全文获取类型
收费全文 | 461篇 |
免费 | 8篇 |
国内免费 | 2篇 |
专业分类
化学 | 202篇 |
力学 | 22篇 |
数学 | 175篇 |
物理学 | 72篇 |
出版年
2024年 | 2篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2019年 | 11篇 |
2018年 | 5篇 |
2017年 | 8篇 |
2016年 | 13篇 |
2015年 | 12篇 |
2014年 | 10篇 |
2013年 | 24篇 |
2012年 | 18篇 |
2011年 | 21篇 |
2010年 | 15篇 |
2009年 | 14篇 |
2008年 | 17篇 |
2007年 | 20篇 |
2006年 | 13篇 |
2005年 | 25篇 |
2004年 | 13篇 |
2003年 | 17篇 |
2002年 | 7篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1999年 | 5篇 |
1998年 | 7篇 |
1997年 | 6篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 2篇 |
1993年 | 8篇 |
1992年 | 12篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 11篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1965年 | 2篇 |
1959年 | 2篇 |
排序方式: 共有471条查询结果,搜索用时 31 毫秒
51.
The aim of this work is to evaluate the efficiency of the determination of As(III) by anodic stripping voltammetry (ASV) using a lateral gold electrode and to study the modifications of the electrode surface during use. Potential waveforms (differential pulse and square wave), potential scan parameters, deposition time, deposition potential and surface cleaning procedure were examined for they effect on arsenic peak intensity and shape. The best responses were obtained with differential pulse potential wave form and diluted 0.25 M HCl as supporting electrolyte. The repeatability, linearity, accuracy and detection limit of the procedure and the interferences of cations and anions in solution were evaluated. The applicability of the procedure for As(III) determination in drinking waters was tested. Cyclic voltammetry (CV) was used to study the electrochemical behaviour of As(III) and for the daily monitoring of electrode surface. Also scanning electron microscopy (SEM) analysis was used to control the electron surface. Finally we evaluated the possibility to apply the equations valid for flow systems also to a stirred system, in order to calculate the number of electrons transferred per molecule during the stripping step. 相似文献
52.
La Ferla B Airoldi C Zona C Orsato A Cardona F Merlo S Sironi E D'Orazio G Nicotra F 《Natural product reports》2011,28(3):630-648
Cancer is one of the major causes of death worldwide. As a consequence, many different therapeutic approaches, including the use of glycosides as anticancer agents, have been developed. Various glycosylated natural products exhibit high activity against a variety of microbes and human tumors. In this review we classify glycosides according to the nature of their aglycone (non-saccharidic) part. Among them, we describe anthracyclines, aureolic acids, enediyne antibiotics, macrolide and glycopeptides presenting different strengths and mechanisms of action against human cancers. In some cases, the glycosidic residue is crucial for their activity, such as in anthracycline, aureolic acid and enediyne antibiotics; in other cases, Nature has exploited glycosylation to improve solubility or pharmacokinetic properties, as in the glycopeptides. In this review we focus our attention on natural glycoconjugates with anticancer properties. The structure of several of the carbohydrate moieties found in these conjugates and their role are described. The structure–activity relationship of some of these compounds, together with the structural features of their interaction with the biological targets, are also reported. Taken together, all this information is useful for the design of new potential anti-tumor drugs. 相似文献
53.
Archiv der Mathematik - Let X be a smooth projective surface and $$L\in \mathrm {Pic}(X)$$ . We prove that if L is $$(2k-1)$$ -spanned, then the set $${\tilde{V}}_k(L)$$ of all nodal and... 相似文献
54.
Luigi Abis Edoardo Merlo Riccardo Po' 《Journal of Polymer Science.Polymer Physics》1995,33(4):691-697
Amorphous (1) and semicrystalline (2) samples of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) have been investigated by cross-polarization/magic angle spinning (CP/MAS) 13C NMR at 26°C (1 and 2), 100°C (1) and 120°C (2) in order to study the phase structure and the local motion of polymer chain segments at temperatures below and close to Tg (120°C). The lineshape of the ethylene unit 13C signal in sample 2 is consistent with the presence of two components which were assigned to trans and gauche conformations. The first component arises mainly from the crystalline regions and the second one from the amorphous part. Cross-polarization curves were traced by changing the contact time between carbon and proton reservoirs. TCH (cross relaxation time) and proton T1p (spin-lattice relaxation time in the rotating frame) values were obtained as best fit parameters by fitting calculated curves to the experimental data. All 13C NMR data are consistent with the presence of highly rigid ethylene units in both semicrystalline and amorphous samples within the temperature range (T) investigated. This result is in disagreement with the 1H NMR wide line spectra which showed a noticeable narrowing of the linewidth with increasing temperature in the same range, hence indicating a great mobility of the chain segments. To account for this discrepancy a qualitative model based on the existence of two distinct dynamic regions, one where motion is highly restricted and the other one where large reorientations of ethylene group torsional angles take place, is suggested. The NMR results led to the conclusion that three structural phases are present in PEN: crystalline, very rigid amorphous, and very mobile amorphous. © 1995 John Wiley & Sons, Inc. 相似文献
55.
Abstract Here we investigate r-canonical embeddings of general k-gonal curves of genus g from the point of view of Caporaso–Sernesi’s reconstruction procedure via odd theta-characteristics.
Keywords: Theta-characteristic, general k-gonal curve, trigonal curve, pluricanonical embedding, Hilbert scheme
Mathematics Subject Classification (2000): 14H50, 14N05 相似文献
56.
In this paper we are interested in gaining local stability insights about the interior equilibria of delay models arising in biomathematics. The models share the property that the corresponding characteristic equations involve delay-dependent coefficients. The presence of such dependence requires the use of suitable criteria which usually makes the analytical work harder so that numerical techniques must be used. Most existing methods for studying stability switching of equilibria fail when applied to such a class of delay models. To this aim, an efficient criterion for stability switches was recently introduced in [E. Beretta, Y. Kuang, Geometric stability switch criteria in delay differential systems with delay dependent parameters, SIAM J. Math. Anal. 33 (2002) 1144–1165] and extended [E. Beretta, Y. Tang, Extension of a geometric stability switch criterion, Funkcial Ekvac 46(3) (2003) 337–361]. We describe how to numerically detect the instability regions of positive equilibria by using such a criterion, considering both discrete and distributed delay models. 相似文献
57.
Fabiano V. Pereira Redouane Borsali Aloir A. Merlo Nadya P. Da Silveira 《Liquid crystals》2004,31(5):655-661
Small angle X-ray scattering was used to examine the new chiral side chain liquid crystalline polyacrylates (P4M and P11M) and their mixtures (2 wt %) in the low molar mass nematogenics 4'-n-pentyl-4-cyanobiphenyl (5CB) and 4'-octyl-4-cyanobiphenyl (8CB). Complementary data were obtained by polarizing optical microscopy. In agreement with previous studies, the mesophases of the bulk polymers show a dependence on the aliphatic spacers linking the mesogenic units to the polymer backbone. Chiral nematic and smectic A1 phases were observed for the polyacrylates with four (P4M) and eleven (P11M) methylene units as spacers, respectively. In solution with 5CB and 8CB, P4M exhibits an injected smectic phase, whereas P11M maintains the smectic arrangement already observed in the bulk, with swollen smectic layers. In all the mixtures, layer stability was found to depend on the liquid crystal used as solvent, as well as on the temperature. At temperatures corresponding to the nematic 5CB and 8CB, the coexistence of two mesophases was observed in the mixtures. Moreover, with the liquid crystal solvents in the isotropic phase, microstructures suspended in the solvent matrix containing the liquid crystalline polymer in the smectic arrangement were detected. 相似文献
58.
Corrado Sarzanini Giovanni Sacchero Maurizio Aceto Ornella Abollino Edoardo Mentasti 《Journal of chromatography. A》1993,640(1-2):127-134
Ion-interaction chromatography of Plasmocorinth B (a disulphonated azo dye) complexes of Co(III), Cu(II), Fe(III), Ga(III), In(III), Ni(II), V(V) and Zr(IV) was studied. The behaviour of two different reversed-phase C18 columns (5 and 10 μm) was compared and an on-line enrichment procedure was developed following the optimization of eluent (pH, ligand concentration, ionic strength and organic modifier). The described technique, applied to the analysis of metal ions at μg/1 levels in natural waters, gave satisfactory precision and accuracy in comparison with inductively coupled plasma atomic emission spectroscopic results. 相似文献
59.
Edoardo Ballico 《Rendiconti del Circolo Matematico di Palermo》1992,41(2):309-314
Here we prove the stability of several higher rank bundles on ? n strongly related with the postulation of projective varieties. As a particular case, we give another proof of the stability of ? n 相似文献
60.