全文获取类型
收费全文 | 40983篇 |
免费 | 5635篇 |
国内免费 | 4971篇 |
专业分类
化学 | 29792篇 |
晶体学 | 627篇 |
力学 | 2005篇 |
综合类 | 396篇 |
数学 | 4508篇 |
物理学 | 14261篇 |
出版年
2024年 | 117篇 |
2023年 | 664篇 |
2022年 | 1288篇 |
2021年 | 1337篇 |
2020年 | 1500篇 |
2019年 | 1492篇 |
2018年 | 1246篇 |
2017年 | 1242篇 |
2016年 | 1777篇 |
2015年 | 1836篇 |
2014年 | 2255篇 |
2013年 | 2877篇 |
2012年 | 3593篇 |
2011年 | 3664篇 |
2010年 | 2636篇 |
2009年 | 2603篇 |
2008年 | 2980篇 |
2007年 | 2595篇 |
2006年 | 2347篇 |
2005年 | 1932篇 |
2004年 | 1594篇 |
2003年 | 1332篇 |
2002年 | 1334篇 |
2001年 | 1035篇 |
2000年 | 837篇 |
1999年 | 709篇 |
1998年 | 541篇 |
1997年 | 434篇 |
1996年 | 455篇 |
1995年 | 376篇 |
1994年 | 375篇 |
1993年 | 294篇 |
1992年 | 287篇 |
1991年 | 263篇 |
1990年 | 235篇 |
1989年 | 186篇 |
1988年 | 150篇 |
1987年 | 117篇 |
1986年 | 115篇 |
1985年 | 150篇 |
1984年 | 105篇 |
1983年 | 93篇 |
1982年 | 67篇 |
1981年 | 61篇 |
1980年 | 54篇 |
1979年 | 40篇 |
1977年 | 43篇 |
1976年 | 43篇 |
1974年 | 52篇 |
1973年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
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
102.
Two molecular imprinting polymer (MIP) monolithic columns with (S)-(-)-1,1'-bi-2-naphthol and (R)-(+)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol as the templating molecules, respectively, have been prepared by in situ polymerization using 4-vinylpyridine and ethylene dimethacrylate as functional monomer and cross-linker, respectively. The columns with good flow-through properties were obtained by changing the molar ratio of the functional monomer and the template molecule. The effects of mobile-phase composition on separation of enantiomers were systematically investigated. The results indicate that hydrophobic interaction in aqueous solution and hydrogen-bonding interaction in ACN between the enantiomers and polymers could play important roles in the retention and resolution. The effects of chromatographic conditions, such as flow rate, column temperature, sample loading, on the enantioseparation were also studied. Further, these two MIP columns show a cross-reactivity. 相似文献
103.
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
104.
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 Cs(σV) model with Ag+ above the plane of NO 3 ? is consistent with the data. 相似文献
105.
A novel route for waste water treatment: photo-assisted Fenton degradation of dye pollutants accumulated in natural polyelectrolyte microshells 总被引:2,自引:0,他引:2
The efficient accumulation of dyes in constructed natural polyelectrolyte microshells under moderate conditions, combined with the photo-assisted Fenton reagent, opens a new route for the effective elimination of dye pollutants from waste water. 相似文献
106.
The hydroboration of cis-2-butene and cis-3-hexene with pure diisopinocampheylborane (IPC2BH) and monoisopinocampheylborane (IPCBH2), both prepared from (+)-α-pinene, were studied. In contrast to IPC2BH, which yielded R-(?)-2-butanol and R-(?)-3-hexanol in 98.4 and 94.5% optical purities, IPCBH2 yielded S-(+)-2-butanol and S-(+)-3-hexanol in 23.6 and 19.7% optical purifies respectively. PMR examination of the methanolyzed products were utilized to establish the species present in the freshly prepared and aged adduct from (+)-α-pinene and BH3 · THF. The results confirm the interpretation previously advanced for the major differences realized in asymmetric hydroboration with the fresh and aged adducts. 相似文献
107.
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
108.
Tiao-Hsu Chang 《中国化学会会志》1966,13(4):143-149
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
109.
The cyclopeptide, squamtin A (1, formula: C39H60O11N8S), was found to crystallized in two pseudopolymorphisms, i.e. 1·(H2O)3.5 and 1·(H2O)3.9. The composition of the amino acids and their linkage sequences are the same. The main differences between the two kinds of crystals lie in the positions and occupancies of the water molecules, the positions of the sulfur atoms and the conformation of the side chains. The absolute configuration of 1 is established by X-ray analysis in combination with the Marfey's analysis of its hydrolysates. 相似文献
110.
Seong J Rohrbacher A Li ZR Janda KC Tao FM Spiegelman F Halberstadt N 《The Journal of chemical physics》2004,120(16):7456-7463
The potential energy surface of He2Ne+ has been reinvestigated using a combination of ab initio and diatomics-in-molecule (DIM) calculations. In contrast to the reports of two recent studies the ion is found to have an asymmetric linear He-Ne-He structure, with no barrier to formation from the separated atoms on the ground-state surface. The He-Ne+ bond lengths at the potential minimum are 1.51 and 1.81 A, and the total bonding energy is 0.717 eV. Comparing the He2Ne+ energy to that of HeNe+, the bonding energy for the second helium atom is 0.06 eV, about 10% of that of the first He atom. The saddle point between the two equivalent minima is a symmetric structure, 0.0074 eV above the potential minimum. A symmetric geometry becomes the overall potential minimum if the 2s hole on the Ne is excluded from the reference states of a multireference configuration interaction calculation. A DIM potential was created for the HenNe+ family of ions. The DIM potential is consistent with the asymmetric He2Ne+ ion serving as a core; it predicts a slightly more asymmetric geometry than the ab initio results. Additional helium atoms form five-membered rings around the bonds of the core ion to fill the first shell and then add to the ends of the cluster. The asymmetric core ion and the highly compact structure help to account for the lack of apparent shell structure in the mass spectrometry of HenNe+ clusters. Finally, we recommend that the value De=0.63+/-0.04 eV be adopted for the ground state of HeNe+. 相似文献