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   71626篇
  免费   10791篇
  国内免费   1540篇
化学   55823篇
晶体学   574篇
力学   1146篇
数学   4260篇
物理学   8314篇
无线电   13840篇
  2024年   50篇
  2023年   369篇
  2022年   467篇
  2021年   992篇
  2020年   2263篇
  2019年   3638篇
  2018年   1921篇
  2017年   1470篇
  2016年   4694篇
  2015年   4608篇
  2014年   4921篇
  2013年   6163篇
  2012年   5198篇
  2011年   4652篇
  2010年   4501篇
  2009年   4530篇
  2008年   4640篇
  2007年   3745篇
  2006年   3426篇
  2005年   3365篇
  2004年   2966篇
  2003年   2661篇
  2002年   3178篇
  2001年   2159篇
  2000年   2018篇
  1999年   941篇
  1998年   483篇
  1997年   454篇
  1996年   439篇
  1995年   302篇
  1994年   287篇
  1993年   296篇
  1992年   259篇
  1991年   202篇
  1990年   200篇
  1989年   163篇
  1988年   118篇
  1987年   106篇
  1986年   72篇
  1985年   117篇
  1984年   83篇
  1983年   72篇
  1982年   88篇
  1981年   79篇
  1980年   52篇
  1979年   61篇
  1978年   64篇
  1977年   55篇
  1976年   53篇
  1975年   49篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Magnolia flower buds are a source of herbal medicines with various active compounds. In this study, differences in the distribution and abundance of major essential oils, phenolic acids, and primary metabolites between white flower buds of Magnolia heptapeta and violet flower buds of Magnolia denudata var. purpurascens were characterised. A multivariate analysis revealed clear separation between the white and violet flower buds with respect to primary and secondary metabolites closely related to metabolic systems. White flower buds contained large amounts of monoterpene hydrocarbons (MH), phenolic acids, aromatic amino acids, and monosaccharides, related to the production of isoprenes, as MH precursors, and the activity of MH synthase. However, concentrations of β-myrcene, a major MH compound, were higher in violet flower buds than in white flower buds, possibly due to higher threonine levels and low acidic conditions induced by comparatively low levels of some organic acids. Moreover, levels of stress-related metabolites, such as oxygenated monoterpenes, proline, and glutamic acid, were higher in violet flower buds than in white flower buds. Our results support the feasibility of metabolic profiling for the identification of phytochemical differences and improve our understanding of the correlated biological pathways for primary and secondary metabolites.  相似文献   
992.
Polypyrrole, formed in an organic solvent, shows higher density and better quality in electrical conductivity and mechanical strength than that formed in water. On electrochemical reduction in an aqueous system the laurylsulfate anion, used as dopant, sticks to the polymer. During reduction a cation moves into the polymer and on oxidation it leaves the polymer. The anion, however, leaves the polymer on reduction in an organic solvent system and cannot penetrate into the polymer on re-oxidation. The redox reaction in the aqueous system reveals a surface reaction process, while the reaction in the organic solvent system undergoes a diffusion-controlled process. This penomenon is consistant with the results of the ion concentration profile.  相似文献   
993.
Background: Ponciri Fructus, a crude drug consisting of the dried immature fruits of Poncirus trifoliata (L.) Raf., is a popular folk medicine used for the treatment of allergy and gastrointestinal disorders in Korea and China. In this study, the anti-adipogenic activity of extracts and isolated compounds were evaluated using 3T3-L1 preadipocytes. Methods: Dried immature fruits were extracted and fractionated into n-hexane, ethyl acetate (EtOAc), n-butanol and water-soluble fractions. The ethanol extract and fractions were tested for anti-adipogenic activity in the 3T3-L1 cell line. The active fractions (n-hexane and EtOAc fractions) were further subjected to chromatographic techniques to isolate and identify active compounds. Furthermore, the isolated compounds were evaluated for their anti-adipogenic activity. Results: Altogether, seven compounds, including two flavonoids, one phytosteroid and four coumarin derivatives, were isolated. Ethanol extract, n-hexane fraction, EtOAc fraction and three isolated compounds (phellopterin, oxypeucedanin and poncirin) showed significant anti-adipogenic activity as observed by reduced lipid deposition in differentiated 3T3-L1 cells. Further, oxypeucedanin downregulated the key adipogenic markers, such as peroxisome proliferator-activated receptors proteins γ (PPAR-γ), sterol response element binding proteins-1 (SREBP-1), CCAAT/enhancer binding proteins-α (C/EBP-α), adipocyte-specific lipid binding proteins (FABP-4), adipocyte fatty acid binding proteins (aP2), lipoprotein lipase (LPL) and leptin. Conclusion: This study indicated that the ethanol extract, hexane fraction and ethyl acetate fraction of P. trifoliata fruits possess strong anti-adipogenic activity, containing the active compounds such as phellopterin, oxypeucedanin and poncirin. Further research is recommended to explore their efficacy and safety in animal and clinical models.  相似文献   
994.
This paper presents design and simulation of a switchable radiative cooler that exploits phase transition in vanadium di-oxide to turn on and off in response to...  相似文献   
995.
Transport in Porous Media - We investigate viscous fluid flows and concurrent fluid-driven deformations in porous media. The hydro-mechanically (H-M) coupled pore-network model (PNM) is developed,...  相似文献   
996.
Applied Magnetic Resonance - Total cerebral volume increases very rapidly in childhood, peaking in early teenage years then declining in adolescence. However, most studies quantified only one or...  相似文献   
997.
Multimodal approaches combined with various nanomaterials and advanced techniques have been developed for synergistic cancer treatment. Among various therapies, conventional chemotherapy (CHT) is a direct cancer treatment that can produce unintended side effects due to nonspecific action on both the tumor and normal cells; patient-friendly photothermal therapy (PTT) may be able to treat embedded tumors in vital regions with minimal invasion but does not guarantee complete removal of cancers. However, the combination of CHT-PTT may provide a promising tool for direct cancer treatment with minimal side effects. In this regard, nanostructured materials, such as gold nanorods with tuned size and surface characteristics, are key components designed to enhance the heating capacity and active or passive delivery of drugs to the tumor site. In this review, the pioneering work synergizing CHT and PTT is summarized, and the current state-of-the-art in the development of inorganic and organic nanocomposites for combinational therapy is described.  相似文献   
998.
Pham  Y.  Yu  Y. K.  Manh  T. V.  Gamzatov  A.  Tartakovsky  D. M.  Yu  S.-C.  Yang  D.-S.  Kim  D.-H. 《Physics of the Solid State》2020,62(5):845-850
Physics of the Solid State - Structural and magnetic properties of Sr-doped Pr0.7Ba0.3MnO3 polycrystallines prepared by solid state reaction are presented. Samples were in monoclinic structure...  相似文献   
999.
Journal of Visualization - Airfoils are mostly inefficient in their off-design conditions. In order to improve the aerodynamic performance of airfoils in these conditions, using an optimized cavity...  相似文献   
1000.
In the last decade,the functionally graded carbon nanotube reinforced composites(FG-CNTRCs)have attracted considerable interest due to their excellent mechanical properties,and the structures made of FG-CNTRCs have found broad potential applications in aerospace,civil and ocean engineering,automotive industry,and smart structures.Here we review the literature regarding the mechanical analysis of bulk CNTR nanocomposites and FG-CNTRC structures,aiming to provide a clear picture of the mechanical modeling and properties of FG-CNTRCs as well as their composite structures.The review is organized as follows:(1)a brief introduction to the functionally graded materials(FGM),CNTRCs and FG-CNTRCs;(2)a literature review of the mechanical modeling methodologies and properties of bulk CNTRCs;(3)a detailed discussion on the mechanical behaviors of FG-CNTRCs;and(4)conclusions together with a suggestion of future research trends.  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