全文获取类型
收费全文 | 26119篇 |
免费 | 2639篇 |
国内免费 | 5209篇 |
专业分类
化学 | 30218篇 |
晶体学 | 140篇 |
力学 | 272篇 |
综合类 | 122篇 |
数学 | 512篇 |
物理学 | 2703篇 |
出版年
2024年 | 30篇 |
2023年 | 372篇 |
2022年 | 527篇 |
2021年 | 870篇 |
2020年 | 1282篇 |
2019年 | 1111篇 |
2018年 | 1114篇 |
2017年 | 1052篇 |
2016年 | 1168篇 |
2015年 | 1093篇 |
2014年 | 1693篇 |
2013年 | 2601篇 |
2012年 | 1511篇 |
2011年 | 1841篇 |
2010年 | 1410篇 |
2009年 | 1554篇 |
2008年 | 1656篇 |
2007年 | 1792篇 |
2006年 | 1641篇 |
2005年 | 1522篇 |
2004年 | 1488篇 |
2003年 | 1171篇 |
2002年 | 732篇 |
2001年 | 551篇 |
2000年 | 556篇 |
1999年 | 463篇 |
1998年 | 408篇 |
1997年 | 370篇 |
1996年 | 311篇 |
1995年 | 323篇 |
1994年 | 274篇 |
1993年 | 216篇 |
1992年 | 217篇 |
1991年 | 158篇 |
1990年 | 119篇 |
1989年 | 109篇 |
1988年 | 90篇 |
1987年 | 55篇 |
1986年 | 51篇 |
1985年 | 57篇 |
1984年 | 49篇 |
1983年 | 27篇 |
1982年 | 44篇 |
1981年 | 39篇 |
1980年 | 29篇 |
1979年 | 25篇 |
1978年 | 28篇 |
1977年 | 29篇 |
1976年 | 36篇 |
1974年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 368 毫秒
971.
Under steady-state conditions, the current equations of the second-order EC, ECE and DISPI reactions at microdisk, microspherical and microring electrodes are derived with the aid of the concept of the reaction layer. The conditions under which these equations would be valid are also discussed. Using these equations, methods to determine the kinetic parameters for the second-order EC, ECE and DISPI reactions are presented. The reduction of 2,6-diphenyl-pyrylium cation and oxidation of triphenylamine were investigated as examples of the second-order EC and ECE reactions. 相似文献
972.
Ralph Jaquet 《Theoretical chemistry accounts》1987,71(6):425-448
We are trying to investigate systematically the application of the finite element method (FEM) for solving the Schrödinger equation. The present paper is devoted to the calculation of vibrational transition probabilities for the collinear reactive system A + BC (i.e. H+H2 and their isotopes). The calculations are fully two-dimensional and the results are compared with earlier FEM calculations and conventional basis set expansion methods using the the R-matrix or S-matrix propagation.We made extensive analysis of FEM on the vector-computer Cyber 205 and developed a vector code for the efficient use in two dimensions, so that in the near future applications even in three dimensions will be possible.For the hydrogen exchange reactions we investigated the following isotope combinations: (a) H + H2, b) H + DH, D + HD and H + MuH (symmetric reaction), (c) D + HH, H + DD and Mu + DD (asymmetric reaction). We calculated the transition probabilities for up to five open vibrational channels and found excellent agreement with known exact values. 相似文献
973.
974.
Differential thermal analysis and thermogravimetry were used to evaluate the effect of some additives, such as CaSO4, CaCl2 and silica fume amorphous silica from ferrosilicon synthesis on the mechanism and kinetics of reactions occurring in fly
ash-Ca(OH)2 system. The accelerating role of these additives was demonstrated from the data relating to Ca(OH)2 consumption in hydrated pastes, determined by TG measurements.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
975.
The total synthesis of the polyenoyltetramic acid polycephalin C is described. Key steps of the synthesis include a double Swern oxidation, double Takai reaction and a double Stille reaction. In addition, the absolute stereochemistry of the ring junction has been determined by synthesis of both isomers and comparison of their CD spectra with natural polycephalin C. 相似文献
976.
The antioxidative effect of flavonols and their glycosides against the peroxidation of linoleic acid has been studied in homogeneous solution (tBuOH/H(2)O, 3:2) and in sodium dodecyl sulfate and cetyl trimethylammonium bromide micelles. The peroxidation was initiated thermally by the water-soluble initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride, and the reaction kinetics were studied by monitoring the formation of linoleic acid hydroperoxides. The synergistic antioxidant effect of the flavonols with alpha-tocopherol (vitamin E) was also studied by following the decay kinetics of alpha-tocopherol and the alpha-tocopheroxyl radical. Kinetic analysis of the antioxidative process demonstrates that the flavonols are effective antioxidants in solution and in micelles, either alone or in combination with alpha-tocopherol. The antioxidative action involves trapping the initiating radicals in solution or in the bulk-water phase of the micelles, trapping the propagating lipid peroxyl radicals on the surface of the micelles, and regenerating alpha-tocopherol by reducing the alpha-tocopheroxyl radical. It was found that the antioxidant activity of the flavonols and their glycosides depends significantly on the position and number of the hydroxy groups, the oxidation potential of the molecule, and the reaction medium. The flavonols bearing ortho-dihydroxy groups possess significantly higher antioxidative activity than those without such functionalities, and the glycosides are less active than their parent aglycones. The activity of the flavonols is higher in micelles than in solution, while the activity of alpha-tocopherol is lower in micelles than in solution. This is because the predominant factor for controlling the activity is the hydrogen-bonding interaction of the antioxidant with the micellar surface in the case of hydrophilic flavonols, while it is the inter- and intramicellar diffusion in the case of lipophilic alpha-tocopherol. 相似文献
977.
Thermodynamic and kinetic parameters have been established for the reaction between the carbon acid, 4-nitrophenylnitromethane,
(4-NPNM), and the base N’-n-propyl-N,N-di-n-propylbenzimidamide, (N’PDPBA), in mesitylene and in chlorobenzene. In some cases deuteron transfer from 4-(D2)NPNM to the base has also been studied. In addition, some results for the proton transfer reaction in tetrahydrofuran have
been collected. Spectrophotometric methods have been employed to monitor the ion-pair product, which is solvatochromic. In
general the solvent dependence of the parameters is as expected, but there is some indication of specific solvation. The kinetic
isotope effects of 11 and 8 in mesitylene and chlorobenzene, respectively, are larger than those predicted classically. However,
as is discussed the n-propyl group on the secondary nitrogen of the base may serve to reduce the extent of tunneling compared
to that in an unsubstituted analogue by a steric effect. 相似文献
978.
偶合反应化学发光法测定痕量银的研究——K4Fe(CN)6—Luminol体系 总被引:5,自引:0,他引:5
将Ag(I)催化K4Fe(CN)6的水合反应与Luminol同Cu(CN)^2-4的化学发光反应相偶合,建立起一种新的灵敏测定痕量银的化学发光分析法,在所研究的优化条件下,该法测定银的选择性较好,检测限达到4.0×10^-13g/mLAg线性浓度范围为1.0×10^-12~1.0×10^-6g/mLAg对4×10^-11g/mLAg(I)溶液连续11次测定的RSD≤3.5%,用于环境水和岩矿中银的 相似文献
979.
改性ZSM—5分子筛在甲苯一段硝化中的应用 总被引:1,自引:0,他引:1
对NaZSM-5分子筛进行改性,在其表面引入铁,镁离子,用作硝硫混酸对甲苯一段硝化反应的催化剂,明显提高产物中对硝基甲苯的生成比例,研究了影响反应结果的因素。 相似文献
980.
G. D. Kolomnikova T. O. Krylova I. V. Chernoglazova P. V. Petrovskii Yu. G. Gololobov 《Russian Chemical Bulletin》1993,42(7):1188-1190
In the presence of trifluoroacetic acid, ethyl 2-cyanoacrylate readily reacts with nucleophilic reagents, such as 2-chloro-1,3,2-benzodioxaphosphole, (EtO)3P, (EtO)2PCl, Ph2PCl, Ph3P, and thiourea. In these reactions the acid proton enters position 2 of the cyanoacrylate, whereas the nucleophilic component enters position 3, in accordance with the electron density distribution in the acrylate. In the absence of trifluoroacetic acid the above reagents, except 2-chloro-1,3,2-benzodioxaphosphole, cause ethyl 2-cyanoacrylate polymerization. The interaction of ethyl 2-cyanoacrylate with 2-chloro-1,3,2-benzodioxaphosphole and trifluoroacetic acid is the first example of a 2-cyanoacrylate taking part in the acid-initiated electrophilic conjugate addition of a weak nucleophile.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1245–1247, July, 1993. 相似文献