全文获取类型
收费全文 | 8843篇 |
免费 | 267篇 |
国内免费 | 24篇 |
专业分类
化学 | 5791篇 |
晶体学 | 68篇 |
力学 | 277篇 |
综合类 | 1篇 |
数学 | 1143篇 |
物理学 | 1854篇 |
出版年
2023年 | 60篇 |
2022年 | 197篇 |
2021年 | 274篇 |
2020年 | 166篇 |
2019年 | 183篇 |
2018年 | 168篇 |
2017年 | 157篇 |
2016年 | 350篇 |
2015年 | 260篇 |
2014年 | 284篇 |
2013年 | 531篇 |
2012年 | 576篇 |
2011年 | 610篇 |
2010年 | 456篇 |
2009年 | 417篇 |
2008年 | 545篇 |
2007年 | 525篇 |
2006年 | 415篇 |
2005年 | 379篇 |
2004年 | 306篇 |
2003年 | 285篇 |
2002年 | 237篇 |
2001年 | 174篇 |
2000年 | 152篇 |
1999年 | 112篇 |
1998年 | 95篇 |
1997年 | 83篇 |
1996年 | 93篇 |
1995年 | 80篇 |
1994年 | 75篇 |
1993年 | 62篇 |
1992年 | 67篇 |
1991年 | 55篇 |
1990年 | 54篇 |
1989年 | 45篇 |
1988年 | 43篇 |
1987年 | 31篇 |
1986年 | 44篇 |
1985年 | 66篇 |
1984年 | 44篇 |
1983年 | 38篇 |
1982年 | 29篇 |
1981年 | 27篇 |
1980年 | 38篇 |
1979年 | 32篇 |
1978年 | 32篇 |
1977年 | 16篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1973年 | 21篇 |
排序方式: 共有9134条查询结果,搜索用时 31 毫秒
971.
da Silva AS Fernandes FC Tognolli JO Pezza L Pezza HR 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,79(5):1881-1885
This article describes a simple, inexpensive, and environmentally friendly method for the monitoring of glyphosate using diffuse reflectance spectroscopy. The proposed method is based on reflectance measurements of the colored compound produced from the spot test reaction between glyphosate and p-dimethylaminocinnamaldehyde (p-DAC) in acid medium, using a filter paper as solid support. Experimental designs were used to optimize the analytical conditions. All reflectance measurements were carried out at 495 nm. Under optimal conditions, the glyphosate calibration graphs obtained by plotting the optical density of the reflectance signal (AR) against the concentration were linear in the range 50-500 μg mL(-1), with a correlation coefficient of 0.9987. The limit of detection (LOD) for glyphosate was 7.28 μg mL(-1). The technique was successfully applied to the direct determination of glyphosate in commercial formulations, as well as in water samples (river water, pure water and mineral drinking water) after a previous clean-up or pre-concentration step. Recoveries were in the ranges 93.2-102.6% and 91.3-102.9% for the commercial formulations and water samples, respectively. 相似文献
972.
Renato Vessecchi Lourenço Marcelo Kobelnik Clóvis Augusto Ribeiro Fernando L. Fernonani 《Journal of Thermal Analysis and Calorimetry》2011,106(3):735-740
The Brazilian sugarcane industry shows a great amount of generated sludge which should be utilized adequately. Two sludge
samples, aerobic and anaerobic, were collected. Both were evaluated by thermogravimetry and differential thermal analysis
(DTA) as well as X-ray power diffraction. These compounds show variations of mass between 30 and 140 °C due to the dehydration
stage. The DTA curves show that the compounds have an exothermic reaction between 450 and 550 °C, which indicates that this
can be used as an energy source. Details concerning the kinetic parameters of the dehydration and thermal decomposition have
also been described here. The kinetic study of these stages was evaluated in open crucibles under nitrogen atmosphere. The
obtained data were evaluated with the isoconversional kinetic method. The results show that different activation energies
were obtained for thermal decomposition. 相似文献
973.
Taís Vanessa Gabbay Alves Eraldo José Madureira Tavares Fauze Ahmad Aouada Charles Alberto Brito Negrão Marcos Enê Chaves Oliveira Anivaldo Pereira Duarte Júnior Carlos Emmerson Ferreira da Costa José Otávio Carréra Silva Júnior Roseane Maria Ribeiro Costa 《Journal of Thermal Analysis and Calorimetry》2011,106(3):717-724
This paper reports the thermal characterization of polyacrylamide-co-methylcellulose hydrogels and the constituent monomers (acrylamide and methylcellulose). Polymeric materials can be used
to produce hydrogels, which can be natural, synthetic, or a mixture. The hydrogels described here were obtained by free radical
polymerization, in the presence of N,N′-methylene-bis-acrylamide as a cross-linker agent. Four acrylamide concentrations were used for the synthesis of hydrogels:
3.6, 7.2, 14.7, and 21.7% (w/v). The materials so obtained were analyzed by TG, DTG, DSC, and FT-IR. The TG curves of acrylamide
and methylcellulose showed three mass loss events. In DSC curves, the acrylamide exhibited one melting peak at 84.5 °C, and
methylcellulose indicated one exothermic event. Nevertheless, acrylamide was considered more stable than methylcellulose.
The TG curves of the hydrogels exhibited three mass loss events, and on the DSC curves, three endothermic events were observed.
It was verified that the different acrylamide proportions influenced the thermic behavior of hydrogels, and that the authors
considered the 7.2% hydrogel a promising drug carrier system. The absorption bands were well defined, confirming the presence
of the functional groups in the samples. 相似文献
974.
Costa EV Pinheiro ML de Souza AD Barison A Campos FR Valdez RH Ueda-Nakamura T Filho BP Nakamura CV 《Molecules (Basel, Switzerland)》2011,16(11):9714-9720
Phytochemical investigation of the branches of Annona foetida Mart. led to isolation from the CH(2)Cl(2) extract of four alkaloids: Atherospermidine (1), described for the first time in this species, liriodenine (2), O-methylmoschatoline (3), and annomontine (4). Their chemical structures were established on the basis of spectroscopic data from IR, MS, NMR (1D and 2D), and comparison with the literature. Compounds 2-4 showed potent trypanocidal effect when evaluated against epimastigote and trypomastigote forms of Trypanosoma cruzi. 相似文献
975.
de Oliveira CS Falcão-Silva Vdos S Siqueira JP Harding DP Lira BF Lorenzo JG Barbosa-Filho JM de Athayde-Filho PF 《Molecules (Basel, Switzerland)》2011,16(3):2023-2031
Two salts of the mesoionic compounds 1,4-diphenyl-5-(5-nitro-2-furanyl)-1,3,4-thiadiazolium-2-thiol chloride (MC-1) and 4-phenyl-5-(5-nitro-2-furanyl)-1,3,4-thiadiazolium-2-phenylamine chloride (MC-2) were synthesized utilizing 1,4-diphenyl-thiosemicarbazide and 5-nitro-2-furoyl chloride as starting materials. Their structures were characterized by IR, 1H-NMR, 13C-NMR and elemental analysis. These compounds were analyzed for their influence on the effectiveness of norfloxacin, tetracycline, and erythromycin (standard antibiotics) against resistant strains of Staphylococcus aureus. MC-1 and MC-2, at sub-inhibitory concentrations of 16 μg/mL, favourably modulated the antibiotic activity of tetracycline by 16- and 32-fold, respectively (MIC), and that of erythromycin by 4-fold. 相似文献
976.
Reis FS Pereira E Barros L Sousa MJ Martins A Ferreira IC 《Molecules (Basel, Switzerland)》2011,16(6):4328-4338
The use of natural products isolated from mushrooms, included inedible species, against infection, cancer diseases and other oxidative-stress related diseases is one of the cornerstones of modern medicine. In the present work, the antioxidant molecule profiles of inedible mushroom species were evaluated and compared with those of edible species. The order of antioxidant abundance found in inedible wild mushrooms was: phenolics > flavonoids > ascorbic acid > tocopherols > carotenoids, similar to that of edible species. Furthermore the same energetic biomolecules were found including the disaccharide trehalose, the monosaccharide alcohol derivative mannitol and the fatty acids palmitic, oleic and linoleic acids. Fomitopsis pinicola revealed a very high phenolics concentration (388 mg GAE/g extract) and powerful antioxidant properties, mainly reducing power (EC??) value 60 μg/mL similar to the standard Trolox?). It could find applications in the prevention of free radical-related diseases as a source of bioactive compounds. 相似文献
977.
Lúcio AS Almeida JR Barbosa-Filho JM Pita JC Branco MV Diniz Mde F Agra Mde F da-Cunha EV da Silva MS Tavares JF 《Molecules (Basel, Switzerland)》2011,16(8):7125-7131
Phytochemical investigation of Anaxagorea dolichocarpa Sprague & Sandwith led to isolation of three azaphenanthrene alkaloids: eupolauramine, sampangine and imbiline 1. Their chemical structures were established on the basis of spectroscopic data from IR, HR-ESI-MS, NMR (including 2D experiments) and comparison with the literature. Sampangine and imbiline 1 are being described in the Anaxagorea genus for the first time. Eupolauramine and sampangine show concentration-dependent antitumoral activity in leukemic cells K562 with IC(50) of 18.97 and 10.95 μg/mL, respectively. 相似文献
978.
Rangel TB Assreuy AM Pires Ade F Carvalho AU Benevides RG Simões Rda C Silva HC Bezerra MJ Nascimento AS Nascimento KS Nagano CS Sampaio AH Delatorre P Rocha BA Fernandes PM Cavada BS 《Molecules (Basel, Switzerland)》2011,16(6):5087-5103
DwL, a lectin extracted from the seeds of Dioclea wilsonii, is a metalloprotein with strong agglutinating activity against rabbit and ABO erythrocytes, inhibited by glucose and mannose. DwL was purified by affinity chromatography on a Sephadex G-50 column and ion exchange chromatography on a HiTrap SP XL column. SDS-PAGE revealed three electrophoretic bands corresponding to the α (25,634 ± 2 Da), β (12,873 ± 2 Da) and γ (12,779 ± 2 Da) chains. Protein sequencing was done by Tandem Mass Spectrometry. The primary sequence featured 237 amino acids and was highly homologous to other reported Diocleinae lectins. A complete X-ray dataset was collected at 2.0 ? for X-Man-complexed DWL crystals produced by the vapor diffusion method. The crystals were orthorhombic and belonged to the space group I222, with the unit-cell parameters a = 59.6, b = 67.9 and c = 109.0 ?. DWL differed in potency from other ConA-like lectins and was found to induce neutrophil migration in rats, making it particularly useful in structural/functional studies of this class of proteins. 相似文献
979.
980.