全文获取类型
收费全文 | 29078篇 |
免费 | 2602篇 |
国内免费 | 2078篇 |
专业分类
化学 | 5298篇 |
晶体学 | 95篇 |
力学 | 2703篇 |
综合类 | 327篇 |
数学 | 18134篇 |
物理学 | 7201篇 |
出版年
2024年 | 48篇 |
2023年 | 262篇 |
2022年 | 524篇 |
2021年 | 554篇 |
2020年 | 671篇 |
2019年 | 862篇 |
2018年 | 748篇 |
2017年 | 826篇 |
2016年 | 880篇 |
2015年 | 715篇 |
2014年 | 1328篇 |
2013年 | 2294篇 |
2012年 | 1417篇 |
2011年 | 1738篇 |
2010年 | 1426篇 |
2009年 | 1889篇 |
2008年 | 1958篇 |
2007年 | 2053篇 |
2006年 | 1710篇 |
2005年 | 1360篇 |
2004年 | 1207篇 |
2003年 | 1208篇 |
2002年 | 1144篇 |
2001年 | 855篇 |
2000年 | 821篇 |
1999年 | 701篇 |
1998年 | 670篇 |
1997年 | 583篇 |
1996年 | 465篇 |
1995年 | 394篇 |
1994年 | 310篇 |
1993年 | 290篇 |
1992年 | 229篇 |
1991年 | 208篇 |
1990年 | 174篇 |
1989年 | 112篇 |
1988年 | 109篇 |
1987年 | 89篇 |
1986年 | 86篇 |
1985年 | 135篇 |
1984年 | 132篇 |
1983年 | 76篇 |
1982年 | 83篇 |
1981年 | 71篇 |
1980年 | 63篇 |
1979年 | 72篇 |
1978年 | 48篇 |
1977年 | 43篇 |
1976年 | 31篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
用直流溅射法制备了6个不同厚度的超薄Ag膜。结合超薄Ag膜的结构特点,采用了Drude模型联合Lorentz Oscillator模型的解谱方法,得到1~6号样品的厚度分别为4.0,6.2,12.5,26.2,30.0和40.6 nm。从拟合结果的消光系数k图谱中发现,在1号到4号样品中分别于430,450,560和570 nm处出现了表面等离子体共振峰(SPR),随膜厚的增加共振峰宽化且峰位红移。最后,利用SPR理论计算出不同厚度Ag薄膜等离子体共振峰出现的位置,并和实验结果进行了比较。 相似文献
992.
针对广义Birkhoff系统动力学,提出广义Birkhoff系统动力学的一类逆问题,研究由已知积分流形来建立广义Birkhoff方程. 这类逆问题的解通常不是唯一的,需给出必要的补充要求. 最后举例说明结果的应用.
关键词:
广义Birkhoff系统
动力学逆问题
积分流形 相似文献
993.
对单模激光增益模型的光强方程加入调频信号,用线性化近似方法计算了以δ函数形式关联的两白噪声驱动下光强的输出功率谱及信噪比. 结果表明,信噪比随抽运噪声和量子噪声强度的变化可出现典型随机共振,受调制信号振幅的影响,信噪比随载波信号频率和调制信号频率的变化出现抑制、单调上升、共振、抑制和共振等几种情况.
关键词:
抽运噪声
单模激光
随机共振
调频信号 相似文献
994.
提出了一种利用电荷耦合器件摄像机获取的辐射信息进行二维弥散介质温度场快速重建的新模型. 与现有的温度场重建模型相比,该模型是以逆向Monte Carlo方法为基础,重建速度更快、效率更高,能更好地描述散射介质. 采用了一个精确的二维温度场作为重建对象,结果表明,即使在存在测量误差的情况下,利用此模型重建出的温度场仍能重现原有精确温度场的特征.
关键词:
温度场重建
逆向Monte Carlo
电荷耦合器件摄像机
反问题 相似文献
995.
Resonances of a hydrogen atom in strong parallel electric and magnetic fields using B-spline basis sets 下载免费PDF全文
The B-spline basis set plus complex scaling method is applied to the numerical calculation of the exact resonance parameters Er and Г/2 of a hydrogen atom in parallel electric and magnetic fields. The method can calculate the ground and higher excited resonances accurately and efficiently. The resonance parameters with accuracies of 10^-9 - 10^-12 for hydrogen atom in parallel fields with different field strengths and symmetries are presented and compared with previous ones. Extension to the calculation of Rydberg atom in crossed electric and magnetic fields and of atomic double excited states in external electric fields is discussed. 相似文献
996.
997.
Yuan J Mei CS Madore B McDannold NJ Panych LP 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,210(1):38-43
The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies. 相似文献
998.
采用化学镀和黏接法制备层状磁电复合材料Ni/PZT/TbFe2,研究其磁电性能及谐振频率随Ni层厚度的变化情况. 结果表明:Ni/PZT/TbFe2层状磁电复合材料与其他结构的磁电性能不同,其一阶弯曲谐振峰值和纵向谐振峰值都很大. 随着Ni层厚度的增加,Ni/PZT/TbFe2层状磁电复合材料的一阶纵向谐振峰值逐渐增大. 结合实验数据和理论计算值得出了材料的一阶弯曲谐振频率fr1和一阶纵向谐振频率f
关键词:
磁电效应
正磁致伸缩
负磁致伸缩
谐振频率 相似文献
999.
A.I. Shames E. Rozenberg Ya.M. Mukovskii E. Sominski A. Gedanken 《Journal of magnetism and magnetic materials》2008,320(14):e8-e11
Electron magnetic resonance (EMR) was studied on crystalline bulk and nanometer-sized samples of the same La0.7Ca0.3MnO3 manganite compound aiming to compare their magnetic homogeneities. The results obtained show that single-crystalline bulk sample is less homogeneous than the nanocrystalline one. Except for higher homogeneity, the nanometer-sized sample also demonstrates a weaker magnetic anisotropy. As a result, well-pronounced coexistence of different magnetic phases (coexistence of ferromagnetic and paramagnetic resonance signals) is observed in the bulk sample, while it is absent in the nanocrystalline one. It is suggested that such strong difference is directly connected to the difference in structural state of the samples. 相似文献
1000.
Sh.B. Abdulvagidov I.K. Kamilov 《Physica C: Superconductivity and its Applications》2008,468(6):453-457
To study a behavior of the thermal conductivity near Tc specific heat and thermal diffusivity of the YBa2Cu3O7−δ high-Tc ceramics were simultaneously measured. Close to Tc = 92.30 K the thermal diffusivity and the thermal conductivity discovered minima and the specific heat – maximum. Quantitative analysis of the influence of thermodynamical fluctuations showed the same power laws with Gaussian exponent equal to 0.5 and existing of crossover from the 3D Gaussian to 3D XY critical behavior in the specific heat and thermal conductivity at the approach to Tc. To explain the minimum in thermal conductivity at Tc we propose a mechanism of scattering of phonons on the superconducting fluctuations. 相似文献