全文获取类型
收费全文 | 3655篇 |
免费 | 77篇 |
国内免费 | 22篇 |
专业分类
化学 | 2607篇 |
晶体学 | 30篇 |
力学 | 94篇 |
数学 | 486篇 |
物理学 | 537篇 |
出版年
2022年 | 18篇 |
2021年 | 27篇 |
2020年 | 33篇 |
2019年 | 37篇 |
2018年 | 28篇 |
2017年 | 30篇 |
2016年 | 64篇 |
2015年 | 58篇 |
2014年 | 62篇 |
2013年 | 187篇 |
2012年 | 175篇 |
2011年 | 217篇 |
2010年 | 103篇 |
2009年 | 82篇 |
2008年 | 212篇 |
2007年 | 217篇 |
2006年 | 191篇 |
2005年 | 179篇 |
2004年 | 184篇 |
2003年 | 146篇 |
2002年 | 139篇 |
2001年 | 39篇 |
2000年 | 36篇 |
1999年 | 38篇 |
1998年 | 39篇 |
1997年 | 45篇 |
1996年 | 57篇 |
1995年 | 52篇 |
1994年 | 54篇 |
1993年 | 42篇 |
1992年 | 35篇 |
1991年 | 34篇 |
1990年 | 44篇 |
1989年 | 35篇 |
1988年 | 46篇 |
1987年 | 45篇 |
1986年 | 41篇 |
1985年 | 76篇 |
1984年 | 55篇 |
1983年 | 54篇 |
1982年 | 66篇 |
1981年 | 56篇 |
1980年 | 61篇 |
1979年 | 41篇 |
1978年 | 37篇 |
1977年 | 41篇 |
1976年 | 26篇 |
1975年 | 27篇 |
1974年 | 38篇 |
1973年 | 32篇 |
排序方式: 共有3754条查询结果,搜索用时 15 毫秒
1.
2.
Ingrid Lukkari Jaromir Ruzicka Gary D. Christian 《Fresenius' Journal of Analytical Chemistry》1993,346(6-9):813-818
Summary Automated methods for the determination of ammonium and ammonia are reviewed, and techniques based on gas diffusion using a semi-permeable membrane were selected for the determination of both total ammonium-nitrogen and free ammonia in fermentation samples. A simple and robust instrument based on sequential injection analysis (SIA) consisting of a piston pump and two selector valves was used. Two different methods of ammonia detection, the Berthelot method and detection using pH-indicators, have been evaluated and compared. The indicator method showed higher reproducibility and the range of determination could easily be adjusted to match the concentrations of the samples. The range of determination could be adjusted from 0.05 mmol/l to 350 mmol/l, depending on choice of acceptor solution. This method has been evaluated with fermentation medium samples and tested on-line in a yeast fermentation process.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday 相似文献
3.
Polyphysa peniculus was grown in artificial seawater in the presence of arsenate, arsenite, monomethylarsonate and dimethylarsinic acid. The separation and identification of some of the arsenic species produced in the cells as well as in the growth medium were achieved by using hydride generation–gas chromatography–atomic absorption spectrometry methodology. Arsenite and dimethylarsinate were detected following incubation with arsenate. When the alga was treated with arsenite, dimethylarsinate was the major metabolite in the cells and in the growth medium; trace amounts of monomethylarsonate were also detected in the cells. With monomethylarsonate as a substrate, the metabolite is dimethylarsinate. Polyphysa peniculus did not metabolize dimethylarsinic acid when it was used as a substrate. Significant amounts of more complex arsenic species, such as arsenosungars, were not observed in the cells or medium on the evidence of flow injection–microwave digestion–hydride generation–atomic absorption spectrometry methodology. Transfer of the exposed cells to fresh medium caused release of most cell–associated arsenicals to the surrounding environment. 相似文献
4.
5.
Gary D. Jaycox 《Journal of polymer science. Part A, Polymer chemistry》2006,44(1):207-218
Azobenzene‐modified polyesters and poly(ester amide)s fitted with chiral, atropisomeric binaphthylene segments were prepared by a series of low‐temperature polycondensation reactions carried out in polar solvent media. When compared with their polyaramide counterparts studied earlier, these materials had significantly improved solubility behaviors and were readily dissolved by a wide range of organic solvents. In solution, each of these constructs underwent photoinduced oscillations in optical rotatory power when subjected to multiple UV‐light/visible‐light illumination cycles that drove trans?cis isomerization reactions along their polymer chains. Light‐regulated chiroptical perturbations were dependent on polymer backbone structures and were further modulated by well‐coordinated temperature fluctuations and by the nature of the solvent medium employed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 207–218, 2006 相似文献
6.
7.
Gary W. Schnuelle S. Swaminathan David L. Beveridge 《Theoretical chemistry accounts》1978,48(1):17-27
A theoretical study of ion hydration using the statistical thermodynamic supermolecule-continuum method is described. The cell and shell methods are used for configurational averaging. Enthalpies, free energies and entropies are calculated for Li+, Na+, K+, F– and Cl– each four coordinated with water. The results are in reasonable accord with experiment. A comparison of the site method, cell method and shell method results is presented. The supermolecule-continuum approach to solvent effects seems to be capable of accommodating essential features for the calculation of solvation energy and solvent effects on structure and properties. 相似文献
8.
Walter K. Dean Gary L. Simon Paul M. Treichel Lawrence F. Dahl 《Journal of organometallic chemistry》1973,50(1):193-207
The accidental but intriguing synthesis of acetatobis(triphenylphosphine)dicarbonylmanganese(I), (CH3CO2)Mn(CO)2[P(C6H5)3]2, has been accomplished by the reaction of NaMn(CO)5 with (CH3)3SiCl followed by the addition of triphenylphosphine and acetic acid. A three-dimensional single-crystal X-ray diffraction analysis has shown an octahedral-like molecule containing a symmetrically oxygen-chelating acetate group, the first such group to be reported in a metal carbonyl complex. The two triphenylphosphine ligands occupy mutually trans positions with the two carbonyl ligands possessing the remaining cis sites in the octahedral complex. The compound crystallizes with four molecules in a monoclinic unit cell of space group symmetry and of dimensions a = 17.744(2) Å, b = 9.692(1) Å, c = 20.004(2) Å, and β = 106.195(4)°. The relatively long MnO(acetate) bond lengths [2.066(6) and 2.069(7) Å] and the relatively short MnCO bond lengths [1.701(12) and 1.760(13) Å] and the relatively short MnP(C6H5)3 bond lengths [2.260(3) and 2.275(3) Å], compared to the corresponding MnCO and MnP(C6H5)3 bond lengths in other manganese carbonyl triphenylphosphine complexes, are rationalized on the basis that the acetate ligand in this molecule functions primarily as a σ-donor. 相似文献
9.
The application of parallel processing techniques to molecular mechanics calculations is evaluated. Using the standard molecular mechanics package, MM2, four different parallel versions of the program are implemented in a four-processor computing environment. A set of 529 test structures is used to compare the efficiency of the parallel versions of MM2 to a standard serial version of the program. Statistics describing execution times and program execution cycles are gathered and analyzed. The effects of parallel processing overhead and computer system load are explored, and the practical utility of parallel processing in molecular mechanics is estimated. The results of these parallelization experiments indicate that for geometry optimizations requiring significant amounts of computing time an improvement in program execution speed approaching 50% is realizable. © 1993 John Wiley & Sons, Inc. 相似文献
10.
The enol of acetone, formed by disproportionation reactions of 1-hydroxy-1-methylethyl radicals, is detected by NMR, spectroscopy during photoreactions of 3-hydroxy-3-methyl-2-butanone in acetonitrile and of acetone in 2-propanol and slowly tautomerizes to acetone. The photolysis of 3-hydroxy-3-methyl-2-butanone is shown to proceed via Type I cleavage, predominantly from an excited triplet state. 相似文献