Change Materials (PCMs) are used to absorb thermal energy during transient heat pulses. They accomplish this by storing energy through the exploitation of the latent heat of fusion during a solid–liquid phase change process. This stored energy is released later in the transient duty cycle. This approach to thermal management allows the material to remain at constant temperature during the transient pulse, absorbing energy while suppressing any temperature rise of the source and protecting it from extreme environmental conditions . PCMs have been used successfully in small scales in solar energy systems, as energy efficient building materials, and in rugged portable electronics for the military and civilian first responders.
While PCMs have been implemented successfully at the small scale, in many larger systems the low thermal conductivity of most PCMs results in a bottlenecking of heat at the source and poor utilization of the PCM mass. Over the past few years our research group has developed a nanoenhanced PCM using unique graphite nanofibers (GNF) with enhanced thermal conductivity. The graphite nanofibers are formed using a catalytic decomposition process and take the form of concentric basal planes along the fiber axis. The type of graphite plane produced is controlled using different catalysts and hydrocarbon gases during the synthesis process.
Preliminary results show that these graphite nanofiber enhanced materials feature an improved transient response time and energy storage capability. However, while we have preliminarily shown enhanced thermal behavior, we need to explore the fundamental energy transport mechanisms behind this improvement and examine how these mechanisms drive the enhancement by applying molecular dynamics (MD) simulations to our new and unique graphite nanofiber styles. These simulations expand the current effort towards the creation of thermally tailored materials. MD simulations will be used to model the nature of energy transport in the different fiber styles and characterize their thermal conductivity in a single fiber.
We will also add stochastic simulations of matrix configurations to understand how the fiber style affects their implementation in nano-enhanced materials. Our proposed work also includes exploration into thermal conductivity enhancement by other types of nanoparticles: carbon nanotubes and alumina particles. Experiments using enhanced PCMs with different types of nanoparticles allows for direct comparison of the influence of each nanoparticle on the PCM thermal conductivity. This experimental comparison provides a basis by which the influence of low-cost GNFs on thermal conductivity enhancement may be compared to that by more expensive nanoparticles.
This website describes an integrated multidisciplinary research and educational program to examine the fundamental nature of thermal transport in nano-enhanced phase change materials during solid-liquid phase transitions in a variety of high storage energy PCMs in thin layers (0.5 mm to 5 mm) for use in building materials, TIMs, and other thin film applications, and in thicker layers (2 cm to 20 cm) for solar energy storage, portable communication devices, electronics thermal management and other high power energy efficient systems.
Graphic: TEM image of a 100 nm diameter herringbone Graphite Nanofiber (GNF).
Dr. Amy Fleischer
Director, NovaTherm Laboratory
Dr. Randy Weinstein
Professor and Chair, Dept. of Chemical Engineering
Dr. Aaron Wemhoff
Director, Multiscale System Analysis Laboratory
- April 2011: Ph.D. student
Masoud Khadem has successfully added the
Tersoff many-body potential to the code
Molecular Dynamics for Arbitrary
Geometries (MDAG). The Tersoff
potential is used for in-plane
intermolecular interactions. The
Lennard-Jones 6-12 potential is used for
cross-plane interactions. Masoud
has also added the Andersen thermostat,
Nose-Hoover thermostat, Gear
Predictor-Corrector algorithm (for use
with the Nose-Hoover thermostat), and
Green-Kubo relations for both pairwise
and many-body potentials. The
Green-Kubo relations are used to predict
- May 2011: Ph.D. student
Masoud Khadem has successfully modeled
the thermal conductivity of single
sheets of graphene. The graphene
thermal conductivity was determined to
be heavily dependent on the sheet size
(smaller graphene sheets have enhanced
boundary scattering) and ensemble
formulation (NVT simulations show a much
larger thermal conductivity than NVE
simulations). The results have
been submitted to the ASME 5th
International Conference on Energy