全文获取类型
收费全文 | 1146篇 |
免费 | 271篇 |
国内免费 | 165篇 |
专业分类
化学 | 538篇 |
晶体学 | 26篇 |
力学 | 235篇 |
综合类 | 15篇 |
数学 | 23篇 |
物理学 | 745篇 |
出版年
2024年 | 6篇 |
2023年 | 15篇 |
2022年 | 42篇 |
2021年 | 38篇 |
2020年 | 36篇 |
2019年 | 36篇 |
2018年 | 42篇 |
2017年 | 51篇 |
2016年 | 82篇 |
2015年 | 71篇 |
2014年 | 68篇 |
2013年 | 89篇 |
2012年 | 93篇 |
2011年 | 88篇 |
2010年 | 60篇 |
2009年 | 88篇 |
2008年 | 55篇 |
2007年 | 86篇 |
2006年 | 58篇 |
2005年 | 50篇 |
2004年 | 57篇 |
2003年 | 41篇 |
2002年 | 41篇 |
2001年 | 57篇 |
2000年 | 31篇 |
1999年 | 28篇 |
1998年 | 28篇 |
1997年 | 34篇 |
1996年 | 26篇 |
1995年 | 14篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 7篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 5篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 7篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1971年 | 1篇 |
1969年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有1582条查询结果,搜索用时 109 毫秒
71.
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
72.
K. Yanada M. Sakairi T. Kikuchi Y. Oya Y. Kojima 《Surface and interface analysis : SIA》2010,42(4):189-193
An in situ artificial micro‐pit fabrication method with an area selective electrochemical measurement technique was applied to investigate the effect of the geometry of artificially formed pits on their localized corrosion behavior in anodized 1000 series aluminum. This technique enables the fabrication of artificial micro‐pits with different aspect ratios (pit depth/pit diameter) in solutions. The aspect ratios of the fabricated artificial micro‐pits in this experiment could be varied from 0.13 to 1.83 by controlling the irradiation time of the focused pulsed YAG laser beam. By applying a constant potential to the final laser‐beam‐irradiated spot in chloride environments, localized dissolution occurred only at the laser beam irradiated area, because the anodic oxide film acted as an insulator. The corrosion current and charge increase with increasing aspect ratio at any applied potential. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
73.
Hsin‐Chung Cheng Chau‐Hsiang Wang Chiung‐Fang Huang Yung‐Kang Shen Yi Lin Dong‐Yea Sheu Yi‐Han Hu 《先进技术聚合物》2010,21(9):632-639
In this study, master of the microlens arrays is fabricated using micro dispensing technology, and then electroforming technology is employed to replicate the Ni mold insert of the microlens arrays. Finally, micro hot embossing is performed to replicate the molded microlens arrays from the Ni mold insert. The resin material is used as the dispensing material, which is dropped on a glass substrate. The resin is exposed to a 380 W halogen light. It becomes convex under surface tension on the glass substrate. A master for the microlens arrays is then obtained. A 150‐nm‐thick copper layer is sputtered on the master as an electrically conducting layer. The electroforming method replicates the Ni mold insert from the master of the microlens arrays. Finally, micro hot embossing is adopted to replicate the molded microlens arrays. The micro hot embossing experiment employs optical films of polymethylmethacrylate (PMMA) and polycarbonate (PC). The processing parameters of micro hot embossing are processing temperature, embossing pressure, embossing time, and de‐molding temperature. Taguchi's method is applied to optimize the processing parameters of micro hot embossing for molded microlens arrays. An optical microscope and a surface profiler are utilized to measure the surface profile of the master, the Ni mold insert and the molded microlens arrays. AFM is employed to measure the surface roughness of the master, the Ni mold insert and the molded microlens arrays. The sag height and focal length are determined to elucidate the optical characteristics of the molded microlens arrays. Copyright © 2009 John & Sons, Ltd. 相似文献
74.
微型气相色谱仪热导检测器放大电路设计 总被引:1,自引:0,他引:1
Agilent公司生产的Agilent 3000+系列色谱仪是微型气相色谱仪(micro GC)的典型代表,其热导检测器的信号放大电路和模数转换器(analog-to-digital convertor, ADC)存在功耗大、工作温度过高等不足.文中分析了micro GC电路的功能需求,从选用低噪声的24 bit Δ-Σ ADC ADS1255入手,设计了高共模电压容限、低噪声的全差分放大电路及其他外围电路,并且对全差分放大电路建立了噪声模型,计算了其噪声理论值,优化了系统设计参数.另外,还设计了一个测试平台,对所设计的全差分放大电路和ADC的性能进行了全面的测试评估,结果表明新设计的热导检测器放大电路与ADC的总噪声(以美国材料与试验协会(ASTM)标准值计)仅为1.25 μV,总功耗降低了3.7 W,满足micro GC的功能需求,而且可靠性高、体积小、结构简单,可用于新一代micro GC的研发和生产. 相似文献
75.
R. M. Shalaby 《Crystal Research and Technology》2010,45(4):427-432
The Sn‐Zn alloys have been considered as lead‐free solders. In this paper, the effect of 0.0, 0.5, 1.0, 1.5 and 2.0 wt.% Indium as ternary additions on melting temperature, structure, microhardness and micro‐creep of the Sn‐9Zn lead‐free solders were investigated. It is shown that the alloying additions of Indium to the Sn‐Zn binary system result in a suppression of the melting point to 187.9 °C. From x‐ray diffraction analysis, a new intermetallic compound phase, designated β‐In3Sn is detected. The formation of an intermetallic compound phase causes a pronounced increase in the electrical resistivity and mechanical strength. Also, an interesting connection between dynamic Young's modulus and the axial ratio (c/a) of the unit cell of the β‐Sn was found in which Young's modulus increases with increasing the axial ratio (c/a). The ternary Sn‐9Zn‐xIn exhibits creep resistance superior to Sn‐9Zn binary alloy. The better creep resistance of the ternary alloy is attributed to solid solution effect and precipitation of In3Sn in the Sn matrix. The addition of small amounts of In is found to refine the effective grain size and consequently, improves hardness. The 89%Sn‐9%Zn‐2%In alloy is a lead‐free solder designed for possible drop‐in replacement of Pb‐Sn solders. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
76.
研究了多元线性阵列探测器的主频、阵元个数和阵元间距对探测器方向性的影响.实验结果表明,采用主频为2~5 MHz,阵元数为5~15,阵元间距为0.3~0.9 mm的多元线性阵列探测器,其方向性好,将有利于光声信号的探测与成像. 相似文献
77.
78.
79.
80.