Ronald J. Warzoha

Effect of graphene layer thickness and mechanical compliance on interfacial heat flow and thermal conduction in solid-liquid phase change materials.

Solid-liquid phase change materials (PCMs) are attractive candidates for thermal energy storage and electronics cooling applications but have limited applicability in state-of-the-art technologies due to their low intrinsic thermal conductivities. Recent efforts to incorporate graphene and multilayer graphene into PCMs have led to the development of thermal energy storage materials with remarkable values of bulk thermal conductivity. However, the full potential of graphene as a filler material for the thermal enhancement of PCMs remains unrealized, largely due to an incomplete understanding of the physical mechanisms that govern thermal transport within graphene-based nanocomposites. In this work, we show that the number of graphene layers (n) within an individual graphene nanoparticle has a significant effect on the bulk thermal conductivity of an organic PCM. Results indicate that the bulk thermal conductivity of PCMs can be tuned by over an order of magnitude simply by adjusting the number of graphene layers (n) from n = 3 to 44. Using scanning electron microscopy in tandem with nanoscale analytical techniques, the physical mechanisms that govern heat flow within a graphene nanocomposite PCM are found to be nearly independent of the intrinsic thermal conductivity of the graphene nanoparticle itself and are instead found to be dependent on the mechanical compliance of the graphene nanoparticles. These findings are critical for the design and development of PCMs that are capable of cooling next-generation electronics and storing heat effectively in medium-to-large-scale energy systems, including solar-thermal power plants and building heating and cooling systems.

Development of Methods to Fully Saturate Carbon Foam With Paraffin Wax Phase Change Material for Energy Storage

In this work, the effect of infiltration method on the saturation rate of paraffin phase change material within graphite foams is experimentally investigated. Graphite foams infiltrated with paraffin have been found to be effective for solar energy storage, but it has been found that it is difficult to completely saturate the foam with paraffin. The effectiveness of the fill will have a significant effect on the performance of the system, but the data on fill ratio are difficult to separate from confounding effects such as type of graphite or phase change material (PCM) used. This will be the first detailed quantitative study that directly isolates the effect of infiltration method on fill ratio of PCM in graphite foams. In this work, the two most commonly reported methods of infiltration are studied under controlled conditions. In fact, the effect of the infiltration method on the paraffin saturation rate is found to be highly significant. It was found that the more commonly used simple submersion technique is ineffective at filling the voids within the graphite foam. Repeated tests showed that at least 25% of the reported open space within the foam was left unfilled. In contrast, it was found that the use of a vacuum oven lead to a complete fill of the foam. These high saturation rates were achieved with significantly shorter dwell times than in previously reported studies and can be of significant use to others working in this area.

Mechanisms of nonequilibrium electron-phonon coupling and thermal conductance at interfaces

We study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an ∼3 nm Ti adhesion layer between the Au film and the non-metal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate with temperature, which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electron-interface scattering is shown to have negligi...

Evaluation of methods to fully saturate carbon foam with paraffin wax phase change material for energy storage

In this work, the effect of infiltration method on the saturation rate of paraffin phase change material within graphite foams is experimentally investigated. Graphite foams infiltrated with paraffin have been found to be effective for energy storage, but it is difficult to completely saturate the foam with paraffin. In this work, two methods are used to infiltrate the foam: a simple submersion technique and a vacuum infiltration technique. The effect of the infiltration method on the paraffin saturation rate is found to be significant with the vacuum system yielding better results.

Molecular Tuning of the Vibrational Thermal Transport Mechanisms in Fullerene Derivative Solutions

Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C60 monoadduct (ICMA), indene-C60 bisadduct (ICBA), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), [6,6]-phenyl C61 butyric acid butyl ester (PCBB), and [6,6]-phenyl C61 butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump-probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C60 molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C60 does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C60 affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications.

Processing and characterization of silicon nitride nanofiber paper

Papers of silicon nitride nanofibers were synthesized by a carbothermal reduction process. These nanofiber papers were synthesized in situ and did not require a secondary processing step. The process utilized silica nanopowders and silica gel as the precursor material. Processing geometry played a crucial role in regulating the growth of the nanofiber papers. Characterization of the nanofiber papers indicated that the nanofibers were of the alpha silicon nitride phase. Both mechanical stiffness and strength of the nanofiber papers were measured. Thermal conductivity and specific heat of the papers were also measured and were found to be lower than many common thermal insulation materials at much smaller thicknesses and were comparable to those values that are typically reported for carbon-nanotube-based buckypaper. Results of the mechanical and thermal characterization indicate that these silicon nitride nanofiber papers can be utilized for specialized thermal insulation applications.

A Computational Study of the Thermal Performance of a 15 kV Solid State Current Limiter Cooled by Immersion in Mineral Oil

This paper outlines the thermal performance of a unique liquid-cooled 15 kV/4000 A Solid State Fault Current Limiter (SSFCL) developed by Silicon Power of Malvern, PA with support of EPRI. The design features an extremely high power system which consumes 96 kW of power in a one phase configuration. The system is submerged in mineral oil coolant and the waste heat is dissipated by internal liquid convection and subsequently through an external radiator system driven by a centrifugal pump. This project numerically explores the effects of various design parameters on the heat dissipation and the resulting effect on the operating temperature of several components within the system.Copyright

High resolution steady-state measurements of thermal contact resistance across thermal interface material junctions

Thermal interface materials (TIMs) are meant to reduce the interfacial thermal resistance (RT) across bare metal contacts in commercial electronics packaging systems. However, there is little scientific consensus governing material design for optimized thermal performance. This is principally due to the inability to separate the effects of the intrinsic material thermal properties from the magnitude of heat flow crossing the TIM-substrate junction (RC). To date, efforts to isolate these effects using standard thermal interface material characterization techniques have not been successful. In this work, we develop an infrared thermography-based steady-state heat meter bar apparatus with a novel in situ thickness measurement system having 0.5 nm sensitivity. These in situ thickness measurements allow us to simultaneously determine RT and RC independently across current state-of-the-art TIMs with ±5% uncertainty. In this work, thermal pastes with bond line thicknesses ranging between 5 and 50 μm are used to illustrate the capability of the apparatus to measure extremely thin materials that are expected to achieve relatively low values of RT. Results suggest that the contribution of the thermal contact resistance to the total thermal resistance can range from 5% to 80% for these materials. This finding highlights the need for appropriate metrology and independent measurements of RC and RT to better optimize thermal interface materials for a number of important electronics applications.

Improved methodology for calculating interfacial thermal resistance and uncertainty for steady-state TIM testers with embedded probes

Efforts to miniaturize electronic components within the semiconductor industry continue to intensify stresses on the primary thermal pathways that are used for heat dissipation in electronics packaging equipment. This is particularly true for heat flow pathways that traverse interfaces. Consequently, an increasing priority for thermal engineers is to design materials that are capable of reducing the impedance to heat flow acrosss device junctions. However, the equipment most often used to measure the resistance to heat flow across interfaces (ASTM D5470) is becoming increasingly insufficient for the characterization of next-generation thermal interface materials (TIMs), as evidenced by the wide variability in the reported results for current state-of-the-art TIMs. Through the use of statistical analyses, we show that one possible reason for these discrepancies is the method by which the temperature difference across the interface is calculated. Additionally, we find that there exists a lack of consideration for many potential sources of positional uncertainty that exist within the measurement system, including: 1) the thermal conductivity mismatch between the thermal probes, the heat meter bars and any interstitial filler material used to increase contact conductance between them, 2) drill drift during manufacturing, 3) the temperature and positional uncertainties of each probe along the length of the heat meter bars, 4) the tolerance associated with the location of each thermal probes junction and 5) the number of thermal probes that are used to determine the temperature difference across the interface. We find that these factors result in an unavoidably large uncertainty in the position of the thermal probes, which produces a significant measurement uncertainty when RT is on the order of 1·10-5 m2·K/W or lower, regardless of the temperature measurement accuracy that can be achieved with the probes. Using numerical simulations, we conduct a parametric study to determine the magnitude of these effects on positional uncertainty. Results suggest that the lowest positional uncertainty is achieved when the thermal probe is significantly less thermally conductive than its surrounding filler, and that drill drift and the uncertainty associated with the location of the actual thermal probe junction account for a significant increase in the overall uncertainty of the measurement. It is expected that these results will allow for the development of steady-state TIM characterization instruments with improved measurement resolutions and for greater consistency between the results of different groups that use thermal probe-based TIM testers.

Development and testing of subambient melt temperature nano-enhanced phase change materials

This work presents the results of an effort to create and thermally characterize sub-ambient phase change materials enhanced with graphite nanofibers. Several samples were created using a paraffin based phase change material with 10% by weight suspended graphite nanofibers. This material has a melting point of 21°C, slightly below standard ambient conditions (23°C). The thermal properties of the samples were experimentally characterized. This paper presents the process used for creation of the materials and the results of the thermal characterization.