全文获取类型
收费全文 | 1804篇 |
免费 | 207篇 |
国内免费 | 143篇 |
专业分类
化学 | 443篇 |
晶体学 | 5篇 |
力学 | 128篇 |
综合类 | 32篇 |
数学 | 856篇 |
物理学 | 690篇 |
出版年
2024年 | 3篇 |
2023年 | 50篇 |
2022年 | 62篇 |
2021年 | 102篇 |
2020年 | 150篇 |
2019年 | 154篇 |
2018年 | 152篇 |
2017年 | 148篇 |
2016年 | 121篇 |
2015年 | 105篇 |
2014年 | 155篇 |
2013年 | 203篇 |
2012年 | 188篇 |
2011年 | 66篇 |
2010年 | 39篇 |
2009年 | 52篇 |
2008年 | 73篇 |
2007年 | 50篇 |
2006年 | 31篇 |
2005年 | 31篇 |
2004年 | 30篇 |
2003年 | 28篇 |
2002年 | 14篇 |
2001年 | 21篇 |
2000年 | 13篇 |
1999年 | 14篇 |
1998年 | 11篇 |
1997年 | 10篇 |
1996年 | 6篇 |
1995年 | 20篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1959年 | 1篇 |
排序方式: 共有2154条查询结果,搜索用时 0 毫秒
61.
根据菲涅尔全内反射相变理论,给出了双菱体λ/4消色差器的结构设计、性能分析和测量方法.由有效通光孔径和光线追迹设计出BK7玻璃在波长532 nm时相位延迟λ/4的双菱体的结构,用作532 nm至1 064 nm波长范围的标准λ/4相位延迟器.理论分析了入射角变化和波长变化对双菱体相位延迟的影响,当入射角变化限制在±4.3°以内时,其影响得到补偿;波长从532 nm到1 064 nm产生的误差为-0.65°.采用椭偏法中的消光技术,分别实测了双菱体在532 nm和1 064 nm波长下的相位延迟为:90.08±0.14°和88.99±0.1°,可知两不同波长产生的相位延迟误差为-1.09°. 相似文献
62.
Simple and robust digital holography for high-resolution imaging 总被引:2,自引:0,他引:2
Based on the point spread function of holographic system,the lateral resolution of digital holographic imaging system without any pre-magnification is studied.The expression of resolution limitation of holographic imaging system is thus presented.We investigate the possibilities to improve the lateral resolution.The simple experimental setup with an off-axis arrangement is built.By using a U.S.Air Force(USAF)test target as microscopic object,the recorded holograms are reconstructed digitally based on the principle of Fresnel diffraction.The lateral resolution of 2.76 μm without any pre-magnification is demonstrated experimentally,which matches the theoretical prediction well. 相似文献
63.
利用非对称非线性函数耦合混沌同步方法,讨论了Chen吸引子的混沌同步问题,数值模拟分析初始值和耦合强度因子的选择对于实现混沌同步的影响. 将非对称非线性函数耦合同步方法进一步推广发展到完全连接网络和由星形子网络构成的复杂大网络混沌同步的研究中. 提供了确定网络中神经元之间混沌同步状态稳定性的误差发展方程,并讨论各个耦合强度因子对网络同步稳定性过程的影响,给出了相应的稳定性范围. 通过数值模拟证明利用非线性函数作为耦合函数,实现完全连接网络、星形子网络构成大网络的混沌同步是有效的. 可以预测在网络的混沌同步
关键词:
非线性耦合函数
Chen吸引子
混沌同步
网络 相似文献
64.
65.
The early stage of secondary structural conversion of amyloid beta (Aβ) to misfolded aggregations is a key feature of Alzheimer's disease (AD). Under normal physiological conditions, Aβ peptides can protect neurons from the toxicity of highly concentrated metals. However, they become toxic under certain conditions. Under conditions of excess iron, amyloid precursor proteins (APP) become overexpressed. This subsequently increases Aβ production. Experimental studies suggest that Aβ fibrillation (main-pathway) and amorphous (off-pathway) aggregate formations are two competitive pathways driven by factors such as metal binding, pH and temperature. In this study, we performed molecular dynamic (MD) simulations to examine the initial stage of conformational transformations of human Aβ (hAβ) and rat Aβ (rAβ) peptides in the presence of Fe2+ and Fe3+ ions. Our results demonstrated that Fe2+ and Fe3+ play key roles in Aβs folding and aggregation. Fe3+ had a greater effect than Fe2+on Aβs’ folding during intermolecular interactions and subsequently, had a greater effect in decreasing structural diversity. Fe2+ was observed to be more likely than Fe3+ to interact with nitrogen atoms from the residues of imidazole rings of His. rAβ peptides are more energetically favorable than hAβ for intermolecular interactions and amorphous aggregations. We concluded that most hAβ structures were energetically unfavorable. However, hAβs with intermolecular β-sheet formations in the C-terminal were energetically favorable. It is notable that Fe2+ can change the surface charge of hAβ. Furthermore, Fe3+ can promote C-terminal folding by binding to Glu22 and Ala42, and by forming stable β-sheet formations on the C-terminal. Fe3+ can also pause the main-pathway by inducing random aggregations. 相似文献
66.
Nonlinear effects such as friction and freeplay on the control surfaces can affect aeroelastic dynamics during flight. In particular, these nonlinearities can induce limit cycle oscillations (LCO), changing the system stability, and because of this it is essential to employ computational methods to predict this type of motion during the aircraft development cycle. In this context, the present article presents a matrix notation for describing the Hénon’s method used to reduce errors when considering piecewise linear nonlinearities in the numerical integration process. In addition, a new coordinate system is used to write the aeroelastic system of equations. The proposal defines a displacement vector with generalized and physical variables to simplify the computational implementation of the Hénon’s technique. Additionally, the article discusses the influence of asymmetric freeplay and friction on the LCO of an airfoil with control surface. The results show that the extended Hénon’s technique provides more accurate LCO predictions, that friction can change the frequency and amplitude of these motions, and the asymmetry of freeplay is important to determine the LCO behavior. 相似文献
67.
68.
A novel diffractive optical element, named phase zone photon sieve
(PZPS), is presented. There are three kinds of phase plates in
PZPSs: PZPS1, PZPS2, and PZPS3. Each of the PZPSs has its own
structure and is made on quartz substrate by etching. The three
PZPSs have stronger diffraction peak intensity than a photon sieve
(PS) when the margin pinhole and zone line width are kept the same.
The PZPS3 can produce a smaller central diffractive spot than the
ordinary PS with the same number of zones on the Fresnel zone plate.
We have given the design method for and the simulation of PZPS and
PS. PZPS has potential applications in optical maskless lithography. 相似文献
69.
70.
Fractional Fourier transformation of an object can be approximated by the object's free-space Fresnel diffraction pattern under some restricted conditions and plane wave illumination according to Hua's method. A better approximation is achieved under least-squared conditions developed in this paper. Simulation results verify that our theoretical development works for any fractional order a compared with the previous approach. 相似文献