全文获取类型
收费全文 | 724篇 |
免费 | 37篇 |
国内免费 | 5篇 |
专业分类
化学 | 504篇 |
晶体学 | 11篇 |
力学 | 30篇 |
数学 | 121篇 |
物理学 | 100篇 |
出版年
2024年 | 2篇 |
2023年 | 8篇 |
2022年 | 17篇 |
2021年 | 21篇 |
2020年 | 32篇 |
2019年 | 37篇 |
2018年 | 24篇 |
2017年 | 23篇 |
2016年 | 43篇 |
2015年 | 27篇 |
2014年 | 30篇 |
2013年 | 69篇 |
2012年 | 71篇 |
2011年 | 46篇 |
2010年 | 50篇 |
2009年 | 38篇 |
2008年 | 29篇 |
2007年 | 39篇 |
2006年 | 26篇 |
2005年 | 19篇 |
2004年 | 26篇 |
2003年 | 9篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 4篇 |
1998年 | 3篇 |
1997年 | 3篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1969年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有766条查询结果,搜索用时 31 毫秒
51.
Fayez H.?OsmanEmail author Fatma A.?El-Samahy I. S.?Ahmed Farag 《Monatshefte für Chemie / Chemical Monthly》2004,135(7):823-831
Summary. The reaction of (E)-(2-oxo-1,2-dihydroindol-3-ylidene)acetic acid esters with dry acetone at 70°C in presence of aluminum oxide as a catalyst led to the formation of an addition product as a mixture of two isomers. When the same reaction was carried out in presence of morpholine as a base, the unexpected spiro-product was obtained beside the two isomers. Methylation of the latter with methyl iodide in acetone in presence of anhydrous potassium carbonate yielded the corresponding methylated products, whereas methylation of the starting material by the same procedure gave an N-methylated product and the unexpected dispiro-compound. The reaction mechanisms are considered and the structural assignments of the new compounds are based on the chemical and spectroscopic evidences. The structure of the dispiro-product was derived by X-ray analysis. 相似文献
52.
In this paper we report the effect of dicationic ‘gemini’ surfactants (CH3)2C16H33N+? (CH2)m? N+C16H33(CH3)2, 2Br? (where m = 4, 5, 6) on the reaction of ninhydrin with DL ‐tryptophan. The gemini surfactant micellar media are comparatively more effective than their conventional monomeric counterpart cetyltrimethylammonium bromide (CTAB) micelles. Also, whereas typical rate constant (kψ) increase and leveling‐off regions, just like CTAB, are observed with geminis, the latter produce a third region of increasing kψ at higher concentrations. These subsequent increases are ascribed to changes in micellar morphologies, consistent with changes in 1H NMR line widths. Quantitative kinetic analysis of the rate constant–[surfactant] data has been performed on the basis of modified pseudophase model. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
53.
M.E. Aydin 《Journal of Physics and Chemistry of Solids》2007,68(9):1770-1773
Electrical transport properties of Ag metal-fluorescein sodium salt (FSS) organic layer-silicon junction have been investigated. The current-voltage (I-V) characteristics of the diode show rectifying behavior consistent with a potential barrier formed at the interface. The diode indicates a non-ideal I-V behavior with an ideality factor higher than unity. The ideality factor of the Ag/FSS/p-Si diode decreases with increasing temperature and the barrier height increases with increasing temperature. The barrier height (φb=0.98 eV) obtained from the capacitance-voltage (C-V) curve is higher than barrier height (φb=0.72 eV) derived from the I-V measurements. The barrier height of the Ag/FSS/p-Si Schottky diode at the room temperature is significantly larger than that of the Ag/p-Si Schottky diode. It is evaluated that the FSS organic layer controls electrical charge transport properties of Ag/p-Si diode by excluding effects of the SiO2 residual oxides on the hybrid diode. 相似文献
54.
Synthesis of three new stable spirodifuranose derivatives (3, 5, and 7), which cannot be obtained easily using ordinary synthetic methods, has been achieved by reduction of 3-O-acetyl and 3-O-methyl derivatives of (4R)-1,2-O-alkylidene-5-eno-4,7-epidioxy-5,6,8-trideoxy-α-D-threo-1,4-furano-4,7-diulo-octoses (1, 4, and 6). 相似文献
55.
Yusuf Kurtulmus T. Tanyalçın Giray Bozkaya Ogˇuzhan Gündüz Ömür Çerçi Fatma Z. Kutay Biltan Ersöz 《Accreditation and quality assurance》2001,6(9-10):424-426
The aim of this study was to compare two immunoassay measurement methods, the ACS:180 Plus (Chiron Diagnostics) kit and the Immulite sPSA (DPC) kit. Method comparison analysis was performed according to EP9-A; approved guideline of NCCLS 51. Serum samples having a wide range of total prostate-specific antigen (PSA) concentrations were evaluated in split-sample analysis. F-test, t-test analysis and regression statistics were performed. In Deming regression analysis the coefficients were as follows; the slope=0.967; y-intercept=-0.148, r=0.989. An acceptable bias was seen since the systematical error was calculated to have a value less than the total allowable error calculated from biological variations. Non-parametric evaluation of the area under ROC curves for ACS:180 Plus and Immulite sPSA were 0.997 and 0.987, respectively. Diagnostic accuracy was at the level of p= 0.000 and no statistical difference was found between the two assay methods. 相似文献
56.
Gunjan Pandey Tasneem Fatma Sudha M. Cowsik Sneha Sudha Komath 《Journal of photochemistry and photobiology. B, Biology》2009,97(2):87-93
Recent research has shown that, like porphyrins, phycocyanin (PC) too can produce singlet oxygen upon excitation with the appropriate radiation and hence could be useful in photodynamic therapy (PDT) for cancer. Unlike porphyrins, PC has the advantage of being a non-toxic, non-carcinogenic, soluble protein. However, the challenge would be to target the fluorescent phycobiliprotein to malignant cells. We report here that the tumor-specific lectin, jacalin, binds PC specifically in a carbohydrate-independent manner and with affinities better than that for porphyrins. Hence the lectin could prove to be a useful carrier for targeted delivery of PC. The interaction involves both ionic and hydrophobic interactions and more than one contact site. 相似文献
57.
A series of homo‐, heterodinuclear and homotrinuclear copper(II) complexes containing a new Schiff base ligand and 1,10‐phenanthroline were synthesized. Based on results of elemental analyses, FTIR, 1H‐ and 13C‐NMR spectra, conductivity measurements and magnetic susceptibility measurements, the complexes had general compositions {[Cu(L)(H2O)M(phen)2](ClO4)2 [M = Cu(II), Mn(II), Co(II)]} and {[Cu3(L)2(H2O)2](ClO4)2}. The metal:L:phen ratio is 2:1:2 for the dinuclear copper(II) complexes and the metal:L ratio was 3:2 for the trinuclear copper(II) complex. The liquid–liquid extraction of various transition metal cations [Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II), Hg(II)] from the aqueous phase to the organic phase was carried out using the diimine–dioxime ligand. It was concluded that the ligand can effectively be used in solvent extraction of copper(II) from the aqueous phase to the organic phase. Furthermore, catalytic activitiy of the complexes for the disproportionation of hydrogen peroxide was also investigated in the presence of imidazole. Dinuclear copper(II)–manganese(II) complex has some similarity to manganese catalase in structure and activity. The interaction between these complexes and DNA has also been investigated by agarose gel electrophoresis; we found that the homo‐ and heterodinuclear copper complexes can cleave supercoiled pBR322 DNA to nicked and linear forms in the presence of H2O2. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
58.
This research work is concerned with the use of N‐benzoyl cyanoacetylhydrazine ( 3 ) in synthesizing several new heterocyclic compounds with potential biological activity, via its reaction with various chemical reagents. The synthesized derivatives have actually exhibited, upon screening, antibacterial and antifungal activities. 相似文献
59.
Emine Özge Karaca Mitat Akkoç Muhammad Nawaz Tahir Cengiz Arıcı Fatma İmik Nevin Gürbüz Sedat Yaşar İsmail Özdemir 《Tetrahedron letters》2017,58(36):3529-3532
A series of seven-membered ditopic ring-expanded N-heterocyclic carbene (dre-NHC) precursors, bearing sterically demanding and electron-rich aryl groups, were synthesised in moderate yields via the reaction of 1,2,4,5-tetrakis(bromomethyl)benzene with the corresponding N,N′-diarylformamidines in the presence of K2CO3 in acetonitrile under an air atmosphere. All new compounds were characterised by HRMS, NMR spectroscopy, and microanalysis, as well as X-ray crystallography for compound 1c. The development of an efficient catalytic system for the Suzuki-Miyaura coupling reaction of aryl chlorides with various boronic acids was also investigated using the in situ generated dre-NHC ligands. 相似文献
60.
The synthesis of well-defined block copolymers from styrene and methyl acrylate via ATRP is discussed in this contribution. Kinetic studies on these block copolymerizations as well as characterization studies were performed to investigate the monomer composition in the respective PS and PMA blocks. MALDI-TOF-MS was performed to clarify the exact number of repeating units of each block and the total number of units in the block copolymer. Block copolymers up to 22 kDa could be analyzed by MALDI-TOF-MS, whereby polymers with PMA as first block showed a large second distribution corresponding to PMA homopolymers. However, SEC demonstrated that only a small amount of homopolymer was present indicating that care needs to be taken with interpreting MALDI-TOF-MS data, which is a qualitative rather than a quantitative technique. 相似文献