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   525篇
  免费   7篇
  国内免费   4篇
化学   285篇
晶体学   9篇
力学   7篇
数学   106篇
物理学   129篇
  2023年   4篇
  2022年   9篇
  2021年   7篇
  2020年   7篇
  2019年   4篇
  2017年   9篇
  2016年   8篇
  2015年   3篇
  2014年   7篇
  2013年   14篇
  2012年   15篇
  2011年   35篇
  2010年   27篇
  2009年   12篇
  2008年   23篇
  2007年   18篇
  2006年   22篇
  2005年   16篇
  2004年   19篇
  2003年   15篇
  2002年   18篇
  2001年   8篇
  2000年   11篇
  1999年   23篇
  1998年   14篇
  1997年   12篇
  1996年   11篇
  1995年   8篇
  1994年   9篇
  1993年   14篇
  1992年   6篇
  1991年   12篇
  1990年   6篇
  1989年   9篇
  1988年   12篇
  1987年   4篇
  1986年   3篇
  1985年   3篇
  1984年   7篇
  1981年   4篇
  1980年   4篇
  1979年   4篇
  1977年   7篇
  1976年   7篇
  1975年   3篇
  1974年   8篇
  1973年   3篇
  1972年   3篇
  1962年   3篇
  1956年   3篇
排序方式: 共有536条查询结果,搜索用时 15 毫秒
121.
122.
123.
A series of isomeric cationic surfactants (S1-S5) bearing a long alkyl chain that carries a 1,4-phenylene unit and a trimethyl ammonium headgroup was synthesized; the location of the phenyl ring within the alkyl tail was varied in an effort to understand its influence on the amphiphilic properties of the surfactants. The cmc's of the surfactants were estimated using ionic conductivity measurements and isothermal calorimetric titrations (ITC); the values obtained by the two methods were found to be in excellent agreement. The ITC measurements provided additional insight into the various thermodynamic parameters associated with the micellization process. Although all five surfactants have exactly the same molecular formula, their micellar properties were seen to vary dramatically depending on the location of the phenyl ring; the cmc was seen to decrease by almost an order of magnitude when the phenyl ring was moved from the tail end (cmc of S1 is 23 mM) to the headgroup region (cmc of S5 is 3 mM). In all cases, the enthalpy of micellization was negative but the entropy of micellization was positive, suggesting that in all of these systems the formation of micelles is both enthalpically and entropically favored. As expected, the decrease in cmc values upon moving the phenyl ring from the tail end to the headgroup region is accompanied by an increase in the thermodynamic driving force (ΔG) for micellization. To understand further the differences in the micellar structure of these surfactants, small-angle neutron scattering (SANS) measurements were carried out; these measurements reveal that the aggregation number of the micelles increases as the cmc decreases. This increase in the aggregation number is also accompanied by an increase in the asphericity of the micellar aggregate and a decrease in the fractional charge. Geometric packing arguments are presented to account for these changes in aggregation behavior as a function of phenyl ring location.  相似文献   
124.
In rapid parallel magnetic resonance imaging, the problem of image reconstruction is challenging. Here, a novel image reconstruction technique for data acquired along any general trajectory in neural network framework, called “Composite Reconstruction And Unaliasing using Neural Networks” (CRAUNN), is proposed. CRAUNN is based on the observation that the nature of aliasing remains unchanged whether the undersampled acquisition contains only low frequencies or includes high frequencies too. Here, the transformation needed to reconstruct the alias-free image from the aliased coil images is learnt, using acquisitions consisting of densely sampled low frequencies. Neural networks are made use of as machine learning tools to learn the transformation, in order to obtain the desired alias-free image for actual acquisitions containing sparsely sampled low as well as high frequencies. CRAUNN operates in the image domain and does not require explicit coil sensitivity estimation. It is also independent of the sampling trajectory used, and could be applied to arbitrary trajectories as well. As a pilot trial, the technique is first applied to Cartesian trajectory-sampled data. Experiments performed using radial and spiral trajectories on real and synthetic data, illustrate the performance of the method. The reconstruction errors depend on the acceleration factor as well as the sampling trajectory. It is found that higher acceleration factors can be obtained when radial trajectories are used. Comparisons against existing techniques are presented. CRAUNN has been found to perform on par with the state-of-the-art techniques. Acceleration factors of up to 4, 6 and 4 are achieved in Cartesian, radial and spiral cases, respectively.  相似文献   
125.
A nanocomposite carbon was prepared by grafting a carbonizable polymer, poly(furfuryl alcohol) (PFA), to a single-wall carbon nanotube (SWNT). The SWNT was first functionalized with arylsulfonic acid groups on the sidewall via a method using a diazonium reagent. Both Raman and FTIR spectroscopies were used to identify the functional groups on the nanotube surface. HRTEM imaging shows that the SWNT bundles are exfoliated after functionalization. Once this state of the SWNTs was accomplished, the PFA-functionalized SWNT (PFA-SWNT) was prepared by in situ polymerization of furfuryl alcohol (FA). The sulfonic acid groups on the surface of the SWNT acted as a catalyst for FA polymerization, and the resulting PFA then grafted to the SWNTs. The surfaces of the SWNTs converted from hydrophilic to hydrophobic when they were wrapped with PFA. The formation of the polymer and the attraction between it and the sulfonic acid groups were confirmed by IR spectra. A nanocomposite carbon was generated by heating the PFA-SWNT in argon at 600 degrees C, a process during which the PFA was transformed to nanoporous carbon (NPC) and the sulfonic acid groups were cleaved from the SWNT. Based upon the Raman spectra and HRTEM images of the composite, it is concluded that SWNTs survive this process and a continuous phase is formed between the NPC and the SWNT.  相似文献   
126.
It is well known that if m is an L p -multiplier for the Fourier transform on \mathbbRn{\mathbb{R}^n} , (1 < p < ∞) then there exists a pseudomeasure σ such that T m fσ * f . A similar problem is discussed for the L p −Fourier multipliers for H{\mathcal{H}} -valued functions on the Heisenberg group, where H{\mathcal{H}} is a separable Hilbert space.  相似文献   
127.
It is well known that if m is an L p -multiplier for the Fourier transform on \({\mathbb{R}^n}\) , (1 < p < ∞) then there exists a pseudomeasure σ such that T m fσ * f . A similar problem is discussed for the L p ?Fourier multipliers for \({\mathcal{H}}\) -valued functions on the Heisenberg group, where \({\mathcal{H}}\) is a separable Hilbert space.  相似文献   
128.
Injection locked Gunn oscillators have been developed to amplify the upconverted millimeter wave frequencies. They are being used in millimeter wave communication systems at Ka, V and W-Bands respectively.  相似文献   
129.
130.
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