全文获取类型
收费全文 | 2225篇 |
免费 | 241篇 |
国内免费 | 571篇 |
专业分类
化学 | 1368篇 |
晶体学 | 75篇 |
力学 | 101篇 |
综合类 | 20篇 |
数学 | 228篇 |
物理学 | 1245篇 |
出版年
2023年 | 19篇 |
2022年 | 57篇 |
2021年 | 65篇 |
2020年 | 69篇 |
2019年 | 62篇 |
2018年 | 71篇 |
2017年 | 92篇 |
2016年 | 71篇 |
2015年 | 101篇 |
2014年 | 126篇 |
2013年 | 143篇 |
2012年 | 129篇 |
2011年 | 148篇 |
2010年 | 137篇 |
2009年 | 162篇 |
2008年 | 181篇 |
2007年 | 171篇 |
2006年 | 143篇 |
2005年 | 143篇 |
2004年 | 120篇 |
2003年 | 82篇 |
2002年 | 67篇 |
2001年 | 72篇 |
2000年 | 63篇 |
1999年 | 49篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 26篇 |
1995年 | 18篇 |
1994年 | 30篇 |
1993年 | 30篇 |
1992年 | 22篇 |
1991年 | 17篇 |
1990年 | 18篇 |
1989年 | 15篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1983年 | 18篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1979年 | 18篇 |
1978年 | 16篇 |
1975年 | 9篇 |
1974年 | 19篇 |
1973年 | 12篇 |
1972年 | 12篇 |
1971年 | 23篇 |
1970年 | 10篇 |
1969年 | 11篇 |
1968年 | 9篇 |
排序方式: 共有3037条查询结果,搜索用时 125 毫秒
1.
介绍了光码分多址系统中常用地址码(一维扩时码、二维码和三维码)的特点,并对它们各自的互相关均值和方差进行了理论分析。基于非相干光码分多址系统中光学相关接收机的基本原理,结合不同的用户地址码,对系统误码率性能进行了分析,得到了接收机最佳判决阈值与地址码基本特性参数和系统同时用户数间的关系。最后,给出了数值仿真结果。结果表明,对于采用特定地址码的光码分多址系统,只有选择合适的接收机判决阈值,系统的误码率性能才能达到最佳。研究结果对光码分多址系统中接收机判决阈值的选取具有一定的参考作用。 相似文献
2.
Let H and K be indefinite inner product spaces. This paper shows that a bijective map φ:B(H)→B(K) satisfies φ(AB B A) =φ(A)φ(5) φ(B) φ(A) for every pair A,B ∈B(H) if and only if either φ(A) = cU AU for all A or φ(A) = cUA U for all A; φsatisfies φ(AB A) = φ(A)φ(B) φ(A) for every pair A,B ∈B(H) if and only if either φ(A) = UAV for all A or φ(A) = UA V for all A, where A denotes the indefinite conjugate of A, U and V are bounded invertible linear or conjugate linear operators with U U = c-1I and V V = cI for some nonzero real number c. 相似文献
3.
A. S. Baranov 《Technical Physics》2004,49(8):1081-1083
A concise proof is given for the expansion of the reciprocal distance between two points in spheroidal harmonics. The proof
is given for an oblate and a prolate spheroid. 相似文献
4.
Grigor'ev S. A. Kalinnikov A. A. Porembskii V. I. Baranov I. E. Borisova E. V. Fateev V. N. 《Russian Journal of Electrochemistry》2004,40(11):1188-1192
A two-dimensional mathematical model for the transport of reactants in a fuel cell with a solid polymer electrolyte is developed. The model is used for analyzing spatial distributions of the concentration of reactants and current density over the cell. The effect of the catalytic-layer activity, reactant speed, bipolar-plate geometry, thickness and porosity of current collector and/or gas-diffusion sublayer, and the reaction mixture composition on the fuel cell efficiency is estimated theoretically and experimentally. 相似文献
5.
6.
Effects of Substrate Temperature on Helium Content and Microstructure of Nanocrystalline Titanium Films 下载免费PDF全文
Helium-charged nanocrystalline titanium films have been deposited by HeAr magnetron co-sputtering. The effects of substrate temperature on the helium content and microstructure of the nanocrystalline titanium films have been studied. The results indicate that helium atoms with a high concentration are evenly incorporated in the deposited titanium films. When the substrate temperature increases from 60℃ to 350℃ while the other deposition'parameters are fixed, the helium content decreases gradually from 38.6 at.% to 9.2at.%, which proves that nanocrystalline Ti films have a great helium storage capacity. The 20 angle of the Bragg peak of (002) crystal planes of the He-charged Ti film shifts to a lower angle and that of (100) crystal plane is unchanged as compared with that of the pure Ti film, which indicates that the lattice parameter c increases and a keeps at the primitive value. The grain refining and helium damage result in the diffraction peak broadening. 相似文献
7.
8.
The atomic structure and magnetic and electric properties of the Cr0.5TiSe2-Cr0.5TiTe2 system of intercalated phases were studied in detail by gradually replacing selenium by tellurium. It was revealed that this replacement changes the crystalline structure from monoclinic in the initial compounds to hexagonal in the compounds containing various types of chalcogen atoms; this is accompanied by disordering of chromium atoms in the van der Waals gaps. The electrical resistance and magnetic characteristics vary nonmonotonically on replacement of selenium by tellurium, which is associated with a change in the degree of atomic disordering during the transition from Cr0.5TiSe2 to Cr0.5TiTe2. 相似文献
9.
N. F. Morozov Yu. K. Startsev Yu. V. Sud’enkov A. A. Suslikov G. A. Baranov A. A. Belyaev 《Technical Physics》2006,51(7):872-877
The radiation hardness and mechanical strength of single-and two-phase glasses are studied for the case when nanosecond laser pulses (λ= 1.06 μm, τ0.5 ≈ 12.5 ns) are focused inside the material. Laser interferometry is applied to measure the displacement of the free surface, find optical breakage thresholds, and carry out the fractographic analysis of damaged regions. It is shown that breakdown channels and damage regions develop in a nonlinear manner according to optical breakdown mechanisms, changing each other with an increase in the laser energy. The strength of the two-phase glass is found to be more than four times that of the single-phase glass, although their elastic properties differ insignificantly. Such a considerable difference in the hardness of these materials with chemically similar constitutents is attributed to the presence of the double-lattice nanometer-scale structure of the two-phase glass. 相似文献
10.
It is well known that potassium ion channels have higher permeability than K ions, and the permeable rate of a single K ion channel is about 108 ions per second. We develop a hierarchical model of potassium ion channel permeation involving ab initio quantum calculations and Brownian dynamics simulations, which can consistently explain a range of channel dynamics. The results show that the average velocity of K ions, the mean permeable time of K ions and the permeable rate of single channel are about 0.92nm/ns, 4.35ns and 2.30 ×10^8 ions/s, respectively. 相似文献