全文获取类型
收费全文 | 139365篇 |
免费 | 29928篇 |
国内免费 | 6240篇 |
专业分类
化学 | 123255篇 |
晶体学 | 1079篇 |
力学 | 6366篇 |
综合类 | 281篇 |
数学 | 17958篇 |
物理学 | 26594篇 |
出版年
2024年 | 289篇 |
2023年 | 5117篇 |
2022年 | 2858篇 |
2021年 | 4357篇 |
2020年 | 7674篇 |
2019年 | 6488篇 |
2018年 | 5248篇 |
2017年 | 3929篇 |
2016年 | 9978篇 |
2015年 | 9895篇 |
2014年 | 9750篇 |
2013年 | 11195篇 |
2012年 | 11730篇 |
2011年 | 8712篇 |
2010年 | 9506篇 |
2009年 | 8984篇 |
2008年 | 6105篇 |
2007年 | 5003篇 |
2006年 | 4059篇 |
2005年 | 7553篇 |
2004年 | 6504篇 |
2003年 | 4603篇 |
2002年 | 3605篇 |
2001年 | 2544篇 |
2000年 | 2023篇 |
1999年 | 1318篇 |
1998年 | 815篇 |
1997年 | 784篇 |
1996年 | 787篇 |
1995年 | 778篇 |
1994年 | 595篇 |
1993年 | 613篇 |
1992年 | 634篇 |
1991年 | 526篇 |
1990年 | 465篇 |
1989年 | 378篇 |
1988年 | 353篇 |
1987年 | 270篇 |
1986年 | 246篇 |
1985年 | 245篇 |
1977年 | 352篇 |
1976年 | 474篇 |
1975年 | 516篇 |
1974年 | 542篇 |
1973年 | 371篇 |
1972年 | 494篇 |
1971年 | 441篇 |
1970年 | 659篇 |
1969年 | 496篇 |
1968年 | 537篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
992.
J?rg Libuda 《Chemphyschem》2004,5(5):625-631
Reaction kinetics on nanometer-scale particles are different from those on extended surfaces of bulk materials. This fact has been utilized for a long time to empirically maximize the performance of heterogeneous catalysts, but the understanding of the underlying effects is poor at the microscopic level. Modern molecular beam-based methods, however, allow us to derive very detailed kinetic information on catalytically active surfaces. In combination with structurally highly controlled model catalysts, microscopic insights into the activity and selectivity of specific reaction centers on catalyst nanoparticles can be obtained. This combined approach is illustrated through simple model reactions. 相似文献
993.
A new fluorescent compound based on calix[4] arene skeleton was synthesized.Its complexation ability with transition metal ions,such as Fe^3 ,Co^2 ,Ni^2 ,Cu^2 ,Zn^2 and Ag^ ,Was investigated by UV-vis and fluorescent spectra. 相似文献
994.
Regulation of lithospermic acid B and shikonin production in Lithospermum erythrorhizon cell suspension cultures 总被引:6,自引:0,他引:6
Cell suspension cultures of Lithospermum erythrorhizon produced a large amount of lithospermic acid B, a caffeic acid tetramer, as well as shikonin derivatives (each ca. 10% of dry wt.) when cultured in shikonin production medium M-9. Various culture factors for increasing the production of lithospermic acid B were investigated. Lithospermic acid B production was inhibited by 2, 4-D or NH4+, whereas it was stimulated by Cu2+. These regulatory patterns were similar to those for the production of shikonin derivatives in these cell cultures, suggestive of close relations and similar metabolic regulation between the production of these compounds. Cultivation under light illumination, however, showed that these metabolisms were independently regulated. In particular, blue light showed a stimulatory effect on lithospermic acid B production, while shikonin production was strongly inhibited, indicative of an effective condition for lithospermic acid B production. 相似文献
995.
Bao Liu Ling Huang Juntao Liu Yao Zhong Xingshu Li Albert S.C. Chan 《Tetrahedron: Asymmetry》2007,18(24):2901-2904
Chiral tridentate N-tosylated aminoimine ligands were used in the Cu(II)-catalyzed enantioselective addition of phenylacetylene to N-aryl arylimines, affording products up to 92% ee. 相似文献
996.
离子色谱法同时测定水中六种阳离子 总被引:3,自引:0,他引:3
研究了用抑制电导离子色谱法一次进样同时测定水中Li^ ,Na^ , NH4^ ,Mg^2 ,Ca^2 的色谱法条件,相对标准偏差小于0.5%,加标回收率在97.5%-104.0%。在水质分析中运用t检验法,比较了用钙、镁离子浓度计算和传统方法测定的总硬度之间是否存在显著性差异。 相似文献
997.
A method is presented for the determination of dihydrostreptomycin in milk, muscle and kidney by reversed-phase ion-pair high-performance liquid chromatography and post-column derivatisation with beta-naphthoquinone-4-sulfonate prior to fluorescence detection. The new sample work-up procedures include acid precipitation of proteins and, in the case of muscle and kidney, removal of fats by solvent extraction followed by solid phase extraction on a cation exchanger. The fluorescence response was linear from 25 to 2000 micrograms l-1 of injected analyte. The detection limits were 10 micrograms kg-1 for milk and 15 micrograms kg-1 for muscle and kidney and the analyte recoveries were on average 93% for milk, 70% for kidney and 75% for muscle. 相似文献
998.
Li J Hong X Li D Zhao K Wang L Wang H Du Z Li J Bai Y Li T 《Chemical communications (Cambridge, England)》2004,(15):1740-1741
Highly luminescent water-soluble CdTe nanorods were prepared with the assistance of the mixed ligand system of cysteine and thioglycolic acid; the aspect ratio and photoluminescence of the CdTe nanorods could be controlled by the refluxing time. 相似文献
999.
Mycotoxins are secondary metabolites, formed by the action of fungi on agricultural crops in the field or during storage. These metabolites are highly toxic to animals and humans and high levels have been measured in agricultural crops. In order to evaluate human risks due to ingestion of mycotoxin-contaminated food different methods have been developed for analysis of mycotoxins in cereals and maize. In this project the focus was on mycotoxins in agricultural soil and the fate of these toxins in the soil-water-plant system. Two different mycotoxins were selected in the study: zearalenone (ZON) produced by species of Fusariumor Aspergillusand ochratoxin A (OTA) produced by species of Penicillium. We developed a method for analysis of these toxins in soil. Soil samples were extracted with methanol-water (9:1) and purified by solid-phase extraction (SPE, C8-columns). The final extract was analysed using high-pressure liquid chromatography (HPLC) with fluorescence detection. A Phenyl Hexyl column was used to separate the toxins. The detection limits obtained were 0.1 and 1.0 microg kg(-1) dry weight (dw) for OTA and ZON, respectively. The developed method has been used for analysis of different soils in connection with growth chamber experiments. The soil types used in the growth chamber experiments were a sandy soil, a sandy clay soil, and a soil with high content of organic matter. The recovery was determined as 85.8 and 93.4% and the repeatability to 5.1 and 12.8% for OTA and ZON, respectively. The reproducibility obtained was 8.5 and 15.0% for soil samples, representing concentration levels from 0.2-30 microg kg(-1) dw (OTA) and from 1.0-100 microg kg(-1) dw (ZON). 相似文献
1000.
S Çak?r E Co?kunP Naumov E Biçer? Bulut H ?çbudakO Çak?r 《Journal of Molecular Structure》2002,608(1):101-107
Aspartame adduct of copper(II) chloride Cu(Asp)2Cl2·2H2O (Asp=aspartame) is synthesized and characterized by elemental analysis, FT IR, UV/vis, ESR spectroscopies, TG, DTG, DTA measurements and molecular mechanics calculations. Aqueous solution of the green solid absorbs strongly at 774 and 367 nm. According to the FT IR spectra, the aspartame moiety coordinates to the copper(II) ion via its carboxylate ends, whereas the ammonium terminal groups give rise to hydrogen bonding network with the water, the chloride ions or neighboring carboxylate groups. The results suggest tetragonally distorted octahedral environment of the copper ions. 相似文献