全文获取类型
收费全文 | 1212篇 |
免费 | 40篇 |
专业分类
化学 | 736篇 |
晶体学 | 7篇 |
力学 | 26篇 |
数学 | 216篇 |
物理学 | 267篇 |
出版年
2023年 | 10篇 |
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 17篇 |
2019年 | 32篇 |
2018年 | 34篇 |
2017年 | 20篇 |
2016年 | 43篇 |
2015年 | 48篇 |
2014年 | 39篇 |
2013年 | 75篇 |
2012年 | 88篇 |
2011年 | 79篇 |
2010年 | 52篇 |
2009年 | 59篇 |
2008年 | 70篇 |
2007年 | 74篇 |
2006年 | 59篇 |
2005年 | 58篇 |
2004年 | 32篇 |
2003年 | 21篇 |
2002年 | 31篇 |
2001年 | 26篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 16篇 |
1993年 | 21篇 |
1992年 | 15篇 |
1990年 | 6篇 |
1988年 | 9篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1967年 | 4篇 |
1966年 | 7篇 |
1965年 | 5篇 |
1964年 | 9篇 |
1963年 | 6篇 |
排序方式: 共有1252条查询结果,搜索用时 218 毫秒
31.
do Nascimento Filho I Schossler P Santos Freitas L Melecchi MI Rodrigues Vale MG Bastos Caramão E 《Journal of chromatography. A》2004,1027(1-2):167-170
In this work a simple method was described for selective extraction of benzoic acid from landfill leachate samples. The samples were submitted to solid-phase extraction (SPE) with XAD-4 resin as the stationary phase and ion-exchange chromatography (IEC) using the ion-exchange resin Amberlyst A-27. The instrumental analysis was performed by gas chromatography with mass spectrometric detection (GC-MSD). Benzoic acid was isolated, identified and quantified. The extraction process is rapid, simple and of low cost. It was also environmental friendly, that is, it was used a minimum amounts of hazardous organic solvents and produced also minimum quantities of residues. 相似文献
32.
Luís C. Duarte Florbela Carvalheiro Inês Neves Francisco M. Gírio 《Applied biochemistry and biotechnology》2005,121(1-3):413-425
Debaryomyces hansenii is a polyol overproducing yeast that can have a potential use for upgrading lignocellulosic hydrolysates. Therefore, the establishment of its tolerance to metabolic inhibitors found in hydrolysates is of major interest. We studied the effects of selected aliphatic acids, phenolic compounds, and furfural. Acetic acid favored biomass production for concentrations <6.0 g/L. Formic acid was more toxic than acetic acid and induced xylitol accumulation (maximum yield of 0.21 g/g of xylose). All tested phenolics strongly decreased the specific growth rate. Increased toxicity was found for hydroquinone, syringaldehyde, and 4-methylcatechol and was correlated to the compound’s hydrophobicity. Increasing the amount of furfural led to longer lag phases and had a detrimental effect on specific growth rate and biomass productivity. 相似文献
33.
The kinetics of the reaction NH2 + NO → N2 + H2O were studied, using a conventional flash photolysis system. A value of k1 = (1.1 ± 0.2) × 1010 & mole?1 s?1 was obtained at room temperature and in the pressure range 2–700 torr in the presence of nitrogen. A slight negative temperature coefficient was observed between 300 and 500 K, equivalent to a negative activation energy of 1.05 ± 0.2 kcal mole?1. 相似文献
34.
Cristina Tealdi Lorenzo Malavasi Giorgio Flor 《Journal of solid state chemistry》2004,177(11):4359-4367
Atomistic computer simulation techniques have been used, for the first time, to reproduce the crystal structure of MgTa2O6 and to investigate the defect chemistry and dopant properties of this material. The calculated defect energetics suggest that the concentration of intrinsic atomic defects in this phase is insignificant and that the system is probably stable to both oxidation and reduction. Dopant solution energy versus ion size trends are found for both isovalent and aliovalent dopant incorporation at Mg and Ta sites. Divalent dopants (e.g. Ca, Cu) preferentially occupy the Mg site whereas dopants with higher charge (e.g. Sc, Zr, Nb) are more favorable on the Ta site. High migration activation energies (>2 eV) predict limited ionic conductivity in this material. 相似文献
35.
Purification of plasmid DNA vectors by aqueous two-phase extraction and hydrophobic interaction chromatography 总被引:4,自引:0,他引:4
The current study explores the possibility of using a polyethyleneglycol(PEG)-ammonium sulphate aqueous two-phase system (ATPS) as an early step in a process for the purification of a model 6.1 kbp plasmid DNA (pDNA) vector. Neutralised alkaline lysates were fed directly to ATPS. Conditions were selected to direct pDNA towards the salt-rich bottom phase, so that this stream could be subsequently processed by hydrophobic interaction chromatography (HIC). Screening of the best conditions for ATPS extraction was performed using three PEG molecular weights (300, 400 and 600) and varying the tie-line length, phase volume ratio and lysate load. For a 20% (w/w) lysate load, the best results were obtained with PEG 600 using the shortest tie-line (38.16%, w/w). By further manipulating the system composition along this tie-line in order to obtain a top/bottom phase volume ratio of 9.3 (35%, w/w PEG 600, 6%, w/w NH4)2 SO4), it was possible to recover 100% of pDNA in the bottom phase with a three-fold increase in concentration. Further increase in the lysate load up to 40% (w/w) with this system resulted in a eight-fold increase in pDNA concentration, but with a yield loss of 15%. The ATPS extraction was integrated with HIC and the overall process compared with a previously defined process that uses sequential precipitations with iso-propanol and ammonium sulphate prior to HIC. Although the final yield is lower in the ATPS-based process the purity grade of the final pDNA product is higher. This shows that it is possible to substitute the time-consuming two-step precipitation procedure by a simple ATPS extraction. 相似文献
36.
37.
38.
(Z)-Ethyl-3-perfluoroalkyl-3-magnesiated crotonates were prepared from (Z)-ethyl-3-perfluoroalkyl-3-iodo enoates by iodine-magnesium exchange reaction with isopropylmagnesium bromide in THF at -78 degrees C. These new reagents reacted with a range of electrophiles, leading to polyfunctional products bearing a perfluoroalkyl group. 相似文献
39.
Inês Nunes Nuno Mesquita Sandra Cabo Verde Maria João Trigo Armando Ferreira Maria Manuela Carolino António Portugal Maria Luísa Botelho 《Radiation Physics and Chemistry》2012,81(12):1943-1946
Parchments are important documents that give testimony for History; therefore these materials should be respected and preserved. Considering incremental biodeterioration problems that have to be faced daily, the Archive of the University of Coimbra (AUC) is involved in different scientific projects in order to evaluate and determine new methods for document decontamination and preservation.The aim of this study was to evaluate gamma radiation effects on the colour and texture of the AUC parchment documents. The assessment of these effects was used to estimate the maximum gamma radiation dose (Dmax) that could guarantee parchment documents′ decontamination treatment, without significant alteration of their physical properties. Parchment samples were exposed to gamma radiation doses ranging from 10 to 30 kGy. The texture and colour of samples were assessed before and after the irradiation procedure, using a texture analyser and an electronic colorimeter. Hardness and springiness were determined based on texture spectra. Lightness (L?), Chroma (C), greenness vs. redness (a*) and yellowness vs. blueness (b*) values were obtained from colorimetric measures. Results indicate no significant effects of gamma radiation on the texture and colour of parchment for the studied doses. 相似文献