全文获取类型
收费全文 | 16704篇 |
免费 | 2868篇 |
国内免费 | 2447篇 |
专业分类
化学 | 12352篇 |
晶体学 | 299篇 |
力学 | 985篇 |
综合类 | 177篇 |
数学 | 1798篇 |
物理学 | 6408篇 |
出版年
2024年 | 73篇 |
2023年 | 329篇 |
2022年 | 614篇 |
2021年 | 620篇 |
2020年 | 634篇 |
2019年 | 704篇 |
2018年 | 576篇 |
2017年 | 563篇 |
2016年 | 756篇 |
2015年 | 790篇 |
2014年 | 1006篇 |
2013年 | 1329篇 |
2012年 | 1521篇 |
2011年 | 1670篇 |
2010年 | 1156篇 |
2009年 | 1130篇 |
2008年 | 1147篇 |
2007年 | 1104篇 |
2006年 | 911篇 |
2005年 | 805篇 |
2004年 | 655篇 |
2003年 | 509篇 |
2002年 | 483篇 |
2001年 | 428篇 |
2000年 | 390篇 |
1999年 | 296篇 |
1998年 | 231篇 |
1997年 | 186篇 |
1996年 | 193篇 |
1995年 | 158篇 |
1994年 | 171篇 |
1993年 | 126篇 |
1992年 | 126篇 |
1991年 | 105篇 |
1990年 | 99篇 |
1989年 | 86篇 |
1988年 | 52篇 |
1987年 | 46篇 |
1986年 | 47篇 |
1985年 | 43篇 |
1984年 | 30篇 |
1983年 | 19篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 14篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1957年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Tian-Yang Wu Juan Liang Jing-Ya Ai Jing-Long Cui Wei-Dong Huang Yi-Lin You Ji-Cheng Zhan 《Molecules (Basel, Switzerland)》2022,27(13)
Mulberry extract has been proven to have the effect of resisting alcohol damage, but its mechanism is still unclear. In this study, the composition of mulberry ethanol extract (MBE) was identified by LC-MS/MS and the main components of MBE were ascertained by measuring. Gastric mucosal epithelial (GES-1) cells were used to elucidate the mechanism of MBE and rutin (the central part of MBE) helped protect against alcohol damage. The results revealed that phenolics accounted for the majority of MBE, accounting for 308.6 mg/g gallic acid equivalents and 108 substances were identified, including 37 flavonoids and 50 non-flavonoids. The treatment of 400 μg/mL MBE and 320 μM rutin reduced early cell apoptosis and the content of intracellular reactive oxygen species, malondialdehyde and increased glutathione. The qPCR results indicated that the MBE inhibits the expression of genes in the mitogen-activated protein kinase (MAPK) pathway, including p38, JNK, ERK and caspase-3; rutin inhibits the expression of p38 and caspase-3. Overall, MBE was able to reduce the oxidative stress of GES-1 cells and regulated apoptosis-related genes of the MAPK pathway. This study provides information for developing anti-ethanol injury drugs or functional foods. 相似文献
62.
Heck reaction is one of the most important carbon-carbon bond forming reactions with wide applications in organic synthesis.Considerable advances of enantiosele... 相似文献
63.
Yanlin Feng Jiali Lin Gang He Li Liang Qijun Liu Jun Yan Qian Yao 《Molecules (Basel, Switzerland)》2022,27(15)
Pomegranate peel extract (PPE), which is abundant in polyphenols, holds immerse prospects for the treatment of airway infection. In this study, water and ethanol of 30%, 50%, and 80% were used to prepare PPE. A total of 18 phenols belonging to 8 categories of polyphenols were identified in PPE by HPLC-MS/MS. The PPE from the four extraction solvents possessed different antioxidant, antibacterial, and anti-inflammatory activities. Principal component analysis revealed that though total flavonoids (TFs), total polyphenols (TPs), and total tannins (TTs) were responsible for the reducing power of PPE, only TFs contributed to the effect of PPE in inhibiting lipid membrane peroxidation. TPs, TTs, and punicalagin were positively correlated with the antibacterial strength against S. aureus while TTs alone contributed to the inhibition of methicillin-resistant S. aureus, implying the crucial role of TT in suppressing bacteria. Meanwhile, TTs was associated with the prevention of IL-6 release. The PPE with higher contents of TPs, TTs, and punicalagin had a weaker capacity to decrease nitric oxide secretion. PPE of 30% ethanol gained the highest integrated score due to its stronger antioxidant, antibacterial, and anti-inflammatory activities. It is a suitable candidate for the therapy of respiratory tract infection. 相似文献
64.
奇偶对相干态的维格纳函数和层析图函数 总被引:5,自引:1,他引:4
利用纠缠态η〉表象下的维格纳算符,重构了奇偶对相干态的维格纳函数.根据维格纳函数在相空间中随变量ρ和γ的变化规律,讨论了奇偶对相干态的非经典性质和量子干涉效应.研究发现,奇偶对相干态总呈现非经典性质,并且当q取奇数时,奇偶对相干态更容易出现非经典性质.奇偶对相干态的量子干涉效应的显著程度与q取值有关,但对于q的同一取值,奇对相干态的量子干涉效应更为显著.利用纠缠态η〉表象下的维格纳算符Δ1,2(ρ,γ)和纠缠态η,τ1,τ2〉的投影算符之间满足的拉东变换,获得了奇偶对相干态的量子层析图函数. 相似文献
65.
Yang Ye Xiang Qi Bing Xu Ying Lin Huan Xiang Liang Zou Xiang-Yang Ye Tian Xie 《Chemical science》2022,13(23):6959
Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. This Ni-catalyzed modular approach displays excellent functional group tolerance and a broad substrate scope, which the creation of a series of 1,4-dienes including several structurally complex natural products and pharmaceutical motifs. Moreover, the coupling strategy has the potential to realize enantiomeric control. The practicality of this transformation is demonstrated through the potent modification of the naturally antitumor active molecule β-elemene.Herein, we present a facile and efficient allylation method via Ni-catalyzed cross-electrophile coupling of readily available allylic acetates with a variety of substituted alkenyl bromides using zinc as the terminal reductant. 相似文献
66.
Yuchao Feng Xia Fan Shu Zhang Miao Yu Tong Wu Ying Liang Changyuan Wang Hongzhi Yang 《Molecules (Basel, Switzerland)》2022,27(12)
An ultra-high-performance liquid chromatography coupled with high-field quadrupole-orbitrap mass spectrometry (UHPLC-QE-MS) histological platform was used to analyze the effects of two thermal processing methods (cooking and steaming) on the nutritional metabolic components of black beans. Black beans had the most amino acids, followed by lipids and polyphenols, and more sugars. Multivariate statistical analysis indicated that heat processing significantly affected the metabolic component content in black beans, with effects varying among different components. Polyphenols, especially flavonoids and isoflavones, were highly susceptible. A total of 197 and 210 differential metabolites were identified in both raw black beans and cooked and steamed black beans, respectively. Cooking reduced the cumulative content of amino acids, lipids, polyphenols, sugars, and nucleosides, whereas steaming reduced amino acid and lipid content, slightly increased polyphenol content, and significantly increased sugar and nucleoside content. Our results indicated that metabolic components were better retained during steaming than cooking. Heat treatment had the greatest impact on amino acids, followed by polyphenols, fatty acids, sugars, and vitamins, indicating that cooking promotes the transformation of most substances and the synthesis of a few. The results of this study provide a basis for further research and development of nutritional products using black beans. 相似文献
67.
对Weyl关于二阶奇异微分方程边值问题谱性质的一个定理进行了推广和改进,使得所讨论的方程形式更一般且结果更详细. 相似文献
68.
第4代太空防风的X射线荧光研究 总被引:8,自引:3,他引:5
利用我国发射的神舟三号飞船搭载防风种子,回收后在地面上筛选繁育,定向培育成新品种获得优良的种质资源。本文用X射线荧光光谱法(XRF)对枝叶和产量等方面占优势的第4代太空及地面组防风药用部分的元素含量进行测定和对比分析。两组样品的元素种类基本相同, 但多种矿质元素含量是太空组>地面组。尤其是太空防风中与性味功效和归经相关的Zn,Fe,Mn和Cu含量比地面组分别提高到4.39,1.23,0.84和0.90倍。太空防风矿质元素指标明显优化。XRF方法可以从整体上对中药材的元素进行测定分析和研究。利用太空搭载防风种子可以筛选出多种元素优化的防风药材新品种。 相似文献
69.
太空诱变桔梗的X射线荧光光谱的测定分析 总被引:6,自引:3,他引:3
选用X射线荧光光谱法(XRF),对我国独创的第4代太空诱变育种桔梗、地面组桔梗和桔梗对照品的元素种类和含量进行测定与分析。结果表明,太空组桔梗中与桔梗理气、化痰功效相关的Zn,Mn,Fe等元素分别比地面组提高了1.9,2.4,0.6倍,其中Zn和Mn元素比对照品分别提高了0.6和1.9倍。即与地面组相比,太空组桔梗更趋近于或优于对照品桔梗。太空组桔梗中多种元素指标得到了大幅度的提升和优化。XRF方法具有方便快捷、灵敏度高、元素和含量范围广以及再现性好等特点,可用于一切能制成粉末样品的其他中药的测定分析。 相似文献
70.
The energy loss of the multi-stage centrifugal pump was investigated by numerical analysis using the entropy generation method with the RNG k-ε turbulence model. Entropy generation due to time-averaged motion and velocity fluctuation was mainly considered. It was found that the entropy generation of guide vanes and impellers account for 71.2% and 23.3% of the total entropy generation under the designed flow condition. The guide vanes are the main hydraulic loss domains and their entropy generation is about 9 W/K, followed by impellers. There are vortices at the tongue of the guide vane inlet as well as flow separations in the impellers, which lead to entropy generation. The fluid impacts the outer surface of the guide vanes, resulting in the increase in entropy generation. There are refluxes near the guide vane tongues which also increase the entropy generation of this part. The entropy generation distribution of the guide vanes and impellers was investigated, which found that the positive guide vane has more entropy generation compared with the reverse guide. The entropy generation of the blade suction surface is higher compared with the pressure surface. This study indicated that the entropy generation method has distinct advantages in the assessment of hydraulic loss. 相似文献