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   1565篇
  免费   98篇
  国内免费   2篇
化学   1238篇
晶体学   3篇
力学   32篇
综合类   1篇
数学   194篇
物理学   197篇
  2024年   4篇
  2023年   28篇
  2022年   27篇
  2021年   31篇
  2020年   51篇
  2019年   34篇
  2018年   22篇
  2017年   17篇
  2016年   80篇
  2015年   75篇
  2014年   69篇
  2013年   77篇
  2012年   102篇
  2011年   103篇
  2010年   73篇
  2009年   54篇
  2008年   99篇
  2007年   104篇
  2006年   97篇
  2005年   89篇
  2004年   78篇
  2003年   57篇
  2002年   34篇
  2001年   27篇
  2000年   16篇
  1999年   21篇
  1998年   10篇
  1997年   10篇
  1996年   6篇
  1995年   4篇
  1994年   6篇
  1993年   6篇
  1992年   11篇
  1991年   10篇
  1990年   4篇
  1989年   4篇
  1988年   4篇
  1986年   5篇
  1985年   5篇
  1984年   11篇
  1983年   8篇
  1982年   8篇
  1981年   6篇
  1980年   8篇
  1979年   6篇
  1978年   6篇
  1977年   9篇
  1976年   9篇
  1975年   5篇
  1916年   3篇
排序方式: 共有1665条查询结果,搜索用时 11 毫秒
41.
A straightforward, versatile approach to the production of protein gradients on planar and spherical particle surfaces is described. The method is based on the spatially controlled oxidation of thiolated surfaces by Au(III) ions generated via the electrochemical oxidation of a gold electrode in a phosphate-buffered saline solution (10 mM PBS, pH 7.2, 150 mM NaCl). Because the gold electrode is in direct contact with the thiolated surfaces, the released Au(III) ions, which are present as Au(III) chloride complexes, give rise to the formation of a surface gradient of Au(I)-thiolate complexes depending on the local redox potential given by the local Au(III) concentration. As is shown on the basis of the use of X-ray photoelectron spectroscopy and fluorescently labeled proteins, the Au(I)-thiolate complexes can subsequently be functionalized with thiolated proteins, yielding surface density protein gradients on micrometer-sized nonconducting polymer beads as well as linear Au(I)-thiolate gradients on planar silicon surfaces.  相似文献   
42.
The scope for the synthesis and investigation of chiral smectic A liquid crystalline (LC) polymers and block copolymers is discussed. LC block copolymers can combine the molecular order of liquid crystals and the supramolecular order typical of block copolymers, thus allowing to attain information on mesomorphic responses in restricted geometries. We present a general overview on several aspects of the syntheses and properties of LC block copolymers, specifically those obtained by starting from azomacroinitiators. These materials can exhibit an electroclinic effect, that is an electrically induced molecular tilt, which is characterized by a linear dependence on the applied field and a very fast response time in the paraelectric smectic A phase. The current progress in their potential application in electrooptics is outlined.  相似文献   
43.
The C 1 s, N 1 s, and O 1 s core level binding energies (BEs) of the functional groups in amino acids (glycine, aspartic acid, glutamic acid, arginine, and histidine) with varied side‐chains and cell‐binding RGD‐based peptides have been determined and characterized by X‐ray photoelectron spectroscopy with a monochromatic Al Kα source. The zwitterionic nature of the amino acids in the solid state is unequivocally evident from the N 1 s signals of the protonated amine groups and the C 1 s signature of carboxylate groups. Significant adventitious carbon contamination is evident for all samples but can be quantitatively accounted for. No intrinsic differences in the XP spectra are evident between two polymorphs (α and γ) of glycine, indicating that the crystallographic differences have a minor influence on the core level BEs for this system. The two nitrogen centers in the imidazole group of histidine exhibit an N 1 s BE shift that is in line with previously reported data for theophylline and aqueous imidazole solutions, while the nitrogen and carbon chemical shifts reflect the unusual guanidinium chemical environment in arginine. It is shown that the complex envelopes of C 1 s and O 1 s photoemission spectra for short‐chain peptides can be analyzed quantitatively by reference to the less complex XP spectra of the constituent amino acids, provided the peptides are of high enough purity. The distinctive N 1 s photoemission from the amide linkages provides an indicator of peptide formation even in the presence of common impurities, and variations in the relative intensities of N 1 s were found to be diagnostic for each of the three peptides investigated (RGD, RGDS, and RGDSC). Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   
44.
Highly enantioselective hydrogenation of β‐alkyl and β‐(ω‐chloroalkyl) substituted β‐keto esters was achieved with Ru catalysts based on chiral diphosphines in EtOH at 50°C under 50‐bar initial hydrogen pressure, affording the corresponding β‐hydroxy esters in >98% ee.  相似文献   
45.
46.
The methodology for ready-made matrix-assisted laser desorption/ionization (MALDI) target plates covered with an optimized thin layer consisting of matrix and nitrocellulose has been developed. Piezoelectric microdispensing enabled sample depositions in a high-density array format of 2000 sample depositions on a conventionally sized target plate (45 x 47 mm). The sample depositions were made reproducibly in a fully automated mode by using an in-house developed computer-controlled piezoelectric flow-through microdispenser. Additionally, the piezoelectric technique facilitated significant analyte enrichment that increased the detection sensitivity. The MS signal was obtained rapidly, generally within ten laser pulses. An airbrush device was used to generate a fine spray of matrix and nitrocellulose dissolved in acetone. The acetone evaporated instantly when reaching the target plate leaving the entire surface with a thin and uniform matrix/nitrocellulose coating consisting of very small crystals of matrix embedded in the nitrocellulose. These crystals acted as a seed-layer on subsequent analyte depositions, rendering homogeneous sample spots when using alpha-cyano-4-hydroxycinnamic acid (CHCA) as matrix. The relative standard deviation of the signal intensity between spots was (20-30)% (n = 30). The detection sensitivity was improved by restricting the sample spot diameter to 300 microm. The spot size was affected by the deposition rate and the evaporation rate of the dispensed sample volume. Mass spectra of a 25-amol peptide mixture deposition were successfully recorded.  相似文献   
47.
2,2-Dibutyl-2-stanna-1,3-dioxacycloalkanes were used as cyclic initiators for the ring-opening polymerization of various lactons. This method exclusively yielded series of macrocyclic polylactones without any competition with linear polymers. Under optimized reaction conditions these macrocyclic polymerizations obey the pattern of “living polymerizations”. The living chain ends allow the syntheses of macrocyclic blockcopolymers. The macrocyclic polylactones react with carboxylic acid chlorides by ring-opening yielding telechelic oligo or polylactones. Furthermore, the tin containing macrocyclic polylactones can be used as difunctional “monomers” for polycondensations with dicarboxylic acid dichlorides.  相似文献   
48.
High-temperature superconductivity exists in layered, square-planar cuprates, but is almost absent in most other Cu(II) compounds and in most Ag(II) and Au(II) compounds. Valence state II is quite unusual in silver and gold and often disproportionates to valence states I and III ("negative-U compounds"). The two-electron difference in oxidation state is suggestive of electron pairing, a prerequisite for superconductivity. In the present paper the connection between disproportionation and geometrical structure on one hand and superconductivity on the other is discussed by using the accepted theory for mixed valence complexes. It is concluded that absence of superconductivity in gold and silver compounds can be connected to the instability of oxidation state II and the large difference in equilibrium geometry between oxidation states I and III.  相似文献   
49.
The structural stability of hen egg white lysozyme in solution and adsorbed to small colloidal silica particles at various surface concentrations was investigated using hydrogen-deuterium (H/D) exchange in combination with mass spectrometry (HDX-MS) and differential scanning calorimetry (DSC). The combination of HDX-MS and DSC allows a full thermodynamic analysis of the lysozyme structure as both the enthalpy and the Gibbs free energy can be derived from the various measurements. Moreover, both HDX-MS and DSC provide information on the relative structural heterogeneity of lysozyme in the adsorbed state compared to that in solution. Results demonstrated that at high surface coverage, the structural stability of lysozyme was only marginally affected by adsorption to silica particles whereas the unfolding enthalpy decreased by more than 10%, meaning that the entropy of lysozyme increased with a similar value upon adsorption. Furthermore, the structural heterogeneity increased considerably. At lower surface concentrations, the structural heterogeneity increased further whereas the enthalpy of unfolding decreased. Further analyses of the HDX-MS experiments clearly indicated that folding/unfolding of lysozyme occurs through a two-domain process. These two domains had a similar amount of structural elements and a difference in stabilization energy of 8 kJ/mol, regardless if lysozyme was in solution or adsorbed to silica.  相似文献   
50.
Several new platinum monolayer protected clusters (MPCs) have been synthesized and characterized. Two methods of platinum reduction were used depending on the solubility of the thiol: sodium borohydride for the water-soluble thiols and lithium triethylborohydride for the organic soluble thiols. In general, reactant solutions containing a 1:1 thiol/Pt ratio yielded the best particles in a single-phase reaction. Higher thiol/Pt ratios produced lower yields of MPCs, while much lower ratios produced gray-black precipitates. The Pt MPCs were used as catalysts to hydrogenate allyl alcohol to propanol by reducing the carbon-carbon double bond. The Pt-mercaptoammonium MPCs were also used as catalysts in the hydrogenation of maleic acid to succinic acid. Differences in the catalytic hydrogenation rates among the various monolayer coatings for MPCs are attributed to the variations in ligand chain length, branching, charged functional groups, packing density, and core size.  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