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   220342篇
  免费   4692篇
  国内免费   2811篇
化学   117320篇
晶体学   3591篇
力学   11545篇
综合类   169篇
数学   23645篇
物理学   71575篇
  2021年   2601篇
  2020年   2753篇
  2019年   3214篇
  2018年   4239篇
  2017年   4185篇
  2016年   5621篇
  2015年   3153篇
  2014年   5287篇
  2013年   9784篇
  2012年   8424篇
  2011年   9668篇
  2010年   7545篇
  2009年   7653篇
  2008年   8797篇
  2007年   8612篇
  2006年   7724篇
  2005年   6773篇
  2004年   6309篇
  2003年   5727篇
  2002年   5712篇
  2001年   6062篇
  2000年   4522篇
  1999年   3574篇
  1998年   3009篇
  1997年   3049篇
  1996年   2878篇
  1995年   2495篇
  1994年   2596篇
  1993年   2424篇
  1992年   2706篇
  1991年   2795篇
  1990年   2684篇
  1989年   2731篇
  1988年   2638篇
  1987年   2617篇
  1986年   2470篇
  1985年   3075篇
  1984年   3131篇
  1983年   2622篇
  1982年   2575篇
  1981年   2545篇
  1980年   2342篇
  1979年   2710篇
  1978年   2671篇
  1977年   2870篇
  1976年   2872篇
  1975年   2664篇
  1974年   2602篇
  1973年   2711篇
  1972年   2085篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
A simple and efficient nitrile-directed meta-C−H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C−H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C−H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles. Notably, the remote meta-selectivity of biphenylnitriles could not be expected from previous results with a macrocyclophane nitrile template. DFT computational studies show that a ligand-containing Pd–Ag heterodimeric transition state (TS) favors the desired remote meta-selectivity. Control experiments demonstrate the directing effect of the nitrile group and exclude the possibility of non-directed meta-C−H activation. Substituted 2-pyridone ligands were found to be key in assisting the cleavage of the meta-C−H bond in the concerted metalation–deprotonation (CMD) process.  相似文献   
122.
123.
Physics of Particles and Nuclei Letters - The problem of spontaneous isotopic symmetry breaking in Nambu–Jona-Lasinio quark models is considered. It is shown that, in models with light...  相似文献   
124.
Gurbatov  S. N.  Pelinovsky  E. N. 《Doklady Physics》2020,65(8):269-272
Doklady Physics - In this paper, we study the statistical characteristics of the Riemann wave and the integral of it. Such problems arise in the physics of fronts’ propagation, in particular,...  相似文献   
125.
126.
127.
Eucommia ulmoides Oliv. (E. ulmoides) is a valuable and nourishing medicinal herb in China that has been used in the treatment of hypertension. Given the fact that most traditional Chinese medicine is mainly used to treat disease, investigating the pharmacokinetics of traditional Chinese medicines in the pathological state is more useful than that in the normal state. However, the differences in the absorption kinetics of active ingredients of E. ulmoides extract between pathological and physiological conditions have not been reported. Therefore, in this study, the rat intestinal in situ circulatory perfusion model was used to investigate the differences in absorption kinetics of seven active ingredients of E. ulmoides extract in normal and spontaneously hypertensive rats, namely, genipinic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, cryptochlorogenic acid, (+)-pinoresinol di-O-β-D -glucopyranoside and (+)-pinoresinol 4′-O-β-D -glucopyranoside. Our results indicate that the pathological state of spontaneous hypertension may change the absorption of active components of E. ulmoides extracts, and these findings may provide a reference for improving the rational use of E. ulmoides in the clinic.  相似文献   
128.
129.
A series of donor-acceptor copolymers with dicyclopenta[cd,jk]pyrene and dicyclopenta[cd,lm]perylene acceptor units was prepared via palladium catalyzed cyclopenta-annulation reactions. The acceptor units were paired with diethynyl containing donor groups based on benzo[1,2-b:4,5-b’]dithiophene, thieno[3,2-b]thiophene, and 4-octyl-4H-dithieno[3,2-b:2′,3′-d]pyrrole to create six polymer variants. The cyclopentannulation polymerization resulted in copolymers with molecular weights (Mn) of 6–14 kDa and broad light absorption in the visible region with band gaps of 1.38–1.85 eV. The synthetic methodology, as well as optoelectronic properties, including thin-film absorption and cyclic voltammetry, of the donor-acceptor copolymers are presented.  相似文献   
130.
Vlasov  V. V.  Rautian  N. A. 《Doklady Mathematics》2021,103(1):10-13
Doklady Mathematics - Integro-differential equations with unbounded operator coefficients in a Hilbert space are studied. The equations under consideration are abstract hyperbolic equations...  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