全文获取类型
收费全文 | 417192篇 |
免费 | 4170篇 |
国内免费 | 1444篇 |
专业分类
化学 | 221859篇 |
晶体学 | 6401篇 |
力学 | 17895篇 |
综合类 | 7篇 |
数学 | 48760篇 |
物理学 | 127884篇 |
出版年
2021年 | 3406篇 |
2020年 | 3512篇 |
2019年 | 4055篇 |
2018年 | 4973篇 |
2017年 | 4823篇 |
2016年 | 7385篇 |
2015年 | 4888篇 |
2014年 | 7273篇 |
2013年 | 19336篇 |
2012年 | 14658篇 |
2011年 | 17867篇 |
2010年 | 12007篇 |
2009年 | 11823篇 |
2008年 | 15971篇 |
2007年 | 16196篇 |
2006年 | 15140篇 |
2005年 | 13755篇 |
2004年 | 12567篇 |
2003年 | 11112篇 |
2002年 | 10877篇 |
2001年 | 12201篇 |
2000年 | 9295篇 |
1999年 | 7293篇 |
1998年 | 6007篇 |
1997年 | 6070篇 |
1996年 | 5633篇 |
1995年 | 5219篇 |
1994年 | 4981篇 |
1993年 | 4922篇 |
1992年 | 5409篇 |
1991年 | 5457篇 |
1990年 | 5190篇 |
1989年 | 5047篇 |
1988年 | 5099篇 |
1987年 | 4994篇 |
1986年 | 4777篇 |
1985年 | 6341篇 |
1984年 | 6710篇 |
1983年 | 5514篇 |
1982年 | 5956篇 |
1981年 | 5827篇 |
1980年 | 5634篇 |
1979年 | 5699篇 |
1978年 | 6027篇 |
1977年 | 5836篇 |
1976年 | 6085篇 |
1975年 | 5531篇 |
1974年 | 5600篇 |
1973年 | 5943篇 |
1972年 | 3756篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
The method described is based on the extraction of uranium with a chloroform solution of phenylacetic acid from slightly acidic solution containing nitrilotriacetic acid, which masks all interfering metals. After stripping into very dilute hydrochloric acid, uranium is reduced with ascorbic acid and determined complexometrically. The method permits reliable determination of uranium in the presence of all quadri-, ter- and bivalent metals investigated, molybdenum(VI), tungsten(VI), and vanadium(V). 相似文献
983.
Hans. R. Kricheldorf William E. Hull 《Journal of polymer science. Part A, Polymer chemistry》1978,16(3):583-595
The tripeptides Phe-Gly-Gly, β-Ala-Gly-Gly, and ?-Aca-Gly-Gly as well as the peptide derivatives δ-isothiocyanatovaleroyl-Gly-Gly and ?-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in 15N to a level of 0.8–0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly)n, (β-Ala-Gly-Gly)n, (δ-Ava-Gly-Gly)n, and (?-Aca-Gly-Gly)n. The 18.24 MHz 15N-NMR spectra of the oligo- and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low viscosity and exhibit a strong negative nuclear Overhauser enhancement of the 15N signals. For comparison, 15N-NMR spectra of the homopolymers (Gly)n, (β-Ala)n, (γ-Abu)n, (δ-Ava)n, and (?-Aca)n were also recorded. The 15N signals from the ω-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The 15N signals from the two glycyl residues are separated by 3–7 ppm. Comparison with the 13C-NMR spectra of the same polymers indicates that 15N-NMR is better suited for the characterization and sequence analysis of these types of polymers. 相似文献
984.
K. Müller R. Henkelmann J. P. Biersack P. Mertens 《Journal of Radioanalytical and Nuclear Chemistry》1977,38(1-2):9-17
The isotopes3He,6Li,7Be and10B exhibit extremely large cross sections, between 1 and 48 kilobarn, for (n, p) or (n, α) reactions with thermal neutrons.
Together with now available extracted thermal neutron fluxes of 10( n·cm−2·sec−1 or more, these reactions present a highly sensitive method of detecting the mentioned light elements in any heavy matrix
material. Through the experimentally determined energy losses of the emitted protons or α-particles, also well resolved depth
profiles can be obtained, as demonstrated here for some relevant examples from semiconductor and fusion technology. 相似文献
985.
Résumé Une méthode d'analyse des impuretés dans l'or métallique par activation neutronique, a été mise au point. Après irradiation
de 15 heures dans un flux de 2·1014 n·cm−2·sec−1, l'échantillon d'or est traité en cellule blindée. La matrice est séparée des impuretés à analyser par des méthodes électrolytiques.
Les impuretés sont généralement séparées en 3 fractions qui sont mesurées par spectrométrie γ. Le traitement des informations
est effectué sur ordinateur IBM 360/91 moyennant un programme écrit au laboratoire. 29 éléments sont détectables par cette
méthode. 相似文献
986.
In an extension of studies of flameless atomizers for atomic-absorption spectrometry, an electrically heated tungsten-rhenium alloy wire loop was examined. Reduction of metallic salts to ground-state metal atoms was accomplished with the high temperature produced by the loop. Lead and nickel were investigated. Experimental parameters such as wavelength, slit width, atomization temperature and sheathing gas flow rate were optimized. Absolute detection limits of 6.6·10?10 and 1.2·10?10, and absolute sensitivities of 7·10?10 and 8·10?11 g of lead, were established for unenclosed and enclosed cells, respectively. The interferences of twenty cations and sixteen anions were studied; foreign cations generally enhanced the lead absorption by retarding its vaporization, allowing the slow detection system to respond more efficiently. Nickel was investigated as a representative less volatile metal; an absolute detection limit of 1.6·10?9 and an absolute sensitivity of 9·10?10 g of nickel were established. 相似文献
987.
Kordes and Nolte are of the opinion that, on melting, Na2W2O7 dissociates according to the reaction whereas Gelsing et al. assume a dissociation into Na+ ions and a mixture of chains of WO4 tetrahedra (WO, W2O, W3O, etc.). In the present paper it is demonstrated that a dissociation according to the ideas of Gelsing et al. can excellently explain the cryometric results found by Kordes and Nolte themselves. The dissociation scheme proposed by Gelsing et al., is even more likely than that proposed by Kordes and Nolte when the results of surface tension measurements reported by Gossink and Stevels are taken into account. 相似文献
988.
D. M. Hirst 《Theoretical chemistry accounts》1971,20(3):292-298
INDO-MO Calculations have been carried out on the anions of a variety of pyridine derivatives for which e.s.r. data have recently become available. The method gives reasonable results for proton hyperfine splitting constants but calculated nitrogen splitting constants are considerably larger than the experimental values. Some -only calculations are included for the pyridine anion. All methods overestimate the spin density at the nitrogen atom. The orbital populations and excess charges for the pyridine molecule are not in agreement with those of an ab initio calculation. 相似文献
989.
A semi-empirical open shell model of sigma bonded systems has been employed to calculate the binding energies for diatomic lithium halides. The repulsion between the atomic cores is taken as the Coulomb repulsion term modified by the presence of electronic cores. This modification has been estimated from the repulsion term in the Rittner potential function. The agreement with the experimental values is quite satisfactory but the extension of the same method to alkali hydrides results in gross discrepancies. The results are discussed in the light of other results. 相似文献
990.
N-methylaminoalkoxides of titanium of the type Ti(OR)4?n(O · CHR′ · CH2 · NR″R?)n where R = Et and Pr1; n = 1–4; and R′ = R″ = H, R? = Me; R′ = H, R″ = R? = Me; R′ = R″ = R? = Me, synthesized by the reactions of titanium alkoxides with aminoalcohols, show interesting variations in their properties like physical state, volatility and molecular complexity. I.r. and p.m.r. spectra of these derivatives have been recorded. A few interchange reactions with methanol and tert-butanol have also been carried out. These aminoalkoxides get cleaved with acetyl chloride and undergo insertion reactions with phenylisocyanate, thus providing the first examples of insertion reactions in such derivatives. 相似文献