低相噪启钥式耦合微波光电振荡器研究北大核心

针对微波通信、雷达、电子战和导航等先进电子系统的应用发展需求,为了实现具有实用价值的极低相噪耦合微波光电振荡信号产生,提出了一种基于Sigma型光纤储能环腔架构的低相噪启钥式耦合微波光电振荡器。该方案通过Sigma型光纤储能技术消除了耦合微波光电振荡器中存在的偏振衰落问题,并且通过有源光纤环腔再生增益特性极大增强了振荡器品质因数,最终实现了10 GHz耦合微波光电振荡器的开机启钥稳定运行,起振信号在10 kHz频偏处的相位噪声达到-139.26 dBc/Hz。     

New nitrene functionalizations onto sidewalls of carbon nanotubes and their spectroscopic analysis

The reactivity of p-toluenesulfonyl, methylsulfonyl and trimethylsilyl nitrene, derived from the corresponding azides, was studied towards single-walled carbon nanotubes (SWCNT) prepared by electric arc or HiPCO (High-pressure CO conversion) methods. The functionalized SWCNTs were analyzed by Raman, IR, and VIS/NIR spectroscopy. The spectroscopic results indicated that covalent modification of the SWCNTs was successful. While the IR measurements gave evidence of successful reaction in all studied cases, the Raman measurements indicated differences in the reactivity of the two tube types and between different nitrenes. VIS/NIR spectrum was measured for reaction with p-toluenesulfonyl azide and it indicated strong changes upon functionalization. In the reaction between p-toluenesulfonyl azide and SWCNTs, for tubes prepared by the electric arc method the Raman signal from the metallic tubes decreased while for HiPCO tubes an opposite effect was observed. For the reaction with p-toluenesulfonyl nitrene the changes in the Raman spectra induced by the reaction were substantial but for the other two nitrenes the changes were more modest. Therefore, control experiments were performed without heating the reactions mixtures and complementary IR analysis was performed for the samples. Based on the analysis, it can be concluded, that nitrenes provide a route for rich chemical modification of carbon nanotubes.     

Effects of lactic acid concentration and spray drying conditions on stickiness of acidified skim milk powder

The effects of lactic acid concentration (LA) and spray drying conditions on stickiness of acidified skim milk powder (SMP), which impacts process efficiency and powder properties, were examined using a fully replicated 2^4-1 fractional factorial design. The wall deposition was significantly affected by LA, and yield was positively influenced by the drying temperature difference. Drying temperature difference significantly affected particle size, moisture content and water activity of acidified SMP. Particle size was also significantly influenced by nozzle airflow rate. Moisture content and water activity were significantly affected by LA, while glass transition temperature was only influenced by the outlet temperature.     

带宽预分配算法的应用空间定义及算法比较

在简略介绍等效带宽、虚带宽等几种带宽预分算法的基础上，定义了算法应用空间，并在此空间内比较了等效带宽、虚带宽及改进的虚带宽算法，给出了这些算法的适用空间，通过比较可看出：改进的虚带宽算法具有更广的适用空间。     

多媒体数据库管理系统的关键技术

本文简述了多媒体数据的特点，提出了多媒体数据库管理系统需要的基本要求，最后讨论了多媒体数据库管理管理系统需要解决的关键性问题。     

一种基于整数小波变换的图像无损压缩方法

由于第二代小波变换可以实现图像的整数到整数的变换,而且图像的恢复质量与变换时边界采用何种延拓方式无关,完全可以克服由第一代小波变换所带来的缺陷。该文利用这个特性,提出了一种基于整数小波变换的图像无损压缩算法。该方法首先将图像进行整数小波变换,然后利用不同子带的小波系数分布特性,对不同的子带采用不同的预测方式,最后将预测误差进行哈夫曼编码。实验结果表明,该方法算法简单,有较好的压缩性能,与JPEG无损压缩模式相比较,有较大的优势。     

碳纤维复合材料拉索的锚固体系及服役性能研究进展

随着国家基础设施建设的发展,桥梁拉索面临着大跨度、长寿命的需求,使轻质高强、耐腐蚀疲劳性能优异的碳纤维增强复合材料拉索具有替代传统钢索的巨大优势。为此,探讨了现有碳纤维复合材料拉索的锚固体系,介绍了碳纤维复合材料拉索锚固体系的工程应用,总结了碳纤维复合材料拉索长期服役性能的研究现状。结果表明,现有的锚固体系可解决跨度小、吨位小的碳纤维复合材料桥梁拉索锚固问题,但因锚具尺寸过大、制造工艺复杂、受力不协同、不便于现场安装等问题,需开发针对大跨度桥梁拉索的高效率锚固体系。由于碳纤维复合材料拉索长期性能的演化规律与性能退化机制较为复杂多样,且在不同服役环境和荷载耦合作用下存在较大差异,需建立碳纤维复合材料拉索长期服役性能的监测技术和寿命预测方法。     

V‐Shaped Thiophene‐Based Oligomers with Improved Electroluminescence Properties

The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film‐forming properties than their linear counterparts. Spin‐coated films show photoluminescence efficiencies up to 50 %. Light‐emitting diodes with spin‐coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10 000 cd m^–2. Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl‐S,S‐dioxide leads an increase in electron affinity by 0.5–0.7 V, analogous to that of the corresponding linear oligomers.     

Drugs related to motor vehicle crashes in northern European countries: a study of fatally injured drivers

The aim of this study was to find which drugs and drug combinations were most common in drivers who died, in particular, in single vehicle crashes where the responsibility for the crash would be referred to the driver killed. The study included all available blood samples from drivers, who died within 24 h of the accident, in the years 2001 and 2002 in the five Nordic countries (total population about 24 million inhabitants). The samples were analysed for more than 200 different drugs in addition to alcohol, using a similar analytical programme and cut-off limits in all countries. In three countries (Finland, Norway and Sweden) blood samples were available for more than 70% of the drivers, allowing representative prevalence data to be collected. 60% of the drivers in single vehicle crashes had alcohol and/or drug in their blood samples, compared with 30% of drivers killed in collisions with other vehicles. In single vehicle accidents, 66% of the drivers under 30 years of age had alcohol and/or drugs in their blood (alcohol only – 40%; drugs only – 12%; alcohol and drugs – 14%). The drugs found were mostly illicit drugs and psychoactive medicinal drugs with warning labels (in 57% and 58% respectively of the drivers under 30 with drugs present). Similar findings were obtained for drivers 30–49 years of age (63% with alcohol and/or drugs). In drivers aged 50 years and above, killed in single vehicle crashes (48% with alcohol and/or drugs) illicit drugs were found in only one case, and psychoactive medicinal drugs were detected less frequently than in younger age groups. In 75% of single vehicle crashes, the driver was under 50 years. Thus, the majority of accidents where the drivers must be considered responsible, occurred with drivers who had recently used alcohol, or drugs, alone or in combination. The drugs involved were often illicit and/or psychoactive drugs with warning labels. Therefore a large proportion of single vehicle accidents appear to be preventable, if more effective measures against driving after intake of alcohol and drugs can be implemented.