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   323161篇
  免费   3899篇
  国内免费   1175篇
化学   176364篇
晶体学   4651篇
力学   13239篇
综合类   8篇
数学   38044篇
物理学   95929篇
  2020年   2062篇
  2019年   2036篇
  2018年   2035篇
  2017年   1957篇
  2016年   4026篇
  2015年   3370篇
  2014年   4569篇
  2013年   14408篇
  2012年   11125篇
  2011年   13791篇
  2010年   8510篇
  2009年   8458篇
  2008年   12773篇
  2007年   13023篇
  2006年   12638篇
  2005年   11560篇
  2004年   10378篇
  2003年   9129篇
  2002年   9011篇
  2001年   10208篇
  2000年   7809篇
  1999年   6207篇
  1998年   4982篇
  1997年   4846篇
  1996年   4927篇
  1995年   4455篇
  1994年   4208篇
  1993年   4046篇
  1992年   4519篇
  1991年   4376篇
  1990年   4113篇
  1989年   3919篇
  1988年   4207篇
  1987年   3857篇
  1986年   3755篇
  1985年   5462篇
  1984年   5546篇
  1983年   4490篇
  1982年   4936篇
  1981年   4956篇
  1980年   4684篇
  1979年   4789篇
  1978年   4787篇
  1977年   4758篇
  1976年   4723篇
  1975年   4617篇
  1974年   4474篇
  1973年   4631篇
  1972年   2636篇
  1971年   1932篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Oxidative addition of 2 molecules of 3,3-dimethylcyclopropene (I) to α,α′-bipyridyl(1,5-cyclooctadiene)nickel(0) (III) gave the title compound (IV) in over 90% yield. Complex IV was characterized by mass, 1H NMR and 13C NMR spectroscopy. Its structure was determined by X-ray diffraction (a 13.7081(2), b 14.638(2), c 9.5139(1) Å, β 110.82(1)°, C2/c, R = 0.05 for 1614 reflections).  相似文献   
902.
The 34S(p, n)34mCl reaction induced by 13-MeV protons is used for the determination of sulphur in copper, nickel and aluminium alloys. The 34mCl is separated by repeated precipitation as silver chloride. The results obtained were 3.08 ± 0.47, 1.47 ± 0.17 and <1μg g-1 for copper, nickel and aluminium alloys, respectively.  相似文献   
903.
The goals and limitations of some clustering methods are briefly reviewed. In order to avoid the usual methods which are very demanding on computer time and space, a new scheme is proposed for updating (generation) of, and retrieval from, large data bases organized as binary trees. The method is based on calculation of three distances at any given vertex l on the level n, A(l,n) instead of two. The respective distances d1(X,Al), d2(X,Ar), d3(AlAr) are calculated between the input vector X and the left and right descendant of the vertex A(l,n), Al and Ar, respectively. The advantage of the method compared to the standard clustering methods or formation of hierarchal trees is that the required memory or computational time is reduced from N2 to approximately N log N (N is the number of clustering objects).  相似文献   
904.
An efficient method for the preparation of crystalline α-hydroxy previtamin D3 is described. Also the stereoselective reduction of 1-keto previtamin 9 is discussed; it was observed that with aluminum hydride 7 is the most abundant product. This stands in contrast with previously reported LiAlH4 and NaBH4 mediated reductions.  相似文献   
905.
A simple method for the separation of hemoglobin chains from hemolysate or globin, by sodium dodecyl sulfate polyacrylamide gel electrophoresis, is described. The alpha, beta, and gamma chains can be clearly separated from each other. The alpha chain has the highest mobility, the beta chain has a slower mobility than the gamma chain, while the delta chain has about the same mobility as the beta chain. Hemoglobins with elongated chains can easily be detected by this method. Tak-beta, elongated by 11 residues, moves much more slowly than betaA but is much faster than alpha Constant Spring which is elongated by 31 residues. Screening of several individuals with slow-moving hemoglobins using this method led to the finding of a case with Hb Tak-beta thalassemia and other carriers of Hb Tak.  相似文献   
906.
Polarographic and cyclovoltammetric investigation of the thiooxalates 16 shows that they are reversibly reduced to the corresponding radical anions. Accordingly, EPR spectra are obtained after in-situ electroreduction. Taking also the electronic spectra into consideration, conclusions about the spin density distribution can be drawn. It is shown thatpπ is higher at the oxygen atoms of oxoesters than at the sulfur atoms of the corresponding thiono esters.  相似文献   
907.
I.R. Spectra of the Phases A B Te3O8 (A = Fe, In, Sc; B = Nb, Ta) and UTe3O9 The infrared spectra of the title compounds have been recorded and discussed in comparison to formerly investigated MIVTe3O8 phases. The spectrum of UTe3O9 (synthetic cliffordite) confirms the presence of uranyl groups and the structural relation of this compound to the fluorite.  相似文献   
908.
The α,β-unsaturated γ,δ-epoxyketone 7 is isomerized almost exclusively to the δ-diketone 9 both upon irradiation in the n → π* absorption band with light of wavelengths above 310 nm (in anhydrous dioxane or benzene solutions) and upon triplet sensitization using acetophenone in benzene. The reaction may be formulated by the cleavage of the Cγ? O oxide bond and the shift of the δ-hydrogen to the γ-position, and thus bears a formal “double bond homology” to the photochemical α,β-epoxyketone rearrangement. Excitation in the π → π* absorption band of 7 with light of wavelength 253,7 nm (in anhydrous dioxane solution) leads to the formation of product 10 as well as to the triplet rearrangement to 9 . With this result a novel partial synthesis of O-acetyl-B-nortestosterone has been accomplished, which has the advantages of fewer steps and higher product yield ( 7 → 10 : ~30% yield) than previously published syntheses. On the basis of the presently available experiments, the mechanism of the transformation 7 → 10 , which constitutes one of the still few examples of enone photoreactions induced selectively from the π,π* excited singlet, remains unknown.  相似文献   
909.
Summary A simple isolation and cleanup procedure for the determination of organophosphorus insecticides and their oxones in animal tissues has been developed. This procedure unites partly extraction of organophosphorus pesticides and their oxones with cleanup by column chromatography. Most polar coextracts remain on the column and in its further processing the eluate does not tend to form emulsion. The process has been studied on model mixtures of 32P- and 14C-labelled pesticides with tissues as well as after application of pesticides to white rats. The average recovery of the method is 90.5%, with a standard deviation of ±5.1%.
Isolierung und Reinigung von Organophosphorinsecticiden und ihren Oxonen aus tierischen Gewehen
Zusammenfassung Ein einfaches Isolations- und Reinigungsverfahren wurde ausgearbeitet. Die Extraktion der Insecticide aus einer Homogenatsäule, die aus Kieselgel und Tiergewebe besteht, umfaßt zum Teil auch die Reinigung der Extrakte, da die polaren Verbindungen an die Säule gebunden bleiben. Der Mittelwert der Ausbeute des Verfahrens wurde im Modellversuch mit 14C- und 32P-markierten Insecticiden zu 90,5% festgestellt. Die Standardabweichung beträgt ± 5,1%.
  相似文献   
910.
A thermodyrmmic study of the complex formation of trivaient lanthanides with hydroxyethylethyienediaminetriacetic acid and other aminoacetic acids. Part III. The determination of the formation constants of mixed complexes by potentiometric titration.The partial stability constants of the mixed and 1:2 complexes Ln—HEDTA—L (where Ln = La, Ce,... and L = glycine, IMDA, NTA, HEDTA and EDTA) have been determined by potentiometric titration at 25°C and at a constant ionic strength of 1 (KCl). The changes in stability of the complexes studied (1:1, 1:2, mixed, protonated and unprotonated) vs. atomic number of the lanthanide are discussed. The changes observed in the trends of the partial and overall stability constants across the lanthanide series are attributed to the decrease in the number of water molecules in the 1:1 LnHEDTA ·xH2O from x = 3 for light lanthanides to x = 2 for heavy ones. However, in this 1:1 complex, HEDTA seems to be a hexadeutate ligand in the La—Sm range of the series and a pentadentate ligand in the Gd—Lu range. Significant differences have been found between the complexes containing four nitrogen atoms, i.e. L = HEDTA, EDTA, and those with three nitrogen atoms i.e. L = glycine, IMDA, NTA.  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