全文获取类型
收费全文 | 164479篇 |
免费 | 28615篇 |
国内免费 | 18166篇 |
专业分类
化学 | 127130篇 |
晶体学 | 1934篇 |
力学 | 8497篇 |
综合类 | 1119篇 |
数学 | 18306篇 |
物理学 | 54274篇 |
出版年
2024年 | 429篇 |
2023年 | 2538篇 |
2022年 | 4690篇 |
2021年 | 5099篇 |
2020年 | 6147篇 |
2019年 | 7128篇 |
2018年 | 5488篇 |
2017年 | 5008篇 |
2016年 | 9141篇 |
2015年 | 9389篇 |
2014年 | 10758篇 |
2013年 | 13587篇 |
2012年 | 14483篇 |
2011年 | 14161篇 |
2010年 | 11205篇 |
2009年 | 10984篇 |
2008年 | 10997篇 |
2007年 | 9505篇 |
2006年 | 8721篇 |
2005年 | 7551篇 |
2004年 | 6054篇 |
2003年 | 5043篇 |
2002年 | 5480篇 |
2001年 | 4296篇 |
2000年 | 3862篇 |
1999年 | 2970篇 |
1998年 | 2188篇 |
1997年 | 1956篇 |
1996年 | 1886篇 |
1995年 | 1647篇 |
1994年 | 1513篇 |
1993年 | 1241篇 |
1992年 | 1092篇 |
1991年 | 936篇 |
1990年 | 792篇 |
1989年 | 594篇 |
1988年 | 463篇 |
1987年 | 405篇 |
1986年 | 409篇 |
1985年 | 316篇 |
1984年 | 219篇 |
1983年 | 182篇 |
1982年 | 142篇 |
1981年 | 94篇 |
1980年 | 70篇 |
1979年 | 48篇 |
1978年 | 30篇 |
1977年 | 29篇 |
1975年 | 30篇 |
1957年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
982.
983.
Ammonia-treated activated carbon has been studied as a support of Ru-Ba catalyst for ammonia synthesis. It is shown that the
introduction of nitrogen leads to a decrease of ammonia synthesis activity for the catalysts with a low Ba/Ru molar ratio,
while no significant changes are obtained for the catalysts with a high Ba/Ru molar ratio, confirming that electronegative
impurities suppress the activity in ammonia synthesis and consume part of the promoters.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
984.
Solid complex of terbium nitrate with a novel tetrapodal ligand, 1,1,1′,1′-tetra(2-pyridinecarboxylester)-di(trimethylpropane) was prepared. This new complex with the formula of [TbL(NO3)](NO3)2·2H2O was charact-erized by elemental analysis, molar conductivity, IR spectroscopy and thermal analysis. At the same time, the luminescent property of the complex was also studied. 相似文献
985.
Fe3Al and Cr18-Ni8 stainless steel were diffusion-bonded in vacuum and a Fe3Al/Cr18-Ni8 interface with reaction layer was formed. Microstructure in the reaction layer at Fe3Al/Cr18-Ni8 interface was analyzed by means of scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA). The growth of reaction layer with heating temperature (T) and holding time (t) was researched. The results indicate that FeAl, Fe3Al, Ni3Al, and alpha-Fe (Al) solid solution are formed in the reaction layer. These phases are favorable to promote the element diffusion and to accelerate the formation of the reaction layer at Fe3Al/Cr18-Ni8 interface. The growth of reaction layer obeys the parabolic law and its thickness (X) is expressed by X2 = 7.5 x 10(-4)exp(-83.59/RT)(t - t0). 相似文献
986.
Xing Yong Wang Jing Nan Cui Wei Min Ren Feng Li Chun Liang Lu Xu Hong Qian State Key Laboratory of Fine Chemicals Dalian University of Technology Dalian China School of Chemical Engineering Technology China University of Mining & Technology Xuzhou China Shanghai Key Laboratory of Chemical Biology East China University of Science Technology Shanghai China 《中国化学快报》2007,18(6):681-684
Baker's yeast mediated reduction of acenaphthenequinone within 4-10 h afforded mono-hydroxyacenaphthenone mainly with low enantioselectivity, the substrate and mono-hydroxyacenaphthenone product almost converted to dihydroxyacenaphthene after 48 h. By control of the reaction time and in the presence of DMF as co-solvent, the reduction of 6-substituted acenaphthenequinones under vigorous agitation afforded the corresponding 2-hydroxyacenaphthenones in 24-84% yields with 10-93% ee. 相似文献
987.
Binyuan Liu Yang Li Boo-gyo Shin Li Zhang Weidong Yan 《Polymer Degradation and Stability》2007,92(5):868-875
The thermal properties and degradation behaviors of poly(norbornene-2,3-dicarboxylic acid dialkyl esters) (PNB-dialkyl esters) (alkyl = Me (PNB-Me), Et (PNB-Et), Pr (PNB-Pr), and Bu (PNB-Bu)) were investigated by thermogravimetric analysis (TGA) in dynamic conditions and by infrared (IR) spectroscopy. The PNB-dialkyl esters show good thermal stability up to 350 °C, and the thermal stability decreases in the order Me > Et > Pr > Bu with the increase in size of side chain. The effect of side-chain size on the thermal degradation behaviors of PNB-dialkyl esters is evidenced by one-step thermal degradation profile for PNB-Me while two-step thermal degradation profile for PNB-Et, PNB-Pr, and PNB-Bu. Transformation is deduced to undergo β-hydrogen elimination and formation of anhydride group in the first stage of thermal degradation reaction according to TGA and IR results for PNB-Et, PNB-Pr, and PNB-Bu. The apparent activation energy and thermal degradation model of PNB-dialkyl esters are estimated by means of Ozawa-Flynn-wall method and Phadnis-Deshpande method, respectively. 相似文献
988.
Antonio Doménech María Teresa Doménech‐Carbó Howell G. M. Edwards 《Electroanalysis》2007,19(18):1890-1900
Multivariate chemometric methods are applied for identifying earth pigments from square‐wave voltammetric measurements performed at pigment‐modified paraffin‐impregnated graphite electrodes allowing for a separation between hematite‐based earths, French ochres, Spanish ochres, siennas, umbers and green earths. This methodology is applied to the identification of the pigments in samples from the ceiling frescoes of Antonio Palomino (dated 1707) in the vaulted nave of the Sant Joan del Mercat church in Valencia (Spain). These frescoes suffered considerable damage by fire during the Spanish Civil War in 1936, resulting in severe chemical and chromatic alterations. Electrochemical data, supported by scanning electron microscopy/energy dispersive X‐ray microanalysis and Raman spectroscopy, indicate that thermal stress induced the transformation of goethite‐based yellow ochres into hematite and magnetite. 相似文献
989.
该文简述了电子显微技术的发展历程,并介绍了现代电子显微镜的新功能。针对生物纳米材料理化性能与功能应用的特殊性,结合研究实例,重点阐述运用电子显微结构表征与原位分析测试技术指导构建新颖纳米结构、揭示材料与细胞/组织相互作用并发挥功能的机制。并在此基础上,展望了电子显微技术在生物纳米材料研究领域的发展方向(大尺寸图像拼接、三维重构、动态原位实时成像)。 相似文献
990.
Chan Sik Cho Tae Kyung Kim Tae‐Jeong Kim Sang Chul Shim Nam Sik Yoon 《Journal of heterocyclic chemistry》2002,39(2):291-294
Nitroarenes are reductively cyclized with 3‐amino‐1‐propanols in dioxane/H2O in the presence of a ruthenium catalyst and tin(II) chloride dihydrate together with isopropanol to afford the corresponding quinolines. A reaction pathway involving initial reduction of nitroarenes to anilines, propanol group transfer from 3‐amino‐1‐propanols to anilines, N‐alkylation of anilines by 3‐anilino‐1‐propanols and heteroannulation of 1,3‐dianilinopropanes is proposed. 相似文献