全文获取类型
收费全文 | 887篇 |
免费 | 42篇 |
国内免费 | 6篇 |
专业分类
化学 | 717篇 |
晶体学 | 6篇 |
力学 | 4篇 |
数学 | 114篇 |
物理学 | 94篇 |
出版年
2023年 | 8篇 |
2022年 | 17篇 |
2021年 | 24篇 |
2020年 | 19篇 |
2019年 | 21篇 |
2018年 | 16篇 |
2017年 | 14篇 |
2016年 | 43篇 |
2015年 | 35篇 |
2014年 | 42篇 |
2013年 | 62篇 |
2012年 | 59篇 |
2011年 | 74篇 |
2010年 | 36篇 |
2009年 | 30篇 |
2008年 | 58篇 |
2007年 | 43篇 |
2006年 | 51篇 |
2005年 | 49篇 |
2004年 | 30篇 |
2003年 | 25篇 |
2002年 | 30篇 |
2001年 | 10篇 |
2000年 | 8篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 11篇 |
1996年 | 8篇 |
1995年 | 6篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1982年 | 4篇 |
1981年 | 3篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1973年 | 2篇 |
1972年 | 4篇 |
1965年 | 7篇 |
排序方式: 共有935条查询结果,搜索用时 31 毫秒
101.
Byssus threads are tough biopolymer produced by mussels (Mytilus viridis) to attach themselves to rocks. These were collected from mussels in their natural habitat (N) and from animals maintained in laboratory condition (L) as a novel support. Byssus thread surfaces were characterized by SEM analysis, chemically modified and used for adsorption of urease. The efficiency of the immobilization was calculated by examining the relative enzyme activity of free and the immobilized urease. The pH stabilities of immobilized urease were higher (0.5 unit) than free enzyme. Immobilized enzymes on byssus (both N and L) when stored at 6 °C retained 50% of its activity after 30 days, but they were more stable in dry condition. The optimum temperature of immobilized enzymes was found to increase (25 °C). A Michaelis-Menten constant (K (m)) value for immobilized urease was also elevated (2.08 mol). 相似文献
102.
Vargis E Kanter EM Majumder SK Keller MD Beaven RB Rao GG Mahadevan-Jansen A 《The Analyst》2011,136(14):2981-2987
In this paper, we examine how variations in normal tissue can influence disease classification of Raman spectra. Raman spectra from normal areas may be affected by previous disease or proximity to areas of dysplasia. Spectra were acquired in vivo from 172 patients and classified into five tissue categories: true normal (no history of disease), previous disease normal (history of disease, current normal diagnosis), adjacent normal (disease on cervix, spectra acquired from visually normal area), low grade, and high grade. Taking into account the various "normal" states of the tissue before statistical analysis led to a disease classification accuracy of 97%. These results indicate that abnormal changes significantly affect Raman spectra, even when areas are histopathologically normal. The sensitivity of Raman spectroscopy to subtle biochemical differences must be considered in order to successfully implement it in a clinical setting for diagnosing cervical dysplasia and cancer. 相似文献
103.
Molnár A Kapros A Párkányi L Mucsi Z Vlád G Hermecz I 《Organic & biomolecular chemistry》2011,9(19):6559-6565
The palladium-catalyzed Suzuki-Miyaura cross-coupling reactions of halo derivatives of 4H-pyrido[1,2-a]pyrimidin-4-one with (het)arylboronic acids allow easy access to (het)aryl and vinyl derivatives of this bicycle in good to excellent yields, even from chloro derivatives. The sequence of reactivity of the halogen in the different positions of the ring system was also investigated. 6-Phenyl-4H-pyrido[1,2-a]pyrimidin-4-one could be prepared by thermal cyclization of isopropylidene (6-phenylpyrid-2-ylamino)methylenemalonate, together with a small amount of 7-phenyl-1,4-dihydro-1,8-naphthyridin-4-one. 相似文献
104.
Abedi A Safari N Amani V Khavasi HR 《Dalton transactions (Cambridge, England : 2003)》2011,40(26):6877-6885
The complexes [Fe(dm4bt)(3)][FeCl(4)](2) (1) and [Fe(dm4bt)(3)][FeBr(4)](2) (2) were prepared from the reaction of 2,2'-dimethyl-4,4'-bithiazole (dm4bt) with FeCl(3)·6H(2)O and FeBr(3), respectively, in methanol. Both complexes were characterized by IR, UV-Vis and (1)H NMR spectroscopy and their structures were studied by single-crystal diffraction. The methylated bithiazole led to high spin Fe(II) centers in the octahedral cation part of complexes 1 and 2 with Fe-N distance of 2.220 ?, in spite of the low spin octahedral Fe(II) complexes with unsubstituted bithiazole ligands. Crystal structure determination of 2 was performed at 90, 120 and 298 K. Temperature reduction to 90 K resulted in a decrease in the Fe-N bond length to 2.206 ? which is still in the range of high spin Fe(II). Complex 1 shows a reversible mechanochromic effect from the crystalline phase to powder form from red to yellow; it also displays reversible photochromism from yellow to green in solution under sunlight. The magnetic behaviour of the complexes was also studied at 2-300 K. The temperature dependence of χ(m)T curves for the two forms of 1, crystal and powder, demands some changes in their magnetic behavior, causing different colors i.e. red and yellow. At low temperatures, χ(m)T decreases where the decrease starts at around 65 K for compound 1, and at around 100 K for compound 2, due to different counterions. The two complexes exhibit antiferromagnetism at around 4 K. 相似文献
105.
Boronate ureas are introduced as a new class of noncovalent catalysts for conjugate addition reactions with enhanced activity. Through intramolecular coordination of the urea functionality to a strategically placed Lewis acid, rate enhancements up to 10 times that of more conventional urea catalysts are observed. The tunable nature of boronate ureas is a particularly attractive feature and enables the rational design of catalysts for optimal performance, in terms of both activity and stereocontrol, in new bond-forming processes. 相似文献
106.
Hafner A Dürrigl M Pepić I Filipović-Grčić J 《Chemical & pharmaceutical bulletin》2011,59(9):1117-1123
The aim of this study was to establish a freeze-drying process for melatonin-loaded lecithin/chitosan nanoparticles (NPs) to preserve their chemical and physical stability for a longer time period that what is possible in an aqueous suspension. Glucose and trehalose were investigated as potential excipients during freeze-drying of NP suspensions. Lecithin/chitosan NPs were characterised by mean diameter and zeta potential, ranging between 117.4 and 328.5 nm and 6.7 and 30.2 mV, respectively, depending on the lecithin type and chitosan content in the preparation. Melatonin loadings were up to 7.1%. For all lecithin/chitosan NPs, no notable differences in the mean particle size, size distribution, zeta potential or melatonin content were observed before or immediately after the lyophilisation process or after 7 months of storage at 4 °C. The residual moisture contents of lyophilisates with glucose and trehalose immediately after the lyophilisation process varied between 4.0-4.8% and 2.4-3.0%, respectively. All lecithin/chitosan NPs had a fully amorphous nature after the freeze-drying process, as indicated by modulated differential scanning calorimetry. NP lyophilisates with glucose had a low glass transition temperature (ca. 5 °C), confirming that lyophilisation with glucose as a cryoprotectant was not appropriate. All lyophilisates with trehalose had a glass transition temperature above the room temperature, allowing formation of the cake without a collapse of the structure, which was capable of preserving its characteristics and appearance following 7 months of storage at 4 °C. 相似文献
107.
We report the design and characterization of a metal ion-imidazole self-assembled monolayer on a gold electrode. The resultant monolayer is well-suited for direct immobilization of histidine and methylene blue-modified peptides. 相似文献
108.
Heretsch P Büttner A Tzagkaroulaki L Zahn S Kirchner B Giannis A 《Chemical communications (Cambridge, England)》2011,47(26):7362-7364
The combination of theoretical and computational studies with organic synthesis and biological investigations has led to exo-cyclopamine. This stable and highly potent derivative of cyclopamine promises big potential as an experimental drug against several types of human cancer. 相似文献
109.
Erande N Gunjal AD Fernandes M Kumar VA 《Chemical communications (Cambridge, England)》2011,47(13):4007-4009
Sugar conformations in the isoDNA strand of isoDNA : RNA duplexes are preferred S-type locked/frozen in contrast to N-type locked conformations preferred in DNA : RNA duplexes. 相似文献
110.
Higgins EM Sherwood JA Lindsay AG Armstrong J Massey RS Alder RW O'Donoghue AC 《Chemical communications (Cambridge, England)》2011,47(5):1559-1561
pK(a) values of 19.8-28.2 are reported for the conjugate acids of a large series of NHCs in water. The effects of ring size, N-substituent and C(4)-C(5) saturation on pK(a) are discussed. 相似文献