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排序方式: 共有8167条查询结果,搜索用时 15 毫秒
101.
102.
An optimized modeling method of 8 × 100 μm AlGaN/GaN-based high electron mobility transistor (HEMT) for accurate continuous wave (CW) and pulsed power simulations is proposed. Since the self-heating effect can occur during the continuous operation, the power gain from the continuous operation significantly decreases when compared to a pulsed power operation. This paper extracts power performances of different device models from different quiescent biases of pulsed current-voltage (I-V) measurements and compared them in order to determine the most suitable device model for CW and pulse RF microwave power amplifier design. The simulated output power and gain results of the models at Vgs = -3.5 V, Vds = 30 V with a frequency of 9.6 GHz are presented. 相似文献
103.
《材料与设计》2015
The effects of plunge depth (PD) revolutionary pitch (RP) and materials arrangement on deformation and softening material at the fractured position of a friction stir welded AA6111 to AA5023 lap joint were investigated. As a result, the main factor determining tensile shear load and fracture was different with the location of soft material. When the soft material was located on top, the softening material and the deformed surface height occurred by friction heat generation of the rotating shoulder. The main factor was the deformed surface height rather than the softening material, and the deformed surface height was decreased with increasing RP. On the other hand, when the soft material was located at the bottom, the movement of the un-bonded line and hooking occurred due to the vertical flow of the rotating probe. The crucial factor was the position of the un-bonded line rather than the height of the hook. The un-bonded line occurred along the interface between two materials affected by the lowest strength, which were deformed toward the hard material affected by a higher joint strength. 相似文献
104.
G. Agarwal H. Gao M. Amirthalingam M.J.M. Hermans 《Science & Technology of Welding & Joining》2018,23(2):134-139
In situ strain evolution during laser welding has been measured by means of digital image correlation to assess the susceptibility of an advanced high strength automotive steel to solidification cracking. A novel method realised using auxiliary illumination and optical narrow bandpass filter allowed strain measurements as close as 1.5?mm from the fusion boundary with good spatial and temporal resolution. A finite-element thermomechanical model of the welding process supports the experimentally measured transverse strain. The validated finite-element numerical model can be used to assess the local strain and associated stress conditions which influences weldability and in particular, solidification cracking. 相似文献
105.
Meng-Nan Feng Yao Chen Dong-Hui Fu Chuan Qu 《Science & Technology of Welding & Joining》2018,23(6):487-500
Mg/Al dissimilar joints with favourable static and dynamic mechanical properties were obtained utilising ultrasonic spot welding and ultrasonic spot weld bonding technology. The fatigue behaviour, fatigue failure mode, and typical fatigue fracture surface morphology of Mg/Al ultrasonic spot welding and ultrasonic spot weld bonding joints were compared and observed to get a better understanding of the fatigue failure mechanism. In general, the Mg/Al ultrasonic spot weld bonding joint exhibited better fatigue performance than the ultrasonic spot welding joint. The two kinds of joints presented different fatigue failure modes with decreasing applied load level. A fatigue life prediction model was established, and the fatigue life prediction results were in good agreement with the experimental fatigue results. 相似文献
106.
对轨道交通用6082-T6铝合金进行MIG焊接,使用光学显微镜、X射线衍射仪、显微硬度计分别对焊接接头的显微组织、相结构与显微硬度进行观察与分析。结果表明,熔合区为柱状晶组织,焊缝主要由树枝晶和胞状晶组成,焊缝中心为等轴晶,焊缝体现出联生结晶的特点。母材相组成为基体α-Al固溶体、β-Mg2Si以及单质Si,焊缝金属相组成主要为α-Al固溶体。热影响区的宽度达31mm,且存在一个软化区。设定应力比R=0,测试了铝合金的疲劳寿命,通过拟合试验数据得到S-N曲线,得到MIG焊接6082-T6铝合金焊接接头的条件疲劳极限为99MPa,为母材的72.26%。用扫描电镜对疲劳断口进行观察和分析,结果表明,疲劳断口分布的二次裂纹促进了疲劳裂纹萌生和扩展,第二相粒子对疲劳裂纹的萌生起着重要的作用,焊缝中的气孔则容易成为疲劳源。稳态扩展区出现大量呈平行趋势且具有规则间距的疲劳条带,瞬断区存在大量韧窝和撕裂棱,体现出韧性断裂的特征。 相似文献
107.
108.
Analysis and performance evaluation of new coding options for space telecommand links – part II: jamming channels 下载免费PDF全文
M. Baldi F. Chiaraluce R. Garello N. Maturo I. Aguilar Sanchez S. Cioni 《International Journal of Satellite Communications and Networking》2015,33(6):527-542
In this paper, we study the performance of telecommand space links affected by pulsed, continuous wave and pseudo‐noise jamming. Countermeasures include coding, interleaving, and direct sequence spread spectrum. Binary and non‐binary low‐density parity‐check codes, parallel turbo codes, and soft‐decision decoded BCH codes are considered. We investigate the impact of different decoding algorithms, also taking into account the role of jamming state information, spreading processing gain and interleaving. The results show that significant gains (up to more than 10 dB) can be achieved in a number of interesting scenarios. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
109.
采用Nd:YAG低功率脉冲激光对0.4 mm纯钛薄片的微孔进行封焊,研究光斑直径对焊缝成形的影响规律,并分析作用于封接区的激光能量变化。研究表明,当光斑直径不大于0.4 mm时小孔能够完全封接,封接区有效厚度随着光斑直径的增大呈先增大后减小的趋势,其最大有效封接厚度约为352μm,封接接头主要由不同形态的α晶粒组成,在封接区边缘存在较大的热影响区。另外,随着光斑直径的增大,封接区上表面熔化面积基本不变,但封接模式由匙孔型向热导型转变,其中热导型模式更适合小孔封接。对作用于封接区的激光能量计算发现,理论能量值与光斑直径成正相关,而实际金属熔化量与光斑直径成反相关,造成此现象的原因是小孔边缘熔化的金属在重力等力的作用下向小孔内流动,从而可以吸收更多的激光能量。 相似文献
110.
采用脉冲激光烧蚀石墨/WS2组合靶,在硅基片上沉积不同碳质量分数的WSx/a-C复合膜。用能谱仪、扫描电子显微镜和X射线衍射仪对薄膜的成分、形貌和微观组织进行了表征。采用纳米压痕仪、涂层附着力划痕仪和球-盘式摩擦磨损试验机对薄膜的硬度、结合力和大气中(相对湿度50~55%)的摩擦学性能进行了测试。结果表明,薄膜的S/W比稳定在2.0左右且形成了(002)择优取向的WS2相。随着薄膜中碳质量分数的增加,薄膜的硬度在36.1%C时出现最高值,结合力随之增大且在52.4%C时达到最高值,摩擦因数先降低后增加,在41.2%C时有最小值0.144。薄膜磨损率在(0.91~1.61)×10-15 m3N-1m-1范围内变化,36.1%C的WSx/a-C复合膜具有最佳耐磨性能。 相似文献