全文获取类型
收费全文 | 79856篇 |
免费 | 24337篇 |
国内免费 | 17809篇 |
专业分类
化学 | 50465篇 |
晶体学 | 1047篇 |
力学 | 5394篇 |
综合类 | 108篇 |
数学 | 10253篇 |
物理学 | 54735篇 |
出版年
2024年 | 478篇 |
2023年 | 879篇 |
2022年 | 1010篇 |
2021年 | 898篇 |
2020年 | 1214篇 |
2019年 | 1709篇 |
2018年 | 1621篇 |
2017年 | 2245篇 |
2016年 | 2446篇 |
2015年 | 2674篇 |
2014年 | 2524篇 |
2013年 | 4409篇 |
2012年 | 4760篇 |
2011年 | 6279篇 |
2010年 | 9531篇 |
2009年 | 9738篇 |
2008年 | 3535篇 |
2007年 | 3027篇 |
2006年 | 2659篇 |
2005年 | 3021篇 |
2004年 | 3882篇 |
2003年 | 3128篇 |
2002年 | 2921篇 |
2001年 | 3181篇 |
2000年 | 2354篇 |
1999年 | 2614篇 |
1998年 | 2205篇 |
1997年 | 1992篇 |
1996年 | 2327篇 |
1995年 | 2761篇 |
1994年 | 2814篇 |
1993年 | 2876篇 |
1992年 | 2446篇 |
1991年 | 2115篇 |
1990年 | 1773篇 |
1989年 | 1861篇 |
1988年 | 1848篇 |
1987年 | 1137篇 |
1986年 | 1190篇 |
1985年 | 852篇 |
1984年 | 965篇 |
1982年 | 889篇 |
1981年 | 736篇 |
1980年 | 772篇 |
1979年 | 525篇 |
1978年 | 532篇 |
1977年 | 633篇 |
1976年 | 1041篇 |
1973年 | 437篇 |
1972年 | 532篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
Conclusions Karatavic acid has been isolated from the resin of the roots ofFerula karatavica Rgl. et Schmalh; this is an ether of umbelliferone and a hydroxy acid C15H24O3 probably having the humulane type of hydrocarbon skeleton. Two possible structural formulas—(I) and (II)—have been proposed for karatavic acid.Khimiya Prirodnykh Soedinenii, Vol. 4, No. 5, pp. 283–287, 1968 相似文献
982.
Summary An emission spectrographic method for the estimation of Ga, In and Tl in semiconductor grade selenium at less than a ppm concentration has been developed. The impurities were separated from 1 g of selenium by volatilizing it from its solution in nitric acid and sulphuric acid. The residue containing the impurities was dissolved in small amounts of 6 M HCl and was loaded along with the washings on 10 mg mixture containing graphite (90%) and sodium chloride carrier (10%) in the electrode crater. The spectra were excited in a d.c. arc carrying 13 A current. The detection limits lie in the range of 0.0025–0.005 ppm. The precision of the method ranges from 7–12% for these elements. The recoveries of impurities from selenium were confirmed using radioactive tracers.
Spektrographische Bestimmung von Gallium, Indium und Thallium in Halbleiterselen
Zusammenfassung Eine 1 g-Probe wird in Salpeter- und Schwefelsäure gelöst und die Verunreinigungen durch Verflüchtigung des Selens isoliert. Sie werden in 6 M HCl gelöst und zusammen mit einem Graphit/NaCl-Gemisch (91) in die Elektrode eingebracht. Zur Anregung dient ein Gleichstrombogen (13 A). Die Nachweisgrenzen liegen im Bereich von 0.0025–0.005 ppm, der Variationskoeffizient bei 7–12%. Die Wiedergewinnung der Verunreinigungen wurde mit Hilfe von Radioindicatoren geprüft.相似文献
983.
M. Yamanaka Y. Takeda S. Tanigawa A. Nishizawa N. Noda J. Fujita M. Takai M. Shimobayashi Y. Hayashi T. Koizumi K. Nagasaka S. Okajima Y. Tsunawaki A. Nagashima 《International Journal of Infrared and Millimeter Waves》1980,1(1):57-76
A twin optically-pumped far-infrared CH3OH laser has been constructed for use in plasma diagnostics. The antisymmetric doublet due to the Raman-type resonant two-photon transition is reproducibly observed at 118.8 m. With the 118.8-m line, it is obtained from the frequency separation of the anti-symmetric doublet that CH3OH absorption line center is 16±1 MHz higher than the pump 9.7-m P(36) CO2 laser line center. It is shown that the Raman-type resonant two-photon transition is useful in order to get several-MHz phase modulation for the far-infrared laser interferometer. Some preliminary performances of this twin laser for the modulated interferometer are described.This work was carried out under the collaborating research program at the Institute of Plasma Physics, Nagoya University, Nagoya 464, Japan. 相似文献
984.
R. Takke M. Niksch W. Assmus B. Lüthi R. Pott R. Schefzyk D. K. Wohlleben 《Zeitschrift für Physik B Condensed Matter》1981,44(1-2):33-39
We give experimental results of magnetic susceptibility, thermal expansion and elastic constant measurements for CeSn3 and CePd3. We interpret these results with a phenomenological scaling function for the free energy and we include a brief discussion of CeAl3. The coupling between the electronic system and the lattice can be described by an electronic Grüneisen parameter g associated with the unstable 4f system which turns out to become a system specific constant far below the fluctuation temperatureT
0. The coupling parameter T0g is an order of magnitude larger than the coupling constant for stable valentRE systems extracted from crystal field effects. A generalization to include magnetic field dependent effects is also given. 相似文献
985.
We use the Periodic Anderson Model (PAM) to describe the transport properties of intermediate valence compounds. The transport quantities of interest are related to the current-current response function. Therefore, a current operator being consistent with the PAM Hamiltonian must be defined. This aim is achieved by defining a proper particle density operator and using the continuity equation. The PAM is treated within the alloy analog approximation, i.e. it is replaced by the sum of two effective single-particle alloy-Hamiltonians, which are treated within the coherent potential approximation (CPA). Then the CPA for transport quantities is used to calculate the current-current response function. It is shown that for all reasonable assumptions for the conduction band and the hybridization dispersion the current vertex corrections vanish within the CPA. For different assumptions concerning the hybridization dispersion, we present numerical results for the temperature and parameter dependence of some transport quantities, in particular the static resistivity. The relevance of these results for the understanding of the typical experimental resistivity behaviour obtained for different intermediate valence compounds and possible shortcomings of our approach are discussed.Work performed within the research program of the Sonderforschungsbereich 125 Aachen-Jülich-Köln, FRG 相似文献
986.
The development of the hydration-hydrolysis processes in the 3CaOSiO2-H2O system is studied by X-ray diffraction in presence of varying contents of a new plasticizer, belonging to the lignosulphonates class. The influence of the additives upon these processes with increasing time is observed and it is shown to depend on the nature and content of the additives and the reaction time. This influence of the additives on the kinetics of the hydration-hydrolysis processes, retardation or acceleration, is due to the strong adsorption of these admixtures on the surface of the anhydrous or partially hydrated particles of the system. The influence of the additives also appears during the development of the formed hydrocompounds according to structure and composition. As a result of these complex actions provided in the forming system (3CaOSiO2-H2O), the mechanical strength is favorized mainly by certain proportions of plasticizer admixtures (0.1% LSC and 2% ADCOM). 相似文献
987.
Summary Amperometric ascorbimetric determinations of cerium(IV) and ferric iron have been carried out at 50°C with two polarized electrodes at 200 and 100 mV respectively. The results obtained are fairly accurate and precise within ±1.0 per cent. A simple method for successive determination of cerium and iron has been developed; and conditions for such estimations have been established. At an acidity of 2.5 M with respect to sulphuric acid, it is possible to ward off the reduction of ferric iron and thereby cerium(IV) is successfully titrated with ascorbic acid in this medium. After completion of the reaction and then lowering the acid concentration to pH 1.5 with aid of ammonium hydroxide, FeIII is titrated with standard ascorbic acid yielding good results. 相似文献
988.
R. Fricke H. -G. Jerschkewitz G. Öhlmann 《Reaction Kinetics and Catalysis Letters》1981,18(3-4):479-483
The effect of alkali metal admixtures (Li, Na, K, Rb, and Cs) on V/SiO2 catalysts was studied by ESR spectroscopy. Alkali metals strongly change the VO2+ spectra. Reduction leads to spectra showing a slow rotational motion of VO2+ species on the support surface. The proposed explanation includes former structural models and the formation of salts of polyvanadium acid.
(Li, Na, K, Rb Cs) V/SiO2 . VO2+. , VO2+ . .相似文献
989.
K. Samikov D. U. Abdullaeva R. Shakirov S. Yu. Yunusov 《Chemistry of Natural Compounds》1980,15(6):728-731
Korsevine and the new alkaloids severine N-oxide and korsemine have been isolated from the combined alkaloids ofKorolkowia sewerzowii Regel. On the basis of the results of a study of the IR and NMR spectra of the alkaloids and also of a correlation with korseveriline and korsevine it has been established that base (I) is severine N-oxide and korsemine has the structure of korsevine 3-D-glucopyranoside.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 823–826, November–December, 1979. 相似文献
990.
T. Borowiecki A. Machocki D. Nazimek J. Barcicki 《Reaction Kinetics and Catalysis Letters》1984,26(3-4):285-289
The coking resistance of six alumina supported nickel catalysts in n-butane steam reforming mainly depends on the average size of nickel crystallites. Thus, by using suitable preparative methods, it is possible to produce good, coking resistant nickel catalysts even with Al2O3 support.
Al2O3 -, , . . ., , - Al2O3.相似文献