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   34858篇
  免费   383篇
  国内免费   330篇
化学   12937篇
晶体学   300篇
力学   1505篇
综合类   12篇
数学   9349篇
物理学   9104篇
无线电   2364篇
  2023年   68篇
  2022年   274篇
  2021年   274篇
  2020年   218篇
  2019年   217篇
  2018年   1424篇
  2017年   1641篇
  2016年   994篇
  2015年   786篇
  2014年   742篇
  2013年   1117篇
  2012年   3577篇
  2011年   2721篇
  2010年   2066篇
  2009年   1759篇
  2008年   808篇
  2007年   806篇
  2006年   761篇
  2005年   4863篇
  2004年   4063篇
  2003年   2370篇
  2002年   567篇
  2001年   352篇
  2000年   128篇
  1999年   207篇
  1998年   140篇
  1997年   105篇
  1996年   74篇
  1995年   55篇
  1994年   68篇
  1992年   189篇
  1991年   169篇
  1990年   178篇
  1989年   138篇
  1988年   133篇
  1987年   107篇
  1986年   79篇
  1985年   93篇
  1984年   76篇
  1983年   64篇
  1982年   59篇
  1980年   62篇
  1979年   52篇
  1976年   100篇
  1975年   49篇
  1974年   52篇
  1973年   72篇
  1972年   51篇
  1970年   55篇
  1968年   52篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
993.
Allocation rules for cooperative games can be manipulated by coalitions merging into single players, or, conversely, players splitting into a number of smaller units. This paper collects some (im)possibility results on merging- and splitting-proofness of (core) allocation rules for cooperative games with side-payments.  相似文献   
994.
An approach to evaluate the second-order exchange-induction energies of symmetry-adapted intermolecular perturbation theory (SAPT) for single-determinant ground-state monomer wavefunctions has been derived. This approach is correct to all orders of the intermonomer overlap, that is, it takes multiple electron exchange between the monomers into account. The resulting formulae can be written in a compact way and implemented efficiently. Here, the method is employed to investigate the performance of the S 2- or single-exchange approximation at the Hartree-Fock-SAPT level. The list of test systems comprises the prototypical van der Waals- and hydrogen-bridge complexes Ne2, Ar–HF, and (H2O)2, but also the systems HeCl?, NeNa+ and Li+F? involving closed-shell ions. It was found that the errors introduced by the S 2-approximation are more pronounced for the second-order exchange-induction energy than for the first-order exchange energy. While these errors tend to be negligible throughout the well region of complexes such as the neon dimer, they start to be significant in the repulsive part of the well regions of systems such as the water dimer, and in particular for the ionic lithium fluoride molecule. The consequences of these findings for the Hartree-Fock level estimate of higher-order induction plus exchange-induction energies, which is frequently employed in SAPT are also discussed.  相似文献   
995.
We present the comaprative study of semileptonic and leptonic decays of Ds, D±and D0meson(D →M l+-αl-β, D → l+αlβ, D → l+αvα;α,β=e,μ) within the framework of R-parity violating the( Rp) Minimal Supersymmetric Standard Model(MSSM). The comparison shows that combination and product couplings,(λβiα λ* ij qor λβqkλ*αj k)contribution to the branching fractions of the said processes(under consideration) is consistent with or comparable to the experimental measurements in most of the cases. However, some cases exist where these contributions are highly suppressed. We identify such cases in our analysis and single out the important ones suitable for exploring in the future and current experiments.  相似文献   
996.
We demonstrate a switchable Q-switched and mode-locked erbium-doped fiber laser (EDFL) operating in the L-band region using the nonlinear polarization rotation effect. The switching operation is achieved by controlling intensity-dependent loss using a polarization controller. In Q-switching mode, the EDFL produces a pulse train with a repetition rate of 21.1 kHz, pulse width of 7.7 #s, and pulse energy of 13.6 nJ. The EDFL also generates a multi-wavelength comb with a very narrow and constant wavelength spacing of 0.045 nm and optical signal-to-noise ratio of at least l0 dB. During mode locking, the EDFL produces stretched pulses with 3-dB bandwidth of 26.2 nm, pulse width of 350 fs, repetition rate of 2.38 MHz, and pulse energy of 48.56 pJ.  相似文献   
997.
Conventional slotted waveguide antennas are well known and prominently used for many military and even marketable applications. This is due to their different advantageous electrical and mechanical characteristics. It is also well known that this type of antennas is a typical array of slots with which the width of the main beam can be controlled in only one plane by using a different number of elements in such antennas. Numerous research, discuss the capability of using an array of such antennas in order to narrow down the main beam in both planes and even achieve a scanning phased array. In this paper, a new and broader approach is presented concerning beam width control of the typical configuration of such antennas. This work presents the ability to control the main lobe in both planes using only one slotted waveguide to a certain extent. Different simulation results are considered at the 2.4 GHz frequency demonstrating improved effectiveness in terms of directivity and high gain as well as minimizing the side-lobe level and mechanically controlling the main lobe of the antenna.  相似文献   
998.
Thermogravimetry, diffuse reflectance infrared Fourier transform spectroscopy, and X-ray photoelectron spectroscopy (XPS) were used for the studying of thermally induced structural changes of polyacrylonitrile (PAN) deposited on the surface of SBA-15 type mesoporous silica. Polymer was introduced onto the support by the precipitation polymerization of acrylonitrile in aqueous suspension of SBA-15. Low temperature transformation (to 723?K) of the deposited PAN was analyzed. It was found that at about 523?K, exothermic cyclization of polymer chains to the so-called ladder form of PAN occurred. However, the total cyclization of PAN required higher carbonization temperatures, at which gradual dehydrogenation followed by graphitization was initiated. XPS revealed that the cyclic form of PAN and a relatively large amount of carbonyl species, formed during the carbonization of the PAN/SBA-15 composite at 623?K, were responsible for the high sorption capacity in the methyl?Cethyl ketone (MEK) vapor elimination. The efficiency in the MEK adsorption was also influenced by the content of PAN-derived carbon deposited on the SBA-15 surface.  相似文献   
999.
Cellular networks planning has been affected by restrictions in the placement inside urban areas. This burden is motivated by public complaints on adverse effects in humans?? health, despite the lack of conclusive scientific evidence. As a result, some regulations are enforcing base station placement at suburban locations aimed toward urban areas. With this configuration, network capacity is severely reduced and, paradoxically, users will suffer from a higher radio exposure since terminals must raise up their power to reach suburban base stations. In this work, we propose a feasible solution to improve network capacity under these restrictive placement regulations in the context of UMTS networks. We introduce a passive receiver subsystem located in the urban area, while the Node-B is kept at the suburban location. Thereby, the power received from the Node-B could be considered negligible inside the urban area (since the active part is far away), and user equipments would transmit at lower power levels (since passive receivers are close to them). The results in this paper reveal that our approach improves network capacity (up to 21%) and notably reduces electromagnetic exposure compared to suburban installations.  相似文献   
1000.
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