Doctor of Philosophy

dissertationThe sintering of nanosized particles (or nanosintering) is an approach to the manufacture of bulk nanocrystalline materials. The goal of nanosintering is to achieve fully densified parts with grain size less than 100 nm. However, in practice, it is very difficult to reach. Due to the extremely small size and the high surface to volume ratio of nanosized powders, nanosintering exhibits a number of different phenomena compared to the sintering of coarse powders. For example, it is generally found that the sintering temperatures of nanosized particles are drastically lower than those of their micron or submicron sized counterparts, and grain growth during heating up is considerably rapider for nanosized powders in comparison with micronsized powders. In order to obtain a comprehensive understanding about these different phenomena during nanosintering, this study, by using tungsten as the example material, aims to examine size dependence of the sintering behavior and further explore the characteristics of densification and grain growth of nanosized powders, especially during initial and intermediate stages of sintering. The nanosized tungsten powder was produced by high energy mechanical milling. It is demonstrated that the sinterability of nanosized tungsten powder, compared with that of coarser powder, is significantly enhanced at lower sintering temperatures, and the enhancement of sintering at low temperatures for nanosized powders can be rationalized by Herring scaling law. The characteristics of densification and grain growth during nanosintering are examined by both nonisothermal heating up and isothermal holding experiments. The experimental results show linear densification behavior during the initial stage of sintering at low temperatures when density is less than ~50% relative density. Grain growth also exhibits a linear behavior during initial and intermediate stages of sintering. The mechanisms for linear densification and linear grain growth during early stage of sintering of nanosized tungsten powder are discussed based on kinetic analysis of experimental data. The evaluation results show surface diffusion is the mass transport mechanism for linear densification and linear grain growth. On the basis of the understanding of the densification and grain growth mechanisms, the general principles for inhibiting grain growth during nanosintering are proposed

Development of a Portable CMOS Time-Domain Fluorescence Lifetime Imager

Modern laboratory equipments to measure the excited-state lifetime of fluorophores usually include an expensive picosecond pulsed-laser excitation source, a fragile photomultiplier tube, and a large instrument body for optics. A portable and robust device to make fluorescence lifetime measurement in nanosecond scale is of great attraction for chemists and biologists. This dissertation reports the development of a portable LED time-domain fluorimeter from an all-solid-state discrete-component prototype to its advanced CMOS integrated circuit implementation. The motivation of the research is to develop a multiplexed fluorimeter for point-of-care diagnosis. Instruments developed by this novel method have higher fill factor, are more portable, and are fabricated at lower cost

Myopic Versus Farsighted Behaviors in a Low-Carbon Supply Chain with Reference Emission Effects

The increased carbon emissions cause relatively climate deterioration and attract more attention of governments, consumers, and enterprises to the low-carbon manufacturing. This paper considers a dynamic supply chain, which is composed of a manufacturer and a retailer, in the presence of the cap-and-trade regulation and the consumers’ reference emission effects. To investigate the manufacturer’s behavior choice and its impacts on the emission reduction and pricing strategies together with the profits of both the channel members, we develop a Stackelberg differential game model in which the manufacturer acts in both myopic and farsighted manners. By comparing the equilibrium strategies, it can be found that the farsighted manufacturer always prefers to keep a lower level of emission reduction. When the emission permit price is relatively high, the wholesale/retail price is lower if the manufacturer is myopic and hence benefits consumers. In addition, there exists a dilemma that the manufacturer is willing to act in a farsighted manner but the retailer looks forward to a partnership with the myopic manufacturer. For a relatively high price of emission permit, adopting myopic strategies results in a better performance of the whole supply chain

NRPA: Neural Recommendation with Personalized Attention

Existing review-based recommendation methods usually use the same model to learn the representations of all users/items from reviews posted by users towards items. However, different users have different preference and different items have different characteristics. Thus, the same word or similar reviews may have different informativeness for different users and items. In this paper we propose a neural recommendation approach with personalized attention to learn personalized representations of users and items from reviews. We use a review encoder to learn representations of reviews from words, and a user/item encoder to learn representations of users or items from reviews. We propose a personalized attention model, and apply it to both review and user/item encoders to select different important words and reviews for different users/items. Experiments on five datasets validate our approach can effectively improve the performance of neural recommendation.Comment: 4 pages, 4 figure

Exploring transcriptional signalling mediated by OsWRKY13, a potential regulator of multiple physiological processes in rice

BACKGROUND Rice transcription regulator OsWRKY13 influences the functioning of more than 500 genes in multiple signalling pathways, with roles in disease resistance, redox homeostasis, abiotic stress responses, and development. RESULTS To determine the putative transcriptional regulation mechanism of OsWRKY13, the putative cis-acting elements of OsWRKY13-influenced genes were analyzed using the whole genome expression profiling of OsWRKY13-activated plants generated with the Affymetrix GeneChip Rice Genome Array. At least 39 transcription factor genes were influenced by OsWRKY13, and 30 of them were downregulated. The promoters of OsWRKY13-upregulated genes were overrepresented with W-boxes for WRKY protein binding, whereas the promoters of OsWRKY13-downregulated genes were enriched with cis-elements putatively for binding of MYB and AP2/EREBP types of transcription factors. Consistent with the distinctive distribution of these cis-elements in up- and downregulated genes, nine WRKY genes were influenced by OsWRKY13 and the promoters of five of them were bound by OsWRKY13 in vitro; all seven differentially expressed AP2/EREBP genes and six of the seven differentially expressed MYB genes were suppressed by in OsWRKY13-activated plants. A subset of OsWRKY13-influenced WRKY genes were involved in host-pathogen interactions. CONCLUSION These results suggest that OsWRKY13-mediated signalling pathways are partitioned by different transcription factors. WRKY proteins may play important roles in the monitoring of OsWRKY13-upregulated genes and genes involved in pathogen-induced defence responses, whereas MYB and AP2/EREBP proteins may contribute most to the control of OsWRKY13-downregulated genes.This work was supported by grants from the National Program of High Technology Development of China, the National Program on the Development of Basic Research in China, and the National Natural Science Foundation of China

Size distribution and diffuse pollution impacts of PAHs in street dust in urban streams in the Yangtze River Delta

Particles of dust washed off streets by stormwater are an important pathway of polyaromatic hydrocarbons (PAHs) into urban streams. This article presented a comprehensive assessment of the size distribution of PAHs in street dust particles, the potential risks of the particles in urban streams, and the sources and sinks of PAHs in the stream network. This assessment was based on measurements of 16 PAHs from the USEPA priority list in street dust particles and river sediments in Xincheng, China. The content of total PAHs ranged from 1629 to 8986 μg/kg in street dust particles, where smaller particles have a higher concentrations. Approximately 55% of the total PAHs were associated with particles less than 250 μm which accounted for 40% of the total mass of street dust. The PAH quantities increased from 2.41 to 46.86 μg/m2 in the sequence of new residential, rising through main roads, old town residential, commercial and industrial areas. The sediments in stream reaches in town were found to be sinks for street dust particle PAHs. The research findings suggested that particle size, land use and the hydrological conditions in the stream network were the factors which most influenced the total loads of PAH in the receiving water bodies.<br/