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   237篇
  免费   12篇
  国内免费   2篇
化学   193篇
数学   25篇
物理学   33篇
  2024年   1篇
  2023年   5篇
  2022年   10篇
  2021年   15篇
  2020年   10篇
  2019年   9篇
  2018年   4篇
  2017年   7篇
  2016年   14篇
  2015年   8篇
  2014年   5篇
  2013年   14篇
  2012年   14篇
  2011年   19篇
  2010年   12篇
  2009年   10篇
  2008年   24篇
  2007年   20篇
  2006年   15篇
  2005年   9篇
  2004年   6篇
  2003年   4篇
  2002年   1篇
  2001年   2篇
  2000年   3篇
  1999年   1篇
  1998年   3篇
  1997年   1篇
  1996年   2篇
  1994年   1篇
  1992年   2篇
排序方式: 共有251条查询结果,搜索用时 31 毫秒
131.
We study the possibility of the existence of extra fermion families and an extra Higgs doublet. We find that requiring the extra Higgs doublet to be inert leaves space for three extra families, allowing for mirror fermion families and a dark matter candidate at the same time. The emerging scenario is very predictive: It consists of a standard model Higgs boson, with a mass above 400 GeV, heavy new quarks between 340 and 500 GeV, light extra neutral leptons, and an inert scalar with a mass below M(Z).  相似文献   
132.
The design of hybrid mesoporous materials incorporating polymeric assemblies as versatile functional units has become a very fertile research area offering major opportunities for controlling molecular transport through interfaces. However, the creation of such functional materials depends critically on our ability to assemble polymeric units in a predictable manner within mesopores with dimensions comparable to the size of the macromolecular blocks themselves. In this work, we describe for the first time the manipulation of the molecular transport properties of mesoporous silica thin films by the direct infiltration of polyelectrolytes into the inner environment of the 3D porous framework. The hybrid architectures were built up through the infiltration-electrostatic assembly of polyallylamine (PAH) on the mesopore silica walls, and the resulting systems were studied by a combination of experimental techniques including ellipso-porosimetry, cyclic voltammetry and X-ray photoelectron spectroscopy, among others. Our results show that the infiltration-assembly of PAH alters the intrinsic cation-permselective properties of mesoporous silica films, rendering them ion-permeable mesochannels and enabling the unrestricted diffusion of cationic and anionic species through the hybrid interfacial architecture. Contrary to what happens during the electrostatic assembly of PAH on planar silica films (quantitative charge reversal), the surface charge of the mesoporous walls is completely neutralized upon assembling the cationic PAH layer (i.e., no charge reversal occurs). We consider this work to have profound implications not only on the molecular design of multifunctional mesoporous thin films but also on understanding the predominant role of nanoconfinement effects in dictating the functional properties of polymer-inorganic hybrid nanomaterials.  相似文献   
133.
The nitroprusside ion [Fe(CN)(5)NO](2-) (NP) reacts with excess HS(-) in the pH range 8.5-12.5, in anaerobic medium ("Gmelin" reaction). The progress of the addition process of HS(-) into the bound NO(+) ligand was monitored by stopped-flow UV/Vis/EPR and FTIR spectroscopy, mass spectrometry, and chemical analysis. Theoretical calculations were employed for the characterization of the initial adducts and reaction intermediates. The shapes of the absorbance-time curves at 535-575 nm depend on the pH and concentration ratio of the reactants, R=[HS(-)]/[NP]. The initial adduct [Fe(CN)(5)N(O)SH](3-) (AH, λ(max) ≈570 nm) forms in the course of a reversible process, with k(ad)=190±20 M(-1)s(-1) , k(-ad)=0.3±0.05 s(-1) . Deprotonation of AH (pK(a)=10.5±0.1, at 25.0 °C, I=1 M), leads to [Fe(CN)(5)N(O)S](4-) (A, λ(max)=535 nm, ε=6000±300 M(-1) cm(-1) ). [Fe(CN)(5)NO](.)(3-) and HS(2)(.)(2-) radicals form through the spontaneous decomposition of AH and A. The minor formation of the [Fe(CN)(5)NO](3-) ion equilibrates with [Fe(CN)(4)NO](2-) through cyanide labilization, and generates the "g=2.03" iron-dinitrosyl, [Fe(NO)(2)(SH)(2)](-) , which is labile toward NO release. Alternative nucleophilic attack of HS(-) on AH and A generates the reactive intermediates [Fe(CN)(5)N(OH)(SH)(2)](3-) and [Fe(CN)(5)N(OH)(S)(SH)](4-) , respectively, which decompose through multielectronic nitrosyl reductions, leading to NH(3) and hydrogen disulfide, HS(2)(-) . N(2)O is also produced at pH≥11. Biological relevance relates to the role of NO, NO(-) , and other bound intermediates, eventually able to be released to the medium and rapidly trapped by substrates. Structure and reactivity comparisons of the new nitrososulfide ligands with free and bound nitrosothiolates are provided.  相似文献   
134.
Essential to (cardiac) 3D ultrasound are 2D matrix array transducer technology and the associated (two-stage) beam forming. Given the large number of degrees of freedom and the complexity of this problem, simulation tools play an important role. Hereto, the impulse response (IR) method is commonly used. Unfortunately, given the large element count of 2D array transducers, simulation times become significant jeopardizing the efficacy of the design process. The aim of this study was therefore to derive a new analytical expression to more efficiently calculate the IR in order to speed up the calculation process. To compare accuracy and computation time, the reference and the proposed method were implemented in MATLAB and contrasted. For all points of observation tested, the IR with both methods was identical. The mean calculation time however reduced in average by a factor of 3.93 ± 0.03 times. The proposed IR method therefore speeds up the calculation time of the IR of an individual transducer element while remaining perfectly accurate. This new expression will be particularly relevant for 2D matrix transducer design where computation times remain currently a bottle neck in the design process.  相似文献   
135.
About 95 % of people diagnosed with glioblastoma die within five years. Glioblastoma is the most aggressive central nervous system tumour. It is necessary to make progress in the glioblastoma treatment so that advanced chemotherapy drugs or radiation therapy or, ideally, two-in-one hybrid systems should be implemented. Tyrosine kinase receptors–inhibitors and boron neutron capture therapy (BNCT), together, could provide a therapeutic strategy. In this work, sunitinib decorated-carborane hybrids were prepared and biologically evaluated identifying excellent antitumoral- and BNCT-agents. One of the selected hybrids was studied against glioma-cells and found to be 4 times more cytotoxic than sunitinib and 1.7 times more effective than 10B-boronophenylalanine fructose complex when the cells were irradiated with neutrons.  相似文献   
136.

An extracellular laccase (Lacc10) was discovered in submerged cultures of Pleurotus ostreatus var. florida bleaching ß-carotene effectively without the addition of a mediator (650 mU/L, pH 4). Heterologous expression in P. pastoris confirmed the activity and structural analyses revealed a carotenoid-binding domain, which formed the substrate-binding pocket and is reported here for the first time. In order to increase activity, 106 basidiospore-derived monokaryons and crosses of compatible progenies were generated. These showed high intraspecific variability in growth rate and enzyme formation. Seventy-two homokaryons exhibited a higher activity-to-growth-rate-relation than the parental dikaryon, and one isolate produced a very high activity (1800 mU/L), while most of the dikaryotic hybrids showed lower activity. The analysis of the laccase gene of the monokaryons revealed two sequences differing in three amino acids, but the primary sequences gave no clue for the diversity of activity. The enzyme production in submerged cultures of monokaryons was stable over seven sub-cultivation cycles.

  相似文献   
137.
A family of transformations of the processes of accumulated residues of linear models is used to construct tests of fit of the models, consistent for any alternative, and focused on alternatives in the direction selected by the user. The resulting tests are asymptotically distribution free, both under the null hypothesis of fit, and under the selected alternatives. An interesting feature is that this distributions do not depend on (possible) parameter estimations.  相似文献   
138.
Current methods used in document examinations are not suitable to associate or discriminate between sources of paper and gel inks with a high degree of certainty. Nearly non-destructive, laser-based methods using laser induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were used to improve the forensic comparisons of gel inks, ballpoint inks and document papers based on similarities in elemental composition. Some of the advantages of these laser-based methods include minimum sample consumption/destruction, high sensitivity, high selectivity and excellent discrimination between samples from different origins. Figures of merit are reported including limits of detection, precision, homogeneity at a micro-scale and linear dynamic range. The variation of the elemental composition in paper was studied within a single sheet, between pages from the same ream, between papers produced by the same plant at different time intervals and between seventeen paper sources produced by ten different plants. The results show that elemental analysis of paper by LIBS and LA-ICP-MS provides excellent discrimination (> 98%) between different sources. Batches manufactured at weekly and monthly intervals in the same mill were also differentiated. The ink of more than 200 black pens was analyzed to determine the variation of the chemical composition of the ink within a single pen, between pens from the same package and between brands of gel inks and ballpoint inks. Homogeneity studies show smaller variation of elemental compositions within a single source than between different sources (i.e. brands and types). It was possible to discriminate between pen markings from different brands and between pen markings from the same brand but different model. Discrimination of ~ 96–99% was achieved for sets that otherwise would remain inseparable by conventional methods. The results show that elemental analysis, using either LA-ICP-MS or LIBS, provides an effective, practical and robust technique for the discrimination of document paper and gel inks with minimum mass removal (9–15 μg) and minimum damage to the document's substrate.  相似文献   
139.
Given a linear bounded selfadjoint operator a on a complex separable Hilbert space ${\mathcal{H}}Given a linear bounded selfadjoint operator a on a complex separable Hilbert space H{\mathcal{H}}, we study the decompositions of a as a difference of two positive operators whose ranges satisfy an angle condition. These decompositions are related to the canonical decompositions of the indefinite metric space (H,á ,  ?a){(\mathcal{H},\langle\,, \,\rangle_a)}, associated to a. As an application, we characterize the orbit of congruence of a in terms of its positive decompositions.  相似文献   
140.
The tetravalently stabilized fullerene cage of C28 is historically the most elusive small fullerene cage observed by employing the laser vaporization synthesis methodology. Its first observation reported by Smalley et al. in 1992 suggests that C28 is potentially the smallest and most stable fullerene ever observed. By using the Krätschmer?Huffman arc discharge synthesis method, we have recently succeeded in synthesizing a series of uranium‐endohedral fullerenes which differ from those reported by Smalley and co‐workers. Intrigued by this interesting mismatch, we tuned our experimental conditions to favor the formation and detection of these missing species. Experiments done using solvents of varying polarity allowed the observation of several empty and uranofullerenes. Extractions with pyridine and o‐DCB allowed for observation of small U@C2n (2n=28, 60, 66, 68, 70) by high resolution Fourier‐Transform Ion Cyclotron Resonance Mass Spectrometry (FT‐ICR MS). This is the first time that U@C28 is observed in soot produced by the Krätschmer‐Huffman arc‐discharge methodology. Carbon cage selection and spin density distribution on the endohedral metallofullerenes (EMFs) U@C60, U@C70, and U@C72 were studied by means of density functional theory (DFT) calculations. A plausible pathway for the formation of U@D3h‐C74 from U@D5h‐C70 through two C2 insertions and one Stone‐Wales rearrangement is proposed.  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