全文获取类型
收费全文 | 7915篇 |
免费 | 1359篇 |
国内免费 | 963篇 |
专业分类
化学 | 5699篇 |
晶体学 | 120篇 |
力学 | 445篇 |
综合类 | 53篇 |
数学 | 1142篇 |
物理学 | 2778篇 |
出版年
2024年 | 18篇 |
2023年 | 167篇 |
2022年 | 266篇 |
2021年 | 276篇 |
2020年 | 302篇 |
2019年 | 296篇 |
2018年 | 250篇 |
2017年 | 260篇 |
2016年 | 422篇 |
2015年 | 404篇 |
2014年 | 535篇 |
2013年 | 590篇 |
2012年 | 752篇 |
2011年 | 770篇 |
2010年 | 503篇 |
2009年 | 455篇 |
2008年 | 510篇 |
2007年 | 400篇 |
2006年 | 409篇 |
2005年 | 373篇 |
2004年 | 324篇 |
2003年 | 240篇 |
2002年 | 225篇 |
2001年 | 186篇 |
2000年 | 127篇 |
1999年 | 186篇 |
1998年 | 124篇 |
1997年 | 95篇 |
1996年 | 112篇 |
1995年 | 113篇 |
1994年 | 93篇 |
1993年 | 58篇 |
1992年 | 58篇 |
1991年 | 60篇 |
1990年 | 68篇 |
1989年 | 45篇 |
1988年 | 29篇 |
1987年 | 23篇 |
1986年 | 29篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1983年 | 14篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1979年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1928年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Isoproterenol (ISO) is a synthetic catecholamine with a powerful cardiac stimulate, bronchial smooth and skeletal muscle relaxation, coronary artery and peripheral vasodilator effects. In this study, a simple, rapid, and selective liquid chromatography–tandem mass spectrometry has been developed for the determination of ISO in rat plasma. Proteins were precipitated in plasma samples with 0.1?g?mL?1 trichloroacetic acid and the ISO/internal standard (IS) were separated on a C8 column. The ISO was detected by a triple-quadrupole mass spectrometer with electrospray ionization source. The transitions of m/z 212.1?→?193.9 for ISO and m/z 285.2?→?193.2 for IS were conducted through multiple reaction monitoring mode. This method was linear over the range of 2–500?n?g?mL?1. The intra- and inter-day assay precision were observed between 3 and 10%. The accuracy was within ±11%. Stability study results demonstrated that ISO was stable in the autosampler at 4°C for 24?hr, and for 8?hr at room temperature. Only one freeze–thaw cycle at ?80°C for 24?hr was found to be stable. The validated method was successfully applied to a pharmacokinetic study of ISO in rats following subcutaneous administration. 相似文献
942.
当前对股利政策的研究主要集中在产权对股利政策的影响、自由现金流对股利政策的影响、公司过渡投资行为对股利政策的影响以及宏观经济波动对股利政策的影响等,而缺乏控股股东对股利政策的影响研究.现实中,控股股东对股利政策具有较大的影响,围绕控股股东对上市公司股利分配倾向的影响进行深入分析.具体而言,以2013-2014年度上证A股数据,建立Log斌ic模型和多元线性回归模型分析控股股东的几方面特征对股利分配倾向产生的多种影响.研究从理论上客观地评价了控股股东对股利分配倾向的影响,现实中对维护中小股东权益有重要意义. 相似文献
943.
Sufficient dimension reduction (SDR) is a paradigm for reducing the dimension of the predictors without losing regression information. Most SDR methods require inverting the covariance matrix of the predictors. This hinders their use in the analysis of contemporary datasets where the number of predictors exceeds the available sample size and the predictors are highly correlated. To this end, by incorporating the seeded SDR idea and the sequential dimension-reduction framework, we propose a SDR method for high-dimensional data with correlated predictors. The performance of the proposed method is studied via extensive simulations. To demonstrate its use, an application to microarray gene expression data where the response is the production rate of riboflavin (vitamin B2) is presented. 相似文献
944.
Wang Jiajia Yin Haixia Chen Jingshuai Niu Helin Mao Changjie Song Jiming Jin Baokang Tian Yupeng Zhang Shengyi 《Journal of Solid State Electrochemistry》2017,21(8):2381-2391
Journal of Solid State Electrochemistry - A hydrophilic polypyrrole/tellurium (PPy/Te) nanocomposite was synthesized by a facile approach in which H2TeO3 was used as the oxidizing agent of pyrrole... 相似文献
945.
The interactions between AgnO-(n=1-8) and H2 (or D2) were explored by combination of the mass spectroscopy experiments and density function theory (DFT) calculations. The experiments found that all oxygen atoms in AgnO-(n=1-8) are inert in the interactions with H2 or D2 at the low temperature of 150 K, which is in contrast to their high reactivity with CO under the same condition. These observations are parallel with the preferential oxidation (PROX) of CO in excess hydrogen catalyzed by dispersed silver species in the condensed phase. Possible reaction paths between AgnO-(n=1-8) and H2 were explored using DFT calculations. The results indicated that adsorption of H2 on any site of AgnO-(n=1-8) is extremely weak, and oxidation of H2 by any kind of oxygen in AgnO-(n=1-8) has an apparent barrier strongly dependent on the adsorption style of the "O". These experiments and theoretical results about cluster reactions provided molecule-level insights into the activity of atomic oxygen on real silver catalysts. 相似文献
946.
Human serum albumin (HSA) is the most abundant plasma protein and has an inherent ability to target tumor cells. It is an excellent candidate for drug delivery. However, HSA cannot form complex with DNA or RNA, because it is negatively charged under physiological conditions. In this work, we reported a simple method to prepare HSA/RNA nanoparticles mainly by physical interaction. Firstly, the solution pH is adjusted to 4.0, under which condition HSA is positively charged. It forms complex with RNA via electrostatic interaction. The solution is then heated at 75 oC for 15 min to stabilize the structure and the size of the formed complex. The HSA/RNA nanoparticle prepared by this method has a diameter about 110 nm and a narrow distribution. It is also stable for days under physiological conditions. Cellular essays demonstrate that these particles exhibit a high cellular uptake efficiency and non-toxicity to HeLa cells. 相似文献
947.
Gui-Fang Qin Qiong-Yun Qin Bing-Fan Long Dong-Ping Wei Yan-Hui Xu Shan-Ju Bao Xian-Hong Yin 《Journal of the Iranian Chemical Society》2017,14(6):1227-1234
Copper (Cu)(II) complexes were synthesized by Ind-3-COOH combined with N-containing auxiliary ligands via a combinatorial strategy involving hydrothermal and solvent-evaporation method. The synthesized complexes had the following formulas: [Cu(Ind-3-COO)2] (1), [Cu(PHEN)(Ind-3-COO)]2·2H2O (2), [Cu2(DPP)(Ind-3-COO)2(H2O)]·H2O, (3) and [Cu(BPY)(Ind-3-COO)2]·4H2O (4). Meanwhile, the symbol abbreviations were listed as follows: Ind-3-COOH = 1H-indazole-3-carboxylic acid, 1,3-bis(4-pyridyl)propane (DPP), 1,10-phenanthroline (PHEN) and 4,4′-bipyridine (BPY). The crystalline structure and spectroscopy of each complex were characterized by single-crystal X-ray diffraction, elemental analysis, Fourier transform infrared spectroscopy and powder X-ray diffraction. The redox reactions in the complexes were then investigated by performing cyclic voltammetry under nitrogen conditions at room temperature. Two pairs of distinctive irreversible reduction potentials were identified, which could be attributed to the processes of Cu(II)–Cu(I) and Cu(I)–Cu(0). 相似文献
948.
Ya-sheng Huang Qi-qi Yu Yin Chen Min-jie Cheng Li-ping Xie 《Chemistry Central journal》2017,11(1):120
Background
The plant Alisma plantago-aquatica Linnaeus, which is widely distributed in southwest of China, is the main material of traditional Chinese medicine “Zexie”. It was used as folk medicine for immune-modulation, anti-tumor, anti-inflammatory and antibacterial. Previous chemical studies on A. plantago-aquatica reported the identification of triterpenes, diterpenes, sesquiterpenes, steroids, alkaloids and phenolic acid. Terpenes and phenolic acid were regard as major secondary metabolites from this medicine plant.Results
A new phenolic acid, plantain A (1), along with four known compounds (2–5) were isolated and identified from A. plantago-aquatica by extensive chromatographic and spectrometric methods. In the present study, the levels of TNF-α, IL-1β, COX-2, PEG2 and TGF-β1 were increased in model group rats, whereas on treatment with the isolated compound (1 and 4) at 50 mg/kg, there was a significant decrease in the cytokine levels. Therefore, the anti-CNP effect of 1 and 4 may be related to their anti-inflammatory properties.Conclusions
A new phenolic acid and four known phenolic compounds were isolated from A. plantago-aquatica. Moreover, compounds 1 and 4 shows significant anti-chronic prostatitis activity in rats.949.
In this paper, in terms of the Fréchet normal cone, we establish exact separation results for finitely many disjoint closed sets in an Asplund space, which supplement the extremal principle and some fuzzy separation theorems. As an application, we provide a new optimality condition for a constraint optimization problem in terms of Fréchet subdifferential and Fréchet normal cone. 相似文献
950.
In this paper, we study a nematic liquid crystals system in three-dimensional whole space ?3 and obtain the time decay rates of the higher-order spatial derivatives of the solution by the method of spectral analysis and energy estimates if the initial data belongs to L1?3 additionally. 相似文献