Characterization and Modeling Research

Pore-Scale modeling of thermal energy storage materials experiencing pulse heating.

posted Jul 3, 2013, 10:12 AM by Galen Jackson   [ updated Jul 3, 2013, 10:17 AM ]

Student: Galen Jackson
Faculty: Timothy S. Fisher
Sponsor: Air Force Research Laboratory
Summary: Rapidly changing surface heat loads on thermal energy storage units require new methods to calculate the temperature response inside the unit, specifically a carbon based foam filled with a phase change material (PCM). Typical thermal energy storage models are composite in nature and involve a mass fraction comparison of the foam and PCM to determine the temperature inside a section of the unit. The new model currently in development separates the foam and PCM into two separate entities.  The foam is districtized into one-dimensional rectangular nodes while the PCM assumed to be spherical pores inside the foam node. The PCM is districtized into nodes and the enthalpy inside each pore is used to determine if a node of the PCM is solid, melting, or liquid. The foam and PCM interact with each other through heat transfer from the foam into the PCM. We anticipate the new model will allow for scenarios in which both liquid and sold stages of a PCM can be accounted for in one section of a thermal energy storage unit.


Microstructure and Transport Modeling of Dense Nanoparticle Assemblies

posted Jun 21, 2013, 11:38 AM by Ishan Srivastava   [ updated Jun 21, 2013, 1:19 PM ]

Student: Ishan Srivastava

Faculty: Timothy S. Fisher

Sponsor: NSF Scalable Nanomanufacturing Program

Summary: A specific class of heterogeneous nanomaterials, viz. dense assemblies and compacts of nanoparticles, are increasingly finding application in the transport, storage and conversion of energy. Economic and scalable manufacturing of such materials allows large-scale development of energy devices such as thermoelectrics, battery cathodes and hydrogen storage materials. Energy transport through these dense nanoparticle assemblies is intricately correlated to microstructure, and the microstructural evolution during manufacturing and operation is not well understood. We have developed structural optimization models to understand nanoparticle assembly under varying external stress states. The model enables us to understand the effect of nanoparticle shape, size (and dispersity therein) and complex interparticle interactions (surface van der Waals forces and elastic repulsive forces) on final microstructure of the assembly. Rich microstructural information, such as particle-particle contact topology, particle cluster topology and other statistical correlations (volume fraction and distribution functions) is derived from the model. This information feeds a network-type model to compute the effective transport properties of such materials.


Representative Paper

Measuring Gas Temperature of a Microwave Plasma Chemical Vapor Deposition Reactor by Coherent Anti-Stokes Raman Scattering Spectroscopy

posted Jun 14, 2013, 8:38 AM by Timothy Fisher   [ updated Jun 18, 2013, 10:25 AM by Alfredo Tuesta ]

Student: Alfredo Tuesta

Faculty: Tim Fisher, Bob Lucht

Sponsor: National Science Foundation

Summary: Carbon nanostructures, such as carbon nanotubes and graphitic petals, have widely become the research topic for many due to their remarkable mechanical and electrical properties. Their potential applications in science and technology depend not only on the research conducted on their properties but also on an understanding of the chemical environment responsible for synthesizing them. In this study, the hydrogen (H­2) of the plasma in a Microwave Plasma Chemical Vapor Deposition (MPCVD) reactor is investigated by Coherent anti-Stokes Raman Scattering (CARS) spectroscopy using the second harmonic of a pulsed Nd:YAG laser at 532nm as the probe and pump beams and a broadband dye at 685nm as the Stokes beam. CARS spectroscopy is a spatially resolved optical technique extensively utilized to explore the concentration and temperature of Raman-active molecules such as H2, N2, and CO2 in combustion and plasma processes. Preliminary results reveal that for plasma generator powers of 300 W and 500 W at 10 Torr, the temperature range is approximately 700-1200 K while for plasma generator powers of 500 W and 700 W at 30 Torr, the temperature range is approximately 1100-1800 K. An improved understanding of the physics and chemistry of the synthesis environment will advance computational modeling efforts and the potential for mass production of nanodevices.

CNT-Graphene Junction Phonon Transmission

posted Jun 14, 2013, 8:37 AM by Timothy Fisher   [ updated Jun 16, 2013, 5:52 AM by Tim Fisher ]

Student: Jingjing Shi

Faculty: Tim Fisher, Xiulin Ruan

Sponsor: Air Force Office of Scientific Research

Summary: Carbon nanotubes and graphene offer high thermal conductivity, in addition to exceptional functions as electronic devices and sensors. However, both materials exhibit significant anisotropy in their thermal conduction that limits their three-dimensional thermal transport performance. Pillared-graphene structures that produce a three-dimensional network combining the grapheme sheets and carbon-nanotubes can address this deficiency. Hierarchical nanostructures inevitably suffer from the interfaces and junctions, however. This work predicts phonon heat transfer within the CNT-graphene junctions from theoretical calculations. The most commonly applied theoretical models for predicting transmission coefficients are the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM). Both models are used in this work for the zigzag and armchair directions of graphene. In order to calculate the transmission coefficient, phonon group velocity is obtained from the slope of the phonon dispersion relation in CNTs and both zigzag and armchair graphene. Eventually, more advanced models such as the atomistic Green’s function method will be applied.

Representative Paper:

Thermo-Mechanical Modeling of Carbon Nanotube Arrays for Thermal Interface Applications

posted Jun 14, 2013, 8:36 AM by Timothy Fisher   [ updated Jun 18, 2013, 10:41 AM by Sridhar Sadasivam ]

Student: Sridhar Sadasivam

Faculty: Timothy S. Fisher

Sponsor: Raytheon/DARPA Nano Thermal Interfaces Program

Summary: A growing interest has developed in the past decade on the use of carbon nanotube (CNT) arrays as thermal interface materials (TIM). This interest on CNT TIMs stems primarily from high thermal conductivity of individual CNTs and mechanical compliance of CNT arrays. Innovative methods for the measurement of mechanical and thermal properties of CNT arrays have been developed by many others. However, modeling of CNT TIMs have mostly been limited to semi-empirical methods without detailed consideration of the CNT array microstructure. We have developed a combined thermo-mechanical model of CNT arrays with coarse-grain methods. The coarse graining of CNTs allows us to model reasonably large numbers of CNTs at practical engineering scales within a reasonable computational time. A mesoscopic thermal network model couples with the coarse grain mechanics model and is used to estimate the diffusive and tip contact resistances of CNT arrays. Other useful information for thermal interface applications such as the effects of surface roughness and fillers such as wax are also expected to be predicted within the framework of the thermal network model.

Representative Paper:

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