全文获取类型
收费全文 | 24977篇 |
免费 | 3623篇 |
国内免费 | 2351篇 |
专业分类
化学 | 18430篇 |
晶体学 | 288篇 |
力学 | 1304篇 |
综合类 | 177篇 |
数学 | 3063篇 |
物理学 | 7689篇 |
出版年
2024年 | 44篇 |
2023年 | 462篇 |
2022年 | 714篇 |
2021年 | 840篇 |
2020年 | 1022篇 |
2019年 | 894篇 |
2018年 | 769篇 |
2017年 | 710篇 |
2016年 | 1175篇 |
2015年 | 1162篇 |
2014年 | 1340篇 |
2013年 | 1890篇 |
2012年 | 2197篇 |
2011年 | 2215篇 |
2010年 | 1509篇 |
2009年 | 1422篇 |
2008年 | 1688篇 |
2007年 | 1449篇 |
2006年 | 1267篇 |
2005年 | 1148篇 |
2004年 | 935篇 |
2003年 | 788篇 |
2002年 | 800篇 |
2001年 | 606篇 |
2000年 | 480篇 |
1999年 | 513篇 |
1998年 | 340篇 |
1997年 | 270篇 |
1996年 | 335篇 |
1995年 | 271篇 |
1994年 | 235篇 |
1993年 | 207篇 |
1992年 | 178篇 |
1991年 | 159篇 |
1990年 | 137篇 |
1989年 | 121篇 |
1988年 | 94篇 |
1987年 | 81篇 |
1986年 | 73篇 |
1985年 | 74篇 |
1984年 | 50篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1981年 | 29篇 |
1980年 | 19篇 |
1979年 | 13篇 |
1976年 | 14篇 |
1975年 | 16篇 |
1959年 | 13篇 |
1957年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
91.
H.L. Li H.T. Lin Y.H. Wu T. Liu Z.L. Zhao G.C. Han T.C. Chong 《Journal of magnetism and magnetic materials》2006
We report on the growth and characterization of delta-doped amorphous Ge:Mn diluted magnetic semiconductor thin films on GaAs (0 0 1) substrates. The fabricated samples exhibit different magnetic behaviors, depending on the Mn doping concentration. The Curie temperature was found to be dependent on both the Mn doping concentration and spacing between the doping layers. A sharp drop in magnetization and rise in resistivity are observed at low temperature in samples with high Mn doping concentrations, which is also accompanied by a negative thermal remanent magnetization (TRM) in the higher temperature range. The temperature at which the magnetization starts to drop and the negative TRM appears show a correlation with the Mn doping concentration. The experimental results are discussed based on the formation of ferromagnetic regions at high temperature and antiferromagnetic coupling between these regions at low temperature. 相似文献
92.
The general expressions for the compliance , Young's modulus E(h k l) and Poisson's ratio υ(h k l) along an arbitrary direction [h k l] are given for cubic crystals. The representation surfaces, for which the length of the radius vector in the [h k l] direction equals to E(h k l) or υ(h k l), are constructed for seven BCC transition metals Cr, Fe, Mo, Nb, Ta, V and W. Neglecting W, which is isotropic, both representation surfaces of Young's modulus and Poisson's ratio are spherical surfaces. The remaining BCC transition metals may be grouped into two classes. In the first group (Cr, Mo, Nb and V) with negative values of sA, Young's modulus surface has eight depressed corners and six rounded protuberances at the centers of the faces. In the second group (Fe and Ta) with positive values of sA, on the contrary, Young's modulus surface has eight rounded protuberance corners and six depressions at the centers of the faces. The contrary conclusions are obtained for Poisson's ratio. 相似文献
93.
A new constant-pressure molecular dynamics (MD) method is developed to simulate the dynamic behavior and structure transition of finite system under external pressure. In this method, no artificial parameter is introduced and the computation overheads are very small. As an application, a hard-soft transition of single wall carbon nanotube (SWCNT) under external pressure is found, which is in agreement with the experiments.
相似文献94.
将心磁信号从干扰噪声中加以提取并有效地消除噪声干扰是心磁信号处理中尤为重要的环节 .从改进算法的角度出发,提出互补型自适应滤波器结构以实现心磁信号的消噪处理.该滤波器针对心磁这类非平稳信号进行设计,有效地解决了常规自适应滤波器应用于心磁信号处理时收敛速度和稳态误差的矛盾.通过仿真实验和心磁实验结果表明,该算法能有效地消除心磁信号的背景噪声和工频干扰噪声.同时该算法也可用于其他非平稳信号的消噪处理.
关键词:
自适应滤波
心磁图
最小均方误差 相似文献
95.
就如何在4英寸热氧化硅衬底上沉积高质量的磁性隧道结纳米多层薄膜材料和如何利用光刻方法微加工制备均匀性较好的磁性隧道结方面做了初步研究,并对磁性隧 道结的磁电性质及其工作特性进行了初步测量和讨论.利用现有的光刻设备和工艺条 件在4英寸热氧化硅衬底上直接制备出的磁性隧道结,其结电阻与面积的积 矢的绝对误差在10% 以内,隧穿磁电阻的绝对误差在7% 以内,样品的磁性隧道结性质具有较好的均匀性和一致性,可以满足研制磁随机存储器存储单元演示器件的基本要求.
关键词:
磁性隧道结
隧穿磁电阻
磁随机存储器
4英寸热氧化硅衬底 相似文献
96.
HaiHuiXIE XiaoYiWEI JiDongWANG MeiFangLIU RenZhouYANG 《中国化学快报》2003,14(6):588-590
A New Annonaceous acetogenin,squamostolide(1),was isolated from the seeds of Annona squamosa.Its structure was elucidated based on spectroscopic methods and comparison with known compounds.It is the first example of Annonaceous acetogenin with each of the two ends of the aliphatic chain bearing a γ-lactone.Thenew compound exhibited cytotoxic activity in vitro against bel-7402 and CNE2 human tumor cell lines. 相似文献
97.
98.
By using the methods of the matrix decomposition and expansion of the hard-edged aperture function into a finite sum of complex Gaussian functions, the recurrence propagation expressions for a flattened Gaussian beam (FGB) through multi-apertured optical imaging systems of B = 0 are derived and illustrated with numerical examples. Comparisons with the straightforward numerical integration of the Collins formula and with the previous work are made. It is shown that the main advantages of our methods and results are the more accuracy and great reduction of computer time. 相似文献
99.
通过保持非耗散介观LC电路的固有频率不变,而使电路参数作阶跃函数变化,就可将介观LC电路由初始的偶相干态制备到压缩偶相干态;在压缩偶相干态下,介观电路系统不仅有非经典的量子压缩效应,而且有非经典的反聚束效应.
关键词:
介观LC电路
单位阶跃函数
压缩算符
压缩偶相干态 相似文献
100.
Yu-hai WU~ 《中国科学A辑(英文版)》2007,50(7):925-940
This paper concerns the number and distributions of limit cycles in a Z_2-equivariant quintic planar vector field.25 limit cycles are found in this special planar polynomial system and four different configurations of these limit cycles are also given by using the methods of the bifurcation theory and the qualitative analysis of the differential equation.It can be concluded that H(5)≥25=5~2, where H(5)is the Hilbert number for quintic polynomial systems.The results obtained are useful to study the weakened 16th Hilbert problem. 相似文献