全文获取类型
收费全文 | 1820篇 |
免费 | 99篇 |
国内免费 | 8篇 |
专业分类
化学 | 1219篇 |
晶体学 | 34篇 |
力学 | 44篇 |
数学 | 102篇 |
物理学 | 310篇 |
无线电 | 218篇 |
出版年
2024年 | 16篇 |
2023年 | 21篇 |
2022年 | 84篇 |
2021年 | 101篇 |
2020年 | 64篇 |
2019年 | 75篇 |
2018年 | 97篇 |
2017年 | 76篇 |
2016年 | 97篇 |
2015年 | 68篇 |
2014年 | 75篇 |
2013年 | 150篇 |
2012年 | 150篇 |
2011年 | 130篇 |
2010年 | 84篇 |
2009年 | 71篇 |
2008年 | 84篇 |
2007年 | 67篇 |
2006年 | 46篇 |
2005年 | 38篇 |
2004年 | 28篇 |
2003年 | 30篇 |
2002年 | 51篇 |
2001年 | 34篇 |
2000年 | 39篇 |
1999年 | 11篇 |
1998年 | 17篇 |
1997年 | 10篇 |
1996年 | 5篇 |
1995年 | 8篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 11篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1982年 | 6篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1974年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有1927条查询结果,搜索用时 46 毫秒
841.
Dr. Abdul Khayum Mohammed Dr. Jésus Raya Ajmal Pandikassala Dr. Matthew A. Addicoat Safa Gaber Mohamed Aslam Dr. Liaqat Ali Dr. Sreekumar Kurungot Prof. Dinesh Shetty 《Angewandte Chemie (International ed. in English)》2023,62(29):e202304313
Hydrogen-bonded organic frameworks (HOFs) are ordered supramolecular solid structures, however, nothing much explored as centimetre-scale self-standing films. The fabrication of such crystals comprising self-supported films is challenging due to the limited flexibility and interaction of the crystals, and therefore studies on two-dimensional macrostructures of HOFs are limited to external supports. Herein, we introduce a novel chemical gradient strategy to fabricate a crystal-deposited HOF film on an in situ-formed covalent organic polymer film (Tam-Bdca-CGHOF). The fabricated film showed versatility in chemical bonding along its thickness from covalent to hydrogen-bonded network. The kinetic-controlled Tam-Bdca-CGHOF showed enhanced proton conductivity (8.3×10−5 S cm−1) compared to its rapid kinetic analogue, Tam-Bdca-COP (2.1×10−5 S cm−1), which signifies the advantage of bonding-engineering in the same system. 相似文献
842.
Nagarathinam M Peedikakkal AM Vittal JJ 《Chemical communications (Cambridge, England)》2008,(42):5277-5288
trans-1,2-Bis(4-pyridyl)ethylene (bpe), containing a C=C bond, has been extensively studied for solid-state photochemical [2+2] cycloaddition reactions, in discrete molecular and metal complexes, hydrogen bonded and coordination polymeric structures. The challenges in orienting a pair or more of bpe molecules in the solid-state using crystal engineering principles, and their photochemical behaviour and implications, based on Schmidt's postulates, are discussed. 相似文献
843.
Moberg V Mottalib MA Sauer D Poplavskaya Y Craig DC Colbran SB Deeming AJ Nordlander E 《Dalton transactions (Cambridge, England : 2003)》2008,(18):2442-2453
Phosphine derivatives of alkylidyne tricobalt carbonyl clusters have been tested as catalysts/catalyst precursors in intermolecular and (asymmetric) intramolecular Pauson-Khand reactions. A number of new phosphine derivatives of the tricobalt alkylidyne clusters [Co3(micro3-CR)(CO)9] (R = H, CO2Et) were prepared and characterised. The clusters [Co3(micro3-CR)(CO)9-x(PR'3)x] (PR'3 = achiral or chiral monodentate phosphine, x = 1-3) and [Co3(micro3-CR)(CO)7)(P-P)] (P-P = chiral diphosphine; 1,1'- and 1,2-structural isomers) were assayed as catalysts for intermolecular and (asymmetric) intramolecular Pauson-Khand reactions. The phosphine-substituted tricobalt clusters proved to be viable catalysts/catalyst precursors that gave moderate to very good product yields (up to approximately 90%), but the enantiomeric excesses were too low for the clusters to be of practical use in the asymmetric reactions. 相似文献
844.
Amoroso AJ Burrows MW Coles SJ Haigh R Farley RD Hursthouse MB Jones M Malik KM Murphy DM 《Dalton transactions (Cambridge, England : 2003)》2008,(4):506-513
The co-ordination of a series of terpyridine-N-oxide ligands to the Cu(ii) ion is reported. Addition of two equivalents of ligand results in the formation of the expected 2 : 1 six co-ordinate product, while the addition of one equivalent of ligand allows the isolation of 1 : 1 species if the ligand has a meridonal binding mode. The five complexes isolated were characterised in the solid state by X-ray crystallography while they are studied in solution using EPR, UV-vis spectroscopy and IR spectroscopy. 相似文献
845.
In this paper, we establish inequalities for polynomials with restricted zeros, which in particular yields interesting generalizations of some Zygmund type inequalities for polynomial. 相似文献
846.
Ahmad Z Mehmood S Fatima I Malik A Ifzal R Afza N Iqbal L Latif M Nizami TA 《Magnetic resonance in chemistry : MRC》2008,46(1):94-98
Salsolins A (1) and B (2), the new triterpenes, have been isolated from the chloroform soluble fraction of Salsola baryosma along with 2alpha,3beta,23,24-tetrahydroxyurs-12-en-28-oic acid (3) reported for the first time from this species. Their structures have been assigned from 1H and 13C NMR spectra, DEPT and by 2D COSY, NOESY, HMQC and HMBC experiments. The compounds 1-3 showed significant antioxidant activity. 相似文献
847.
Hussain M Zahra DN Hussain SM Ahmed E Ahmad I Malik A Ahmed Z 《Magnetic resonance in chemistry : MRC》2008,46(3):274-277
Two new steroids provisionally named as pakisteroid-A (1) and pakisteroid-B (2) have been isolated in crystalline form from Abutilon pakistanicum. Their structures have been assigned as 3-O-beta-D-glucopyranosyl-stigmasta-5,11(12)-diene (1) and 24beta-ethylcholesta-5, 9(11), 22E-trien-3beta-benzoate (2), respectively through extensive NMR studies. 相似文献
848.
Studies on rhizosphere microbial diversity of some commercially important medicinal plants 总被引:2,自引:0,他引:2
Karthikeyan B Jaleel CA Lakshmanan GM Deiveekasundaram M 《Colloids and surfaces. B, Biointerfaces》2008,62(1):143-145
A preliminary study was made on four medicinal plants viz., Ocimum sanctum L., Coleus forskholii Briq, Catharanthus roseus (L.) G. Don. and Aloe vera in order to identify and enumerate the rhizosphere, non-rhizosphere and diazotrophic microorganisms in soil. The diazotrophic bacterial population studied includes Azospirillum, Azotobacter and Pseudomonas. The rhizosphere bacterial populations were 23.33 x 10(6)g(-1) in O. sanctum followed by C. roseus (20.46 x 10(6)g(-1)), A. vera (18.44 x 10(6)g(-1)) and C. forskholii (16.64 x 10(6)g(-1)). The fungi populations were 19.44 x 10(4)g(-1) in C. roseus, 18.66 x 10(4)g(-1) in O. sanctum, 16.44 x 10(4)g(-1) in A. vera and 14.22 x 10(4)g(-1) in C. forskholii. The actinomycetes population was 12.22 x 10(5)g(-1) in O. sanctum, 10.44 x 10(5)g(-1) in C. roseus, 8.44 x 10(5)g(-1) in A. vera and 6.22 x 10(5)g(-1) in C. forskholii. The diazotrophic bacterial population of Azospirillum, Azotobacter and Pseudomonas is 8.2 x 10(4)g(-1), 12 x 10(4)g(-1), 6 x 10(4)g(-1) in the rhizosphere soil. In all the four medicinal plants the microbial population is more in the rhizosphere soil, when compared to non-rhizosphere soil. These results are helpful in developing a biofertilizer consortium for these commercially grown medicinal plants. 相似文献
849.
Jaleel CA Gopi R Manivannan P Gomathinayagam M Sridharan R Panneerselvam R 《Colloids and surfaces. B, Biointerfaces》2008,62(2):312-318
The variations in antioxidant potentials and indole alkaloid content were studied in the present investigation, in two varieties (rosea and alba) of Catharanthus roseus, an important herb used in traditional as well as modern medicine, exposed to water deficit stress. The antioxidant and alkaloid profiles were estimated from root, stem, leaf, flowers and pods. The antioxidant potentials were examined in terms of non-enzymatic antioxidant molecules and activities of antioxidant enzymes. The non-enzymatic antioxidant molecules studied were ascorbic acid (AA), -tocopherol (-toc) and reduced glutathione (GSH). The estimated antioxidant enzymes were superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), peroxidase (POX) and polyphenol oxidase (PPO). The antioxidant concentrations and activities of antioxidant enzymes were high under water deficit stress in all parts of the plants. Indole alkaloid content was high in the roots of rosea variety in response to stress when compared to alba variety. 相似文献
850.
Liu Z Mutlib AE Wang J Talaat RE 《Rapid communications in mass spectrometry : RCM》2008,22(21):3434-3442
A robust bioanalytical method capable of measuring acetyl and palmitoyl carnitines was developed and validated. Application of hydrophilic interaction chromatography (HILIC) enabled retention of these highly polar and difficult to analyze compounds on a silica HPLC column. The chromatography was conducted with a high percentage of an organic component in the mobile phase, allowing high sensitivity for the pre-existing positively charged quaternary ammonium ions by electrospray ionization mass spectrometry. Successful application of the method to reliably quantify naturally occurring acyl carnitines in mouse plasma depended on the use of corresponding deuterated analogues. The specificity of the method, achieved through the use of stable isotope labeled compounds in combination with a mass spectral multiple reaction monitoring technique, permitted a non-invasive assessment of the overall change in the levels of these acyl carnitines in the plasma of intact animals administered peroxisome proliferator activated receptor (PPAR) agents. These acyl carnitines, as carriers of the corresponding long-chain fatty acids for transport into mitochondria, can be employed as potential biomarkers for significant alteration in the beta-oxidation process in an intact animal. 相似文献