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   15483篇
  免费   2319篇
  国内免费   4434篇
化学   16520篇
晶体学   273篇
力学   374篇
综合类   103篇
数学   1744篇
物理学   3222篇
  2024年   42篇
  2023年   251篇
  2022年   477篇
  2021年   584篇
  2020年   841篇
  2019年   659篇
  2018年   582篇
  2017年   657篇
  2016年   813篇
  2015年   769篇
  2014年   927篇
  2013年   1589篇
  2012年   1051篇
  2011年   1105篇
  2010年   910篇
  2009年   972篇
  2008年   1019篇
  2007年   1019篇
  2006年   837篇
  2005年   802篇
  2004年   782篇
  2003年   680篇
  2002年   601篇
  2001年   535篇
  2000年   381篇
  1999年   346篇
  1998年   336篇
  1997年   260篇
  1996年   234篇
  1995年   207篇
  1994年   193篇
  1993年   204篇
  1992年   167篇
  1991年   134篇
  1990年   136篇
  1989年   152篇
  1988年   137篇
  1987年   160篇
  1986年   144篇
  1985年   134篇
  1984年   131篇
  1983年   94篇
  1982年   33篇
  1981年   28篇
  1980年   20篇
  1979年   23篇
  1978年   15篇
  1977年   13篇
  1976年   12篇
  1974年   14篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
牛顿弦截法预估校正迭代格式的收敛阶   总被引:2,自引:0,他引:2  
研究如下形式的牛顿弦截法的预估校正(P.C.)格式:P(预估):~xk+1=xk-(xk-xk-1)f(xk)f(xk)-f(xk-1)C(校正):xk+1=xk-(~xk+1-xk)f(xk)f~(xk+1)-f(xk)证明了它的收敛阶为2.618.  相似文献   
52.
本文从光学角度出发提出了一种经济实用、精度高的测量亚共析碳钢中碳的含量的新方法。阐述了该方法的测量原理,即以金相试样表面对光的反射程度来判定其含碳量,并给出了有关经验公式及数据处理方法。  相似文献   
53.
The reactions of ethylene glycol mononitrate and glycerol 1,3-dinitrate with methylphosphonic dichloride afforded new nitroxyalkyl methylphosphonates.  相似文献   
54.
In this paper, we report that the phase transformation of Ni-B, Ni-P diffusion barriers deposited electrolessly on Cu, for the reason that the Ni-P layer is a more effective diffusion barrier than the Ni-B layer. The Ni3B crystallized was decomposed to Ni and B2O3 above 400 °C and the Ni3P crystallized was decomposed to Ni and P2O5 above 600 °C respectively in Ar atmosphere. Also, the Ni3B was decomposed to Ni and free B above 400 °C and the Ni3P was decomposed to Ni and free P above 600 °C respectively in H2 atmosphere. The decomposed Ni formed a solid solution with Cu. The Cu diffusion occurred above 400 °C for Ni-B layer and above 600 °C for Ni-P layer, respectively. Because the decomposition temperature of Ni-P layer is about 200 °C higher than that of Ni-B layer, the Ni-P layer is a more effective barrier for Cu than the Ni-B layer.  相似文献   
55.
Different-sized aerosols were collected by an Andersen air sampler to observe the detailed morphology of the black carbon (BC) aerosols which were separated chemically from the other accompanying aerosols, using a Scanning Electron Microscope equipped with an Energy Dispersive X-ray Spectrometer (SEM-EDX). The results indicate that most BC aerosols are spherical particles of about 50 nm in diameter and with a homogeneous surface. Results also show that these particles aggregate with other aerosols or with themselves to form larger agglomerates in the micrometer range. The shape of these 50-nm BC spherical particles was found to be very similar to that of BC particles released from petroleum-powered vehicular internal combustion engines. These spherical BC particles were shown to be different from the previously reported fullerenes found using Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry (MALDI-TOF-MS).  相似文献   
56.
Let G be a finite group. For semi-free G-manifolds which are oriented in the sense of Waner [S. Waner, Equivariant RO(G)-graded bordism theories, Topology and its Applications 17 (1984) 1-26], the homotopy classes of G-equivariant maps into a G-sphere are described in terms of their degrees, and the degrees occurring are characterised in terms of congruences. This is first shown to be a stable problem, and then solved using methods of equivariant stable homotopy theory with respect to a semi-free G-universe.  相似文献   
57.
Following detoxification of the liquid hydrolysate produced in a corn stover pretreatment process, inhibitor levels are seen to increase with the re-addition of solids for the ensuing hydrolysis and fermentation processes. The solids that were separated from the slurry before detoxification of the liquor contain approx 60% (w/w) moisture, and contamination occurs owing to the diffusion of inhibitors from the moisture entrained in the porous structure of the corn stover solids into the bulk fluid. This evidence suggests the need for additional separation and detoxification steps to purge residual inhibitors entrained in the moisture in the solids. An overliming process to remove furans from the hydrolysate failed to reduce total organic acids concentration, so acids were removed by treatment with an activated carbon powder. Smaller carbon doses proved more efficient in removing organic acids in terms of grams of acid removed per gram of carbon powder. Sugar adsorption by the activated carbon powder was minimal.  相似文献   
58.
We study the inverse scattering problem for the nonlinear Schrödinger equation and for the nonlinear Klein–Gordon equation with the generalized Hartree type nonlinearity. We reconstruct the nonlinearity from knowledge of the scattering operator, which improves the known results.  相似文献   
59.
In 1934, Romanoff proved that there are a positive proportion natural numbers which can be expressed as the sum of a prime and a power of 2. In this paper, a quantitative version of this theorem is given. We show that the proportion is larger than 0.0868 and for a positive proportion of odd integers the number of such representations is between 1 and 16.  相似文献   
60.
FI代数的滤子   总被引:14,自引:8,他引:6  
研究FI代数的滤子理论,建立F滤子、P滤子、Q滤子、C滤子之间的联系.  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