全文获取类型
收费全文 | 4207篇 |
免费 | 625篇 |
国内免费 | 368篇 |
专业分类
化学 | 579篇 |
晶体学 | 28篇 |
力学 | 383篇 |
综合类 | 53篇 |
数学 | 992篇 |
物理学 | 1356篇 |
无线电 | 1809篇 |
出版年
2024年 | 13篇 |
2023年 | 50篇 |
2022年 | 75篇 |
2021年 | 89篇 |
2020年 | 79篇 |
2019年 | 83篇 |
2018年 | 99篇 |
2017年 | 122篇 |
2016年 | 147篇 |
2015年 | 144篇 |
2014年 | 229篇 |
2013年 | 257篇 |
2012年 | 269篇 |
2011年 | 303篇 |
2010年 | 238篇 |
2009年 | 300篇 |
2008年 | 286篇 |
2007年 | 367篇 |
2006年 | 286篇 |
2005年 | 220篇 |
2004年 | 201篇 |
2003年 | 201篇 |
2002年 | 150篇 |
2001年 | 141篇 |
2000年 | 128篇 |
1999年 | 97篇 |
1998年 | 100篇 |
1997年 | 83篇 |
1996年 | 72篇 |
1995年 | 65篇 |
1994年 | 63篇 |
1993年 | 37篇 |
1992年 | 32篇 |
1991年 | 26篇 |
1990年 | 21篇 |
1989年 | 19篇 |
1988年 | 16篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1974年 | 4篇 |
1973年 | 3篇 |
排序方式: 共有5200条查询结果,搜索用时 15 毫秒
91.
The molecular intrinsic characteristic contour (MICC) is defined based on the classical turning point of electron movement in a molecule. Three typical organic molecules, I.e. Methane, methanol and formic acid, were employed as examples for detailed introduction of our method. Investigations on the cross-sections of MICC provide important information about atomic size changing in the process of forming molecules. The electron density distributions on the MICCs of these molecules were calculated and shown for the first time. Results showed that the electron density distribution on the MICC correlates closely with molecular chemical properties, and it provides a new insight into molecular boundary. 相似文献
92.
以去离子水与乙二醇的混合液(体积分数分别为36.5%,48.7%,59.0%及71.2%,以下简称混合液)作为同轴传输线的绝缘介质,进行了μs级高电压负充电条件下的正电极击穿实验,研究了混合液的击穿电压、击穿时间、相对介电常数及电阻率与体积分数的关系。实验结果表明:在充电电压为20kV时,71.2%的混合液比36.5%的混合液的平均击穿电压提高25.1%,平均击穿时间延长10.49%,而相对介电常数减小868%。同时,随着充电时间的缩短,混合液的击穿电压提高。 相似文献
93.
《高分子化学》课程是大部分工科化工、材料类专业的必修课程,如何在工科专业高分子化学课程教学中体现工科特色,成为大家关注的热点.针对什么是工科特色,提出了本人的见解.并就如何在教学过程中体现工科特色介绍了本人的探索与尝试. 相似文献
94.
Mohamed Charif Soufiane El Adraoui Mounirh Khalid Asmaa Zugari Mohamed I. Yaich Mohsine Khalladi 《Optik》2014
The current density (JE), the piecewise linear recursive convolution (PLRC) and the constant convolution recursive (CRC) techniques are developed and integrated into the transmission line matrix (TLM) algorithm. A comparison of these three schemes according to the criteria of accuracy and CPU time is presented. Numerical experiments show that JE provides the most accurate solution and requires the least CPU time; PLRC is nearly as accurate but consumes more CPU time than CRC. 相似文献
95.
In this work we present an optical lattice setup to realize a full Dirac Hamiltonian in 2+1 dimensions. We show how all possible external potentials coupled to the Dirac field can arise from perturbations of the existing couplings of the honeycomb lattice pattern. This greatly simplifies the proposed implementations, requiring only spatial modulations of the intensity of the laser beams to induce complex non-Abelian potentials. We finally suggest several experiments to observe the properties of the quantum field theory in the setup. 相似文献
96.
97.
In the dynamics analysis and synthesis of a con-trolled system, it is important to know for what feedback gains can the controlled system decay to the demanded steady state as fast as possible. This article presents a sys-tematic method for finding the optimal feedback gains by taking the stability of an inverted pendulum system with a delayed proportional-derivative controller as an example. First, the condition for the existence and uniqueness of the stable region in the gain plane is obtained by using the D-subdivision method and the method of stability switch. Then the same procedure is used repeatedly to shrink the stable region by decreasing the real part of the rightmost charac-teristic root. Finally, the optimal feedback gains within the stable region that minimizes the real part of the rightmost root are expressed by an explicit formula. With the optimal feedback gains, the controlled inverted pendulum decays to its trivial equilibrium at the fastest speed when the initial val-ues around the origin are fixed. The main results are checked by numerical simulation. 相似文献
98.
For a spatial characteristic, there exist commonly fat-tail frequency distributions of fragment-size and -mass of glass, areas enclosed by city roads, and pore size/volume in random packings. In order to give a new analytical approach for the distributions, we consider a simple model which constructs a fractal-like hierarchical network based on random divisions of rectangles. The stochastic process makes a Markov chain and corresponds to directional random walks with splitting into four particles. We derive a combinatorial analytical form and its continuous approximation for the distribution of rectangle areas, and numerically show a good fitting with the actual distribution in the averaging behavior of the divisions. 相似文献
99.
《Physics letters. A》2019,383(19):2272-2276
Magnetic field design is important in cylindrical Hall thrusters and using permanent magnets to generate magnetic field is very promising in the future. In two typical permanent magnet configurations (i.e., ring and cylindrical configurations) of cylindrical Hall thrusters, discharge characteristics are compared in this paper through the experiments and simulations. The study shows that the cylindrical configuration can bring about higher thruster performance in the same working condition. The reason is that the potential drop of the cylindrical configuration is mainly concentrated in the channel, which is beneficial for the electrons to obtain energy to promote the ionization of the propellant. However, the voltage regulation range of the cylindrical configuration is lower because the anode is more easily overheated. 相似文献
100.
The charge generation mechanism of organic heterojunction (OHJ) consisted of 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) and different hole transporting materials (HTMs) are studied systematically by current-voltage (I–V) and capacitance-voltage measurements. The analysis of I–V characteristics of the devices based on OHJs at forward and reverse voltages by comparing the thickness of HTM layers finds that a forward and reverse symmetrical I–V curve is observed at thin HTM layers and the forward current becomes larger than the reverse current with the increase of HTM thickness, fully illustrating the effectiveness of OHJ charge generation. Moreover, the I–V characteristics at different temperatures indicate that the efficient charge generation is originated from electron tunneling rather than diffusion. And the C–V and capacitance-frequency (C–F)characteristics further illustrate the highly efficient charge generation ability of OHJs so that the charge density is as high as 4.5 × 1017 cm−3, guaranteeing the high conductivity of OHJs, which is very beneficial to developing highly efficient OLEDs using OHJs as charge injector and generator. 相似文献