全文获取类型
收费全文 | 1383篇 |
免费 | 49篇 |
国内免费 | 4篇 |
专业分类
化学 | 890篇 |
晶体学 | 14篇 |
力学 | 51篇 |
数学 | 168篇 |
物理学 | 313篇 |
出版年
2023年 | 13篇 |
2022年 | 30篇 |
2021年 | 27篇 |
2020年 | 16篇 |
2019年 | 20篇 |
2018年 | 22篇 |
2017年 | 22篇 |
2016年 | 45篇 |
2015年 | 40篇 |
2014年 | 41篇 |
2013年 | 84篇 |
2012年 | 88篇 |
2011年 | 90篇 |
2010年 | 65篇 |
2009年 | 85篇 |
2008年 | 79篇 |
2007年 | 91篇 |
2006年 | 68篇 |
2005年 | 59篇 |
2004年 | 44篇 |
2003年 | 35篇 |
2002年 | 52篇 |
2001年 | 39篇 |
2000年 | 45篇 |
1999年 | 26篇 |
1998年 | 15篇 |
1997年 | 18篇 |
1996年 | 15篇 |
1995年 | 14篇 |
1994年 | 5篇 |
1993年 | 11篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 14篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 8篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 3篇 |
1974年 | 3篇 |
1972年 | 1篇 |
1966年 | 1篇 |
1935年 | 1篇 |
排序方式: 共有1436条查询结果,搜索用时 15 毫秒
21.
Maria de Fátima V. Marques Danielle Ramos Juliana D. Rego 《European Polymer Journal》2004,40(11):2583-2589
Ethylene and different amounts of 1,7-octadiene were copolymerized using the metallocene catalyst system ethylidene-bis(fluorenyl) zirconium dichloride and methylaluminoxane (MAO) at both 50 and 90 °C. The catalyst activity has slightly increased with the addition of low amounts of the diene in relation to the homopolymerization of ethylene. The obtained polymers were characterized according to their melting temperature (Tm) and crystallinity degree (xc) by differential scanning calorimetry (DSC). Weight-average molecular weight (Mw) and polydispersity were determined by gel permeation chromatography (GPC). Diene contents in the copolymer were obtained through the FTIR spectroscopy. The results indicated that at polymerization temperature of 90 °C, crosslinking bonds in the obtained copolymers were low, differently from what was observed at 50 °C. The diene content in the copolymer achieved more than 3 mol% and the comonomer conversion was around 15%. Moreover, the obtained copolymers have Mw around 100,000 and large polydispersity. 相似文献
22.
Joo Bosco P. Da Silva Mozart N. Ramos 《International journal of quantum chemistry》1992,43(2):215-219
Ab initio MP 2/4-31G ** calculations indicate that the most stable form of C3NH is bent and singlet and that the linear structure corresponds to a maximum. The effect of changing the CNH angle on the total energy is slight, but it is quite pronounced on the molecular polarity. The wider angle tends to increase the polarity of C3NH. MP 2/4-31G ** calculations predict a difference of polarity between linear and bent structures of 0.8 D. 相似文献
23.
The application of a program written in BASIC for an IBM/PC interfaced with a commercial spectrofluorimeter enables variable-angle scanning fluorescence spectra to be obtained, without modification of the instrument. The technique is effective for determinations of the components of mixtures of carbaryl, fuberidazol and warfarin, all of which are pesticides with intrinsic fluorescence and closely overlapping profiles. The proposed method permitted simultaneous determinations with RSD of less than 2.5% and recoveries of 99–110%. 相似文献
24.
The feasibility of comprehensive two-dimensional gas chromatography with electron-capture detection (GC x GC-ECD) for the enantioseparation of chiral PCBs from other possible interfering compounds has been evaluated. Three commercially available enantioselective beta-cyclodextrin-based capillary columns (Chirasil-Dex, BGB-172 and BGB-176SE) have been tested as first-dimension columns. Three non-enantioselective stationary phases (HT-8, BPX-50 and Supelcowax-10) were combined with the enantioselective columns to allow the unambiguous determination of the enantiomers of the target chiral PCBs. Each enantioselective first-dimension column tested was able to separate into enantiomers different PCB congeners, but in all cases, the use of Supelcowax-10 as second-dimension column provided the most satisfactory results. The Chirasil-Dex x Supelcowax-10 column combination allowed the determination of the enantiomeric fraction (EF) of PCBs 84, 91, 95, 132, 136, 149, 174 and 176 in the working standard solution, while that of congener 135 was hindered. The BGB-172 x Supelcowax-10 column set allowed a proper EF determination of congeners 45, 84, 131, 132, 135, 171, 174 and 183, while that of PCB 91 was interfered with co-elutants. The column combination BGB-176SE x Supelcowax-10 allowed the determination of all congeners that this enantioselective stationary phase was able to separate into enantiomers, i.e. PCBs 45, 91, 95, 136, 149 and 176. These column combinations have also been evaluated for the simultaneous determination of the 12 congeners with a toxic equivalency factor assigned by the WHO (PCBs 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, 189) and the seven indicator congeners (PCBs 28, 52, 101, 118, 138, 153 and 180), and evaluated for the analysis of food samples. 相似文献
25.
Information content of data and variables and types of weighting in least-squares regression methods
Algorithms are given for evaluating the relative amount of useful information related to a particular parameter which is carried by individual data points and intervals of the variables. The algorithms provide an efficient means of using the information contained in a set of data. Applications to the optimization of weighting in regression methods are described. Several informational and combined informational-statistical types of weighting are studied as a means of improving the accuracy and precision of the parameters obtained by non-linear regression. 相似文献
26.
This research was aimed
to investigate the role of clay on the combustion and kinetic behavior of
crude oils in limestone matrix. For this purpose, simultaneous TG (thermogravimetry)
and DTA (differential thermal analysis) experiments were performed at three
different heating rates as 10–15 and 20°C min–1,
respectively. A uniform trend of decreasing activation energies was observed
with the addition of clay. It was concluded that clays surface area affects
the values of Arrhenius constant, while it is the catalytic properties of
clay, which lower the activation energies of all the reactions, involved in
the combustion process. 相似文献
27.
The aim of the present work is to characterize the pigments present in the kernel of four native maize varieties related to the races Arrocillo, Cónico, Peruano, and Purepecha to determine their possible use as natural dyes. Total anthocyanin content is determined by a conventional spectrophotometric method, and anthocyanin analysis is done by high-performance liquid chromatography. The stability of the pigment at pH is also evaluated. The four maize samples contained anthocyanin in both the pericarp and aleurone layer. Total anthocyanin content among samples ranged from 54 mg/100 g of sample to 115 mg/100 g of sample. Anthocyanin profiles are almost the same among the four samples. Differences are observed only in the relative percentage of each anthocyanin. The anthocyanins identified are cyanidin-3-glucoside, pelargonidin- 3-glucoside, peonidin-3-glucoside, cyanidin-3-(6" malonylglucoside), and cyanidin-3-(3",6"-dimalonylglucoside). Anthocyanin extracts showed similar behavior in solutions with different pH. From pH 1-6 lambdamax values are maintained almost constant; however, above this pH value, a marked increase is observed in the bathochromic shifts, but the bluish color did not continue to change above pH 8. 相似文献
28.
The effects of anionic, non-ionic, and cationic surfactants and thallium(I) nitrate on the paper-substrate room-temperature phosphorescence (RTP) of eleven phenothiazine derivatives are described. Enhancement factors within the range 1.5–10 are observed after addition of sodium dodecylsulfate, dodecyltrimethylammonium chloride and thallium(I) nitrate. The influence of surfactant on phosphor characteristics as well as effects of moisture are discussed. Limits of detection were in the range 0.2–2 ng. Results are also reported for p-aminobenzoic acid, carbazole and o-terphenyl. 相似文献
29.
Radul has recently introduced a map from the Lie algebra of differential operators on the circle of W
n
. In this Letter, we extend this map to W
KP
(q)
, a recently introduced one-parameter deformation of WKP - the second Hamiltonian structure of the KP hierarchy. We use this to give a short proof that W is the algebra of additional symmetries of the KP equation. 相似文献
30.
José M. Figueroa-O'Farrill Javier Mas Eduardo Ramos 《Communications in Mathematical Physics》1993,158(1):17-43
The KP hierarchy is hamiltonian relative to a one-parameter family of Poisson structures obtained from a generalized Adler map in the space of formal pseudodifferential symbols with noninteger powers. The resulting W-algebra is a one-parameter deformation of WKP admitting a central extension for generic values of the parameter, reducing naturally to W
n
for special values of the parameter, and contracting to the centrally extended W1+, W and further truncations. In the classical limit, all algebras in the one-parameter family are equivalent and isomorphic tow
KP. The reduction induced by setting the spin-one field to zero yields a one-parameter deformation of
which contracts to a new nonlinear algebra of the W-type.Address after October 1993: Queen Mary and Westfield College, UK 相似文献