Carbon nanotube-guided thermopower waves

Thermopower waves are a new concept for the direct conversion of chemical to electrical energy. A nanowire with large axial thermal diffusivity can accelerate a self-propagating reaction wave using a fuel coated along its length. The reaction wave drives electrical carriers in a thermopower wave, creating a high-power pulse of as much as 7 kW/kg in experiments using carbon nanotubes. We review nanomaterials designed to overcome limitations of thermoelectricity and explore the emerging scientific and practical outlook for devices using thermopower waves

New concepts in energy and mass transport within carbon nanotubes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 164-177).The unique structure of carbon nanotubes (CNTs) contributes to their distinguished properties, making them useful in nanotechnology. CNTs have been explored for energy transport in next-generation, such as light-emitting diodes, field-effect transistors, and phonon wave guides due to their high axial electrical and thermal conductivity. Also, their subnanometer scale with atomically smooth surfaces is promising for selective mass transport in nanoscale, such as molecular transport, selective gas permeation, and nanofluidics. The first part of this thesis considers CNTs as substrates for guided chemical reactivity and thermal waves for energy generation. Coupling an exothermic chemical reaction with a nanowire possessing a high axial thermal conductivity creates a self-propagating reactive wave. Such waves are realized using a 7-nm cyclotrimethylene-trinitramine (TNA) annular shell around a CNT and are amplified by 104 times the bulk TNA value, propagating more than 2 m/s, with an effective thermal conductivity of 1.28 ± 0.2 kW/m/K at 2860 K. Thermally excited carriers in the direction of the propagating reaction produces a concomitant electrical pulse of high specific power, as large as 7 kW/kg, that we identify as a thermopower wave. The specific power increases with a decreasing system size, resulting in usually efficient sub-micron and nano-sized pulse power sources. In the second portion, we develop a nanopore platform using the interior of a single walled carbon nanotube (SWNT) for study of single ion transport. Such pores can undergo a resonance in ion transport such that coherent waveforms are generated (CR). The asymmetric electrostatic barriers at their ends show that above the threshold bias, traversing the nanopore end is not rate limiting and that the pore blocking behavior of two parallel nanotubes follows an idealized Markov process. We report two channels undergoing this CR simultaneously, the dynamics of ion transport for different cations (Li+, Na+, K+, Cs+) and the effect of varying the applied voltage on transport across the SWNT channel. Finally, the diameter and temperature dependence (1-2 nm) of ion transport shows the distinct trend in dwell time and blockade current that study its transfer mechanism by proton 'hop' and 'turn', and single ion transport.by Wonjoon Choi.Ph.D

Incremental placement for timing optimization

An incremental timing driven placement algorithm is presented. We introduce a fast path-based analytical approach for timing improvement. Our method achieves timing optimization by reducing the enclosing bounding boxes of selected nets on critical paths. Furthermore, this technique tries to minimize modifications to the initial placement while improving the delay of the circuit incrementally. Two contributions of this work are 1) efficient conversion of a path-based timing minimization problem to a geometric net-constraint problem and 2) minimal modification of a placement to improve timing. Our technique can take an initial placement from any algorithm and improve timing iteratively. The experiments show that the proposed approach is very efficient. 1

Fish Forensics: Exposing Discrepancies in Contemporary Species of Sculpin

The McCloud River is an aquatic sanctuary in Northern California, whose roaring waterfalls, luscious fish and scenic views attract hordes of recreationists. A lesser known fact is that the McCloud holds a scientific significance which rivals its capabilities as an entertainer: it contains the endemic native species known as McCloud River Redband Trout. This suggests that other endemic species may thrive in the McCloud. Our attention was brought to three species of fish from family Cottidae, namely the Pit, Riffle and Prickly Sculpin. Individuals from said groups were sampled from select rivers in Central/Northern California including the McCloud, and their genomic information was used in a statistical procedure known as a principal component analysis (PCA). This analysis compiles and rearranges data points in a way that accentuates variation between and within loci. The first PCA affirmed that Prickly sculpin were far different from Pit and Riffle Sculpin and those samples collected at the mouth of the McCloud River grouped with Prickly Sculpin. It also suggested that an extra group existed in the McCloud River and Hot Springs Creek which had genetic characteristics intermediate between known Pit and Riffle Sculpin. A second PCA was conducted to investigate this newly emerged group, and the results showed that this new group was significantly different from both Pit and Riffle sculpin. Additionally within it were two distinct groups of fish, one in the McCloud and one in Hot Springs Creek. This indicates that either there was secondary contact between the Pit and Riffle sculpin which led to a now-independent hybrid species, or this new group diverged from the same ancestor as Riffle and Pit Sculpin. Either way, our results differentiate McCloud River Sculpin from currently known species and suggest a full investigation is needed to unearth additional endemic species in the McCloud

Fish Forensics: Exposing Discrepancies in Contemporary Species of Sculpin

The McCloud River is an aquatic sanctuary in Northern California, whose roaring waterfalls, luscious fish and scenic views attract hordes of recreationists. A lesser known fact is that the McCloud holds a scientific significance which rivals its capabilities as an entertainer: it contains the endemic native species known as McCloud River Redband Trout. This suggests that other endemic species may thrive in the McCloud. Our attention was brought to three species of fish from family Cottidae, namely the Pit, Riffle and Prickly Sculpin. Individuals from said groups were sampled from select rivers in Central/Northern California including the McCloud, and their genomic information was used in a statistical procedure known as a principal component analysis (PCA). This analysis compiles and rearranges data points in a way that accentuates variation between and within loci. The first PCA affirmed that Prickly sculpin were far different from Pit and Riffle Sculpin and those samples collected at the mouth of the McCloud River grouped with Prickly Sculpin. It also suggested that an extra group existed in the McCloud River and Hot Springs Creek which had genetic characteristics intermediate between known Pit and Riffle Sculpin. A second PCA was conducted to investigate this newly emerged group, and the results showed that this new group was significantly different from both Pit and Riffle sculpin. Additionally within it were two distinct groups of fish, one in the McCloud and one in Hot Springs Creek. This indicates that either there was secondary contact between the Pit and Riffle sculpin which led to a now-independent hybrid species, or this new group diverged from the same ancestor as Riffle and Pit Sculpin. Either way, our results differentiate McCloud River Sculpin from currently known species and suggest a full investigation is needed to unearth additional endemic species in the McCloud

Hierarchical Global Floorplacement Using Simulated Annealing and Network

Floorplanning large designs with many hard macros and IP blocks of various sizes is becoming an increasingly important and challenging problem. This paper presents a global floorplacement method that combines a hierarchical simulated annealing floorplanning method with a partitioning-based global placement technique. A novel area migration method formulated as a min-cost, max-flow network flow problem is used to improve area utilization, and provide a communication mechanism between the partitioning engine and the placement method for better design quality. The network flow area migration method can be used in managing incremental changes in the design as well. Our global placement wire length is 12% better than the detailed placement wire length of a previous work, while our global placement is almost 8 times faster than their global placement

Rapid Electromechanical Transduction on a Single-Walled Carbon Nanotube Film: Sensing Fast Mechanical Loading via Detection of Electrical Signal Change

Carbon nanotubes (CNTs) have been widely explored as next generation embedded-strain-pressure sensors. However, most investigations of CNT sensors did not consider the response time as a critical factor, although the ultrafast sensing of mechanical deformation is very important for the detection of dynamic loading or impact, such as in reactive armor systems. Owing to the low capacitance that shortens the response time of the electrical resistance changes induced by mechanical deformation, CNTs are expected to detect rapid electromechanical transduction without delay. Herein, we fabricate single-walled carbon nanotube (SWNT) films on diverse substrates, and evaluate their applications as sensors to detect rapid electromechanical transduction on a macroscopic scale. Under repetitive, high-speed mechanical loading, the SWNT films generate consistent electrical signal changes, which are accurately synchronized with their strain and the beginning of the deformation

Interpreting Internal Activation Patterns in Deep Temporal Neural Networks by Finding Prototypes

Deep neural networks have demonstrated competitive performance in classification tasks for sequential data. However, it remains difficult to understand which temporal patterns the internal channels of deep neural networks capture for decision-making in sequential data. To address this issue, we propose a new framework with which to visualize temporal representations learned in deep neural networks without hand-crafted segmentation labels. Given input data, our framework extracts highly activated temporal regions that contribute to activating internal nodes and characterizes such regions by prototype selection method based on Maximum Mean Discrepancy. Representative temporal patterns referred to here as Prototypes of Temporally Activated Patterns (PTAP) provide core examples of subsequences in the sequential data for interpretability. We also analyze the role of each channel by Value-LRP plots using representative prototypes and the distribution of the input attribution. Input attribution plots give visual information to recognize the shapes focused on by the channel for decision-making

An experimental study on the thermal performance of cellulose-graphene-based thermal interface materials

In this study, an innovative thermal interface material (TIM) paper based on a composite of cellulose and graphene is investigated experimentally. Six types of commercially-available papers: a wool paper; an aqua satin; a merit paper; a new craft board; and two oriental traditional papers (Bulgyeong and Daerye) are used to fabricate the paper-graphene composites via bar coating and a slot die coating. The fabricated TIM papers are lightweight, flexible and robust against tensile strength. The in-plane and through-plane thermal conductivities of the TIM papers are measured using a laser-flash-method (LFM). The measured in-plane thermal conductivities are of the order of 5 W/m-K, whereas the through-plane thermal conductivities are of the order of 0.1 W/m-K. These results suggest that the addition of graphene significantly enhance the in-plane thermal conductivity of papers, while the through-plane thermal conductivities are not significantly improved. The mechanical properties of the TIM papers are also tested. This work provides a new possibility for development of next-generation thermal interface materials with good thermal and mechanical properties