全文获取类型
收费全文 | 248173篇 |
免费 | 3156篇 |
国内免费 | 1472篇 |
专业分类
化学 | 122737篇 |
晶体学 | 3228篇 |
力学 | 8889篇 |
综合类 | 61篇 |
数学 | 24388篇 |
物理学 | 65706篇 |
无线电 | 27792篇 |
出版年
2020年 | 1894篇 |
2019年 | 2091篇 |
2018年 | 2681篇 |
2017年 | 2734篇 |
2016年 | 3934篇 |
2015年 | 2594篇 |
2014年 | 3890篇 |
2013年 | 9425篇 |
2012年 | 8131篇 |
2011年 | 9957篇 |
2010年 | 7220篇 |
2009年 | 7291篇 |
2008年 | 9865篇 |
2007年 | 10227篇 |
2006年 | 9713篇 |
2005年 | 8906篇 |
2004年 | 8034篇 |
2003年 | 7138篇 |
2002年 | 7002篇 |
2001年 | 7806篇 |
2000年 | 6098篇 |
1999年 | 4569篇 |
1998年 | 3888篇 |
1997年 | 3890篇 |
1996年 | 3754篇 |
1995年 | 3365篇 |
1994年 | 3511篇 |
1993年 | 3360篇 |
1992年 | 3512篇 |
1991年 | 3585篇 |
1990年 | 3399篇 |
1989年 | 3284篇 |
1988年 | 3097篇 |
1987年 | 2790篇 |
1986年 | 2792篇 |
1985年 | 3621篇 |
1984年 | 3650篇 |
1983年 | 3025篇 |
1982年 | 3263篇 |
1981年 | 3025篇 |
1980年 | 2792篇 |
1979年 | 2956篇 |
1978年 | 3176篇 |
1977年 | 3177篇 |
1976年 | 3199篇 |
1975年 | 2952篇 |
1974年 | 3040篇 |
1973年 | 3087篇 |
1972年 | 2411篇 |
1971年 | 1941篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
The Systems AI/CoI2 (A = Alkali Metal, Tl, Ag) and the Crystal Chemistry of the Double Halides AnCoX(n+2) with X = Cl, Br, I The systems AI/CoI2 (A = Cs, Rb, K, Tl, Na, Ag) were investigated by differencethermal analysis. The systems of NaI and AgI are found to be eutectical. A compound A2CoI4 always exists in the other systems. Cs2CoI4 crystallizes in the β-K2SO4 type with a coordination number (C.N.) for Cs equal to 9/10. Results obtained with single crystal technique reveal for the first time that among the double halides Rb2CoI4 is of the monoclinic Sr2GeS4 type (C. N. for Rb = 6(+2)). The compounds K2CoJ4, Tl2CoJ4, T-K2CoBr4, and T-Tl2CoBr4 are isotypic. Both structure groups are characterized by isolated CoX42? tetrahedra. Reflectance spectra and magnetic susceptibilities can be explained on the basis of crystal field theory. – Our results close presently existing gaps in the knowledge on systems of CoBr2 and CoCl2 too. 相似文献
962.
L. Van Den Enden H.J. Geise H.P. Figeys P. Geerlings C. Van Alsenoy 《Journal of Molecular Structure》1976,33(1):69-77
The structure of tricyclo-(3.1.002,4)exane has been determined by gas phase electron diffraction. The molecule has an inversion centre. The mean carbon—carbon bond length, averaged over both three- and four-membered rings is 1.508 A. A model with equal C-C bond lengths fits to the measured diffraction intensities. The four-membered ring is planar with valency angles of 90°, while the carbon atoms of the three-membered rings form isosceles triangles. Ab initio quantum mechanical calculations at the STO-3G level support this geometry. The valency angle CCC (between three- and four-membered rings) for the equilateral four-membered ring model has been found experimentally to be 109.9°. The average C-H bond distance (1.080 Å) is small as a result of increased s-character in these bonds in agreement with reported INDO—LMO calculations. 相似文献
963.
N. O. Saldabol L. L. Zeligman Yu. Yu. Popelis S. A. Giller 《Chemistry of Heterocyclic Compounds》1975,11(1):45-50
The action of 1 mole of bromine on 6-(2′-furyl)imidazo[2,1-b]thiazole, its 2-methyl-, 3-methyl-, and 2,3-dimethyl-substituted derivatives, and their hydrobromides in chloroform and glacial acetic acid was studied. The bromination of bases containing a methyl group in the 3 position leads primarily to the 5′-derivatives with respect to the furan ring in chloroform, whereas in the remaining cases 5-bromo derivatives with respect to the imidazothiazole system are formed. Compounds of the latter type are formed by the action of bromine in glacial acetic acid or of sodium hypobromite in alkaline media on the bases. The hydrobromides are brominated in both acetic acid and chloroform, regardless of the substituents in the thiazole ring, primarily in the 5′ position of the furan ring. The structures of the bromination products were proved by means of alternative syntheses, thin-layer chromatography, and the PMR spectra. 相似文献
964.
D. H. Beebe C. E. Gordon R. N. Thudium M. C. Throckmorton T. L. Hanlon 《Journal of polymer science. Part A, Polymer chemistry》1978,16(9):2285-2301
A quantitative procedure has been developed for characterizing the complete microstructure of polymers of 1,3-pentadiene, including the tacticity of any crystalline component. This can be accomplished by a combination of infrared spectroscopy, X-ray crystallinity, and 300-MHz NMR spectroscopy. A series of high structural purity polymers were synthesized with a series of previously unreported mixed microstructures. These samples were characterized by using the three techniques mentioned, including the previously unreported 300-MHz NMR data. With those results a 60-MHz NMR/IR method of spectroscopy was developed to determine the composition of poly(1,3-pentadiene)s in terms of percent cis-1,2-, cis-1,4-, trans-1,4-, and 3,4-pentadiene units. 相似文献
965.
V. M. Lembrikov V. V. Volkova L. V. Konyakhina Z. N. Korneva O. V. Pokidova 《Russian Journal of Applied Chemistry》2005,78(8):1209-1212
Data on the kinematic viscosity of phosphoric acid solutions in technical-grade tributyl phosphate and in an industrial extract from a technological purification system based on tributyl phosphate, which contain 0–18% phosphoric acid in terms of P2O5, at 10–70°C are presented. 相似文献
966.
D. Bagatti M. C. Cantone A. Giussani S. Ridone C. Birattari M. L. Bonardi F. Groppi A. Martinotti S. Morzenti M. Gallorini E. Rizzio 《Journal of Radioanalytical and Nuclear Chemistry》2005,263(2):515-520
Summary The nuclear properties of 186gRe make it a useful agent for radionuclide therapy and imaging. The coordination compound [186gRe]Re-HEDP has proved to be a successful bone seeking agent for palliation of metastatic bone pain. Chemical, radiochemical and radionuclidic purity of commercial radiopharmaceutical [186gRe]Re-HEDP have been checked by means by γ- and β-spectrometries, INAA and paper radio-chromatography. The results indicate a good radionuclidic purity, with levels of contamination from the short-lived 188Re well below the required specifications. After injection of the radiopharmaceutical, the radiochemical measurements conducted in vivo, on biological matrices, blood, plasma and urine, have shown that, entering the systemic circulation, 186gRe dissociates from the bis-phosphonate complex as hydrosoluble [186gRe]ReO4-, and the two chemical species follow different biokinetics. 相似文献
967.
Yu. L. Khromova 《Colloid Journal》2006,68(1):115-119
The preparation of the alginate-CaSO4-KCl/NH4Cl gel phases is described. Dispersion of calcium sulfate in solutions with a low chloride content is found to lead to the formation of homogeneous alginate gels. The dependences of mechanical and absorptive properties of the gels on the chloride concentration are studied, and a possibility of practical application of the compositions thus prepared is shown. The effect of the KCl and NH4Cl additives on gelation is explained by attainment of the optimal rate of CaSO4 dissolution, which favors uniform distribution of Ca2+ cations in the bulk of the arising gel structure. 相似文献
968.
The herb low cudweed has yielded for the first time a new acylated flavone glycoside, for which the structure of 3,4,5,7-tetrahydroxy-6-methoxyflavone 7-O-(6-O-caffeyl--D-glucopyranoside) has been established. In addition, 6-methoxyluteolin, 6-hydroxyluteolin 7-O--D-glucopyranoside, and scutellarein 7-O--D-glucopyranoside have been isolated. Identification was made on the basis of UV, IR, PMR, and mass spectra, the products of alkaline and acid hydrolyses, and the results of elementary analyses, melting points, and specific rotations.Vitebsk State Medical Institute, All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 483–487, July–August, 1979. 相似文献
969.
A. S. Knyazev A. I. Boronin O. V. Vodyankina S. V. Koshcheev L. N. Kurina 《Kinetics and Catalysis》2005,46(1):144-150
The chemical composition, structure, and properties of an H3PO4-promoted polycrystalline silver foil are studied by XPS. The surface of the doped sample contains oxygen in two states with binding energies of 531.1 and 533.0 eV. Heat treatment results in silver clusters distributed in the polyphosphate matrix. It follows from TDS data that the surface of the promoted catalyst contains a strongly bound oxygen species that desorbs at ~900 K. It is believed that the silver clusters stabilized by the phosphate matrix are active sites on the surface of the phosphorus-promoted catalyst.Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 153–160. Original Russian Text Copyright © 2005 by Knyazev, Boronin, Vodyankina, Koshcheev, Kurina. 相似文献
970.
Khairitdinova E. D. Tsirlina E. M. Spirikhin L. V. Fedorov N. I. Efremov Yu. Ya. Yunusov M. S. 《Russian Chemical Bulletin》2003,52(9):2078-2080
A new norditerpenoid alkaloid has been isolated from the roots of Delphinium cuneatum. On the basis of 1H, 13C NMR, IR, and mass spectra, the structure of the alkaloid was established as 1,6,14-trimethoxy-7,8-dihydroxy-18-(2-methyl)succinylanthranoyloxyaconane (16-demethoxymethyllycaconitine). The roots of Delphinium cuneatum also contain the known alkaloid methyllycaconitine and N,N"-di(methoxycarbonyl)-3,4-diaminotoluene. 相似文献