全文获取类型
收费全文 | 2128篇 |
免费 | 65篇 |
国内免费 | 5篇 |
专业分类
化学 | 1573篇 |
晶体学 | 12篇 |
力学 | 38篇 |
数学 | 279篇 |
物理学 | 296篇 |
出版年
2023年 | 18篇 |
2022年 | 51篇 |
2021年 | 68篇 |
2020年 | 56篇 |
2019年 | 52篇 |
2018年 | 33篇 |
2017年 | 33篇 |
2016年 | 66篇 |
2015年 | 57篇 |
2014年 | 56篇 |
2013年 | 124篇 |
2012年 | 120篇 |
2011年 | 164篇 |
2010年 | 110篇 |
2009年 | 99篇 |
2008年 | 135篇 |
2007年 | 144篇 |
2006年 | 116篇 |
2005年 | 144篇 |
2004年 | 101篇 |
2003年 | 53篇 |
2002年 | 48篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 29篇 |
1998年 | 14篇 |
1997年 | 16篇 |
1996年 | 23篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 16篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 8篇 |
1973年 | 6篇 |
1972年 | 4篇 |
排序方式: 共有2198条查询结果,搜索用时 250 毫秒
981.
We determine shape-preserving regions and we describe a general setting to generate shape-preserving families for the 2-points
Hermite subdivision scheme introduced by Merrien (Numer. Algorithms 2:187–200, [1992]). This general construction includes the shape-preserving families presented in Merrien and Sablonníere (Constr. Approx.
19:279–298, [2003]) and Pelosi and Sablonníere (C
1 GP Hermite Interpolants Generated by a Subdivision Scheme, Prépublication IRMAR 06–23, Rennes, [2006]). New special families are presented as particular examples. Nonstationary and nonuniform versions of such schemes, which
produce smoother limits, are discussed.
相似文献
982.
Ana C. Q. Silva Bárbara Pereira Nicole S. Lameirinhas Paulo C. Costa Isabel F. Almeida Patrícia Dias-Pereira Inês Correia-Sá Helena Oliveira Armando J. D. Silvestre Carla Vilela Carmen S. R. Freire 《Macromolecular bioscience》2023,23(1):2200323
The aim of this study is to prepare dissolvable biopolymeric microneedle (MN) patches composed solely of sodium carboxymethylcellulose (CMC), a water-soluble cellulose derivative with good film-forming ability, by micromolding technology for the transdermal delivery of diclofenac sodium salt (DCF). The MNs with ≈456 µm in height displayed adequate morphology, thermal stability up to 200 °C, and the required mechanical strength for skin insertion (>0.15 N needle−1). Experiments in ex vivo abdominal human skin demonstrate the insertion capability of the CMC_DCF MNs up to 401 µm in depth. The dissolution of the patches in saline buffer results in a maximum cumulative release of 98% of diclofenac after 40 min, and insertion in a skin simulant reveals that all MNs completely dissolve within 10 min. Moreover, the MN patches are noncytotoxic toward human keratinocytes. These results suggest that the MN patches produced with CMC are promising biopolymeric systems for the rapid administration of DCF in a minimally invasive manner. 相似文献
983.
Lilian Cristina Trevizan Dário Santos Jr. Ricardo Elgul Samad Nilson Dias Vieira Jr. Cassiana Seimi Nomura Lidiane Cristina Nunes Iolanda Aparecida Rufini Francisco José Krug 《Spectrochimica Acta Part B: Atomic Spectroscopy》2008
Laser induced breakdown spectroscopy (LIBS) has become an analytical tool for the direct analysis of a large variety of materials in order to provide qualitative and/or quantitative information. However, there is a lack of information for LIBS analysis of agricultural and environmental samples. In this work a LIBS system has been evaluated for the determination of macronutrients (P, K, Ca, Mg) in pellets of vegetal reference materials. An experimental setup was designed by using a Nd:YAG laser operating at 1064 nm and an Echelle spectrometer with ICCD detector. The plasma temperature was estimated by Boltzmann plots and instrumental parameters such as delay time, lens-to-sample distance and pulse energy were evaluated. Certified reference materials as well as reference materials were used for analytical calibrations of P, K, Ca, and Mg. Most results of the direct analysis of plant samples by LIBS were in reasonable agreement with those obtained by ICP OES after wet acid decomposition. 相似文献
984.
Electroactive nanostructured membranes have been produced by the layer-by-layer (LbL) technique, and used to make electrochemical enzyme biosensors for glucose by modification with cobalt hexacyanoferrate redox mediator and immobilisation of glucose oxidase enzyme. Indium tin oxide (ITO) glass electrodes were modified with up to three bilayers of polyamidoamine (PAMAM) dendrimers containing gold nanoparticles and poly(vinylsulfonate) (PVS). The gold nanoparticles were covered with cobalt hexacyanoferrate that functioned as a redox mediator, allowing the modified electrode to be used to detect H2O2, the product of the oxidase enzymatic reaction, at 0.0 V vs. SCE. Enzyme was then immobilised by cross-linking with glutaraldehyde. Several parameters for optimisation of the glucose biosensor were investigated, including the number of deposited bilayers, the enzyme immobilisation protocol and the concentrations of immobilised enzyme and of the protein that was crosslinked with PAMAM. The latter was used to provide glucose oxidase with a friendly environment, in order to preserve its bioactivity. The optimised biosensor, with three bilayers, has high sensitivity and operational stability, with a detection limit of 6.1 μM and an apparent Michaelis–Menten constant of 0.20 mM. It showed good selectivity against interferents and is suitable for glucose measurements in natural samples. 相似文献
985.
Gaweda S Morán MC Pais AA Dias RS Schillén K Lindman B Miguel MG 《Journal of colloid and interface science》2008,323(1):75-83
Fluorescence microscopy was used to investigate the conformational changes of individual T4 DNA molecules induced by different compacting agents, namely the cationic surfactants, cetyltrimethylammonium bromide (CTAB) and chloride (CTAC), iron(III), lysozyme, and protamine sulfate. A protocol for establishing size estimates is suggested to obtain reproducible results. Observations show that in the presence of lysozyme and protamine sulfate, DNA molecules exhibit a conformational change from an elongated coil structure to compact globules, usually interpreted as a first-order transition. The maximum degree of compaction that is attained when iron(III) or CTAB (CTAC) are used as compacting agents is considerably smaller, and intermediate structures (less elongated coils) are visible even for high concentrations of these agents. Dynamic light scattering experiments were carried out, for some of the systems, to assess the reliability of size estimates from fluorescence microscopy. 相似文献
986.
Benassi M Moraes LA Cabrini LG Dias LC Aguilar AM Romeiro GA Eberlin LS Eberlin MN 《The Journal of organic chemistry》2008,73(14):5549-5557
Reactions of the model acylium ion (CH3)2N-C(+)=O with acyclic, exocyclic, and spiro acetals of the general formula R(1)O-CR(3)R(4)-OR(2) were systematically evaluated via pentaquadrupole mass spectrometry. Characteristic intrinsic reactivities were observed for each of these classes of acetals. The two most common reactions observed were hydride and alkoxy anion [R(1)O(-) and R(2)O(-)] abstraction. Other specific reactions were also observed: (a) a secondary polar [4(+) + 2] cycloaddition for acetals bearing alpha,beta-unsaturated R(3) or R(4) substituents and (b) OH(-) abstraction for exocyclic and spiro acetals. These structurally diagnostic reactions, in conjunction with others observed previously for cyclic acetals, are shown to reveal the class of the acetal molecule and its ring type and substituents and to permit their recognition and distinction from other classes of isomeric molecules. 相似文献
987.
988.
Corr SA Byrne SJ Tekoriute R Meledandri CJ Brougham DF Lynch M Kerskens C O'Dwyer L Gun'ko YK 《Journal of the American Chemical Society》2008,130(13):4214-4215
Using a one-step procedure we have prepared magnetic fluids comprising of polyelectrolyte stabilized magnetite nanoparticles. These nanocomposites are comprised of linear, chain-like assemblies of magnetic nanoparticles, which can be aligned in parallel arrays by an external magnetic field. We have shown the potential use of these materials as contrast agents by measuring their MR response in live rats. The new magnetic fluids have demonstrated good biocompatibility and potential for in vivo MRI diagnostics. 相似文献
989.
Clemente-León M Coronado E Dias JC Soriano-Portillo A Willett RD 《Inorganic chemistry》2008,47(14):6458-6463
The synthesis, structure, and magnetic properties of a novel oxalate-based bimetallic magnet obtained by using the chiral (S)-trimethyl-(1-phenyl-ethyl)-ammonium, ((S)-[PhCH(CH3)N(CH3)3](+)), cation as template is reported. This compound can be formulated as [(S)-[PhCH(CH3)N(CH3)3]][Mn(CH3CN)2/3Cr(ox)3] x (CH3CN)_(solvate), and it crystallizes in the chiral trigonal space group P3. It shows a distorted two-dimensional honeycomb structure formed by Mn(II) and Cr(III) ions connected through oxalate anions with [(S)-[PhCH(CH3)N(CH3)3](+) cations and solvent molecules intercalated between the oxalate layers. Two-thirds of the Mn(II) ions of the honeycomb anionic network are heptacoordinated. This compound behaves as a soft ferromagnet with an ordering temperature of 5.6 K. 相似文献
990.
Huang F Nagvekar DS Slebodnick C Gibson HW 《Journal of the American Chemical Society》2005,127(2):484-485
The first supramolecular star polymer based on pseudorotaxane host-guest complexation was prepared from statistical complexation of a homotritopic tris(crown ether) host and monotopic paraquat-terminated polystyrene guest in solution. The formation of this supramolecular star polymer was confirmed by proton NMR characterization and viscosity studies. 相似文献