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   104334篇
  免费   3847篇
  国内免费   2959篇
化学   42953篇
晶体学   1115篇
力学   7997篇
综合类   181篇
数学   34707篇
物理学   24187篇
  2024年   80篇
  2023年   471篇
  2022年   844篇
  2021年   857篇
  2020年   885篇
  2019年   966篇
  2018年   11033篇
  2017年   10865篇
  2016年   7060篇
  2015年   1949篇
  2014年   1673篇
  2013年   2227篇
  2012年   6017篇
  2011年   12839篇
  2010年   7182篇
  2009年   7605篇
  2008年   8243篇
  2007年   10324篇
  2006年   1622篇
  2005年   2504篇
  2004年   2596篇
  2003年   2827篇
  2002年   1805篇
  2001年   911篇
  2000年   877篇
  1999年   603篇
  1998年   560篇
  1997年   449篇
  1996年   554篇
  1995年   392篇
  1994年   375篇
  1993年   332篇
  1992年   281篇
  1991年   250篇
  1990年   240篇
  1989年   216篇
  1988年   192篇
  1987年   158篇
  1986年   168篇
  1985年   181篇
  1984年   140篇
  1983年   120篇
  1982年   96篇
  1981年   94篇
  1980年   98篇
  1979年   89篇
  1978年   70篇
  1976年   61篇
  1974年   61篇
  1973年   73篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The complexing process proceeding in the NiII–thiocarbohydrazide (H2N–H–NC(=S)–NH–NH2)–propanone triple system in EtOH solution and nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix has been studied. It has been found that in the first case, template synthesis leading, as a minimum, to formation of three coordination compounds of NiII with (N,N,S,S)-donor tetradentate ligands having NiL1, NiL2 and NiL3compositions where L1 is 4,6,6-trimethyl-2,3,7,8-tetraazanonen-3-di(thiohydrazide)-1,9, L2 is 4,6,6,12-tetrametyl-1,9-dithio-2,3,7,8,10,11-hexaazatridekadien-3,11-hydrazide-1 and L3 is 2,8,10,10,16-pentamethyl-5,13-dithio-3,4,6,7,11,12,14,15-octaazaheptadekatrien-2,7,15 is observed, whereas in the gelatin-immobilized matrix, a complexing process in the system considered does not occur.  相似文献   
32.
Summary This paper discusses the application of X-ray photoelectron spectroscopy to the determination of the chemical composition of sulfur and nitrogen species in air pollution particulates. Core electron chemical shift measurements are augmented by the determination of relative concentrations and volatility of particulate species.
Untersuchung von Verunreinigungsteilchen in der Luft durch Röntgenphotoelektronen-Spektroskopie
Zusammenfassung Die Anwendung der Röntgenphotoelektronen-Spektroskopie auf die Bestimmung der chemischen Zusammensetzung von Schwefel und Stickstoff enthaltenden Verunreinigungsteilchen in Luft wird diskutiert. Messungen der chemischen Verschiebung werden ergänzt durch die Bestimmung der relativen Konzentrationen und der Flüchtigkeit der Verbindungen.
Presented at the 6th Annual Symposium on Recent Advances in the Analytical Chemistry of Pollutants, April 21–23, 1976; Vienna, Austria.  相似文献   
33.
Summary Thermal decomposition of metal-organic complexes of nickel, cobalt and iron has given catalysts which are very effective at about 900° for the conversion of carbon dioxide, water and other oxygencontaining sample decomposition products to carbon monoxide in the direct determination of oxygen in organic compounds when using a modified Unterzaucher type apparatus. A copper catalyst similarly prepared required a temperature of 1030° whereas a manganese complex decomposition product was ineffective.
Katalysatoren zur Sauerstoffbestimmung in organischen Substanzen
Zusammenfassung Durch thermische Zersetzung metallorganischer Komplexe von Ni, Co und Fe erhält man Katalysatoren, die die Umwandlung von CO2, H2O und anderen sauerstoffhältigen Zerfallsprodukten zu CO bei etwa 900° C bei der direkten Sauerstoffbestimmung in einer modifizierten Unterzaucher-Apparatur sehr wirksam fördern. Ein ähnlich hergestellter Cu-Katalysator erfordert 1030° C und das Zersetzungsprodukt eines Mn-Komplexes ist unwirksam.
  相似文献   
34.
A detailed study of the Raman and infrared spectral line shapes and line parameters of aqueous solutions (both H2O and D2O) of AgNO3 is interpreted in terms of an equilibrium between “free” ions and the ion pair Ag+NO 3 ? . An association constant of 0.1M ?1 was obtained from both the 717 cm?1 and 1047 cm?1 line intensities. Spectral features suggest a significant degree of covalence in the interaction. A CsV) model with Ag+ above the plane of NO 3 ? is consistent with the data.  相似文献   
35.
Adsorption, desorption and activity of acid phosphatase on various soil colloidal particles and pure clay minerals were studied. Higher adsorption amounts and low percentage of desorption of acid phosphatase were found on fine soil clays (<0.2 μm). Electrostatic force and ligand exchange are the major driving forces that are involved in the adsorption of enzymes on soil clays. More enzyme molecules were adsorbed on soil clays in the presence of organic components. However, enzymes on organic clays were more easily released. One-third of the enzyme on goethite was adsorbed via ligand exchange process. Some other interactions, such as van der Waals force, hydrophobic force and hydrogen bonding may be more important in the adsorption of enzyme on kaolinite and the enzyme in this system cannot be easily removed. Coarse clays (0.2–2 μm) and inorganic soil clays had higher affinities for enzyme molecules than fine clays and organic clays, respectively. The activity of enzyme bound on soil clays was inhibited and the thermal stability was increased in the presence of organic matter. Data obtained in this study are helpful for a better understanding of the interactions of enzymes with inorganic and organic constituents in soil and associated environments.  相似文献   
36.
Shen YH  Li SH  Li RT  Han QB  Zhao QS  Liang L  Sun HD  Lu Y  Cao P  Zheng QT 《Organic letters》2004,6(10):1593-1595
Both coriatone (1). a novel highly oxygenated picrotoxane-type sesquiterpene, and corianlactone (2). with an unprecedented sesquiterpene basic skeleton, named coriane, were isolated from Coriaria nepalensis Wall. The structures of 1 and 2 were determined by analysis of their two-dimensional NMR data, and the structure of 2 was confirmed by X-ray analysis. Compounds 1 and 2 showed no remarkable inhibitory activity toward K(562) cells. They are cytotoxic with IC(50) > 50 microg/mL (cis-platinim: IC(50) = 0.49 microg/mL).  相似文献   
37.
The fragmentation patterns of 3-methyl-2-butyl trifluoroacetate and 2-methyl-2-butyl trifluoroacetate were investigated by GC/MS/MS, with electron impact and collision-induced dissociation, on regular and isotope-labeled (deuterium and 18O) esters. The atoms found in various fragments could be traced back to specific positions in the parent molecules. In this way, molecular rearrangements potentially occurring during the formation of esters by trifluoroacetolysis of 3-methyl-2-butyl p-toluenesulfonate or trifluoroacetic acid addition to various 2-methylbutenes could be shown. Rearrangements also occurred during the fragmentation, particularly during the expulsion of the small fragments CO, C2H4 and F2CO. For the decompositions of oxygen-containing ions these rearrangements were highly specific. By contrast, alkyl cations lead to fragments that are fully scrambled (statistical label distribution). Alkene radical cations ([CnH2n]+ ˙) fragment to daughter ions that are extensively, but less than statistically scrambled. Hydrogen scrambling may also occur in fluoroalkyl cation fragments.  相似文献   
38.
The acid dissociation constants of dihalogenated 8-hydroxyquinaldines and the stability constants of their chelates with Zn[II], Ni[II], Co[II], Pb[II], Cd[II], Mn[II], and Mg[II] have been determined. The Calvin-Bjerrum potentiometric titration technique was used in studying the behavior of these substances. The order of decreasing stability of the metal chelates Zn, Nc>Co>Pb>Cd>Mn>Mg is similar to the stability sequence generally obtained for divalent metals, and the order of decreasing stability of dihalogenated 8-hydroxyquinaldines to form the metal chelates with metal ions is I>Br>Cl. Spot test reactions of dihalogenated 8-hydroxyquinaldines and 8-hydroxyquinolines with the metal ions also have been investigated.  相似文献   
39.
The different aspects of Vitamin D3 determination and the data reported in the literature are discussed. The spectrophotometric determination of Vitamin D3 in cod-liver oil is carried out after the alkaline saponification, extraction of unsaponified parts, precipitation of accompanying sterols and the column- and thin-layer chromatographic purification and separation of vitamin D3 on 40 cm plates from other vitamins. The dyestuff α-naphtholbenzein is suited well as standard substance for the better location and identification of vitamin D3 zone on the thin-layer plate. The results obtained from the chemical method were checked through the simultaneous biological determination.  相似文献   
40.
The mixture of silver iodide and ferrocene is used to prepare the membrane of iodide ion-selective electrode. The interference of sulfide can be reduced by the addition of cupric ion in the test solution. Ethylene glycol is determined by potentiometric titration using iodine ion-selective electrode as an indicator electrode.  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