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   17650篇
  免费   3110篇
  国内免费   2398篇
化学   7594篇
晶体学   162篇
力学   3607篇
综合类   175篇
数学   1594篇
物理学   10026篇
  2024年   11篇
  2023年   178篇
  2022年   493篇
  2021年   505篇
  2020年   694篇
  2019年   549篇
  2018年   523篇
  2017年   662篇
  2016年   703篇
  2015年   576篇
  2014年   978篇
  2013年   1420篇
  2012年   1092篇
  2011年   1241篇
  2010年   992篇
  2009年   1121篇
  2008年   1161篇
  2007年   1094篇
  2006年   1139篇
  2005年   922篇
  2004年   920篇
  2003年   806篇
  2002年   676篇
  2001年   615篇
  2000年   524篇
  1999年   411篇
  1998年   468篇
  1997年   357篇
  1996年   329篇
  1995年   283篇
  1994年   272篇
  1993年   234篇
  1992年   225篇
  1991年   175篇
  1990年   141篇
  1989年   128篇
  1988年   122篇
  1987年   72篇
  1986年   54篇
  1985年   42篇
  1984年   47篇
  1983年   20篇
  1982年   40篇
  1981年   23篇
  1980年   16篇
  1979年   18篇
  1978年   18篇
  1977年   14篇
  1973年   16篇
  1957年   16篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
针对考虑几何和材料非线性的石英晶体板厚度剪切振动和弯曲振动的方程组,利用扩展伽辽金法对该方程组进行转化和求解,分别获得了强烈耦合的厚度剪切振动模态和弯曲振动模态的频率响应关系,绘制了不同振幅比和不同驱动电压影响下的频率响应曲线图。数值计算结果表明可以选取石英晶片的最佳长厚比尺寸来避免两种模态的强烈耦合。驱动电压的变化将引起石英晶体谐振器厚度剪切振动频率的明显改变,必须将振动频率的漂移值控制在常用压电声波器件的允许值之内。扩展伽辽金法对石英晶体板非线性振动方程组的求解为非线性有限元分析和偏场效应分析奠定了基础。  相似文献   
2.
Pesticide residue in vegetables has been considered as a serious food safety problem across the whole world. This study investigates a novel advanced oxidation process (AOP), namely the coupled free chlorine/ultrasound (FC/US) process for the removal of three typical pesticides from lettuce. The removal efficiencies of dimethoate (DMT), trichlorfon (TCF) and carbofuran (CBF) from lettuce reached 86.7%, 79.8% and 71.3%, respectively by the FC/US process. There existed a synergistic effect in the coupled FC/US process for pesticide removal and the synergistic factors reached 22.3%, 19.0% and 36.4% for DMT, TCF and CBF, respectively. Based on the analysis of mass balance of pesticides, the synergistic effect was probably attributed to the efficient oxidation of pesticides both in vegetables and in water by the generated free radicals and FC. The surface area and surface structure of vegetables strongly affected the removal of pesticides by FC/US. The removal efficiency of DMT increased from 80.9% to 88.1% as solution pH increased from 5.0 to 8.0, and then decreased to 84.1% when solution pH further increased to 9.0. When the ultrasonic frequency changed from 20 to 40 kHz, a remarkable improvement in pesticide removal by FC/US was observed. As the FC concentration increased from 0 to 15 mg L–l, the removal efficiencies of pesticides increased firstly, and then became stagnant when the FC concentration further increased to 25 mg L–l. The pesticide degradation pathways based on the identified intermediates were proposed. The total chlorophyll content was reduced by less than 5% after the FC/US process, indicating a negligible damage to the quality of vegetables. It suggests that the FC/US process is a promising AOP for pesticides removal from vegetables.  相似文献   
3.
Ultrasound can accelerate and change the reaction process and is widely used in the field of hydrogen production and storage. In this study, ultrasound (US) and AlOOH suspension (AH) are used to promote hydrogen production from Al hydrolysis. The results indicate that both US and AH greatly shorten the induction time and enhance the hydrogen production rate and yield. The promoting effect of US and AH on Al hydrolysis originates from the acoustic cavitation effect and catalytic effect, respectively. When AH is used in combination with US, Al hydrolysis has the best hydrogen production performance and the hydrogen yield can reach 96.6 % within 1.2 h, because there is a synergistic effect on Al hydrolysis between AH and US. Mechanism analyses reveal that the micro-jets and local high temperature environment arising from acoustic cavitation improve the catalytic activity of AlOOH, while the suspended AlOOH particles enhance the cavitation effect of US. This work provides a novel and feasible method to promote hydrogen production from Al hydrolysis.  相似文献   
4.
本文采用有限体积法对激光熔覆过程的温度场分布和熔体流动进行了数值模拟。基于CALPHAD相图法计算了基体和粉末的热物理性质,采用三维热源精确预测了凝固过程和温度分布,研究了Marangoni对流对熔池尺寸的影响。在熔池凝固过程中模拟所得出的温度梯度和凝固速度,预测了熔覆层凝固组织的演变趋势,相应的显微组织与实验结果吻合较好。  相似文献   
5.
We discuss a new gravitational effect that the wave packet of a free-fall quantum particle undergoes a spin-dependent transverse shift in Earth's gravitational field. This effect is similar to the geometric spin Hall effect (GSHE) (Aiello 2009 et al Phys. Rev. Lett. 103 100401 ), and can be called gravity-induced GSHE. This effect suggests that the free-fall wave packets of opposite spin-polarized quantum particles can be split in the direction perpendicular to spin and gravity.  相似文献   
6.
Lijun Ni 《中国物理 B》2022,31(12):128504-128504
We report the temperature dependence of the spin pumping effect for Y3Fe5O12 (YIG, 0.9 μm)/NiO (tNiO)/W (6 nm) (tNiO = 0 nm, 1 nm, 2 nm, and 10 nm) heterostructures. All samples exhibit a strong temperature-dependent inverse spin Hall effect (ISHE) signal Ic and sensitivity to the NiO layer thickness. We observe a dramatic decrease of Ic with inserting thin NiO layer between YIG and W layers indicating that the inserting of NiO layer significantly suppresses the spin transport from YIG to W. In contrast to the noticeable enhancement in YIG/NiO (tNiO ≈ 1-2 nm)/Pt, the suppression of spin transport may be closely related to the specific interface-dependent spin scattering, spin memory loss, and spin conductance at the NiO/W interface. Besides, the Ic of YIG/NiO/W exhibits a maximum near the TN of the AF NiO layer because the spins are transported dominantly by incoherent thermal magnons.  相似文献   
7.
Kelvin−Helmholtz (KH) instability is a fundamental fluid instability that widely exists in nature and engineering. To better understand the dynamic process of the KH instability, the influence of the tangential velocity on the compressible KH instability is investigated by using the discrete Boltzmann method based on the nonequilibrium statistical physics. Both hydrodynamic and thermodynamic nonequilibrium (TNE) effects are probed and analyzed. It is found that, on the whole, the global density gradients, the TNE strength and area firstly increase and decrease afterwards. Both the global density gradient and heat flux intensity in the vertical direction are almost constant in the initial stage before a vortex forms. Moreover, with the increase of the tangential velocity, the KH instability evolves faster, hence the global density gradients, the TNE strength and area increase in the initial stage and achieve their peak earlier, and their maxima are higher for a larger tangential velocity. Physically, there are several competitive mechanisms in the evolution of the KH instability. (i) The physical gradients increase and the TNE effects are strengthened as the interface is elongated. The local physical gradients decrease and the local TNE intensity is weakened on account of the dissipation and/or diffusion. (ii) The global heat flux intensity is promoted when the physical gradients increase. As the contact area expands, the heat exchange is enhanced and the global heat flux intensity increases. (iii) The global TNE intensity reduces with the decreasing of physical gradients and increase with the increasing of TNE area. (iv) The nonequilibrium area increases as the fluid interface is elongated and is widened because of the dissipation and/or diffusion.  相似文献   
8.
This paper presents a nonlinear thickness-shear vibration model for onedimensional infinite piezoelectric plate with flexoelectricity and geometric nonlinearity. The constitutive equations with flexoelectricity and governing equations are derived from the Gibbs energy density function and variational principle. The displacement adopted here is assumed to be antisymmetric through the thickness due to the thickness-shear vibration mode. Only the shear strain gradient through the thickness is considered in the present model. With geometric nonlinearity, the governing equations are converted into differential equations as the function of time by the Galerkin method. The method of multiple scales is employed to obtain the solution to the nonlinear governing equation with first order approximation. Numerical results show that the nonlinear thickness-shear vibration of piezoelectric plate is size dependent, and the flexoelectric effect has significant influence on the nonlinear thickness-shear vibration frequencies of micro-size thin plates. The geometric nonlinearity also affects the thickness-shear vibration frequencies greatly. The results show that flexoelectricity and geometric nonlinearity cannot be ignored in design of accurate high-frequency piezoelectric devices.  相似文献   
9.
Jing-Peng Song 《中国物理 B》2022,31(3):37401-037401
Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.  相似文献   
10.
Bin-Hao Du 《中国物理 B》2022,31(7):77201-077201
We study theoretically Josephson effect in a planar ballistic junction between two triplet superconductors with p-wave orbital symmetries and separated by a two-dimensional (2D) semiconductor channel with strong Rashba spin-orbit coupling. In triplet superconductors, three types of orbital symmetries are considered. We use Bogoliubov-de Gennes formalism to describe quasiparticle propagations through the junction and the supercurrents are calculated in terms of Andreev reflection coefficients. The features of the variation of the supercurrents with the change of the strength of Rashba spin-orbit coupling are investigated in some detail. It is found that for the three types of orbital symmetries considered, both the magnitudes of supercurrent and the current-phase relations can be manipulated effectively by tuning the strength of Rashba spin-orbit coupling. The interplay of Rashba spin-orbit coupling and Zeeman magnetic field on supercurrent is also investigated in some detail.  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