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   1333篇
  免费   39篇
  国内免费   2篇
化学   1133篇
晶体学   8篇
力学   15篇
数学   93篇
物理学   125篇
  2024年   1篇
  2023年   6篇
  2022年   51篇
  2021年   45篇
  2020年   19篇
  2019年   19篇
  2018年   22篇
  2017年   17篇
  2016年   46篇
  2015年   25篇
  2014年   59篇
  2013年   73篇
  2012年   81篇
  2011年   95篇
  2010年   87篇
  2009年   68篇
  2008年   94篇
  2007年   98篇
  2006年   86篇
  2005年   70篇
  2004年   58篇
  2003年   36篇
  2002年   48篇
  2001年   19篇
  2000年   15篇
  1999年   13篇
  1998年   8篇
  1997年   16篇
  1996年   13篇
  1995年   15篇
  1994年   7篇
  1993年   5篇
  1992年   1篇
  1991年   7篇
  1990年   8篇
  1989年   8篇
  1988年   4篇
  1986年   2篇
  1985年   5篇
  1984年   1篇
  1983年   2篇
  1982年   4篇
  1981年   3篇
  1980年   2篇
  1979年   2篇
  1978年   1篇
  1977年   2篇
  1976年   2篇
  1975年   4篇
  1971年   1篇
排序方式: 共有1374条查询结果,搜索用时 31 毫秒
241.
The PTV (Programmable Temperature Vaporizer) is a split/splitless injector which allows the sample to be introduced at a relatively low temperature, thus affording accurate and reproducible sampling. After injection the PTV is rapidly heated to transfer the vaporized components into the capillary column. This type of injector has proved to be an efficient tool for the evaluation of fatty acids, essential oils, and pesticides in food analysis. In this work the suitability of PTV for the analysis of milk fat purity by the Official EU method was evaluated. This method is based on the gas chromatographic determination of triglycerides only according to their total number of carbon atoms followed by the application of formulae deriving from multiple linear regressions. The analysis is currently carried out with a packed column or a short capillary column and an on-column injection system. Several samples of pure milk fat and mixtures of milk fat with foreign fat were analyzed with the same capillary column and by using both PTV and on-column injection systems. The results show that the gas chromatographic profile obtained by PTV is comparable with that obtained by the on-column injector, while repeatability and reproducibility data meet the requirements indicated in the Official Method. Therefore, this study demonstrates that it is possible to use the PTV instead of the on-column injector to determine the purity of milk fat with this method.  相似文献   
242.
Metallo‐supramolecular chemistry offers possibilities for the construction of stimuli‐responsive polymeric materials where the environment can have a large impact on the reversibility and strength of interactions between the individual components. The potential of manipulating the strength of the intermolecular non‐covalent bonds can result in impressive modifications of the metallo‐supramolecular structure and, subsequently, produces changes in the properties of the designed material. The present feature article provides an overview on recent developments in the field of metallo‐polymerization of chelating terpyridyl and analogues ligands. Synthetic strategies are described followed by a discussion regarding the characterization and the application of the reviewed metallo‐supramolecular structures, mainly based on terpyridines.

  相似文献   

243.
It is experimentally shown that a single ionization electron can be detected by various xenon-based electroluminescent detectors with optical readout, such as a high-pressure cylindrical gas counter, a two-phase detector with gas amplification, and a two-phase detector with a plane-parallel and uniform electric field. The sensitivity of the detectors was 7, 18, and 15 photoelectrons respectively. The coordinate resolution of the two-phase detector for electrons resulting from field emission at the cathode was σ X,Y = 5.2 mm.  相似文献   
244.
R. Arnaldi  K. Banicz  K. Borer  J. Castor  B. Chaurand  W. Chen  C. Cicalò  A. Colla  P. Cortese  S. Damjanovic  A. David  A. de Falco  A. Devaux  L. Ducroux  H. En’yo  J. Fargeix  A. Ferretti  M. Floris  A. F?rster  P. Force  N. Guettet  A. Guichard  H. Gulkanyan  J. Heuser  M. Keil  L. Kluberg  Z. Li  C. Louren?o  J. Lozano  F. Manso  P. Martins  A. Masoni  A. Neves  H. Ohnishi  C. Oppedisano  P. Parracho  P. Pillot  T. Poghosyan  G. Puddu  E. Radermacher  P. Ramalhete  P. Rosinsky  E. Scomparin  J. Seixas  S. Serci  R. Shahoyan  P. Sonderegger  H. J. Specht  R. Tieulent  A. Uras  G. Usai  R. Veenhof  H. K. W?hri 《The European Physical Journal C - Particles and Fields》2009,64(1):1-18
The NA60 experiment has measured muon pair production in In–In collisions at 158 AGeV at the CERN SPS. This paper presents a high statistics measurement of φμ μ meson production. Differential spectra, yields, mass and width are measured as a function of centrality and compared to previous measurements in other colliding systems at the same energy. The width of the rapidity distribution is found to be constant as a function of centrality, compatible with previous results. The decay muon polar angle distribution is measured in several reference frames. No evidence of polarization is found as a function of transverse momentum and centrality. The analysis of the p T spectra shows that the φ has a small radial flow, implying a weak coupling to the medium. The T eff parameter measured in In–In collisions suggests that the high value observed in Pb–Pb in the kaon channel is difficult to reconcile with radial flow alone. The absolute yield is compared to results in Pb–Pb collisions: though significantly smaller than measured by NA50 in the muon channel, it is found to exceed the NA49 and CERES data in the kaon channel at any centrality. The mass and width are found to be compatible with the PDG values at any centrality and at any p T : no evidence for in-medium modifications is observed.  相似文献   
245.
The title compound, [Hf(C11H23N2)2Cl2], is a monomeric hafnium(IV) complex containing two bidentate amidinate ligands and two cis Cl atoms. The crystals are triclinic (space group ) and there is one independent six‐coordinate monomer with a highly distorted octa­hedral geometry in the asymmetric unit. The reported structure is the first hafnium–amidinate complex to be characterized successfully by single‐crystal X‐ray diffraction.  相似文献   
246.
A general and highly efficient synthetic protocol under phase transfer catalytic condition has been established for the synthesis of fused tetracyclic oxazocinoquinolone analogues which served as the precursors for novel biaryl quinolones using microwave assisted Suzuki cross coupling reaction.  相似文献   
247.
In this work, a non-chromatographic procedure for the on-line determination of ultratraces of V(V) and V(IV) is presented. The method involves a solid phase extraction-flow injection system coupled to electrothermal atomic absorption spectrometry (SPE-FI-ETAAS). The system holds two microcolumns (MC) set in parallel and filled with lab-made mesoporous silica functionalized with 3-aminopropyltriethoxy silane (APS) and mesoporous silica MCM-41, respectively. The pre-concentration of V(V) is performed by sorption onto the first MC (C1) filled with APS at pH 3, whilst that of V(IV) is performed by sorption onto the second column (C2) filled with mesoporous silica MCM-41 at pH 5. Aqueous samples containing both analytes are loaded and, after pre-concentration (pre-concentration factor PCF = 10, sorption flow rate = 1 mL min−1, sorption time = 10 min), they are eluted in separate vessels with hydroxylammonium chloride (HC) 0.1 mol L−1 in HCl 0.5 mol L−1 (elution volume = 1 mL, elution flow rate = 0.5 mL min−1). Afterwards, both analytes are determined through ETAAS with graphite furnace. Under optimized conditions, the main analytical figures of merit for V(V) and V(IV) are, respectively: detection limits (3 s): 0.5 and 0.6 μg L−1, linear range: 2-100 μg L−1 (both analytes), sensitivity: 0.015 and 0.013 μg−1 L and sample throughput: 6 h−1 (both analytes). Recoveries of both species were assayed in different water samples. Validation was performed through certified reference materials for ultratraces of total vanadium in river water.  相似文献   
248.
Leading light : A series of zinc(II) bis‐terpyridine complexes (see picture) is investigated by means of DFT calculations combined with Bader's quantum theory of atoms in molecules. Raman spectroscopy experiments and studies of the electro‐optical properties of the complexes in solution and the solid state are also performed to examine their potential as new emissive materials in light‐emitting devices.

  相似文献   

249.
Blocking is an important step before an enzyme-linked immunosorbent assay (ELISA) can be performed. It reduces non-specific binding to the microtiter plate to a minimum. For detecting food allergens by means of ELISA, the problem with protein blocking solutions is obvious. The blocker might interfere with the antibodies of the assay and leads to false positive results. Therefore, other blocking solutions are greatly needed. There are some alternatives like synthetic blockers or carbohydrates. Comparisons of these different blocking agents, namely proteins, carbohydrates, and synthetic blockers, were made at different reaction conditions. The incubation periods and temperatures were varied, as well as the pH. The best combinations were evaluated and compared, in respect of their blocking efficiency. The two best non-proteinaceous blockers, i.e. polyvinylalcohol and Ficoll, were subsequently applied to ELISA tests for the determination of α-casein and peanut. The study showed that Ficoll and PVA did as well as BSA in buffer solution. Therefore, they can be considered as alternative blocking reagents for ELISA, especially for the detection of food allergens.  相似文献   
250.
A rapid and simple short-end injection capillary zone electrophoresis method was developed for the quantification of plasma uric acid. The separation was performed in an uncoated fused-silica capillary (50 μm ID, 60 cm total length, 10.2 cm effective length) by using as a background electrolyte a 75 mmol/L glycylglycine solution titrated with NaOH 5 mol/L to pH 9.0, a voltage of 28 kV, a cartridge temperature of 15 °C, and direct UV detection at 292 nm. Under optimized conditions, uric acid was determinate in little more than 1 min (1.076 minutes). In order to verify the accuracy of the analysis, urate levels were measured in 543 apparently healthy volunteers by the new assay and our previous method, and the obtained data were compared by Passing–Bablock regression, Bland–Altman test, and a new regression-based approach, which showed a good agreement between two methods.  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