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   52091篇
  免费   10966篇
  国内免费   3808篇
化学   51577篇
晶体学   464篇
力学   1773篇
综合类   175篇
数学   3975篇
物理学   8901篇
  2025年   4篇
  2024年   137篇
  2023年   376篇
  2022年   595篇
  2021年   904篇
  2020年   2142篇
  2019年   3152篇
  2018年   1478篇
  2017年   1155篇
  2016年   4058篇
  2015年   4189篇
  2014年   4202篇
  2013年   5050篇
  2012年   4196篇
  2011年   3457篇
  2010年   3796篇
  2009年   3694篇
  2008年   3293篇
  2007年   2571篇
  2006年   2265篇
  2005年   2383篇
  2004年   2012篇
  2003年   1853篇
  2002年   2615篇
  2001年   1905篇
  2000年   1684篇
  1999年   790篇
  1998年   410篇
  1997年   342篇
  1996年   349篇
  1995年   330篇
  1994年   283篇
  1993年   234篇
  1992年   166篇
  1991年   185篇
  1990年   124篇
  1989年   100篇
  1988年   93篇
  1987年   60篇
  1986年   64篇
  1985年   43篇
  1984年   29篇
  1983年   31篇
  1982年   23篇
  1981年   18篇
  1980年   5篇
  1979年   4篇
  1963年   2篇
  1957年   4篇
  1936年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
L ‐Proline is found to be an efficient catalyst for the synthesis of tetrahydrofuro[3,4‐b]quinoline‐1,8(3H,4H)‐dione derivatives. This protocol is novel and has the advantages of mild condition, high yield, and easy operation. J. Heterocyclic Chem., (2012).  相似文献   
992.
Wang L  Cai W  Zhang X  Xu J 《Optics letters》2012,37(13):2730-2732
The properties of surface plasmons localized at the interface between graphene and Kerr-type nonlinear substrates are investigated analytically. Although the relative propagation distance remains the same, the dispersion of graphene plasmons may be affected much by the inevitable nonlinear effect of substrates. Specifically, the wavelength of graphene plasmons can be tuned by adjusting the nonlinear permittivity of substrates.  相似文献   
993.
超短超强激光与稀薄等离子体相互作用的数值研究   总被引:2,自引:0,他引:2  
陆全明  王水 《光学学报》1998,18(5):37-540
用一维粒子模拟方法(Particle-in-Cell)数值研究了超短超强激光(Iλ^2〉10^18W.μm^2/cm^2)与稀薄等离子体的相互作用过程,结果表明,超短超强激光与稀薄等相互作用后,在等离子体中激发起尾波和拉曼(Raman)波,它们的波长和频率的值与解析解符合得很好;同时在尾波的作用下,等离子体的部分电子被加速的很高的速度,甚至接近光速。  相似文献   
994.
This paper describes the quality of convergence to an infinitely divisible law relative to free multiplicative convolution. We show that convergence in distribution for products of identically distributed and infinitesimal free random variables implies superconvergence of their probability densities to the density of the limit law. Superconvergence to the marginal law of free multiplicative Brownian motion at a specified time is also studied. In the unitary case, the superconvergence to free Brownian motion and that to the Haar measure are shown to be uniform over the entire unit circle, implying further a free entropic limit theorem and a universality result for unitary free Lévy processes. Finally, the method of proofs on the positive half-line gives rise to a new multiplicative Boolean to free Bercovici–Pata bijection.  相似文献   
995.
光纤偏振态模式分布的干涉测量方法   总被引:1,自引:0,他引:1  
王涛  周柯江 《光学学报》1997,17(6):37-740
研究一种高精度单模光纤偏振态模式耦合干涉测量方法,运用短相干长度光源,克服了传统的强度型测量光纤及光纤无源器件方法的缺点,该系统不仅可使测量精度达到-80dB,且可以精确地测量耦合点位置,位置分辨率小于10mm,并给出对Lyot型光纤去偏器的测量结果。  相似文献   
996.
Phytochemical study of the ethanol extract of the seeds of Aesculus assamica led to the isolation of two new triterpenoid saponins. The structure of the new compounds were elucidated on the basis of spectral data to be 28-O-acetyl-21-O-(4-O-angeloyl)-6-deoxy-beta-glucopyranosyl-3-O-[beta-glucopyranosyl(1-2)-O-[beta-glucopyranosyl(1-4)]-beta-glucuronopyranosyl]protoaescigenin (1), and 21-O-(4-O-angeloyl)-6-deoxy-beta-glucopyranosyl-3-O-[beta-glucopyranosyl(1-2)-O-[beta-glucopyranosyl(1-4)]-beta-glucuronopyranosyl]protoaescigenin (2). Their in vitro bioactivity against plant pathogenic fungus Pyricularia oryzae and cytotoxicity against K562 and HCT-15 cell lines were evaluated.  相似文献   
997.
Two hydrophobic vinyl saccharide monomers based on D ‐glucose and D ‐fructose were polymerized by employing the reversible addition‐fragmentation transfer (RAFT) miniemulsion polymerization technique to prepare well‐designed glycopolymers. Three dithiobenzoate‐RAFT agents [S?C(Ph)S? R], 1‐phenylethyl dithiobenzoate (PED), 2‐phenylprop‐2‐yl dithiobenzoate (PPD), and 2‐cyanoprop‐2‐yl dithiobenzoate (CPD), were used to control the growth of polymer chains. The best results were obtained in the presence of the PPD‐RAFT agent and the formed polymers have polydispersity index's (PDI) lower than 1.15. Under adequate miniemulsion polymerization conditions, a glycopolymer with PDI of 1.1 and molecular weight of 5 × 104 g/mol has been successfully synthesized in a short reaction time of 100 min. Furthermore, some block copolymers containing saccharide segment with butyl or methyl methacrylate were prepared. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   
998.
A steep electric pulsed field with low intensity (150–250 V/cm) and relative long time (10 min) was applied to adherent liver cancer cell line SMMC-7721 and the liver cell line HL-7702. Results showed that the electric field with intensity of 200 and 250 V/cm could trigger cell apoptosis, whereas the SMMC-7721 cell was more sensitive to the electric stimulation than the HL-7702 cell. Laser Scanning Confocal Microscope (LSCM) was used to measuring the real-time change of cytosolic free Ca2+ concentration. When cells were exposed electric pulses with 100 V/cm intensity for 10 min, there was no significant change of intracellular calcium concentration. With the intensity increased to 200 and 250 V/cm, intracellular calcium concentration decreased significantly. Results demonstrated the relationship between the apoptosis and change of intracellular calcium concentration. And the steep electric pulsed field can be used to the cancer therapy.  相似文献   
999.
A highly effective Meldrum's acid catalyzed synthesis of bis(indol‐3‐yl)methanes has been developed. This reaction provides the corresponding bis(indol‐3‐yl)methanes in high yields after short reaction time in the presence of 2 mol % catalyst in water under ultrasonic condition.  相似文献   
1000.
Forty‐five years after the point de départ [Hohenberg and Kohn, Phys Rev, 1964, 136, B864] of density functional theory, its applications in chemistry and the study of electronic structures keep steadily growing. However, the precise form of the energy functional in terms of the electron density still eludes us—and possibly will do so forever [Schuch and Verstraete, Nat Phys, 2009, 5, 732]. In what follows we examine a formulation in the same spirit with phase space variables. The validity of Hohenberg–Kohn–Levy‐type theorems on phase space is recalled. We study the representability problem for reduced Wigner functions, and proceed to analyze properties of the new functional. Along the way, new results on states in the phase space formalism of quantum mechanics are established. Natural Wigner orbital theory is developed in depth, with the final aim of constructing accurate correlation‐exchange functionals on phase space. A new proof of the overbinding property of the Müller functional is given. This exact theory supplies its home at long last to that illustrious ancestor, the Thomas–Fermi model. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012  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