全文获取类型
收费全文 | 1035篇 |
免费 | 39篇 |
国内免费 | 8篇 |
专业分类
化学 | 741篇 |
晶体学 | 6篇 |
力学 | 23篇 |
数学 | 112篇 |
物理学 | 200篇 |
出版年
2023年 | 2篇 |
2022年 | 17篇 |
2021年 | 29篇 |
2020年 | 21篇 |
2019年 | 21篇 |
2018年 | 18篇 |
2017年 | 20篇 |
2016年 | 42篇 |
2015年 | 23篇 |
2014年 | 45篇 |
2013年 | 92篇 |
2012年 | 68篇 |
2011年 | 83篇 |
2010年 | 48篇 |
2009年 | 41篇 |
2008年 | 74篇 |
2007年 | 69篇 |
2006年 | 51篇 |
2005年 | 41篇 |
2004年 | 29篇 |
2003年 | 25篇 |
2002年 | 43篇 |
2001年 | 21篇 |
2000年 | 25篇 |
1999年 | 11篇 |
1998年 | 11篇 |
1997年 | 6篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 2篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1970年 | 1篇 |
1965年 | 1篇 |
1948年 | 3篇 |
1894年 | 1篇 |
排序方式: 共有1082条查询结果,搜索用时 15 毫秒
121.
We use Riccati's equations and the ordinary and exponential dichotomies to get simple recurrent formulae for the asymptotic integration of linear systems subjected to L
p
-perturbations with arbitrary p 1. Moreover, we establish conditions which are necessary and sufficient for the persistence of one-dimensional invariant subbundles for the linear system under L
p
-perturbations. In this way, we prove the sharp nature of the well known Levinson and Hartman–Wintner asymptotic theorems. 相似文献
122.
Pinto LF Kundu S Brogueira P Cruz C Fernandes SN Aluculesei A Godinho MH 《Langmuir : the ACS journal of surfaces and colloids》2011,27(10):6330-6337
We report on a new type of liquid crystalline cellulosic films with light controllable reversible wettability. The films are prepared from a thermotropic cellulose derivative functionalized with azo-containing groups. These groups exhibit dynamic changes in interfacial properties in response to UV irradiation. The UV irradiation induces trans-to-cis isomerization in the azobenzene moiety, which causes a conformational change in the upper molecular layers of the thin films. These changes originate a hydrophobic to comparatively hydrophilic transformation of the surface. The reversible wettability of the surface results from the cis/trans photo and thermal isomerization. The UV-vis absorption spectra, as well as contact angle measurements with UV irradiation, clearly support the understanding of the phenomenon. This type of surface design enables the amplification of molecular level conformational transitions to macroscopic changes in interface properties using the means of isomerism. This opens new opportunities in surface engineering using eco-friendly cellulose manipulation. 相似文献
123.
A sensitive procedure is presented for the voltammetric determination of nickel. The procedure involves an adsorptive accumulation of nickel 1-nitroso-2-napthol (NN) complex on a bismuth film electrode prepared ex situ by electrodeposition. The most suitable operating conditions and parameters such as pH, ligand concentration (CNN), adsorptive potential (Eads), adsorptive time (tads), scan rate and others were selected and the determination of nickel in aqueous solutions using the standard addition method was possible. The adsorbed Ni-NN complex gives a well defined cathodic stripping peak current at −0.70 V, which was used for the determination of nickel in the concentration range of 10.0-70.0 μg L−1 (pH 7.5; CNN 6.5 μmol L−1; Eads −0.30 V; tads 60 s) with a detection limit of 0.1 μg L−1. The relative standard deviation for a solution containing 10.0 μg L−1 of Ni(II) was 3.5% (n = 4). The proposed method was validated determining Ni(II) in certified reference waste water (SPS-WW1) and Certified Reference Water for Trace Elements (TMDA 51.3) with satisfactory results. Then lake water samples were analyzed. 相似文献
124.
Cíntia Salomão Pinto Zarth Andreas Koschella Annett Pfeifer Susann Dorn Thomas Heinze 《Cellulose (London, England)》2011,18(5):1315-1325
The homogeneous conversion of cellulose dissolved in N-methyl-2-pyrrolidone/LiCl and 1-N-butyl-3-methylimidazolium chloride with N-methyl-2-pyrrolidone, ε-caprolactam, N-methyl-ε-caprolactam, and N-methyl-2-piperidone in the presence of p-toluenesulphonic acid chloride was studied. Depending on the reaction conditions, novel cellulose esters with degree of substitution
(DS) values ranging from 0.12 to 1.17 could be prepared. The structure of the amino group containing cellulose esters was
elucidated by elemental analysis, FTIR- and NMR spectroscopy. NMR spectroscopy revealed an almost complete esterification
of position 6 of the anhydroglucose unit at DS of 1. The conversion can be conducted between room temperature and 40 °C, while
side-reactions became predominant at 60 °C. Starting with DS of 0.24, the samples were soluble both in water and dimethyl
sulphoxide. The derivatives described are capable of forming polyelectrolyte complexes. The samples were stable at room temperature
in aqueous solution at pH 2 and 7. Lower viscosities were found for samples with higher DS in aqueous solution at comparable
molar mass. 相似文献
125.
García Pinto C Herrero Martín S Pérez Pavón JL Moreno Cordero B 《Analytica chimica acta》2011,(1):129-136
A method based on simplified QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) extraction followed by large-injection volume-fast gas chromatography and mass spectrometry detection has been developed for the determination of trihalomethanes (chloroform, bromodichloromethane, dibromochloromethane and bromoform) and BTEX (benzene, toluene, ethylbenzene and xylenes) in soil samples.The simplified version of QuEChERS used meets the requirements of the “green chemistry” and provides reliable results with high sample throughput, low solvent consumption, little labour and the use of materials commonly employed in laboratories. The GC device used is equipped with a programmable temperature vaporizer (PTV), with a liner packed with Tenax-TA®. Using the solvent-vent mode, the PTV allows the injection of large volumes of sample, affording an improvement in the sensitivity of the method. The chromatographic conditions used here allowed the separation of the compounds in less than 5.50 min. Good linearity was obtained for all the target compounds, with highly satisfactory repeatability and reproducibility values. The limits of detection were in the 0.2 to 15 μg kg−1 range. The method was validated by the analysis of two certified reference materials. 相似文献
126.
Sequential injection analysis systems have been extensively exploited in the last decades for the implementation of enzyme based assays aiming the evaluation of enzyme activity or the determination of specific analytes. The most prominent aspects of the automation of enzymatic assays in these systems are discussed in this review. Special attention is devoted to the mode of enzyme manipulation in homogeneous or heterogeneous media and to the comparison with batch and flow injection enzyme methodologies. The possibility of implementing strategies for the enhancement of selectivity in specific determinations is also addressed. The more recent trends in this field are discussed focusing mainly on the miniaturization resorting to the lab-on valve platform as well as on the bead injection concept. 相似文献
127.
Dörr FA Kovačevi B Maksi ZB Pinto E Volmer DA 《Journal of the American Society for Mass Spectrometry》2011,22(11):2011-2020
The aim of this study was to investigate the unusual gas-phase dissociation behavior of two epimer pairs of protonated gonyautoxins
(GTX) following electrospray ionization in comparison to their deprotonated counterparts. The chemical structures of the investigated
GTX1-4 variants vary in their substitution pattern at N-1 and the stereochemical orientation of the hydroxysulfate group at C-11
(11α for GTX1/2 versus 11β for GTX3/4). The direct comparison of mass spectra in positive and negative ion modes illustrated two distinct features: first, an intriguing
difference between protonated 11α and 11β species, where 11α conformations exhibited almost complete dissociation of [M +
H]+ ions via facile SO3 elimination, while 11β species remained mostly intact as [M + H]+; and second, the lack of such differences for the deprotonated counterparts. In this study, we propose an acid-catalyzed
elimination mechanism from density functional theory calculations, initiated by a proton transfer of a guanidinium proton
to the hydroxysulfate group with simultaneous SO3 release, which is only possible for the 11α conformation based on intramolecular distances. The same mechanism explains the
lack of a comparable SO3 loss in the negative ion mode. CID experiments supported this proposed mechanism for GTX1 and GTX2. Computational modeling of product ions seen in the CID spectra of GTX3 and GTX4 established that the lowest energy dissociation pathway for the 11β epimers is elimination of water with the possibility
for further SO3 release from the intermediate product. Experimental data for structurally analogous decarbamoyl gonyautoxins confirmed the
evidence for the GTX compounds as well as the proposed elimination mechanisms. 相似文献
128.
F.G.M. Cunha A.W.M. Pinto P.R. Oliveira J.X. da Cruz Neto 《Applied mathematics and computation》2011,218(8):4523-4532
We obtain a class of primal affine scaling algorithms which generalize some known algorithms. This class, depending on a r-parameter, is constructed through a family of metrics generated by −r power, r ? 1, of the diagonal iterate vector matrix. We prove the so-called weak convergence of the primal class for nondegenerate linearly constrained convex programming. We observe the computational performance of the class of primal affine scaling algorithms, accomplishing tests with linear programs from the NETLIB library and with some quadratic programming problems described in the Maros and Mészáros repository. 相似文献
129.
Pinto RC Stenlund H Hertzberg M Lundstedt T Johansson E Trygg J 《Analytica chimica acta》2011,(2):127-131
To find and ascertain phenotypic differences, minimal variation between biological replicates is always desired. Variation between the replicates can originate from genetic transformation but also from environmental effects in the greenhouse. Design of experiments (DoE) has been used in field trials for many years and proven its value but is underused within functional genomics including greenhouse experiments. We propose a strategy to estimate the effect of environmental factors with the ultimate goal of minimizing variation between biological replicates, based on DoE. DoE can be analyzed in many ways. We present a graphical solution together with solutions based on classical statistics as well as the newly developed OPLS methodology.In this study, we used DoE to evaluate the influence of plant specific factors (plant size, shoot type, plant quality, and amount of fertilizer) and rotation of plant positions on height and section area of 135 cloned wild type poplar trees grown in the greenhouse. Statistical analysis revealed that plant position was the main contributor to variability among biological replicates and applying a plant rotation scheme could reduce this variation. 相似文献
130.
de Oliveira CS Falcão-Silva Vdos S Siqueira JP Harding DP Lira BF Lorenzo JG Barbosa-Filho JM de Athayde-Filho PF 《Molecules (Basel, Switzerland)》2011,16(3):2023-2031
Two salts of the mesoionic compounds 1,4-diphenyl-5-(5-nitro-2-furanyl)-1,3,4-thiadiazolium-2-thiol chloride (MC-1) and 4-phenyl-5-(5-nitro-2-furanyl)-1,3,4-thiadiazolium-2-phenylamine chloride (MC-2) were synthesized utilizing 1,4-diphenyl-thiosemicarbazide and 5-nitro-2-furoyl chloride as starting materials. Their structures were characterized by IR, 1H-NMR, 13C-NMR and elemental analysis. These compounds were analyzed for their influence on the effectiveness of norfloxacin, tetracycline, and erythromycin (standard antibiotics) against resistant strains of Staphylococcus aureus. MC-1 and MC-2, at sub-inhibitory concentrations of 16 μg/mL, favourably modulated the antibiotic activity of tetracycline by 16- and 32-fold, respectively (MIC), and that of erythromycin by 4-fold. 相似文献