高功率微波作用下限幅器尖峰泄漏特性

针对限幅器在高功率微波（HPM）作用下的尖峰泄漏问题,基于搭建的HPM注入实验平台和电路仿真开展了研究。研究结果表明：当注入功率超过6 dBm时,限幅器会出现尖峰泄漏现象,限幅器泄漏尖峰的上升沿与脉宽随着注入功率的增加而减小,而绝对尖峰泄漏功率随注入功率的增加呈增长趋势,平顶泄漏功率呈近似“线性增加-缓慢下降-小幅增长”趋势。并且,实验结果显示：HPM脉宽与重频对限幅器尖峰泄漏特性基本无影响,其泄漏特性变化规律与单次脉冲的基本一致;尖峰泄漏能量随注入功率的增加而降低。     

介质材料二次电子发射特性对微波击穿的影响

以介质填充的平行板放电结构为例,本文主要研究了介质填充后微波低气压放电和微放电的物理过程.为了探究介质材料特性对微波低气压放电和微放电阈值的影响,本文采用自主研发的二次电子发射特性测量装置,测量了7种常见介质材料的二次电子发射系数和二次电子能谱.依据二次电子发射过程中介质表面正带电的稳定条件,计算了介质材料稳态表面电位与二次电子发射系数以及能谱参数的关系.在放电结构中引入与表面电位相应的等效直流电场后,依据电子扩散模型和微放电中电子谐振条件,分别探讨了介质表面稳态表面电位的大小对微波低气压放电和微放电阈值的影响.结果表明,介质材料的二次电子发射系数以及能谱参数越大,介质材料的稳态表面电位也越大,对应的微波低气压放电和微放电阈值也越大.所得结论对于填充介质的选择有一定的理论指导价值.     

Toward Uniform Optical Properties of Carbon Dots

Carbon dots possess versatile optical properties that have prompted their investigation in applications including photocatalysis, photovoltaics, imaging, and drug delivery, among others. However, the preparation of these nanodots is accompanied by the formation of fluorophores and intermediates, which can be difficult to separate. In the absence of thorough purification protocols, the reported optical properties are often heterogeneous, which hinders understanding of their physicochemical and optical properties and concrete application development. Here, two hydrophilic carbon dot systems starting with citric acid and diethylenetriamine are prepared. The impact of purification, including dialysis, ultrafiltration, and organic washes, on the properties of the dots is demonstrated. It is shown that monitoring the purification endpoint using near-infrared, fluorescence, and absorbance spectroscopies is possible. Moreover, it is demonstrated that fluorescence quantum yields can be a reliable tool to determine the purification endpoint. This work shows that even carbon dots derived from the same chemical precursors can have different purification profiles and purification requirements. However, the developed approach can be used to determine the proper purification procedure and endpoint for any carbon dot system regardless of the starting materials. Finally, it is envisioned that this work can be easily extended toward the purification of other hydrophilic nanomaterials.     

常规速调管空间功率合成微波源

基于常规速调管,研制了用于开展空间功率合成实验的微波源系统。介绍了50 MW超大功率速调管发射机的系统组成、大功率速调管、相位控制及高功率测试系统等。重点阐述了超大功率速调管发射机的关键技术,并针对超大功率发射机的设计难点提出了具体的解决方案。为提高超大功率发射机的可靠性,设计了基于绝缘栅双极晶体管（IGBT）的感应叠加型全固态调制器。给出了超大功率发射机的测试结果,并探讨了超大功率速调管发射机的改进方法。最后对基于常规速调管,研制用于空间功率合成微波源系统的发展进行了展望。     

Influence of sintering temperature and pH on the phase transformation,particle size and anti-reflective properties of RHA nano silica powders

Nano silica powders were synthesized from rice husk ash, the most silica-rich raw material, using alkaline extraction followed by acid precipitation. The phase transformation during sintering, the influence of sintering temperature and pH on the particle size and anti-reflective properties of nano silica were investigated. The results showed that the amorphous SiO₂ sintered at 600°C were transformed to a cristobalite structures completely during the sintering process at 800°C and 1100°C. With the increasing sintering temperature and pH, the particle size distributions (d₅₀) were increased respectively in the range of 62–84, 192–240, and 283–329?nm at sintering temperatures of 600°C, 800°C, and 1100°C. When the sintering temperatures were increased at 1100°C, 98.15% and 96.84% of transmittances were obtained respectively at the highest and lowest points of the anti-reflection band and could be used for anti-reflective applications.     

Growth of polymer crystals during annealing

Observations by transmission electron microscopy are reported on the processes involved in polymer crystal growth during annealing. The observations suggest that crystal growth occurs by two processes. One process involves the melting of those regions of the crystals in which the melting point is lower than the annealing temperature. The polymer melt due to the melting process gradually becomes incorporated into the unmolten crystals, resulting in crystal growth. The alternative process is solid-state crystal growth by the migration of the amorphous region between crystallites.     

Microwave Photonic Phase Shifter Based on an Integrated Dual-polarization Dual-parallel Mach-Zehnder Modulator without Optical Filter

ABSTRACT

A frequency-doubled microwave photonic phase shifter (MPPS) without optical filter is proposed. The MPPS is based on an integrated dual-polarization dual-parallel Mach-Zehnder modulator (DP-DPMZM) and a polarization modulator (PolM). The DP-DPMZM with a 90° polarization rotator in one arm is used to generate an optical carrier suppressed double sideband (OCS-DSB) signal with orthogonal polarization, and the PolM with two modes opposite phase modulation is used to introduce the optical phase shift between the two orthogonally polarized tones. Simulations show that the MPPS can realize a continuously tunable phase shift of 360° with only one DC bias voltage, and is not sensitive to the optical carrier wavelength and microwave signal frequency since no optical filter is used.