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   1287篇
  免费   31篇
  国内免费   9篇
化学   808篇
晶体学   7篇
力学   88篇
数学   148篇
物理学   276篇
  2022年   26篇
  2021年   19篇
  2020年   7篇
  2019年   17篇
  2018年   7篇
  2017年   6篇
  2016年   25篇
  2015年   15篇
  2014年   33篇
  2013年   54篇
  2012年   72篇
  2011年   92篇
  2010年   39篇
  2009年   62篇
  2008年   79篇
  2007年   61篇
  2006年   71篇
  2005年   67篇
  2004年   35篇
  2003年   44篇
  2002年   39篇
  2001年   27篇
  2000年   31篇
  1999年   14篇
  1998年   10篇
  1997年   15篇
  1996年   33篇
  1995年   17篇
  1994年   15篇
  1993年   23篇
  1992年   21篇
  1991年   23篇
  1990年   15篇
  1989年   14篇
  1988年   15篇
  1987年   8篇
  1986年   13篇
  1985年   21篇
  1984年   12篇
  1983年   10篇
  1982年   15篇
  1981年   9篇
  1980年   9篇
  1978年   7篇
  1977年   24篇
  1976年   11篇
  1975年   5篇
  1974年   7篇
  1973年   6篇
  1972年   5篇
排序方式: 共有1327条查询结果,搜索用时 15 毫秒
51.
The first excited state of137Ba has been excited by the inelastic scattering of accelerator-produced neutrons. The energy of this state at 283.5 keV is not in agreement with the generally accepted value of 279.2 keV, but is in accord with other recent measurements. No evidence for a doublet of states near this energy is found.  相似文献   
52.
With the accelerated accumulation of genomic sequence data, there is a pressing need to develop computational methods and advanced bioinformatics infrastructure for reliable and large-scale protein annotation and biological knowledge discovery. The Protein Information Resource (PIR) provides an integrated public resource of protein informatics to support genomic and proteomic research. PIR produces the Protein Sequence Database of functionally annotated protein sequences. The annotation problems are addressed by a classification-driven and rule-based method with evidence attribution, coupled with an integrated knowledge base system being developed. The approach allows sensitive identification, consistent and rich annotation, and systematic detection of annotation errors, as well as distinction of experimentally verified and computationally predicted features. The knowledge base consists of two new databases, sequence analysis tools, and graphical interfaces. PIR-NREF, a non-redundant reference database, provides a timely and comprehensive collection of all protein sequences, totaling more than 1,000,000 entries. iProClass, an integrated database of protein family, function, and structure information, provides extensive value-added features for about 830,000 proteins with rich links to over 50 molecular databases. This paper describes our approach to protein functional annotation with case studies and examines common identification errors. It also illustrates that data integration in PIR supports exploration of protein relationships and may reveal protein functional associations beyond sequence homology.  相似文献   
53.
Intermolecular interactions drive the vast majority of condensed phase phenomena from molecular recognition to protein folding to particle adhesion. Complex energy barriers encountered in these interactions include contributions from van der Waals forces, hydrogen bonding, and solvent medium. With the spectacular exception of hydrophobic interactions, contributions from the medium are usually considered secondary. We report a variable temperature force microscopy study of the interactions between several hydrogen bonds in different solvents that challenges this point of view. Surprisingly, we observed an increase in the strength of the interaction between carboxylic acid groups in ethanol as the temperature increased. Moreover, when we switched to a nonpolar solvent we observed the opposite behavior: The binding force decreased as the temperature increased. Kinetic model of bond dissociation provided quantitative interpretation of our measurements. We attributed the observed phenomena to a large entropic contribution from the ordered solvent layers that are forming on the probe and sample surfaces upon detachment. The observed reversal in the force vs temperature trend is a manifestation of a transition between thermodynamic and kinetic regimes of unbinding predicted by the model. Our results indicate that entropic barriers dominated by the interactions of solvent molecules with the surface exist in a much wider variety of systems than previously thought.  相似文献   
54.
A 10 w/o Pd/C catalyst from Johnson Matthey has been examined by three different techniques: chemisorption, line broadening of X-ray diffraction, and TEM (transmission electron microscopy) to investigate the size of the palladium crystallites. The dispersion of this catalyst was determined to be 20%. Among these techniques, the X-ray diffraction was found to be the most convenient method. Through this method, the dispersion of Pd was found to decrease on catalyzing hydroxylamine reactions at 330 K. The deactivation of Pd/C for these reactions was correlated to the sintering of palladium. H2 chemisorption at 373 K was found to be a good way to accurately measure the number of active palladium sites in the Pd/C catalysts.  相似文献   
55.
The water extract of Gracilaria tenuistipitata have been found to be protective against oxidative stress-induced cellular DNA damage, but the biological function of the ethanolic extracts of G. tenuistipitata (EEGT) is still unknown. In this study, the effect of EEGT on oral squamous cell cancer (OSCC) Ca9-22 cell line was examined in terms of the cell proliferation and oxidative stress responses. The cell viability of EEGT-treated OSCC cells was significantly reduced in a dose-response manner (p < 0.0001). The annexin V intensity and pan-caspase activity of EEGT-treated OSCC cells were significantly increased in a dose-response manner (p < 0.05 to 0.0001). EEGT significantly increased the reactive oxygen species (ROS) level (p < 0.0001) and decreased the glutathione (GSH) level (p < 0.01) in a dose-response manner. The mitochondrial membrane potential (MMP) of EEGT-treated OSCC cells was significantly decreased in a dose-response manner (p < 0.005). In conclusion, we have demonstrated that EEGT induced the growth inhibition and apoptosis of OSCC cells, which was accompanied by ROS increase, GSH depletion, caspase activation, and mitochondrial depolarization. Therefore, EEGT may have potent antitumor effect against oral cancer cells.  相似文献   
56.
The influence of electroosmotic flow (EOF) on the electrophoretic behavior of a particle is investigated by considering a rigid sphere in a charge-regulated, zwitterionic cylindrical pore filled with an aqueous solution containing multiple ionic species. This extends conventional analyses to a more general and realistic case. Taking a pore with pK(a) = 7 and pK(b) = 2 (point of zero charge is pH = 2.5) filled with an aqueous NaCl solution as an example, several interesting results are observed. For instance, if pH < 5.5, the particle mobility is influenced mainly by boundary effect, and is influenced by both EOF and boundary effects if pH ≥ 5.5. If pH is sufficiently high, the particle behavior is dominated by EOF, which might alter the direction of electrophoresis. The ratio of (pore radius/particle radius) influences not only the boundary effect, but also the strength of EOF. If the boundary effect is insignificant, the mobility varies roughly linearly with log(bulk salt concentration). These findings are of practical significance to both the interpretation of experimental data and the design of electrophoresis devices.  相似文献   
57.
This study incorporates morphine into a molecularly imprinted polymer (MIP) for the amperometric detection of morphine. The polymer, poly(3,4-ethylenedioxythiophene), PEDOT, is an electroactive film that catalyzes morphine oxidation and lowers the oxidization potential on an indium tin oxide (ITO) electrode. The MIP-PEDOT modified electrode is prepared by electropolymerizing PEDOT onto an ITO electrode in a 0.1 M LiClO4 solution with template addition (morphine). After template molecule extraction, the oxidizing current of the MIP-PEDOT modified electrode is measured in a 0.1 M KCl solution (pH = 5.3) at 0.75 V (versus Ag/AgCl/sat’d KCl) with the morphine concentration varying in the 0.1-5 mM range. A linear range, displaying the relationship between steady-state currents and morphine concentrations, from 0.1 to 1 mM, is obtained. The proposed amperometric sensor could be used for morphine detection with a sensitivity of 91.86 μA/cm2 per mM. A detection limit of 0.2 mM at a signal-to-noise ratio of 3 is achieved. Moreover, the proposed method can discriminate between morphine and its analogs, such as codeine.  相似文献   
58.
Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene- and -1,3,5-triene-tethered nitrile oxides gave tricyclic isoxazolines as a single stereoisomer in most cases. The relative stereochemistry of tricycle-fused isoxazolines resulting from 1,3-dipolar cycloaddition of cyclo-1,3-diene-tethered nitrile oxides is cis-cis, whereas from cyclohepta-1,3,5-triene-tethered nitrile oxides the cis-trans isomer predominates.  相似文献   
59.
60.
Rapid solvent‐free microwave‐assisted headspace solid‐phase microextraction (MA‐HS‐SPME) coupled with gas chromatography‐mass spectrometry (GC‐MS) was developed to determine synthetic polycyclic and nitro‐aromatic musks in fish samples. Four commonly used synthetic musks, galaxolide (HHCB), tonalide (AHTN), musk xylene (MX) and musk ketone (MK) were employed in the method development and validation. The parameters (microwave irradiation time, irradiation power, amount of water addition, pH value and addition of NaCl) affecting the extraction efficiency of analytes from fish slurry were systematically investigated and optimized. The best extraction conditions were achieved when the fish sample 2‐g mixed with 4‐mL methanol and 15‐mL deionized water (containing 4 g of NaCl, pH 2.0 in a 40‐mL sample‐vial) was microwave irradiated at 80 watt for 5 min. The limits of quantification (LOQ) were 0.4 to 1.2 ng/g in 2‐g of wet tissue. The precision for these analytes, as indicated by relative standard deviations, were less than 9% for both intra‐ and inter‐day analysis. Accuracy, expressed as the mean extraction recovery, was between 80 to 92%. A standard addition method was used to quantitate these four synthetic musks, and the total concentrations ranged from 2.1 to 23.1 ng/g in various fish samples.  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