全文获取类型
收费全文 | 261224篇 |
免费 | 2567篇 |
国内免费 | 812篇 |
专业分类
化学 | 141957篇 |
晶体学 | 3879篇 |
力学 | 11099篇 |
综合类 | 12篇 |
数学 | 26703篇 |
物理学 | 80953篇 |
出版年
2018年 | 2034篇 |
2017年 | 1947篇 |
2016年 | 3602篇 |
2015年 | 2421篇 |
2014年 | 3585篇 |
2013年 | 10615篇 |
2012年 | 7850篇 |
2011年 | 10041篇 |
2010年 | 6672篇 |
2009年 | 6485篇 |
2008年 | 9094篇 |
2007年 | 9200篇 |
2006年 | 8897篇 |
2005年 | 8402篇 |
2004年 | 7406篇 |
2003年 | 6621篇 |
2002年 | 6555篇 |
2001年 | 7326篇 |
2000年 | 5616篇 |
1999年 | 4509篇 |
1998年 | 3929篇 |
1997年 | 3823篇 |
1996年 | 3588篇 |
1995年 | 3466篇 |
1994年 | 3261篇 |
1993年 | 3090篇 |
1992年 | 3555篇 |
1991年 | 3610篇 |
1990年 | 3365篇 |
1989年 | 3369篇 |
1988年 | 3321篇 |
1987年 | 3228篇 |
1986年 | 3077篇 |
1985年 | 4095篇 |
1984年 | 4305篇 |
1983年 | 3558篇 |
1982年 | 4006篇 |
1981年 | 3728篇 |
1980年 | 3730篇 |
1979年 | 3840篇 |
1978年 | 4095篇 |
1977年 | 3915篇 |
1976年 | 3980篇 |
1975年 | 3812篇 |
1974年 | 3737篇 |
1973年 | 3944篇 |
1972年 | 2490篇 |
1971年 | 2065篇 |
1968年 | 2030篇 |
1967年 | 2087篇 |
排序方式: 共有10000条查询结果,搜索用时 17 毫秒
951.
Quantitative determination of nitroglycerin in human plasma by capillary gas chromatography with electron-capture detection 总被引:2,自引:0,他引:2
A sensitive method for the selective determination of nitroglycerin at concentrations down to 50 pg/ml in human plasma is described. After the addition to plasma of a known amount of butane-1,2,4-triol trinitrate as internal standard, both compounds are extracted into hexane. Nitroglycerin is then quantitated by capillary gas chromatography with electron-capture detection. 相似文献
952.
A. Simonits S. Jovanović F. De Corte L. Moens J. Hoste 《Journal of Radioanalytical and Nuclear Chemistry》1984,82(1):169-179
When making use of some single comparator or absolute standardization methods in reactor neutron and in epicadmium neutron activation analysis, the knowledge of the effective resonance energy (
) is essential to correct for the effect of the nonideal epithermal flux distribution on the analysis result.
can be calculated from neutron resonance data, but when these are incomplete, not accurate or even not known at all, experimental determination should be considered. Such a method, providing both
and the resonance integral to 2200 ms–1 cross-section ratio (QO), is described in this paper. Results are given for 11 isotopes. 相似文献
953.
Raman spectra of a series of alkali-halide/BH?4 (and BD?4 crystals have been obtained. These spectra show some interesting examples of Fermi resonance type interactions between the stretching mode levels and overtone and combination band levels of the bending modes. Two resonances will be considered: (i) that between ν1 and 2ν4(A1), and (ii) that between ν3, 2ν4 (F2) and (ν2+ν4) (F2).The F2 resonance between ν3, 2ν4 and ν2+ν4 appears in the infrared spectrum and it has been studied on several occasions. However the equivalent Raman spectrum is of interest because the relative intensities of the bands are significantly different to those shown by the infrared spectrum.In the A1 (and E) Raman spectrum of the stretching mode region there are two strong bands for each for the 10B and 11B isotopes. The ν1 would not be expected to show any 10B and 11B splitting, but the observed bands are both closely resonating mixtures of ν1 and 2ν4(A1). In fact the analysis shows that the stronger band has the higher proportion of 2ν4 character, and the larger isotopic shift of the more intense band can then be seen to be reasonable. 相似文献
954.
The replacement of phenylethanethiolate (SC2Ph) ligands on 1.1 nm (core diameter) Au38(SC2Ph)24 monolayer-protected clusters (MPCs) with varied amounts of triphenylphosphine (PPh3) is investigated in methylene chloride. UV-vis spectra suggest that changes in the MPC Au core size occur when large amounts (> 10 equiv moles per cluster) of PPh3 are reacted with Au38(SC2Ph)24. 1H and 31P NMR spectra following the addition of smaller amounts (< 5 equiv moles) of PPh3 indicate that the reaction liberates a AuISC2Ph complex, as opposed to a SC2Ph thiol, disulfide, or anion. A 1H NMR kinetic study shows that the exchange is surprisingly rapid, even faster than exchanges of thiolates with other thiolates, at room temperature and in air. The reaction is slowed when cooled or conducted under Ar. The difference in potentials of the initial one-electron voltammetric reduction and oxidation of Au38(SC2Ph)24 decreases slightly upon reaction with small amounts of PPh3. 相似文献
955.
The displacements of the methyl substituents away from the metal and out of the cyclopentadienyl ring plane are compared in sterically crowded (C(5)Me(5))(3)M complexes vs sterically normal f-element complexes in an attempt to evaluate the utility of this parameter in predicting unusual (C(5)Me(5))(1-) ring reactivity. The out-of-plane displacements of 16 sterically crowded tris(cyclopentadienyl) complexes of general formula (C(5)Me(5))(3)M, (C(5)Me(4)R)(3)M (R = Et, (i)Pr, (t)()Bu, SiMe(3)), (C(5)Me(5))(3)MX (X = anion), and (C(5)Me(5))(3)ML (L = neutral ligand) are compared with [(C(5)Me(5))(2)U](2)(C(6)H(6)), (C(5)Me(5))(2)Sm(PC(4)H(2)(t)Bu(2)), and 33 representative examples of f-element bis(cyclopentadienyl) complexes with normal cyclopentadienyl behavior and coordination numbers ranging from 6 to 10. In general, the methyl displacement values of sterically crowded complexes overlap with those in the other complexes, which demonstrates that the basis of the structural distortions is complex. However, if the most extreme out-of-plane displacement in each of the sterically crowded complexes is examined vs the analogous maximum out-of-plane displacement in less crowded systems, there appears to be a basis for predicting cyclopentadienyl reactivity. 相似文献
956.
The rates of decomposition and the energies of activation were determined for the thermal dissociation of NiSO4, CuSO4, Cu2(SO4)O, ZnSO4and Zn3(SO4)2O. Linear, parabolic and logarithmic rate laws were found. Kinetics are largely determined by the rate of energy transfer. The reasons for the formation of basic sulfate intermediates are discussed. 相似文献
957.
Photosynthetic suspension cultures of higher plants offer an alternative approach to biomass production, potentially yielding
cellulosic material and protein on a continuous year-round basis. A bench-top hybrid photobioreactor was developed to study
photomixotrophic and photoautotrophic growth of Glycine max as a model system. Maximum biomass doubling times for photomixotrophic
and photoautotrophic growth were 1.87 and 3.92 d, respectively. The presence of exogenous sugars resulted in photomixotrophic
growth, reduced chlorophyll levels, and a reduction in photosynthetically-evolved oxygen. Depletion of carbohydrates from
the medium coincided with the beginning of stationary phase and an increase in oxygen evolution by the cells. A second growth
phase, prolonging cell viability, could be initiated by increasing the carbon dioxide from 2 to 5%, just before the onset
of stationary phase. Biomass from bioreactor cultured cells proved resistant to enzymatic attack without pretreatment. Composition
of the biomass was 7.8% lignin, 20.7% α-cellulose, 23% hemicellulose, 5.5% starch, 14.5% protein and 6.5% nucleic acids. 相似文献
958.
959.
Bestimmung des Oxydationszustandes in Vanadiumkomplexen mit Hilfe von V—K-Röntgenabsorptionsspektren
Determination of the Oxydation State in Vanadium Complexes using V—K-Absorption Spectra The compounds resulting from the reaction of oxovanadium(IV) sulfate with 8-quinolinols in air were characterized by magnetic moments and V—K-absorption spectra. 8-quinolinol and its Mannich derivates (“CMAB-oxine”, “CMAP-oxine”) form vanadium(V) complexes. The oxovanadium(IV) compound of 7-methyl-8-quinolinol initially formed is not very stable in solution and tends to oxydation. Only the oxovanadium(IV) complex of 2-methyl-8-quinolinol has a good stability corresponding to that of β-diketone compounds. 相似文献
960.
A new technique is presented for study of the thermal behaviour of materials. Experimental and theoretical bases are established for the limiting temperatures of thermal decomposition of different polymers.
Zusammenfassung Es wird ein neues Verfahren zur Untersuchung des thermischen Verhaltens von Stoffen. vorgestellt. Es wurden experimentelle und theoretische Grundlagen zur Bestimmung von thermischen Zersetzungstemperaturen verschiedener Polymere erstellt.
. .相似文献