Functional Device Research


Electron Emission from Cx(BN) Graphite Petals

posted Jun 14, 2013, 8:48 AM by Timothy Fisher   [ updated Jun 18, 2013, 10:20 AM by Patrick McCarthy ]

Student: Patrick McCarthy

Faculty: Tim Fisher

Summary: Carbon based nanomaterials have been studied extensively as energy generation devices via thermionic and photoemission characterization of carbon nanotubes, graphene petals, and chemical modifications thereof. Potassium intercalated graphene petals exhibit a low work function (energy barrier) and high electron emission intensity. Good optical and thermal properties of the carbon structures reinforces the potential of these materials. However, thermionic emission from these structures is limited by the deintercalation of potassium from between planar layers of graphene at elevated temperatures. This work will attempt to limit deintercalation of potassium and increase thermal stability via boron nitride (BN) modification of graphene petals. BN is an analogue of graphene (i.e. consisting of stacks of planar, hexagonal atomic layers). Limited mobility of oxygen within Cx(BN) has been demonstrated leading to decreased carbon burnoff. This may in turn limit potassium mobility within the lattice allowing for increased stability at high temperatures. Electron energy distributions (EEDs) will be recorded utilizing a hemispherical energy analyzer (HEA) with current density measurements to be taken as well. Initial EEDs recording exhibit trends of increased thermal stability.


Results from Cx(BN) studies indicate that while boron nitride modification produces multiple work function peaks while retaining a low work function emission peak, it rapidly losses emission intensity above 580 K. Conversely, non-modified graphitic petals intercalated with potassium show remarkable increases in emission intensity up to 980 K.


Image: Photoemission electron energy distributions resulting from solar simulator illumination of potassium intercalated graphitic petals, Cx(BN) petals, and hydrogen plasma treated Cx(BN) petals.


Representative Paper: http://avspublications.org/jvstb/resource/1/jvtbd9/v28/i2/p423_s1


Graphene-Based Nanostructures for Electrochemical Energy Storage

posted Jun 14, 2013, 8:47 AM by Timothy Fisher   [ updated Jun 18, 2013, 10:29 AM by Guoping Xiong ]

Student: Guoping Xiong

Faculty: Tim Fisher, Ron Reifenberger

Sponsor: AFOSR MURI on ‘Nanofabrication of Tunable 3D Nanotube Architectures’

Summary: Small crystalline graphitic petals (GPs), or carbon nanowalls (or nanosheets) containing a few layers of graphene have interesting industrial applications because they grow roughly perpendicular to a substrate and dramatically increase the surface area from which they grow. The GPs are thin, containing only a few graphitic layers, and can be catalyst-free, suggesting they might be a source of free-standing graphitic material. This work involves growth of GPs on various substrates (e.g., buckypaper, carbon cloth, silicon, quartz) for conventional supercapacitor, on-chip planar micro-supercapacitor and lithium ion battery electrodes. The graphitic petals with high specific area and high electrical conductivity can also be used as templates for pseudocapacitive materials (e.g., metal oxide and conducting polymers) in electrochemical energy storage application. Interdigitated GP electrodes on insulating substrates are fabricated using conventional photolithography techniques for micro-supercapacitor application.  Boron and nitrogen (B, N) modified graphene-based materials are fabricated using a facile chemical method for lithium ion battery electrode application


Paper:
http://www.sciencedirect.com/science/article/pii/S0925963512001276http://www.sciencedirect.com/science/article/pii/S0378775312017144

Carbon-based Nanostructured Surfaces for Enhanced Phase-Change Cooling

posted Jun 14, 2013, 8:46 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:54 AM by Tim Fisher ]

Student: Arun Selvaraj Kousalya

Faculty: Tim Fisher

Sponsor: Air Force Office of Scientific Research

Summary: Phase-change cooling schemes have emerged as a prominent thermal management solution for evolving electronic circuit architectures because of their potential to achieve high heat dissipation rates while maintaining uniform device temperatures. Three types of carbon-based nanostructured surfaces are tested in this work. The initial part of this work focused on enhancing the flow boiling performance of carbon nanotube (CNT)-coated copper surfaces with low-intensity photonic excitation. The oxidation resistance of graphene-coated copper surfaces exposed to forced convection boiling systems was later studied to observe a 65% reduction in oxidation with graphene-coated copper surfaces as opposed to uncoated surfaces. Graphitic petal-decorated carbon nanotubes are currently being investigated as a means of enhancing the flow-boiling performance. A means of altering the wettability of a nanostructured boiling surface by low power plasma treatment is presented as an approach to study the effect of contact angle on boiling performance of a nanostructured surface.

Representative Paper: http://apl.aip.org/resource/1/applab/v100/i7/p071601_s1

Carbon Nanotube Thermal Interface Materials

posted Jun 14, 2013, 8:45 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:56 AM by Tim Fisher ]

Student: Stephen Hodson

Faculty: Tim Fisher

Sponsor: Raytheon Integrated Systems / DARPA

Challenges

  • Strengthen thermal connection between two surfaces
  • Mitigate weak bonding at heterogenous interfaces
  • Address differences in phonon dispersion & density of states
  • Combat wave constriction affects at nanoscale contacts
  • Maintain performance under thermal cycling and stability conditions
  • Accurately characterize the CNT array (density, diameter distribution, and height)

Approach

  • Growth of vertically aligned multi-walled CNT arrays on various substrates
  • Development of bonding techniques that include post-growth functionalization and coating
  • Assess thermal performance by photoacoustic and ns-thermoreflectance techniques
  • Model statistical nature of CNT array characteristics (density and height)

Selected publications

Palladium Thiolate Bonding of Carbon Nanotube Thermal Interfaces

Characterization of Metallically Bonded CNT-Based TIMs Using a High Accuracy 1D Steady-State Technique

Microsupercapacitor Aging with an Electroreflectance Method

posted Jun 14, 2013, 8:44 AM by Timothy Fisher   [ updated May 4, 2015, 6:31 PM by Tim Fisher ]

Student: Kimberly Saviers

Faculty: Tim Fisher, Ali Shakouri

Sponsor: AFOSR

Summary: Supercapacitors bridge the gap between traditional capacitors and batteries in that they can achieve both high power density and high energy density in one device. Because they are new devices in the marketplace, it is imperative that their aging behavior is thoroughly understood. Microsupercapacitors were fabricated and evaluated over millions of charge/discharge cycles using an electroreflectance method in order to understand their aging behavior. The devices were constructed of gold electrodes by traditional photolithography etching techniques. Eight interdigitated fingers comprise each electrode with separation on the order of microns. The devices were continuously charged and discharged while capacitance and electroreflectance signals were periodically recorded. The electroreflectance technique measures light reflected from the device during the charge/discharge cycle. Because charge accumulates in the device, the reflectivity property of the electrodes changes throughout the charge/discharge cycle. This allows for knowledge of the spatial resolution of the charge accumulation in the device. Here, this information is used to understand the evolution of charge accumulation behavior over millions of cycles.


Representative Paper: http://onlinelibrary.wiley.com/doi/10.1002/elan.201300238/abstract



Graphitic Petal Nanosheets for Electrochemical Biosensing

posted Jun 14, 2013, 8:43 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:57 AM by Tim Fisher ]

Student: Kwesi Adarkwa

Faculty: Tim Fisher, Marshall Porterfield (ABE)

Sponsor: Purdue TRASK Fund

Summary: Hybridization of nanoscale metals and carbon nanotubes into composite nanomaterials has produced some of the best-performing sensors to date. The challenge remains to develop scalable nanofabrication methods that are amenable to the development of sensors with broad sensing ranges. This work involves a scalable nanostructured biosensor based on multilayered graphene petal nanosheets (MGPNs), Pt nanoparticles, and a biorecognition element (glucose oxidase). The combination of zero-dimensional nano- particles on a two-dimensional support that is arrayed in the third dimension creates a sensor platform with exceptional characteristics. The versatility of the biosensor platform is demonstrated by altering biosensor performance (i.e., sensitivity, detection limit, and linear sensing range) through changing the size, density, and morphology of electrodeposited Pt nanoparticles on the MGPNs. This work enables a robust sensor design that demonstrates exceptional performance with enhanced glucose sensitivity (0.3 μM detection limit, 0.01–50 mM linear sensing range), a long stable shelf-life (>1 month), and a high selectivity over electroactive, interfering species commonly found in human serum samples.

Representative Paper: http://onlinelibrary.wiley.com/doi/10.1002/adfm.201200551/abstract

CNT-based Thermal Interface Materials for High-Temperature Applications

posted Jun 14, 2013, 8:42 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:59 AM by Tim Fisher ]

Student: Menglong Hao

Faculty: Tim Fisher, Tim Sands

Sponsor: Department of Energy / General Motors

Summary: Thermoelectric Modules (TEM) are being designed to harvest heat at a higher temperature not only because of a higher conversion efficiency, but also because many thermal energy sources that are traditionally too hot to for conversion by other means may become accessible with high temperature TEMs. However, the harsh working environment also imposes serious challenges to all components of TEMs. A reliable Thermal Interface Material (TIM) is crucial to the successful implementation of TEMs. In addition to mechanical robustness, a good TIM should have low thermal resistance in order to maximize the electric power output of the module. At high working temperatures, differences in coefficients of thermal expansion on both sides of an interface are the most common cause of the failure. TIMs based on carbon nanotube (CNT) arrays developed in our group has achieved much success in terms of reducing thermal resistance. CNT-based TIMs are also ideal candidates for high temperature applications because their excellent mechanical compliance could substantially reduce thermal fatigue failures. BN (boron nitride) treatment is used to extend the thermal stability of CNTs.  CNTs in combination with brazing alloys are being studied to further optimize the thermomechanical properties of interfaces in TEMs as well as in other high temperature applications. A high temperature testing rig for TIMs is being developed with the schematic shown above (courtesy of Kim Saviers).

Flash-Boiling and Desorption from a Mesoporous Carbon Boron Nitride Foam for Rapid Thermal Energy Storage

posted Jun 14, 2013, 8:40 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:57 AM by Tim Fisher ]

Student: Jeffrey D. Engerer

Faculty: Tim Fisher

Sponsor: Air Force Research Laboratory

Summary: Technological advancement has necessitated innovations in thermal transport to match the increasing power densities resultant of continual device scaling and growing power consumption. For high-performance applications, single-phase convection cooling is no longer a viable option due to its inability to dissipate the thermal load, thus constraining the performance of the host system.  With this challenge in mind, we present a transient method for the rapid adsorption of thermal energy by flash boiling, which is induced by the rapid depressurization of the working fluid.  To further promote heat transfer, highly graphitized carbon-based foams are chosen for their high thermal conductivity, adsorptive properties, and open mesoporous structure.  A chemical surface modification of the carbon foam consisting of boron and nitrogen is utilized for its demonstrated improvement of adsorptivity. We anticipate that this combination of technologies will achieve instantaneous peak cooling rates rivaling other advanced 
technologies in a manner appropriate for transient thermal events.  

       

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