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   5113篇
  免费   169篇
  国内免费   40篇
化学   3573篇
晶体学   54篇
力学   180篇
数学   432篇
物理学   1083篇
  2024年   23篇
  2023年   44篇
  2022年   329篇
  2021年   288篇
  2020年   178篇
  2019年   169篇
  2018年   212篇
  2017年   143篇
  2016年   218篇
  2015年   199篇
  2014年   213篇
  2013年   365篇
  2012年   347篇
  2011年   371篇
  2010年   213篇
  2009年   175篇
  2008年   172篇
  2007年   191篇
  2006年   142篇
  2005年   149篇
  2004年   91篇
  2003年   88篇
  2002年   76篇
  2001年   75篇
  2000年   56篇
  1999年   58篇
  1998年   28篇
  1997年   29篇
  1996年   44篇
  1995年   40篇
  1994年   31篇
  1993年   35篇
  1992年   34篇
  1991年   42篇
  1990年   20篇
  1989年   21篇
  1988年   30篇
  1987年   29篇
  1986年   28篇
  1985年   41篇
  1984年   24篇
  1983年   24篇
  1982年   22篇
  1981年   15篇
  1980年   30篇
  1979年   21篇
  1978年   27篇
  1977年   28篇
  1976年   18篇
  1973年   11篇
排序方式: 共有5322条查询结果,搜索用时 15 毫秒
991.
In this article, a deterministic model is formulated to perform a thorough investigation of the transmission dynamics of influenza. In particular, our model takes into account the effects of medication as well as hospitalization. An in-depth stability analysis of the model is performed, and it is subsequently shown that the model is locally, as well as globally asymptotically stable, when R0 > 1. It is also shown that there exists a unique endemic equilibrium whenever R0 > 1. After estimating the effective contact rate, we estimate the basic reproduction number, using both an ordinary least squares and generalized least squares methodology. We also estimated confidence intervals for the effective contact rate using parametric bootstrapping. Furthermore, we perform uncertainty and sensitivity analysis to recognize the impact of crucial model parameters on R0. In addition, using ideas from the optimal control theory, optimal medication and hospitalization strategies are proposed to eliminate the disease.  相似文献   
992.
Optical emission spectroscopy and Langmuir probe are used to investigate the low pressure inductively coupled Ar–N2 plasmas as function of rf power, filling pressure and Ar content in N2 discharge. It is observed that the active species generation, dissociation fraction and electron temperature significantly depends on discharge parameters and may be used to optimize the plasma reactor. Mixture of SWCNTs and MWCNTs are treated for different treatment time (0–120 min) at optimum discharge conditions. Changes induced in the elemental composition, surface morphology, crystallographic structure, and structural disorder in the plasma irradiated CNTs are analyzed by EDX, FTIR, SEM, XRD and Raman spectroscopy, respectively. Ar–N2 mixture plasma treatment of CNTs lead to a significant increase in the electrical conductivity, modify the microstructure and induce structural disorder and cause a transition of crystalline phase from well crystalline to an amorphous structure.  相似文献   
993.
Palm oil is an edible vegetable oil derived from lipid‐rich fleshy mesocarp tissue of oil palm (Elaeis guineensis Jacq.) fruit and is of global economic and nutritional relevance. While the understanding of oil biosynthesis in plants is improving, the fundamentals of oil biosynthesis in oil palm still require further investigations. To gain insight into the systemic mechanisms that govern oil synthesis during oil palm fruit ripening, the proteomics approach combining gel‐based electrophoresis and mass spectrometry was used to profile protein changes and classify the patterns of protein accumulation during these complex physiological processes. Protein profiles from different stages of fruit ripening at 10, 12, 14, 15, 16, 18 and 20 weeks after anthesis (WAA) were analysed by two‐dimensional gel electrophoresis (2DE). The proteome data were then visualised using a multivariate statistical analysis of principal component analysis (PCA) to get an overview of the proteome changes during the development of oil palm mesocarp. A total of 68 differentially expressed protein spots were successfully identified by matrix‐assisted laser desorption/ionisation‐time of flight (MALDI‐TOF/TOF) and functionally classified using ontology analysis. Proteins related to lipid production, energy, secondary metabolites and amino acid metabolism are the most significantly changed proteins during fruit development representing potential candidates for oil yield improvement endeavors. Data are available via ProteomeXchange with identifier PXD009579. This study provides important proteome information for protein regulation during oil palm fruit ripening and oil synthesis.  相似文献   
994.
995.
Research on Chemical Intermediates - Hydrogen peroxide (H2O2) is commercially produced by catalytic oxidation of anthrahydroquinone, which is energy-intensive. Electrochemical production of...  相似文献   
996.
Polycrystalline bulk ferromagnetic insulating (FMI) Ln0.85Ca0.15MnO3 (Ln=Nd, Pr and Sm) samples are prepared by standard solid-state reaction route and characterized. Powder X-ray diffraction (XRD) data of the manganites show single-phase character. Existing theoretical models predict that the high temperature (T>θD/2, θD being the Debye temperature) dc conductivity (σdc) of these samples is due to adiabatic small polaron-hopping conduction. Greaves’ and Mott's variable range hopping (VRH) conduction mechanisms are not suitable to explain the σdc data at low temperature (T<θD/2).  相似文献   
997.
The gravitational instability of an infinitely extending homogenous plasma endowed with several physical mechanisms, namely Hall currents, finite conductivity, ion viscosity and thermal conductivity is considered. The main result is that the various parameters play different physical roles in the perturbed problem. Jeans' criterion is analyzed in the framework of Tsallis' statistics for possible modifications due to the presence of nonextensive effects. A simple generalization of the Jeans' criterion is obtained and the standard values are obtained in the limiting case q=1, q being the nonextensive parameter.  相似文献   
998.
The relationship between the diffusivity D n and the mobility μ n of chemically doped organic n-type semiconductors exhibiting a disordered band structure is presented. These semiconductors have a Gaussian-type density of states. So, calculations have been performed to elucidate the dependence of D n /μ n on the various parameters of this Gaussian density of states. Y. Roichman and N. Tessler (Appl. Phys. Lett. 80:1948, 2002), and subsequently Peng et al. (Appl. Phys. A 86:225, 2007), conducted numerical simulations to study this diffusivity–mobility relationship in organic semiconductors. However, almost all other previous studies of the diffusivity–mobility relationship for inorganic semiconductors are based on Fermi–Dirac integrals. An analytical formulation has therefore been developed for the diffusivity/mobility relationship for organic semiconductors based on Fermi–Dirac integrals. The D n /μ n relationship is general enough to be applicable to both non-degenerate and degenerate organic semiconductors. It may be an important tool to study electrical transport in these semiconductors.  相似文献   
999.
In this study six unsymmetrical thiourea derivatives, 1-isobutyl-3-cyclohexylthiourea (1), 1-tert-butyl-3-cyclohexylthiourea (2), 1-(3-chlorophenyl)-3-cyclohexylthiourea (3), 1-(1,1-dibutyl)-3-phenylthiourea (4), 1-(2-chlorophenyl)-3-phenylthiourea (5) and 1-(4-chlorophenyl)-3-phenylthiourea (6) were obtained in the laboratory under aerobic conditions. Compounds 3 and 4 are crystalline and their structure was determined for their single crystal. Compounds 3 is monoclinic system with space group P21/n while compound 4 is trigonal, space group R3:H. Compounds (1–6) were tested for their anti-cholinesterase activity against acetylcholinesterase and butyrylcholinesterase (hereafter abbreviated as, AChE and BChE, respectively). Potentials (all compounds) as sensing probes for determination of deadly toxic metal (mercury) using spectrofluorimetric technique were also investigated. Compound 3 exhibited better enzyme inhibition IC50 values of 50, and 60 µg/mL against AChE and BChE with docking score of −10.01, and −8.04 kJ/mol, respectively. The compound also showed moderate sensitivity during fluorescence studies.  相似文献   
1000.
Pyrazol-l'-ylpyridines undergo electrophilic substitution reactions (bromination, chlorination, and nitration) preferentially in the pyrazole ring. There is some evidence of the mutual influence of the pyrazole and the pyridine ring on the reactivity of the system. Some modifications of the substituents were also carried out. A dihydro derivative of a new ring system, pyrazolo[1′,2′-a]pyrido[2,1-c][l,2,4]triazine was also obtained.  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