首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2292篇
  免费   337篇
  国内免费   122篇
化学   958篇
晶体学   8篇
力学   274篇
综合类   59篇
数学   593篇
物理学   859篇
  2024年   5篇
  2023年   27篇
  2022年   156篇
  2021年   124篇
  2020年   86篇
  2019年   72篇
  2018年   69篇
  2017年   108篇
  2016年   123篇
  2015年   73篇
  2014年   111篇
  2013年   173篇
  2012年   143篇
  2011年   142篇
  2010年   96篇
  2009年   132篇
  2008年   105篇
  2007年   127篇
  2006年   112篇
  2005年   96篇
  2004年   93篇
  2003年   82篇
  2002年   58篇
  2001年   59篇
  2000年   54篇
  1999年   48篇
  1998年   43篇
  1997年   27篇
  1996年   17篇
  1995年   20篇
  1994年   15篇
  1993年   26篇
  1992年   8篇
  1991年   19篇
  1990年   13篇
  1989年   12篇
  1988年   12篇
  1987年   14篇
  1986年   9篇
  1985年   11篇
  1984年   6篇
  1983年   3篇
  1982年   4篇
  1980年   2篇
  1979年   2篇
  1976年   2篇
  1975年   2篇
  1970年   2篇
  1959年   2篇
  1957年   2篇
排序方式: 共有2751条查询结果,搜索用时 15 毫秒
71.
针对运动预测过程中单一匹配准则的不足提出了一种联合匹配准则,能较好地处理小目标的灰度渐变。基于运动预测过程中不断进行的像素块匹配运算提出了一种新的像素抽样算法,该算法使得运算量大大减少,匹配精度逼近全像素匹配算法。这两种技术的综合应用使得整个运动预测效率有较大改善。  相似文献   
72.
A nondestructive selection technique for predicting ionizing radiation effects of commercial metal-oxide-semiconductor (MOS) devices has been put forward. The basic principle and application details of this technique have been discussed. Practical application for the 54HC04 and 54HC08 circuits has shown that the predicted radiation-sensitive parameters such as threshold voltage, static power supply current and radiation failure total dose are consistent with the experimental results obtained only by measuring original electrical parameters. It is important and necessary to choose suitable information parameters. This novel technique can be used for initial radiation selection of some commercial MOS devices.  相似文献   
73.
李鹤  杨周  张义民  闻邦椿 《物理学报》2011,60(7):70512-070512
根据Takens定理,研究了混沌时间序列相空间重构嵌入维数的选取问题.提出了基于径向基函数神经网络预测模型性能的嵌入维数估计方法,即根据嵌入维数与混沌时间序列预测模型性能的变化关系来确定嵌入维数.通过对几种典型混沌动力学系统的数值验证,结果表明该方法能够确定出合适的相空间重构嵌入维数. 关键词: 混沌 相空间重构 嵌入维数 预测  相似文献   
74.
Kushida and Kushida found that FM radio waves from stations at distances over-the-horizon are received before earthquakes. Based on this finding, since the mid-1990’s, the Kushidas have been practicing “Earthquake Precursor Detection Experiment”. The performance of the Kushida method during 2000–2003 has been evaluated by checking their predictions against the actual seismicity. During the period, there were 92 Kushida predictions mentioning the possibility of M ≥ 5.5 event, whereas there were 49 M ≥ 5.5 earthquakes in the Japanese region. If the criteria for successful prediction are set as: the errors in date is less than one day, epicentral position is roughly within specified area, and error in M is less than 0.5, the success rate was 20% and the alarm rate was 12%. If we relax the criteria to: the errors in dates within 10 days, epicenter within additional 100 km of specified area and the magnitude error less than 1.0, the success rate was 40% and the alarm rate was 27%. These rates may look insufficient for a practical prediction method. Considering, however, the fact that no other short-term prediction has ever been made in Japan so far it is a significant achievement. Moreover, it was found that in almost all failed predictions, meaningful signals were detected although the interpretations were incorrect. This indicates that the method is promising provided further investigation is carried out. The same evaluation at the M ≥ 6.0 level showed that the general performance was similar to the M ≥ 5.5 level, except that both success rate and alarm rate were lower at the M ≥ 6.0 level. If this unexpected finding is real, it might be inherent to the methodology using scattering of short-wave length radio waves as suggested by M. Hayakawa and may contain important information in understanding the earthquake physics and LAI-coupling. The results of the present study indicate strongly that the earthquake prediction research using anomalous transmission of VHF FM radio waves should be enhanced in parallel with complementary research in other frequency ranges.  相似文献   
75.
We report analytical and numerical studies of surface correlations in finite, homogeneously polarizable, classical Coulomb systems placed in an insulating or conducting environment. Their purpose is to understand the phenomenological, shape-dependent laws of electrostatics, from the point of view of statistical mechanics; we focus on the knowledge of the dielectric susceptibility of the system, a quantity proportional to the equilibrium fluctuation of the system's instantaneous polarization per unit volume. This goal has been achieved for a system in a conducting state. The picture is that the shape-dependent part of the susceptibilities results from the action of unbounded observables (the second moments of the instantaneous polarization of the system) on long-range surface correlations and that the relations of electrostatics are verified by means of shape-dependent thermodynamic limits. This picture is supported (i) by exact solutions and asymptotic analysis of the Debye-Hückel approximation of multicomponent plasmas in disks and spheres with insulating and conducting environment and also in ellipses in a vacuum, and (ii) by computer simulations of a one-component plasma in a disk with different environments, notably a conducting environment with permeable and impermeable wall. These observations have revealed for the first time the reason why the susceptibility of a conducting disk in a conductor with impermeable walls diverges linearly with the radius of the disk: this is due to the occurrence of long-range radial correlations in the conductor. These findings are quantitatively interpreted in terms of a novel canonical Debye-Huckel approximation as contrasted to the ordinary grand canonical version. Lastly a fresh look at the problem of the surface correlations of a conductor in a vacuum, which places the observer close to the surface of the conductor but in the vacuum, is presented and applied to the disk, the ellipse, the cylinder, the sphere, and the wedge.  相似文献   
76.
徐歆  胡水清  陶超  杜功焕 《应用声学》2003,22(5):36-40,44
本文应用Short[8-10]改进了的短时非线性预测方法对正常语速的汉语音节和短语进行了分析!究,揭示了汉语语音中浊音和清音的短时非线性预测能力的差异,并且发现这种差异即使在强背景噪声下仍能用短时非线性预测方法加以辨别。这些为浊音和清音的切分提供了一种可能性手段。  相似文献   
77.
李军  刘君华 《物理学报》2005,54(10):4569-4577
提出了一种新颖的广义径向基函数神经网络模型,其径向基函数(RBF)的形式由生成函数确定.然后,给出了易实现的梯度学习算法,同时为了进一步提高网络的收敛速度和网络性能,又给出了基于卡尔曼滤波的动态学习算法.为了验证网络的学习性能,采用基于卡尔曼滤波算法的新型广义RBF网络预测模型对Mackey-Glass混沌时间序列和Henon映射进行了仿真.结果表明,所提出的新型广义RBF神经网络模型能快速、精确地预测混沌时间序列,是研究复杂非线性动力系统辨识和控制的一种有效方法. 关键词: 广义径向基函数神经网络 卡尔曼滤波 梯度下降学习算法 混沌时间序列 预测  相似文献   
78.
吴晗平 《应用光学》2005,26(4):30-32
在试验数据有限的情况下,通过分析激光测距系统工作模式和建立系统可靠性模型解决了小批量、复杂的某型激光测距系统可靠性评价问题。根据以前相似产品的试验数据,采用相似电路法对系统可靠性进行预计。根据激光晶体、染料片、全反射镜、聚光腔等元器件的寿命,对激光器寿命进行预计,并在实际使用中得到了验证。这对系统设计和使用具有重要参考作用。  相似文献   
79.
介绍了光纤寿命预期的疲劳实验方法和筛选实验方法。采用这两种方法,利用电子万能材料试验机和光纤筛选复绕机对室温下光纤进行了疲劳性能实验。实验得出,在张力小于850g的情况下,制导光纤的寿命可以达到10年以上;光纤的韦伯参数md和疲劳参数nd的大小影响光纤寿命。实验表明,当光纤的使用状态受力较大时,或使用贮存状态相差较大时,宜选用动态方法进行寿命预期;当贮存和使用状态受力变化不大时,宜选用筛选复绕方法预期光纤寿命。制导光纤寿命模型宜选用动态疲劳模型,通信光纤可选用筛选实验模型。实验还表明,提高光纤的抗疲劳因子nd和减小光缆中光纤的应变,可以更好地保持光缆中光纤的使用寿命和可靠性。  相似文献   
80.
Artificial neural networks are trained to forecast the plasma disruption in HL-2A tokamak. Optimized network architecture is obtained. Saliency analysis is made to assess the relative importance of different diagnostic signals as network input. The trained networks can successfully detect the disruptive pulses of HL-2A tokamak. The results obtained show the possibility of developing a neural network predictor that intervenes well in advance for avoiding plasma disruption or mitigating its effects.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号