首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1253篇
  免费   39篇
  国内免费   8篇
化学   881篇
晶体学   12篇
力学   53篇
数学   134篇
物理学   220篇
  2024年   6篇
  2023年   22篇
  2022年   49篇
  2021年   54篇
  2020年   49篇
  2019年   58篇
  2018年   53篇
  2017年   33篇
  2016年   52篇
  2015年   44篇
  2014年   56篇
  2013年   137篇
  2012年   104篇
  2011年   74篇
  2010年   65篇
  2009年   43篇
  2008年   45篇
  2007年   39篇
  2006年   46篇
  2005年   22篇
  2004年   27篇
  2003年   16篇
  2002年   15篇
  2001年   16篇
  2000年   21篇
  1999年   11篇
  1998年   6篇
  1997年   9篇
  1996年   4篇
  1995年   10篇
  1994年   11篇
  1992年   4篇
  1991年   3篇
  1989年   3篇
  1987年   9篇
  1986年   3篇
  1985年   5篇
  1984年   4篇
  1983年   7篇
  1982年   5篇
  1981年   4篇
  1979年   6篇
  1978年   8篇
  1977年   3篇
  1976年   6篇
  1974年   7篇
  1966年   2篇
  1960年   5篇
  1959年   3篇
  1958年   2篇
排序方式: 共有1300条查询结果,搜索用时 15 毫秒
991.
992.
Vitamin E is known as an essential vitamin, and many studies had demonstrated the importance of vitamin E throughout the reproductive process, such as miscarriage, premature birth, preeclampsia, and intrauterine growth restriction, which could be caused by a lack of vitamin E during pregnancy. Its potent antioxidant properties can counteract the oxidative stress induced by oxygen free radicals and imbalance of oxidative-antioxidant levels, hence it may play a role in maintaining the normal function of the female reproductive system. Despite the fact that vitamin E is acknowledged as the substance needed for reproduction, its beneficial effects on female fertility, gynaecological health, and diseases are still poorly understood and lacking. Therefore, the goal of this paper is to provide a summary of the known roles of vitamin E supplementation in women for gynaecological health and reproductive-related diseases, as well as its future perspective.  相似文献   
993.
The medicinal herb, lemon balm (Melissa officinalis L.), which is high in rosmarinic acid (RA), has well-known therapeutic value. The goals of this study were to investigate the effects of methyl jasmonate (MeJA) on RA content, total phenolic content (TPC), and total flavonoid content (TFC), as well as changes in expression of their biosynthesis-related key genes (MoPAL, Mo4CL, and MoRAS) in Iranian lemon balm ecotypes, as first reported. Our results revealed that MeJA doses significantly increase the RA content, TPC, and TFC in both ecotypes compared with the control samples. Additionally, the higher expression levels of MoPAL, Mo4CL, and MoRAS following treatment were linked to RA accumulation in all treatments for both Iranian lemon balm ecotypes. After 24 h of exposure to 150 µM MeJA concentration, HPLC analysis showed that MeJA significantly increased RA content in Esfahan and Ilam ecotypes, which was about 4.18- and 7.43-fold higher than untreated plants. Our findings suggested that MeJA has a considerable influence on RA, TPC, and TFC accumulation in MeJA-treated Iranian M. officinalis, which might be the result of gene activation from the phenylpropanoid pathway. As a result of our findings, we now have a better understanding of the molecular processes behind RA production in lemon balm plants.  相似文献   
994.
This study demonstrates the presence of CH-π interaction in poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO-1) due to an aggregate formation of PFO-1 in the liquid state. The absorption spectra of PFO-1 in certain solvents at low concentrations showed a single band at 390 nm. However, when using high concentrations, a new band at 437 nm appeared. This band is due to the aggregate formation of PFO-1. The aggregate formation occurs as a result of the CH interaction of the n-alkyl side chains with π-electrons in the benzene ring. The optical characteristics of another conjugated polymer of poly [9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl] (PFO-2) were investigated to confirm the CH-π interaction. The absorption showed only one wavelength at 390 nm without any new band at the end of the spectrum, even at higher concentrations and lower temperatures. The main reason for the absence of aggregate formation in PFO-2 is the sterical hindrance caused by the branched alkyl side chains. In addition, Density Functional Theory (DFT) was used to compute the HOMO–LUMO transitions, electron charge distribution, and frontier molecular orbitals for each polymer. The Mulliken charge distribution and demonstrated a notable difference in the reactivity of the alkyl side chain, confirming the higher ability of PFO-1 to form CH-π bonds. docking model emphasized that the band at 437 nm could be attributed to the interaction between CH in the n-alkyl side chain and π bonds in the aromatic rings of PFO-1.  相似文献   
995.
This note looks at the two similarity solutions of the Navier Stokes equations in polar coordinates. In the second solution an initial value problem is reduced into generalized stationary KDV and hence integrable.  相似文献   
996.
Siddha medicine is one of the oldest medical systems in the world and is believed to have originated more than 10,000 years ago and is prevalent across ancient Tamil land. It is undeniable that inhibitor preferences rise with increasing solubility in water due to the considerations pertaining to the bioavailability and the ease of which unabsorbed residues can be disposed of. In this study, we showed the phytochemical discrimination of Saussurea costus extracted with water at room temperature as a green extraction procedure. A total of 48 compounds were identified using gas chromatography-mass spectrometry (GC-MS). The fatty acids had a high phytochemical abundance at 73.8%, followed by tannins at 8.2%, carbohydrates at 6.9%, terpenoids at 4.3%, carboxylic acids at 2.5%, hydrocarbons at 2.4%, phenolic compounds at 0.2%, and sterols at 1.5%. Of these compounds, 22 were docked on the active side and on the catalytic dyad of His41 and Cys145 of the main protease of SARS-CoV-2 (Mpro). Eight active inhibitors were carbohydrates, five were fatty acids, three were terpenoids, two were carboxylic acids, one was a tannin, one was a phenolic compound, and one was a sterol. The best inhibitors were 4,8,13-Cyclotetradecatriene-1,3-diol, 1,5,9-trimethyl-12-(1-methylethyl), Andrographolide, and delta.4-Androstene-3.beta.,17.beta.-diol, with a binding affinity that ranged from −6.1 kcal/mol to −6.5 kcal/mol. The inhibitory effect of Saussurea costus of SARS-CoV-2 entry into the cell was studied using a pseudovirus with Spike proteins from the D614G variant and the VOC variants Gamma and Delta. Based on the viral cycle of SARS-CoV-2, our results suggest that the Saussurea costus aqueous extract has no virucidal effect and inhibits the virus in the events after cell entry. Furthermore, the biological activity of the aqueous extract was investigated against HSV-1 virus and two bacterial strains, namely Staphylococcus aureus ATCC BAA 1026 and Escherichia coli ATCC 9637. According to this study, an enormous number of water-soluble inhibitors were identified from Saussurea costus against the Mpro, and this is unprecedented as far as we know.  相似文献   
997.
Natural frequencies are important dynamic characteristics of a structure. Therefore, the exact solution pertaining to free vibration of stepped circular plate elastically restrained against rotation, translation, and internal elastic ring support resting on an arbitrary variable elastic foundation using Green Function is presented in this paper. Thus, an accurate and direct modeling technique is introduced for modeling stepped circular plate on an arbitrary variable elastic foundation with arbitrary boundary conditions and internal elastic ring support. The effect of the translational along with rotational support flexibilities, as well as, the elastic coefficient of Winkler foundation and other parameters are assessed. Finally, some numerical examples are shown in order to present the efficiency and simplicity of the Green Function in the new formulation.  相似文献   
998.
Phthalates are ubiquitous environmental contaminants, and frequent biological monitoring of their metabolites (as toxic species of phthalates in human body) is highly recommended. A novel dispersive liquid–liquid microextraction (DLLME) coupled with Gas Chromatography–Mass Spectrometry (GC-MS) has been developed for the determination of seven phthalate metabolites in human plasma for the first time. Plasma proteins were efficiently precipitated by adding of 0.2mg dry trichloroacetic acid to 10 mL plasma samples, incubation and centrifuging. For DLLME, a mixture of extraction solvent (chlorobenzene, 10 µL) and dispersive solvent (acetonitrile, 750 µL) were rapidly injected into 5.0 mL aqueous sample for the formation of cloudy solution, the analytes in the sample were extracted into the fine droplets of chlorobenzene. After extraction, phase separation was performed by centrifugation and the enriched analytes in the sedimented phase were subjected to GC-MS analysis. All important parameters affecting DLLME performance were investigated and optimised. Under the optimum extraction condition, the method yields a linear calibration curve for all target analytes in the concentration range from 5 to 5000 ng mL?1. The limits of detection and relative standard deviations for all phthalate metabolites were between 1.21–2.09 ng mL?1 and 4.8–6.8%, respectively. This is a very simple, rapid and reproducible method, which requires low volume of sample and toxic solvents.  相似文献   
999.
A novel eco‐friendly and effective electromembrane extraction method combining high‐performance liquid chromatography with UV detection was developed for the enrichment and determination of capecitabine. Tragacanth‐silver nanoparticles conjugated gel was prepared by dissolving the tragacanth powder in synthesized silver nanoparticles solution and was used as a green membrane in electromembrane extraction. The porosity and presence of silver nanoparticles in the gel were characterized by field emission scanning electron microscopy. This new electromembrane extraction approach uses neither organic solvent nor carrier agents to extract the target analyte. The best electromembrane extraction efficiency was obtained by using 4.0 mm membrane gel thickness containing 2.5% w/v of tragacanth gum, donor phase pH = 5.0, acceptor phase pH = 3.0, applied voltage 50 V, extraction time 20 min, and agitation rate 500 rpm. During method validation under the optimized conditions, good linearity dynamic range between 1 and 500 ng/mL with the coefficient of determination (R2) = 0.998 was obtained. Limit of detection and Limit of quantitation were estimated to be 0.84 and 1.0 ng/mL, respectively. Finally, the applicability of this method in real samples was confirmed by an acceptable performance in extraction and determination of capecitabine in human plasma samples.  相似文献   
1000.
Two simple and sensitive high performance liquid chromatographic (HPLC) methods have been developed for the simultaneous determination of three different quinolones: enrofloxacin, lomefloxacin and ofloxacin in their pure and dosage forms, one with reversed phase HPLC and the other with ion-pair HPLC. In reversed phase HPLC, method (A), the mobile phase consists of 2.18% aqueous solution of KH2PO4 with pH adjusted to 2.4 ± 0.2 with acetonitrile (80:20; v/v), the mobile phase pumped at flow rate of 1.2 ml min?1. A Neucleosil C18 column (10 μm, 100 Å), 250 mm length × 4.6 mm diameter was utilized as stationary phase. Detection was affected spectrophotometrically at 294 nm. While in ion-pair HPLC, method (B), the mobile phase was aqueous solution of 0.65% sodium perchlorate and 0.31% ammonium acetate adjusted to pH 2.2 ± 0.2 with orthophosphoric acid: acetonitrile (81:19; v/v), the mobile phase pumped at flow rate of 1.5 ml min?1. A μ bondapack C18 column (10 μm, 100 Å), 250 mm length × 4.6 mm diameter was utilized as stationary phase. Detection was affected spectrophotometrically at 294 nm. Linearity ranges for enrofloxacin, lomefloxacin and ofloxacin were 4.0–108, 7.0–112 and 8.0–113 μg ml?1, respectively using method A and 8.0–112, 7.0–112 and 5.0–105 μg ml?1, respectively applying method B. Minimum detection limits obtained were 0.013, 0.023 and 0.035 μg ml?1 for enrofloxacin, lomefloxacin and ofloxacin, respectively using method A, and 0.028, 0.023 and 0.011 μg ml?1 using method B. The proposed methods were further applied to the analysis of enrofloxacin in injection and tablets containing the ofloxacin and lomefloxacin drugs, and the results were satisfied.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号