全文获取类型
收费全文 | 3831篇 |
免费 | 700篇 |
国内免费 | 608篇 |
专业分类
化学 | 3251篇 |
晶体学 | 67篇 |
力学 | 186篇 |
综合类 | 75篇 |
数学 | 430篇 |
物理学 | 1130篇 |
出版年
2024年 | 14篇 |
2023年 | 85篇 |
2022年 | 125篇 |
2021年 | 171篇 |
2020年 | 209篇 |
2019年 | 206篇 |
2018年 | 189篇 |
2017年 | 173篇 |
2016年 | 220篇 |
2015年 | 249篇 |
2014年 | 288篇 |
2013年 | 339篇 |
2012年 | 361篇 |
2011年 | 361篇 |
2010年 | 309篇 |
2009年 | 282篇 |
2008年 | 260篇 |
2007年 | 233篇 |
2006年 | 186篇 |
2005年 | 145篇 |
2004年 | 97篇 |
2003年 | 89篇 |
2002年 | 91篇 |
2001年 | 70篇 |
2000年 | 53篇 |
1999年 | 49篇 |
1998年 | 23篇 |
1997年 | 18篇 |
1996年 | 13篇 |
1995年 | 24篇 |
1994年 | 14篇 |
1993年 | 20篇 |
1992年 | 16篇 |
1991年 | 7篇 |
1990年 | 20篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1985年 | 8篇 |
1982年 | 5篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1972年 | 12篇 |
1971年 | 6篇 |
1970年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有5139条查询结果,搜索用时 15 毫秒
81.
82.
A method for the isotopic determination of selenium in biological matrices is described. The method is based on hydride generation inductively coupled plasma mass spectrometry (ICP-MS). The development is specifically related to the requirements of stable isotope tracer studies in human subjects. The method is based on isotope dilution using 82Se as the in vitro spike and can quantify the 74Se and 77Se contents of samples. It involves wet oxidation (HNO3 - H2O2 or HNO3 - HClO4) of the 82Se-spiked matrix, reduction to selenite by boiling with HCl followed by measurement of the isotope ratios (82Se/77Se and 74Se/77Se) in the gas stream (H2Se) generated from on-line reduction of the sample selenite with NaBH4. Compared with the isotopic signal resulting from a selenite solution containing 5 ng ml-1 of Se, the total sample blank contributions at m/z = 74, 77 and 82 were less than 5% of the respective isotope signal. Worst-case absolute detection limits were 0.2-0.9 ng of Se, depending on the isotope used. Ion beam intensity ratios were measured with an over-all precision [relative standard deviation (RSD)] of 1% for both isotope pairs. Measured ratios (MRa/b) were stable during a given day's operation within the expected precision of the measurements but varied for different days. The magnitude of MRa/b was generally independent of the nature of the matrix. Highly linear relationships were found between ion beam intensity ratios (MRa/b) and the corresponding true isotope ratios for calibration solutions whose isotope ratios had been altered by as much as one order of magnitude.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
83.
Lewis acid/base addition between Ln(NO3)3 · 6H2O (Ln = Pr, Nd, Sm, Eu, Tb and Lu) and H2salen [H2salen = N,N′-ethylenebis(salicylideneimine)] gives rise to an array of coordination polymeric structures. Crystal structural analysis reveals that Salen effectively functions as a bridging ligand in these compounds. The size of the lanthanide ions controls the structures of these Salen lanthanide complexes. Two representative structures with one dimensional and two dimensional topologies, viz. [Pr(H2salen)(NO3)3(CH3OH)2]n (1) and [Ln(H2salen)1.5(NO3)3]n [Ln = Pr (2), Nd (3), Sm (4), Eu (5), Tb (6) and Lu (7)] are reported. Luminescent spectra of complexes 4 and 5 exhibit characteristic metal-centered emission lines. However, the characteristic luminescence of the terbium(III) ion is not observed either in solution or in the solid state of complex 6. 相似文献
84.
The crystal structure of [Cd(BDA)(phen)2(H2O)](H2O)2 (1) (BDA=6,6′-dibromo-2,2′-dimethoxy-1,1′-binaphthylene-4,4′-disulfonate, phen=1,10-phenanthroline)consists of a cadmium center whose coordination environment can be best described as a slightly distorted octahedron defined four nitrogen atoms from two phen ligands and two oxygen atoms differently from BDA ligand and water. There are strong hydrogen-bonding interactions between water and sulfonate group of BDA ligands to construct the 3D network. CCDC: 277921. 相似文献
85.
J. Somlai B. Kanyár R. Bodnár CS. Németh Z. Lendvai 《Journal of Radioanalytical and Nuclear Chemistry》1996,207(2):437-443
A significant dose contribution on the population could be derived from coal slags used as isolation material. Extremely high natural activities are measured in the coal slag, derived from the region of the settlement Ajka, Hungary. In some buildings monitored, the elevated -doses were nearly 5–10 times higher than the world average ones. The annual average indoor radon concentrations from the slag exceeded 400 Bq/m3 and in some cases up to 1200 Bq/m3. Due to the elevated exposure and the radon concentrations in the dwellings the annual dose was estimated to 8–24 mSv/y more than 5–10 times of the world average one. 相似文献
86.
AbstractAn investigation on seeds of Brucea javanica led to the acquisition of a new quassinoid, 20-hydroxyyadanzigan (1), along with five known quassinoids (2–6). The structure of the new compound was elucidated on the basis of extensive spectral analysis. All of the compounds were assayed for their anticomplement activities through classical and alternative pathways. Compounds 1–6 exhibited potent anticomplement activity with CH50 and AP50 values of 0.032–0.075?mg/mL and 0.061–0.118?mg/mL, respectively. Moreover, the structure-activity relationships of these compounds are discussed. 相似文献
87.
Yong Hong Wang Ting Fang Tian Xing Qin Liu Guang Yao Meng 《Journal of membrane science》2006,280(1-2):261-269
The high-quality tubular titania MF membranes are successfully prepared by dip-coating techniques and systematically investigated with regard to their corrosive resistances. The experiments show that dispersants PAA and anatase powder were preferably employed to prepare desired suspensions with solid loading 10–15 wt.% and that suspensions properties significantly affect the final membranes quality. The titania MF membranes with pure water permeability 742.42 l m−2 h−1 bar−1 and 0.1 μm pore diameter have been obtained using the prepared suspensions. The corrosion resistance of titania membranes exhibits that the anatase layers are more stable than the alumina supports in boiling corrosive medium and that the poor quality titania MF membranes become more deteriorated due to its top layer pore blockages and fouling layer formation, which provides a wide range of practical application fields in very hash environments with reliable data supports. 相似文献
88.
Chen-Hanson Ting 《中国化学会会志》1969,16(4):137-142
The Raman spectra of the p-polyphenyls: benzene, biphenyl, p-terphenyl, and p-quaterphenyl, are compared with their electronic spectra. The relative intensity of the double bond stretching mode around 1600 Kaisers to that of the breathing mode around 1000K increases steadily when the phenyl chain is lengthened. This intensity ratio can be calculated using our theory proposed previously.1 The results are in very good agreement with the experimental observations 相似文献
89.
The Raman spectra of powdered biphenyl, p-terphenyl, and p-quaterphenyl are compared with each other and also with the Raman spectrum of benzene. Most Raman lines in polyphenyls can be considered as derived from normal vibrations in benzene. The intense lines are thus assigned and the correlations between them are established. 相似文献
90.