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   28946篇
  免费   3834篇
  国内免费   2107篇
化学   5725篇
晶体学   98篇
力学   4850篇
综合类   496篇
数学   11408篇
物理学   12310篇
  2024年   50篇
  2023年   280篇
  2022年   585篇
  2021年   804篇
  2020年   909篇
  2019年   794篇
  2018年   749篇
  2017年   1051篇
  2016年   1240篇
  2015年   908篇
  2014年   1552篇
  2013年   2282篇
  2012年   1633篇
  2011年   1908篇
  2010年   1643篇
  2009年   1994篇
  2008年   1840篇
  2007年   1865篇
  2006年   1532篇
  2005年   1363篇
  2004年   1258篇
  2003年   1061篇
  2002年   1009篇
  2001年   799篇
  2000年   758篇
  1999年   665篇
  1998年   611篇
  1997年   455篇
  1996年   401篇
  1995年   375篇
  1994年   345篇
  1993年   287篇
  1992年   268篇
  1991年   197篇
  1990年   173篇
  1989年   134篇
  1988年   127篇
  1987年   134篇
  1986年   110篇
  1985年   133篇
  1984年   136篇
  1983年   78篇
  1982年   103篇
  1981年   68篇
  1980年   32篇
  1979年   39篇
  1978年   31篇
  1977年   28篇
  1976年   16篇
  1973年   22篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
We study the well-posedness of a system of one-dimensional partial differential equations modeling blood flows in a network of vessels with viscoelastic walls. We prove the existence and uniqueness of maximal strong solution for this type of hyperbolic/parabolic model. We also prove a stability estimate under suitable nonlinear Robin boundary conditions.  相似文献   
992.
The catalytic hydrolysis of bis(4‐nitrophenyl)phosphate (BNPP) and bis(2,4‐dinitrophenyl)phosphate (BDNPP) catalyzed by α‐nucleophiles in gemini micellar media was investigated at 27 °C. The cationic gemini surfactants, i.e., alkanediyl‐α‐ω‐bis(hydroxyethylmethylhexadecylammonium bromide) (16‐s‐16 MEA 2Br?, where s = 4 and 6) were used. Nucleophilic reactivity of α‐nucleophiles such as hydroperoxide (HOO?), acetohydroxamate (AHA?), and butane 2,3‐dione monoximate ions (BDMO?) were compared. The kinetic rate data were treated by applying the pseudophase model. The cationic gemini surfactants show unusual rate acceleration toward the cleavage of phosphodiesters with nucleophiles. These studies reveal that the hydroperoxide ion shows the highest catalytic activity reported so far with an unprecedented acceleration rate, 107 times faster than that of the uncatalyzed reaction. The possible mechanism for the BNPP and BDNPP cleavage promoted by α‐nucleophiles is proposed on the basis of kinetic analysis. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   
993.
We investigate the behavior of optical absorption of boron–nitride nanotubes (6,0) in the context of Hubbard model at the paramagnetic sector. GW approximation has been implemented in order to make self-energy matrix of electronic system. Afterwards, the real and imaginary parts of transverse dielectric functions have been obtained using linear response theory. The results show that the frequency gap in the optical absorption decreases with Coulomb repulsion strength. Moreover the results show that the local Coulomb interaction leads to the appearance of the excitonic effects in the optical spectrum. Finally the effects of electronic concentration on the frequency behavior of imaginary part of dielectric function have been investigated.  相似文献   
994.
The extinction phenomenon induced by multiplicative non-Gaussian Levy noise in a tumor growth model with immune response is discussed. Under the influence of the stochastic immune rate, the model is analyzed in terms of a stochastic differential equation with multiplicative noise. By means of the theory of the infinitesimal generator of Hunt processes, the escape probability, which is used to measure the noise-induced extinction probability of tumor cells, is explicitly expressed as a function of initial tumor cell density, stability index and noise intensity. Based on the numerical calculations, it is found that for different initial densities of tumor cells, noise parameters play opposite roles on the escape probability. The optimally selected values of the multiplicative noise intensity and the stability index are found to maximize the escape probability.  相似文献   
995.
《Current Applied Physics》2014,14(3):287-293
Lumped parameter models for describing dynamics of the plasma channel in a parallel-plate plasma gun are compared with the experimental results obtained from two plasma guns with different rail geometries. Comparison between the experiments and the numerical calculations reveals that the lumped parameter models can be utilized to describe the dynamic motion of the plasma channel quite well. Parametric study shows that minimizing the line inductance and increasing the charging voltage on a capacitor as well as minimizing the gas injection time for reducing the mass of the plasma channel are the key factors to increase the flow velocity of the plasma jet ejected from the plasma gun.  相似文献   
996.
Soil classification systems are widely used for quickly and easily summarizing soil properties and provide a shorthand method of communication between scientists, engineers, and end-users. Two of the most widely used soil classification systems are the United States Department of Agriculture (USDA) textural soil classification system and the Unified Soil Classification System (USCS). Unfortunately, not all soil map units are classified according to the USDA or USCS systems, and previous attempts to provide a crosswalk table have been inconsistent. Random Forest machine learning model was used to create a USCS prediction model using USDA soil property variables. Important variables for predicting USCS code from available soil properties were USDA soil textures, percent organic material, and available water storage. Prediction error rates less than 2% were achieved compared to error rates of approximately 40% using crosswalk methods.  相似文献   
997.
An anomalous increase in radon concentration was measured at the Nakaizu observatory on the Izu Peninsula prior to the 2011 Tohoku earthquake using a custom-made radon counter. Since the increase was more than three times the standard deviation of radon concentration variations over 35 years of recorded data, it is considered likely that it is a precursor to the earthquake. Following the earthquake, the radon concentration decreased exponentially to the background level. The anomalous increase is explained using a modified volatilization model containing three important aquifer parameters: the groundwater saturation ratio, the fracture surface area per unit volume, and the porosity. The modified model can also explain the radon concentration behavior prior to the 1978 Izu-Oshima-Kinkai earthquake.  相似文献   
998.
The perturbation of multiqubit systems by an external noise can induce various effects like decoherence, stochastic resonance and anti-resonance, and noise-shielding. We investigate how the appearance of these effects on disentanglement time depends on the initial preparation of the systems. We present results for 2-, 3- and 4-qubit chains in various arrangements and observe a clear dependence on the combination of initial geometry of the state space and the placement of noise. Finally, we see that temperature can play a constructive role for the control of these noise induced effects.  相似文献   
999.
《Analytical letters》2012,45(13):2485-2496
Abstract

Blood samples were assayed for PSA values immediately after sampled or by standing for an assigned period. Variation of the free‐ (f), total‐ (t) values, and the free to total (f/t) ratios were determined. Mathematical models were used to interpret the phenomena of deviation. Smaller values of PSA values changed randomly with time and temperature of standings, resulting in varying f/t ratios, while larger initial PSA values were relatively unaffected to any significant extent.

Model interpreted that the changes of PSA values might be caused by higher temperature and time of standings, and conformational participation was also possible.  相似文献   
1000.
The article is concerned with mathematical models for media with oscillating inclusions. These models consist of mutually connected equations, one of which is the wave equation for carrying medium and others are equations of motion for partial oscillators. To close these models, we use cubic and nonlocal equations of state for the carrying medium. Travelling wave solutions to these models are studied in detail. Using qualitative analysis methods, the phase space is shown to contain periodic, homo- and heteroclinic trajectories. Moreover, in the case of nonlocal models we observe the creation of quasiperiodic and chaotic regimes. Bifurcations of localized regimes are studied via the Poincaré section technique.  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