全文获取类型
收费全文 | 15593篇 |
免费 | 2089篇 |
国内免费 | 475篇 |
专业分类
化学 | 810篇 |
晶体学 | 17篇 |
力学 | 733篇 |
综合类 | 129篇 |
数学 | 2580篇 |
物理学 | 2506篇 |
无线电 | 11382篇 |
出版年
2024年 | 81篇 |
2023年 | 258篇 |
2022年 | 394篇 |
2021年 | 452篇 |
2020年 | 415篇 |
2019年 | 347篇 |
2018年 | 383篇 |
2017年 | 553篇 |
2016年 | 605篇 |
2015年 | 634篇 |
2014年 | 1010篇 |
2013年 | 1226篇 |
2012年 | 979篇 |
2011年 | 1128篇 |
2010年 | 850篇 |
2009年 | 878篇 |
2008年 | 1020篇 |
2007年 | 1081篇 |
2006年 | 909篇 |
2005年 | 776篇 |
2004年 | 631篇 |
2003年 | 559篇 |
2002年 | 483篇 |
2001年 | 405篇 |
2000年 | 294篇 |
1999年 | 266篇 |
1998年 | 264篇 |
1997年 | 187篇 |
1996年 | 188篇 |
1995年 | 151篇 |
1994年 | 122篇 |
1993年 | 109篇 |
1992年 | 95篇 |
1991年 | 51篇 |
1990年 | 58篇 |
1989年 | 41篇 |
1988年 | 35篇 |
1987年 | 35篇 |
1986年 | 17篇 |
1985年 | 52篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 21篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1979年 | 18篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1974年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
用场方法研究了相对论速调管放大器辐射频率的可调性。发现调制电流频率的可调性与一阶Bessel函数的周期相关,对应相同的一阶Bessel函数值,可以有多个不同数值的宗量,调制电流的频率与Bessel函数宗量数值包括中间腔与提取腔之间的距离和电子束的传输速度等因素相联系。理论估计的调制电流频率可调范围与实验测得的结果符合得很好。相对论速调管放大器辐射频率的可调性还取决于提取腔的设计。 相似文献
92.
研究了 2 0 0 0年 MCM— B题 ,将问题推广到有三层干扰的复杂情况 ,给出并严格证明了 ( k,1,1)问题的最优解 相似文献
93.
M. Brunken L. Casper H. Genz C. Hessler S. Khodyachykh A. Richter 《Optics & Laser Technology》2003,35(5):349-353
A compact autocorrelator suitable for the measurement of the optical pulse length of the infrared free electron laser (IR-FEL) at the S-DALINAC is described. It allows to investigate the energy transfer process from the electrons into the laser light within the undulator of the FEL. The autocorrelator utilizes the second harmonic generation (SHG) and it provides the possibility of a background-free measurement at wavelengths between 4 and 10 μm. Simulations for three different crystals used for SHG show that the largest efficiency for SHG and the best temporal resolution is achieved with a ZnGeP2 crystal at a length of 2 mm. A test experiment at the free electron laser for infrared experiments confirmed the expected high efficiency of about 7.5% of SHG for the ZnGeP2 crystal as well as the easy handling and fast adjustment of the system. 相似文献
94.
95.
《Electroanalysis》2005,17(3):255-262
Characteristics of mass transport and potential distribution applicable to microfluidic electrochemical flow cell devices has been modelled using the finite element method. A flexible, automatic grid generation algorithm has been combined with an a‐posteriori error indication technique presented by Nann and Heinze to allow irregular cell geometries to be modelled. The code has been applied to the problem of steady state generator – detector linear sweep voltammetry in a channel flow cell showing the effects of IR drop on the voltammetric response of each electrode. 相似文献
96.
This paper considers the estimation problem for a trigonometric regression model with the noise specified by the Ornstein–Uhlenbeck
process with unknown parameter. We propose a sequential procedure which ensures a prescribed mean square precision uniformly
in the nuisance parameter. The asymptotic behaviour of the procedure duration mean has been studied.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
97.
This paper investigates regression quantiles (RQ) for unstable autoregressive models. The uniform Bahadur representation of the RQ process is obtained. The joint asymptotic distribution of the RQ process is derived in a unified manner for all types of characteristic roots on or outside the unit circle. It involves stochastic integrals in terms of a sequence of independent and identically distributed multivariate Brownian motions with correlated components. The related L-estimator is also discussed. The asymptotic distributions of the RQ and the L-estimator corresponding to the nonstationary componentwise arguments can be transformed into a function of a normal random variable and a sequence of i.i.d. univariate Brownian motions. This is different from the analysis based on the LSE in the literature. As an auxiliary theorem, a weak convergence of a randomly weighted residual empirical process to the stochastic integral of a Kiefer process is established. The results obtained in this paper provide an asymptotic theory for nonstationary time series processes, which can be used to construct robust unit root tests. 相似文献
98.
In this paper we consider the problem of estimating an unknown joint distribution which is defined over mixed discrete and continuous variables. A nonparametric kernel approach is proposed with smoothing parameters obtained from the cross-validated minimization of the estimator's integrated squared error. We derive the rate of convergence of the cross-validated smoothing parameters to their ‘benchmark’ optimal values, and we also establish the asymptotic normality of the resulting nonparametric kernel density estimator. Monte Carlo simulations illustrate that the proposed estimator performs substantially better than the conventional nonparametric frequency estimator in a range of settings. The simulations also demonstrate that the proposed approach does not suffer from known limitations of the likelihood cross-validation method which breaks down with commonly used kernels when the continuous variables are drawn from fat-tailed distributions. An empirical application demonstrates that the proposed method can yield superior predictions relative to commonly used parametric models. 相似文献
99.
100.
A generalisation of the Burg algorithm to vector spaces is described. This can be used to subsample time series that have narrow frequency bandwidth, thereby reducing the computation time and the complexity of the autoregressive model required. The technique is preferable to conventional subsampling. 相似文献