The air electrode, which reduces oxygen (O2), is a critical component in energy generation and storage applications such as fuel cells and metal/air batteries. The highest current densities are achieved with platinum (Pt), but in addition to its cost and scarcity, Pt particles in composite electrodes tend to be inactivated by contact with carbon monoxide (CO) or by agglomeration. We describe an air electrode based on a porous material coated with poly(3,4-ethylenedioxythiophene) (PEDOT), which acts as an O2 reduction catalyst. Continuous operation for 1500 hours was demonstrated without material degradation or deterioration in performance. O2 conversion rates were comparable with those of Pt-catalyzed electrodes of the same geometry, and the electrode was not sensitive to CO. Operation was demonstrated as an air electrode and as a dissolved O2 electrode in aqueous solution.

High Rates of Oxygen Reduction over a Vapor

Review of spray cooling – Part 1: Single-phase and nucleate boiling regimes, and critical heat flux

We review the current state-of-the-art graphene-enhanced thermal interface materials for the management of heat in the next generation of electronics. Increased integration densities, speed and power of electronic and optoelectronic devices require thermal interface materials with substantially higher thermal conductivity, improved reliability, and lower cost. Graphene has emerged as a promising filler material that can meet the demands of future high-speed and high-powered electronics. This review describes the use of graphene as a filler in curing and non-curing polymer matrices. Special attention is given to strategies for achieving the thermal percolation threshold with its corresponding characteristic increase in the overall thermal conductivity. Many applications require high thermal conductivity of composites, while simultaneously preserving electrical insulation. A hybrid filler approach, using graphene and boron nitride, is presented as a possible technology providing for the independent control of electrical and thermal conduction. The reliability and lifespan performance of thermal interface materials is an important consideration towards the determination of appropriate practical applications. The present review addresses these issues in detail, demonstrating the promise of graphene-enhanced thermal interface materials compared to alternative technologies.

https://iopscience.iop.org/article/10.1088/1361-6528/abc0c6/pdf

Thermal interface materials with graphene fillers: review of the state of the art and outlook for future applications

Aircraft thermal management: Practices, technology, system architectures, future challenges, and opportunities

The objective ofthis paper was to investigate the cooling performance of 16-nozzle spray array using FC-72 as the working fluid in variable-gravity conditions. A flight-test experiment was modified to accommodate a 16-nozzle spray array, which was then tested in the parabolic flight trajectory environment of NASA's C-9 reduced-gravity aircraft. The 16-nozzle array was designed to cool a 25.4 x 25.4 mm 2 area on a thick-film resistive heater used to simulate an electronic component. Flight tests were conducted over the course of two flight weeks (each week consisting of four flights and each flight consisting of 40 to 60 parabolas). The mass flow rate through the 16-nozzle spray array ranged from 13.1 ≤ m ≤ 21.3 g/s. The heat flux at the thick-film resistor ranged from 2.9 ≤ q" ≤ 25 w / 2 , the subcooling of the working fluid ranged from 1.6 ≤ ΔT sc ≤ 18.4°C, the saturation temperature ranged from 37.4 ≤ T sat ≤ 47.2°C, and the absorbed air content in the working fluid was C = 10.1,14.3 and 16.8% by volume. The spray chamber pressure ranged from 42 ≤ P ≤ 78 kPa and the acceleration ranged from -0.02 ≤ a ≤ -2.02 g. Two-phase cooling was emphasized, but some single-phase data were also collected. A one-dimensional model was used to predict the heater surface temperature from the heat input and mean heater base temperature. It was found that the cooling performance was enhanced in microgravity over terrestrial and elevated gravity. In addition, a sudden degradation in performance was found at high mass flow rates in microgravity, possibly due to liquid buildup on the surface between the nozzle impact zones. A high degree of subcooling was found to be beneficial, but the dissolved air content had little effect on the heat transfer performance in either microgravity or elevated gravity.

Cooling Performance of a 16-Nozzle Array in Variable Gravity

: The objective of this thesis was to investigate the cooling performance of a 16-nozzle spray array, using FC-72 as the working fluid, in variable gravity conditions with additional emphasis on fluid management and flow stability A flight test experiment was modified to accommodate a 16-nozzle spray array, which was then tested in the parabolic flight trajectory environment of NASA's C-9 reduced gravity aircraft The 16-nozzle array was designed to cool a 254 by 254 [mm] area on a thick film resistive heater used to simulate electronic components Data was taken and reduced as a result of flight tests conducted over the course of two flight weeks (each week consisting of four flights, each flight consisting of 40 to 60 parabolas) The flight tests were conducted in order to examine gravity effects on spray cooling performance and to evaluate a novel liquid-vapor separator design The mass flow rate through the 16-nozzle spray array ranged from 131< m< 213 [g/s] for the spray cooling analysis and 14<m< 35 [g/s] for the separator evaluation The heat flux at the thick film resistor ranged from 29< q"<25 [W/cm2], the subcooling of the working fluid

/pdf/the-effect-of-variable-gravity-on-the-cooling-performance-of-llpmtz9nhm.pdf

The Effect of Variable Gravity on the Cooling Performance of a 16-Nozzle Spray Array

Carbon nanotubes (CNTs) are known for high thermal conductivity and have potential use as nano-radiators or heat exchangers. This paper focuses on the thermal performance of carpet-like arrays of vertically aligned CNTs on solid graphite substrates with the idea of investigating their behavior as a function of carpet dimensions and predicting their performance as thermal interface material (TIM) for electronic device cooling. Vertically aligned CNTs were grown on highly oriented pyrolytic graphite (HOPG) substrate, which creates a robust and durable all-carbon hierarchical structure. The multi-layer thermal analysis approach using Netzsch laser flash analysis system was used to evaluate their performance as a function of carpet height, from which their thermal properties can be determined. It was seen that the thermal resistance of the CNT array varies linearly with CNT carpet height, providing a unique way of decoupling the properties of the CNT carpet from its interface. This data was used to estimate the thermal conductivity of individual multi-walled nanotube strands in this carpet, which was about 35 W/m-K. The influence of CNT carpet parameters (aerial density, diameter, and length) on thermal resistance of the CNT carpet and its potential advantages and limitations as an integrated TIM are discussed.

Aligned Carbon Nanotube Arrays Bonded to Solid Graphite Substrates: Thermal Analysis for Future Device Cooling Applications

An innovative liquid‐vapor separator design has been incorporated into a spray cooling experiment, which was flown recently on the NASA C‐9 reduced gravity aircraft. This design is the latest in a series of designs that have been flown in microgravity and elevated gravity in relation to an experimental investigation into spray cooling performance with regard to possible application to high‐performance aircraft undergoing combat maneuvers. Initially, significant problems occurred during flight testing due to the inability of the reservoir to separate the liquid coolant from the vapor under microgravity conditions, which resulted in a loss of coolant to the simulated electronic device that was being spray cooled. Improvements in the design of the liquid‐vapor separator are described in this paper, and experimental data showing the performance of several generations of liquid‐vapor separators are presented. The final separator design went through more rigorous evaluation to compare performance at multiple fi...

Qualitative Evaluation of a Liquid‐Vapor Separator Concept in Micro‐Gravity Conditions

Vertically-aligned carbon nanotubes (VA-CNTs) have a large surface area, high electronic conductivity and electrochemical accessibility, and mechanical/chemical/electrochemical stability. These unique properties make VA-CNTs promising electrode materials for energy conversion and storage devices, including fuel cells, lithium batteries, and supercapacitors. This chapter provides an overview on recent development of VA-CNT electrodes with and without heteroatom-doping for efficient energy conversion and storage by summarizing our work on the discovery of nitrogen-doped VA-CNTs as a highly active cathode for ORR in fuel cells, vertically aligned nitrogen doped coral-like carbon fiber arrays (VA-NCCFs) as a high-performance air cathode in Li-air batteries, as well as VA-CNTs and their 3D derivatives as porous electrodes in high-performance Li-ion batteries and supercapacitors.

Levi Elston

Papers

The Effect of Variable Gravity on the Cooling Performance of a 16-Nozzle Spray Array

Aligned Carbon Nanotube Arrays Bonded to Solid Graphite Substrates: Thermal Analysis for Future Device Cooling Applications

Qualitative Evaluation of a Liquid‐Vapor Separator Concept in Micro‐Gravity Conditions

Vertically-Aligned Carbon Nanotubes for Electrochemical Energy Conversion and Storage

Experimental Validation of a Combined Thermal Management and Power Generation System using a Multi-Mode Rankine Cycle