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   1502854篇
  免费   32093篇
  国内免费   8233篇
化学   652789篇
晶体学   19993篇
力学   74015篇
综合类   104篇
数学   238804篇
物理学   362593篇
无线电   194882篇
  2021年   14703篇
  2020年   17333篇
  2019年   17491篇
  2016年   29162篇
  2015年   21577篇
  2014年   32734篇
  2013年   78494篇
  2012年   39081篇
  2011年   34629篇
  2010年   38460篇
  2009年   43573篇
  2008年   38397篇
  2007年   34956篇
  2006年   43334篇
  2005年   34440篇
  2004年   36446篇
  2003年   34477篇
  2002年   35728篇
  2001年   34784篇
  2000年   31140篇
  1999年   28724篇
  1998年   27343篇
  1997年   27223篇
  1996年   26800篇
  1995年   24604篇
  1994年   24061篇
  1993年   23435篇
  1992年   23083篇
  1991年   23299篇
  1990年   22027篇
  1989年   21702篇
  1988年   20834篇
  1987年   19609篇
  1986年   18446篇
  1985年   24947篇
  1984年   26165篇
  1983年   22162篇
  1982年   23606篇
  1981年   22815篇
  1980年   22056篇
  1979年   21876篇
  1978年   23216篇
  1977年   22754篇
  1976年   22349篇
  1975年   21040篇
  1974年   20636篇
  1973年   21196篇
  1972年   15369篇
  1968年   13044篇
  1967年   13146篇
排序方式: 共有10000条查询结果,搜索用时 307 毫秒
161.
Designs, Codes and Cryptography - Given an integer $$k\ge 3$$ and a group G of odd order,  if there exists a 2-(v, k, 1)-design and if v is sufficiently large then there...  相似文献   
162.
Journal of Analytical Chemistry - Silver nanoparticles modified with Eu3+ ions increase the intensity of the fluorescence signal by a factor of 125 in an analytical system containing doxycycline as...  相似文献   
163.
Computational Optimization and Applications - We consider a continuous-time optimization method based on a dynamical system, where a massive particle starting at rest moves in the conservative...  相似文献   
164.
The health monitoring has been studied to ensure integrity of design of engine structure by detection, quantification, and prediction of damages. Early detection of faults may allow the downtime of maintenance to be rescheduled, thus preventing sudden shutdown of machines. In cylinder pressure developed, vibrations and noise emissions data provide a rich source of information about condition of engines. Monitoring of vibrations and noise emissions are novel non-intrusive methodologies for which positioning of various transducers are important issue. The presented work shows applicability of these diagnosis methodologies adopted in case of diesel engines. The effects of changing various fuel injection parameters was analyzed. Scope of using non-intrusive technique has been analyzed by changing locations of microphone. Novelty of this worklies in exploring signal processing methods for various locations around the engine test set up. Various frequency ranges of contributing noise and vibration sources were identified. Time-Frequency analysis showed the onset of various cyclic. Based on the identification of various frequency bands, it is possible to device suitable filters in order to extract more information.  相似文献   
165.
Herein, we report a Mott-Schottky catalyst by entrapping cobalt nanoparticles inside the N-doped graphene shell (Co@NC). The Co@NC delivered excellent oxygen evolution activity with an overpotential of merely 248 mV at a current density of 10 mA cm–2 with promising long-term stability. The importance of Co encapsulated in NC has further been demonstrated by synthesizing Co nanoparticles without NC shell. The synergy between the hexagonal close-packed (hcp) and face-centered cubic (fcc) Co plays a major role to improve the OER activity, whereas the NC shell optimizes the electronic structure, improves the electron conductivity, and offers a large number of active sites in Co@NC. The density functional theory calculations have revealed that the hcp Co has a dominant role in the surface reaction of electrocatalytic oxygen evolution, whereas the fcc phase induces the built-in electric field at the interfaces with N-doped graphene to accelerate the H+ ion transport.  相似文献   
166.
Crystallography Reports - Two LiInSe2 single crystals, grown under different conditions, have been studied. Characteristics of these crystals for neutron detection have been compared using...  相似文献   
167.
Journal of Radioanalytical and Nuclear Chemistry - In this paper a study for the selection of an NPE (nonylphenol ethoxylates)—free cocktail is discussed in order to be used for our routine...  相似文献   
168.
169.
A known strategy for improving the properties of layered oxide electrodes in sodium-ion batteries is the partial substitution of transition metals by Li. Herein, the role of Li as a defect and its impact on sodium storage in P2-Na0.67Mn0.6Ni0.2Li0.2O2 is discussed. In tandem with electrochemical studies, the electronic and atomic structure are studied using solid-state NMR, operando XRD, and density functional theory (DFT). For the as-synthesized material, Li is located in comparable amounts within the sodium and the transition metal oxide (TMO) layers. Desodiation leads to a redistribution of Li ions within the crystal lattice. During charging, Li ions from the Na layer first migrate to the TMO layer before reversing their course at low Na contents. There is little change in the lattice parameters during charging/discharging, indicating stabilization of the P2 structure. This leads to a solid-solution type storage mechanism (sloping voltage profile) and hence excellent cycle life with a capacity of 110 mAh g-1 after 100 cycles. In contrast, the Li-free compositions Na0.67Mn0.6Ni0.4O2 and Na0.67Mn0.8Ni0.2O2 show phase transitions and a stair-case voltage profile. The capacity is found to originate from mainly Ni3+/Ni4+ and O2-/O2-δ redox processes by DFT, although a small contribution from Mn4+/Mn5+ to the capacity cannot be excluded.  相似文献   
170.
Radiophysics and Quantum Electronics - A well-known simplified model of turbulent convection from local sources of buoyancy and/or momentum acting for a short time has been developed and...  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