全文获取类型
收费全文 | 148092篇 |
免费 | 36230篇 |
国内免费 | 31395篇 |
专业分类
化学 | 57613篇 |
晶体学 | 1583篇 |
力学 | 6134篇 |
综合类 | 800篇 |
数学 | 11486篇 |
物理学 | 60216篇 |
综合类 | 77885篇 |
出版年
2024年 | 755篇 |
2023年 | 1371篇 |
2022年 | 2137篇 |
2021年 | 2258篇 |
2020年 | 3124篇 |
2019年 | 5751篇 |
2018年 | 5854篇 |
2017年 | 6901篇 |
2016年 | 7197篇 |
2015年 | 7816篇 |
2014年 | 8382篇 |
2013年 | 9735篇 |
2012年 | 9849篇 |
2011年 | 11414篇 |
2010年 | 14736篇 |
2009年 | 14583篇 |
2008年 | 8152篇 |
2007年 | 7533篇 |
2006年 | 6507篇 |
2005年 | 6530篇 |
2004年 | 7185篇 |
2003年 | 5576篇 |
2002年 | 5151篇 |
2001年 | 5084篇 |
2000年 | 4108篇 |
1999年 | 3612篇 |
1998年 | 2706篇 |
1997年 | 2482篇 |
1996年 | 2614篇 |
1995年 | 3053篇 |
1994年 | 3174篇 |
1993年 | 3338篇 |
1992年 | 2887篇 |
1991年 | 2587篇 |
1990年 | 2116篇 |
1989年 | 2173篇 |
1988年 | 2203篇 |
1987年 | 1328篇 |
1986年 | 1368篇 |
1985年 | 987篇 |
1984年 | 1041篇 |
1982年 | 923篇 |
1981年 | 768篇 |
1980年 | 796篇 |
1979年 | 557篇 |
1978年 | 543篇 |
1977年 | 642篇 |
1976年 | 1046篇 |
1973年 | 444篇 |
1972年 | 535篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
D. V. Sokolskii A. K. Zharmagambetova S. G. Mukhamedzhanova E. A. Bekturov S. E. Kudaibergenov 《Reaction Kinetics and Catalysis Letters》1987,33(2):387-392
The title complexes exhibit high catalytic activity, selectivity and stability in hydrogenation of allyl alcohol. The structure of a polymeric ligand (i.e. the position of the nitrogen atom in the pyridine ring) affects catalyst activity. According to the hydrogenation rates of allyl alcohol, the complexes studied are arranged in the sequence: P4VP-Pd>P2VP-Pd>P2M5VP-Pd.
, - - . ( ) . : 4->2-Pd>25-.相似文献
992.
M. V. Reshetnyak S. G. Il''in N. K. Utkina A. I. Yanovskii Yu. T. Struchkov 《Chemistry of Natural Compounds》1988,24(3):360-363
DL-Allantoin (I) has been isolated from an alcoholic extract of a marine spongeAxinellidae gen. sp., and derivatives of it have been obtained: 1, 8-diacetyl-DL-allantoin (II) and 1,8-diacetyl-3-methyl-DL-allantoin (III). The crystal and molecular structures of (III) have been established by X-ray structural analysis.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, USSR Academy of Sciences, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 422–426, May–June, 1988. 相似文献
993.
Summary Using 75Se as a radiotracer, the preatomization behaviour of selenium in the graphite furnace was studied. The selenium forms investigated included Se(-II)-methionine, selenite, and selenate in a 0.2% HNO3 solution, and in a 0.2% HNO3 solution containing 1% NaCl. The effect of nickel nitrate and of the mixture of palladium/magnesium nitrates as matrix modifiers and of boron nitride coating of the graphite tube on the behaviour of selenium was investigated. The best stabilization effect for all oxidation states of selenium in the conventional graphite tube was achieved by using the mixture Pd/Mg. A considerable degree of modifier-free stabilization of selenuium could be achieved in boron nitride coated tubes. After the conversion of Se(IV) to a volatile piaselenol, a quantitative preatomization separation of Se(IV) from Se(VI) in the boron nitride coated tube was possible. However problems with these newtype tubes still to be solved include the need to increase the thermal stability of the coating. 相似文献
994.
The reactions of 3-carene and -terpinene with dialkyl disulfides have been studied.S. V. Kurashov Kazan' State Medical Institute. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 624–626, September–October, 1990. 相似文献
995.
20-Deoxoluteone has been synthesized from sclareol. Sclareol was brominated with phosphorus tribromide to form a mixture of primary allyl bromides, from which, by the malonic synthesis, a mixture of bicyclogeranylgeranylacetic acids was obtained which was cyclized with fluorosulfonic acid to form a mixture of two diastereomeric -lactones. The predominating lactone was converted by successive reduction with lithium tetrahydroaluminate, oxidation with oxalyl chloride in dimethyl sulfoxide, reaction with methylmagnesium iodide, and oxidation by the chromium trioxide/pyridine complex into 20-deoxyluteone.Institute of Chemistry, Moldavian SSR Academy of Science, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 346–353, May–June, 1990. 相似文献
996.
Summary Atmospheric precipitation and aerosol samples are characterized by a complex mixture of several organic compounds. A simple method for the simultaneous determination of the main compound classes by GC/MS is presented. In detail, seasonal variations of C8–C32 fatty acids in precipitation in a semirural area have been studied. Total fatty acids concentrations of 7–53 g/l were detected. Summer rain is characterized by high amounts of fatty acids >C20 and lower amounts of C11–C20 species. Two effects may be responsible: larger emission rates of fatty acids during vegetation periods and an increased influence of vapour phase due to higher temperatures during summer. CPI values showed no general trend; lower CPI values for winter rain could not be observed. This would be expected if anthropogenic sources play an important role. Monounsaturated fatty acids (C16:1, C18:1) were more abundant during winter than in summer. High concentrations of polyunsaturated fatty acids (C18:x) could be detected during summer. High relative concentrations of the potential oxidation products of the unsaturated species, nonanedioic acid and w-oxononanoic acid, are associated with relatively low concentrations of their precursors. 相似文献
997.
B. S. Balzhinimaev V. E. Ponomarev G. K. Boreskov A. A. Ivanov V. S. Sheplev E. M. Sadovskaya 《Reaction Kinetics and Catalysis Letters》1985,28(1):81-88
Diffusion and solubility coefficients of O2, SO2 and SO3 in the active component of vanadium catalysts have been determined by a high-speed relaxation method. Processes of their dissolution are shown to be quasi-equilibrium with respect to catalytic reactions.
O2, SO2 SO3 ,相似文献
998.
Jií Davídek Miloslava Neméthová Josef Seifert 《Fresenius' Journal of Analytical Chemistry》1977,287(4-5):286-287
Summary An indirect polarographic method for the determination of phosmet was elaborated. It is based on the fact that N-hydroxymethylphthalimide formed by the reaction of phosmet with sulphuric acid is polarographically active and exhibits a behaviour during electrolysis at a Hg dropping-electrode similar to that of phthalimide. The method has been applied to the determination of phosmet in apples. The sensitivity is 0.1 ppm.
Indirekte polarographische Bestimmung von Phosmet O,O-Dimethyl-S-(phthalimidomethyl)dithiophosphat
Zusammenfassung Das Verfahren beruht darauf, daß N-Hydroxymethylphthalimid, das bei der Reaktion von Phosmet mit Schwefelsäure gebildet wird, polarographisch aktiv ist und sich bei der Elektrolyse mit der Quecksilber-Tropfelektrode ähnlich wie Phthalimid verhält. Die Methode wurde zur Bestimmung von Phosmet in Äpfeln angewandt. Die Empfindlichkeit beträgt 0,1 ppm.相似文献
999.
M. Kh. Dzhukharova Z. Saatov N. D. Abdullaev N. K. Abubakirov 《Chemistry of Natural Compounds》1995,30(6):680-683
A new brahuisterone glycoside — silenoside F — has been isolated from the epigeal part ofSilene brahuica Bois. (fam. Caryophyllaceae). Its structure has been established by an analysis of spectral characteristics: 3,5,14,22R,25-pentahydroxy-5-cholest-6-one 3-O--D-glucopyranoside.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, fax (3712) 89 14 75. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 734–737, November–December, 1994. 相似文献
1000.
S. S. Basson L. D. C. Bok S. R. Grobler 《Fresenius' Journal of Analytical Chemistry》1972,260(2):117-119
Summary N-Bromosuccinimide has been successfully applied in the determination of 1–50 mg of W(CN)8
4–in approximately 6 M hydrochloric acid using Naphthol Blue Black indicator. The average error is ±0.42% and the average standard deviation is ±0.06 mg. The method is not applicable to the determination of Mo(CN) 8
4– ions. Tetrakis-(N-hydro-2-carbocyclic acid-pyridinium)octacyanotungstate(IV) and -molybdate(IV) were used as primary standards for the preparation of standard Mo(IV) and W(IV) solutions. The composition of the latter compounds was verified by analysis and single crystal X-ray data were obtained.
Titrimetrische Bestimmung von Octacyanowolframat(IV)-ionen mit N-Bromsuccinimid
Zusammenfassung N-Bromsuccinimid wurde mit Erfolg zur Bestimmung von 1–50 mg W(CN)8 4–-Ionen in ungefähr 6 M Salzsäure mit Naphthol-Blau-Schwarz als Indicator verwendet. Der mittlere Fehler beträgt ±0,42 %, die mittlere Abweichung ±0,06 mg. Die Methode ist nicht brauchbar für die Bestimmung von Mo(CN)8 4–-Ionen. Tetrakis(N-hydro-2-carbonylsäure-pyridinium) octacyanowolframat(IV) und -molybdat(IV) wurden als Primärstandards für die Herstellung von Standard-Mo(IV)- und -W(IV)-Lösungen benutzt. Die Zusammensetzung dieser Verbindungen wurde durch Analyse kontrolliert und die Einkristall-Röntgen-Daten wurden festgestellt.相似文献