全文获取类型
收费全文 | 836篇 |
免费 | 126篇 |
国内免费 | 82篇 |
专业分类
化学 | 453篇 |
晶体学 | 7篇 |
力学 | 27篇 |
综合类 | 7篇 |
数学 | 9篇 |
物理学 | 541篇 |
出版年
2024年 | 2篇 |
2023年 | 23篇 |
2022年 | 56篇 |
2021年 | 94篇 |
2020年 | 68篇 |
2019年 | 26篇 |
2018年 | 34篇 |
2017年 | 49篇 |
2016年 | 50篇 |
2015年 | 29篇 |
2014年 | 70篇 |
2013年 | 77篇 |
2012年 | 46篇 |
2011年 | 41篇 |
2010年 | 33篇 |
2009年 | 29篇 |
2008年 | 30篇 |
2007年 | 43篇 |
2006年 | 38篇 |
2005年 | 19篇 |
2004年 | 25篇 |
2003年 | 29篇 |
2002年 | 21篇 |
2001年 | 16篇 |
2000年 | 17篇 |
1999年 | 18篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有1044条查询结果,搜索用时 140 毫秒
61.
激光超声的原理及其在固体中的应用 总被引:5,自引:0,他引:5
激光超声是指脉冲激光所产生的脉冲超声。激光超声是较新的产生超声的方法。它的主要特色是,可以遥发遥收,这样就有可能在高温、有危险辐射等恶劣环境下以及在样品运动的生产线上进行超声检测。皮秒级和飞秒级的激光所激发的超声近来又用来研究固体中电子和声子的相互作用。文章介绍了固体中激光超声的声学性能及两种产生机理:热弹机理和电子机理。 相似文献
62.
The use of non-contact laser techniques for the generation of ultrasound has extended the limits of the application of traditional ultrasonic techniques. This paper focusses on the use of one such non-contact laser technique, known as ‘optical fibre array’, to generate shear and surface waves. The shear wave experimental directivity pattern results are presented and compared with the theoretical results of a single source and an array source. The experimental directivity results for the surface wave are also presented, and compared with the theoretical results. The data show that the array enhances signal generation in the forward direction for both shear and surface waves. The array gain is also discussed. The receiver for the directivity measurements was a contact piezoelectric transducer. 相似文献
63.
64.
65.
梁顺全 《广东微量元素科学》1999,6(5):72-73
用B超对119例晚期妊娠出现高腹压征候群的病人分析。结果显示,该症与孕妇体位、妊娠期间胎位,羊水的多少,初产孕妇及一些不确定的体外倒转引起的孕妇腔静脉受压部位及程度关系密切。认为在仰卧位或截石位的羊水偏少和早破水的晚期孕妇,若出现不明原因头晕眼花,大汗淋漓,心慌心跳,甚至休克,应迅速改变体位,及时纠正。 相似文献
66.
Non-contact protein microarray fabrication using a procedure based on liquid bridge formation 总被引:1,自引:1,他引:0
Michael Hartmann Johan Sjödahl Mårten Stjernström Johan Redeby Thomas Joos Johan Roeraade 《Analytical and bioanalytical chemistry》2009,393(2):591-598
Contemporary microarrayers of contact or non-contact format used in protein microarray fabrication still suffer from a number
of problems, e.g. generation of satellite spots, inhomogeneous spots, misplaced or even absent spots, and sample carryover.
In this paper, a new concept of non-contact sample deposition that reduces such problems is introduced. To show the potential
and robustness of this pressure-assisted deposition technique, different sample solutions known to cause severe problems or
to be even impossible to print with conventional microarrayers were accurately printed. The samples included 200 mg mL–1 human serum albumin, highly concentrated sticky cell adhesion proteins, pure high-salt cell-lysis buffer, pure DMSO, and
a suspension of 5-μm polystyrene beads. Additionally, a water-immiscible liquid fluorocarbon, which was shown not to affect
the functionality of the capture molecules, was employed as a lid to reduce evaporation during microarray printing. The fluorocarbon
liquid lid was shown to circumvent hydrolysis of water-sensitive activated surfaces during long-term deposition procedures. 相似文献
67.
The exact frequency shift of an AFM non-uniform probe with an elastically restrained root, subjected to van der Waals force, is derived. The original distributed system is considered and then its exact fundamental solutions and the general frequency equation are derived. Results are compared with those by the force gradient method and the perturbation method. The effects of several parameters on the sensitivity of measurement are investigated. Results show that the interpretation of frequency shift by using the force gradient method is unsatisfactory. The smaller the amplitude of oscillation and the tip–surface distance are, the larger the frequency shift. The design of a taper beam is recommended for increasing the sensitivity of measurement. 相似文献
68.
69.
An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element. 相似文献
70.
Numerical simulations of partial elements excitation for hemispherical high-intensity focused ultrasound phased transducer 下载免费PDF全文
Yanqiu Zhang 《中国物理 B》2021,30(7):78704-078704
The hemispherical phased transducer maximizes the coverage of the skull and the ultrasonic energy per unit area of the skull is minimized, thereby reducing the risk of skull burns, but the transducer has a small focal area adjustment range, increasing the focal length of treatment is an urgent question for this type of transducer. In this paper, a three-dimensional high-intensity focused ultrasound (HIFU) transcranial propagation model is established based on the human head structure. The finite difference time domain (FDTD) is combined with the Westervelt acoustic wave nonlinear propagation equation and Penne's biological heat conduction equation for numerical simulation of the sound pressure field and temperature field. Forming a treatable focal area in a small-opening hemispherical transducer with a small amount of numerical simulation calculation focusing at a set position to determine the minimum partial excitation area ratio of focusing. And then, applying these preliminary results to a large-opening diameter hemispherical transducer and the temperature field formed by it or full excitation is studied. The results show that the focus area with the excitation area ratio of less than 22% moves forward to the transducer side when the excitation sound is formed. When the excitation area ratio is greater than or equal to 23%, it focuses at the set position. In the case of partial incentives, using 23% of the partial array, the adjustable range of the treatable focal area formed in the three-dimensional space is larger than that of the full excitation. 相似文献