UC San Diego

Nanostructured materials exhibit useful properties that are not found in the same materials in bulk form. (1) Dramatically increase surface and area and roughness of nanostructured materials is advantageous for evaporative cooling, which is one of the main subjects in this dissertating, due to the strong capillary effect and high density of gas-liquid-solid triple junctions. (2) In magnetic materials, when the size of the magnetic material is decreased to submicron or nano size, magnetic coercivity increases since magnetic domains are confined by the size of the material. (3) Also, unique optical properties of materials in nano size can be utilized for thermal managing and energy saving application.

In Chapter 2 of this dissertation, I described the demonstration of using nanoporous membranes in evaporative cooling. Nanoporous membranes have been proposed and theoretically shown as a promising candidate for high heat flux evaporative heat transfer. However, the experimentally demonstrated heat flux has so far been significantly lower than the theoretical prediction, which has cast doubt on the feasibility of achieving a high heat flux from nanoporous membranes. Here we carried out evaporative heat transfer experiments using isopropyl alcohol (IPA) through anodized aluminum oxide (AAO) membranes. For membranes with a 200nm average pore size on a 0.5cm2 size area, we demonstrated a high evaporative heat flux of 210W/cm2 based on the overall AAO surface area, or ~400W/cm2 if only the pore area is considered. This heat flux is close to the theoretical value of 572 W/cm^2 (based on the pore area) for IPA evaporation through nanoporous membranes. Using time synchronized high-speed images, it was verified that evaporation was the main heat transfer mode in the high heat flux regime. The demonstration of high heat flux evaporation through nanoporous membranes, close to the theoretical limit and on a relatively large area (0.5cm2), is significant for the future development of high heat flux thermal management technology for electronic devices.

In chapter 3, I presented a noble technique to achieve exchange coupling of hard phase magnetic materials and soft phase magnetic materials. Exchange coupled spring magnets have been suggested as a possible replacement of rare earth contained strong magnets. We have chosen LPT-MnBi as hard phase and FeCo as soft phase magnetic material, as many early conducted theoretical modeling suggests. The optimal size of hard phase magnet for the exchange coupling (approximately twice of single magnetic domain size, ~2µm for MnBi) was achieved by conventional ball milling process, and the shell layer of FeCo was deposited by a noble process called sonic agitation assisted physical vapor deposition (SAA-PVD). TEM image and EDX mapping shows uniform coating of FeCo outer shell layer on the MnBi core. The thickness of the shell layer was in the range of 10~35nm which is slightly less than twice of single domain size of FeCo. Magnetic remanence of ball milled MnBi particles was increased from Ms = 36 emu/g to 51 emu/g after SAPVD process while the coercivity was slghtly decreased from 1.1T to 1.0T. (BH)max of the particles after the SAA-PVD process was about 2.5MGOe. Smooth demagnetization curve that resembles that of single magnetic materials and high increase of magnetic remanence suggest that exchange coupling was achieved.

In chapter 4, a new route to synthesize thermochromic VO2 particles and properties of the film using the particles was presented. A temperature responding fully reversible metal to insulator phase transition (MIT) accompanied by a change of optical properties only found in Vanadium dioxide monoclinic phase (VO2 (M)) has potential for huge energy saving application by controlling the amount of infrared (IR) light enter into buildings. More synthetic routes are still worthy to be explored because of difficulty in mass production of VO2 (M). In this work, we demonstrated a combination of thermal decomposition method subsequent ball milling process to produce pure VO2 (M) particles. The size of the synthesized particles was between 20 and 200nm. IR modulating smart film was fabricated by blade casting mixture of synthesized VO2 (M) particles and PVP on PET film. The thickness of the film was about 300nm and particles were uniformly dispersed in the film. Despite the irregular shape of the particles and the fact that few portion of the synthesized particles exceeding suggested optimal size range, the transmittance of little less than 40% and the IR modulation of about 20% which values are practically useful was achieved.