全文获取类型
收费全文 | 14745篇 |
免费 | 962篇 |
国内免费 | 858篇 |
专业分类
化学 | 10527篇 |
晶体学 | 45篇 |
力学 | 2489篇 |
综合类 | 110篇 |
数学 | 1004篇 |
物理学 | 2390篇 |
出版年
2024年 | 13篇 |
2023年 | 200篇 |
2022年 | 541篇 |
2021年 | 398篇 |
2020年 | 394篇 |
2019年 | 395篇 |
2018年 | 406篇 |
2017年 | 566篇 |
2016年 | 669篇 |
2015年 | 572篇 |
2014年 | 608篇 |
2013年 | 957篇 |
2012年 | 993篇 |
2011年 | 950篇 |
2010年 | 763篇 |
2009年 | 926篇 |
2008年 | 761篇 |
2007年 | 878篇 |
2006年 | 687篇 |
2005年 | 727篇 |
2004年 | 566篇 |
2003年 | 450篇 |
2002年 | 388篇 |
2001年 | 324篇 |
2000年 | 252篇 |
1999年 | 238篇 |
1998年 | 206篇 |
1997年 | 199篇 |
1996年 | 177篇 |
1995年 | 193篇 |
1994年 | 145篇 |
1993年 | 137篇 |
1992年 | 114篇 |
1991年 | 114篇 |
1990年 | 111篇 |
1989年 | 80篇 |
1988年 | 86篇 |
1987年 | 83篇 |
1986年 | 69篇 |
1985年 | 46篇 |
1984年 | 44篇 |
1983年 | 18篇 |
1982年 | 53篇 |
1981年 | 17篇 |
1980年 | 9篇 |
1979年 | 10篇 |
1978年 | 15篇 |
1976年 | 3篇 |
1971年 | 4篇 |
1957年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Hasalettin Deligöz 《Journal of inclusion phenomena and macrocyclic chemistry》2006,55(3-4):197-218
Azocalixarenes, which have a conjugated chromophore azo (–N=N–) group in p-positions, are synthesized in “one-pot” procedures in satisfactory yields. Their structures in the solid and solution are elucidated by UV–vis, FT-IR, 1H and 13C NMR spectroscopic methods as well as elemental analysis techniques. Some of them are complexed with metal ions (alkali, alkaline-earth and transition metal ions) under neutral conditions. A wide variety of applications is expected by the functionalization of the side arms. The selective liquid–liquid extraction of various alkali, alkaline-earth and transition metal cations from the aqueous phase to the organic phase is carried out by using azocalixarenes. Furthermore, the synthesized azocalixarenes are utilized for selective extraction of Fe3+ cations from the aqueous phase to the organic phase. The effects of varying pH and solvent upon the absorption ability of azocalixarenes substituted with electron-donating and electron-withdrawing groups at their o-, m-, p-positions are examined. The thermal behaviours of azocalixarenes containing upper rim functionalized groups are investigated by means of thermogravimetry (TG), differential thermal analysis (DTA) and differential thermogravimetry (DTG). 相似文献
52.
Peter Hald Martin Bremholm Jacques Chevallier Bo B. Iversen 《Journal of solid state chemistry》2006,179(8):2674-2680
Highly crystalline anatase TiO2 nanoparticles have been synthesised in less than 1 min in a supercritical propanol-water mixture using a continuous flow reactor. The synthesis parameter space (T, P, concentration) has been explored and the average particle size can be accurately controlled within 10-18 nm with narrow size distributions (2-3 nm). At subcritical conditions amorphous products are obtained, whereas a broad range of T and P in the supercritical regime gives 11-14 nm particles. At high temperature and pressure, the particles size increase to 18 nm. The nanoparticles have been extensively characterised with powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) with excellent agreement on size and size distribution parameters. The SAXS analysis suggests disk-shaped particles with diameters that are approximately double the height. For comparison, a series of conventional autoclave sol-gel syntheses have been carried out. These also produce phase-pure anatase nanoparticles, but with much broader size distributions and at much longer synthesis times (hours). The study demonstrates that synthesis in supercritical fluids is a very promising method for manipulating the size and size distribution of nanoparticles, thus removing one of the key limitations in many applications of nanomaterials. 相似文献
53.
A rapid and simple fractionation procedure using solid-phase extraction (SPE) cartridges was developed for an accurate determination of aliphatic and polycyclic aromatic hydrocarbons in petroleum residues and further application in chemical fingerprinting of oil spills by gas chromatography-mass spectrometry (GC-MS). Among the adsorbents evaluated, SiO2/C3-CN exhibited the best selectivity, providing, by elution with n-hexane (4 ml) and n-hexane-CH2Cl2 (1:1) (5 ml), two well-resolved aliphatic and aromatic hydrocarbon fractions, with recoveries of 97 +/- 7.2 and 99.7 +/- 13.9%, respectively. The SPE fractionation procedure was compared with the conventional silica-alumina adsorption chromatography showing similar results but practical advantages in terms of reproducibility, analysis time, solvent reduction and cost. Moreover, is particularly suitable for routine analysis with a high sample throughput. The developed methodology was tested in the characterization of fuel-oil samples collected along the Spanish north-west coast, after the Prestige oil spill accident. 相似文献
54.
Possible use of room temperature ionic liquids (RTILs) as chelate extraction solvent was evaluated by using 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF6]) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim][PF6]). These RTILs showed high extraction performance for divalent metal cations with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (Htta). The extracted metals were back-extracted into 1 mol dm−3 nitric acid quantitatively. Furthermore, the extracted species were estimated as neutral hydrated complexes M(tta)2(H2O)n (n= 1 or 2) for M = Ni, Cu and Pb and anionic complexes M(tta)3− for M = Mn, Co, Zn and Cd. 相似文献
55.
《Analytica chimica acta》2002,458(1):95-102
An easy, fast and reliable analytical method is proposed to determine the concentration of volatile phenols (ethyl- and vinylphenols) in fino sherry wines. The technique employed is a single stage sample treatment by solid-phase extraction (SPE) following a simple, fast procedure that enables 12 samples to be extracted simultaneously and requires a small volume sample and little time. Subsequently, the extracts are analyzed by gas chromatography (GC) with flame ionization detection. The method proposed has been applied to the study of fino sherry wines affected by microbial contamination with yeasts of the Brettanomyces genus, and the relationship of these yeasts with the concentrations of volatile phenols present in this wine. 相似文献
56.
特征线谱提取是舰船目标识别的一个重要研究环节,常采用传统的DEMON谱分析方法,处理过程中,一般对舰船噪声时域信号未予抑噪,低信噪比情况下,传统DEMON谱分析性能差。对此,提出一种采用遗传算法优化变分模态分解方法,用于分解舰船噪声原时域信号,获得抑制噪声后的舰船噪声重构信号,进而有效提取了舰船目标噪声幅度调制特征线谱。该方法首先采用遗传算法优化变分模态分解的两个关键输入参数(分解所取模态个数和惩罚因子),对变分模态分解得到的各阶固有模态分量加以判别,去除噪声主导分量,保留信号主导分量,使重构舰船噪声信号显著抑制了干扰噪声,然后对降噪后的重构信号进行频谱分析,获得目标噪声调制特征线谱。理论分析、仿真和实验数据处理结果表明,相比传统DEMON谱分析法,基于遗传算法优化变分模态分解的舰船噪声特征线谱提取方法具有更好的噪声抑制能力,所获取的舰船噪声幅度调制特征线谱信噪比明显高于传统DEMON方法,具有一定优势,前景良好。 相似文献
57.
58.
Andrey G. Kazakov Taisya Y. Ekatova Julia S. Babenya Sergey S. Belyshev Vadim V. Khankin Alexander A. Kuznetsov Sergey E. Vinokurov Boris F. Myasoedov 《Molecules (Basel, Switzerland)》2022,27(10)
A new method of production of one of the most widely used isotopes in nuclear medicine, 177Lu, with high chemical purity was developed; this method includes irradiation of the HfO2 target with bremsstrahlung photons. The irradiated target was dissolved in HF and then diluted and placed onto a column filled with LN resin. Quantitative sorption of 177Lu could be observed during this process. The column later was rinsed with the mixture of 0.1 M HF and 1 M HNO3 and then 2 M HNO3 to remove impurities. Quantitative desorption of 177Lu was achieved by using 6 M HNO3. The developed method of 177Lu production ensures high purification of this isotope from macroquantities of hafnium and zirconium and radioactive impurities of carrier-free yttrium. The content of 177mLu in 177Lu in photonuclear production was determined. Due to high chemical and radionuclide purity, 177Lu obtained by the developed method can be used in nuclear medicine. 相似文献
59.
A turnout switch machine is key equipment in a railway, and its fault condition has an enormous impact on the safety of train operation. Electrohydraulic switch machines are increasingly used in high-speed railways, and how to extract effective fault features from their working condition monitoring signal is a difficult problem. This paper focuses on the sectionalized feature extraction method of the oil pressure signal of the electrohydraulic switch machine and realizes the fault detection of the switch machine based on this method. First, the oil pressure signal is divided into three stages according to the working principle and action process of the switch machine, and multiple features of each stage are extracted. Then the max-relevance and min-redundancy (mRMR) algorithm is applied to select the effective features. Finally, the mini batch k-means method is used to achieve unsupervised fault diagnosis. Through experimental verification, this method can not only derive the best sectionalization mode and feature types of the oil pressure signal, but also achieve the fault diagnosis and the prediction of the status of the electrohydraulic switch machine. 相似文献
60.
薄层色谱-紫外可见分光光度法测定食品中的苏丹红Ⅰ号 总被引:1,自引:0,他引:1
建立了测定食品中苏丹红Ⅰ号新方法:薄层色谱-紫外可见分光光度法.方法的线性回归方程为:A=0.00583 0.15105ρ,在0.50~10.0 μg/mL之间苏丹红Ⅰ号的浓度与吸光度之间呈线性关系,相关系数R=0.9992,检出限为0.05 μg/mL.测定结果与国家标准HPLC法对照,令人满意.可用于食品中苏丹红Ⅰ号的测定. 相似文献