全文获取类型
收费全文 | 254篇 |
免费 | 1篇 |
专业分类
化学 | 233篇 |
数学 | 2篇 |
物理学 | 20篇 |
出版年
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 7篇 |
2014年 | 2篇 |
2013年 | 6篇 |
2012年 | 5篇 |
2011年 | 13篇 |
2010年 | 8篇 |
2008年 | 4篇 |
2007年 | 10篇 |
2006年 | 7篇 |
2005年 | 10篇 |
2004年 | 11篇 |
2003年 | 9篇 |
2002年 | 8篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 5篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 9篇 |
1985年 | 5篇 |
1984年 | 10篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 7篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1971年 | 2篇 |
1970年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有255条查询结果,搜索用时 15 毫秒
81.
82.
83.
84.
85.
86.
Narins PM Feng AS Lin W Schnitzler HU Denzinger A Suthers RA Xu C 《The Journal of the Acoustical Society of America》2004,115(2):910-913
Several groups of mammals such as bats, dolphins and whales are known to produce ultrasonic signals which are used for navigation and hunting by means of echolocation, as well as for communication. In contrast, frogs and birds produce sounds during night- and day-time hours that are audible to humans; their sounds are so pervasive that together with those of insects, they are considered the primary sounds of nature. Here we show that an Old World frog (Amolops tormotus) and an oscine songbird (Abroscopus albogularis) living near noisy streams reliably produce acoustic signals that contain prominent ultrasonic harmonics. Our findings provide the first evidence that anurans and passerines are capable of generating tonal ultrasonic call components and should stimulate the quest for additional ultrasonic species. 相似文献
87.
Koblitz JC Stilz P Pflästerer W Melcón ML Schnitzler HU 《The Journal of the Acoustical Society of America》2011,130(5):3090-3099
Reduction of echolocation call source levels in bats has previously been studied using set-ups with one microphone. By using a 16 microphone array, sound pressure level (SPL) variations, possibly caused by the scanning movements of the bat, can be excluded and the sonar beam aiming can be studied. During the last two meters of approach flights to a landing platform in a large flight room, five big brown bats aimed sonar beams at the landing site and reduced the source level on average by 7 dB per halving of distance. Considerable variation was found among the five individuals in the amount of source level reduction ranging from 4 to 9 dB per halving of distance. These results are discussed with respect to automatic gain control and intensity compensation and the combination of the two effects. It is argued that the two effects together do not lead to a stable echo level at the cochlea. This excludes a tightly coupled closed loop feed back control system as an explanation for the observed reduction of signal SPL in landing big brown bats. 相似文献
88.
89.
90.
Wavelength stabilized 980 nm uncooled pump laser modules for erbium-doped fiber amplifiers 总被引:2,自引:0,他引:2
Tomas Pliska Sebastian Arlt Rainer Bttig Tim Kellner Isabella Jung Nicolai Matuschek Pascal Mauron Bernd Mayer Stefan Mohrdiek Jürgen Müller Susanne Pawlik Hans-Ulrich Pfeiffer Berthold Schmidt Boris Sverdlov Stefan Teodoropol Jrg Troger Bernd Valk Christoph Harder 《Optics and Lasers in Engineering》2005,43(3-5):271
We review the development of wavelength stabilized 980 nm pump laser modules without active temperature stabilization for applications in erbium-doped fiber amplifiers. Operation over a wide temperature range with an output power exceeding 400 mW at an ambient temperature of 70°C is demonstrated. The overall reliability of uncooled modules is estimated to be well below 500 FIT at all operating conditions. Such devices are made possible by continuous development and steady improvement of the pump laser chip, the optimization of the fiber Bragg grating stabilization scheme, careful design of the module package, and extended reliability analysis on the basis of stress tests as well as field data. 相似文献