全文获取类型
收费全文 | 804篇 |
免费 | 18篇 |
国内免费 | 1篇 |
专业分类
化学 | 487篇 |
晶体学 | 1篇 |
力学 | 13篇 |
数学 | 55篇 |
物理学 | 267篇 |
出版年
2023年 | 2篇 |
2022年 | 16篇 |
2021年 | 20篇 |
2020年 | 22篇 |
2019年 | 13篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 20篇 |
2015年 | 26篇 |
2014年 | 32篇 |
2013年 | 67篇 |
2012年 | 43篇 |
2011年 | 60篇 |
2010年 | 44篇 |
2009年 | 67篇 |
2008年 | 44篇 |
2007年 | 37篇 |
2006年 | 41篇 |
2005年 | 20篇 |
2004年 | 35篇 |
2003年 | 18篇 |
2002年 | 18篇 |
2001年 | 18篇 |
2000年 | 16篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 9篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 17篇 |
1993年 | 6篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 3篇 |
1989年 | 2篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有823条查询结果,搜索用时 31 毫秒
101.
Thayana da C. Alves Mariele R. S. Gonçalves Francyne C. S. Correia Virgínia C. da Silva Paulo T. de Sousa Jr. Mário G. de Carvalho Raimundo Braz‐Filho Evandro L. Dall'Oglio 《Helvetica chimica acta》2014,97(11):1469-1474
From the hexane and MeOH extracts of Annona coriacea Mart . (Annonaceae) seeds, two novel acetogenins, coriapentocins A and B ( 1 and 2 , resp.) were isolated. The known acetogenin bullacin ( 3 ) was also isolated from the hexane extract. The structures of compounds 1 – 3 were elucidated by NMR and MS analysis, and relative configurations were established by comparison with literature data. 相似文献
102.
G.D. Saraiva J.A. Lima Jr.F.F. de Sousa P.T.C. FreireJ. Mendes Filho A.G. Souza Filho 《Vibrational Spectroscopy》2011,55(1):101-106
Temperature dependent Raman study of l-ascorbic acid has been performed from 15 to 418 K. Changes in the wavenumber vs. temperature plots for some internal modes were interpreted as conformational molecular change and the discontinuity in the wavenumber vs. temperature plots along with the appearance of a new vibrational mode in the temperature range 200-270 K suggests that l-ascorbic acid undergoes a structural phase transition. For temperatures higher than 300 K, no relevant modification was observed on the Raman spectra thus indicating a stable structure at high temperatures. Additionally, a correlation between OH stretching wavenumber and the behavior of hydrogen bond is also made. 相似文献
103.
Laura L. G. Fuentes Sarita C. Rabelo Rubens Maciel Filho Aline C. Costa 《Applied biochemistry and biotechnology》2011,163(5):612-625
The objective of this work was to determine the optimum conditions of sugarcane bagasse pretreatment with lime to increase
the enzymatic hydrolysis of the polysaccharide component and to study the delignification kinetics. The first stage was an
evaluation of the influence of temperature, reaction time, and lime concentration in the pretreatment performance measured
as glucose release after hydrolysis using a 23 central composite design and response surface methodology. The maximum glucose yield was 228.45 mg/g raw biomass, corresponding
to 409.9 mg/g raw biomass of total reducing sugars, with the pretreatment performed at 90°C, for 90 h, and with a lime loading
of 0.4 g/g dry biomass. The enzymes loading was 5.0 FPU/dry pretreated biomass of cellulase and 1.0 CBU/dry pretreated biomass
of β-glucosidase. Kinetic data of the pretreatment were evaluated at different temperatures (60°C, 70°C, 80°C, and 90°C),
and a kinetic model for bagasse delignification with lime as a function of temperature was determined. Bagasse composition
(cellulose, hemicellulose, and lignin) was measured, and the study has shown that 50% of the original material was solubilized,
lignin and hemicellulose were selectively removed, but cellulose was not affected by lime pretreatment in mild temperatures
(60–90°C). The delignification was highly dependent on temperature and duration of pretreatment. 相似文献
104.
Izumi E Ueda-Nakamura T Dias Filho BP Veiga Júnior VF Nakamura CV 《Natural product reports》2011,28(4):809-823
Here, we review studies that have investigated the activity of plant-derived compounds against Trypanosoma cruzi, the etiologic agent of Chagas’ disease. In the last decade, more than 300 species belonging to almost 100 families have been evaluated for activity, and here we describe the compounds isolated; 85 references are cited. 相似文献
105.
Milton C. Lopes Filho Evelyne Miot Helena J. Nussenzveig Lopes 《Journal of Nonlinear Science》2011,21(5):685-703
The vortex-wave system is a coupling of the two-dimensional vorticity equation with the point-vortex system. It is a model
for the motion of a finite number of concentrated vortices moving in a distributed vorticity background. In this article,
we prove existence of a weak solution to this system with an initial background vorticity in L
p
, p>2, up to the time of first collision of point vortices. 相似文献
106.
Ccallata HJ Tomaz Filho L Watanabe S 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(4):1272-1277
Natural silicate mineral of zoisite, Ca(2)Al(3)(SiO(4))(Si(2)O(7))O(OH), has been investigated concerning γ-radiation, UV-radiation and high temperature annealing effects on thermoluminescence (TL). X-ray diffraction (XRD) measurement confirmed zoisite structure and X-ray fluorescence (XRF) analysis revealed besides Si, Al and Ca that are the main crystal components, other oxides of Fe, Mg, Cr, Na, K, Sr, Ti, Ba and Mn which are present in more than 0.05 wt%. The TL glow curve of natural sample contains (130-150), (340-370) and (435-475)°C peaks. Their shapes indicated a possibility that they are result of composition of two or more peaks strongly superposed, a fact confirmed by deconvolution method. Once pre-annealed at 600°C for 1h, the shape of the glow curves change and the zoisite acquires high sensitivity. Several peaks between 100 and 400°C appear superposed, and the high temperature peak around 435°C cannot be seen. The ultraviolet radiation, on the other hand, produces one TL peak around 130°C and the second one around 200°C and no more. 相似文献
107.
Luiz K. C. de Souza Juliana J. R. Pardauil José R. Zamian Geraldo N. da Rocha Filho Carlos E. F. da Costa 《Journal of Thermal Analysis and Calorimetry》2011,106(2):355-361
Al-modified MCM-41, La-modified MCM-41, and Ce-modified MCM-41 mesoporous materials were prepared with different molar ratios
(Si/M = 10; 25; 50; 100 and 200) at room temperature. The materials were characterized using XRD, BET–BJH, and TG–DTA. The
XRD showed four peaks, due to the ordered hexagonal array of parallel silica tubes, which could be indexed as (100), (110),
(200), and (210), assuming a hexagonal unit cell. The surface area decreased as the concentration of the metal incorporated
in the material increased. The thermal stability of the materials was around 650 °C. The CeO2 phase made the mass transfer process more difficult, hindering Hofmann degradation and favoring oxidation. 相似文献
108.
Costa EV Pinheiro ML de Souza AD Barison A Campos FR Valdez RH Ueda-Nakamura T Filho BP Nakamura CV 《Molecules (Basel, Switzerland)》2011,16(11):9714-9720
Phytochemical investigation of the branches of Annona foetida Mart. led to isolation from the CH(2)Cl(2) extract of four alkaloids: Atherospermidine (1), described for the first time in this species, liriodenine (2), O-methylmoschatoline (3), and annomontine (4). Their chemical structures were established on the basis of spectroscopic data from IR, MS, NMR (1D and 2D), and comparison with the literature. Compounds 2-4 showed potent trypanocidal effect when evaluated against epimastigote and trypomastigote forms of Trypanosoma cruzi. 相似文献
109.
Façanha Filho PF Jiao X Freire PT Lima JA dos Santos AO Henry PF Yokaichiya F Kremner E Bordallo HN 《Physical chemistry chemical physics : PCCP》2011,13(14):6576-6583
We have studied the amino acid L-leucine (LEU) using inelastic neutron scattering, X-rays and neutron diffraction, calorimetry and Raman scattering as a function of temperature, focusing on the relationship between the local dynamics of the NH(3), CH(3), CH(2) and CO(2) moieties and the molecular structure of LEU. Calorimetric and diffraction data evidenced two novel phase transitions at about 150 K (T(1)) and 275 K (T(2)). The dynamical susceptibility function, obtained from the inelastic neutron scattering results, shows a re-distribution of the intensity of the vibrational bands that can be directly correlated with the phase transitions observed at T(1) and T(2), as well as with the already reported phase transition at T(3) = 353 K. Through the analysis of the Raman modes, the new structural arrangement observed below T(1) was related to conformational modifications of the CH and CH(3) groups, while the behavior of the N-H stretching vibration, ν(NH(3)), gave insight into the intermolecular N-H…O interactions. The observation of changes in the translational symmetry in the crystalline lattice, as well as anharmonic dynamics, allows for localized motions in LEU. 相似文献
110.
In this work we study the asymptotic behavior of viscous incompressible 2D flow in the exterior of a small material obstacle. We fix the initial vorticity ω0 and the circulation γ of the initial flow around the obstacle. We prove that, if γ is sufficiently small, the limit flow satisfies the full-plane Navier–Stokes system, with initial vorticity ω0 + γδ, where δ is the standard Dirac measure. The result should be contrasted with the corresponding inviscid result obtained by the authors in Iftimie et al. (Comm. Part. Differ. Eqn. 28, 349–379 (2003)), where the effect of the small obstacle appears in the coefficients of the PDE and not only in the initial data. The main ingredients of the proof are L
p
− L
q
estimates for the Stokes operator in an exterior domain, a priori estimates inspired on Kato’s fixed point method, energy estimates, renormalization and interpolation. 相似文献