全文获取类型
收费全文 | 155228篇 |
免费 | 6272篇 |
国内免费 | 5934篇 |
专业分类
化学 | 71503篇 |
晶体学 | 1585篇 |
力学 | 11374篇 |
综合类 | 581篇 |
数学 | 39674篇 |
物理学 | 42717篇 |
出版年
2025年 | 108篇 |
2024年 | 920篇 |
2023年 | 1767篇 |
2022年 | 2544篇 |
2021年 | 2992篇 |
2020年 | 3311篇 |
2019年 | 3076篇 |
2018年 | 12148篇 |
2017年 | 11768篇 |
2016年 | 8884篇 |
2015年 | 4072篇 |
2014年 | 4112篇 |
2013年 | 5265篇 |
2012年 | 9624篇 |
2011年 | 16165篇 |
2010年 | 9527篇 |
2009年 | 9823篇 |
2008年 | 10600篇 |
2007年 | 12299篇 |
2006年 | 3804篇 |
2005年 | 4393篇 |
2004年 | 4006篇 |
2003年 | 3825篇 |
2002年 | 2692篇 |
2001年 | 1783篇 |
2000年 | 1740篇 |
1999年 | 1742篇 |
1998年 | 1550篇 |
1997年 | 1327篇 |
1996年 | 1436篇 |
1995年 | 1206篇 |
1994年 | 1105篇 |
1993年 | 984篇 |
1992年 | 839篇 |
1991年 | 738篇 |
1990年 | 633篇 |
1989年 | 574篇 |
1988年 | 457篇 |
1987年 | 420篇 |
1986年 | 360篇 |
1985年 | 353篇 |
1984年 | 246篇 |
1983年 | 214篇 |
1982年 | 185篇 |
1981年 | 157篇 |
1980年 | 132篇 |
1979年 | 94篇 |
1978年 | 99篇 |
1975年 | 74篇 |
1973年 | 84篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
J A Simmons E G Freedman S B Stevenson L Chen T J Wohlgenant 《The Journal of the Acoustical Society of America》1989,86(4):1318-1332
The ability of the echolocating bat, Eptesicus fuscus, to detect a sonar target is affected by the presence of other targets along the same axis at slightly different ranges. If echoes from one target arrive at about the same delay as echoes from another target, clutter interference occurs and one set of echoes masks the other. Although the bat's sonar emissions and the echoes themselves are 2 to 5 ms long, echoes (of approximately equal sensation levels--around 15 dB SL) only interfere with each other if they arrive within 200 to 400 microseconds of the same arrival time. This figure is an estimate of the integration time of the bat's sonar receiver for echoes. The fine structure of the clutter-interference data reflects the reinforcement and cancellation of echoes according to their time separation. When clutter interference first occurs, the waveforms of test and cluttering echoes already overlap for much of their duration. The masking effect underlying clutter interference appears specifically due to overlap, not between raw echo waveforms, but between the patterns of mechanical excitation created when echoes pass through bandpass filters equivalent to auditory-nerve tuning curves. While the time scale of clutter interference is substantially shorter than the duration of echo waveforms, it still is much longer than the eventual width of a target's range-axis image expressed in terms of echo delay. 相似文献
942.
943.
A very simple method for reducing probehead background signal in one-pulse 1H nuclear magnetic resonance (NMR) spectra is presented. Two one-pulse spectra are recorded, the first with pulse length tp1, the second with an L-times longer pulse, e.g. with L = 2. The second spectrum scaled by 1/L is subtracted from the first. Since the weak pulses experienced by spins outside the coil are in the linear regime, the background from outside the coil is effectively subtracted out. The background suppression efficiency is approximately 1.5b2, where b is the ratio of the B1 field inside the coil relative to that outside the coil. Experimentally, background suppression by at least a factor of 10 was achieved. Examples of background suppression in 1H wideline as well as 1H fast magic-angle spinning (MAS) one-pulse spectra of clay and polymer samples are shown. 相似文献
944.
An efficiency of 8.2% is demonstrated for a diode-pumped passively Q-switched self-stimulated Raman laser with an a-cut mixed vanadate crystal, Nd:Gd0.8Y0.2VO4. At 2.2 W of incident pump power, the self-stimulated Raman laser produces pulses as short as 660 ps at a Stokes wavelength of 1175 nm with 2.7 microJ of energy per pulse at a 66-kHz repetition rate. 相似文献
945.
John F. Gunion 《Pramana》2004,62(2):283-305
I summarize the basic theory and selected phenomenology for the Higgs boson(s) of the standard model, the minimal supersymmetric
model and some extensions thereof, including the next-to-minimal supersymmetric model. 相似文献
946.
Bao-Quan?Ai Xian-Ju?Wang Guo-Tao?Liu Hui-Zhang?Xie De-Hua?Wen Wei?Chen Liang-Gang?LiuEmail author 《The European Physical Journal B - Condensed Matter and Complex Systems》2004,37(4):523-526
We present a study of the transport of a Brownian particle moving in a periodic symmetric potential in the presence of asymmetric unbiased fluctuations. The particle is considered to move in a medium with periodic space dependent friction. By tuning the parameters of the system, the direction of the current exhibits reversals, both as a function of temperature as well as the amplitude of rocking force. We found that the mutual interplay between the opposite driving factors is the necessary term for current reversals.Received: 11 October 2003, Published online: 9 April 2004PACS:
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 02.50.Ey Stochastic processes - 87.10. + e General theory and mathematical aspects 相似文献
947.
Electron cyclotron resonance (ECR) plasma is produced and studied in a small cylindrical system. Microwave power is delivered
by a CW magnetron at 2.45 GHz in TE10 mode and launched radially to have extraordinary (X) wave in plasma. The axial magnetic field required for ECR in the system
is such that the first two ECR surfaces (B = 875.0 G andB = 437.5 G) reside in the system. ECR plasma is produced with hydrogen with typical plasma density ne as 3.2 × 1010 cm-3 and plasma temperature Te between 9 and 15 eV. Various cut-off and resonance positions are identified in the plasma system. ECR heating (ECRH) of the
plasma is observed experimentally. This heating is because of the mode conversion of X-wave to electron Bernstein wave (EBW)
at the upper hybrid resonance (UHR) layer. The power mode conversion efficiency is estimated to be 0.85 for this system. The
experimental results are presented in this paper. 相似文献
948.
949.
Jyotsna Galgale Nupinder Kaur Preeti Singh Manik Manake N. K. Gaur R. K. Singh 《Pramana》2004,63(2):413-417
An extended three-body force shell model (ETSM) has been applied to investigate the static and dynamic properties of KCN
x
Cl1−x
for the compositionx = 0.56 and 1.0 at 300 K. The phonon dispersion curves computed by us are compared with the single crystal neutron diffraction
data. The unusual features of these curves are the upward curvature seen in some of the acoustic branches. This is a result
ofK-dependent softening of the phonon due to translation-rotation coupling. The transverse acoustic branch is more soft near
the zone centre. 相似文献
950.
A New Cohomology Theory of Orbifold 总被引:7,自引:0,他引:7
Based on the orbifold string theory model in physics, we construct a new cohomology ring for any almost complex orbifold. The key theorem is the associativity of this new ring. Some examples are computed.Both authors partially supported by the National Science Foundation 相似文献