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   189877篇
  免费   5679篇
  国内免费   2982篇
化学   105955篇
晶体学   2701篇
力学   8782篇
综合类   193篇
数学   19735篇
物理学   61172篇
  2021年   1659篇
  2020年   1905篇
  2019年   1886篇
  2018年   1769篇
  2017年   1758篇
  2016年   3247篇
  2015年   2804篇
  2014年   3531篇
  2013年   9020篇
  2012年   8030篇
  2011年   9762篇
  2010年   6102篇
  2009年   6103篇
  2008年   8536篇
  2007年   8504篇
  2006年   8089篇
  2005年   7494篇
  2004年   6542篇
  2003年   5647篇
  2002年   5487篇
  2001年   6422篇
  2000年   4951篇
  1999年   3931篇
  1998年   2969篇
  1997年   2975篇
  1996年   2873篇
  1995年   2565篇
  1994年   2460篇
  1993年   2265篇
  1992年   2533篇
  1991年   2500篇
  1990年   2192篇
  1989年   2114篇
  1988年   2111篇
  1987年   2046篇
  1986年   1927篇
  1985年   2818篇
  1984年   2792篇
  1983年   2230篇
  1982年   2415篇
  1981年   2289篇
  1980年   2239篇
  1979年   2173篇
  1978年   2243篇
  1977年   2199篇
  1976年   2129篇
  1975年   2120篇
  1974年   2023篇
  1973年   2105篇
  1972年   1199篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
We report the discovery of 57 new fir laser lines from13CD3OH molecule optically pumped by a waveguide CO2 laser of 300 MHz tunability. For all lines, precise frequency offset measurements between the CO2 line center and the center of the absorbing13CD3OH line were performed using the transferred Lamb-Dip technique. We have also measured directly the frequency of seven FIR laser lines by heterodyning with already known laser lines. We present a complete list of all known laser lines (134) and frequency measurements (24) for this molecule.Work supported by CNPq, FAPESP, FAEP-Brasil, and CNR-Italia  相似文献   
992.
By using the two-point space correlation function an equation for the power spectral density for a random Langmuir field has been derived. The dispersion relation for a monochromatic wave is regained for a delta spectrum. For a Gaussian spectrum, the maximum growth rate is less than that for a monochromatic wave. For a “meander spectrum”, the growth rate is increased with the width of the spectrum in the first stage then decreased for further increase of the width.  相似文献   
993.
An analysis of the effects of combined evaporation and quench periods on the formation of asymmetric membranes by the phase inversion technique is presented. The model for the evaporation period assumes binary diffusion and includes composition dependence in the mutual diffusion coefficient and free convection mass transfer in the gas phase. The quench period model includes the use of 4 composition-dependent diffusion coefficients and incorporates a modified from of the interface jump mass balance which enables efficient numerical analysis of composition gradients in the film at the beginning of the quench period. Results for the effects of evaporation-period variable, including: evaporation time, initial cast film thickness, casting surface dimension, and vapor-phase composition, clearly indicate that significant effects of the evaporation period on membrane structure formation and its reproducibility occur within 20 seconds.  相似文献   
994.
A theoretical approach to calculate the time evolution of magnetization during a CPMG pulse sequence of arbitrary parameter settings is developed and verified by experiment. The analysis reveals that off-resonance effects can cause systematic reductions in measured peak amplitudes that commonly lie in the range 5–25%, reaching 50% in unfavorable circumstances. These errors, which are finely dependent upon frequency offset and CPMG parameter settings, are subsequently transferred into erroneousT2values obtained by curve fitting, where they are reduced or amplified depending upon the magnitude of the relaxation time. Subsequent transfer to Lipari–Szabo model analysis can produce significant errors in derived motional parameters, with τeinternal correlation times being affected somewhat more thanS2order parameters. A hazard of this off-resonance phenomenon is its oscillatory nature, so that strongly affected and unaffected signals can be found at various frequencies within a CPMG spectrum. Methods for the reduction of the systematic error are discussed. Relaxation studies on biomolecules, especially at high field strengths, should take account of potential off-resonance contributions.  相似文献   
995.
提出并详细分析了一种基于时分复用技术倍乘光脉冲重复率的方法:在2×2光纤耦合器的一个输入口与一个输出口间,接入时延为脉冲列半周期奇数倍的一段光纤使之形成环状连接循环耦合,而在另一输出口获得光脉冲。详细分析了该系统中光纤耦合器耦合比、插入损耗、时延光纤长度的要求,偏差的影响及其调节法,给出了理论公式、结果的计算机模拟与实验介绍;与经典的马赫陈德尔干涉仪接法及近年提出的Sagnac环接法进行了比较。该方法特别适用于对窄脉冲列进行串接复用实现重复率的多次倍乘,从而得到数千兆赫至上百千兆赫的高重复率光脉冲  相似文献   
996.
997.
Summary A new approach to cancer treatment using low-energy gamma-rays is discussed. Cultures of bone marrow mononuclear cells collected from normal donors and patients with chronic myelogenous leukaemia have been studied after the addition of hematin, at different concentrations, and irradiation by a M?ssbauer gamma-ray source. Growth stimulation has been observed when hematin is added to normal bone marrow cultures, while growth inhibition is observed when hematin is added to leukaemic cultures. The effect becomes then more pronounced when hematin is used in combination with M?ssbauer gamma-rays.  相似文献   
998.
C. H. Ryter  J. Schmid 《Order》1994,11(3):257-279
We show that it is a NP-complete problem to decide whether a finite poset arises as the (Birkhoff) dual of the Frattini sublattice of some finite distributive lattice.This work was supported in part by Swiss NSF grant 20-32644.91.  相似文献   
999.
Corrections to the last two equations of Ref. 1 are given.  相似文献   
1000.
Spectroscopic determinations of the concentrations associated with the onset of intermolecular interaction in solutions of poly(ethylene terephthalate) (PET) are described. The intersegment forces perturb the electronic spectra and induce ground-state aggregation. These phenomena become observable at different chain densities, reflecting the influence of concentration on the coil dimensions, as well as the dynamical nature of the interactions. An estimate of the equilibrium constant for PET dimer formation in hexafluoroisopropanol is reported.  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