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   769篇
  免费   58篇
  国内免费   14篇
化学   534篇
晶体学   8篇
力学   33篇
数学   113篇
物理学   153篇
  2023年   4篇
  2022年   26篇
  2021年   41篇
  2020年   37篇
  2019年   47篇
  2018年   53篇
  2017年   55篇
  2016年   69篇
  2015年   37篇
  2014年   39篇
  2013年   89篇
  2012年   78篇
  2011年   72篇
  2010年   42篇
  2009年   32篇
  2008年   18篇
  2007年   28篇
  2006年   17篇
  2005年   9篇
  2004年   14篇
  2003年   6篇
  2002年   4篇
  2001年   3篇
  2000年   2篇
  1999年   1篇
  1997年   2篇
  1996年   2篇
  1994年   2篇
  1992年   1篇
  1989年   2篇
  1988年   2篇
  1987年   1篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1979年   2篇
排序方式: 共有841条查询结果,搜索用时 31 毫秒
91.
92.
93.
94.
Flat plates, both single and in tandem or side by side arrangement, are widely used in many engineering applications. Despite vast investigations of the flow structures and wakes downstream of these bluff bodies, this unsteady phenomenon yet remains a fundamental issue in many industrial applications. This paper reviews the state of the art concerning the flow over flat plates in different arrangements focusing on plates normal to the flow. Turbulent wake regions are discussed for the flat plates in side by side or tandem arrangement. Numerical studies are reviewed with emphasis on the realized turbulent models. The effect of the chosen turbulence model on the prediction of the wake region is discussed.  相似文献   
95.
Nonlinear Dynamics - In this paper, a numerical simulation of an anomalous reaction–diffusion process in two-dimensional space with a nonlinear source term is presented. An efficient and...  相似文献   
96.
A multicomponent electrochemical sensor, with two nanometer-scale components in sensing matrix/electrode, was used to simultaneous determination of levodopa (LD) and tyramine (TR) in pharmaceutical and diet samples. Multiwall carbon nanotubes (MWCNTs) were used as carbonaceous materials in the electrode construction. 5-amino-3',4'-dimethoxy-biphenyl-2-ol (5ADMB) was used as electron mediator and Pt nanoparticles (nPt) as a catalyst. The 5ADMB catalyzes the oxidation of LD to the corresponding catecholamine, which is electrochemically reduced back to LD. Preparation of this electrode was very simple and modified electrode showed good properties at electrocatalytic oxidization of LD and TR. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of LD and TR has been explored at the modified electrode. Differential pulse voltammetry peak currents of LD and TR increased linearly with their concentrations at the ranges of 0.50–100.0 μM and 0.60–100.0 μM, respectively. Also, the detection limits for LD and TR were 0.31 and 0.52 μM, respectively. The electrode exhibited an efficient catalytic response with good reproducibility and stability.  相似文献   
97.
Herein we report a fluorescent method for sensitive and selective detection of uranyl ions using CdTe quantum dots functionalized with mercaptopropionic acid, which the fluorescence of the quantum dots could be quantitatively quenched through electron transfer mechanism. The detection limit of the method was estimated to be 4 nM, less than the maximum allowed content of 130 nM for uranyl in drinking water defined by the U.S. Environmental Protection Agency. Furthermore, the probe was successfully applied in detection of uranyl ions in real samples, demonstrating its potential practical applications for monitoring of uranyl ions in environment.  相似文献   
98.

In this study, the corrosion behavior of Zircaloy was investigated in the presence and absence of copper–graphene nanocomposites coating. The coating was prepared employing Hummers’ and electrochemical reduction methods. The morphology of copper–graphene nanocomposites coating was studied using scanning electron microscopy (SEM). Corrosion behavior was investigated employing dynamic polarization and electrochemical impedance spectroscopy (EIS) tests in a solution containing lithium hydroxide (LiOH), boric acid (H3BO3), and deionized water. The results showed that corrosion resistance of Zircaloy increased with introduction of copper–graphene nanocomposites coatings. The lowest corrosion rate was attained in the Zircaloy with copper–graphene nanocomposites coating (corrosion rate: 0.040 mm/year). An approximately 20 times decrease in the corrosion rate was observed in the Zircaloy with copper–graphene nanocomposites coating when compared to the un-coated Zircaloy (corrosion rate: 0.831 mm/year).

  相似文献   
99.
We investigated geometry, energy, ${\nu_{{\text{N--H}}}}$ harmonic frequencies, 14N nuclear quadrupole coupling tensors, and ${n_{\rm O}\to \sigma _{{\text{N--H}}}^\ast}$ charge transfer properties of (acetamide) n clusters, with n = 1 ? 7, by means of second-order Møller-Plesset perturbation theory (MP2) and DFT method. Dependency of dimer stabilization energies and equilibrium geometries on various levels of theory was examined. B3LYP/6-311++G** calculations revealed that for acetamide clusters, the average hydrogen-bonding energy per monomer increases from ?26.85 kJ mol?1 in dimer to ?35.12 kJ mol?1 in heptamer; i.e., 31% cooperativity enhancement. The n-dependent trend of ${\nu_{{\text{N--H}}}\,{and}\,^{14}}$ N nuclear quadrupole coupling values were reasonably correlated with cooperative effects in ${r_{{\text{N--H}}}}$ bond distance. It was also found that intermolecular ${n_{\rm O}\to \sigma_{{\text{N--H}}}^\ast}$ charge transfer plays a key role in cooperative changes of geometry, binding energy, ${\nu_{{\text{N--H}}}}$ harmonic frequencies, and 14N electric field gradient tensors of acetamide clusters. There is a good linear correlation between 14N quadrupole coupling constants, C Q (14N), and the strength of Fock matrix elements (F ij ). Regarding the ${n_{\rm O}\to \sigma_{{\text{N--H}}}^\ast}$ interaction, the capability of the acetamide clusters for electron localization, at the N–H· · ·O bond critical point, depends on the cluster size and thereby leads to cooperative changes in the N–H· · ·O length and strength, N–H stretching frequencies, and 14N quadrupole coupling tensors.  相似文献   
100.
Yazdi AS  Razavi N  Yazdinejad SR 《Talanta》2008,75(5):1293-1299
Dispersive liquid–liquid microextraction (DLLME) coupled with gas chromatography–flame ionization detection (GC–FID) was applied for the determination of two tricyclic antidepressant drugs (TCAs), amitriptyline and nortriptyline, from water samples. This method is a very simple and rapid method for the extraction and preconcentration of these drugs from environmental sample solutions. In this method, the appropriate mixture of extraction solvent (18 μL Carbon tetrachloride) and disperser solvent (1 mL methanol) are injected rapidly into the aqueous sample (5.0 mL) by syringe. Therefore, cloudy solution is formed. In fact, it is consisted of fine particles of extraction solvent which is dispersed entirely into aqueous phase. The mixture was centrifuged and the extraction solvent is sedimented on the bottom of the conical test tube. 2.0 μL of the sedimented phase is injected into the GC for separation and determination of TCAs. Some important parameters, such as kind of extraction and disperser solvent and volume of them, extraction time, pH and ionic strength of the aqueous feed solution were optimized. Under the optimal conditions, the enrichment factors and extraction recoveries were between 740.04–1000.25 and 54.76–74.02%, respectively. The linear range was (0.005–16 μg mL−1) and limits of detection were between 0.005 and 0.01 μg mL−1 for each of the analytes. The relative standard deviations (R.S.D.) for 4 μg mL−1 of TCAs in water were in the range of 5.6–6.4 (n = 6). The performance of the proposed technique was evaluated for determination of TCAs in blood plasma.  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