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   171007篇
  免费   2013篇
  国内免费   716篇
化学   91929篇
晶体学   2461篇
力学   7603篇
综合类   7篇
数学   17703篇
物理学   54033篇
  2021年   991篇
  2020年   1112篇
  2019年   1126篇
  2018年   1153篇
  2017年   1167篇
  2016年   2329篇
  2015年   1908篇
  2014年   2484篇
  2013年   7686篇
  2012年   6264篇
  2011年   7935篇
  2010年   4881篇
  2009年   5030篇
  2008年   7299篇
  2007年   7282篇
  2006年   7007篇
  2005年   6533篇
  2004年   5852篇
  2003年   5044篇
  2002年   4953篇
  2001年   6026篇
  2000年   4557篇
  1999年   3474篇
  1998年   2624篇
  1997年   2617篇
  1996年   2567篇
  1995年   2275篇
  1994年   2161篇
  1993年   2025篇
  1992年   2343篇
  1991年   2308篇
  1990年   2021篇
  1989年   2005篇
  1988年   2028篇
  1987年   1960篇
  1986年   1871篇
  1985年   2774篇
  1984年   2752篇
  1983年   2206篇
  1982年   2391篇
  1981年   2272篇
  1980年   2226篇
  1979年   2174篇
  1978年   2243篇
  1977年   2199篇
  1976年   2129篇
  1975年   2118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 11 毫秒
111.
Cavity ring-down spectroscopy (CRDS) is used to measure the NO mole fraction formed in the burnt gases of low-pressure premixed flames. It is shown that the line-of-sight absorption is greatly increased by the contribution of the NO molecules surrounding the burner. This contribution has been quantified by developing a mathematical procedure taking into account the spatial and spectral features of the CRDS measurement. Calculations have been undertaken in the general case of a stable species not consumed in the flame. The most sensitive parameter is the temperature both in the flame and outside the flame. Simulations allow the selection of the best spectroscopic transitions for a given flame (i.e. a given temperature profile), ensuring the weakest influence of the inaccuracy affecting the temperature determination. High quantum states belonging to the A–X (0–1) band of NO have been found to be the most valuable and have led to a NO mole fraction determination with an accuracy of ±13%. NO absorption in the flame was completely masked using the A–X (0–0) band. Finally, the prompt-NO mole fraction formed in a methane/air flame stabilized at 33 Torr is obtained by combining CRDS and laser induced fluorescence techniques. Received: 12 October / Revised version: 1 February 2002 / Published online: 14 March 2002  相似文献   
112.
Paracchini  C.  Roman&#;  L. 《Il Nuovo Cimento D》1994,16(10):1835-1841
Il Nuovo Cimento D - The applicability of the Kosterlitz-Thouless theory to HTSC is examined. The model initially presented in its general lines for bidimensional systems is then applied to...  相似文献   
113.
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   
114.
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   
115.
Corrections to the last two equations of Ref. 1 are given.  相似文献   
116.
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   
117.
An integral constitutive equation which includes strain-coupling effects is proposed. The constitutive equation is more general than the K-BKZ equation, and it is shown that this equation can explain various important aspects of double-step stress-relaxation data.  相似文献   
118.
Five different analytical schemes for examining isothermal and nonisothermal degradation of polymers were reviewed and found inadequate for describing multistage decomposition. The different schemes were experimentally tested using thermogravimetric analysis data for an ethylene-vinyl acetate (EVA) copolymer, which exhibited a well-behaved two-step decomposition process in a nitrogen environment. Based on these experimental and analytical findings, a generalized methodology was developed capable of describing the exhibited complex decomposition behavior of polymers by combining two or more kinetic mechanisms in a series or parallel arrangement. This composite of combinational methodology thus extends established analytical schemes for describing complex decomposition of polymers in a rational manner, incorporating both experimental and theoretical considerations.  相似文献   
119.
The 3'-iodonucleoside 4 and the 3'-O-methylsulfonylthymidine 9 have been synthesized by condensation of silylated uracils 2 with methyl 5-O-tert-butyldiphensilyl-2,3-dideoxy-3-iodo-D-threo-pentofuran oside (3) and methyl 5-O-tert-butyldiphenylsilyl-2-deoxy-3-methylsulfonyl-D-erythro- pentofuranoside (8), respectively. The nucleoside 4 and 9 produced the corresponding 2',3'-didehydro-2',3'-dideoxynucleosides 5 in an elimination reaction on treatment with sodium methoxide. The compounds 5b showed no antiviral activity against HIV-1.  相似文献   
120.
This paper presents the results of an experimental study undertaken on the irradiation process of polyethylene tubing in an electron beam. Isodoses of distribution through material thickness in case of irradiation in one or two irradiation directions have been analyzed.  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