Thermal Conductivity, Heat Capacity, and Elastic Constants of Water-Soluble Polymers and Polymer Blends

We use time-domain thermoreflectance (TDTR), and the generation and detection of longitudinal and surface acoustic waves, to study the thermal conductivity, heat capacity, and elastic properties of thin films of poly­(vinyl alcohol) (PVA), poly­(acrylic acid) (PAA), polyacrylamide (PAM), poly­(vinyl­pyrrolidone) (PVP), methyl cellulose (MC), poly­(4-styrene­sulfonic acid) (PSS), poly­(<i>N</i>-acryloyl­piperidine) (PAP), poly­(methyl methacrylate) (PMMA), and a polymer blend of PVA/PAA. The thermal conductivity of six water-soluble polymers in the dry state varies by a factor of ≈2, from 0.21 to 0.38 W m^–1 K^–1, where the largest values appear among polymers with a high concentration of hydrogen bonding (PAA, PAM, PSS). The longitudinal elastic constants range from 7.4 to 24.5 GPa and scale linearly with the shear elastic constants, suggesting a narrow distribution of Possion’s ratio 0.35 < ν < 0.40. The thermal conductivity increases with the average sound velocity, as expected based on the model of the minimum thermal conductivity. The thermal conductivity of polymer blends of PVA (0.31 W m^–1 K^–1) and PAA (0.37 W m^–1 K^–1) is in agreement with a simple rule of mixtures

Electroluminescence in Aligned Arrays of Single-Wall Carbon Nanotubes with Asymmetric Contacts

High quantum efficiencies and low current thresholds are important properties for all classes of semiconductor light emitting devices (LEDs), including nanoscale emitters based on single wall carbon nanotubes (SWNTs). Among the various configurations that can be considered in SWNT LEDs, two terminal geometries with asymmetric metal contacts offer the simplest solution. In this paper, we study, experimentally and theoretically, the mechanisms of electroluminescence in devices that adopt this design and incorporate perfectly aligned, horizontal arrays of individual SWNTs. The results suggest that exciton mediated electron–hole recombination near the lower work-function contact is the dominant source of photon emission. High current thresholds for electroluminescence in these devices result from diffusion and quenching of excitons near the metal contact

Two-dimensional simulation of cold start processes for proton exchange membrane fuel cell with different hydrogen flow arrangements

© 2020 Hydrogen Energy Publications LLC Proton exchange membrane (PEM) fuel cells with an off-gas recirculation anode (ORA) or dead-ended anode (DEA) are widely adopted in engineering. However, those two hydrogen flow arrangements may cause anodic water and nitrogen accumulation in comparison with the flow-through anode (FTA) mode, which causes significant performance degradation. In this paper, a two-dimensional cold-start model is developed with detailed consideration of water phase changes and the nitrogen crossover phenomenon. A simplified electrochemical module is built to calculate the current density distribution in the model. The simulation results are consistent with the experimental data at both subzero temperatures and normal operating temperatures. The effects of hydrogen flow arrangements, flow configurations, and startup strategies are investigated during startup process from subzero to normal operating temperatures. Much less ice is generated in counter-flow cases than in co-flow cases during constant current operation. A relatively lower startup voltage can effectively shorten the cold-start process and enhance the cold-start capacity for the PEM fuel cell. The ORA mode has the best hydrogen flow arrangement due to its general abilities, including higher hydrogen utilization efficiency, higher anodic nitrogen tolerance, better output performance and better startup capability

Nanosoldering Carbon Nanotube Junctions by Local Chemical Vapor Deposition for Improved Device Performance

The performance of carbon nanotube network (CNN) devices is usually limited by the high resistance of individual nanotube junctions (NJs). We present a novel method to reduce this resistance through a nanoscale chemical vapor deposition (CVD) process. By passing current through the devices in the presence of a gaseous CVD precursor, localized nanoscale Joule heating induced at the NJs stimulates the selective and self-limiting deposition of metallic nanosolder. The effectiveness of this nanosoldering process depends on the work function of the deposited metal (here Pd or HfB₂), and it can improve the on/off current ratio of a CNN device by nearly an order of magnitude. This nanosoldering technique could also be applied to other device types where nanoscale resistance components limit overall device performance

Summary table of parameters.

<p>Summary table of parameters.</p

Table of beta (or <i>β</i>) multiplier (<i>θ</i>) with respect to different age groups.

<p>Table of beta (or <i>β</i>) multiplier (<i>θ</i>) with respect to different age groups.</p

The structural diagram of the ABM-SEIR in Shenzhen.

<p>Within each classes, SEIR model structure is applied (see ODE <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177514#pone.0177514.e002" target="_blank">System (1)</a>). Within a school, if a class reaches the pre-defined outbreak threshold, there will be possible disease transmission to non-outbreak classes, to which we name “class-class transmission”. This transmission will vanish whenever the number of cases in the outbreak classes becomes lower than the outbreak threshold.</p

Varicella incidence distribution in Shenzhen by district from 2013 to 2015.

<p>The shade represents the levels of varicella incidence, cases are per 100,000 population within each district.</p

A summary table of the adjusted average number of schools, classes, and distribution of students in Shenzhen.

<p>Students per class, <i>N</i>_{<i>j</i>,<i>i</i>}, was given by <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177514#pone.0177514.e003" target="_blank">Eq (2)</a>). The information was obtained from Shenzhen Education Bureau [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0177514#pone.0177514.ref020" target="_blank">20</a>].</p

Weekly number of varicella confirmations in each year from 2010 to 2015, per 1,000,000 population.

<p>Weekly varicella cases is computed using <i>LOESS</i> model.</p