全文获取类型
收费全文 | 4895篇 |
免费 | 73篇 |
国内免费 | 18篇 |
专业分类
化学 | 2716篇 |
晶体学 | 22篇 |
力学 | 105篇 |
数学 | 521篇 |
物理学 | 1622篇 |
出版年
2020年 | 37篇 |
2019年 | 35篇 |
2018年 | 28篇 |
2016年 | 69篇 |
2015年 | 56篇 |
2014年 | 62篇 |
2013年 | 201篇 |
2012年 | 172篇 |
2011年 | 206篇 |
2010年 | 115篇 |
2009年 | 95篇 |
2008年 | 186篇 |
2007年 | 247篇 |
2006年 | 207篇 |
2005年 | 222篇 |
2004年 | 226篇 |
2003年 | 159篇 |
2002年 | 132篇 |
2001年 | 109篇 |
2000年 | 108篇 |
1999年 | 63篇 |
1998年 | 46篇 |
1997年 | 62篇 |
1996年 | 68篇 |
1995年 | 55篇 |
1994年 | 68篇 |
1993年 | 82篇 |
1992年 | 109篇 |
1991年 | 72篇 |
1990年 | 82篇 |
1989年 | 67篇 |
1988年 | 49篇 |
1987年 | 63篇 |
1986年 | 66篇 |
1985年 | 104篇 |
1984年 | 92篇 |
1983年 | 73篇 |
1982年 | 70篇 |
1981年 | 82篇 |
1980年 | 61篇 |
1979年 | 67篇 |
1978年 | 68篇 |
1977年 | 61篇 |
1976年 | 53篇 |
1975年 | 67篇 |
1974年 | 70篇 |
1973年 | 80篇 |
1972年 | 57篇 |
1971年 | 41篇 |
1970年 | 40篇 |
排序方式: 共有4986条查询结果,搜索用时 15 毫秒
21.
The application of convolution potential voltammetry to questions of metal complexation is described. Theoretical relations are derived to show that the stability constants may be directly related to the shift in the peak potential of the semiderivative wave, provided the complexes are labile. Equations are also given for inert and quasilabile complexes. Stability constants for the PbClx and CdClx systems are reported, illustrating the use of convolution techniques with linear scan voltammetry and with linear scan anodic stripping voltammetry. Advantages of convolution techniques are discussed. 相似文献
22.
The mass spectra of phenyl 2-pyridyl, phenyl 3-methoxy-2-pyridyl, phenyl 4-methoxy-2-pyridyl, phenyl 5-methoxy-2-pyridyl, phenyl 6-methoxy-2-pyridyl ketones, phenyl 3-pyridyl and phenyl 6-methoxy-3-pyridyl ketones, and phenyl 4-pyridyl ketone were studied. The major fragmentation pathway of all the ketones results in the formation of[C6H5CO]+ and [C5H4NCO]+ type ions. Another fragmentation path is the loss of carbon monoxide with formation of an [M ? CO]+ ion after skeletal rearrangement. 相似文献
23.
The stoichiometry of the various stages involved in the thermal decomposition of ammonium metavanadate has been shown to correspond to a stepwise decrease in the ratio of ammonia and water to V2O5, with V2O5 being the final product in vacuum, in air and in argon. In ammonia, VO2 is formed. The actual stages and intermediates are dependent upon the prevailing atmosphere. Chemical analyses, together with infrared absorption spectra and X-ray powder data, have enabled the intermediates and products to be characterized and the structural changes involved in the decomposition to be discussed.
We wish to acknowledge helpful comments from Prof. L. Glasser and Dr. N. H. Agnew and financial support from the National Institute for Metallurgy, Johannesburg, South Africa. 相似文献
Zusammenfassung Es wurde gezeigt, daß die verschiedenen Stufen in der thermischen Zersetzung von Ammoniummetavanadat dem stufenweisen stöchiometrischen Verlust von Ammonia und Wasser entsprechen. In Vakuum, Sauerstoff und Argon ist V2O5, in Ammoniak VO2 das Endprodukt. Die Zwischenprodukte der einzelnen Stufen sind von der umgebenden Gasatmosphäre abhängig. Durch chemische, infrarot- und röntgenspektroskopische Analyse gelang es, diese zu charakterisieren und so die durch die Zersetzung hervorgerufenen strukturellen Umlagerungen zu deuten.
Résumé On montre que les différentes étapes de la décomposition thermique du métavanadate d'ammonium correspondent à la diminution progressive de l'eau et de l'ammoniac par rapport à V2O5; cet oxyde constitue le produit final dans le vide, dans l'air et dans l'argon. Dans l'ammoniac, c'est VO2 qui se forme. Les étapes respectives et les intermédiaires dépendent de l'atmosphère qui prévaut. A l'aide de l'analyse chimique, des spectres d'absorption infrarouge et des données de rayons X sur poudre, on a pu caractériser les intermédiaires et les produits formés, ainsi que les changements structuraux provoqués par la décomposition.
, V2O5. , V2O5. . , , .
We wish to acknowledge helpful comments from Prof. L. Glasser and Dr. N. H. Agnew and financial support from the National Institute for Metallurgy, Johannesburg, South Africa. 相似文献
24.
25.
A survey of the solid-state polymerizability of 2- and 4-vinylpyridine (Vpy) complexes of divalent cobalt, nickel, copper, and zinc chlorides has been carried out. The influence of metal ion, ligand, coordination stereochemistry, and crystal structure is discussed. The tetrahedral modification of the complex Co(4-Vpy)2Cl2 has been found to undergo a particularly facile thermal polymerization below its melting point to yield high molecular weight poly-4-vinylpyridine. The polymer is shown to be conventional atactic head-to-tail poly-4-vinylpyridine. Thermogravimetric methods and optical microscopy have been used to study the thermal polymerization of single crystals of the complex. Results indicate a diffusion-controlled mechanism in which defective regions in the crystal act as nuclei for the polymerization. 相似文献
26.
An amplitude modulation of the electron spin echo envelope has been observed for radiation-produced trapped electrons in 10 M NaOD/D2O at 77°K but not in 10 M N2OH/H2O. The modulation has been simulated theoretically by generalizing the single crystal model of Rowan et al. to disordered systems. The modulation has been interpreted as due to dipolar interactions with deuterons in molecules of the second solvation shell around the trapped electrons. 相似文献
27.
28.
Thitivat Suebsaeng Charles A. Wilkie Vern T. Burger Janet Carter Charles Eric Brown 《Journal of polymer science. Part A, Polymer chemistry》1984,22(4):945-957
The solid reaction products from pyrolysis of polyethylene terephthalate in the presence and absence of red phosphorus were characterized by CP/MAS 13C-NMR, FR-IR, and MAS 31P-NMR spectroscopy. Over the temperature range of 300–400°C, polyethylene terephthalate was converted in a sealed vial to a highly crosslinked polymer of terephthalic acid. Pyrolysis in the presence of red phosphorus, which functions as a flame retardant by increasing the amount of char, yielded an intractible polyaromatic phosphate ester. After thermal cleavage of polyethylene terephthalate with formation of free carboxyl and vinyl ester groups, there are two competing reaction pathways. The smaller molecular weight fragments may enter the vapor phase where they undergo further degradation primarily to CO2, CO, and acetaldehyde, as described by others. However, if volatilization of the oligomeric fragments is inhibited, an alternate reaction pathway gives rise to the formation of highly crosslinked char. Red phosphorus decreases the volatility of the oligomeric fragments by converting them to phosphates and thereby enhances char formation. 相似文献
29.
Dichloro-2,3,3-trimethyl-1-platinocyclobutane, , prepared by the metathetical reaction of Zeise's dimer and 1,1,2-trimethylcyclopropane, reacts with pyridine in chloroform to produce a platinum complex of 2,3-dimethyl-1-butene . Reaction of I with pyridine at low temperatures (ca. ?40° C) leads to a pyridine addition compound (III) in which the platinocyclobutane ring remains intact. The thermal isomerization of III, which may be conveniently studied using NMR, produces a mixture of II and the free olefin. 相似文献
30.
High-performance liquid chromatography was used to assay serum acid and alkaline phosphatase. Samples were incubated with adenosine-5'-monophosphoric acid (AMP) in a buffer of required pH, 5'-nucleotidase was inhibited with Ni2+ ions, and the phosphatase activity was determined by measuring the concentration of the reaction product, adenosine. The analysis time, after the incubation is terminated, is short (7 min), and the assay is quantitative and reproducible. Complete separation of the reaction product from the substrate and the naturally occurring serum constituents and the high sensitivity of the ultraviolet detection system eliminate some of the problems commonly encountered in spectrophotometric assays. 相似文献