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   489篇
  免费   8篇
化学   305篇
晶体学   6篇
力学   8篇
数学   22篇
物理学   156篇
  2023年   4篇
  2021年   8篇
  2019年   1篇
  2018年   4篇
  2017年   1篇
  2016年   6篇
  2015年   5篇
  2014年   5篇
  2013年   9篇
  2012年   21篇
  2011年   14篇
  2010年   17篇
  2009年   11篇
  2008年   17篇
  2007年   35篇
  2006年   26篇
  2005年   25篇
  2004年   21篇
  2003年   15篇
  2002年   25篇
  2001年   12篇
  2000年   25篇
  1999年   12篇
  1998年   11篇
  1997年   11篇
  1996年   9篇
  1995年   11篇
  1994年   17篇
  1993年   23篇
  1992年   11篇
  1991年   17篇
  1990年   7篇
  1989年   6篇
  1988年   10篇
  1987年   3篇
  1986年   1篇
  1985年   3篇
  1984年   3篇
  1983年   3篇
  1982年   5篇
  1981年   6篇
  1980年   1篇
  1979年   2篇
  1978年   4篇
  1977年   2篇
  1976年   2篇
  1975年   2篇
  1974年   4篇
  1973年   1篇
  1968年   1篇
排序方式: 共有497条查询结果,搜索用时 15 毫秒
411.
Two novel nonsymmetrical disc‐shaped molecules 1 and 2 based on 3,3′‐bis(acylamino)‐2,2′‐bipyridine units were synthesized by means of a statistical approach. Discotic 1 possesses six chiral dihydrocitronellyl tails and one peripheral phenyl group, whereas discotic 2 possesses six linear dodecyloxy tails and one peripheral pyridyl group. Preorganization by strong intramolecular hydrogen bonding and subsequent aromatic interactions induce self‐assembly of the discotics. Liquid crystallinity of 1 and 2 was determined with the aid of polarized optical microscopy, differential scanning calorimetry, and X‐ray diffraction. Two columnar rectangular mesophases (Colr) have been identified, whereas for C3‐symmetrical derivatives only one Colr mesophase has been found. 1 In solution, the molecularly dissolved state in chloroform was studied with 1H NMR spectroscopy, whereas the self‐assembled state in apolar solution was examined with optical spectroscopy. Remarkably, these desymmetrized discotics, which lack one aliphatic wedge, behave similar to the symmetric parent compound. To prove that the stacking behavior of discotics 1 and 2 is similar to that of reported C3‐symmetrical derivatives, a mixing experiment of chiral 1 with C3‐symmetrical 13 has been undertaken; it has shown that they indeed belong to one type of self‐assembly. This helical J‐type self‐assembly was further confirmed with UV/Vis and photoluminescence (PL) spectroscopy. Eventually, disc 2 , functionalized with a hydrogen‐bonding acceptor moiety, might perform secondary interactions with molecules such as acids.  相似文献   
412.
Oligonucleotides composed of 1′,5′‐anhydro‐arabino‐hexitol nucleosides belonging to the L series (L ‐HNA) were prepared and preliminarily studied as a novel potential base‐pairing system. Synthesis of enantiopure L ‐hexitol nucleotide monomers equipped with a 2′‐(N6‐benzoyladenin‐9‐yl) or a 2′‐(thymin‐1‐yl) moiety was carried out by a de novo approach based on a domino reaction as key step. The L oligonucleotide analogues were evaluated in duplex formation with natural complements as well as with unnatural sugar‐modified oligonucleotides. In many cases stable homo‐ and heterochiral associations were found. Besides Tm measurements, detection of heterochiral complexes was unambiguously confirmed by LC‐MS studies. Interestingly, circular dichroism measurements of the most stable duplexes suggested that L ‐HNA form left‐handed helices with both D and L oligonucleotides.  相似文献   
413.
The first experimental evidence of parallel momentum transport generated by the up-down asymmetry of a toroidal plasma is reported. The experiments, conducted in the Tokamak à Configuration Variable, were motivated by the recent theoretical discovery of ion-scale turbulent momentum transport induced by an up-down asymmetry in the magnetic equilibrium. The toroidal rotation gradient is observed to depend on the asymmetry in the outer part of the plasma leading to a variation of the central rotation by a factor of 1.5-2. The direction of the effect and its magnitude are in agreement with theoretical predictions for the eight possible combinations of plasma asymmetry, current, and magnetic field.  相似文献   
414.
415.
The gas barrier properties of ethylene vinyl alcohol copolymer (EVOH) against oxygen, carbon dioxide and water vapor have been widely investigated in relation to different material characteristics, environmental conditions and new processing technologies. Recently, EVOH is gaining more attention as a barrier material against other gases and organic substances such as aromas, flavors, fuels, chemicals (e.g., BTEX), and as a functional barrier, e.g., to avoid mineral oil migration. This review contains an update on permeability data of EVOH emphasizing its potential as a barrier material for new and versatile applications in food and pharmaceutical packaging, agriculture, construction, automotive, etc.  相似文献   
416.
By utilizing a fully floating double electrical probe system, the conductivity of a linear atmospheric pressure plasma jet, utilizing nitrogen as process gas, was measured. The floating probe makes it possible to measure currents in the nanoamp range, in an environment where capacitive coupling of the probes to the powered electrodes is on the order of several kilovolts. Using a chemical kinetic model, the production of reactive nitrogen oxide and hydrogen-containing species through admixture of ambient humid air is determined and compared to the measured gas conductivity. The chemical kinetic model predicts an enhanced diffusion coefficient for admixture of O2 and H2O from ambient air of 2.7 cm2 s?1, compared to a literature value of 0.21 cm2 s?1, which is attributed to rapid mixing between the plasma jets and the surrounding air. The dominant charge carriers contributing to the conductivity, aside from electrons, are NO+, NO2 ? and NO3 ?. Upon admixture of O2 and H2O, the dominant neutral products formed in the N2 plasma jet are O, NO and N2O, while O2(1Δg) singlet oxygen is the only dominant excited species.  相似文献   
417.
418.
We derive the expression of the reference distribution function for magnetically confined plasmas far from the thermodynamic equilibrium. The local equilibrium state is fixed by imposing the minimum entropy production theorem and the maximum entropy (MaxEnt) principle, subject to scale invariance restrictions. After a short time, the plasma reaches a state close to the local equilibrium. This state is referred to as the reference state. The aim of this Letter is to determine the reference distribution function (RDF) when the local equilibrium state is defined by the above mentioned principles. We prove that the RDF is the stationary solution of a generic family of stochastic processes corresponding to an universal Landau-type equation with white parametric noise. As an example of application, we consider a simple, fully ionized, magnetically confined plasmas, with auxiliary Ohmic heating. The free parameters are linked to the transport coefficients of the magnetically confined plasmas, by the kinetic theory.  相似文献   
419.
An ab initio study of the importance of the phosphorus atom in PHY, PY2, POY2 (Y=–H, –CH3, –NH2, –OH and –F) functional groups is presented. Methods like HF, DFT, MP(2,4), QCISD(T), CCSD(T) are used to estimate structural and energetic behaviour and obtain charge distribution. The radicals and their positive and negative ions are considered as well as neutral and negatively charged organic compounds in order to compare these situations. By the use of different properties, the influence of the phosphorus containing groups and the importance of the phosphorus atom in these substituents is discussed and clarified.  相似文献   
420.
The synthesis and the full characterization of two new linear bisphosphonates (tetraethyl(N-tert-butyl-1-aminoethan-1,1-diyl)bisphosphonate and tetraethyl(N-sec-butyl-1-aminoethan-1,1-diyl)bisphosphonate), and the first analysis of the ESR spectra of the corresponding nitroxides is reported. The preliminary results of theoretical calculations on model compounds suggest a small B0 (in the McConnell equation). The results of bisphosphonate ester and bisphosphonic acid are similar. The discrepancies of P coupling for the diphosphorus compound stems from B2 that is different when the dihedral angle is larger than 90°.  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