科研成果 by Type: Conference Proceedings

基于细观力学复合球模型研究了含非均匀界面相球形颗粒填充复合材料的有效比热和其他热弹性性质,讨论了界面相性质沿径向的分布对有效性质的影响。首先,将不均匀界面相沿径向离散为多个同心球壳,假设每个球壳内材料性质是均匀的,不同球壳内的材料性质沿径向满足一定的分布。基于上述离散模型,利用含界面相的复合球模型,推导了复合材料的有效比热及有效体积模量、有效热膨胀系数的数值求解表达式。预测的有效热膨胀系数与实验结果吻合良好。另外,结果表明,界面相弹性模量和热膨胀系数的径向分布均对有效比热有较大的影响;而有效热膨胀系数虽然与界面相的模量和热膨胀系数都有关,但只有界面相热膨胀系数的径向分布对其影响较大。

提出了一种新的细观模型用以反映含连通开孔的饱和孔隙介质的结构特性。将流体作为基体,以取向随机分布的长柱形固体作为等效夹杂,利用Mori-Tanaka方法求解了饱和孔隙介质在排水和不排水状态时的有效弹性模量。该模型预测枫丹白露砂岩等岩土类饱和孔隙介质的有效弹性性质与实验相符很好。

本文用边界元方法求解颗粒填充复合材料的变形和界面分离问题,采用多子域法实现颗粒和基体之间的界面连续和分离条件,并采用主从自由度法(Master-Slave Elimination)[1]求解了界面从完美结合到分离的过程。算例表明,边界元法在此类问题中具有精度高和容易实施的优势。

阿富汗战争即将结束,有必要研究这场战争的历史影响。本文认为,这场以反恐为开端的战争同时也是一场政权更迭战争。推翻塔利班政权、通过新宪法、确立新的国体和政体,是战争最重要的产物。阿富汗塔利班作为反叛力量及其同跨国激进极端主义力量的联手、卡尔扎伊政权合法性缺失、美国—巴基斯坦—阿富汗之间错综复杂的关系,以及美国推动建立的阿富汗问题若干多边机制、地区大国介入阿富汗等,一系列源自于战争、政权更迭和地缘政治交互作用的产物,将对阿富汗和中南亚地区未来产生深远影响。

This paper proposes an energy recovery rectifier suitable for the use in outphasing and/or linear amplification with nonlinear components (LINC) transmitters. The proposed application oriented rectifier consists of a high-efficiency resistive rectifier and a stepped-impedance resonator (SIR) which stores the energy in order to reduce load sensitivity of the circuit. The rectifier is designed including harmonic frequency control to improve conversion efficiency and to provide a resistive input impedance at the fundamental frequency. The proposed rectifier has been implemented in hybrid technology. The fabricated circuit provides peak RF-to-DC efficiency of 73.5 % at 2.3 GHz and more than 60 % over a dynamic range of 8 dB. Furthermore, measurements show good agreement with simulation results.

This paper presents a dual-band rectifying circuit for wireless power transmission working at 2.45 GHz and 5.8 GHz. A modified dual-band matching network is adopted to realize the highly efficient dual-band rectifier. Source-pull simulations are performed, to determine the proper impedances at the two different frequencies. The proposed dual-band rectifier has been implemented and the measurements show good agreement with simulation. With a dual-band input matching network, the measurement results for an input power level of 10 mW show peak RF-to-DC efficiencies of 66.8 % and 51.5 % at 2.45 GHz and 5.8 GHz respectively.

This paper proposes a highly selective bandpass filter suitable for lowpass delta-sigma RF transmitters. The proposed filter is characterized by a low insertion loss, high selectivity and a transfer function tailored for filtering the close-up out-of-band noise of lowpass delta-sigma transmitters. The circuit design is based on a modified stub-loaded ring resonator structure. The proposed filter has been implemented and the measurements show good agreement with simulation. The proposed filter provides a fractional 3-dB bandwidth of 14.6 %, an insertion loss of less than 1.3 dB, a suppression of more than 15 dB on both sides of desired band, and a sharp cut-off frequency response.

北京大学教育学院教育技术系承办的2013年"新媒体与学习"暑期学校,从2013年7月15日到26日在北大举行。本暑期学校经过精心筹备和严密组织,在研究生院和计算中心的大力支持下,第一次采用面授加网络在线的混合式学习方式,较大幅度地增加了招生数量、扩大了受益面;借助课程管理系统较为严格地对全体学员的考勤、作业、测试和论文情况进行监控,有效地保障了教学质量;通过小组合作汇报等方式促进所有学员的参与和协作学习;采用同步课程视频直播和在线讨论等方式,确保在线学员取得与面授学生差别不大的学习体验;授课专家中一半来自海外,三分之一的授课采用全英文进行,国际化程度较高。根据考核结果,在录取的485名学员中,共有297名学员取得结业证书和学分,结业率为61.2%。51名学员获得优秀论文奖,65名学员获得优秀学员奖,39名学员获得优秀小组成员奖。对学生测验成绩的分析表明,面授学生和在线学生的以测验成绩为标志的学习效果无统计意义上的显著性差异。作为一次中规模混合式学习的研究生课程实践,本项教学研究结果对于全世界范围内正在广泛开展的大规模开放在线课程(MOOC)具有一定的借鉴和参考价值。