科研成果 by Year: 2019

With wearable electronic devices arising, a flexible hybrid energy harvester that is capable to continuously harvest multi-types of energy and seamlessly integrate with human body draws great attentions. In this paper, we introduce a novel self-cleaning flexible hybrid energy harvesting system which includes a groove-shape micro/nanostructured haze thin film (GHF), a flexible power management circuit, and a hybrid energy harvester is integrated by a flexible organic solar cells (F-OSC) with an autonomous single-electrode triboelectric nanogenerator (AS-TENG) via one common-electrode. This system allows for simutaneously harvesting both solar and mechanical energy through two separate parts (i.e. the top F-OSC and the bottom AS-TENG). The flexible power management circuit simultaneously utilizes the large current of the solar cell and the high voltage of the TENG. In addition, GHF with excellent optical properties, large surface area and super-hydrophobicity has been introduced into the hybrid cell, which serves not only as a triboelectric layer to increase the surface charge density of the AS-TENG, but also as a light-trapping layer to improve the photoelectric conversion efficiency (PCE) of the F-OSC. Meanwhile, GHF helps this device to achieve unique functions, such as dust-proof, self-cleaning and self-encapsulating, which significantly improve the stability and repeatability of hybrid power unit in practical applications.

In back-end analog/mixed-signal (AMS) design flow, well generation persists as a fundamental challenge for layout compactness, routing complexity, circuit performance and robustness. The immaturity of AMS layout automation tools comes to a large extent from the difficulty in comprehending and incorporating designer expertise. To mimic the behavior of experienced designers in well generation, we propose a generative adversarial network (GAN) guided well generation framework with a post-refinement stage leveraging the previous high-quality manually-crafted layouts. Guiding regions for wells are first created by a trained GAN model, after which the well generation results are legalized through post-refinement to satisfy design rules. Experimental results show that the proposed technique is able to generate wells close to manual designs with comparable post-layout circuit performance.

To implement the Wigner branching random walk, the particle carrying a signed weight, either $-1$ or $+1$, is more friendly to data storage and arithmetic manipulations than that taking a real-valued weight continuously from $-1$ to $+1$. The former is called a signed particle and the latter a weighted particle. In this paper, we propose two efficient strategies to realize the signed-particle implementation. One is to interpret the multiplicative functional as the probability to generate pairs of particles instead of the incremental weight, and the other is to utilize a bootstrap filter to adjust the skewness of particle weights. Performance evaluations on the Gaussian barrier scattering (2D) and a Helium-like system (4D) demonstrate the feasibility of both strategies and the variance reduction property of the second approach. We provide an improvement of the first signed-particle implementation that partially alleviates the restriction on the time step and perform a thorough theoretical and numerical comparison among all the existing signed-particle implementations. Details on implementing the importance sampling according to the quasi-probability density and an efficient resampling or particle reduction are also provided.

A field campaign was conducted from November to December 2017 at the campus of Peking University (PKU) to investigate the formation mechanism of the winter air pollution in Beijing with the measurement of hydroxyl and hydroperoxyl radical (OH and HO2) with the support from comprehensive observation of trace gases compounds. The extent of air pollution depends on meteorological conditions. The daily maximum OH radical concentrations are on average 2.0 × 106 cm−3 and 1.5 × 106 cm−3 during the clean and polluted episodes, respectively. The daily maximum HO2 radical concentrations are on average 0.4 × 108 cm−3 and 0.3 × 108 cm−3 during the clean and polluted episodes, respectively (diurnal averaged for one hour bin). A box model based on RACM2-LIM1 mechanism can reproduce the OH concentrations but underestimate the HO2 concentrations by 50% during the clean episode. The OH and HO2 concentrations are underestimated by 50% and 12 folds during the polluted episode, respectively. Strong dependence on nitric oxide (NO) concentration is found for both observed and modeled HO2 concentrations, with the modeled HO2 decreasing more rapidly than observed HO2, leading to severe HO2 underestimation at higher NO concentrations. The OH reactivity is calculated from measured and modeled species and inorganic compounds (carbon monoxide (CO), NO, and nitrogen dioxide (NO2)) make up 69%–76% of the calculated OH reactivity. The photochemical oxidation rate denoted by the OH loss rate increases by 3 times from the clean to polluted episodes, indicating the strong oxidation capacity in polluted conditions. The comparison between measurements at PKU site and a suburban site from one previous study shows that chemical conditions are similar in both urban and suburban areas. Hence, the strong oxidation capacity and its potential contribution to the pollution bursts are relatively homogeneous over the whole Beijing city and its surrounding areas.

The US-China relationship is one of the most important bilateral relationships in today’s world politics. Conflict and cooperation between the two major powers affect regional and global stability and the dynamics of the international system. What are the patterns of the ebbs and flows of this relationship? And how can the US-China conflict and cooperation be explained by reciprocity, policy inertia, and influence of an important third party? Scholars have used machine-code event data and time-series tools to analyze the three factors in American-Soviet relations. This research extends the existing research and applies new data and methods to trace and explain the variations in US-China interactions. We rely on data from event data from the Global Data on Events Location and Tone (GDELT) and obtain a sample of directed actions of China, the United States, and Russia/U.S.S.R. towards one another between 1979 and 2017, totaling 3,957,479 daily records. We first apply multivariate change-point analysis and locate three structural breaks in the 38 years. For each of the four sub-periods, we specify a Vector Autoregressive Model and utilize the Impulse Response Functions to estimate the mutual effects of six time series of directed actions among China, the U.S., and Russia. Then we build signed and directed networks using country dyads as nodes and estimates as edges to summarize and interpret the interdependence among dyadic interactions in this triangle. The paper has three major empirical findings. First, the most important factor behind the US actions toward China is policy inertia that could reflect the effect of domestic politics on American foreign policy decision- making. But China’s action towards the US are equally determined by reciprocity, policy inertia, and Russia as the third party. Second, we find mutual reciprocity in the China-US interactions, but reciprocity is highly assymmetric---the responses of the US to China’s actions are much weaker than China’s responses to the US actions, which could explain why US-China cooperation is often difficult to reach. Thirdly, the China- US-Russia triangle is featured by a logic of “balance of power” --- Russia and China are more cooperative to each other when their relations with the US get more conflictual, and vice versa. The pattern shows that the cooperation between China and Russia is a tactic to increase their leverage to settle conflicts with the US. Furthermore, there is no stable reciprocity between China and Russia, which means that a China-Russia alliance is unlikely to become true despite that the foreign policy rhetoric and diplomatic gestures seem to suggest an emergence of such an alliance.

【背景】 煤化工企业排放的废水中含有大量难降解、高毒性的有机污染物，采用以高效降解菌为基础的生物强化技术对其进行处理，是一种经济可行的策略；而促进高效降解菌在载体材料表面的生物膜形成，有助于提升生物膜法废水处理系统的效能。【目的】 探究一株吡啶高效降解菌 Pseudomonas sp.ZX08 的生物膜形成过程和特性，识别不同的环境因子如温度、 pH、 Na+、 K+、Ca2+、 Mg2+等对其生物膜形成的影响规律，为实现人工调控其在实际废水处理系统中的成膜过程提供理论依据。【方法】 采用改良的微孔板生物膜培养与定量方法，以单因子影响实验测定不同条件下菌株在 12 孔板内的生物膜形成量和浮游态细菌量；采用激光共聚焦显微镜(confocal laser scanning microscope，CLSM)观察和分析生物膜的结构特征。【结果】 Pseudomonas sp. ZX08 菌株具有良好的吡啶降解性能，且生物膜形成能力较强， CLSM 观察到其在载体表面形成的生物膜可达到 40-50 mm；生物膜外层的活细胞比例更高，分泌的胞外蛋白也更多。 ZX08 菌株的生物膜形成量具有明显的周期性变化特征， 12 h、48 h 的生物膜量是相对峰值。 ZX08 生物膜形成的最适温度是 25 °C，最适 pH范围是 7.0-9.0；较高浓度的 NaCl (>0.6 mol/L)和 KCl (>0.4 mol/L)均对 ZX08的生物膜形成有显著的抑制作用；一定范围内(0-16 mmol/L) Ca2+浓度的提高可以促进 ZX08 在 12 孔板底部固-液界面生物膜的形成，浓度更高时则显著抑制生物膜的形成；一定范围内(0-16 mmol/L) Mg2+浓度的提高对 ZX08 生物膜形成有促进作用，但促进幅度不大。【结论】 吡啶降解菌 Pseudomonas sp. ZX08 的生物膜形成能力较强，未来在实际废水处理系统中应用时需要综合考虑各种环境因子对其生物膜形成的影响。