全文获取类型
收费全文 | 1152篇 |
免费 | 47篇 |
国内免费 | 7篇 |
专业分类
化学 | 783篇 |
晶体学 | 19篇 |
力学 | 60篇 |
数学 | 135篇 |
物理学 | 209篇 |
出版年
2023年 | 20篇 |
2022年 | 18篇 |
2021年 | 55篇 |
2020年 | 39篇 |
2019年 | 47篇 |
2018年 | 45篇 |
2017年 | 30篇 |
2016年 | 66篇 |
2015年 | 35篇 |
2014年 | 38篇 |
2013年 | 69篇 |
2012年 | 74篇 |
2011年 | 83篇 |
2010年 | 53篇 |
2009年 | 57篇 |
2008年 | 51篇 |
2007年 | 49篇 |
2006年 | 41篇 |
2005年 | 34篇 |
2004年 | 37篇 |
2003年 | 17篇 |
2002年 | 20篇 |
2001年 | 9篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 6篇 |
1995年 | 7篇 |
1994年 | 6篇 |
1993年 | 16篇 |
1992年 | 9篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1976年 | 3篇 |
1973年 | 3篇 |
1965年 | 3篇 |
1959年 | 2篇 |
排序方式: 共有1206条查询结果,搜索用时 156 毫秒
21.
An extraction‐spectrophotometric method for the determination of trace amounts of copper and cobalt based on their extraction into n‐pentanol with 0. carboxyphenylhydrazoethylacetoacetate (O.CPHEAA) was per formed. Copper was extracted from pH 6.0 – 8.0 and ionic strength 0.5 M – KCl. The maximum absorption of the extracted Cu(II) ‐ O.CPHEAA complex (1:1 & 1:2 species) occurs at 415 nm. The proposed method succeeded in as saying a concentration of 3–63 μg per 10 mL of n‐pentanol (? = 1.25 × 104L mol?1 cm?1). The method failed to ex tract cobalt ion into various organic sol vents over a pH range of 2–11. The suggested method is highly selective and sensitive according to a wide scheme of interference studied. Copper in some plant samples was accurately estimated using the suggested method. The obtained results and the results of the AAS method were consistent. The reproducibility test shows a relative standard deviation of 1%. Sandell sensitivity for A = 0.001 is 5 × 10?3 μg cm?2. 相似文献
22.
Ya‐Ching Shen Yun‐Sheng Lin Shaw‐Man Hsu Ashraf Taha Khalil Shih‐Sheng Wang Ching‐Te Chien Yao‐Haur Kuo Chang‐Hung Chou 《Helvetica chimica acta》2007,90(7):1319-1329
The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells. 相似文献
23.
Catalysis of mixed oxide LaMnO3 was studied for the decomposition of hydrogen peroxide (H2O2). The catalyst was -irradiated in open petri dishes, vacuum, dry oxygen and moist oxygen. LaMnO3 irradiated in moist oxygen showed highest catalytic activity. X-ray photoelectron spectroscopic (XPS) studies were carried out to investigate the surface modifications occurred during -irradiaiton of LaMnO3. No significant change in the surface was noticed in LaMnO3 irradiated in vacuum and dry oxygen. However, LaMnO3 irradiated in moist oxygen and in open petri dishes showed the reduction of transition metal (MN3+ to Mn2+) which in turn leads to the formation of chemisorbed superoxide ions (O
2
–
) and surface carbonate species (CO
3
2–
). The latter processes decreases the electrical conductivity by trapping the charge carriers. The hydrated electron generated by the radiolysis of moisture reduces the transition metal. A qualitative molecular orbital model has been proposed for the chemisorption of O
2
–
on the reduced transition metal centers (Mn2+). 相似文献
24.
Saxena C Wang H Kavakli IH Sancar A Zhong D 《Journal of the American Chemical Society》2005,127(22):7984-7985
In this communication, we report the ultrafast dynamics of resonance energy transfer in a blue-light photoreceptor, Vibrio cholerae cryptochrome. The transfer was observed to occur in 60 ps. We also studied the local rigidity and solvation around the binding site of the photoantenna molecule. The results for the first time show energy transfer in cryptochrome suggesting some mechanistic similarities between photolyase that repairs damaged DNA and cryptochrome that mediates blue-light signaling. 相似文献
25.
Balkis Al‐Saleh Morsy Ahmed El‐Apasery Rasha Safwat Abdel‐Aziz Mohamed Hilmy Elnagdi 《Journal of heterocyclic chemistry》2005,42(4):563-566
The 3‐anilinoenones 3a,b were prepared from the corresponding 3‐dimemyl‐aminopropenones. The reactivity of 3a,b towards a variety of carbon and nitrogen nucleophiles as well as naphthoquinones is reported. 相似文献
26.
The strong affinity of natural ionic polysaccharides for certain metal ions is exploited in the design of a new class of voltammetric sensing devices. In particular, carbon-paste electrodes containing pectic and alginic acids are used for the nonelectrolytic collection and subsequent voltammetric determination of copper and lead, respectively. Cyclic and differential pulse voltammetry are used to quantify the accumulated ions. The response is characterized with respect to modifier loading, preconcentration period, metal concentration, reproducibility, possible interferences and other variables. Titrimetric experiments illustrate the potential of polysaccharide electrodes for speciation work. Preliminary data are also given for analogous measurements of copper at heparin-modified electrodes. Detection limits are 1 mug/ml and the relative standard deviation is 4.8%. 相似文献
27.
Acid dissociation constants of acetoacetanilide abbreviated (AAA); Orthocarboxy-phenylhydrazoacetoacetanilide (o-CPHAAA); and orthocarboxy-phenylhydrazo-parachloro-acetoacetanilide (o-CPH-p-Cl-AAA) and chelate stability constants of the corresponding anions with Cu+2, Ni+2, CO+2, Zn+2, Mn+2, and Cd+2 ions are reported. The infra-red spectra of o-CPHAAA and o-CPH-p-Cl-AAA ligands and their chelates for copper and nickel are discussed. 相似文献
28.
Abdel‐Zaher Abdel‐Aziz Elassar 《中国化学会会志》2006,53(4):901-907
Methylquinoxaline derivative 1 undergoes bromination to give bromomethyl quinoxaline 2 , which could be transferred to cyanomethyl quinoxaline 3 . The latter compound 3 readily coupled with arene diazonium salts to give hydrazone derivatives 5a‐c . Compounds 5a,b reacted with active methylene reagents to give the target ring system, quinoxalinylpyridazine derivatives 8a‐c . Compound 1 reacted with DMFDMA to give the enamine derivative 9 , which coupled with arene diazonium salts to give the aldehydic hydrazone derivatives 10a‐c . Compound 10 reacted with active methylene compounds to give quinoxalinylpyridazine derivatives. UV characterization of some of the prepared compounds was reported. 相似文献
29.
Christelle Titier Jean-Pierre Pascault Mohamed Taha Boris Rozenberg 《Journal of polymer science. Part A, Polymer chemistry》1995,33(1):175-184
Epoxy-amine methacrylated prepolymers are prepared in a 1-step synthesis by the reaction of Bisphenol A diglycidyl ether, glycidyl methacrylate, and different diamines. The adaptation of these reactions to production processes with a reduced reaction time requires a kinetic control and the use of efficient catalysts. The comparative kinetic treatment of the epoxy-amine reaction in the frame of Horie's and Rozenberg's kinetic schemes have been made. Cases where an efficient acid catalyst is used are also examined. Structural studies of these reactions established the occurrence of a Michael amine/double bond addition, when some amines are used. © 1995 John Wiley & Sons, Inc. 相似文献
30.
A. A. Abd El Raheem Abdel Aziz Amin A. S. Moustafa 《Fresenius' Journal of Analytical Chemistry》1960,172(5):347-356
Summary Omega Chrome Fast Blue 2G has been found to be a suitable indicator for the detection of the equivalence point of calcium, magnesium and manganese in the presence of cadmium, nickel, and aluminium, when they are masked by the addition of cyanide or triethanolamine. It is also a suitable indicator for the back titration of aluminium using calcium or magnesium as a back titrant.This dye has also been found to be a suitable reagent for the colorimetric determination of calcium, magnesium, manganese and thorium. 相似文献