全文获取类型
| 收费全文 | 899篇 |
| 免费 | 118篇 |
| 国内免费 | 40篇 |
专业分类
| 化学 | 68篇 |
| 晶体学 | 10篇 |
| 力学 | 88篇 |
| 综合类 | 30篇 |
| 数学 | 527篇 |
| 物理学 | 334篇 |
出版年
| 2025年 | 7篇 |
| 2024年 | 12篇 |
| 2023年 | 23篇 |
| 2022年 | 31篇 |
| 2021年 | 23篇 |
| 2020年 | 24篇 |
| 2019年 | 23篇 |
| 2018年 | 31篇 |
| 2017年 | 41篇 |
| 2016年 | 26篇 |
| 2015年 | 28篇 |
| 2014年 | 49篇 |
| 2013年 | 52篇 |
| 2012年 | 66篇 |
| 2011年 | 40篇 |
| 2010年 | 42篇 |
| 2009年 | 41篇 |
| 2008年 | 49篇 |
| 2007年 | 69篇 |
| 2006年 | 54篇 |
| 2005年 | 45篇 |
| 2004年 | 38篇 |
| 2003年 | 37篇 |
| 2002年 | 24篇 |
| 2001年 | 26篇 |
| 2000年 | 20篇 |
| 1999年 | 25篇 |
| 1998年 | 20篇 |
| 1997年 | 13篇 |
| 1996年 | 13篇 |
| 1995年 | 6篇 |
| 1994年 | 13篇 |
| 1993年 | 8篇 |
| 1992年 | 4篇 |
| 1991年 | 4篇 |
| 1990年 | 7篇 |
| 1989年 | 2篇 |
| 1987年 | 2篇 |
| 1986年 | 2篇 |
| 1984年 | 1篇 |
| 1983年 | 1篇 |
| 1981年 | 3篇 |
| 1980年 | 2篇 |
| 1979年 | 1篇 |
| 1978年 | 1篇 |
| 1977年 | 2篇 |
| 1970年 | 1篇 |
| 1967年 | 1篇 |
| 1959年 | 1篇 |
| 1957年 | 1篇 |
排序方式： 共有1057条查询结果，搜索用时 0 毫秒
101.
Guancai Xie Xiaoyue Zhang Xiao Ouyang Prof. Qi Xin Beidou Guo Prof. Jian Ru Gong 《ChemCatChem》2019,11(24):6332-6338
For photoelectrochemical (PEC) devices, light irradiation direction changes photogenerated charge distribution and thus will affect the charge collection efficiency of the photoelectrodes. Herein, we studied the influence of irradiation direction on charge collection properties of α-Fe2O3 photoanodes with different geometries, and demonstrated the controlled charge collection by modifying surface geometry. An irradiation direction-dependent surface charge recombination was identified. In comparison with backside irradiation, frontside irradiation was found to be favorable to obtain high photocurrent because of the effective hole collection, except for the thick planar film due to its high surface recombination. This problem can be solved by patterning the planar film surface with nanonet geometry to promote surface charge transfer. As a result, the nanonet photoanode exhibited a higher photocurrent than the planar counterpart under frontside irradiation. Our finding provides useful guidance on designing high efficiency PEC devices. 相似文献
102.
We study existence and uniqueness of solutions for a general class of second order abstract differential equations with state-dependent delay. Some examples related to partial differential equations with state dependent delay are presented. 相似文献
103.
In this paper, we are concerned with a class of abstract second-order nonlocal Cauchy problem with impulsive conditions in Banach spaces. First, we study the existence of mild solutions for a class of second-order nonlocal Cauchy problem with impulsive conditions in Banach spaces on an interval [0,a]. Later, we study a couple of cases where we can establish the existence of global solutions for a class of abstract second-order nonlocal Cauchy problem with impulsive conditions in Banach spaces. We apply our theory to study the existence of solutions for impulsive partial differential equations. 相似文献
104.
In this paper, we study the power allocation problem for an orthogonal frequency division multiplexing (OFDM)-based cognitive radio (CR) system. In a departure from the conventional power allocation schemes available in the literature for OFDM-based CR, we propose power allocation schemes that are augmented with spectral shaping. Active interference cancellation (AIC) is an effective spectral shaping technique for OFDM-based systems. Therefore, in particular, we propose AIC-based optimal and suboptimal power allocation schemes that aim to maximize the downlink transmission capacity of an OFDM-based CR system operating opportunistically within the licensed primary users (PUs) radio spectrum in an overlay approach. Since the CR transmitter may not have the perfect knowledge about the instantaneous channel quality between itself and the active PUs, the interference constraints imposed by each of the PUs are met in a statistical sense. We also study an optimal power allocation scheme that is augmented with raised cosine (RC) windowing-based spectral shaping. For a given power budget at the CR transmitter and the prescribed statistical interference constraints by the PUs, we demonstrate that although the on-the-run computational complexity of the proposed AIC-based optimal power allocation scheme is relatively higher, it may yield better transmission rate for the CR user compared to the RC windowing-based power allocation scheme. Further, the AIC-based suboptimal scheme has the least on-the-run computational complexity, and still may deliver performance that is comparable to that of the RC windowing-based power allocation scheme. The presented simulation results also show that both the AIC-based as well as the RC windowing-based power allocation schemes lead to significantly higher transmission rates for the CR user compared to the conventional (without any spectral shaping) optimal power allocation scheme. 相似文献
105.
106.
Michael M. Tung Javier Ibáñez Emilio Defez Jorge Sastre 《Mathematical Methods in the Applied Sciences》2020,43(14):8237-8252
This work deals with the simulation of a two-dimensional ideal lattice having simple tetragonal geometry. The harmonic character of the oscillators give rise to a system of second-order linear differential equations, which can be recast into matrix form. The explicit solutions which govern the dynamics of this system can be expressed in terms of matrix trigonometric functions. For the derivation we employ the Lagrangian formalism to determine the correct solutions, which extremize the underlying action of the system. In the numerical evaluation we develop diverse state-of-the-art algorithms which efficiently tackle equations with matrix sine and cosine functions. For this purpose, we introduce two special series related to trigonometric functions. They provide approximate solutions of the system through a suitable combination. For the final computation an algorithm based on Taylor expansion with forward and backward error analysis for computing those series had to be devised. We also implement several MATLAB programs which simulate and visualize the two-dimensional lattice and check its energy conservation. 相似文献
107.
Reconfigurable intelligent surface (RIS), a planar metasurface consisting of a large number of low-cost reflecting elements, has received much attention due to its ability to improve both the spectrum and energy efficiency (EE) by reconfiguring the wireless propagation environment. In this paper, we propose a base station (BS) beamforming and RIS phase shift optimization technique that maximizes the EE of a RIS-aided multiple-input–single-output system. In particular, considering the system circuits’ energy consumption, an EE maximization problem is formulated by jointly optimizing the active beamforming at the BS and the passive beamforming at the RIS, under the constraints of each user’ rate requirement, the BS’s maximal transmit power budget and unit-modulus constraint of the RIS phase shifts. Due to the coupling of optimization variables, this problem is a complex non-convex optimization problem, and it is challenging to solve it directly. To overcome this obstacle, we divide the problem into active and passive beamforming optimization subproblems. For the first subproblem, the active beamforming is given by the maximum ratio transmission optimal strategy. For the second subproblem, the optimal phase shift matrix at the RIS is obtained by exploiting sine cosine algorithm (SCA). Moreover, for this case where each reflection element’s working state is controlled by a circuit switch, each reflection element’s switch value is optimized with the aid of particle swarm optimization algorithm. Finally, numerical results verify the effectiveness of our proposed algorithm compared to other algorithms. 相似文献
108.
109.
110.
Color image watermarking has become essential and important copyright protection or authentication scheme. It is noted that most of the existing color image watermarking algorithms are performed only in the single domain (spatial domain or frequency domain), and not to integrate these merits of the different domains. By utilizing the generating principle and distribution feature of the direct current (DC) coefficient, a novel blind watermarking algorithm is proposed for color host images in this paper. Firstly, the Y luminance of host image is divided into 8 × 8 sub-blocks and the DC coefficients of each block are directly calculated in the spatial domain without DCT transform. Secondly, according to the watermark information and the quantization step, the DC coefficients are calculated and their increments are further utilized to modify directly the values of all pixels in the spatial domain instead of the DCT domain to embed watermark. When watermark extraction, only the watermarked image and the quantization step are needed in the spatial domain. Experimental results show that the proposed method not only can resist both traditional signal processing attacks and geometric attacks, but also has more efficient in computational complexity. Comparisons also demonstrate the advantages of the method. 相似文献