全文获取类型
收费全文 | 51362篇 |
免费 | 11914篇 |
国内免费 | 3879篇 |
专业分类
化学 | 51599篇 |
晶体学 | 584篇 |
力学 | 1510篇 |
综合类 | 269篇 |
数学 | 4593篇 |
物理学 | 8600篇 |
出版年
2024年 | 22篇 |
2023年 | 246篇 |
2022年 | 344篇 |
2021年 | 652篇 |
2020年 | 1759篇 |
2019年 | 3103篇 |
2018年 | 1556篇 |
2017年 | 1176篇 |
2016年 | 4027篇 |
2015年 | 4223篇 |
2014年 | 4354篇 |
2013年 | 5305篇 |
2012年 | 4252篇 |
2011年 | 3665篇 |
2010年 | 3942篇 |
2009年 | 3955篇 |
2008年 | 3636篇 |
2007年 | 2904篇 |
2006年 | 2585篇 |
2005年 | 2604篇 |
2004年 | 2209篇 |
2003年 | 1946篇 |
2002年 | 2717篇 |
2001年 | 1870篇 |
2000年 | 1695篇 |
1999年 | 702篇 |
1998年 | 289篇 |
1997年 | 218篇 |
1996年 | 185篇 |
1995年 | 141篇 |
1994年 | 125篇 |
1993年 | 103篇 |
1992年 | 91篇 |
1991年 | 81篇 |
1990年 | 63篇 |
1989年 | 72篇 |
1988年 | 62篇 |
1987年 | 39篇 |
1986年 | 33篇 |
1985年 | 35篇 |
1984年 | 18篇 |
1983年 | 25篇 |
1982年 | 15篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 14篇 |
1978年 | 18篇 |
1977年 | 8篇 |
1976年 | 10篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 122 毫秒
101.
In this paper we consider the numerical approximation of steady and unsteady generalized Newtonian fluid flows using divergence free finite elements generated by the Powell–Sabin–Heindl elements. We derive a priori and a posteriori finite element error estimates and prove convergence of the method of successive approximations for the steady flow case. A priori error estimates of unsteady flows are also considered. These results provide a theoretical foundation and supporting numerical studies are to be provided in Part II. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
102.
Generation of UV laser light by stimulated Raman scattering in D2, D2/Ar and D2/He using a pulsed Nd:YAG laser at 355nm
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A pulsed Nd:YAG laser at 355nm is used to pump Raman cell filled with D_2, D_2/Ar and D_2/He. With adequately adjusted parameters, the maximum photon conversion efficiency of the first-order Stokes light (S_1, 396.796nm) reaches 33.33% in D_2/Ar and the stability of S_1 in pure D_2 is fairly high, the energy drift being less than 10% when the pump energy drifts in the range of 5%. The conversion efficiency and stability, which are functions of the composition and pressure of the Raman medium and the energy of pump laser, are investigated. The result has been used to optimize the laser transmitter system for a differential absorption lidar system to measure NO_2 concentration profiles. 相似文献
103.
104.
105.
GM(1,1)的周期修正模型及其在泉水流量预测中的应用 总被引:6,自引:2,他引:4
本文阐述了 GM( 1 ,1 )的残差周期修正模型 ,并将它应用于山西神头泉的流量预测中 ,建立了神头泉流量的 GM( 1 ,1 )周期修正模型 ,对其流量进行了预测 ,为神头泉域岩溶水资源开发利用和保护提供了科学依据 相似文献
106.
Ana Luísa Daniel‐da‐Silva João Carlos Moura Bordado José Miguel Martín‐Martínez 《Journal of Polymer Science.Polymer Physics》2007,45(22):3034-3045
The degree of phase separation in several moisture‐cured poly(urethane urea)s (PUUs) was studied by FTIR spectroscopy, wide angle X‐ray diffraction (WAXD), and temperature‐modulated differential scanning calorimetry (TMDSC). This latter technique was shown to be particularly useful in analysing the degree of phase separation in PUU polymers. Both phase mixing and phase segregation coexisted in the PUUs and the degree of phase separation increased as the urea hard segment (HS) content in the PUU increased. The maximum solubility of urea HSs into the polyol soft segments (SSs) was achieved for 50 wt % urea HS content in diol‐based PUUs, whereas for triol‐based PUUs the highest solubility between HS and SS was reached for lower urea HS amount. Finally, the higher the urea HS content the higher the extent of phase separation in the PUU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3034–3045, 2007 相似文献
107.
Mid-infrared generation based on a periodically poled LiNbO3 optical parametric oscillator
下载免费PDF全文
![点击此处可从《中国物理》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We report a nanosecond Nd:YVO_4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO_3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm. 相似文献
108.
J. M. Martínez‐Burgos R. Benavente E. Prez M. L. Cerrada 《Journal of Polymer Science.Polymer Physics》2003,41(11):1244-1255
Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003 相似文献
109.
110.
C. Foces‐Foces M. Lpez‐Rodríguez C. Prez 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(2):o95-o97
The title compound, C12H21NO3, crystallizes in two polymorphic forms, viz. the tetragonal form described here and the monoclinic form described previously [Foces‐Foces, López‐Rodríguez, Pérez, Martín & Pérez‐Hernández (2007). Cryst. Growth Des. 7 , 905–911]. The differences in the conformations of the hydroxymethyl and methylaminocarbonyl substituents have important consequences in the hydrogen‐bond interaction motifs and, therefore, in the packing arrangements. These forms are concomitant polymorphs with melting points differing by 3 K. 相似文献