270 research outputs found

    Analysis of the performance of different implementations of a heuristic method to optimize forest harvest scheduling

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    Research ArticleFinding an optimal solution of forest management scheduling problems with even flow constraints while addressing spatial concerns is not an easy task. Solving these combinatorial problems exactly with mixed-integer programming (MIP) methods may be infeasible or else involve excessive computational costs. This has prompted the use of heuristics. In this paper we analyze the performance of different implementations of the Simulated Annealing (SA) heuristic algorithm for solving three typical harvest scheduling problems. Typically SA consists of searching a better solution by changing one decision choice in each iteration. In forest planning this means that one treatment schedule in a single stand is changed in each iteration (i.e. one-opt move). We present a comparison of the performance of the typical implementation of SA with the implementation where up to three decision choices are changed simultaneously in each iteration (i.e. treatment schedules are changed in more than one stand). This may allow avoiding local optimal. In addition, the impact of SA ÔÇô parameters (i.e. cooling schedule and initial temperature) are tested. We compare our heuristic results with a MIP formulation. The study case is tested in a real forest with 1000 stands and a total of 213116 decision choices. The study shows that when the combinatorial problem is very large, changing simultaneously the treatment schedule in more than one stand does not improve the performance of SA. Contrarily, if we reduce the size of the problem (i.e. reduce considerably the number of alternatives per stand) the two-opt moves approach performs betterinfo:eu-repo/semantics/publishedVersio

    The catalytic potential of high-k dielectrics for graphene formation

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    The growth of single and multilayer graphene nano-flakes on MgO and ZrO2 at low temperatures is shown through transmission electron microscopy. The graphene nano-flakes are ubiquitously anchored at step edges on MgO (100) surfaces. Density functional theory investigations on MgO (100) indicate C2H2 decomposition and carbon adsorption at step-edges. Hence, both the experimental and theoretical data highlight the importance of step sites for graphene growth on MgO

    A progressive hedging approach to solve harvest scheduling problem under climate change

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    Due to the long time horizon typically characterizing forest planning, uncertainty plays an important role when developing forest management plans. Especially important is the uncertainty related to recently human-induced global warming since it has a clear impact on forest capacity to contribute to biogenic and anthropogenic ecosystem services. If the forest manager ignores uncertainty, the resulting forest management plan may be sub-optimal, in the best case. This paper presents a methodology to incorporate uncertainty due to climate change into forest management planning. Specifically, this paper addresses the problem of harvest planning, i.e., defining which stands are to be cut in each planning period in order to maximize expected net revenues, considering several climate change scenarios. This study develops a solution approach for a planning problem for a eucalyptus forest with 1000 stands located in central Portugal where expected future conditions are anticipated by considering a set of climate scenarios. The model including all the constraints that link all the scenarios and spatial adjacency constraints leads to a very large problem that can only be solved by decomposing it into scenarios. For this purpose, we solve the problem using Progressive Hedging (PH) algorithm, which decomposes the problem into scenario sub-problems easier to solve. To analyze the performance of PH versus the use of the extensive form (EF), we solve several instances of the original problem using both approaches. Results show that PH outperforms the EF in both solving time and final optimality gap. In addition, the use of PH allows to solve the most di cult problems while the commercial solvers are not able to solve the EF. The approach presented allows the planner to develop more robust management plans that incorporate the uncertainty due to climate change in their plansinfo:eu-repo/semantics/publishedVersio

    In situ observations of freestanding single-atom-thick gold nanoribbons suspended in graphene

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    Bulk gold's attributes of relative chemical inertness, rarity, relatively low melting point and its beautiful sheen make it a prized material for humans. Recordings suggest it was the first metal employed by humans dating as far back to the late Paleolithic period approximate to 40 000 BC. However, at the nanoscale gold is expected to present new and exciting properties, not least in catalysis. Moreover, recent studies suggest a new family of single-atom-thick two-dimensional (2D) metals exist. This work shows single-atom-thick freestanding gold membranes and nanoribbons can form as suspended structures in graphene pores. Electron irradiation is shown to lead to changes to the graphene pores which lead to dynamic changes of the gold membranes which transition to a nanoribbon. The freestanding single-atom-thick 2D gold structures are relatively stable to electron irradiation for extended periods. The work should advance the development of 2D gold monolayers significantly.Web of Scienceart. no. 200043

    Graphene: Piecing it together

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    Graphene has a multitude of striking properties that make it an exceedingly attractive material for various applications, many of which will emerge over the next decade. However, one of the most promising applications lie in exploiting its peculiar electronic properties which are governed by its electrons obeying a linear dispersion relation. This leads to the observation of half integer quantum hall effect and the absence of localization. The latter is attractive for graphene-based field effect transistors. However, if graphene is to be the material for future electronics, then significant hurdles need to be surmounted, namely, it needs to be mass produced in an economically viable manner and be of high crystalline quality with no or virtually no defects or grains boundaries. Moreover, it will need to be processable with atomic precision. Hence, the future of graphene as a material for electronic based devices will depend heavily on our ability to piece graphene together as a single crystal and define its edges with atomic precision. In this progress report, the properties of graphene that make it so attractive as a material for electronics is introduced to the reader. The focus then centers on current synthesis strategies for graphene and their weaknesses in terms of electronics applications are highlighted.Comment: Advanced Materials (2011

    Electron Beam-Induced Reduction of Cuprite

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    Cu-based materials are used in various industries, such as electronics, power generation, and catalysis. In particular, monolayered cuprous oxide (Cu2O) has potential applications in solar cells owing to its favorable electronic and magnetic properties. Atomically thin Cu2O samples derived from bulk cuprite were characterized by high-resolution transmission electron microscopy (HRTEM). Two voltages, 80 kV and 300 kV, were explored for in situ observations of the samples. The optimum electron beam parameters (300 kV, low-current beam) were used to prevent beam damage. The growth of novel crystal structures, identified as Cu, was observed in the samples exposed to isopropanol (IPA) and high temperatures. It is proposed that the exposure of the copper (I) oxide samples to IPA and temperature causes material nucleation, whereas the consequent exposure via e-beams generated from the electron beam promotes the growth of the nanosized Cu crystals

    Hydrophilic non-precious metal nitrogen-doped carbon electrocatalysts for enhanced efficiency in oxygen reduction reaction

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    Exploring the role of surface hydrophilicity of non-precious metal N-doped carbon electrocatalysts in electrocatalysis is challenging. Herein we discover an ultra-hydrophilic non-precious carbon electrocatalyst, showing enhanced catalysis efficiency on both gravimetric and areal basis for oxygen reduction reaction due to a high dispersion of active centres

    In situ N-doped graphene and Mo nanoribbon formation from Mo2Ti2C3 MXene monolayers

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    Since the advent of monolayered 2D transition metal carbide and nitrides (MXenes) in 2011, the number of different monolayer systems and the study thereof have been on the rise. Mo2Ti2C3 is one of the least studied MXenes and new insights to this material are of value to the field. Here, the stability of Mo2Ti2C3 under electron irradiation is investigated. A transmission electron microscope (TEM) is used to study the structural and elemental changes in situ. It is found that Mo2Ti2C3 is reasonably stable for the first 2 min of irradiation. However, structural changes occur thereafter, which trigger increasingly rapid and significant rearrangement. This results in the formation of pores and two new nanomaterials, namely, N-doped graphene membranes and Mo nanoribbons. The study provides insight into the stability of Mo2Ti2C3 monolayers against electron irradiation, which will allow for reliable future study of the material using TEM. Furthermore, these findings will facilitate further research in the rapidly growing field of electron beam driven chemistry and engineering of nanomaterials.Web of Scienceart. no. 190711
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