1985年 | 139篇 |
1984年 | 101篇 |
1983年 | 45篇 |
1982年 | 87篇 |
1981年 | 110篇 |
1980年 | 79篇 |
1979年 | 78篇 |
1978年 | 55篇 |
1977年 | 59篇 |
1976年 | 36篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
921.
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
922.
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
923.
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
924.
925.
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
926.
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
927.
928.
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
929.
The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes. 相似文献
930.
二氧化碳和氧气的快速电化学方法联合测定 总被引:4,自引:0,他引:4
根据O2和CO2在二甲亚砜(DMSO)介质中Pt电极上的伏安特性,采用微电极并结构计算机控制的电位调制技术,建立了快速联合测定CO2和O2的调制电位脉冲库仑法和调制电位脉冲电流法,取得了满意的结果。一次测量时间快达40ms。CO2检测范围0 ̄10%(体积百分比);O2检测范围不受限制。通过编程设计特定的调制电位-时间波形,可保持连续检测的长期稳定性。 相似文献