全文获取类型
收费全文 | 47812篇 |
免费 | 1654篇 |
国内免费 | 165篇 |
专业分类
化学 | 32051篇 |
晶体学 | 357篇 |
力学 | 855篇 |
综合类 | 4篇 |
数学 | 7328篇 |
物理学 | 9036篇 |
出版年
2023年 | 339篇 |
2022年 | 519篇 |
2021年 | 731篇 |
2020年 | 772篇 |
2019年 | 784篇 |
2018年 | 938篇 |
2017年 | 882篇 |
2016年 | 1675篇 |
2015年 | 1380篇 |
2014年 | 1438篇 |
2013年 | 2921篇 |
2012年 | 3121篇 |
2011年 | 3448篇 |
2010年 | 2105篇 |
2009年 | 1862篇 |
2008年 | 2972篇 |
2007年 | 2885篇 |
2006年 | 2594篇 |
2005年 | 2357篇 |
2004年 | 1924篇 |
2003年 | 1554篇 |
2002年 | 1325篇 |
2001年 | 1015篇 |
2000年 | 897篇 |
1999年 | 694篇 |
1998年 | 515篇 |
1997年 | 443篇 |
1996年 | 591篇 |
1995年 | 413篇 |
1994年 | 421篇 |
1993年 | 450篇 |
1992年 | 421篇 |
1991年 | 312篇 |
1990年 | 300篇 |
1989年 | 242篇 |
1988年 | 234篇 |
1987年 | 244篇 |
1986年 | 235篇 |
1985年 | 306篇 |
1984年 | 291篇 |
1983年 | 220篇 |
1982年 | 222篇 |
1981年 | 193篇 |
1980年 | 171篇 |
1979年 | 171篇 |
1978年 | 143篇 |
1977年 | 139篇 |
1976年 | 131篇 |
1975年 | 114篇 |
1973年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
191.
Please forward books for review to the book review editor: Magdolna Hargittai, Structural Chemistry Research Group of the Hungarian Academy of Sciences, Eötvös University, H-1431 Budapest, Pf. 117, Hungary 相似文献
192.
R. Andreu I. Gayn J. Garín J. Orduna M. Savirn 《Journal of mass spectrometry : JMS》1994,29(6):321-325
The electron impact ionization mass spectra of 4,5-bis(alkylthio)-1,3-dithiole-2-thiones and their 1,2-dithiole-3-thione isomers were studied by accurate mass measurements and linked scans. The relative abundance of ions formed following the extrusion of S2, CS or CS2 allows an unambiguous isomer differentiation. Isomerization of molecular ions was studied by means of metastable ion analysis and collision-induced dissociation. The order of reactivity was analogous to that observed in isomerization under photochemical conditions. 相似文献
193.
The electron impact ionization mass spectra of o-, m- and p-bis(1,4-dithiafulven-6-yl)benzenes were studied by means of accurate mass measurements, metastable analysis and collision-induced dissociation. Differences observed in the spectra of the ortho isomers are due to a cyclization reaction leading to molecular ions with the same structure as those generated from certain cyclic compounds, as confirmed by comparison of linked scans at constant B/E of metastable and collisionally activated molecular ions. Parallels of this cyclization of molecular ions with their electrochemical or acid-induced isomerization are also discussed. 相似文献
194.
Oxyethylene macrocyclic compounds (crown-ethers) act as ligands of intracrystalline cations of certain layered silicates as montmorillonites. Stable intercalation materials are formed which are used to prepare organic-inorganic membranes by encapsulating these intercalation compounds with a poly-butadiene thin coating. Electrochemical Impedance Spectroscopy (EIS) is used to study the resulting composite membranes in contact with aqueous electrolytes. From the impedance plots, the ionic resistance of the membranes is obtained. The thickness of the polybutadiene coating is an important factor determining the ability of ions to pass across the membrane. Marked differences in the ionic resistance are observed as a function of the nature of the interlayer macrocyclic compound. For non-intercalated montmorillonite membranes, the ionic resistance is strongly reduced, whereas for some crown-ether intercalated materials such as 18-crown-6 and dibenzo 24-crown-8, iono-selective membranes are obtained. Concerning the nature of the electrolyte, cations exhibiting greater hydration energies show higher difficulties to pass through the membrane and, consequently, the ionic resistance increases. 相似文献
195.
Manuel Fernández-López Eduardo García-Río Demir N. Kupeli 《Annals of Global Analysis and Geometry》2002,21(1):1-13
A characterization of time functions on a spacetime is made by using theMöbius equation. It is shown that a time function characterized in this wayyields past timelike geodesic incompleteness and local Lorentzian warpedproduct decomposition of spacetime, provided that the stress-energy tensoris a fluid. Also, by imposing additional assumptions on the stress-energytensor and global analytic structure of the spacetime, more restrictivedecompositions closer to Robertson–Walker spacetimes are obtained. 相似文献
196.
197.
J. A. Murillo J. M. Lemus L. F. García 《Analytical and bioanalytical chemistry》1994,349(10-11):761-767
First-derivative spectrophotometric method with a “zero-crossing” technique of measurement has been used for the quantification of two-components mixtures of cephalothin and clavulanic acid. As the absorption bands of these drugs overlap, both direct and derivative spectrophotometric methods have been investigated and evaluated by a rigorous statistical analysis of the experimental data. The first-derivative spectrophotometric method was found to be more accurate, direct and reproducible. Beer's law was valid over the concentration range 2.0–28.0 mg/l for both compounds. The detection limits of cephalothin and clavulanic acid, at a 0.05 level of significance, were calculated to be 0.13 and 0.15 mg/l. The method was applied for determining these antibiotics in mixtures, some of them containing inject-able dosage forms of cephalothin, and so to determine both compounds in saline and glucosed physiological sera. 相似文献
198.
A new concept of orthogonality in real normed linear spaces is introduced. Typical properties of orthogonality (homogeneity, symmetry, additivity, ...) and relations between this orthogonality and other known orthogonalities (Birkhoff, Boussouis, Unitary-Boussouis and Diminnie) are studied. In particular, some characterizations of inner product spaces are obtained. 相似文献
199.
Mária Meiarová Katarína Hubinská tefan Toma Burkhard Koch Albrecht Berkessel 《Monatshefte für Chemie / Chemical Monthly》2007,138(11):1181-1186
Summary.
N-Toluensulfonyl-l-prolin amide was tested as catalyst in the enantioselective Michael addition of carbonyl compounds to (E)-β-nitrostyrene in nine ionic liquids under different reaction conditions. The reaction rates and enantioselectivities were
strongly dependent on the ionic liquids. Change of enantioselectivity was observed too and it is attributed to both the cation
and the anion of ionic liquid. The best yields (up to 98%) and enantioselectivity (70% ee) of product were obtained in a basic ionic liquid [bmim]BF4 at room temperature. 相似文献
200.
Juan F García-Reyes Pilar Ortega-Barrales Antonio Molina-Díaz 《Analytical sciences》2007,23(4):423-428
A combined approach based on solid-phase optosensing and multicommutation principles has been applied to develop a method for the simultaneous analysis of two pharmaceuticals (naproxen and salicylic acid) in biological fluids. The multicommuted flow-through optosensor was based on direct native fluorescence measurements of both analgesics using a non-polar sorbent (C18 silica gel) as a solid sensing zone. The flow system was controlled by Java-written home-made software and designed using three-way solenoid valves for an independent automated manipulation of sample and carrier solutions. Using an optimized sampling time, the method was calibrated in the range of 1 - 25 and 5 - 200 ng mL(-1). The obtained detection limits were 0.3 and 1.3 ng mL(-1) for naproxen and salicylic acid, respectively, with RSD (%) values of better than 2% for both analytes. The proposed methodology was successfully applied to urine, serum and pharmaceutical preparations. Recovery percentages ranging from 96.1 to 104% were obtained for both analytes. 相似文献