全文获取类型
收费全文 | 15121篇 |
免费 | 553篇 |
国内免费 | 47篇 |
专业分类
化学 | 10255篇 |
晶体学 | 235篇 |
力学 | 466篇 |
综合类 | 1篇 |
数学 | 1172篇 |
物理学 | 3592篇 |
出版年
2024年 | 65篇 |
2023年 | 172篇 |
2022年 | 448篇 |
2021年 | 431篇 |
2020年 | 472篇 |
2019年 | 530篇 |
2018年 | 493篇 |
2017年 | 467篇 |
2016年 | 683篇 |
2015年 | 490篇 |
2014年 | 775篇 |
2013年 | 1292篇 |
2012年 | 1140篇 |
2011年 | 1171篇 |
2010年 | 774篇 |
2009年 | 601篇 |
2008年 | 744篇 |
2007年 | 748篇 |
2006年 | 575篇 |
2005年 | 499篇 |
2004年 | 388篇 |
2003年 | 325篇 |
2002年 | 266篇 |
2001年 | 159篇 |
2000年 | 135篇 |
1999年 | 107篇 |
1998年 | 75篇 |
1997年 | 110篇 |
1996年 | 97篇 |
1995年 | 82篇 |
1994年 | 76篇 |
1993年 | 98篇 |
1992年 | 100篇 |
1991年 | 79篇 |
1990年 | 72篇 |
1989年 | 73篇 |
1988年 | 50篇 |
1987年 | 47篇 |
1986年 | 43篇 |
1985年 | 66篇 |
1984年 | 65篇 |
1983年 | 55篇 |
1982年 | 48篇 |
1981年 | 41篇 |
1980年 | 48篇 |
1979年 | 63篇 |
1978年 | 50篇 |
1977年 | 60篇 |
1976年 | 44篇 |
1975年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Deepika Kushwaha S. N. Upadhyay Pradeep Kumar Mishra 《Applied biochemistry and biotechnology》2018,184(4):1247-1262
Growths of Lyngbya limnetica and Oscillatoria obscura were investigated at varying pH, light intensity, temperature, and trace element concentration with a view to optimize these parameters for obtaining the maximum carbohydrate content. The maximum growth for both strains was obtained at pH 9.0 and temperature 20 ± 3 °C using a light intensity of 68.0 μmol m?2 s?1 with continuous shaking. Growth under the nitrogen starvation condition affected the carbohydrate content more compared to the phosphorus starvation, and maximum concentrations were found as 0.660 and 0.621 g/g of dry biomass for L. limnetica and O. obscura, respectively. Under the optimized nitrogen-rich conditions, the specific growth rates for the two strains were found to be 0.187 and 0.215 day?1, respectively. The two-stage growth studies under nitrogen-rich (stage I) followed by nitrogen starvation (stage II) conditions were performed, and maximum biomass and carbohydrate productivity were found as 0.088 and 0.423 g L?1 day?1 for L. limnetica. This is the first ever attempt to evaluate and optimize various parameters affecting the growth of cyanobacterial biomass of L. limnetica and O. obscura as well as their carbohydrate contents. 相似文献
992.
A novel vitamin B6 cofactor derived anion sensor L for the selective colorimetric detection of acetate has been developed by the condensation of pyridoxal and 2-aminothiophenol. The sensor L showed a noteworthy change in the visible region of the spectrum and was detected by the ‘naked-eye’ for both acetate and fluoride anions in DMSO but selectively for acetate in DMSO/H2O (88:12, v/v). The anion recognition ability of L was investigated by spectroscopic (UV–vis and 1H NMR) and DFT methods. 相似文献
993.
An ‘on-water’, efficient, high yielding, expeditious method has been developed for the synthesis of 1,4-dihydropyridine (1,4-DHP) derivatives via an one-pot multi-component condensation of dimedone or 4-hydroxycoumarine, aldehydes, and ammonium acetate using Fe3O4@SiO2 nanoparticles as a recyclable heterogeneous catalyst. This method takes advantage of the fact that water, a green solvent is used in combination with Fe3O4@SiO2 nanoparticles as catalyst which can be easily recovered magnetically and reused for further runs. 相似文献
994.
Mahender Reddy Marri Swamy PerakaArun Kumar Macharla Naresh MamedaSrujana Kodumuri Narender Nama 《Tetrahedron letters》2014
A new and efficient protocol is described for the one-pot synthesis of benzimidazoles from a variety of aryl alcohols and 1,2-diaminoarenes. The yields were ranging from moderate to excellent. Moreover, the present method is utilizing alcohols instead of aldehydes and the reactions are carried out under solvent- and catalyst-free conditions, offering an environmentally benign process. 相似文献
995.
An efficient, novel, and concise one pot regio- and chemoselective synthesis of benzo[a]phenazines (4) and naphtho[2,3-d]imidazoles (8) has been accomplished in excellent yields by nucleophilic substitution reaction of 2,3-dichloro-1,4-naphthoquinone (1) with o-phenylenediamine (2) and benzamidines (7) respectively ‘in H2O’ using base and micelles (SDS) as catalyst. Analog reaction of 2,3-dichloro-1,4-naphthoquinone (1) with 2-aminobenzenethiol (9) under identical conditions led to formation of a mixture of benzo[b]phenothiazine (10), benzo[a]phenothiazine (11), and benzo[a]-1,4-benzothiazino-3,2-phenothiazine (12) in 17%, 23%, and 57% yields, respectively. 相似文献
996.
Sanjukta A. Kumar Neha Thakur Harshala J. Parab Shailaja P. Pandey Rakesh N. Shinde Ashok K. Pandey Sangita D. Kumar A.V.R. Reddy 《Analytica chimica acta》2014
A visual strip has been developed for sensing iron in different aqueous samples like natural water and fruit juices. The sensor has been synthesized by UV-radiation induced graft polymerization of acrylamide monomer in microporous poly(propylene) base. For physical immobilization of iron selective reagent, the in situ polymerization of acrylamide has been carried out in the presence of 1,10-phenanthroline. The loaded strip on interaction with Fe(II) in aqueous solution turned into orange red color and the intensity of the color was found to be directly proportional to the amount of Fe(II) in the aqueous sample. The minimal sensor response with naked eye was found for 50 ng mL−1 of Fe in 15 min of interaction. However, as low as 20 ng mL−1 Fe could be quantified using a spectrophotometer. The detection limit calculated using the 3s/S criteria, where ‘s’ is the standard deviation of the absorbance of blank reagent loaded strip and ‘S’ is the slope of the linear calibration plot, was 1.0 ng mL−1. The strip was applied to measure Fe in a variety of samples such as ground water and fruit juices. 相似文献
997.
Kyle Clagg Sara Hold Archana Kumar Stefan G. Koenig Remy Angelaud 《Tetrahedron letters》2019,60(1):5-7
The direct application of Knochel-Hauser derivative arylmagnesium halides in Kumada-Corriu couplings is described. By utilizing the in situ-generated Grignard reagents, the transmetalation with stoichiometric zinc salts to known Negishi coupling conditions is avoided, thereby streamlining the transformation. Several aromatic hetereocycles participate in the deprotonation and couple with a variety of phenyl iodides and bromides in isolated yields up to 81%. The parent system is demonstrated on 6?g scale with 79% yield, and monitored via ReactIR to show the stability of the Grignard and progression of the deprotonation/CC coupling reaction. 相似文献
998.
This study compared eight major ginsenosides (Rg1, Rg2, Rf, Re, Rd, Rc, Rb1 and Rb2) between Panax sokpayensis and Panax bipinnatifidus collected from Sikkim Himalaya, India. High-performance liquid chromatographic analysis revealed that all major ginsenosides were present in the rhizomes of P. sokpayensis except ginsenoside Rc, whereas ginsenoside Rf, Rc and Rb2 were not detected in P. bipinnatifidus. 相似文献
999.
Summary The formation of ternary complexes of the MAL3– type [where M = CuII, NiII and ZnII ; A = nitrilotriacetic acid (NTA); L = 1-hydroxy-2-naphthoic acid (1,2 HNA) and 2-hydroxy-1-naphthoic acid (2,1 HNA)] have been studied potentiometrically in 50% v/v aqueous — ethanol (25° and µ = 0.1). Under identical conditions the binary complexes of the 1,2- and 2,1-HNA ligands have also been examined. The values of mixed ligand formation constants KMAL have been found to be lower than KML (first step formation constant of binary complexes) and even less than
(second step formation constant of binary complexes). 相似文献
1000.
A chemical flux of sulfur hexafluoride (SF6) in conjunction with low-energy Ar-ion bombardment has been used for chemically assisted ion beam etching (CAIBE) of silicon and silicon dioxide. The study has shown a large degree of independent control over the selectivity and anisotropy in dry etching. The total etch rate could be controlled by varying either the Ar-ion milling parameters or the chemical flux of SF6. Etch rate enhancement of 7–8 for silicon and 3–4 for silicon dioxide have been obtained over pure physical etching. 相似文献