全文获取类型
收费全文 | 15929篇 |
免费 | 2541篇 |
国内免费 | 1858篇 |
专业分类
化学 | 11260篇 |
晶体学 | 156篇 |
力学 | 984篇 |
综合类 | 122篇 |
数学 | 1645篇 |
物理学 | 6161篇 |
出版年
2024年 | 33篇 |
2023年 | 313篇 |
2022年 | 307篇 |
2021年 | 446篇 |
2020年 | 583篇 |
2019年 | 539篇 |
2018年 | 443篇 |
2017年 | 415篇 |
2016年 | 684篇 |
2015年 | 633篇 |
2014年 | 879篇 |
2013年 | 1114篇 |
2012年 | 1294篇 |
2011年 | 1416篇 |
2010年 | 960篇 |
2009年 | 945篇 |
2008年 | 1059篇 |
2007年 | 903篇 |
2006年 | 810篇 |
2005年 | 812篇 |
2004年 | 621篇 |
2003年 | 523篇 |
2002年 | 571篇 |
2001年 | 458篇 |
2000年 | 409篇 |
1999年 | 381篇 |
1998年 | 322篇 |
1997年 | 279篇 |
1996年 | 299篇 |
1995年 | 227篇 |
1994年 | 256篇 |
1993年 | 203篇 |
1992年 | 211篇 |
1991年 | 175篇 |
1990年 | 135篇 |
1989年 | 147篇 |
1988年 | 93篇 |
1987年 | 69篇 |
1986年 | 75篇 |
1985年 | 60篇 |
1984年 | 34篇 |
1983年 | 35篇 |
1982年 | 27篇 |
1981年 | 21篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 11篇 |
1974年 | 12篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Chirped fiber grating was used in dispersion compensator. We tried to use the same phase mask to write fiber grating of different wavelength with two methods to reduce the price of the fiber grating. 相似文献
83.
Optical characterization of bromo-alkoxyl-substituted copper phthalocyanine derivative thin films 总被引:2,自引:0,他引:2
A series of thin films of copper phthalocyanine derivatives with bromine and alkoxyl substituents [CuPc(OC8H17)4Brm, m=0,1,2,4] were prepared on single-crystal silicon substrates by spin coating. The ellipsometric spectra of the films have been studied on a rotating analyzer–polarizer type of scanning ellipsometer. The optical, dielectric constants and absorption coefficients of the films in 500–800 nm wavelength region were obtained. The results show that the complex refractive index, dielectric constants and absorption coefficient of these thin films are influenced regularly by bromine substituent on conjugated macrocycle. It was found that there are approximately linear relationships between the resonance absorption wavelengths of the films and the average number of bromine atoms substituted on the phthalocyanine ring. 相似文献
84.
Tetraalkyltin complexes, SnR4 (R = Me, Et, Pr, Bu) could react with Pt/HY at 193, 243, 273 and 333 K, respectively. The reactions occurred on the surface of the zeolite and the organotin grafted zeolites were characterized in detail. The framework and the microporous structure of the grafted Pt/HY zeolites were retained. However, the modified zeolites showed better size selectivity in the absorption of hydrocarbons. 相似文献
85.
The influence of hydrogen bonds on the physical and chemical properties of hydrogen bonding fluid system of A a D d type is investigated from two viewpoints by the principle of statistical mechanics. In detail, we proposed two new ways that can be used to obtain the equilibrium size distribution of the hydrogen bonding clusters, and derived the analytical expression of a relationship between the hydrogen bonding free energy and hydrogen bonding degree. For the nonlinear hydrogen bonding systems, it is shown that the sol-gel phase transition can take place under proper conditions, which is further proven to be a kind of geometrical phase transition rather than a thermodynamic one. Moreover, several problems associated with the geometrical phase transition and liquid-solid phase transition in nonlinear hydrogen bonding systems are discussed. 相似文献
86.
The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional
solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time
play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t
1/2, which is consistent with the theoretical model.
Supported by the National Science Foundation of China (Grant No. 50395102) 相似文献
87.
王浚岭 《高等学校计算数学学报》2007,29(4):311-323
1引言与记号单调线性互补问题和线性规划问题的原始-对偶路径跟踪算法,1989年的文献[1、2]分别首先提出。以后又出现了一些改进的算法。早期的原始-对偶路径跟踪算法及其改进算法的迭代点列大都是在包含中心路径C的一个2-范数的窄邻域里,这种可行内点算法通常理论上具有最好的迭代复杂性O(n~(1/2)L),但是由于窄邻域极大地限制了迭代步长,实 相似文献
88.
A combined programming and iteration algorithm for finite element analysis of three-dimensional contact problems 总被引:3,自引:0,他引:3
Comparing with two-dimensional contact problems, three-dimensional frictional contact problems are more difficult to deal
with, because of the unknown slip direction of the tangential force and enormous computing time. In order to overcome these
difficulties, a combined PQP (Parametric Quadratic Programming) and iteration method is derived in this paper. The iteration
algorithm, which alleviates the difficulty of unknown slip direction, is used along with the PQP method to cut down computing
costs. Numerical example is given to demonstrate the validity of the present algorithm.
The project supported by the Machinary and Electronics Ministry of China 相似文献
89.
90.
放射源的辐射指纹能起到标识和鉴别放射性物体的作用.在涉及核弹头不可逆销毁过程的深度核裁军核查中,核弹头的辐射指纹对标识和鉴别裁减下来的核弹头将起到关键作用.预先研究辐射指纹的有关技术,如识别机理的研究,将有助于深度核裁军的核弹头核查技术发展.以实验室放射源为研究对象,探索了类型γ射线能谱指纹的同一性识别机理.类型γ射线指纹识别机理的研究,就是要找出一种合适的方法,以较高的置信度,描述两个正在进行比较的γ射线指纹是否为同一放射源的指纹.采用了谱形比较法,并用谱相似度概念来描述两个指纹的相似程度.在谱形比较思想的指导下,编制了放射源类型指纹识别软件,并通过放射源同一性的识别实验验证了软件的有效性,同时研究了谱相似度随统计涨落和测量条件,如时间、源强和本底等因素的变化情况.研究结果表明:1)用相似度概念来描述两个指纹的相似程度,回答两个待比较的γ射线能谱是否代表同一类型放射源,是切实可行的;2)该识别机理只具备识别放射源类型的能力,而对同一类型、差异甚微的放射源个体还不能识别
关键词:
γ射线能谱指纹
辐射指纹
识别机理
核查技术 相似文献