全文获取类型
收费全文 | 608篇 |
免费 | 39篇 |
国内免费 | 19篇 |
专业分类
化学 | 49篇 |
力学 | 21篇 |
综合类 | 7篇 |
数学 | 1篇 |
物理学 | 115篇 |
综合类 | 473篇 |
出版年
2024年 | 2篇 |
2023年 | 1篇 |
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 6篇 |
2019年 | 12篇 |
2018年 | 12篇 |
2017年 | 11篇 |
2016年 | 11篇 |
2015年 | 18篇 |
2014年 | 23篇 |
2013年 | 25篇 |
2012年 | 42篇 |
2011年 | 32篇 |
2010年 | 25篇 |
2009年 | 23篇 |
2008年 | 28篇 |
2007年 | 36篇 |
2006年 | 18篇 |
2005年 | 32篇 |
2004年 | 30篇 |
2003年 | 41篇 |
2002年 | 33篇 |
2001年 | 23篇 |
2000年 | 25篇 |
1999年 | 11篇 |
1998年 | 19篇 |
1997年 | 19篇 |
1996年 | 16篇 |
1995年 | 8篇 |
1994年 | 11篇 |
1993年 | 14篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1981年 | 1篇 |
排序方式: 共有666条查询结果,搜索用时 296 毫秒
411.
412.
A theoretical model for the evaluation of measurement uncertainty of a sound level meter (hereafter as `SLM') calibration by comparison method in an anechoic room was developed. Through this model, the uncertainties in the semi-automatic calibration and that in the full-automatic calibration were estimated for the recently developed SLM calibration system. In order to estimate the standard uncertainty against the SLM positioning, which is a significant uncertainty component, the sound field curve-fitting formulae were adopted. The validity of the curve-fitting method was proven by the similarity of the spatial distributions of radiation sound field produced by the plane circular piston source and that by the cone shape source. A linear equation was used to fit the measurements of the sound field distribution along the radiation axis. A quadratic equation was used to fit the measurements along the radial axis normal to the radiation axis. The fitting parameters gave us the sensitivity coefficients of the propagation of the uncertainty. In addition, one of the quadratic fitting parameters was found to be a positional uncertainty itself. Using this model, the expanded uncertainties were evaluated for the semi-automatic and full-automatic calibration of SLM. 相似文献
413.
414.
分析了采用标准电能表对三相三线电能表进行检验时,不同测量方式的优缺点。三表法测量无间断点,校验过程中需改变电流元件接线;矢量法采样精度高,校验时需标准表与系统电压中性点连接;两表法测量有间断点,校验时需改变电压元件接线。实验结果表明,三表法和矢量法比两表法相对误差较小。 相似文献
415.
Ran Qiquan 《物理与工程》1998,(3)
本文描述用于照射量仪校准工作的薄窗监测电离室的结构、性能及使用方法.就重复性而言,电离室的读数在0.2%以内相符;用于校准现场仪器时,在中等过滤的情况下误差小于1%,重过滤下不大于2%. 相似文献
416.
在区域地质构造分析的基础上,采用空芯包体测量方法,对东欢索井田两个水平进行了地应力测量,得出东欢坨矿地应力场属于水平应力场,井田内存在着较大的水平构造应力的结论。测量成果为井下工程设计和稳定性分析、巷道合理布局和锚杆支护设计提供了依据。并为进一步研究井田岩体应力状态和应力场的区域预测奠定了基础。 相似文献
417.
418.
磁暴等空间天气事件会引起热层中性成分O和N2浓度的显著变化,故常将氧原子和氮分子的柱密度之比O/N2作为电离层热层受扰动的标志。研究表明,O和N2柱密度之比O/N2与远紫外气辉OⅠ 135.6 nm和N2 Lyman-Birge-Hopfield(LBH)的柱辐射强度之比135.6/LBH具有很好的相关性,因此远紫外光学遥感探测对于监测磁暴等空间天气事件显得尤为重要。自20世纪70年代以来,国外就开展了远紫外气辉电离层探测的研究工作,并相继发射了多颗相关卫星,尤其是美国、日本以及瑞典等国家。而我国在轨运行的星载光学遥感探测仪器中,只有一些工作在微波波段、可见光波段的载荷,还没有在远紫外波段工作的遥感探测仪器,直到2017年11月风云三号D气象卫星的成功发射,卫星上搭载的电离层光度计是我国首台星载远紫外气辉遥感探测载荷,它为我们提供了一系列具有自主知识产权的远紫外探测数据,为开展电离层O/N2特性的研究奠定了基础。首先阐述了利用柱辐射强度之比135.6/LBH来反演热层中性成分柱密度之比O/N2的理论依据。其次,基于MSISE-00大气模型,利用AURIC来仿真计算135.6/LBH与O/N2之间的比例系数,然后利用电离层光度计实时观测的远紫外气辉数据来反演电离层O/N2,进一步验证磁暴期间电离层中性成分受扰动的情况。在数据处理过程中,采用了切比雪夫滤波器滤波方式针对数据中的带外杂散光进行了处理,进一步抑制了杂散光信号对远紫外光谱信号的影响。最后,将电离层光度计O/N2的反演结果与国外光学遥感载荷全球紫外成像仪GUVI(Global Ultraviolet Imager)的结果进行对比,结果显示二者对磁暴的响应一致,O/N2的产品误差RMS约为0.319 6,文中对造成两者差异的可能原因做出了初步分析,为后续工作的开展奠定了基础。此次研究,首次利用我国自主研发的远紫外气辉遥感探测载荷进行数据反演和分析,这对我国电离层远紫外遥感探测技术的发展具有重要意义。 相似文献
419.
Vortex shedding from short circular cylinders with a slit was studied using a flow visualization and amplitude spectrum analysis of a thermoanemometry probe signal. It was found that a circular cylinder with a slit and concave rear surface produces stronger vortices than other bluff cylinders but that these vortices are very vulnerable to the end wall conditions. It was established that two small splitter plates (tails) fixed directly behind the cylinder at the end walls effectively isolate the vortices shed from the cylinder from the end wall boundary layer effects. For this arrangement a perfect regularity of vortex shedding and almost constant Strouhal number were achieved in the Reynolds number test range of about 250 to 43,000.On a leave from Technical University, 60965 Poznan, Piotrowo 3, Poland. 相似文献
420.
Alexandr Gre
enko 《Journal of Terramechanics》2007,44(1):123-131
Dr. Bekker’s first book Theory of land locomotion offers in fact two different concepts of thrust generation on soft ground with respect to the slip: (a) as the push of grousers causing horizontal soil “distortion” and (b) as the shear force in the failure plane linked with the shear deformation. Bekker preferred the second concept and backed it up by the unique shear-ring measuring technique. To clear up the matter, the author decided to re-examine the thrust generation by a track plate experimentally in field conditions. The tests have shown that the initial stage of thrust generation in compressible ground is always horizontal soil compression by grousers, which divides the soil under a track into separate blocks initially at rest. This compression increases at least to the transition point, when a block is sheared off simultaneously at the bottom and in both lateral planes and starts sliding along the channel formed by the preceding grouser. The analysis of these measurements enabled to define the compressive displacement of the face of the soil block (travel of the grouser) appurtenant to the mentioned transition point, useful to define the thrust–slip curve. The case may also be described by the conventional shear stress–shear displacement relationship, imagined to take place in the bottom failure plane, however, namely the “shear displacement” is rather an unusual quantity. 相似文献