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   1958篇
  免费   39篇
  国内免费   3篇
化学   1289篇
晶体学   15篇
力学   30篇
数学   333篇
物理学   333篇
  2022年   35篇
  2021年   19篇
  2020年   23篇
  2019年   26篇
  2018年   22篇
  2017年   32篇
  2016年   28篇
  2015年   24篇
  2014年   43篇
  2013年   101篇
  2012年   116篇
  2011年   96篇
  2010年   61篇
  2009年   70篇
  2008年   123篇
  2007年   112篇
  2006年   99篇
  2005年   85篇
  2004年   74篇
  2003年   59篇
  2002年   57篇
  2001年   29篇
  2000年   22篇
  1999年   15篇
  1997年   13篇
  1996年   27篇
  1995年   19篇
  1994年   23篇
  1993年   17篇
  1992年   24篇
  1991年   14篇
  1990年   16篇
  1989年   21篇
  1988年   24篇
  1987年   19篇
  1986年   14篇
  1985年   31篇
  1984年   36篇
  1983年   25篇
  1982年   39篇
  1981年   30篇
  1980年   23篇
  1979年   20篇
  1978年   29篇
  1977年   19篇
  1976年   21篇
  1974年   15篇
  1973年   30篇
  1972年   13篇
  1971年   12篇
排序方式: 共有2000条查询结果,搜索用时 15 毫秒
981.
A detailed careful analysis of the infrared resonance (IR) spectra of polystyrene (PSt), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN) and their co-mixtures were performed. Through this study the absorption peak area to weight ratios as well as working curves were obtained to test for their reliability as well as their suitability. Satisfactory results were achieved and these working curves were then used to measure the polymerized components of binary and ternary co-polymers. By investigating the acquired data we conclude that the monomer preferential polymeric sequence is St > MMA > AN. A quantitative method to measure P (St/AN/MMA) concentrations by IR spectroscopy is proposed in this work.  相似文献   
982.
There is a need for the preparation of enantiomerically pure compounds for various applications. An efficient approach to achieve this goal is asymmetric catalysis. The chiral catalyst is usually prepared from a chiral auxiliary, which itself is derived from a natural product or by resolution of a racemic precursor. The use of non‐enantiopure chiral auxiliaries in asymmetric catalysis seems unattractive to preparative chemists, since the anticipated enantiomeric excess (ee) of the reaction product should be proportional to the ee value of the chiral auxiliary (linearity). In fact, some deviation from linearity may arise. Such nonlinear effects can be rich in mechanistic information and can be synthetically useful (asymmetric amplification). This Review documents the advances made during the last decade in the use of nonlinear effects in the area of organometallic and organic catalysis.  相似文献   
983.
Functionalized spiroacetals have been easily prepared in a one‐pot three‐component coupling process that involves the reaction of pentynol derivatives, salicylaldehydes, and amines in the presence of catalytic amounts of a palladium(II) complex (see scheme). Alternatively, oxygen‐substituted spiroacetals can be obtained by using orthoesters as the third component.

  相似文献   

984.
Three new coordination complexes, 2{[Co(L1)2]ClO4} · 0.5CH3OH (1), [Mn(L2)2] (2), and [Cu(HL2)(L2)]ClO4 · 2H2O (3) have been synthesized from two tridentate N,N,O-donor hydrazone ligands HL1, 2-acetylpyridine-salicyloylhydrazone, and HL2, 2-benzoylpyridine-salicyloylhydrazone, respectively and thoroughly characterized by elemental analysis, FT-IR, UV–Vis, electrochemical, and room temperature magnetic susceptibility measurements. Structures of the complexes have been unequivocally established by single crystal X-ray diffraction technique. Structural analysis reveals that 1 consists of two chemically similar but crystallographically independent cationic [Co(L1)2]+ units and 2 consists of a neutral [Mn(L2)2] molecule while 3 consists of a cationic [Cu(HL2)(L2)]+ unit. Metal ions display distorted octahedral geometry in 1 and 2 while in 3 it shows a distorted square pyramidal geometry. Ligand conformations around the metal ions are stabilized by the presence of intra-ligand hydrogen bonding in all the complexes. Structure of 3 reveals that a perchlorate ion linked to the complex by hydrogen bonding via a water molecule.  相似文献   
985.
Let D be the circulant digraph with n vertices and connection set {2,3,c}. (Assume D is loopless and has outdegree 3.) Work of S. C. Locke and D. Witte implies that if n is a multiple of 6, c{(n/2)+2,(n/2)+3}, and c is even, then D does not have a hamiltonian cycle. For all other cases, we construct a hamiltonian cycle in D.  相似文献   
986.
Single-component pulse response studies were used to compare the retention and transport behavior of small molecules, proteins, and a virus on commercially available monolithic and perfusive ion-exchangers. Temporal distortion and extra-column effects were corrected for using a simple algorithm based on the method of moments. It was found that temporal distortion is inversely related to the number of theoretical plates. With increasing bioparticle size, retention increased and the transition from a non-eluting to a non-adsorbing state with increasing ionic strength became more abrupt. Both of these observations are qualitatively explained by calculations of particle-surface electrostatic attractive energy. Calculations also suggest that, for sufficiently large bioparticles, such as viruses or cells, hydrodynamic drag can promote elution. Under non-adsorbing conditions, plate height increased only weakly with flow rate and the skew remained unchanged. With increasing retention, plate height increased dramatically for proteins. Plate height was scaled by permeability rather than bead diameter to enable comparison among different stationary phases.  相似文献   
987.
Seven new uranyl vanadates with mono-protonated amine or tetramethylammonium used as structure directing cations, (C2NH8)2{[(UO2)(H2O)][(UO2)(VO4)]4}·H2O (DMetU5V4) (C2NH8){[(UO2)(H2O)2][(UO2)(VO4)]3}·H2O (DMetU4V3), (C5NH6)2{[(UO2)(H2O)][(UO2)(VO4)]4}·H2O (PyrU5V4), (C3NH10){[(UO2)(H2O)2][(UO2)(VO4)]3}·H2O (isoPrU4V3), (N(CH3)4){[(UO2)(H2O)2][(UO2)(VO4)]3}·H2O (TMetU4V3), (C6NH14){[(UO2)(H2O)2][(UO2)(VO4)]3}·H2O (CHexU4V3), and (C4NH12){[(UO2)(H2O)][(UO2)(VO4)]3} (TButU4V3) were prepared from mild-hydrothermal reactions using dimethylamine, pyridine, isopropylamine, tetramethylammonium hydroxide, cyclohexylamine and tertiobutylamine, respectively, with uranyl nitrate and vanadium oxide in acidic medium. The structures were solved using single-crystal X-ray diffraction data. The compounds exhibit three-dimensional uranyl-vanadate inorganic frameworks built from uranophane-type uranyl-vanadate layers pillared by uranyl polyhedra with cavities in between occupied by protonated organic moieties. In the uranyl-vanadate layers the orientations of the vanadate tetrahedra give new geometrical isomers leading to unprecedented pillared systems and new inorganic frameworks with U/V=4/3. Crystallographic data: (DMetU5V4) orthorhombic, Cmc21 space group, a=15.6276(4), b=14.1341(4), c=13.6040(4) Å; (DMetU4V3) monoclinic, P21/n space group, a=10.2312(4), b=13.5661(7), c=17.5291(7) Å, β=96.966(2); (PyrU5V4), triclinic, P1 space group, a=9.6981(3), b=9.9966(2), c=10.5523(2) Å, α=117.194(1), β=113.551(1), γ=92.216(1)°; (isoPrU4V3) monoclinic, P21/n space group, a=10.3507(1), b=13.6500(2), c=17.3035(2) Å, β=97.551(1)°; (TMetU4V3) orthorhombic, Pbca space group, a=17.1819(2), b=13.6931(1), c=21.4826(2) Å; (CHexU4V3), triclinic P−1 space group, a=9.8273(6), b=11.0294(7), c=12.7506(8) Å, α=98.461(3), β=96.437(3), γ=105.955(3)°; (TButU4V3), monoclinic, P21/m space group, a=9.8048(4), b=17.4567(8), c=15.4820(6) Å, β=106.103(2).  相似文献   
988.
An analytical method using an optical probe in a photoelectrochemical cell for the sensitive and selective determination of aqueous Hg2+ is presented. A previously synthesized Hg2+ selective chemosensor, proven to be Hg2+ sensitive up to 2 μg L−1, has been immobilized onto indium tin oxide (ITO) electrodes in a composite form with polyaniline. The coated ITO electrode was placed in a photoelectrochemical cell under closed circuit conditions in which the optical recognition of the chemosensor was converted to a measurable signal. A composite of the fluorescent chemosensor, Rhodamine 6G derivative (RS), and polyaniline (PANI) was immobilized on ITO glass plates and subjected to photovoltage measurements in the absence and presence of Hg2+. The optical responses of the coated electrode were used to determine the sensitivity and selectivity of the immobilized sensor to Hg2+ in the presence of background ions. The optical response of the PANI-dye coated electrode increased linearly with increasing Hg2+ concentration in the range 10-150 μg L−1, with a detection limit of 6 μg L−1.  相似文献   
989.
990.
Patel MV  Tovar AR  Lee AP 《Lab on a chip》2012,12(1):139-145
A novel on-chip microfluidic switch is demonstrated that utilizes the acoustic microstreaming generated by an oscillating air-liquid interface to switch cells/particles into bifurcating microchannels. The air-liquid interface of the Lateral Cavity Acoustic Transducers (LCATs) can be actuated by an external acoustic energy source causing the interface to oscillate. The oscillating interface results in the generation of vortex-like microstreaming flow within a localized region of the surrounding liquid. This streaming was utilized here to deflect cells/particles into a collection outlet. It was demonstrated that the switching zone could be controlled by varying the actuation time of the LCAT. An LCAT based microfluidic switch is capable of achieving theoretical switching rates of 800 cells/particles per second. It was also demonstrated that K562 cells could be switched into a collection channel with cell viability comparable to that of controls as determined by Trypan blue exclusion assay.  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