全文获取类型
收费全文 | 7948篇 |
免费 | 758篇 |
国内免费 | 744篇 |
专业分类
化学 | 2640篇 |
晶体学 | 58篇 |
力学 | 267篇 |
综合类 | 165篇 |
数学 | 2726篇 |
物理学 | 3594篇 |
出版年
2024年 | 28篇 |
2023年 | 76篇 |
2022年 | 242篇 |
2021年 | 237篇 |
2020年 | 231篇 |
2019年 | 195篇 |
2018年 | 219篇 |
2017年 | 237篇 |
2016年 | 270篇 |
2015年 | 223篇 |
2014年 | 445篇 |
2013年 | 607篇 |
2012年 | 390篇 |
2011年 | 480篇 |
2010年 | 387篇 |
2009年 | 501篇 |
2008年 | 545篇 |
2007年 | 564篇 |
2006年 | 443篇 |
2005年 | 370篇 |
2004年 | 315篇 |
2003年 | 342篇 |
2002年 | 272篇 |
2001年 | 235篇 |
2000年 | 261篇 |
1999年 | 187篇 |
1998年 | 173篇 |
1997年 | 170篇 |
1996年 | 159篇 |
1995年 | 100篇 |
1994年 | 72篇 |
1993年 | 65篇 |
1992年 | 52篇 |
1991年 | 46篇 |
1990年 | 39篇 |
1989年 | 25篇 |
1988年 | 37篇 |
1987年 | 22篇 |
1986年 | 30篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 10篇 |
1982年 | 31篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 18篇 |
1978年 | 13篇 |
1976年 | 2篇 |
1970年 | 1篇 |
1968年 | 2篇 |
排序方式: 共有9450条查询结果,搜索用时 15 毫秒
61.
依据多媒体教育资源各层次特征及其关系,通过定义资源媒体特征空间,构建了教育资源库层次索引模型,各层次特征间的映射规则将资源的高层语义特征映射到低层媒体特征,达到了扩充描述语义的目的,提高了查全率;结合学科本体,规范了资源标注内容和用户查询条件的描述,减少了人为因素对查询项和索引项匹配相似度计算的影响,提高了查准率. 相似文献
62.
63.
64.
详细介绍用等效折射率概念设计短波通滤光片的原理和计算方法。根据原理和方法,选择二氧化钛(TiO2)作为高折射率材料、二氧化硅(SiO2)作为低折射率材料。首先从理论上计算出用这2种材料设计的波长λ=950~1150nm的短波通滤光片所需要的周期数,然后给出短波通滤光片的主膜系和光谱曲线。由于据此周期数设计出的膜系光谱曲线在750~810nm处的透过率不符合要求,因此对该膜系进行了改进。依照改进的设计进行多次制备,最终制备出了符合要求的短波通滤光片,找到了最佳制备工艺和方法。最后,对制备出来的短波通滤光片薄膜进行了各种环境实验。实验结果表明,膜层的各项指标符合设计要求。 相似文献
65.
All-electron full-potential linearized augmented plane-wave calculations of the surface energy, work function, and interlayer spacings of close-packed metal surfaces are presented, in particular, for the free-electron-like metal surfaces, Mg(0 0 0 1) and Al(1 1 1), and for the transition metal surfaces, Ti(0 0 0 1), Cu(1 1 1), Pd(1 1 1), and Pt(1 1 1). We investigate the convergence of the surface energy as a function of the number of layers in the slab, using the Cu(1 1 1) surface as an example. The results show that the surface energy, as obtained using total energies of the slab and bulk from separate calculations, converges well with respect to the number of layers in the slab. Obviously, it is necessary that bulk and surface calculations are performed with the same high accuracy. Furthermore, we discuss the performance of the local-density and generalized gradient approximations for the exchange-correlation functional in describing the various surface properties. 相似文献
66.
Density functional theory (DFT) slab calculations, mainly using the generalised gradient approximation, have been used to investigate the minimum energy structures of molecular SO2 and SO3 on Cu(1 1 1) and Ni(1 1 1) surfaces. On Ni(1 1 1) the optimal local adsorption structures are in close agreement with experimental results for both molecular species obtained using the X-ray standing wavefield technique, although for adsorbed SO2 the energetic difference between two alternative lateral positions of the lying-down molecule on the surface is marginally significant. On Cu(1 1 1) the results for adsorbed SO2, in particular, were sensitive to the DFT functional used in the calculations, but in all cases failed to reproduce the experimentally-established preference for adsorption with the molecular plane perpendicular to the surface. This result is discussed in the context of previously published DFT results for these species adsorbed on Cu(1 0 0). The optimal geometry found for SO3 on Cu(1 1 1) is similar to that on Ni(1 1 1), providing agreement with experiment regarding the molecular orientation but not the adsorption site. 相似文献
67.
68.
By introducing a four-layer step-index waveguide modeling, the characteristics of long-period fiber grating (LPFG) with an nm-thick film overlay, which has higher refractive index than that of fiber cladding are investigated in detail. The influence of both the overlay thickness and refractive index on the tuning ability of LPFG is analyzed. The numerical results demonstrate that spectral response of LPFG is divided into three distinct regions as the overlay deposition increases and the shift of resonant wavelength is drastic in some special thickness range. In conjunction with higher-order cladding mode coupling and fiber cladding etching, the sensitivity of LPFG to the overlay refractive index is enhanced significantly and over 120 nm resonant wavelength tunable range is achieved. 相似文献
69.
Daniel Quillen 《K-Theory》1989,3(3):205-246
We construct a spectral sequence to study cyclic cohomology for an extension A = R/I of algebras. When R is a free algebra we describe the cyclic cohomology of A in terms of traces defined on R or powers of I. Explicit cyclic cocycles representing the cyclic cohomology class belonging to the trace are constructed as analogues of Chern character and Chern-Simons forms.Dedicated to Alexander Grothendieck 相似文献
70.
Jipu Ma 《中国科学A辑(英文版)》1997,40(12):1233-1238
A generalized dimension is further developed. Here subtraction and addition of two generalized dimensions are defined, so
that the operations: ∞ ± n = ∞, ∞ + ∞ = ∞, which used to play an inflexible role, are refined and moreover, ∞ - ∞, which used
to be meaningless, is done in sense. Then generalized index for semi-Fred-holm operators is developed to wholeB(H), i.e. all of bounded linear operators in Hilbert spaceH. Theorem 2.2 is proved with an example, which is in contradiction to a known proposition for semi-Fredholm operators in form,
practically a refined result of the known proposition. Then, it is proved thatB(H) is the union of countably many disjoint arewise connected sets over all the generalized dimensions ofB(H).
Project supported by the National Natural Science Foundation of China 相似文献