全文获取类型
收费全文 | 1694篇 |
免费 | 49篇 |
国内免费 | 16篇 |
专业分类
化学 | 987篇 |
晶体学 | 19篇 |
力学 | 39篇 |
数学 | 214篇 |
物理学 | 500篇 |
出版年
2024年 | 7篇 |
2023年 | 30篇 |
2022年 | 34篇 |
2021年 | 38篇 |
2020年 | 43篇 |
2019年 | 49篇 |
2018年 | 43篇 |
2017年 | 40篇 |
2016年 | 64篇 |
2015年 | 37篇 |
2014年 | 52篇 |
2013年 | 102篇 |
2012年 | 130篇 |
2011年 | 140篇 |
2010年 | 72篇 |
2009年 | 78篇 |
2008年 | 101篇 |
2007年 | 82篇 |
2006年 | 74篇 |
2005年 | 65篇 |
2004年 | 48篇 |
2003年 | 36篇 |
2002年 | 38篇 |
2001年 | 23篇 |
2000年 | 33篇 |
1999年 | 14篇 |
1998年 | 21篇 |
1997年 | 17篇 |
1996年 | 15篇 |
1995年 | 21篇 |
1994年 | 16篇 |
1993年 | 16篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 7篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1973年 | 8篇 |
排序方式: 共有1759条查询结果,搜索用时 0 毫秒
21.
The discolouration, that occurs in virgin poly(ethylene terephthalate) - PET during melt processing, was studied using various bulk and surface analytical techniques. Proton nuclear magnetic resonance (1H NMR) was used to study the bulk chemical changes occurring in the polymer during thermo-oxidative degradation. Chemical derivatisation with trifluoroacetic anhydride (TFAA) was used to label the hydroxyl groups introduced on the polymer surface by thermal oxidation.From the surface analysis studies using photoacoustic Fourier transform infrared spectroscopy (PA/FT-IR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and X-ray photoelectron spectroscopy (XPS) it was evident that colour formation starts initially with the hydroxylation of the terephthalic ring. Further, the formation of additional carbonyl functionalities and conjugated chromophoric systems complete the colour formation process. 相似文献
22.
P. Ramachandra T. Siva Rama Krishna Gautam R. Desiraju 《Journal of Chemical Sciences》1989,101(4):327-334
The compound 3-hydroxy-6-(4′-nitro)phenylazopyridine (1) was observed to incorporate several organic solvents of crystallisation, forming stable clathrates. These clathrates decompose
upon heating and the host material is chemically transformed. Single crystal X-ray diffraction studies on the ethanol clathrate
lead to a reasonable model for the host structure. But the guest molecules are severely disordered. Alternatively, the compound
forms twinned crystals. The chemically closely related compound2 does not form clathrates. 相似文献
23.
Dutta D Sarma TK Chowdhury D Chattopadhyay A 《Journal of colloid and interface science》2005,283(1):153-159
In this paper we report a new idea for synthesizing polyaniline in an ordinary filter paper. The synthesis was carried out by a process in which aqueous acidic aniline solution and the oxidizing agent H(2)O(2) was added to the paper drop by drop and in sequence. Uniform polymerization could be obtained with the addition of reagents in either sequence. The polymer formation led to a green coloration of the paper. Formation of the emeraldine salt of polyaniline was confirmed by UV-vis and FTIR spectroscopy. Scanning electron microscopic measurements were made for surface characterization of the polymer formed in the paper. The same paper was used as a sensor for ammonia in vapor and in solution, for acid and base as well as endpoint indication, and also to filter acids and bases. We found that, using the polymer-containing paper, ammonia concentrations in a solution as low as 14 ppm could be measured. 相似文献
24.
Landa Purushottam Unnikrishnan V. B. Dattatraya Gautam Rawale Mansi Gujrati Surya Dev Mishra Sajeev T. K. Neelesh C. Reddy Srinivasa Rao Adusumalli Ram Kumar Mishra Vishal Rai 《Chemical science》2020,11(48):13137
Analytically pure proteins are indispensable for diverse applications, including therapeutics. Here, we report a methodology where a single amino acid, glycine, enables metal-free protein purification. This robust platform is enabled by a Gly-tag resin for site-specific capture, enrichment, and release through chemically triggered C–C bond dissociation by resonance-assisted electron density polarization.Gly-tag resin precisely captures and releases a protein with one glycine at the N-terminus. The user-friendly protocol delivers analytically pure protein free of metal contaminants. 相似文献
25.
The Hexagallane [Ga6{SiMe(SiMe3)2}6] and the closo‐Hexagallanate [Ga6{Si(CMe3)3}4 (CH2C6H5)2]2— — the Transition to an Unusual precloso‐Cluster The closo hexagallanate [Ga6R4(CH2Ph)2]2— (R = SitBu3) as well as the hexagallane Ga6R6 (R = SiMe(SiMe3)2) with only six cluster electron pairs were isolated from reactions of “GaI” with the corresponding silanides. The structure of the latter is derived from an octahedron by a Jahn‐Teller‐distortion and is different from the capped trigonal bipyramidal one expected by the Wade‐Mingos rules. Both compounds were characterized by X‐ray crystallography. The bonding is discussed with simplified Ga6H6 and Ga6H62— models via DFT methods. 相似文献
26.
D. K. Bhattacharyya N. C. Dutta A. De 《Journal of Radioanalytical and Nuclear Chemistry》1990,140(1):121-131
The uptake of 22 cations at tracer concentrations has been studied over hydrous tin dioxide exchanger material. A granular variety of tin dioxide was prepared from the reaction of tin(IV) chloride with NaOH solution, and the formula of the material was ascertained to be SnO2·1.7 H2O. Radiochemical separation of carrier-free234Th from238U and113mIn from113Sn was achieved over a tin dioxide column. The separated products were of high radionuclidic purity. The overall separation procedures are very simple and quick with quantitative yield. 相似文献
27.
Perring M Dutta S Arafat S Mitchell M Kenis PJ Bowden NB 《Langmuir : the ACS journal of surfaces and colloids》2005,21(23):10537-10544
This article describes mild methods to directly assemble, functionalize, and pattern monolayers of undecylenic acid on hydrogen-terminated Si(111). These monolayers were assembled under very mild conditions from a neat solution of undecylenic acid containing 0.1 mol % 4-(decanoate)-2,2,6,6-tetramethylpiperidinooxy at room temperature without the need for UV light. Because of these mild conditions, monolayers exposing carboxylic acids could be assembled in one step without the need to protect the acid prior to its assembly. The monolayers were extensively characterized by horizontal attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle goniometry. The monolayers bonded to the silicon surface preferentially through the olefin with no detectable bonds between the carboxylic acids and silicon. The crystallinity of the monolayer was studied by infrared spectroscopy through the antisymmetric--v(a)(CH(2))--and symmetric--v(s)(CH(2))--stretches for methylene. Because it is important for future applications to assemble functional surfaces, methods to react the acid-terminated monolayers with trifluoroacetic anhydride and triethylamine to yield a symmetric anhydride on the monolayer were studied. These anhydrides were reacted with a variety of milligram-quantity amines to yield amide-terminated surfaces. This method was general, and a variety of amines could be bonded to the monolayer. The stabilities of these monolayers upon exposure to ambient conditions and under a variety of solvents were described. Because patterned monolayers have found wide applications, we have developed methods to pattern 1-octadecylamine and poly(ethylenimine) on the micrometer scale using soft lithography. In addition, polymer brushes of polynorbornene with thicknesses from 32 to 150 nm were grown from monolayers patterned with the Grubbs' catalyst. The patterned surfaces were imaged by scanning electron microscopy, scanning probe microscopy, and ellipsometry to determine the thicknesses of the patterns and the fidelity of the method. 相似文献
28.
Kaushik Chanda Milan Ch. Dutta Enamul Karim Jai N. Vishwakarma 《Journal of heterocyclic chemistry》2004,41(4):627-631
1‐(Aralkyl/aryl)‐3‐(alkyyaralkyl)‐5‐aroyl‐1,2,3,4‐tetrahydropyrimidines ( 2a‐c ) have been synthesized by dethiomethylation of 5‐aroyl‐6‐methylthio‐1,2,3,4‐tetrahydropyrimidines ( 1a‐c ). An alternative one‐pot synthetic strategy has been developed for the title compounds 2a‐t by the reaction of enaminones 3 with pri mary amine and formaldehyde in refluxing methanol in good yields. 相似文献
29.
30.