首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   61697篇
  免费   4062篇
  国内免费   4005篇
化学   36822篇
晶体学   947篇
力学   4187篇
综合类   779篇
数学   9572篇
物理学   17457篇
  2024年   76篇
  2023年   536篇
  2022年   1369篇
  2021年   1303篇
  2020年   1370篇
  2019年   1371篇
  2018年   1248篇
  2017年   1533篇
  2016年   2073篇
  2015年   1749篇
  2014年   2395篇
  2013年   4492篇
  2012年   3472篇
  2011年   3359篇
  2010年   2951篇
  2009年   3672篇
  2008年   3615篇
  2007年   4010篇
  2006年   3449篇
  2005年   3002篇
  2004年   2951篇
  2003年   2413篇
  2002年   1904篇
  2001年   1573篇
  2000年   2293篇
  1999年   1874篇
  1998年   1749篇
  1997年   1074篇
  1996年   999篇
  1995年   924篇
  1994年   839篇
  1993年   692篇
  1992年   607篇
  1991年   492篇
  1990年   422篇
  1989年   303篇
  1988年   284篇
  1987年   182篇
  1986年   152篇
  1985年   184篇
  1984年   142篇
  1983年   77篇
  1982年   129篇
  1981年   96篇
  1980年   63篇
  1979年   91篇
  1978年   53篇
  1977年   33篇
  1976年   33篇
  1973年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The surface chemical composition of raw unscoured cotton was successfully investigated by the surface analytical techniques X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The presence of non-cellulosic material at the fibre surface was established and determined to be a complex mixture of fatty acids, alcohols, alkanes, esters and glycerides. The effect of scouring and bleaching was to reduce the surface concentration of these materials but even after aqueous processing some non-cellulosic material residue was still detected at the fibre surface.  相似文献   
82.
The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10?8 and 1.3 × 10?5 Pa and at temperatures ranging from 273 to 550 K has been investigated in situ with X‐ray photoelectron spectroscopy (XPS), ellipsometry and high resolution elastic recoil detection analysis (HERDA). Quantitative analysis of the XPS spectra showed a clear oxygen deficiency with respect to MgO for the initial oxide. HERDA measurements confirmed this relatively low oxygen content in the thin oxide layers formed. Ellipsometry measurements showed that the electronic structure of the initially formed oxide differs significantly from that of bulk MgO. The band gap values at room temperature for the oxide layers investigated are clearly smaller than the value for bulk MgO. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
83.
The molecular and crystal structures of the two racemic diastereomeric 3,4-dihydropyrromethenone derivatives1 and2 of configurations (Z) and (E) were determined at 93 K (1, 2) and at room temperature (1). From an analysis of the crystallographically observed temperature coefficients a pseudorotation flexibility of the pyrrolidinone ring in the crystal is deduced. In both compounds a nearly orthogonal arrangement between the two heterocyclic ring systems is observed, which is ascribed to the steric bulk of the substituents in positions 1 and 3 of the pyrrolidinone ring.Herrn Prof.Josef Schurz zum 60. Geburtstag gewidmet.  相似文献   
84.
Environmental analyses show that the air which we breathe, and which is so essential to life, is in general a mixture of gaseous, liquid, and solid components. The solid airborne particles, whose concentration, homogeneity, chemical composition, size, and shape can vary over wide ranges, and whose origin may be “natural” or “artificial” are referred to as “dust”. Dust particles can act, inter alia, as condensation nuclei, catalysts, and directly as hazardous materials. Unfortunately, we still know far too little about dust. Dust analysis is extremely difficult and challenging, even for modern analytical chemistry; it is still far from being fully automated. The simultaneous determination of as many “dust parameters” as possible, and particularly the synoptic consideration of all available data against a background of physicochemical and technological knowledge on the development, transformation, and effects of dust, are summarized as “integrated dust analysis”.  相似文献   
85.
根据Fe(Ⅲ)、Mo(Ⅵ)对H2O2氧化邻氨基酚的催化反应速率不同,结合流动注射停留技术,建立了同时测定Fe(Ⅲ)、Mo(Ⅵ)的速差动力学分光光度法.测定铁和钼的线性范围分别为0.1~2.5μg/mL和0.5~15μg/mL.除W(Ⅵ)的允许量较小外,其它离子不干扰测定.用本法测定了合成样液中铁和钼含量,结果满意.  相似文献   
86.
Sharpless asymmetric dihydroxylation and aminohydroxylation of (E)-dimethyl-2-alkylidene glutarates 2 were shown to afford enantio-enriched or enantiopure highly functionalized γ-butyrolactones 3 and 7.  相似文献   
87.
The molecular structure of samarium dibromide has been studied by electron diffraction at T exp = 1250(50) K. The molecule has C2v symmetry; the internuclear distance Rg(Sm–Br) = 274.5(5) pm; g =131(6)°. The vibration frequencies were estimated from the experimental values of the mean square vibration amplitudes.  相似文献   
88.
The methylation and allylation of 6-(3-indolyl)indolo[2,3-b]carbazole were studied, and its trimethyl and mono-, di-, and triallyl derivatives were obtained.  相似文献   
89.
The bromination of 3-allyl-4-hydroxy-2-oxo-1,2-dihydroquinoline by molecular bromine is accompanied by the closure of a five membered furan ring and gives the corresponding 2-bromomethyl-3,9-dihydro-2H-furo[2,3-b]quinolin-4-one. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1677–1686, November, 2007.  相似文献   
90.
O-Methyloximes and nitrones of a series of substituted 4-[2-hydroxy-2-(1H-1,2,4-triazol-1-ylmethyl)cyclohexylidenemethyl]benzaldehydes have been synthesized, and their fungicidal activity has been assayed. All of them exhibit low fungicidal activity in thein vitro assays and high activity in thein vivo assays; theO-methyl oximes are more active than the corresponding nitrones.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2082–2087, December, 1993.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号