全文获取类型
收费全文 | 8693篇 |
免费 | 975篇 |
国内免费 | 528篇 |
专业分类
化学 | 2370篇 |
晶体学 | 73篇 |
力学 | 916篇 |
综合类 | 41篇 |
数学 | 2666篇 |
物理学 | 2858篇 |
无线电 | 1272篇 |
出版年
2024年 | 29篇 |
2023年 | 122篇 |
2022年 | 165篇 |
2021年 | 211篇 |
2020年 | 278篇 |
2019年 | 262篇 |
2018年 | 218篇 |
2017年 | 259篇 |
2016年 | 309篇 |
2015年 | 243篇 |
2014年 | 365篇 |
2013年 | 786篇 |
2012年 | 384篇 |
2011年 | 439篇 |
2010年 | 356篇 |
2009年 | 453篇 |
2008年 | 504篇 |
2007年 | 531篇 |
2006年 | 464篇 |
2005年 | 414篇 |
2004年 | 426篇 |
2003年 | 354篇 |
2002年 | 328篇 |
2001年 | 299篇 |
2000年 | 265篇 |
1999年 | 235篇 |
1998年 | 201篇 |
1997年 | 204篇 |
1996年 | 135篇 |
1995年 | 124篇 |
1994年 | 124篇 |
1993年 | 101篇 |
1992年 | 83篇 |
1991年 | 95篇 |
1990年 | 51篇 |
1989年 | 34篇 |
1988年 | 40篇 |
1987年 | 37篇 |
1986年 | 35篇 |
1985年 | 44篇 |
1984年 | 34篇 |
1983年 | 22篇 |
1982年 | 32篇 |
1981年 | 19篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1974年 | 5篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
141.
利用MBE方法在(001)衬底上成功地生长密度大、尺寸小、发红光的InAlAs/AlGaAs量子点结构。通过原子力显微镜观察表明,InAlAs量子的密度和大小都随覆盖厚度的增加而增大;发现Al原子的表面迁移率决定InAlAs量子点的形貌,光荧光谱证实了量子点的发光峰值在红光范围,并结合形貌的统计得到了量子点的发光峰展宽主要昌受量子点的横向尺寸影响。 相似文献
142.
Philippe Bertani Jsus Raya Pierre Reinheimer Rgis Gougeon Luc Delmotte Jrme Hirschinger 《Solid state nuclear magnetic resonance》1999,13(4):255-229
19F/29Si Hartmann–Hahn continuous wave cross-polarization (CP) has been applied under fast magic-angle spinning (MAS) to a powder sample of octadecasil. Strong oscillations occur during CP on a sideband matching condition between the isolated 29Si–19F spin pairs formed by the silicons in the D4R units and the fluoride anions. The magnitude of the dipolar coupling constant was deduced directly from the line-splitting between the intense singularities of the Pake-like patterns obtained by Fourier transformation of the oscillatory polarization transfer. The corresponding Si–F internuclear distance, r=2.62±0.05 Å, is found to be in very good agreement with the X-ray crystal structure and the value of 2.69±0.04 Å recently reported from rotational echo double resonance (REDOR) and transferred echo double resonance (TEDOR) nuclear magnetic resonance (NMR) experiments. Furthermore, the CP technique is still reliable under fast MAS where both REDOR and TEDOR sequences suffer from severe artefacts due to finite pulse lengths. In octadecasil, a spinning frequency of 14 kHz is shown to be necessary for an effective suppression of 19F–19F spin diffusion. The influences of experimental missettings and radiofrequency (RF) field inhomogeneity are taken into account. 相似文献
143.
144.
In modern telephone exchanges, subscriber lines are usually connected to the so-called subscriber line modules. These modules serve both incoming and outgoing traffic. An important difference between these two types of calls lies in the fact that in the case of blocking due to all channels busy in the module, outgoing calls can be queued whereas incoming calls get busy signal and must be re-initiated in order to establish the required connection. The corresponding queueing model was discussed recently by Lederman, but only the model with losses has been studied analytically. In the present contribution, we study the model which takes into account subscriber retrials and investigate some of its properties such as existence of stationary regime, derive explicit formulas for the system characteristics, limit theorems for systems under high repetition intensity of blocked calls and limit theorems for systems under heavy traffic. 相似文献
145.
146.
Mutual mass diffusion and thermal diffusion has been investigated in poly(dimethylsiloxane)/ poly(ethylmethylsiloxane) (PDMS/PEMS) polymer blends of equal weight fractions. Molar masses ranged from below 1 to over 20 kg/mol. Both the mutual mass (D) and the thermal diffusion (DT) coefficient contain a thermally activated factor with an activation temperature of 1415 K. The molar mass dependence of DT is due to an end-group effect of the local friction coefficient. The thermal diffusion coefficient in the limit of long chains and infinite temperature is DT0, = - 1.69×10-7cm2(sK)-1. The Soret coefficient ST of blends far enough away from a critical point is proportional to the static structure factor S(q = 0). 相似文献
147.
The scope of this paper is to present a nonlinear error estimation and correction for Navier-Stokes and Reynolds-averaged Navier-Stokes equations. This nonlinear corrector enables better solution or functional output predictions at fixed mesh complexity and can be considered in a mesh adaptation process. After solving the problem at hand, a corrected solution is obtained by solving again the problem with an added source term. This source term is deduced from the evaluation of the residual of the numerical solution interpolated on the h/2 mesh. To avoid the generation of the h/2 mesh (which is prohibitive for realistic applications), the residual at each vertex is computed by local refinement only in the neighborhood of the considered vertex. One of the main feature of this approach is that it automatically takes into account all the properties of the considered numerical method. The numerical examples point out that it successfully improves solution predictions and yields a sharp estimate of the numerical error. Moreover, we demonstrate the superiority of the nonlinear corrector with respect to linear corrector that can be found in the literature. 相似文献
148.
We analytically find the diffusion of overdamped active Brownian particles (ABPs) constrained to move along curved one-dimensional channels. The autonomous motion of these particles is achieved by a projection of their internal propulsion force along the channels' long section. In particular, the diffusion of ABPs moving on one-dimensional channels with a form of a circle, an ellipse, and a limacon of second order is analysed. To characterise the effect of substrate's geometry and self-propulsion on their diffusion, analytical expressions for the ABPs short- and long-time variances, as well as their steady angular probability density functions are offered. Curvature effects are found to reduce the time an ABP reaches its steady state. Our theoretical results are validated using Brownian dynamics simulations. This model may be relevant for experiments dealing with catalytic driven systems, bacteria, and tumour cell dispersion in one-dimensional channels. 相似文献
149.
We introduce a new flexible mesh adaptation approach to efficiently compute a quantity of interest by the finite element method. Efficiently, we mean that the method provides an evaluation of that quantity up to a predetermined accuracy at a lower computational cost than other classical methods. The central pillar of the method is our scalar error estimator based on sensitivities of the quantity of interest to the residuals. These sensitivities result from the computation of a continuous adjoint problem. The mesh adaptation strategy can drive anisotropic mesh adaptation from a general scalar error contribution of each element. The full potential of our error estimator is then reached. The proposed method is validated by evaluating the lift, the drag, and the hydraulic losses on a 2D benchmark case: the flow around a cylinder at a Reynolds number of 20. 相似文献
150.
Asymptotic results for weighted means of linear combinations of independent Poisson random variables
Rita Giuliano Claudio Macci 《Stochastics An International Journal of Probability and Stochastic Processes》2020,92(4):497-518
ABSTRACT In this paper we prove the large deviation principle for a class of weighted means of linear combinations of independent Poisson distributed random variables, which converge weakly to a normal distribution. The interest in these linear combinations is motivated by the diffusion approximation in Lansky [On approximations of Stein's neuronal model, J. Theoret. Biol. 107 (1984), pp. 631–647] of the Stein's neuronal model (see Stein [A theoretical analysis of neuronal variability, Biophys. J. 5 (1965), pp. 173–194]). We also prove an analogue result for sequences of multivariate random variables based on the diffusion approximation in Tamborrino, Sacerdote, and Jacobsen [Weak convergence of marked point processes generated by crossings of multivariate jump processes. Applications to neural network modeling, Phys. D 288 (2014), pp. 45–52]. The weighted means studied in this paper generalize the logarithmic means. We also investigate moderate deviations. 相似文献