Enhanced thermoelectric figure of merit in vertical graphene junctions

In this work, we investigate thermoelectric properties of junctions consisting of two partially overlapped graphene sheets coupled to each other in the cross-plane direction. It is shown that because of the weak van-der Waals interactions between graphene layers, the phonon conductance in these junctions is strongly reduced, compared to that of single graphene layer structures, while their electrical performance is weakly affected. By exploiting this effect, we demonstrate that the thermoelectric figure of merit can reach values higher than 1 at room temperature in junctions made of gapped graphene materials, for instance, graphene nanoribbons and graphene nanomeshes. The dependence of thermoelectric properties on the junction length is also discussed. This theoretical study hence suggests an efficient way to enhance thermoelectric efficiency of graphene devices.Comment: 6 pages, 4 figures, submitte

Carbon assessment for Robusta coffee production systems in Vietnam: a case study in Dak Lak

Carbon assessments have proliferated to identify climate friendly practices in Arabica producing systems, though little attention is given to Robusta. In this study, we evaluated the climate impact of Robusta production via quantification of carbon stock and greenhouse gas (GHG) emissions in the intensive shaded and unshaded coffee farms of the world’s largest Robusta producing region, Vietnam’s Central Highlands. We find due to the linear relationship between fertilizer use and yields, emissions from input use on a per unit product basis are not significantly different between the intensive and less intensive systems. However, when accounting for carbon sequestered in shade and coffee trees, the less intensive systems are carbon positive (sequestering more than they emit) per unit of green coffee bean produced

Impact of fishery association management on fishery community

Tam Giang lagoon system has got an area size of around 22.100 ha; a length of 68 km along the coast of central Vietnam. In which, the animal system is abundant, including many kinds of shrimp, fish and crab. Vinh Giang is a commune which belongs to the Tam Giang lagoon system. It is a place, where fishing activities started long time ago and developed strong fishery culture. There are many fishery groups who use this resource, including fixed gears fishing, mobile gears fishing and aquaculture. The numbers of fishers are increasing; meanwhile the resource under harvest is limited. Accordingly, conflict becomes common among household groups in aquatic resource exploitation and use. They use many methods to increase their efficiency, like event destructive fishing. This situation led to overexploitation and depletes resources. The local government tries to take some measures in resource management, but the effectiveness was not significant. Specially, they give emphasis for administrative management mechanism and coercion to reduce approach level, exploit resource in general and fisheries management in particular. The centralized management mechanism didn’t seem suitable in common pool resource (Tuyen, 2008). Thus, the most important is to renew management system in aquaculture, fishing and other resource in lagoon. The fisheries management system improverment is implemented comprehensive in fishing technology, fishing method and management regulations in order to reach a goal that is not harmful to environment and biodiversity in aquatic resource. Although, the Local Government and other organizations coordinated management in aquculture and fishery is there, the connection has not been close enough to bring a change. The fishing and aquculture area planning were not suitable and clearly. In addition, whereas, government organization, local government weren’t enough official, time, expenditure and resource to remain management activities on large area. Fisher has not participated actively in in the protection of aquatic resource in the community yet. Because of this, the overexploitation continuied growing from time to time. These reasons affect aquatic resource and fisher’s livelihood.To solve this problem, the community has to clear property right regime to reinforce aquatic resource management capacity (Tuyen, 2010). Therefore, the fishery management model base on community was establishment in many communes. In which, fishery association is subject of the model. Moreover, Vinh Giang commune, that the first commune is right allocated in water surface resource management, exploitation and using. The nature of the community based fisheries management model is decentralized in participation of fisher and local government. Fishery association is social-professional organization. The participation of fisher in the association is voluntary; they participate in activities such as: fishing, aquaculture, consuming, processing and logistic services fisheries. Fishery association establishment created the change in aquatic resource management. In which, it included the change in the number of fisher, area, fishing equipment, yield, income and fisher’s awareness. From this research, it will show the advantages and disadvantages in aquatic resource management process, the changes and improvements in the livelihood of fisher through support by fishery association.</jats:p

Improvement and analysis of paper properties by adding modified polysaccharides

Polysaccharides are now popularly used in paper technology since they are able to improve mechanical properties of the paper. Xylan and pectin are two natural polymers that have a wide range of applications nowadays. These two polysaccharides can be used in their native forms as well as derivatives. In this study, xylan and pectin were modified to obtain cationic derivatives before adding into the paper for enhancement. The work was motivated by the fact that xylan and pectin share the same negative surface properties as cellulose. This leads to a slight repulsion effect between them once they are close. Their cationic derivatives, therefore, are believed to strengthen the paper due to electrostatic interactions between two oppositely charged objects beside hydrogen bonds. To fulfil the purposes, oat spelt xylan and apple pectin were chemically modified using different modification methods, including oxidation, amidation, and a combination of these two methods. A quantitative method to determine the adsorbed amount and relative retention capacity of xylan onto paper using high performance liquid chromatography was also developed. The results show that oat spelt and apple pectin, as well as their modified derivatives, can improve the paper mechanical properties. The oat spelt xylan significantly enhances the tensile strength, however, this contribution has a saturation level in correlation with the relative retention capacity of the xylan onto cellulose. The best retention capacity is achieved when 5% of xylan is added. This finding provides a practical approach to the application of xylan as a paper additive in consideration of economic issues. Although the application of apple pectin and its amidated derivatives in paper is rather rare, some interesting points have been discovered in this work. Among the apple pectin samples used, only the ones with low degree of esterification give a slight enhancement in tensile index at 5% dosage. The results also suggest that the dosage of the pectin added into the paper should not exceed 5% due to its viscous property at high concentrations