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   31929篇
  免费   1333篇
  国内免费   148篇
化学   21069篇
晶体学   224篇
力学   605篇
数学   4036篇
物理学   5482篇
无线电   1994篇
  2023年   229篇
  2022年   249篇
  2021年   437篇
  2020年   569篇
  2019年   550篇
  2018年   423篇
  2017年   357篇
  2016年   916篇
  2015年   775篇
  2014年   943篇
  2013年   1525篇
  2012年   1932篇
  2011年   2159篇
  2010年   1239篇
  2009年   1048篇
  2008年   1815篇
  2007年   1758篇
  2006年   1722篇
  2005年   1587篇
  2004年   1398篇
  2003年   1074篇
  2002年   1094篇
  2001年   596篇
  2000年   477篇
  1999年   435篇
  1998年   422篇
  1997年   420篇
  1996年   405篇
  1995年   362篇
  1994年   377篇
  1993年   354篇
  1992年   316篇
  1991年   231篇
  1990年   269篇
  1989年   229篇
  1988年   232篇
  1987年   214篇
  1986年   214篇
  1985年   333篇
  1984年   330篇
  1983年   252篇
  1982年   284篇
  1981年   250篇
  1980年   253篇
  1979年   225篇
  1978年   261篇
  1977年   268篇
  1976年   190篇
  1975年   198篇
  1974年   219篇
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
41.
This communication shows an algorithm used in the drug infusion system discussed in a recent paper1 to be incorrect. The error is corrected, and a new algorithm is presented.  相似文献   
42.
43.
描述汽车显示器架构中的外部和内部接口及整合选型方案.  相似文献   
44.
45.
In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k p/(Rs) with ak p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T 1), spin-spin relaxation times (T 2), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT 2 measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T 1 of the narrow line is significantly shorter thanT 1 of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.  相似文献   
46.
We relate the equisingular deformation theory of plane curve singularities and sandwiched surface singularities. We show the existence of a smooth map between the two corresponding deformation functors and study the kernel of this map. In particular we show that the map is an isomorphism when a certain invariant is large enough.  相似文献   
47.
In this paper we analyze the bottom of the energy-momentum spectrum of the translation invariant Nelson model, describing one electron linearly coupled to a second quantized massive scalar field. Our results are valid for all values of the coupling constant and include an HVZ theorem, non-degeneracy of ground states, existence of isolated groundstates in dimensions 1 and 2, non-existence of ground states embedded in the bottom of the essential spectrum in dimensions 3 and 4, (i.e., at total momenta where no isolated groundstate eigenvalue exists), and we study regularity and monotonicity properties of the bottom of the essential spectrum, as a function of total momentum. Communicated by Joel Feldman submitted 04/11/04, accepted 17/02/05  相似文献   
48.
Robust speed control of a low damped electromechanical system with backlash is studied, controlled load angular speed being not measured. The proposed control strategy combines a Luenberger observer (load angular speed and load torque disturbance estimations) and a robust CRONE controller. The observer provides estimation of the load angular speed and of the disturbance torque applied on the load. Through the computation of only three independent parameters (as many as a PID controller), the CRONE controller permits to ensure the robust speed control of the load in spite of plant parametric variations and speed observation errors. The proposed control strategy is applied to a four mass experimental test bench.  相似文献   
49.
A self-referencing, optical modulation technique was used to measure the negative luminescence efficiencies of an array of mid-wave infrared HgCdTe photodiodes with cutoff wavelength 4.6 μm as a function of sample temperature. The internal efficiency at a wavelength of 4 μm was 93% at 295 K, and nearly independent of temperature in the 240–300 K range. This corresponds to an apparent temperature reduction >50 K at room temperature and >30 K at 240 K. Moreover, the reverse-bias saturation current density was only 0.13 A/cm2. The measured transmission and emission spectra were simulated using empirical HgCdTe absorption formulas from the literature.  相似文献   
50.
Poly{bis(4,4′‐tert‐butyl‐2,2′‐bipyridine)–(2,2′‐bipyridine‐5,5′‐diyl‐[1,4‐phenylene])–ruthenium(II)bishexafluorophosphate} ( 3a ), poly{bis(4,4′‐tert‐butyl‐2,2′‐bipyridine)–(2,2′‐bipyridine‐4,4′‐diyl‐[1,4‐phenylene])–ruthenium(II)bishexafluorophosphate} ( 3b ), and poly{bis(2,2′‐bipyridine)–(2,2′‐bipyridine‐5,5′‐diyl‐[1,4‐phenylene])–ruthenium(II)bishexafluorophosphate} ( 3c ) were synthesized by the Suzuki coupling reaction. The alternating structure of the copolymers was confirmed by 1H and 13C NMR and elemental analysis. The polymers showed, by ultraviolet–visible, the π–π* absorption of the polymer backbone (320–380 nm) and at a lower energy attributed to the d–π* metal‐to‐ligand charge‐transfer absorption (450 nm for linear 3a and 480 nm for angular 3b ). The polymers were characterized by a monomodal molecular weight distribution. The degree of polymerization was approximately 8 for polymer 3b and 28 for polymer 3d . © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2911–2919, 2004  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