全文获取类型
收费全文 | 2394篇 |
免费 | 49篇 |
国内免费 | 8篇 |
专业分类
化学 | 1194篇 |
晶体学 | 25篇 |
力学 | 127篇 |
数学 | 240篇 |
物理学 | 865篇 |
出版年
2022年 | 27篇 |
2021年 | 30篇 |
2020年 | 37篇 |
2019年 | 31篇 |
2018年 | 35篇 |
2017年 | 45篇 |
2016年 | 59篇 |
2015年 | 39篇 |
2014年 | 69篇 |
2013年 | 151篇 |
2012年 | 139篇 |
2011年 | 140篇 |
2010年 | 89篇 |
2009年 | 95篇 |
2008年 | 125篇 |
2007年 | 103篇 |
2006年 | 75篇 |
2005年 | 91篇 |
2004年 | 76篇 |
2003年 | 65篇 |
2002年 | 54篇 |
2001年 | 35篇 |
2000年 | 38篇 |
1999年 | 38篇 |
1998年 | 22篇 |
1997年 | 23篇 |
1996年 | 31篇 |
1995年 | 26篇 |
1994年 | 31篇 |
1993年 | 42篇 |
1992年 | 33篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 34篇 |
1988年 | 23篇 |
1987年 | 19篇 |
1986年 | 18篇 |
1985年 | 20篇 |
1984年 | 27篇 |
1983年 | 19篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1978年 | 26篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1975年 | 21篇 |
1974年 | 20篇 |
1973年 | 14篇 |
排序方式: 共有2451条查询结果,搜索用时 15 毫秒
81.
MoSe2 and WSe2 nanotubes are obtained by the reduction of the corresponding triselenides in hydrogen or by the decomposition of the ammonium selenometallates in a hydrogen atmosphere. 相似文献
82.
Sachindra Kumar Datta Sachindra Nath Saha 《Fresenius' Journal of Analytical Chemistry》1961,184(3):177-184
Summary A chromotropic azo dye, DSNADNS, prepared from chromotropic acid and 1-amino-8-naphthol-3,6-disulfonic acid has been found to produce quantitative precipitation of zirconium in acid medium. The blue-violet complex formed is insoluble in all organic solvents commonly used for solvent extraction work, but is soluble in a number of liquid organic bases and solutions of ammonium salts in water showing pink to violet colouration. Spectrophotometric studies as to the nature of the complex in pyridine, triethanolamine ammonium acetate, oxalate and carbonate solutions and the analytical possibility of these solutions have been made. The complex appears to decompose in pyridine, ammonium acetate and ammonium oxalate solutions, but it is fairly stable in ammonium carbonate and triethanolamine solutions. The absorbance peaks of the dye in these solutions occur at 500 and 520 nm respectively, while the complex in triethanolamine and ammonium carbonate both shows absorbance maxima at 540 nm. The solution of the complex in these two solvents obeys Beer's law within a concentration range 4 to 25 mg of zirconium per litre. The molar absorbance coefficients of the complex in triethanolamine and ammonium carbonate are respectively 686 and 823, which indicate poor sensitivity. The solution of the complex in the former solvent is more stable than that in the latter and an analytical procedure for the spectrophotometric determination of zirconium, with little interference from foreign ions, may be developed with ease in the triethanolamine solution.
Zusammenfassung Ein Azofarbstoff der Chromotropsäure, DSNADNS, der aus Chromotropsäure und 1-Amino-8-naphthol-3,6-disulfonsäure dargestellt werden kann, ergibt mit Zirkonium in saurer Lösung eine quantitative Fällung. Der blauviolett gefärbte Komplex ist in allen üblichen organischen Lösungsmitteln unlöslich, löst sich jedoch in einigen flüssigen organischen Basen sowie wäßrigen Lösungen von Ammoniumsalzen mit rosa bis violetter Färbung. Die Lösungen des Komplexes in Pyridin, Triäthanolamin sowie in Ammoniumacetat, -oxalat und -carbonatlösungen und ihre analytische Verwendbarkeit werden spektrophotometrisch untersucht. In Pyridin sowie in Ammoniumacetat und -oxalatlösungen scheint sich der Komplex zu zersetzen, während er in Triäthanolamin und Ammoniumcarbonatlösungen ziemlich beständig ist. Das Absorptionsmaximum des Farbstoffs in diesen beiden Lösungsmitteln liegt bei 500 bzw. 520 nm, der Komplex weist in beiden Fällen ein Maximum bei 540 nm auf. Das Beersche Gesetz wird in beiden Lösungsmitteln von 4–25 mg Zr/l erfüllt. Der molare Absorptionskoeffizient beträgt 686 bzw. 823. Die Lösung des Komplexes in Triäthanolamin ist stabiler als in Ammoniumcarbonatlösung. Eine spektrophotometrische Bestimmung von Zirkonium, die nur wenig von Fremdionen gestört wird, könnte daher gut in Triäthanolaminlösung durchgeführt werden.相似文献
83.
R. Krishna Mohan Rao Kallury Pramod V. Upadhyaya Tumbalam G. Surendra Nath Vankipuram R. Srinivasan 《Journal of mass spectrometry : JMS》1977,12(5):307-312
The mass spectra of eight 1,2,4-triazole derivaties have been recorded and found tao reveal extensive hydrogen and skeletal migrations. The structures of the fragments have been confirmed by deuterium labelling and exact mass measurement. The compounds revealed striking differences in their spectra depending on the nature of the substituents. 相似文献
84.
H. K. Sinha O. K. Abou-Zied M. Ludwiczak A. Maciejewski R. P. Steer 《Chemical physics letters》1994,230(6):547-554
The S2-S0 fluorescence excitation spectra ofxanthione (XT) and azulene (AZ) complexed with 1 or 2 molecules of the C1 to C10 n-alkanes and the C1 to C6 perfluoro-n-alkanes have been measured. The 1:1 complexes exhibit microscopic solvent shifts, Δ
gn, which are larger for the alkanes than the corresponding perfluoroalkanes, despite the larger molecular polarizability of the latter. The values of Δ
gn increase monotonically with carbon number of the adduct to C10 in 1:1 n-alkane complexes with XT and to C5 in 1:1 n-alkane complexes with AZ. However, Δ
gn exhibits no further increase beyond C2 in 1:1 perfluoro-n-alkane complexes with XT and beyond C3 in 1:1 perfluoro-n-alkane complexes with AZ. The results are interpreted in terms of a model in which the n-alkanes stretch out along the long axis of the chromophore and ‘wet’ its surface whereas the perfluoro-n-alkanes with carbon numbers 3 away from the surface of the chromophore and are ‘non-wetting’. 相似文献
85.
The retroaldol-aldol isomerization of the epimeric hydroxyketones and was found to yield predominantly and , respectively, under kinetic conditions in the presence of chelating lithium cations. In hydroxylic medium is the preferred kinetic product. Under equilibration conditions and predominate. 相似文献
86.
A theoretical model for the multiline TEA CO2 laser has been developed which takes into account the overlap of the P(20) line of (0001) (1000) regular band transition with the R(23) line of (0111) (1110) hot band transition. The model is used for the study of the influence of different parameters like laser gas temperature, gas mixture and pump rates on laser intensities of different rotational lines. This study explains the experimental results of the high power multiline TEA CO2 laser very well. 相似文献
87.
The effect of buoyancy force on laminar boundary layer in two dimensional flow and heat transfer along a semi-infinite vertical surface, when the velocity of the on-coming stream oscillates in magnitude about a steady mean, is analysed. Two separate solutions valid for low and high frequency ranges are developed. It is found that for low frequency oscillations the phase angles of oscillatory components of skin friction and the rate of heat transfer increase as the Grashof number increases. For very high frequencies, the velocity field is of shear-wave type unaffected by the mean flow; the phase lead in the skin friction is, then, /4 and the rate of heat transfer fluctuation indicates a phase lag of /2 over that of the free stream oscillations.The authors are thankful to referee for many useful suggestions. 相似文献
88.
Nath M Jairath R Eng G Song X Kumar A 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(13-14):3155-3161
New diorganotin(IV) derivatives of the general formula R2Sn(Umb)2 (where R = n-Bu, n-Oct and Ph; Umb = umbelliferone anion) have been synthesized either by the reaction of R2SnO with umbelliferone under azeotropic removal of water or by the reaction of R2SnCl2 with sodium salt of umbelliferone. Further, the adducts of the general formula R2Sn(Umb)2.phen (where R = n-Bu and n-Oct; phen = 1,10-phenanthroline) have also been synthesized by the interaction of R2Sn(Umb)2 with 1,10-phenanthroline. The bonding and coordination behavior in these derivatives are discussed on the basis of IR and 119Sn M?ssbauer spectroscopic studies in solid state. Their coordination behavior in solution is discussed by the multinuclear (1H, 13C and 119Sn) NMR spectral studies. The M?ssbauer and IR studies indicate that umbelliferone acts as a monoanionic bidentate ligand in R2Sn(Umb)2 coordinating through O(7) and O(1). A distorted octahedral geometry around tin has been proposed for R2Sn(Umb)2 as well as for R2Sn(Umb)2.phen in solid state. The newly synthesized derivatives have been tested for their anti-inflammatory and cardiovascular activities. The average LD50 value >1000 mg kg(-1) of these compounds indicates their safety margin. 相似文献
89.
Summary The kinetics of oxidation of 2-methoxyethanol and 2ethoxyethanol by vanadium (V) in aqueous sulphuric acid medium has been studied. The order of reaction with respect to the oxidant and substrate has been found to.be unity. The energy of activation has also been calculated graphically. A probable mechanism has been proposed. 相似文献
90.
The structure of the local anaesthetic benzocaine was solved by direct methods and refined to anR of 0·12 for 531 observed reflections. The packing of the molecule is stabilised by N−H … O hydrogen-bonds (2–97 ?). The alkyl
chain attached to the benzene ring is intrans-trans conformation. The benzoic moiety shows a quinonoid character as found in some other local anaesthetics.
DCB contribution No. 699 相似文献