全文获取类型
收费全文 | 1111篇 |
免费 | 33篇 |
国内免费 | 6篇 |
专业分类
化学 | 744篇 |
晶体学 | 4篇 |
力学 | 21篇 |
数学 | 225篇 |
物理学 | 156篇 |
出版年
2023年 | 12篇 |
2022年 | 30篇 |
2021年 | 32篇 |
2020年 | 13篇 |
2019年 | 37篇 |
2018年 | 24篇 |
2017年 | 25篇 |
2016年 | 38篇 |
2015年 | 44篇 |
2014年 | 42篇 |
2013年 | 86篇 |
2012年 | 61篇 |
2011年 | 68篇 |
2010年 | 50篇 |
2009年 | 54篇 |
2008年 | 62篇 |
2007年 | 74篇 |
2006年 | 41篇 |
2005年 | 31篇 |
2004年 | 36篇 |
2003年 | 30篇 |
2002年 | 36篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 16篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1996年 | 18篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 5篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 3篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 10篇 |
1955年 | 2篇 |
1931年 | 2篇 |
排序方式: 共有1150条查询结果,搜索用时 15 毫秒
861.
Dmitry V. Bavykin Vera P. Dubovitskaya Alexander V. Vorontsov V. N. Parmon 《Research on Chemical Intermediates》2007,33(3-5):449-464
The effect of synthesis conditions on morphology and catalytic activity has been studied for hydrothermal preparation of TiO2 from acidified aqueous TiOSO4 solution. It was found that the increase in TiOSO4 and H2SO4 concentration results in the increase of photocatalytic activity of produced TiO2, as it was revealed by steady-state gas-phase oxidation of acetone and ethanol vapors in a flow-circulating reactor. TiOSO4 concentration exerts strong influence on the shape of the produced TiO2 particles. At TiOSO4 concentration less than 0.1 wt%, hydrolysis gives rise to hedgehog-like agglomerates consisting of spliced-blade TiO2 nanocrystals. At higher concentration of TiOSO4, the resultant TiO2 consists of round agglomerates of 5–10 nm primary particles. The size of secondary particles depends mainly on the H2SO4 concentration. The increase of the time of hydrothermal treatment results in the enhancement of TiO2 photocatalytic activity that reaches a maximum. Among different acids (HCl, HClO4, HNO3, H3PO4 and CH3COOH) added during hydrolysis of TiOSO4, sulfuric and acetic acids had the best effect on photocatalytic activity of TiO2. The results obtained can help to finely tune this TiO2 preparation method in order to obtain desirable particles size, shape and activity. 相似文献
862.
The fat-soluble vitamins A and E in milk samples were determined by fluorescence at room temperature in an aqueous media of micellar solutions. Different types of surfactants were studied; the cationic hexadecyltrimethylammonium bromide (CTAB), the anionic sodium dodecylsulfate (SDS) and the non-ionic polyoxyethylene(23)laurylether (Brij 35). The detection limits ranged between 50 and 90 ng.L-1 for both vitamins in CTAB and Brij 35. The method has been applied to the determination of vitamins A and E in milk samples. 相似文献
863.
Zorica M. Bugar?i? Marijana P. Gavrilovi? Vera M. Divac 《Monatshefte für Chemie / Chemical Monthly》2007,40(9):149-151
The procedure employs phenylselenyl chloride or bromide, pent-4-en-1-ol, and additives, like pyridine and silver(I) salts,
to generate the cyclic ether of tetrahydrofuran type in high yields. A catalytic amount of additive leads to higher yields,
but equimolar amounts achieved almost quantitative yields under extremely mild experimental conditions. The effect of the
halide ion of the selenylating reagent is not significant. 相似文献
864.
By reaction of (C5H5)2Ti(μ-S2)2C6H10 3 with S2Cl2 7,8,9,10,11,12-hexathiaspiro-[5.6]dodecane 4 is prepared (yield 51%) and characterized by UV, IR, Raman, mass, and NMR spectra (1H, 13C). The seven-membered CS6 ring undergoes pseudorotation in solution. With S7Cl2 the complex 3 yields 7,8,9,10,11,12,13,14,15,16,17-undecathiaspiro[5.11]heptadecane 5 (yield 23%). The yellow, monoclinic crystals of 5 consist of spirocyclic C6H10S11 molecules with the C6 ring in a chair-conformation while the CS11 ring is of the same conformation as cyclododecasulfur S12. UV, IR, Raman, mass and NMR-spectra of 5 are reported. A mixture of dichlorosulfanes SnCl2 (n = 1 -8) reacts with 3 to give the homologous series C6H10Sm which was characterized by reversed-phase HPLC for m = 5 – 14. 相似文献
865.
Vera Zeidan 《Applied Mathematics and Optimization》1984,11(1):97-109
We are concerned with developing sufficiency criteria for the generalized problem of Bolza, where the Hamiltonian is neither concave-convex nor differentiable. The core of the approach is the modified Hamilton-Jacobi inequality which leads to a new type of sufficient conditions. This latter is then used to derive new first order and known second order sufficient conditions.The publication of this report has been made possible due to a grant of the Fonds FCAC for the help and support of research. 相似文献
866.
Dimitri I. Kochubey Vladimir A. Rogov Vera P. Babenko 《Reaction Kinetics and Catalysis Letters》2007,90(1):167-177
The rate of thiophene decomposition was shown to be independent of the type of chalcogens used in catalysts MoX2/Al2O3, where X = S, Se. On the contrary, the rate of selenophene decomposition was shown to be higher on catalysts MoSe2 than that on MoS2. This observation suggests that the decomposition proceeds on anion vacancies. The decomposition of either thiophene over
MoSe2 or selenophene over MoS2 results in the formation of partially substituted chalcogenides. At that, the molar ratios of the substituted chalcogen to
Mo were shown to coincide in both cases. The fact that the rate of the thiophene decomposition does not depend on the degree
of anion exchange indicates that the decomposition is not associated with hydrogenolysis. 相似文献
867.
A practical cross-coupling of amides with alkynyl bromides using catalytic CuSO(4).5H(2)O and 1,10-phenanthroline is described here. This catalytic protocol is more environmentally friendly than the use of CuCN or copper halides and provides a general entry for syntheses of ynamides including various new sulfonyl and heteroaromatic amine substituted ynamides. Given the interest in ynamides, this N-alkynylation of amides should be significant for the future of ynamides in organic synthesis. 相似文献
868.
Santoyo E García R Martínez-Frías J López-Vera F Vera SP 《Journal of chromatography. A》2002,953(1-2):279-286
Micellar electrokinetic capillary chromatography (MEKC) coupled with sample stacking and polarity switching was investigated for the determination of Viagra (sildenafil citrate, SC) and its metabolite (UK-103,320, UK) in human serum in the concentration range of clinical interest. Human serum samples spiked with SC and UK were eluted with methanol from a C18 cartridge, the extract was evaporated and regenerated in a solution that contained 1 mM phosphate buffer (pH 12.3) and 20% methanol. The MEKC separation was performed using an injection time of 275 s, a polarity switching time of 93 s, a phosphate buffer, (pH 12.3, 15 mM) containing 25 mM sodium dodecyl sulfate as separation electrolyte and a fused-silica capillary. The analysis takes about 6 min and gives satisfactory inter-day precision with respect to migration times and linear responses over the 80-900 ng/ml concentration range investigated for SC and UK. Intra-day RSDs (n=4 graphs) for the slopes of the calibration graphs were 4.86% for SC and 3.50% for UK. Inter-day RSDs for the slopes were 4.37% for SC and 5.39% for UK. Detection limits (S/N=3) were about 17 ng/ml for both compounds in human serum. A 1-ml volume of blood serum was necessary to do this determination. 相似文献
869.
A method is described for the simultaneous determination of low concentration of Ca, Cr, Cu, Fe, Mn and Ni in nuclear-grade uranium dioxide by x-ray fluroescence spectrometry, without the use of chemical treatment. The lower limits of detection range from 2 μg g?1 for nickel and manganese to 5 μg g?1 for copper. Samples are prepared in the form of double-layer pellets with boric acid as a binding agent. Standards are prepared in a U3O8 matrix, which is more chemically stable than UO2 and has similar matrix behaviour. The correlation coefficients for calibration curves are better than 0.999. Errors range from 2.4% for chromium to 6.8% for nickel. 相似文献
870.
P. Lukkari J. Hannuksela M.-L. Mattinen M. Virolainen Vera M. A. Hkkinen M.-L. Riekkola 《Journal of separation science》1990,13(3):170-172
The on-line coupled LC-GC technique was applied to the analysis of several metal chelates of N,N-diethyldithiocarbamic acid. A 10-port valve interface was used to couple the LC and GC instruments. Conditions during sample transfer into the GC gave fully concurrent solvent evaporation. The chelates investigated were separated with a short apolar fused silica column. LC preseparation was made with cyano or amino phases using a hexane/dichloromethane mixture as eluent. On-line LC-GC combination seems to be very suitable for the separation of the metal chelates studied. 相似文献