全文获取类型
收费全文 | 47022篇 |
免费 | 1555篇 |
国内免费 | 18篇 |
专业分类
化学 | 28769篇 |
晶体学 | 254篇 |
力学 | 769篇 |
综合类 | 1篇 |
数学 | 8850篇 |
物理学 | 9952篇 |
出版年
2023年 | 334篇 |
2022年 | 312篇 |
2021年 | 499篇 |
2020年 | 679篇 |
2019年 | 620篇 |
2018年 | 958篇 |
2017年 | 878篇 |
2016年 | 1695篇 |
2015年 | 1412篇 |
2014年 | 1365篇 |
2013年 | 3062篇 |
2012年 | 2926篇 |
2011年 | 2748篇 |
2010年 | 1825篇 |
2009年 | 1536篇 |
2008年 | 2417篇 |
2007年 | 2201篇 |
2006年 | 1972篇 |
2005年 | 2028篇 |
2004年 | 1753篇 |
2003年 | 1454篇 |
2002年 | 1266篇 |
2001年 | 960篇 |
2000年 | 966篇 |
1999年 | 669篇 |
1998年 | 516篇 |
1997年 | 441篇 |
1996年 | 586篇 |
1995年 | 427篇 |
1994年 | 496篇 |
1993年 | 447篇 |
1992年 | 478篇 |
1991年 | 400篇 |
1990年 | 444篇 |
1989年 | 363篇 |
1988年 | 373篇 |
1987年 | 330篇 |
1986年 | 322篇 |
1985年 | 451篇 |
1984年 | 413篇 |
1983年 | 324篇 |
1982年 | 329篇 |
1981年 | 343篇 |
1980年 | 284篇 |
1979年 | 275篇 |
1978年 | 270篇 |
1977年 | 261篇 |
1976年 | 278篇 |
1974年 | 250篇 |
1973年 | 263篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
993.
994.
A derivatographic method has been developed for the identification and determination of the pyrites content of bauxites and clay minerals. If the sample contains at least 0.2% of pyrite, then at 400–450° a weight increase is indicated on the DTG curve, while a heat liberation is shown on the DTA curve. The amplitude of or the area under, these maxima is proportional to the pyrites content of the sample, although the correlation is not linear. In addition, the effect of experimental conditions on the thermal decomposition of pyrites has been studied. 相似文献
995.
The theory is given of errors of the title method based on the solvent extraction reaction Aaq+Baq⇌ABorg, the minimum amounts which can be determined are assessed, the desirable values of ξ are recommended and the precision of
the title method is compared with that of the concentration dependent distribution method. Tables are given allowing a rapid
calculation of the error of determination for any reasonable concentration of the substance to be determined, any extractant
concentration and a chosen value of ξ if the equilibrium constant of the above reaction is known. 相似文献
996.
J. Sanmartín M. R. Bermejo J. A. García-Vázquez J. Romero A. Sousa J. Strähle A. Brodbeck 《Transition Metal Chemistry》1993,18(2):187-190
Summary The electrochemical oxidation of anodic metal (nickel or cobalt) in MeCN solutions of 1-hydroxy-2-pyridinethione (HPT) gives [Ni(PT)2], [Co(PT)2] or [Co(PT)3]. When 1,10-phenanthroline (phen) or 2,2-bipyridine (bipy) are added to the electrolytic phase the product is a complex, [Ni(PT)2L] or [Co(PT)2L] (L = bipy or phen). The i.r., u.v. and 1H- and 13C-n.m.r. spectra of the complexes are discussed.This paper was presented at the 5th Inorganic Chemistry Meeting of the Royal Spanish Chemical Society, Tossa de Mar, Girona, Spain, September 1991. 相似文献
997.
Sílvia S. P. R. Almeida Joo J. R. Fraústo Da Silva Armando J. L. Pombeiro 《Journal of organometallic chemistry》1993,450(1-2)
Treatment of a THF solution of trans-[ReCl(N2)(dppe)2] (dppe = Ph2PCH2CH2PPh2) with a 1-alkyne HCCR (R =tBu, CO2Me, CO2Et, or C6H4Me-4), in the presence of Tl[BF4]/[NH4][BF4], under sunlight, affords the corresponding carbyne-fluoro complexes trans-[ReF(CCH2R)(dppe)2][BF4] in an unprecedented single-pot synthesis. Further reaction with [BU4N]OH leads to the vinylidenefluoro compounds trans-[ReF(=C=CHR)(dppe)2] (R = CO2Me, CO2Et, or C6H4Me-4). 相似文献
998.
K. Mészáros Szécsényi K. Tomor G. Pokol 《Journal of Thermal Analysis and Calorimetry》1994,41(1):125-134
The phase diagram for the AgNO3?KNO3 system has been determined using differential scanning calorimetry (DSC). Eutectic point has been found at 391 K andX Ag=0.580 mole fraction AgNO3. The DSC curves indicate the existence of an intermermediate compound (AgNO3·KNO3) in the KNO3-rich region of the phase diagram. This compound was identified in the solid phase by X-ray diffraction. The melting and the crystallization processes were followed with the aid of a hot stage microscope, too. 相似文献
999.
Roca M Andrés J Moliner V Tuñón I Bertrán J 《Journal of the American Chemical Society》2005,127(30):10648-10655
The way in which enzymes influence the rate of chemical processes is still a question of debate. The protein promotes the catalysis of biochemical processes by lowering the free energy barrier in comparison with the reference uncatalyzed reaction in solution. In this article we are reporting static and dynamic aspects of the enzyme catalysis in a bimolecular reaction, namely a methyl transfer from S-adenosylmethionine to the hydroxylate oxygen of a substituted catechol catalyzed by catechol O-methyltransferase. From QM/MM optimizations, we will first analyze the participation of the environment on the transition vector. The study of molecular dynamics trajectories will allow us to estimate the transmission coefficient from a previously localized transition state as the maximum in the potential of mean force profile. The analysis of the reactive and nonreactive trajectories in the enzyme environment and in solution will also allow studying the geometrical and electronic changes, with special attention to the chemical system movements and the coupling with the environment. The main result, coming from both analyses, is the approximation of the magnesium cation to the nucleophilic and the hydroxyl group of the catecholate as a result of a general movement of the protein, stabilizing in this way the transition state. Consequently, the free energy barrier of the enzyme reaction is dramatically decreased with respect to the reaction in solution. 相似文献
1000.
An expression for the extreme values of mean-square amplitudes of vibrations in polyatomic molecules has been derived which permits estimation of the mean-square amplitude without solving the vibrational problem. This expression can be improved for the stretching and scissoring modes when the assignment of frequencies is known. In turn, the corresponding vibrational frequency may be estimated from the experimental value of the mean-square amplitude. The mean-square amplitudes of the butadiene-1,3 molecule are considered as an example. 相似文献