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   25870篇
  免费   4602篇
  国内免费   3117篇
化学   18770篇
晶体学   333篇
力学   1535篇
综合类   201篇
数学   2620篇
物理学   10130篇
  2024年   19篇
  2023年   520篇
  2022年   620篇
  2021年   911篇
  2020年   1087篇
  2019年   1094篇
  2018年   941篇
  2017年   824篇
  2016年   1317篇
  2015年   1251篇
  2014年   1485篇
  2013年   1954篇
  2012年   2369篇
  2011年   2337篇
  2010年   1703篇
  2009年   1636篇
  2008年   1744篇
  2007年   1563篇
  2006年   1485篇
  2005年   1298篇
  2004年   974篇
  2003年   819篇
  2002年   867篇
  2001年   620篇
  2000年   500篇
  1999年   503篇
  1998年   420篇
  1997年   407篇
  1996年   365篇
  1995年   319篇
  1994年   266篇
  1993年   205篇
  1992年   190篇
  1991年   181篇
  1990年   142篇
  1989年   121篇
  1988年   90篇
  1987年   59篇
  1986年   77篇
  1985年   62篇
  1984年   38篇
  1983年   31篇
  1982年   29篇
  1981年   23篇
  1980年   18篇
  1975年   13篇
  1974年   8篇
  1973年   9篇
  1972年   9篇
  1965年   7篇
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
951.
In this work, a novel scheme is proposed to realize joint remote preparation regarding a class of four-qubit cluster-like entangled states based on optimal positive operator-valued measurements. To obtain the preparation of desired states, two tripartite non-maximally entangled states are taken as quantum channels, and devoted to building the robust connections amongst the states’ senders and receiver. It turns out that the present scheme enables one to achieve joint remote state preparation with certain success probability (SP) and unit fidelity. Moreover, several nontrivial issues are discussed, including the improvement of SP and the correlation between SP and the employed channels.  相似文献   
952.
953.
The new resonances X(3872) and Λ c (2940) are considered as weakly bound states of other hadrons in a molecule scenario using phenomenological Lagrangians.  相似文献   
954.
In this paper, a tri-neuron BAM neural network model with multiple delays is considered. We show that the connection topology of the network plays a fundamental role in classifying the rich dynamics and bifurcation phenomena. There is a wide range of different dynamical behaviors which can be produced by varying the coupling strength. By choosing the connected weights c 21 and c 31 (the connection weights through the neurons from J-layer to I-layer) as bifurcation parameters, the critical values where a Bogdanov–Takens bifurcation occurs are derived. Then, by computing the normal forms for the system, the bifurcation diagrams are obtained. Furthermore, some interesting phenomena, such as saddle-node bifurcation, pitchfork bifurcation, homoclinic bifurcation, heteroclinic bifurcation and double limit cycle bifurcation are found by choosing the different connection strengths. Some numerical simulations are given to support the analytic results.  相似文献   
955.
Poly(ether carbonate)s (PPCs) with carbonate unit (CU) content ranging from 57.8 to 97.1% and number average molecular weight (Mn) around 100 kg/mol were conveniently prepared via copolymerization of CO2 and propylene oxide under combinatorial catalyst of rare earth ternary (RET) complex and double metal cyanide (DMC) complex. Enhancement of catalytic activity and reduction of propylene carbonate byproduct were realized due to synergetic effect of the two metal catalysts, where DMC can be activated in the presence of RET. Solubility fractionation confirmed that the obtained PPCs were copolymers, not physical blends of each polymer. Thermal performances of the PPCs were closely related to their CU content, their glass transition temperatures (Tg) were tunable in the range of 6.7–36.3 °C, which decreased with decreasing CU content, while their thermal stabilities were enhanced significantly, an increase of 50.5 °C in 50% weight loss temperature was observed when CU content decreased from 97.1 to 57.8%. Both shear storage modulus and complex viscosity increased with increasing CU content, which became more obvious at lower frequency, featuring well with the CU content in the PPCs. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012  相似文献   
956.
The fabrication of three‐dimensional (3D) electrospun composite scaffolds was presented in this study. Layers of electrospun meshes made from composites of poly(lactide‐co‐glycolide acid) (PLGA) and hydroxyapatite (HA) were stacked and sintered using pressurized gas. Three HA concentrations of 5, 10, and 20 wt % were tested, and the addition of the HA nanoparticles decreased the tensile mechanical properties of the meshes with 20 wt % HA. However, after the gas absorption process, the fibers within the mesh sintered, which improved the mechanical properties more than twofold. The fabrication of 3D, porous, electrospun scaffolds was also demonstrated. The resulting 3D scaffolds had open porosity of up to 70% and modulus of ~20 MPa. This technique improves on the current electrospinning technology by overcoming the challenges of depositing a thick, 3D structure. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011  相似文献   
957.
The inclusion complexes of uniconazole [(E)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-lyl)-1-penten-3-ol, UCZ] with two cyclodextrin derivatives, hydroxypropyl-β-cyclodextrin (HP-β-CD) and methylated-β-cyclodextrin (Me-β-CD), were prepared and characterized by 1H NMR and FT-IR. The phase solubility of UCZ and HP-β-CD, UCZ and Me-β-CD, which displays the ability of CDs complexation and solubilization, was studied in aqueous solutions and phosphate buffer solutions (PBS) with different property pH values (6.2, 7.2, 8.0). The solubility results indicated that the pH of PBS showed more enhancement on the interaction of HP-β-CD and UCZ than Me-β-CD with the increasing pH value, and the optimal pH value for complexation of UCZ and HP-β-CD, UCZ and Me-β-CD was at 8.0 and at 7.2, respectively. These were also determined by UCZ release behavior and dissolution studies of the complexes in solid state.  相似文献   
958.
959.
Ni@CeO2 core–shell catalysts were synthesized via a facile surfactant-assisted hydrothermal method and their catalytic performance in the dry reforming of methane (DRM) reaction was evaluated. A variety of techniques including XRD, N2 adsorption–desorption, SEM, TEM, TPO, TGA were employed to characterize the prepared or spent catalysts. The encapsulation by the CeO2 shell, on one side, can restrict the sintering and growth of Ni nanoparticles under harsh reaction conditions. On the other side, compared to the conventional shell material of SiO2, CeO2 can provide more lattice oxygens and vacancies, which is helpful to suppress coke deposition. Consequently, the Ni@CeO2 core–shell catalysts exhibited better catalytic activity and stability in the DRM reaction with respect to the referenced Ni@SiO2 core–shell catalysts and Ni/CeO2 supported catalysts.  相似文献   
960.
Abstract

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d 1-4′-heptylaniline and 4-n-pentyloxybenzylidene-4′-heptylaniline-2,3,5,6-d 4. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J 1 (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.  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