88 research outputs found
The Renaissance of Black Phosphorus
One hundred years after its first successful synthesis in the bulk form in
1914, black phosphorus (black P) was recently rediscovered from the perspective
of a two-dimensional (2D) layered material, attracting tremendous interest from
condensed matter physicists, chemists, semiconductor device engineers and
material scientists. Similar to graphite and transition metal dichalcogenides
(TMDs), black P has a layered structure but with a unique puckered single layer
geometry. Because the direct electronic band gap of thin film black P can be
varied from 0.3 to around 2 eV, depending on its film thickness, and because of
its high carrier mobility and anisotropic in-plane properties, black P is
promising for novel applications in nanoelectronics and nanophotonics different
from graphene and TMDs. Black P as a nanomaterial has already attracted much
attention from researchers within the past year. Here, we offer our opinions on
this emerging material with the goal of motivating and inspiring fellow
researchers in the 2D materials community and the broad readership of PNAS to
discuss and contribute to this exciting new field. We also give our
perspectives on future 2D and thin film black P research directions, aiming to
assist researchers coming from a variety of disciplines who are desirous of
working in this exciting research field.Comment: 23 pages, 6 figures, perspective article, appeared online in PNA
Individual Parametric Insurance Product Design
This report presents a design of a parametric insurance product for individual consumers in two neighboring countries AmbernĎŠa and PalČŤmĎŠnĎŠa. Unlike traditional insurance, this product issues a predetermined payout to a policyholder when a pre-agreed event has been triggered. By conducting analyses on given health data in the countries, we first projected individual losses and calculated premiums according to gender, age and risk factor information. Then we defined triggering events and modeled the payout scheme for our product. Comprehensive strategies are also provided for marketing and risk mitigation
Meta-learning for Multi-variable Non-convex Optimization Problems: Iterating Non-optimums Makes Optimum Possible
In this paper, we aim to address the problem of solving a non-convex
optimization problem over an intersection of multiple variable sets. This kind
of problems is typically solved by using an alternating minimization (AM)
strategy which splits the overall problem into a set of sub-problems
corresponding to each variable, and then iteratively performs minimization over
each sub-problem using a fixed updating rule. However, due to the intrinsic
non-convexity of the overall problem, the optimization can usually be trapped
into bad local minimum even when each sub-problem can be globally optimized at
each iteration. To tackle this problem, we propose a meta-learning based Global
Scope Optimization (GSO) method. It adaptively generates optimizers for
sub-problems via meta-learners and constantly updates these meta-learners with
respect to the global loss information of the overall problem. Therefore, the
sub-problems are optimized with the objective of minimizing the global loss
specifically. We evaluate the proposed model on a number of simulations,
including solving bi-linear inverse problems: matrix completion, and non-linear
problems: Gaussian mixture models. The experimental results show that our
proposed approach outperforms AM-based methods in standard settings, and is
able to achieve effective optimization in some challenging cases while other
methods would typically fail.Comment: 15 pages, 8 figure
Numerical and Experimental Study on the Multiobjective Optimization of a Two-Disk Flexible Rotor System
With the ever-increasing requirement for the thrust to weight ratio, the rotational speed of modern aeroengine is increasingly improved; thus most of the aeroengine rotor is flexible. Some dynamic problems, such as excessive vibration, appear due to the increase of the rotation speed of the aeroengine. The aim of this study is to reduce the vibration level of the flexible rotor system through optimum design. A laboratory scale two-disk flexible rotor system representing a typical aeroengine rotor system is designed. A combinational optimization strategy coupling the rotordynamics calculation software ANSYS and the multidisciplinary optimization software ISIGHT is proposed to optimize the rotor system. The positions of the disks are selected as the design variables. Constraints are imposed on critical speeds. The disks’ amplitudes and bearings’ transmitted forces are chosen as the optimization objectives. Using this strategy, the optimal positions of the two disks are obtained. The numerical optimization results are verified by the experiments based on the test rig. The results show a significant vibration level reduction after optimization
Design, fabrication and characterization of terahertz quantum-well photodetectors
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 139-144).Terahertz (THz) photodetectors are important in the fully exploration and development of electromagnetic spectrum. However, a fast and sensitive THz photodetector ready for array integration is not available. A THz Quantum-well Photodetector (QWP) has the properties of being fast, sensitive, low-noise, and narrow-band. It is promising for large-array integration for THz imaging. In addition, a THz QWP-LED (Light-Emitting Diode) can upconvert the THz to NIR radiation and useful for THz imaging in Focal-Plane Array (FPA). In this thesis project, the theory of the intersubband transition and carrier transport in a THz QWP is studied. The designe, fabrication, measurement-system buildup, and characterization are performed. In addition, the design and fabrication process of THz QWP-LED are also carried out. Such devices have potential applications in many fields such as biology, medication and security.by Shengxi Huang.S.M
Recovery of Rare Earth Elements from Phosphate Rock by Hydrometallurgical Processes - A Critical Review
Generally, phosphate rock contains about 0.05wt% rare earth elements (REEs) on average. And the world commercial phosphate rock production is estimated to be 250 million tons per year, which makes phosphate rock a potential new REEs resource. However, low content of REEs in phosphate rock leads to the technical challenges and cost overages that hindered the commercial recovery of REEs. In this paper, an overview of achievements aiming at solving the challenges is given. Based on the decomposition processes of phosphate rock by H2SO4, HNO3, HCl, H3PO4, various REEs recovery processes via crystallization, precipitation, solvent extraction and ion exchange methods are systematically reviewed. In H2SO4 processes, REEs are recovered based on the removal of impurities from phosphoric acid and phosphogypsum since the main challenge is the diluted content of REEs in these products. In the case of HCl, HNO3 and H3PO4 processes, REEs entirely transfers into leaching solution and the recovery research are mainly focused on REEs extraction from leaching solutions. For REE extraction from high phosphorus content leaching solutions, crystallization, precipitation, and ion exchange methods are currently inconsiderable due to the high energy consumption, impurity involvement and low efficiency, respectively. Solvent extraction seems to be the potential promising method in terms of its good overall performance. Finally, recommendations to promote the development of REEs recovery technologies from phosphate rock are provided.
Quantized Thermoelectric Hall Effect Induces Giant Power Factor in a Topological Semimetal
Thermoelectrics are promising by directly generating electricity from waste
heat. However, (sub-)room-temperature thermoelectrics have been a long-standing
challenge due to vanishing electronic entropy at low temperatures. Topological
materials offer a new avenue for energy harvesting applications. Recent
theories predicted that topological semimetals at the quantum limit can lead to
a large, non-saturating thermopower and a quantized thermoelectric Hall
conductivity approaching a universal value. Here, we experimentally demonstrate
the non-saturating thermopower and quantized thermoelectric Hall effect in the
topological Weyl semimetal (WSM) tantalum phosphide (TaP). An ultrahigh
longitudinal thermopower Sxx= 1.1x10^3 muV/K and giant power factor ~525
muW/cm/K^2 are observed at ~40K, which is largely attributed to the quantized
thermoelectric Hall effect. Our work highlights the unique quantized
thermoelectric Hall effect realized in a WSM toward low-temperature energy
harvesting applications.Comment: 54 pages total, 5 main figures + 22 supplementary figures. To appear
in Nature Communications (2020
Defects Vibrations Engineering for Enhancing Interfacial Thermal Transport
To push upper boundaries of effective thermal conductivity in polymer
composites, a fundamental understanding of thermal transport mechanisms is
crucial. Although there is intensive simulation research, systematic
experimental investigation on thermal transport in polymer composites is
limited. To better understand thermal transport processes, we design polymer
composites with perfect fillers (graphite) and defective fillers (graphite
oxide); we choose polar polyvinyl alcohol (PVA) as a matrix model; and we
identify how thermal transport occurs across heterogeneous interfaces. Measured
thermal conductivities of in PVA/defective filler composites is higher than
those of PVA/perfect filler composites, while measured thermal conductivities
in defective fillers are lower than those of perfect fillers. An effective
quantum mechanical model is developed, showing that the vibrational state of
the defective level plays a critical role in enhancing the thermal conductivity
with increased defect concentration. Our experimental and model results have
suggested that defects in polymer composites may enhance thermal transport in
polymer composites by promoting vibrational resonant couplings.Comment: Enclosed: (i) Main Manuscript, including 5 main figures. (ii)
Supplementary Information, including 16 Supplementary Figures and one
self-contained theoretical sectio
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