全文获取类型
收费全文 | 5745篇 |
免费 | 152篇 |
国内免费 | 42篇 |
专业分类
化学 | 4280篇 |
晶体学 | 34篇 |
力学 | 158篇 |
数学 | 637篇 |
物理学 | 830篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 85篇 |
2019年 | 75篇 |
2018年 | 55篇 |
2017年 | 54篇 |
2016年 | 106篇 |
2015年 | 96篇 |
2014年 | 100篇 |
2013年 | 231篇 |
2012年 | 359篇 |
2011年 | 436篇 |
2010年 | 206篇 |
2009年 | 166篇 |
2008年 | 364篇 |
2007年 | 351篇 |
2006年 | 384篇 |
2005年 | 402篇 |
2004年 | 301篇 |
2003年 | 248篇 |
2002年 | 263篇 |
2001年 | 87篇 |
2000年 | 97篇 |
1999年 | 64篇 |
1998年 | 63篇 |
1997年 | 53篇 |
1996年 | 94篇 |
1995年 | 70篇 |
1994年 | 55篇 |
1993年 | 43篇 |
1992年 | 43篇 |
1991年 | 46篇 |
1990年 | 29篇 |
1989年 | 29篇 |
1988年 | 28篇 |
1987年 | 29篇 |
1986年 | 23篇 |
1985年 | 42篇 |
1984年 | 46篇 |
1983年 | 39篇 |
1982年 | 36篇 |
1981年 | 48篇 |
1980年 | 43篇 |
1979年 | 24篇 |
1978年 | 47篇 |
1977年 | 30篇 |
1976年 | 34篇 |
1975年 | 43篇 |
1974年 | 31篇 |
1973年 | 31篇 |
排序方式: 共有5939条查询结果,搜索用时 15 毫秒
31.
32.
33.
34.
Superheated water eluent capillary liquid chromatography 总被引:4,自引:0,他引:4
A capillary scale reverse phase liquid chromatography (LC) system using a super hot water eluent is described. The system, constructed in-house from readily available components, has been shown to operate at temperatures as high as 370 °C and pressures in excess of 10 000 psi. The capability of the system is demonstrated with the separation of a mixture of polar and non-polar benzene derivatives on polybutadiene and elemental carbon modified zirconia packings with or without temperature gradients. Six benzene derivatives can be separated in 2 min. 相似文献
35.
One unit of S(IV) (SO2 or SHO3?) is oxidized per 2 units of [NiIII(cyclam)] species to obtain sulfate. Kinetic analyses have been done by varying the acidities (0.013 ? [H+] ? 1.0 M) and halide concentrations (0.000 ? [X?] ? 0.012 M; X=Cl and Br) at constant ionic strength (μ = 1.0 M). The rate law that incorporates the [X?] and [H+] dependence is ?d[NiIII]T/dt=2k[NiIII]T[S(IV)]T where 2k={ka[H+] + kbK + kK′X[H+] [X?] + kK′XK[X?]} {[H+] + K}?1 {1 + K′X[X?]}?1, here ka=87 ± 7 M?1 s?1, kb=(2.5 ± 0.5)×103 M?1 s?1 and pK = 1.8 ± 0.2. Rate constants ka and kb are attributed to the reactions of [NiIII(cyclam) (H2O)2]3+ with SO2 and SHO3?, respectively. Monohalo species apparent equilibrium constants K′Cl=(1600 ± 400) M?1 and K′Br=(190 ± 20) M?1 and rate constants k=80 ± 8 M?1 s?1 and k = 140 ± 15 M?1 s?1 are ascribed to the protonated pathway, involving the [NiIII(cyclam) (H2O)X]2+ and SO2(aq) reaction pairs. The other two rate constants of k=(5 ± 1)×103 M?1 s?1 and k=(3.1 ± 0.5)×104 M?1 s?1, refer to the deprotonated pathway and are assigned to the [NiIII(cyclam) (H2O)X]2+ /SHO3? redox couple. A deuterium H2O / D2O isotope effect of 2.1–2.8 can be attributed partially to an equilibrium isotope effect at low acidity though a small kinetic isotope (2.5 ± 0.5) effect is evident for the dihydrogen sulfito pathway, ka. The kinetic isotope effect and the absence of sulfite radical scavenging effects are explained by a mechanism entailing migration of a hydride from sulfur to the NiIII center to produce a NiIII—H species, which rapidly comproportionates, and S(VI). © 1993 John Wiley & Sons, Inc. 相似文献
36.
Scott Charles D. Woodward Charlene A. Thompson James E. Blankinship Stephen L. 《Applied biochemistry and biotechnology》1990,24(1):799-815
Applied Biochemistry and Biotechnology - Both oxidative and reductive enzymes can be utilized to enhance coal solubilization in aqueous and organic media. Aerobic solubilization was carried out... 相似文献
37.
Stulz E Scott SM Ng YF Bond AD Teat SJ Darling SL Feeder N Sanders JK 《Inorganic chemistry》2003,42(20):6564-6574
The synthesis of linear multiporphyrin arrays with mono- and bisphosphine-substituted porphyrins as ligand donors and ruthenium(II) or rhodium(III) porphyrins as ligand acceptors is described. With appropriate amounts of the building blocks mixed, linear dimeric and trimeric arrays have been synthesized and analyzed by (1)H NMR and (31)P NMR spectroscopy. The Ru/Rh acceptor porphyrins can be located either at the periphery or in the center of the array. Likewise, the monophosphine porphyrins can be positioned at the periphery, thus allowing a high degree of freedom in the overall composition of the arrays. This way, both donor and acceptor porphyrins can act as chain extenders or terminators. One of the trimeric complexes with two nickel and one ruthenium porphyrin has also been analyzed by X-ray crystallography. Attempts have also been made to synthesize higher order arrays by mixing appropriate amounts of the porphyrins; however, from the NMR data it cannot be concluded if monodisperse five, seven, or nine porphyrin arrays are present or if the solutions are composed of a statistical mixture of smaller and larger arrays. 相似文献
38.
It is shown that a thiolamide antioxidant, 4-mercaptoacetamido diphenylamine (MADA) can be reacted with both polypropylene and polyethylene in a mechanochemical process. The resulting bound antioxidant concentrates (MADA-B) can be used for the thermal and photostabilisation of the parent polymers. The adduct is very stable to solvent extraction and is also very effective as a high temperature antioxidant for polypropylene subjected to water leaching.The uv stabilising mechanism of MADA-B is believed to be related to the regenerative process involved in the mechanism of stabilisation by hindered nitroxyl radicals. 相似文献
39.
40.
An expert system for classifying and identifying low-resolution mass spectra of toxic and related compounds was developed with an expert shell program. The shell system used was an inexpensive, rule-building software package with an implementation of the ID3 algorithm. Seventy-eight target compounds were used to establish classes previously found by SIMCA class modeling. The six classes included nonhalobenzenes; chlorobenzenes; bromoalkanes and bromoalkenes; mono- and di-chloroalkanes and the analogous alkenes; tri-, tetra- and penta-chloroalkanes and the analogous alkenes; and unknowns. Identification modules for the target compounds were forward-chained to the classification modules. An expert system based on binary-encoded mass spectra, with 17 masses selected on the basis of information content, gave 97 and 86% classification accuracy for training and test spectra, respectively. Identification accuracy was 77 and 80%, respectively. An expert system was also developed which was based on ternary encoding of the mass spectra of 108 training compounds using 25 masses. Ternary encoding has many of the advantages of binary encoding, without the disadvantages. This latter system was tested with the spectra of thirty compounds found in field samples or potential air pollutants. The classification accuracy for training and test spectra was 99 and 97%, respectively. The identification accuracy was 96 and 93%, respectively. With proper precautions, the rule-building expert system can be very effective in spectral classification and identification problems. 相似文献