全文获取类型
收费全文 | 1313篇 |
免费 | 67篇 |
国内免费 | 1篇 |
专业分类
化学 | 1039篇 |
晶体学 | 7篇 |
力学 | 11篇 |
数学 | 107篇 |
物理学 | 217篇 |
出版年
2024年 | 1篇 |
2023年 | 12篇 |
2022年 | 26篇 |
2021年 | 57篇 |
2020年 | 43篇 |
2019年 | 55篇 |
2018年 | 30篇 |
2017年 | 37篇 |
2016年 | 65篇 |
2015年 | 49篇 |
2014年 | 59篇 |
2013年 | 77篇 |
2012年 | 120篇 |
2011年 | 121篇 |
2010年 | 65篇 |
2009年 | 60篇 |
2008年 | 83篇 |
2007年 | 80篇 |
2006年 | 79篇 |
2005年 | 56篇 |
2004年 | 39篇 |
2003年 | 30篇 |
2002年 | 36篇 |
2001年 | 11篇 |
2000年 | 10篇 |
1999年 | 11篇 |
1998年 | 13篇 |
1997年 | 11篇 |
1996年 | 6篇 |
1995年 | 3篇 |
1994年 | 7篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 7篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1943年 | 1篇 |
排序方式: 共有1381条查询结果,搜索用时 15 毫秒
1.
2.
We report the synthesis, characterization, thermal and dilute-solution properties of zwitterionic copolymers of the sulfobetaine type, where the proportion of ionic groups is varied over a wide range. Two different structures are considered: poly (2-vinyl pyridine sulfopropyl betaine) and poly (4-vinyl pyridine sulfopropyl betaine). Synthesis is carried out to obtain random copolymers which are further characterized and their ionic content determined. At low proportions of ionic groups, the behavior is similar to that observed in ionomers, with substantial differences between the two structures, Increased interaction among the zwitterionic groups at higher charge densities is evidenced, which leads to the formation of clusters and agglomerates. An increase in the intrinsic viscosity with salt content in aqueous solutions is also observed. This behavior is analogous to that of homopolymers with 100% ionic content, but with particular features of interest. 相似文献
3.
Carolina Pereira De Souza Margareth Spangler Andrade Bernardo Ruegger Almeida Neves 《Microscopy and microanalysis》2002,8(6):509-517
In this work, effective, yet simple, recycling mechanisms for used scanning probe microscopy (SPM) tips were implemented. Comprising a tip profile characterization methodology and specific cleaning procedures, which decontaminate SPM tips whether the contamination nature is known or not, such routes were optimized during numerous tests with brand new, previously used, and already discarded categories of SPM tips. The results show that if the used tip suffered contamination only, but no physical damages, during its scanning lifetime, it becomes readily available for reutilization after the cleansing process, characterizing a recycling route. On the other hand, if the tip went through wear and breakages during its utilization, it still can be decontaminated, but may not be directly reutilized due to its inadequate physical profile. Nevertheless, the methodology developed in this work may yet be applied as part of a more complex recycling route. 相似文献
4.
J. G. Cardoso 《International Journal of Theoretical Physics》1997,36(7):1641-1649
A set of simple identities is utilized to build up new projective twistor diagrams for massless free fields of arbitrary spin
in real Minkowski space. It is effectively shown that the inner structure of the configurations which arise out of implementing
the relevant techniques has characteristics that are different from those of the conventional diagrams associated with the
Kirchoff-D'Adhemar-Penrose integral expressions. A nonhomogeneous version of the configurations is also provided. 相似文献
5.
Carolina de Gregorio Lemont B. Kier Lowell H. Hall 《Journal of computer-aided molecular design》1998,12(6):557-561
A structure-activity analysis of a series of steroids binding to corticosteroid-binding globulin was made using the electrotopological state index for each atom in the molecule. Two indices were found to correlate well with the binding affinity. The indices encode structural characteristics in the A and the D rings of the steroids in the study. One of the indices was formulated as the difference between two indices in the A ring. The two were not intercorrelated, suggesting that the composite index signals the influence of structure changes in or near the A ring that can be monitored by the composite index. This is a new observation using this structure-activity method. It is suggested that this model makes some contributions towards detection of the pharmacophore. 相似文献
6.
Fernández-Rivas C Méndez M Nieto-Oberhuber C Echavarren AM 《The Journal of organic chemistry》2002,67(15):5197-5201
The intramolecular reaction of allylsilanes and allylstannanes with alkynes proceeds catalytically in the presence of Pt(II), Pd(II), Ru(II), and Au(III) chlorides. Although more limited, AgOTf also catalyzes the cyclization. Usually, PtCl2 as the catalyst in methanol or acetone gives the best results. The reaction proceeds by exo attack of the allyl nucleophile on the alkyne to form five- or six-membered ring carbocycles. The reaction generally proceeds with anti stereoselectivity. However, a terminally substituted trimethylsilyl derivative reacts by a syn-type addition. The intermediate alkenylpalladium complex has been trapped with allyl chloride to form an allylated derivative with an additional carbon-carbon bond. 相似文献
7.
Cabrerizo FM Petroselli G Lorente C Capparelli AL Thomas AH Braun AM Oliveros E 《Photochemistry and photobiology》2005,81(5):1234-1240
Pterins are heterocyclic compounds with important biological functions, and most of them may exist in two acid-base forms in the pH range between 3 and 13 in aqueous solution. In this work, the photophysical properties of acid and basic forms of six compounds of the pterin family (6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT], N-methylfolic acid [MFA], and pteroic acid [PA]) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety and the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single-photon-counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) and short hydrocarbon chain (4 carbon atoms) (RPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (Phi(F)) of the first group of compounds are relatively high (>/=0.4), whereas MFA and PA exhibit very small Phi(F) values (=0.01). 相似文献
8.
The phosphoramidate-like reaction of 1,3-dithiole derived N-(diethoxyphosphinyl)hydrazones with formyl derivatives of 1,3-dithiole affords the longest aza-analogues of extended tetrathiafulvalenes with a polyenic spacer reported to date. Their structural and electrochemical properties are discussed. 相似文献
9.
M. Costa-Ferreira A. Dias C. Maximo M. J. Morgado G. Sena-Martins J. Cardoso Duarte 《Applied biochemistry and biotechnology》1994,44(3):231-242
Production of xylanolytic enzymes by anAspergillus niger CCMI 850 isolate was investigated in batch cultures. The effect of the composition of a fermentation medium that did not
include chemical inducers, on β-xylanase, β-xylosidase, α-l-arabinofuranosidase, and total cellulase activity was studied. With 4% xylan as the carbon source, about 65 U/mL of β-xylanase
was obtained, whereas the total cellulase activity was undetectable, under the specified conditions. This β-xylanase activity
represents the highest reported for a wild-type strain ofA. niger. The effect of pH and temperature on the activity of β-xylanase was studied. Partial characterization of the β-xylanase showed
that with insoluble birchwood as substrate theK
m
andV
max were 0.3 mM and 19 μmol/min, respectively. Aspects of using the crude β-xylanase preparation for applications in the pulp and paper industry
were discussed. 相似文献
10.
[reaction: see text]. Rate constants for hydrogen abstraction from phenols by a prefluorescent-TEMPO probe are reported. The nitroxide is employed as a potential model of peroxyl radicals. The probe works by nitroxide suppression of the fluorescence of the chromophore. The fluorescence is restored when the nitroxide abstracts a hydrogen atom to produce the diamagnetic hydroxylamine. The phenols studied in this project exhibited rate constants between 0.003 and 0.2 M(-1) s(-1). A deuterium isotope effect of 10 for TROLOX confirms that the mechanism is dominated by hydrogen transfer. 相似文献