Nanostructure Dictating Thermal Transport in Heterogeneous Material Systems

Bauer, Matthew, Mechanical and Aerospace Engineering - School of Engineering and Applied Science, University of Virginia
Norris, Pamela, Department of Mechanical and Aerospace Engineering, University of Virginia

Advances in microfabrication technology have led to novel devices requiring the support of microscale material systems with optimized mechanical and thermal properties. Control of thermal energy is necessary for the successful performance of such microdevices. Heterogenous material systems with nanoscale structural features present an opportunity to tune the unique combinations of properties sought for microscale devices outside the limit of traditional, homogenous material systems. The structure can dictate the paths of individual thermal energy carriers which ultimately influence the operating temperature and rate of heat removal from the device. However, the complexity involved in understanding thermal transport in a material system comprised of multiple materials makes predicting the effective thermal conductivity challenging.

This study examines how systematically changing fabrication processes can tune structural attributes in heterogenous systems and thereby tune thermal transport within the system. The 3ω measurement technique is used to determine the effective thermophysical properties of the systems of interest. The 3ω technique utilizes a thin metal line as both the heat source and thermometer. An alternating current passes through the line at varying frequency. The third harmonic voltage drop is measured via a lock-in amplifier. The experimental technique is advanced to isolate thermal properties in scenarios where the more traditional experimental usage would not be possible by using a heating wire not directly fabricated onto the sample. This allows for measurement of systems on which a metal line could not otherwise be deposited nor maintained while a controlled loading pressure is applied to the material system.

Some microdevices such as chemical sensors have shown peak efficiency at elevated operating temperatures. Homogenous material systems rarely exhibit thermal conductivities much below 1 W/m K. In order to minimize the power required to maintain such operating temperatures, a superior form of insulation with microscale dimensions is sought. Monolithic aerogels have been shown to be capable of achieving thermal conductivities below 0.01 W/m K. Their unique microstructure is critical to achieving such a low thermal conductivity. New fabrication processes have shown thin film aerogels capable of achieving much of the unique structure obtained by their monolithic counterparts. In this study, microfabrication processes are developed to determine the thermal transport properties of thin film silica aerogels as well as their direct applicability as thermal insulation in microscale applications. These methods are built upon by systematically tuning the fabrication conditions of the aerogel films. Changes in structure caused by the varied fabrication conditions are measured with grazing-incidence small angle x-ray scattering. The effects of fabrication and the microstructure of the aerogel films on the thermal conductivity and heat capacity of the system are considered.

Carbon allotropes have shown a tremendous range of achievable thermal transport properties as well as mechanical properties. Considering only individual carbon fibers, thermal conductivities spanning four orders of magnitude have been measured. Modeling thermal transport is complicated in carbon fiber composite systems in which the intrinsic properties of individual fibers, their collective density and orientation, the properties of the host material (air in this work), and the interactions between the fibers as well as between the fiber and the host all influence the effective thermal conductivity of the system. The system can be further altered by changing the pressure of the atmosphere, the applied load of the system, or by heat treatment of the system. To give confidence to predictive thermal modeling of such a system, a bidirectional modification of 3ω was employed to systematically determine the thermal transport of such a system. Fabrication and testing conditions are systematically altered to isolate the roles through which specific attributes of the composite contribute to thermal transport. Furthermore, Raman spectroscopy and small angle x-ray scattering are employed to isolate properties contributing to the thermal conductivity of the individual fibers. Predictive thermal modeling has been employed to explain measured results and better illuminate how further tuning of the system could be used to achieve desired thermal properties.

Individual carbon nanotubes have been shown to have thermal conductivities well over 1,000 W/m K. Such high thermal transport properties are desired for removing heat generated by computer processors. However, attempts to obtain ultra-high thermal conductivities in composite carbon nanotube systems have not yielded the expected success. Vertically aligned carbon nanotube arrays (VACNT), which could be used in microscale heat removal applications, are most commonly grown using chemical vapor deposition (CVD) techniques. To gain insight into the effective thermal conductivity of VACNTs, arrays are grown under systematically varied growth conditions using CVD methods and the thermal conductivity measured with the bidirectional 3ω modification. Raman spectroscopy and transmission electron miscroscopy are then employed to gain insight into the microstucure and ultimately relate its properties to the measured thermal conductivity.

The individual study of thin film silica aerogels, turbostratic carbon fiber networks, and vertically aligned carbon nanotube arrays aims to provide insight into fabrication mechanisms that can be used to alter thermal transport. Relating the varying microstructure of the systems to measured thermal conductivity values enhances the understanding of which mechanisms are most heavily influencing thermal transport. Conversely, the role of specific structural features in tuning thermal transport can be better understood, improving the ability to engineer novel microscale technologies to control thermal transport within a device.

PHD (Doctor of Philosophy)
microscale heat transfer, aerogels, carbon nanofibers, carbon nanotubes
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