全文获取类型
收费全文 | 217130篇 |
免费 | 1594篇 |
国内免费 | 630篇 |
专业分类
化学 | 118823篇 |
晶体学 | 3277篇 |
力学 | 8380篇 |
综合类 | 6篇 |
数学 | 23493篇 |
物理学 | 65375篇 |
出版年
2020年 | 1996篇 |
2019年 | 2280篇 |
2018年 | 3216篇 |
2017年 | 3137篇 |
2016年 | 4305篇 |
2015年 | 2407篇 |
2014年 | 3849篇 |
2013年 | 9321篇 |
2012年 | 6976篇 |
2011年 | 8510篇 |
2010年 | 6127篇 |
2009年 | 5973篇 |
2008年 | 8207篇 |
2007年 | 8271篇 |
2006年 | 7695篇 |
2005年 | 7006篇 |
2004年 | 6229篇 |
2003年 | 5688篇 |
2002年 | 5547篇 |
2001年 | 5964篇 |
2000年 | 4728篇 |
1999年 | 3482篇 |
1998年 | 3017篇 |
1997年 | 3044篇 |
1996年 | 2854篇 |
1995年 | 2347篇 |
1994年 | 2462篇 |
1993年 | 2496篇 |
1992年 | 2634篇 |
1991年 | 2679篇 |
1990年 | 2590篇 |
1989年 | 2558篇 |
1988年 | 2487篇 |
1987年 | 2465篇 |
1986年 | 2379篇 |
1985年 | 3076篇 |
1984年 | 3216篇 |
1983年 | 2571篇 |
1982年 | 2844篇 |
1981年 | 2717篇 |
1980年 | 2518篇 |
1979年 | 2747篇 |
1978年 | 2854篇 |
1977年 | 2872篇 |
1976年 | 2848篇 |
1975年 | 2701篇 |
1974年 | 2690篇 |
1973年 | 2823篇 |
1972年 | 1880篇 |
1967年 | 1834篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
Sadovskii Yu. S. Solomoichenko T. N. Kas'yan A. O. Golodaeva E. A. Anikanova S. V. Kas'yan L. I. Savelova V. A. 《Russian Journal of Organic Chemistry》2004,40(1):50-56
The nucleophilic reactivity of amines of the norbornane, norbornene, and adamantane series toward ptoluenesulfonyl chloride and diphenyl chlorophosphate in acetonitrile at 25°C is determined mainly by the steric factor. Parameters characterizing spatial accessibility of the reaction center in the amine molecule have been determined. Cage-like substituents show no appreciable effect on the amine reactivity, as compared to common alkyl groups. 相似文献
872.
Zusammenfassung Für Elementbestimmungen in Abwasser wird ein Röntgenfluorescenzverfahren beschrieben. Zur Probenvorbereitung werden die Elemente durch Gefriertrocknung auf einem inerten Träger angereichert. Diese Anreicherung ist nicht selektiv und erfaßt praktisch alle in Abwasser gelösten und suspendierten Verunreinigungen. Für die Berechnung der Elementkonzentrationen aus den gemessenen Fluorescenzintensitäten wird ein Matrixkorrekturverfahren angegeben, welches dem unterschiedlichen Absorptionsverhalten der Probenmatrix Rechnung trägt. Die Massenabsorptionskoeffizienten werden durch Messung der diffus gestreuten Bremsstrahlung bzw. der Comptonstreustrahlung ermittelt. Die Selbstabsorption des zu bestimmenden Elementes wird rechnerisch berücksichtigt. Das Korrekturverfahren kann auf alle mit Röntgenfluorescenzanalyse bestimmbaren Elemente angewendet werden.
Determination of elements in waste water by x-ray fluorescence analysis: Sample preparation and matrix correction method
Summary For sample preparation the elements are enriched upon an inert carrier by freeze-drying. This enriching procedure is not selective and collects all dissolved and suspended contaminations from the waste water. In order to calculate the element concentrations from the measured fluorescence intensities a matrix correction method is given which considers the different absorption behaviour of the matrix. The mass absorption coefficients are obtained by measuring the scattered X-ray tube spectrum or a Compton scattered X-ray tube line, respectively. The self-absorption of the element to be determined is taken into account. The correction method is applicable to all elements detectable by X-ray fluorescence.
Herrn Dr. H. Wagner danken wir für das stete Interesse, die zahlreichen konstruktiven Diskussionen und die Hilfe beim Abfassen des Manuskripts. 相似文献
873.
É. A. Karakhanov S. Kh. Sharipova M. I. Rozhkova E. A. Viktorova 《Chemistry of Heterocyclic Compounds》1978,14(1):15-16
It is shown that the principal pathway in the reaction of benzofuran with a solution of lithium in hexametapol is opening of the furan ring of benzofuran to give a dilithium derivative. Mono- and dialkylation products are formed when the reaction mixture is treated with alkylating agents. It was established by the use of alkylating agents with different degrees of hardness that the oxygen atom is a softer polarizable center than the sp-hybridized carbanion center.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 22–23, January, 1978. 相似文献
874.
E. J. Baran 《Monatshefte für Chemie / Chemical Monthly》1973,104(6):1653-1659
The mean amplitudes of vibration of ICl2
–, IBr2
–, BrCl2
– and ClF2
– have been calculated, from known spectroscopic data, in the temperature range between O° and 1000°K. Also generalized mean-square amplitudes and values for theBastiansen—Morino shrinkage effect are reported. The results are briefly discussed.
Mit 1 Abbildung 相似文献
Mit 1 Abbildung 相似文献
875.
A new method is proposed for routine determinations of oxidizable and reducible substances. A mathematical development and criticism of the method of Schierjott has led to a fundamental modification of the earlier experimental conditions (ratio of the oxidized/reduced forms of the reagent at the start and end of the redox reaction). This improvement allows a considerable increase in the sensitivity of the method and is particularly advantageous for the determination of substances of low reactivity, e.g. carbohydrates. Other advantages are a reduction in the quantity and thus cost of the necessary reagent, and a decreased risk of precipitation of the substances involved. The mathematical treatment of the proposed version allows a rigorous theoretical comparison of the two methods. An evaluation of the quality of the approximations introduced into both methods is also proposed, based on the calculation of the deviation (absolute and relative differences) of approximated values from theoretical values. The final part of the work deals with optimization of the different principal parameters, i.e. concentration of the reagent, temperature and reaction time. 相似文献
876.
M.I. Bruce O.M.Abu Salah R.E. Davis N.V. Raghavan 《Journal of organometallic chemistry》1973,64(3):C48-C50
In contrast to a related iron—copper compound, the complex (π-C5H5)Ru(PPh3)2(C2Ph)CuCl is shown to be monomeric, and contains linear dicoordinate copper(I); the reactions of this and similar complexes with metal carbonyls are sources of new mixed-metal clusters, such as (π-C5H5)Fe2 Ru(C2Ph)(CO)6 (PPh3). 相似文献
877.
G. N. Dorofeenko E. I. Sadekova V. M. Goncharova 《Chemistry of Heterocyclic Compounds》1970,6(10):1218-1222
The properties of 1,3-dimethyl-6,7-dialkoxy-2-benzopyrylium salts were studied. The heterocyclic oxygen atom is readily replaced by nitrogen by reaction of the 2-benzopyrylium salts with primary amines, hydrazine, phenylhydrazine, glycine ethyl ester, and hydroxylamine; reaction with secondary amines leads to naphthalene derivatives. The activities of the methyl groups in the 1 and 3 positions of the heterocyclic ring are compared.For Communication IX, see [6].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1308–1312, October, 1970. 相似文献
878.
Kiselev V. D. Kashaeva E. A. Konovalov A. I. 《Russian Journal of General Chemistry》2002,72(11):1793-1796
The catalytic effect of AlCl3 on the Diels-Alder reaction of 9,10-dimethylanthracene with maleic anhydride is dramatically weakened on addition of a competing n donor, dibutyl ether. The n,v complex of aluminum chloride with dibutyl ether is considerably more stable than the complex with maleic anhydride; therefore, the catalytic effect is suppressed even at comparable concentrations of these n donors. 相似文献
879.
Liposomes with encapsulated carboxyfluorescein were used in an affinity-based assay to provide signal amplification for small-volume fluorescence measurements. Microfluidic channels were fabricated by imprinting in a plastic substrate material, poly(ethylene terephthalate glycol) (PETG), using a silicon template imprinting tool. Streptavidin was linked to the surface through biotinylated-protein for effective immobilization with minimal nonspecific adsorption of the liposome reagent. Lipids derivatized with biotin were incorporated into the liposome membrane to make the liposomes reactive for affinity assays. Specific binding of the liposomes to microchannel walls, dependence of binding on incubation time, and nonspecific adsorption of the liposome reagent were evaluated. The results of a competitive assay employing liposomes in the microchannels are presented. 相似文献
880.
Strong adhesion and smooth conductive surface via graft polymerization of aniline on a modified glass fiber surface 总被引:1,自引:0,他引:1
The goal of this paper is to prepare surface-conductive glass fibers. The fibers were first hydroxylated, after which they were reacted with 3-bromopropyltrichlorosilane to form a bromopropylsilane monolayer, which was further reacted with aniline to substitute the bromine atoms. The surface-bound aniline molecules were used as active sites for the graft polymerization of polyaniline (PANI). The composition, structure and the morphology of the modified glass fiber surface were examined by X-ray photoelectron spectroscopy, scanning electron microscopy, and Fourier transform infrared spectroscopy. The method generated a smooth and homogenous thin layer of PANI on the surface of the glass fiber with a surface conductivity of about 6 S/cm, hence larger than the usual value ( approximately 1 S/cm) of the bulk polyaniline. 相似文献