全文获取类型
收费全文 | 3628篇 |
免费 | 143篇 |
国内免费 | 34篇 |
专业分类
化学 | 2251篇 |
晶体学 | 17篇 |
力学 | 90篇 |
数学 | 587篇 |
物理学 | 860篇 |
出版年
2023年 | 17篇 |
2022年 | 44篇 |
2021年 | 50篇 |
2020年 | 61篇 |
2019年 | 56篇 |
2018年 | 49篇 |
2017年 | 56篇 |
2016年 | 92篇 |
2015年 | 84篇 |
2014年 | 94篇 |
2013年 | 175篇 |
2012年 | 235篇 |
2011年 | 241篇 |
2010年 | 155篇 |
2009年 | 137篇 |
2008年 | 200篇 |
2007年 | 220篇 |
2006年 | 224篇 |
2005年 | 269篇 |
2004年 | 195篇 |
2003年 | 159篇 |
2002年 | 142篇 |
2001年 | 36篇 |
2000年 | 47篇 |
1999年 | 32篇 |
1998年 | 38篇 |
1997年 | 28篇 |
1996年 | 54篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 42篇 |
1992年 | 27篇 |
1991年 | 17篇 |
1990年 | 28篇 |
1989年 | 24篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 13篇 |
1985年 | 30篇 |
1984年 | 22篇 |
1983年 | 25篇 |
1982年 | 22篇 |
1981年 | 26篇 |
1980年 | 25篇 |
1979年 | 17篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1975年 | 18篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有3805条查询结果,搜索用时 250 毫秒
71.
A coal tar pitch and a petroleum vacuum residue have been separated by TLC using pyridine, acetonitrile, toluene and pentane to develop the chromatograms. The bands of material detected were recovered in 1-methyl-2-pyrrolidinone (NMP) solvent and examined by size-exclusion chromatography (SEC) in NMP eluent. The relation between elution time in SEC and mobility on the TLC plate indicated that molecular size increased steadily with increasing immobility on the plate. This relation was reinforced by UV fluorescence spectroscopy in that the fluorescence moved to longer wavelengths with increasing immobility. The molecular size of the material excluded from the porosity of the SEC column remains undefined; some excluded material was found in all of the fractions from both samples. The valley of zero intensity separating the retained material from the excluded material may suggest a change of structure from near-planar in the retained region to three-dimensional in the excluded region. 相似文献
72.
A new technique is described for estimating the pure component spectra from a set of linearly independent spectra. The process is one of generalised spectral subtraction in which an iterative combination of multivariate linear least-squares analysis and matrix transformation is applied to the input data to give estimates of the number of independent components in the original mixed spectra. This technique is applicable to bipolar data (e.g. from e.p.r. spectra) as well as absorption spectra determined by any spectroscopic technique, provided that the spectra may be reasonably assumed to be an additive mixture of unknown pure components. Numerical model examples are given together with an experimental application to electron spin resonance. 相似文献
73.
Y. Li N. Obando F. Tschen R. J. Morgan 《Journal of Thermal Analysis and Calorimetry》2006,85(1):125-129
Thermal analysis of phenylethynyl end-capped imide oligomer AFR-PEPA-4
was performed to characterize cure reaction, thermal stabilities and semicrystalline
behavior of AFR-PEPA-4 oligomer and its cured polyimide. Cured AFR-PEPA-4
polyimide showed high T
gs
up to 418°C. Both AFR-PEPA-4 oligomer and polyimide exhibit excellent
thermal stabilities comparable to PETI-5 polyimides. AFR-PEPA-4 imide oligomer
has a T
m of 330°C
and exhibits spherulite crystalline morphology in the film. The crystallinity
in AFR-PEPA-4 films could not be regenerated under any annealing conditions
after the initial melt. 相似文献
74.
De Jonghe S Lamote I Venkataraman K Boldin SA Hillaert U Rozenski J Hendrix C Busson R De Keukeleire D Van Calenbergh S Futerman AH Herdewijn P 《The Journal of organic chemistry》2002,67(3):988-996
The synthesis of a new series of D-erythro-homoceramide analogues is described. Several synthetic approaches were investigated. Homoceramides can be successfully synthesized from L-homoserine as chiral building block and a protected Weinreb-amide as a key intermediate. The synthesis of short-chain analogues with a heptyl side chain, as well as with a phenyl residue in the sphingoid part (instead of the naturally occurring tridecyl side chain), was effected. The homoceramides 15-17 and 24 were investigated for their potential to reverse the inhibitory effect of fumonisin B(1) on axonal growth. Unfortunately, none of the tested compounds showed any biological activity due to their lack of metabolism to glucosylhomoceramide. 相似文献
75.
76.
Monir Uddin Antonis G. Hatzidimitriou Maria Lalia-Kantouri Chris Tsiamis 《Structural chemistry》1997,8(2):131-139
The infrared and electronic excitation spectra of a series of new nickel(II) chelates containing an-diimine (or nitrogenous base, enR) and the anion of a-dione (1,3-ketoenol or 1,3-ketoester,H) were obtained in the solid state and in solution. The composition and the overall structure of the new chelates in the solid state depends on ligand concentration, the substituents within the-dionato moiety and the counterion present. The IR and electronic excitation spectra of [Ni(enR)2
]NO3 and [Ni(enR)(O2NO)] indicate, in conjunction with other physicochemical measurements, bidentate coordination of the ligands, while the spectral shifts reveal replacement of the (O,O)nitrato group by basic solvents. The existence of the nickel(II) in an octahedral environment is confirmed by an X-ray structure analysis of [Ni(dpamH)2acac]NO3 · CH3OH (monoclinic, space groupP2l/n, a=17.296(1),b=7.462(1),c=21.604(3) å,=95.65(1),Z=4, R=0.0534,Rw=0.0600), where dpamH denotes the 2,2-dipyridylamine. 相似文献
77.
In this paper, we report the first observation of metal-enhanced S(2) emission at room and low temperature (77K). The S(2) emission intensity of Azulene is enhanced by close proximity to Silver island films (SiFs). In this regard, a ≈ 2-fold higher S(2) fluorescence intensity of Azulene was observed from SiFs as compared to a glass control sample. This suggests that S(2) excited states can couple to surface plasmons and enhance S(2) fluorescence yields, a helpful observation in our understanding the interactions between plasmons and lumophores, and our continued efforts to develop a unified plasmon-lumophore/fluorophore theory. 相似文献
78.
Joachim G. Liehr Richard M. Caprioli John H. Beynon Roger P. Morgan Wilhelm J. Richter 《Journal of mass spectrometry : JMS》1980,15(3):157-160
The oxygen rearrangement in molecular ions of 3-phenylpropionates has been investigated with the aid of mass analyzed ion kinetic energy spectra. Elimination of an allyl radical followed by expulsion of ketene from the molecular ion of allyl 3-phenylpropionate is shown to result in formation of protonated benzaldehyde. The oxygen rearrangement has been found to be inoperative in ionized methyl 3-methyl-3-phenylbutyrate. [M ? CH3 ? CH2CO]+ ions in the spectrum of the latter compound are formed by elimination of the 3-methyl substituent and subsequent methoxy migration. 相似文献
79.
Esna Killian Kevin Blann Annette Bollmann John T. Dixon Sven Kuhlmann Munaka C. Maumela Hulisani Maumela David H. Morgan Palesa Nongodlwana Matthew J. Overett Mari Pretorius Karola Hfener Peter Wasserscheid 《Journal of molecular catalysis. A, Chemical》2007,270(1-2):214-218
A systematic study was conducted on the Cr catalysed tri- and tetramerisation of ethylene using bis(diphenylphosphino)amine ligands with N-aryl functionality. This study revealed that the oligomerisation reaction product selectivity is primarily dependent on the structure and size of the N-aryl groups.
Addition of sufficient steric bulk to the N-phenyl group via ortho-alkyl substitution increased the combined 1-hexene and 1-octene selectivity (overall alpha selectivity) to above 82% at an overall 1-octene selectivity of 56%. The introduction of a single carbon spacer between the N-atom and the aryl-moiety, as well as the addition of branching on this carbon, resulted in further selectivity improvements, achieving an overall 1-octene selectivity of 64% and an overall alpha selectivity of 84%. This was obtained at catalyst productivities in excess of 1,000,000 g/g Cr/h. 相似文献
80.
FitzGerald PA Carr MW Davey TW Serelis AK Such CH Warr GG 《Journal of colloid and interface science》2004,275(2):649-658
Dimeric poly(ethylene oxide) surfactants (or nonionic gemini surfactants) with the structure (Cn-2H2n-3CHCH2O(CH2CH2O)mH)2(CH2)6 (or GemnEm), where n is the alkyl length and m is the average number of ethylene oxides per head group, were synthesized. Surfactants were synthesized with alkyl chain lengths n = 12, 14, and 20 and m = 5, 10, 15, 20, and 30. Water solubilities and cloud temperatures at 1 wt% were determined by measuring turbidity as a function of temperature. Cloud temperatures increase with m and decrease with n, as observed for conventional surfactants. For large m the cloud temperatures were all above 100 degrees C. Surfactants with small m (i.e., n = 12, 14, m = 5 and n = 20, m = 10) were insoluble at room temperature, forming two-phase mixtures. Critical micelle concentrations (CMCs) were measured using a pyrene fluorescence method and are all in the range of 10(-7) to 10(-6) M, with the lowest values from the surfactants with large n and small m. CMCs of mixtures with both anionic and nonionic conventional (monomeric) surfactants were well described by an ideal mixing model. 相似文献