全文获取类型
收费全文 | 1845篇 |
免费 | 65篇 |
国内免费 | 11篇 |
专业分类
化学 | 1616篇 |
晶体学 | 7篇 |
力学 | 19篇 |
数学 | 195篇 |
物理学 | 84篇 |
出版年
2023年 | 10篇 |
2022年 | 49篇 |
2021年 | 44篇 |
2020年 | 29篇 |
2019年 | 27篇 |
2018年 | 16篇 |
2017年 | 24篇 |
2016年 | 51篇 |
2015年 | 59篇 |
2014年 | 56篇 |
2013年 | 82篇 |
2012年 | 149篇 |
2011年 | 142篇 |
2010年 | 104篇 |
2009年 | 95篇 |
2008年 | 117篇 |
2007年 | 130篇 |
2006年 | 118篇 |
2005年 | 118篇 |
2004年 | 102篇 |
2003年 | 79篇 |
2002年 | 67篇 |
2001年 | 24篇 |
2000年 | 26篇 |
1999年 | 22篇 |
1998年 | 16篇 |
1997年 | 16篇 |
1996年 | 11篇 |
1995年 | 21篇 |
1994年 | 9篇 |
1993年 | 3篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 9篇 |
1979年 | 2篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 2篇 |
1936年 | 1篇 |
排序方式: 共有1921条查询结果,搜索用时 15 毫秒
991.
992.
Alejandra Isabel Vargas-Segura Sonia Yesenia Silva-Belmares Elda Patricia Segura-Ceniceros Juan Alberto Ascacio-Valdés Luis Méndez-González 《Natural product research》2020,34(18):2672-2676
AbstractThe aim of the present study was to perform a screening of extracts obtained from 15 medicinal plants using water (at 25 and 90?°C) or ethanol (at 25?°C), to bactericidal activity against cariogenic S. mutans ATCC 25175, as well as to carry out the preliminary phytochemical characterization of the extracts and HPLC/MS assay for selected extracts. The extractions were carried out for 5?h at 400?rpm. Only five from 45 tested extracts were selected based on their antibacterial activity. The IC50 varied from of 28?ppm for Quercus ilex up to 250?ppm for Jatropha dioica. Different polyphenolic and quinic acids, flavonoids, anthocyanin or tyrosol were identified by HPLC/MS in selected extracts from Rosa gallica L., Jatropha dioica Sessé, Mimosa tenuiflora (Willd.) Poir, Quercus ilex L., and Solanum nigrum. The obtained results confirm that selected extracts are good candidates to be used for cariogenic bacteria control. 相似文献
993.
Archismita Misra Christian Zambrzycki Gabriele Kloker Anika Kotyrba Montaha H. Anjass Isabel Franco Castillo Scott G. Mitchell Robert Güttel Carsten Streb 《Angewandte Chemie (International ed. in English)》2020,59(4):1601-1605
Filtration is an established water‐purification technology. However, due to low flow rates, the filtration of large volumes of water is often not practical. Herein, we report an alternative purification approach in which a magnetic nanoparticle composite is used to remove organic, inorganic, microbial, and microplastics pollutants from water. The composite is based on a polyoxometalate ionic liquid (POM‐IL) adsorbed onto magnetic microporous core–shell Fe2O3/SiO2 particles, giving a magnetic POM‐supported ionic liquid phase (magPOM‐SILP). Efficient, often quantitative removal of several typical surface water pollutants is reported together with facile removal of the particles using a permanent magnet. Tuning of the composite components could lead to new materials for centralized and decentralized water purification systems. 相似文献
994.
John W. Goodby Isabel M. Saez Stephen J. Cowling Julita S. Gasowska Robert A. MacDonald Susan Sia 《Liquid crystals》2013,40(6-7):567-605
In this article we investigate the complexity of the molecular architectures of liquid crystals based on rod-like mesogens. Starting from simple monomeric systems founded on fluoroterphenyls, we first examine the effects of aromatic core structure on mesophase formation from the viewpoint of allowable polar interactions, and then we model these interactions as a function of terminal aliphatic chain length. By incorporating a functional group at the end of one, or both, of the aliphatic chains we study the effects caused by intermolecular interfacial interactions in lamellar phases, and in particular the formation of synclinic or anticlinic modifications. We then develop these ideas with respect to dimers, trimers, tetramers, etc. We show, for dendritic systems, that at a certain level of molecular complexity the local mesogenic interactions become irrelevant, and it is gross molecular shape that determines mesophase stability. The outcome of these studies is to link the complexity of the molecular interactions at the nanoscale level, which lead to the creation of the various liquid-crystalline polymorphs, with the formation of mesophases that are dependent on complex shape dependencies for mesoscopic supermolecular architectures. 相似文献
995.
Estefanía Concha-Graña Isabel Turnes-Carou Purificación López-Mahía Darío Prada-Rodríguez 《International journal of environmental analytical chemistry》2013,93(4):416-433
The monitoring of organochlorine pesticides has raised a great concern in the last years due to their toxicity (some of them are carcinogenic and endocrine disruptor compounds) and persistence. European Directive 2008/105/EC establishes very restrictive levels for organochlorine pesticides in surface waters. Therefore, simple, fast, highly sensitive and low cost analytical methods are required to detect and quantify these pollutants in water. In the present work, four procedures for extraction and determination are proposed and compared for the analysis of 28 organochlorine pesticides in tap, surface and sea waters. The suitability of each method of analysis was evaluated for each kind of water. The extraction methods proposed were: two solid-phase extraction methods using C18 laminar disk and Oasis HLB cartridges, a solid-phase microextraction procedure using a polydimethylsiloxane/divinylbenzene (PDMS/DVB) fibre, and a micro liquid–liquid extraction procedure using ethyl acetate as solvent. Determination of pesticides was performed by large volume on-column injector-gas chromatography-electron capture detection (LVOCI-GC-ECD), splitless-GC-ECD and GC-MS (mass spectrometry). All methods present a good sensitivity with method detection limits lower than 10?ng?L?1, good accuracy with recoveries between 75 and 120% (with some exceptions) and good precision (relative standard deviations <15%), according to the Commission Decision 2002/657/EC criteria. The advantages and disadvantages of each method are discussed in terms of the green chemistry principles, the figures of merit and the matrix effect. This work tries to be a useful guidance for routine and control analysis laboratories. 相似文献
996.
Isabel María Moreno Silvia Pichardo Rosario Moyano A. Gustavo González Ana María Cameán 《International journal of environmental analytical chemistry》2013,93(13):1000-1013
A new HPLC–DAD method has been developed to identify and quantify free microcystins (MC) in biological samples from fish (intestine and liver). The toxins were extracted from 500 mg sample with a mixture of methanol–water (85 : 15, v/v) and the extracts obtained were purified employing immunoaffinity columns (IAC). The purification step was optimised by a full factorial 32 design. MC were separated using conventional C18 column and an acetonitrile-acidified water (pH 3) gradient. Detection and quantification limits resulted equal for the two toxins assayed (MC-RR and MC-LR) and were 0.15 and 0.5 µg g?1, respectively. The accuracy for each MC in liver samples were 96% (range 80–113%) for MC-RR and 101% (range 93–118%) for MC-LR. The results were slightly lower for intestine samples, with recoveries ranging between 85% (75–93%) for MC-RR and 88% (80–97%) for MC-LR. The proposed method was applied for the determination of free MC in fish intoxicated with these toxins, in order to determine its utility to evaluate the potential risks for human health if MC-contaminated fish are consumed. The results showed the transference of MC-LR from cyanobacterial cells to fish tissues. 相似文献
997.
Dayan Rodríguez-Torres Juan Carlos García-Ramos Juan Manríquez Rafael Moreno-Esparza Mario Altamirano Lozano Ignacio González Isabel Gracia-Mora Lena Ruiz-Azuara Rene Antaño Lopez Luis Ortiz-Frade 《Polyhedron》2009,28(6):1186-1190
Preliminary proliferation assays in human tumor cervix line HeLa, using the coordination compound [Cu(pdto)H2O]2+ (pdto = 1,8-bis-(2-pyridyl)-3,6-dithiaoctane) and its precursors Cu(NO3)2 · 2.5H2O and pdto, were carried out. The results showed that the copper complex has a behavior similar to that of the reference drug cis-platin. No biological activity for the non-coordinated ligand and the copper salt was found. It was established by cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy, that the complex [Cu(pdto)H2O]2+ presents an electrochemical reversible Cu(II)/Cu(I) reduction, in acetonitrile solution, meanwhile, the copper salt Cu(NO3)2 · 2.5H2O exhibited an electrochemical irreversible behavior. A comparison between biological and electrochemical results corresponding to [Cu(pdto)H2O]2+ and Cu(NO3)2 · 2.5H2O let us to proposed, the electrochemical reversibility, as one important factor in the antitumoral activity of the copper complex. Due to the nature of the studies presented in this work, other factors like intercalation properties with DNA cannot be neglected in the antitumoral activity of the complex. 相似文献
998.
Tatiana R. Amarante Isabel S. Gonalves Filipe A. Almeida Paz 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(4):m168-m170
The reaction between fluorenyllithium and Mo(η3‐C3H5)Cl(NCMe)2(CO)2 led to the isolation of di‐μ3‐chlorido‐di‐μ3‐hydroxido‐tetrakis[(η3‐allyl)dicarbonylmolybdenum(II)]–9‐fluorenone–tetrahydrofuran (1/1/1), [Mo4(C3H5)4Cl2(OH)2(CO)8]·C4H8O·C13H8O. The tetrametallic Mo4 unit constitutes the first example of a complex containing simultaneously two μ3‐OH groups and two μ3‐Cl anions capping the metallic trigonal prism. The four crystallographically independent Mo2+ centres exhibit distorted octahedral geometry with the η3‐allyl groups being trans‐coordinated to a μ3‐OH group and the carbonyl groups occupying the equatorial plane. Space‐filling tetrahydrofuran and 9‐fluorenone molecules are engaged in strong O—H...O hydrogen‐bonding interactions with Mo4(η3‐allyl)4Cl2(OH)2(CO)8 complexes. 相似文献
999.
Maria Isabel Gomes Salema Ana Paula Barbosa-Povoa Augusto Q. Novais 《European Journal of Operational Research》2007
In this work the design of a reverse distribution network is studied. Most of the proposed models on the subject are case based and, for that reason, they lack generality. In this paper we try to overcome this limitation and a generalized model is proposed. It contemplates the design of a generic reverse logistics network where capacity limits, multi-product management and uncertainty on product demands and returns are considered. A mixed integer formulation is developed which is solved using standard B&B techniques. The model is applied to an illustrative case. 相似文献
1000.
The paper analyzes dollarization in the Turkish economy given the evidence on dedollarization signals. On conducting a Vector Autoregression (VAR) model, the empirical evidence suggests that dollarization has mostly been shaped by macroeconomic imbalances as measured by exchange rate depreciation volatility, inflation volatility and expectations. Furthermore, the generalized impulse response function (IRF) analysis, in addition to the analysis of variance decomposition (VDC) gives support to the notion that dollarization seems to sustain its persistent nature, thus hysteresis still prevails. Hence, unfavorable macroeconomic conditions apparently contribute to dollarization while dollarization itself contains inertia. Furthermore, dedollarization that presumably started after 2001 has lost headway after May 2006. Thus, it seems too early to conclude that dollarization changed its route to dedollarization. 相似文献