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   34893篇
  免费   5911篇
  国内免费   5156篇
化学   25935篇
晶体学   649篇
力学   1891篇
综合类   431篇
数学   3969篇
物理学   13085篇
  2024年   97篇
  2023年   613篇
  2022年   1031篇
  2021年   1165篇
  2020年   1303篇
  2019年   1417篇
  2018年   1120篇
  2017年   1058篇
  2016年   1584篇
  2015年   1650篇
  2014年   1958篇
  2013年   2559篇
  2012年   3203篇
  2011年   3103篇
  2010年   2421篇
  2009年   2243篇
  2008年   2532篇
  2007年   2243篇
  2006年   2195篇
  2005年   1980篇
  2004年   1546篇
  2003年   1259篇
  2002年   1340篇
  2001年   1085篇
  2000年   839篇
  1999年   715篇
  1998年   527篇
  1997年   434篇
  1996年   432篇
  1995年   366篇
  1994年   363篇
  1993年   292篇
  1992年   212篇
  1991年   223篇
  1990年   149篇
  1989年   137篇
  1988年   110篇
  1987年   63篇
  1986年   62篇
  1985年   78篇
  1984年   63篇
  1983年   38篇
  1982年   33篇
  1981年   29篇
  1980年   23篇
  1979年   14篇
  1978年   10篇
  1977年   10篇
  1957年   8篇
  1936年   4篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
The construction of maximally entangled bases for the bipartite system \(\mathbb {C}^{d}\otimes \mathbb {C}^{d}\) is discussed firstly, and some mutually unbiased bases with maximally entangled bases are given, where 2≤d≤5. Moreover, we study a systematic way of constructing mutually unbiased maximally entangled bases for the bipartite system \(\mathbb {C}^{d}\otimes \mathbb {C}^{d^{k}}\).  相似文献   
92.
李骏  张陈陈 《高分子学报》2021,53(2):139-150
社会学家提出并论证了家庭教养方式存在阶级或阶层差异的理论命题,定量研究虽然不断对此进行跨学科检验,但过多测量的是具体的教养行为而非整体的教养方式。这个问题在国内相关研究中被延续了下来,甚至带来结论性的争议。转向测量类型化的教养方式,使用职业、教育和综合地位作为阶层的三重指标,可以重新检验中国城市社会阶层之间是否存在教养方式的显著区别。经过对上海和全国数据中不同测量方法的交叉验证,得出如下结论:教养方式确实存在阶层之间的差异,经济社会地位越高的家庭越可能采取权威型或民主型教养方式,而非专制型或放任型教养方式。  相似文献   
93.
Main observation and conclusion Phytochemical investigations on Tabernaemontana divaricata led to the isolation of seven undescribed monoterpenoid indole alka-l...  相似文献   
94.
Excitable media,such as cells,can be polarized and magnetized in the presence of an external electromagnetic field.In fact,distinct geometric deformation can be induced by the external electromagnetic field,and also the capacitance of the membrane of cell can be changed to pump the field energy.Furthermore,the distribution of ion concentration inside and outside the cell can also be greatly adjusted.Based on the theory of bio-electromagnetism,the distribution of field energy and intracellular and extracellular ion concentrations in a single shell cell can be estimated in the case with or without external electric field.Also,the dependence of shape of cell on the applied electronic field is calculated.From the viewpoint of physics,the involvement of external electric field will change the gradient distribution of field energy blocked by the membrane.And the intracellular and extracellular ion concentration show a certain difference in generating timevarying membrane potential in the presence of electric field.When a constant electric field is applied to the cell,distinct geometric deformation is induced,and the cell triggers a transition from prolate to spherical and then to oblate ellipsoid shape.It is found that the critical frequency in the applied electric field for triggering the distinct transition from prolate to oblate ellipsoid shape obtains smaller value when larger dielectric constant of the cell membrane and intracellular medium,and smaller conductivity for the intracellular medium are used.Furthermore,the effect of cell deformation is estimated by analyzing the capacitance per unit area,the density of field energy,and the change of ion concentration on one side of cell membrane.The intensity of external applied electric field is further increased to detect the change of ion concentration.And the biophysical effect in the cell is discussed.So the deformation effect of cells in electric field should be considered when regulating and preventing harm to normal neural activities occurs in a nervous system.  相似文献   
95.
Many authors have suggested new forms to describe the surface anchoring energy of the liquid crystal-wall interface, replacing the Rapini-Papoular (RP) formula g s = (1/2) A sin2 theta. If the RP function is considered as the primary approximation, and a lowest order modification is included, then the surface anchoring energy can be represented by g s = (1/2) A sin2 theta(1 + zeta sin2 theta). zeta characterizes the modification to the RP formula and varies for the different energy forms. It is well known that the RP formula predicts a second order Freedericksz transition. This paper points out that the transition can be first order if the modification is taken into account, in which case at the threshold point the tilt angle of the director at the middle layer of the cell, thetam, is finite. The conditions for the existence of the first order transition are obtained; zeta < 0 is required for a first order transition. The approximate expression of the threshold field is also given.  相似文献   
96.
The phase composition and properties of microemulsion system of Titron N-101 / n-pentanol/ n-octane/ water (ammonia solution) were studied. CeO2 nanopaticles were prepared by the reaction of cerous nitrate.  相似文献   
97.
98.
Two new thermally crosslinkable hole‐transporting polymers, X‐PTPA and X‐PCz, were synthesized via Yamamoto coupling reactions. The number‐averaged molecular weights (Mn) of X‐PTPA and X‐PCz were found to be 45,000 and 48,000, respectively, and therewith, polydispersity indices were of 1.8 and 1.7, respectively. Thermally crosslinked X‐PTPA and X‐PCz exhibit excellent solvent resistance and stable optoelectronic properties. The UV–visible maximum absorption peaks of X‐PTPA and X‐PCz in the thin film state are at 389 and 322 nm, respectively. The HOMO levels of X‐PTPA and X‐PCz are estimated to be ?5.27 and ?5.39 eV, respectively. Multilayered devices (ITO/crosslinked X‐PTPA or X‐PCz/SY‐PPV/LiF/Al) were fabricated with SY (SuperYellow) as the emitting layer. The maximum efficiency of the multilayered device with a crosslinked X‐PTPA layer is approximately three times higher than that of the device without a crosslinked X‐PTPA layer and much higher than that of the crosslinked X‐PCz device. This result can be explained by the observations that crosslinked X‐PTPA produces increased electron accumulation within the emitter, SY, and also efficient exciton formation due to improved charge balance. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 5111–5117  相似文献   
99.
A flexible electrochromic active matrix addressed display, including 8 × 8 pixels, is demonstrated by using solution processing based on standard printing and coating manufacturing techniques. Each organic electrochromic display (OECD) pixel and its corresponding organic electrochemical transistor (OECT) are located on different sides of the flexible PET substrate. Electronic vias generated through the plastic substrate connects each OECD pixel with one addressing OECT. When comparing this display with actively addressed OECDs with all its components located on the same side, the present approach based on this electronic via substrate provides an enhanced pixel resolution and a relatively more simplified manufacturing process. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012  相似文献   
100.
DABCO-catalyzed dimerization of Morita–Baylis–Hillman acetates to synthesize a series of 3-alkyl-4-(E)-alkenyl-cyclohex-1-ene-1,4-dicarbonyl compounds in excellent yields with modest to excellent diastereoselectivity is reported. A plausible reaction mechanism is also proposed on the basis of previous literature and preliminary investigation the asymmetric version of the reaction.  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