全文获取类型
收费全文 | 3975篇 |
免费 | 202篇 |
国内免费 | 117篇 |
专业分类
化学 | 2253篇 |
晶体学 | 54篇 |
力学 | 166篇 |
综合类 | 6篇 |
数学 | 710篇 |
物理学 | 645篇 |
综合类 | 460篇 |
出版年
2024年 | 9篇 |
2023年 | 59篇 |
2022年 | 97篇 |
2021年 | 118篇 |
2020年 | 141篇 |
2019年 | 116篇 |
2018年 | 115篇 |
2017年 | 91篇 |
2016年 | 151篇 |
2015年 | 128篇 |
2014年 | 157篇 |
2013年 | 301篇 |
2012年 | 268篇 |
2011年 | 302篇 |
2010年 | 191篇 |
2009年 | 181篇 |
2008年 | 252篇 |
2007年 | 232篇 |
2006年 | 204篇 |
2005年 | 207篇 |
2004年 | 150篇 |
2003年 | 142篇 |
2002年 | 142篇 |
2001年 | 89篇 |
2000年 | 53篇 |
1999年 | 61篇 |
1998年 | 36篇 |
1997年 | 35篇 |
1996年 | 26篇 |
1995年 | 26篇 |
1994年 | 22篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1977年 | 4篇 |
1975年 | 4篇 |
1973年 | 4篇 |
1948年 | 3篇 |
排序方式: 共有4294条查询结果,搜索用时 15 毫秒
111.
López F Harutyunyan SR Minnaard AJ Feringa BL 《Journal of the American Chemical Society》2004,126(40):12784-12785
A highly enantioselective Cu-catalyzed addition of Grignard reagents to acyclic aliphatic enones is described. In the presence of 5 mol % of CuBr.SMe2 and 6 mol % of JosiPhos diphosphine aliphatic enones react with Grignard reagents to provide beta-substituted linear ketones with high yields, regio-, and enantioselectivities. 相似文献
112.
New hybrid organic-inorganic gels have been obtained by reaction of 1,4-butanediol, on tetramethoxysilane Si(OMe)4 dissolved in CCl4. This reaction does not require water and leads to the formation of polymeric transparent materials.Infrared, 29Si and 13C NMR spectroscopy shows that interchange reactions between OMe groups of alkoxide and -O-(CH2)4-O of 1,4-butanediol occurred, leading to the monolithic transparent gels in which both organic (Si-O-(CH2)4-O-Si) and inorganic (Si-O-Si) bridges are formed. 相似文献
113.
Wendt-Pienkowski E Huang Y Zhang J Li B Jiang H Kwon H Hutchinson CR Shen B 《Journal of the American Chemical Society》2005,127(47):16442-16452
Fredericamycin (FDM) A, a pentadecaketide featuring two sets of peri-hydroxy tricyclic aromatic moieties connected through a unique chiral spiro carbon center, exhibits potent cytotoxicity and has been studied as a new type of anticancer drug lead because of its novel molecular architecture. The fdm gene cluster was localized to 33-kb DNA segment of Streptomyces griseus ATCC 49344, and its involvement in FDM A biosynthesis was proven by gene inactivation, complementation, and heterologous expression experiments. The fdm cluster consists of 28 open reading frames (ORFs), encoding a type II polyketide synthase (PKS) and tailoring enzymes as well as several regulatory and resistance proteins. The FDM PKS features a KSalpha subunit with heretofore unseen tandem cysteines at its active site, a KSbeta subunit that is distinct phylogenetically from KSbeta of hexa-, octa-, or decaketide PKSs, and a dedicated phosphopantetheinyl transferase. Further study of the FDM PKS could provide new insight into how a type II PKS controls chain length in aromatic polyketide biosynthesis. The availability of the fdm genes, in vivo characterization of the fdm cluster in S. griseus, and heterologous expression of the fdm cluster in Streptomyces albus set the stage to investigate FDM A biosynthesis and engineer the FDM biosynthetic machinery for the production of novel FDM A analogues. 相似文献
114.
医疗垃圾属于一类危险废物,必须进行无害化处理。文章论述了医疗垃圾的特性和危害性,对重庆市三峡库区目前医疗垃圾管理和处理的现状进行了分析,针对其特点,提出对三峡库区医疗垃圾有效的管理措施和进行分区集中处理,具体分析了医疗垃圾处理技术方案:分类收集、密闭运输、预处理、焚烧、完全填埋等。通过无害化处理后,杜绝医疗垃圾对环境的污染,以保护三峡库区生态环境和水资源。 相似文献
115.
吸附制冷系统中吸附床的传热传质分析及结构设计 总被引:5,自引:1,他引:5
对化学吸附制冷系统中吸附剂颗粒之间以及整个吸附床的传热传质特性进行了分析,并总结了目前国内外强化吸附床传热传质性能的主要措施和方法.分析比较了两种用于化学吸附制冷的典型吸附床结构,在此基础上设计了一种新型的吸附床. 相似文献
116.
介绍了客户关系管理(CRM)的概念及内涵,通过对我国旅行社客户关系管理现状的分析,提出了我国旅行社实施CRM系统的两种基本策略,即自行建设策略和外包策略,并探讨了实施CRM时应处理好的几个问题。 相似文献
117.
118.
Jordi Badosa Josep Calbó Richard Mckenzie Ben Liley Josep‐Abel González Bruce Forgan Charles N. Long 《Photochemistry and photobiology》2014,90(4):941-951
Cloud effects on UV Index (UVI) and total solar radiation (TR) as a function of cloud cover and sunny conditions (from sky images) as well as of solar zenith angle (SZA) are assessed. These analyses are undertaken for a southern‐hemisphere mid‐latitude site where a 10‐years dataset is available. It is confirmed that clouds reduce TR more than UV, in particular for obscured Sun conditions, low cloud fraction (<60%) and large SZA (>60°). Similarly, local short‐time enhancement effects are stronger for TR than for UV, mainly for visible Sun conditions, large cloud fraction and large SZA. Two methods to estimate UVI are developed: (1) from sky imaging cloud cover and sunny conditions, and (2) from TR measurements. Both methods may be used in practical applications, although Method 2 shows overall the best performance, as TR allows considering cloud optical properties. The mean absolute (relative) differences of Method 2 estimations with respect to measured values are 0.17 UVI units (6.7%, for 1 min data) and 0.79 Standard Erythemal Dose (SED) units (3.9%, for daily integrations). Method 1 shows less accurate results but it is still suitable to estimate UVI: mean absolute differences are 0.37 UVI units (15%) and 1.6 SED (8.0%). 相似文献
119.
120.