全文获取类型
收费全文 | 2555篇 |
免费 | 48篇 |
国内免费 | 3篇 |
专业分类
化学 | 1553篇 |
晶体学 | 22篇 |
力学 | 69篇 |
数学 | 394篇 |
物理学 | 568篇 |
出版年
2023年 | 14篇 |
2022年 | 49篇 |
2021年 | 49篇 |
2020年 | 35篇 |
2019年 | 32篇 |
2018年 | 70篇 |
2017年 | 73篇 |
2016年 | 64篇 |
2015年 | 80篇 |
2014年 | 72篇 |
2013年 | 118篇 |
2012年 | 166篇 |
2011年 | 157篇 |
2010年 | 100篇 |
2009年 | 61篇 |
2008年 | 121篇 |
2007年 | 121篇 |
2006年 | 99篇 |
2005年 | 290篇 |
2004年 | 241篇 |
2003年 | 157篇 |
2002年 | 49篇 |
2001年 | 44篇 |
2000年 | 38篇 |
1999年 | 18篇 |
1998年 | 17篇 |
1997年 | 20篇 |
1996年 | 14篇 |
1995年 | 19篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 18篇 |
1990年 | 12篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1973年 | 5篇 |
1972年 | 3篇 |
1968年 | 5篇 |
1961年 | 3篇 |
排序方式: 共有2606条查询结果,搜索用时 15 毫秒
81.
Jelem Restrepo José Salazar Simón E. López 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):2311-2320
Abstract We describe a rapid and convenient methodology for the preparation of diverse 3-aryl and 3-trifluoromethyl 4H-1,2,4-benzothiadiazine-1,1-dioxides in a one-pot microwave-promoted reaction between 2-aminobenzenesulfonamides and benzaldehydes, trifluoroacetic acid or benzoic acids. GRAPHICAL ABSTRACT 相似文献
82.
Hyun Ook Seo Dae Han Kim Kwang-Dae Kim Eun Ji Park Chae Won Sim Young Dok Kim 《Adsorption》2013,19(6):1181-1187
Adsorption and desorption of toluene on bare and TiO2-coated silica with a mean pore size of 15 nm under dry and humid conditions were studied using toluene breakthrough curves and temperature programmed desorption (TPD) of toluene and CO2. Two TiO2/silica samples (either partially or fully covered with TiO2) were prepared with 50 and 200 cycles of TiO2 atomic layer deposition (ALD), respectively. The capacity of silica to adsorb toluene improved significantly with TiO2-thin film coating under dry conditions. However, toluene desorption from the surface due to displacement by water was more pronounced for TiO2-coated samples than bare samples under humid conditions. In TPD experiments, silica with a thinner TiO2 film (50-ALD cycled) had the highest reactivity for toluene oxidation to CO2 both in the absence and presence of water. Toluene adsorption and oxidation reactivity of silica can be controlled by modifying the silica surface with small amount of TiO2 using ALD. 相似文献
83.
Georgina M. S. Alves Júlia M. C. S. Magalhães Helena M. V. M. Soares 《Electroanalysis》2013,25(5):1247-1255
A simple, fast, sensitive and greener voltammetric procedure for simultaneous analysis of nickel (Ni) and cobalt (Co) by square wave adsorptive cathodic stripping voltammetry (SW‐AdCSV) using a solid bismuth vibrating electrode is presented for the first time. The procedure enables to determine Ni together with Co, in ammonia buffer 0.1 M (pH 9.2) and in the presence of oxygen, and involves an adsorptive accumulation of metal‐dimethylglyoxime (Ni‐DMG and Co‐DMG) complexes on the electrode surface. For Ni and Co, the detection limits, obtained with 30 s of accumulation time, were 0.6 and 1.0 µg L?1, respectively. The method was free of metals (Cd2+, Cr3+, Cr6+, Cu2+, Fe3+ and Pb2+ up to 50 µg L?1, Al3+ and Mn2+ up to 500 µg L?1; Zn2+ up to 300 µg L?1) interferences up to the concentrations mentioned in brackets. The proposed method was validated for simultaneous determination of Ni and Co in a certified reference surface and river waters with good results. 相似文献
84.
Cristina García‐Simón Marc Garcia‐Borràs Dr. Laura Gómez Dr. Isaac Garcia‐Bosch Dr. Sílvia Osuna Prof. Marcel Swart Dr. Josep M. Luis Prof. Concepció Rovira Prof. Manuel Almeida Dr. Inhar Imaz Prof. Daniel Maspoch Dr. Miquel Costas Dr. Xavi Ribas 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1445-1456
The metal‐directed supramolecular synthetic approach has paved the way for the development of functional nanosized molecules. In this work, we report the preparation of the new nanocapsule 3? (CF3SO3)8 with a A4B2 tetragonal prismatic geometry, where A corresponds to the dipalladium hexaazamacrocyclic complex Pd‐1 , and B corresponds to the tetraanionic form of palladium 5,10,15,20‐tetrakis(4‐carboxyphenyl)porphyrin ( 2 ). The large void space of the inner cavity and the supramolecular affinity for guest molecules towards porphyrin‐based hosts converts this nanoscale molecular 3D structure into a good candidate for host–guest chemistry. The interaction between this nanocage and different guest molecules has been studied by means of NMR, UV/Vis, ESI‐MS, and DOSY experiments, from which highly selective molecular recognition has been found for anionic, planar‐shaped π guests with association constants (Ka) higher than 109 M ?1, in front of non‐interacting aromatic neutral or cationic substrates. DFT theoretical calculations provided insights to further understand this strong interaction. Nanocage 3? (CF3SO3)8 can not only strongly host one single molecule of M(dithiolene)2 complexes (M=Au, Pt, Pd, and Ni), but also can finely tune their optical and redox properties. The very simple synthesis of both the supramolecular cage and the building blocks represents a step forward for the development of polyfunctional supramolecular nanovessels, which offer multiple applications as sensors or nanoreactors. 相似文献
85.
Photooxidation of glycated and non‐glycated phosphatidylethanolamines monitored by mass spectrometry
Tânia Melo Eduarda M. P. Silva Cláudia Simões Pedro Domingues M. Rosário M. Domingues 《Journal of mass spectrometry : JMS》2013,48(1):68-78
Phosphatidylethanolamines (PE) are one of the major components of cells membranes, namely in skin and in retina, that are continuously exposed to solar UV radiation being major targets of photooxidation damage. In addition, due to the presence of the free amine group, PE can also undergo glycation, in hyperglycemic conditions which may increase the susceptibility to oxidation. The aim of this study is to develop a model, based on mass spectrometry (MS) analysis, to identify photooxidative degradation of selected PE (POPE: PE 16:0/18:1, PLPE: PE 16:0/18:2, PAPE: PE 16:0/20:4) and glycated PEs due to UV irradiation. Photooxidation products were analysed by electrospray ionization MS (ESI‐MS) and tandem MS (ESI‐MS/MS) in positive and negative mode. Emphasis is placed in the influence of glycation in the generation of distinct photooxidation products. ESI‐MS spectra of PE after UV photo‐irradiation showed mainly hydroperoxy derivatives, due to oxidation of unsaturated fatty acyl chains. Glycated PE gave rise to several new photooxidation products formed due to oxidative cleavages of the glucose moiety, namely between C1 and C2, C2 and C3, and C5 and C6 of this sugar unit. These new products were identified by ESI‐MS/MS in positive mode showing distinct neutral loss depending on the different structure of the polar head group. These new identified advanced glycated photooxidation products may have a deleterious role in the etiology of diabetic retinopathy and in diabetic retinal microvascular complications. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
86.
Aarón Escrig‐Doménech Isabel Ten‐Doménech Ernesto F. Simó‐Alfonso José M. Herrero‐Martínez 《Journal of separation science》2013,36(14):2283-2290
Monolithic stationary phases based on octadecyl acrylate for CEC using different initiating systems (UV irradiation, thermal, and chemical initiation) in the presence of lauroyl peroxide as initiator were synthesized. For each initiation mode, the influence of the porogenic solvent composition on both the morphological and electrochromatographic properties of the resulting monoliths was investigated. Under optimal conditions, excellent efficiencies for the photochemically and chemically polymerized monoliths (minimum plate heights of 6.9–10.7 and 6.5–12.6 μm, respectively) were achieved. Thermally initiated columns gave lower efficiency values, permeabilities, and longer analysis times compared to these initiating systems. The produced monolithic stationary phases were evaluated in terms of reproducibility and gave RSD values below 9.2, 10.6, and 9.8% for UV, thermally, and chemically initiated columns, respectively. 相似文献
87.
Ariana Borges Souza Monique Rodrigues Moreira Carly Henrique Gambeta Borges Marília Rodrigues Simão Jairo Kenupp Bastos João Paulo Barreto de Sousa Sérgio Ricardo Ambrosio Rodrigo Cassio Sola Veneziani 《Biomedical chromatography : BMC》2013,27(3):280-283
The Copaifera species (Leguminoseae) are popularly known as ‘copaíba’ or ‘copaíva’ and are grown in the states of Amazonas, Pará and Ceará in northern Brazil. The oleoresins obtained from these species have been extensively used owing to their pharmacological potential and their application in cosmetic and pharmaceutical preparations. In the present study, the development and validation of a novel, rapid and efficient RP‐HPLC methodology for the analysis of the diterpene (?)‐copalic acid (CA), pointed out as the only chemical marker of the Copaifera genus, are described. The regression equation (Y = 26,707x ? 29,498) was obtained with good linearity (r2 = 0.9993) and the limits of quantification and detection were 9.182 and 3.032 µg/mL, respectively. The precision and the accuracy of the method were adequate (lower than 4%). Finally, the validation parameters evaluated were satisfactorily met, so the developed method represents a suitable tool for application in the quality control of such natural products. Further studies aiming to develop analytical methodologies for each Copaifera species using a more representative number of chemical markers should be performed. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
88.
Suk-Ku Kang Hyun-Sung Cho Hyeong-Su Sim Beon-Kyu Kim 《Journal of carbohydrate chemistry》2013,32(6):807-812
Abstract In connection with our projects on the synthesis of biologically active 5-hydroxyalkan-4-olides which have a chiral 2.3-diol unit,1 we have carried out the synthesis of (4R,5S)-(?)- and (4R,5S)-(+)-L-factors (1).2 the proposed autoregulators from Streptomyees griseus, and muricatacin (2),3 a biologically active constituent from the seeds of Annona muricata L. via 2.3-dihydroxy aldehydes derived from D-glucose. Hex-3-enofuranose4 was prepared by the elimination of thetriflate derived from D-glucose. 相似文献
89.
Application of matrix solid-phase dispersion to the determination of a new generation of fungicides in fruits and vegetables 总被引:8,自引:0,他引:8
A method based on matrix solid-phase dispersion (MSPD) and gas chromatography to determine eight fungicides in fruits and vegetables is described. Fungicide residues were identified and quantified using nitrogen-phosphorus detection and electron-capture detection connected in parallel and confirmed by mass spectrometric detection. The method required 0.5 g of sample, C18 bonded silica as dispersant sorbent, silica as clean-up sorbent and ethyl acetate as eluting solvent. Recoveries from spiked orange, apple, tomato, artichoke, carrot and courgette samples ranged from 62 to 102% and relative standard deviations were less than 15% in the concentration range 0.05-10 mg kg(-1). Detection and quantitation limits ranged 3-30 microg kg(-1) and 10-100 microg kg(-1), respectively, with linear calibration curves up to 10 mg kg(-1). The analytical characteristics of MSPD compared very favourably with the results of a classical multiresidue method, which uses ethyl acetate and anhydrous sodium sulphate for the extraction. 相似文献
90.
E. A. Guimares Costa F. De Brito Mota A. Ferreira Da Silva 《International journal of quantum chemistry》1992,41(3):517-524
The density of states is calculated for a random distribution of donor-pairs of hydrogenlike impurities in three- and two-dimensional systems. Recent investigations of the hydrogen molecule in the alternant–molecular–orbital approximation are here extended. We found that the lowest excited state 1Σu (i.e., H+H?), which is optically connected to the ground state, plays a relevant role in the absorption spectra of semiconductor systems. 相似文献