Browsing by Subject "Thermal conductivity"

Now showing 1 - 3 of 3
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    High-power and high-aspect-ratio optical coatings by atomic layer deposition
    (2011-02) Gabriel, Nicholas Theodore
    In high-power applications, optical coatings must meet rigorous thermomechanical and damage threshold standards in addition to performing the desired optical function, which includes filters, beam splitters, anti-reflection coatings, and high-reflectivity mirrors. We investigate several aspects of high-power coatings and the particular suitability of atomic layer deposition (ALD) to meet many of the design goals. After reviewing the origin of thermal expansion in solids, techniques for its measurement in thin films, and the unique characteristics of ALD, we look at the ability to predict a coating's thermal deformation. Coatings using ALD alumina and hafnia are demonstrated to have very consistent refractive indices, growth rates, thermal expansion coefficients, and biaxial moduli, which together enable a priori design of "thermally invariant" mirrors that maintain high reflectivity without changing shape with temperature. We have also characterized the undesired crystallization of ALD hafnia that can lead to roughness at thicknesses relevant to optical coatings. A nanolaminate strategy is explored, where ultrathin layers of alumina---less than 1 nanometer thick---are inserted periodically to disrupt the growth of hafnia crystallites. The hafnia-rich nanolaminates, near 100 nanometers in total thickness, are found to be amorphous and smooth down to very low concentrations of alumina and have a predictable decrease in refractive index with increasing alumina concentration. The thermal conductivity of ALD alumina and hafnia along with a series of nanolaminates is characterized in detail, focusing on the effect of interfaces in the nanolaminate films. The room-temperature thermal conductivity of the partially-crystalline pure hafnia film is 1.7 W/(m K), whereas all nanolaminates fall in the range of 1 to 1.2 W/(m K). Cryogenic measurements to 30 K show that this 30-40% reduction is likely due to the amorphous nature of the nanolaminates rather than the effect of interface resistance, and the thermal conductivity closely follows that expected for fully-disordered hafnia. A unique feature of ALD is its ability to conformally coat very high-aspect-ratio structures, like nanoscale holes and trenches. We investigate this at the mixed length scale of many common optical systems, with at least one dimension on the order of centimeters, another as low as several micrometers, and with nanoscale thickness precision. An example is coating the inside of a hollow glass capillary waveguide. We find that ALD alumina considerably outperforms hafnia under such conditions and quantify the difference using a large-area wedge structure with cross-section varying from about 20 micrometers to over a millimeter. The alumina process hardly notices the constrained geometry, whereas hafnia shows variation in thickness and refractive index consistent with non-ideal ALD growth mechanisms. Both coatings remain quite repeatable, with the resonance of a Fabry-Perot filter behaving as predicted except at the deepest regions of the wedge.
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    Investigation of the thermal parameters of reclaimed asphalt materials with applications to asphalt recycling
    (2014-08) DeDene, Christopher D.
    Asphalt concrete is the third most widely used resource in the world, next to Portland Cement Concrete and water. In the United States alone, over 550 million tons of hot mix asphalt (HMA) are produced at more than 4,000 asphalt plants across the country. With over 94% of the paved roads in the United States surfaces with asphalt concrete, it's safe to say asphalt pavement is what America drives on. However, a majority of today's pavement projects are geared towards rehabilitation and reconstruction of existing pavements, rather than construction of new roads. While it is true that asphalt pavement is 100% recyclable and it is the most recycled material in America, the reality is most roads contain no more than 20% recycled material. There are many factors that prohibit new road construction in excess of 20% recycled content, and this thesis aims to explore just one of those factors - the thermodynamics of hot mix asphalt pavement recycling. Most research that is investigating the use of high amounts of Reclaimed Asphalt Pavement (RAP) have been based on empirical trials. This work has approached the issue of pavement recycling by measuring the thermal properties of recycled asphalt, examining the thermodynamic limits of asphalt drum mixing, and by modeling asphalt mixing drums using finite element techniques to determine the amount of time required to achieve full melting inside of asphalt drums. It was found that for many different drum configurations, there is insufficient retention time for RAP to reheat. This insufficient heating could cause premature failures in asphalt pavements using high percentages of RAP. A secondary goal of this thesis is to explore the benefits of using the waste mining material, taconite tailings, in new asphalt pavements. This research shows there is thermodynamic benefit gained by using taconite tailings because they can be heated faster than traditional aggregates. This heating supplies more heat to RAP, which in turn, may allow for more of the recycled asphalt pavement to be incorporated into new asphalt pavements.
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    Understanding and Tailoring Thermal Transport in Materials and Across Interfaces
    (2022-12) Zhang, Yingying
    As a rapid development of synthesis and processing of materials at the nanometer scale, the continuous trend of electronic miniaturization leads to unprecedented power densities within today’s devices. This results in excessive operating temperatures, which becomes the bottleneck of device performance. The challenges in the thermal management of devices (i.e., the ability to remove and/or redistribute heat generation during the device operation) lead to a desire to engineer new materials to lower temperatures and improve reliability. Especially, at reduced length and time scales, size effects and interfaces become dominant, and physical properties such as thermal conductivity of materials may deviate from their bulk properties significantly. Therefore, a better understanding and further manipulation of thermal transport at the micro/nanoscale is necessary to improve overall device performance. In this dissertation, I present four research projects on thermal transport in different material systems, including ultrawide bandgap semiconductors, amorphous materials, perovskite oxides, and metal/semiconductor interfaces. First, I experimentally study the thickness-dependent thermal conductivity of beta-phase gallium oxide (beta-Ga2O3) by conducting time-domain thermoreflectance (TDTR) measurements. By comparing with model calculations, I attribute the dependence of thermal conductivity on thickness to the pronounced phonon-boundary scattering as the film thickness gets smaller. Second, I reveal the impact of hydrogen atoms on thermal transport in hydrogenated amorphous silicon (a-Si:H) films. It is found that the thermal conductivity of a-Si:H films monotonically decreases as the hydrogen concentration increases. By combining the experimental results with the theoretical calculations based on the effective medium approximation, I successfully decompose the impacts of different heat carriers (propagons and diffusons) on thermal transport. I conclude that such a significant reduction in the thermal conductivity of a-Si:H originates from the hydrogenation-induced material softening, the decrease in density, and phonon-defect scattering. Third, after understanding the impacts of different factors on thermal transport, I further investigate the thermal properties of La0.5Sr0.5CoO3-delta (LSCO) films, as a model system offering great flexibility in structural engineering. I demonstrate the ability to continuously tune the thermal conductivity of LSCO films by a factor of over 5. Such a large modulation factor is achieved via a room-temperature electrolyte-gate-induced non-volatile topotactic phase transformation from perovskite (with oxygen non-stoichiometry delta ≈ 0.1) to an oxygen-vacancy-ordered brownmillerite phase (with delta = 0.5), accompanied by a metal-insulator transition. Combining TDTR and electronic transport measurements, model analyses based on molecular dynamics and Boltzmann transport, and structural characterization by X-ray diffraction, I uncover and deconvolve the effects of these transitions on heat carriers, including electrons and lattice vibrations. In parallel with experimental studies, I also make advances in theories to better evaluate interfacial thermal conductance. I come up with a modified model, named the mixed mismatch model, to consider the roughness/bonding at the interface. The model uses interfacial roughness as an input to determine the proportions of specular and diffuse transmission and thus can predict the interfacial thermal conductance for real materials to a certain degree of accuracy.