Sol-gel preparation and characterization of nanoporous ZnO/SiO2 coatings with broadband antireflection properties

Nanoporous ZnO/SiO₂ bilayer coatings were prepared on the surface of glass substrates via sol-gel dip-coating process. The structural, morphological and optical properties of the coatings were characterized. The refractive indices of ZnO layer and SiO₂ layer are 1.34 and 1.21 at 550 nm, respectively. The transmittance and reflectance spectra of the coatings were investigated and the broadband antireflection performance of the bilayer structure was determined over the solar spectrum. The solar transmittances in the range of 300-1200 nm and 1200-2500 nm are increased by 6.5% and 6.2%, respectively. The improvement of transmittance is attributed to the destructive interference of light reflected from interfaces between the different refractive-index layers with an optimized thickness. Such antireflection coatings of ZnO/SiO₂ provide a promising route for solar energy applications.     

Broadband noise reduction by circular multi-cavity mufflers operating in multimodal propagation conditions

In this study, sound propagation through a circular duct with non-locally lining is investigated both numerically and experimentally. The liner concept is based on perforated screens backed by air cavities. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. This gives rise to complex scattering mechanisms among duct modes which renders the muffler more effective over a broader frequency range. This work emanates from the Cleansky European HEXENOR project which aim is to identify the best multi-cavity muffler configuration for reduction of exhaust noise from helicopter turboshaft engines. Here, design parameters are the cavity dimensions in both longitudinal and azimuthal directions. The best cavity configuration must in addition fit weight specifications which implies that the number of walls separating each cavity should be chosen as small as possible. To achieve these objectives, the scattering matrix of the lined duct section is obtained experimentally for two specific muffler configurations operating in multimodal propagation conditions. The good agreement with numerical predictions serves to validate the perforate plate impedance model used in our calculation. Finally, given an incident acoustic pressure which is representative of typical combustion noise spectrum, the best cavity configuration achieving the maximum overall acoustic Transmission Loss is selected numerically. The study also illustrates how the acoustic performances are dependent on the nature of the incident field.     

稳定自由基存在下苯乙烯聚合的加速剂

研究了稳定自由基存在下苯乙烯的活性聚合,发现在β－酮酸酯－乙酰乙酸乙酯,乙二酸二乙酯,1,3－二酮－乙酰丙酮的少量存在下,苯乙烯聚合速率显著增加,分子量可控,分子量在布较窄。而乙酰丙酮较大量存在下,聚苯乙烯分散性稍微变宽。     

Broadband dielectric characterization of piezoelectric poly(vinylidene fluoride)thin films between 278 K and 308 K

This work describes the dielectric properties of piezoelectric poly(vinylidene fluoride) (PVDF) thin films in the frequency and temperature ranges relevant for usual applications. We measured the isothermal dielectric relaxation spectra of commercial piezoelectric PVDF thin films between 10 Hz to 10 MHz, at several temperatures from 278 K to 308 K. Measurements were made for samples in mechanically free and clamped conditions, in the direction of the poling field (perpendicular to the film). We found that the imaginary part of the dielectric relaxation spectra of free and clamped PVDF samples is dominated by a peak, above 100 kHz, that can be characterized by a Havriliak-Negami function. The characteristic time follows an Arrhenius dependence on temperature. Moreover, the spectra of the free PVDF samples show two additional peaks at low frequencies which are associated with mechanical relaxation processes. Our results are important for the characterization of piezoelectric PVDF, particularly after the stretching and poling processes in thin films, and for the design and characterization of a broad range of ultrasonic transducers.     

基于MR-ARQ协议的多业务QoS保证机制研究

针对无线通信中高误码率、高突发误帧、频谱资源紧张等特点,对多拒绝自动请求重传(MR-ARQ)协议相比于传统ARQ协议在提高系统吞吐量、降低系统平均传输时延和时延抖动等方面的优势性能进行了理论分析。在此基础上,针对不同业务传输时对误码率、传输时延和时延抖动的不同要求,研究了基于MR-ARQ协议的宽带无线接入系统(BWA)中多业务混合传输的服务质量保证(QoS)机制,提出了多速率重传策略、基于业务区分的多队列机制以及上行集中带宽分配等3种系统性能优化方案。最后对系统在采用上面3种方案前后的性能进行了仿真和比较分析,验证了方案的可行性和良好效果。     

TD-LTE RAN承载技术方案分析

通过分析TD-LTE RAN承载需求,结合中国移动的TD网络建设现状,对中国移动的TD-LTE RAN承载技术方案进行描述和分析对比。     

Fabrication of biomimic GaAs subwavelength grating structures for broadband and angular-independent antireflection

We combine interferometric lithography and inductively coupled plasma etching to fabricate GaAs subwavelength grating (SWG) which mimics the moth eye structures. Through the modification of morphology parameters, including profile, height and packing fraction, tapered, high-aspect-ratio and closely-packed GaAs SWGs are obtained. The measurement of spectral reflectance of the fabricated SWGs shows that reflection has been dramatically reduced compared to the polished GaAs surface. Particularly, the optimized SWG structures exhibit an average reflection below 5% in the wavelengths ranging from 350 to 900 nm. Furthermore, the angular-independent property is demonstrated by measuring the reflectance versus varying angles of incidence at 532 and 632.8 nm wavelengths.     

宽带雷达中频回波长脉冲压缩采样重构中的分段重叠处理及其机理分析

宽带雷达中频回波在频域上稀疏。根据压缩感知理论,可对其进行压缩采样并从压缩采样结果中以很高的概率重构出原信号。中频回波压缩采样的重构需要的矩阵尺寸由中频回波的长度决定。在长脉冲情况下,重构所需的矩阵尺寸大,占用内存资源过多,处理速度慢。针对此问题,论文将分段重叠处理引入重构过程。论文首先详细给出了引入分段重叠处理之后的重构方法,接着通过严格数学推导,证明了频谱泄漏与局部时域序列的关系具有如下特点:远离谱峰的谱值受时域序列两端的影响要强于靠近谱峰的谱值,利用该特点说明了分段重叠处理抑制误差的机理。实测数据实验表明了分段重叠处理在重构质量上优于分段不重叠处理,在对存储资源的要求上远低于不分段处理。     

宽带无线移动通信网络优化及相关发展策略思考

在介绍宽带无线移动发展趋势所采用的新技术的基础上，论述了宽带无线移动通信网络优化及相关发展策略的一些问题。