全文获取类型
收费全文 | 27677篇 |
免费 | 374篇 |
国内免费 | 41篇 |
专业分类
化学 | 24800篇 |
晶体学 | 65篇 |
力学 | 137篇 |
数学 | 1713篇 |
物理学 | 1377篇 |
出版年
2021年 | 250篇 |
2020年 | 202篇 |
2019年 | 225篇 |
2016年 | 319篇 |
2015年 | 313篇 |
2014年 | 261篇 |
2013年 | 568篇 |
2012年 | 595篇 |
2011年 | 679篇 |
2010年 | 414篇 |
2009年 | 368篇 |
2008年 | 576篇 |
2007年 | 622篇 |
2006年 | 546篇 |
2005年 | 533篇 |
2004年 | 482篇 |
2003年 | 393篇 |
2002年 | 330篇 |
1988年 | 208篇 |
1987年 | 239篇 |
1986年 | 225篇 |
1985年 | 315篇 |
1984年 | 311篇 |
1982年 | 243篇 |
1981年 | 190篇 |
1980年 | 419篇 |
1979年 | 333篇 |
1978年 | 1126篇 |
1977年 | 881篇 |
1976年 | 307篇 |
1974年 | 187篇 |
1968年 | 238篇 |
1967年 | 209篇 |
1966年 | 241篇 |
1965年 | 274篇 |
1964年 | 243篇 |
1963年 | 184篇 |
1962年 | 377篇 |
1961年 | 292篇 |
1960年 | 389篇 |
1959年 | 301篇 |
1958年 | 305篇 |
1956年 | 239篇 |
1955年 | 230篇 |
1937年 | 231篇 |
1936年 | 214篇 |
1935年 | 231篇 |
1934年 | 184篇 |
1933年 | 250篇 |
1930年 | 217篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Paolo Ferruti Elisabetta Ranucci Amedea Manfredi Nicolò Mauro Elena Ferrari Riccardo Bruni Francesco Colombo Patrizia Mussini Manuela Rossi 《Journal of polymer science. Part A, Polymer chemistry》2012,50(24):5000-5010
Traditional precipitation methods for inorganic micropollutant removal from waters are increasingly being replaced by sorption methods based on both natural and synthetic materials. In this context, two novel effective heavy metal ions absorbers are presented. These resins, LYMA and LMT85, were crosslinked poly(amidoamine)s carrying amine and carboxyl groups in their repeating units. In particular, the LYMA‐repeating unit contains one carboxyl and two amine groups and is a mimic of L ‐lysine, whereas LMT85 contains two amine and five carboxyl groups and is a mimic of EDTA. Both resins were prepared at moderate cost by simple eco‐friendly procedures. The heavy metal ion set adopted as benchmark was Cu2+, Cd2+, Pb2+, Zn2+, Ni2+, and Co2+. LYMA proved selective for Cu2+ and Ni2+, the other ions tested being negligibly absorbed, whereas LMT85 proved capable of rapidly and quantitatively absorbing all the ions tested either singly or in mixed solution. The absorption process was reversible, and the resins were easily regenerated by acidification. The absorption of several metal ions imparted intense coloring to the resins, a feature possibly exploitable for analytical purposes. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 相似文献
993.
Itziar Irigoien Concepcion Arenas Elena Fernández Francisco Mestres 《Computational Statistics》2010,25(2):241-255
This paper, arising from population studies, develops clustering algorithms for identifying patterns in data. Based on the
concept of geometric variability, we have developed one polythetic-divisive and three agglomerative algorithms. The effectiveness
of these procedures is shown by relating them to classical clustering algorithms. They are very general since they do not
impose constraints on the type of data, so they are applicable to general (economics, ecological, genetics...) studies. Our
major contributions include a rigorous formulation for novel clustering algorithms, and the discovery of new relationship
between geometric variability and clustering. Finally, these novel procedures give a theoretical frame with an intuitive interpretation
to some classical clustering methods to be applied with any type of data, including mixed data. These approaches are illustrated
with real data on Drosophila chromosomal inversions. 相似文献
994.
995.
996.
Riet Dams Elena Bernabe Anna Nicoletti Claudio Loda Luca Martini Damiano Papini 《Rapid communications in mass spectrometry : RCM》2010,24(17):2650-2654
The introduction of Quality by Design (QbD) in Drug Development has resulted in a greater emphasis on chemical process understanding, in particular on the origin and fate of impurities. Therefore, the identification and quantitation of low level impurities in new Active Pharmaceutical Ingredients (APIs) play a crucial role in project progression and this has created a greater need for sensitive and selective analytical methodology. Consequently, scientists are constantly challenged to look for new applications of traditional analytical techniques. In this context a normal‐phase liquid chromatography/electrospray ionization mass spectrometry (LC/ESI‐MS) method was developed to determine the amount of a de‐fluorinated analogue impurity in Casopitant Mesylate, a new API under development in GlaxoSmithKline, Verona. Normal‐phase LC provided the selectivity needed between our target analyte and Casopitant, while a single quadrupole mass spectrometer was used to ensure the sensitivity needed to detect the impurity at <0.05%w/w. Standard solutions and samples were prepared in heptane/ethanol (50:50, v/v) containing 1% of 2 M NH3 in ethanol; the mobile phase consisted of heptane/ethanol (95:5, v/v) with isocratic elution (flow rate: 1.0 mL/min, total run time: 23 min). To allow the formation of ions in solutions under normal‐phase (apolar) conditions, a post‐column infusion of a solution of 0.1% v/v of formic acid in methanol was applied (flow rate: 200 µL/min). The analysis was carried out in positive ion mode, monitoring the impurity by single ion monitoring (SIM). The method was fully validated and its applicability was demonstrated by the analysis of real‐life samples. This work is an example of the need for selective and accurate methodology during the development of a new chemical entity in order to develop an appropriate control strategy for impurities to ultimately ensure patient safety. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
997.
Paul Podsiadlo Galyna V. Krylova Arnaud Demortière Elena V. Shevchenko 《Journal of nanoparticle research》2011,13(1):15-32
In this article, we review the state-of-the-art in the preparation and characterization of multicomponent self-assembled superlattices
of colloidal nanoparticles with core sizes in the range of 2–20 nm and interparticle spacing less than 2 nm down to intimate
contact stemming from sintering. Several aspects of the field are discussed, including: structural organization, the role
of particle size distribution, key interparticle forces at play, and methods of investigation of the structures. Contrary
to the extensively studied colloidal crystals composed of microscale particles, the nanoparticles possess unique size-dependent
properties, such as electronic, optical, or magnetic, which when combined into periodic structures can potentially lead to
new collective states stemming from precise positioning of the nanocolloids. As such, we examine a number of emerging applications
of this new class of metamaterials. Finally, we speculate on the potential impact of these materials, the new directions,
and the challenges for the researchers. 相似文献
998.
999.
M. Elena Mart��n M. Luz S��nchez Jos�� C. Corchado Aurora Mu?oz-Losa Ignacio Fdez. Galv��n Francisco J. Olivares del Valle Manuel A. Aguilar 《Theoretical chemistry accounts》2011,128(4-6):783-793
The possible influence of the solvent Stark effect (SSE) on the solvatochromic shift in electron transitions has been analyzed by using the ASEP/MD (averaged solvent electrostatic potential from molecular dynamics) method. With this purpose, four molecules, two polar (acrolein and formaldehyde) and two non-polar (p-difluorobenzene and trans-difluoroethene) have been studied in solvents of diverse polarity. Independently of the nature of the system we found that the contribution of SSE on the average value of the solvent shift or on the multipole moment values is negligible. In the case of centro-symmetric molecules, our results permit to discard the SSE as cause of the solvent shift found, which must be assigned to the electrostatic interaction of the solute quadrupole and higher multipoles with the solvent. As the SSE values provide also a measure of the errors introduced by the mean field approximation (MFA), these results indicate that MFA permits a very accurate determination of the solvent shift at the same time that it reduces drastically the computational cost. Finally, a new procedure suited to the ASEP/MD method has been presented that permits to estimate the inhomogeneous broadening of spectral bands, complementing the information provided by mean field theories. This procedure does not need additional quantum calculations and its computational cost is minimal. 相似文献
1000.