Thermophysical properties study of micro/nanoscale materials

Thermal transport in low-dimensional structure has attracted tremendous attentions because micro/nanoscale materials play crucial roles in advancing micro/nanoelectronics industry. The thermal properties are essential for understanding of the energy conversion and thermal management. To better investigate micro/nanoscale materials and characterize the thermal transport, pulse laser-assisted thermal relaxation 2 (PLTR2) and transient electrothermal (TET) are both employed to determine thermal property of various forms of materials, including thin films and nanowires. As conducting polymer, Poly(3-hexylthiophene) (P3HT) thin film is studied to understand its thermal properties variation with P3HT weight percentage. 4 P3HT solutions of different weight percentages are compounded to fabricate thin films using spin-coating technique. Experimental results indicate that weight percentage exhibits impact on thermophysical properties. When percentage changes from 2% to 7%, thermal conductivity varies from 1.29 to 1.67 W/m*K and thermal diffusivity decreases from 10-6 to 5×10-7 m2/s. Moreover, PLTR2 technique is applied to characterize the three-dimensional anisotropic thermal properties in spin-coated P3HT thin films. Raman spectra verify that the thin films embrace partially orientated P3HT molecular chains, leading to anisotropic thermal transport. Among all three directions, lowest thermal property is observed along out-of-plane direction. For in-plane characterization, anisotropic ratio is around 2 to 3, indicating that the orientation of the molecular chains has strong impact on the thermal transport along different directions. Titanium dioxide (TiO2) thin film is synthesized by electrospinning features porous structure composed by TiO2 nanowires with random orientations. The porous structure caused significant degradation of thermal properties. Effective thermal diffusivity, conductivity, and density of the films are 1.35~3.52×10-6 m2/s, 0.06~0.36 W/m*K, and 25.8~373 kg/m3, respectively, much lower than bulk values. Then single anatase TiO2 nanowire is synthesized to understand intrinsic thermophysical properties and secondary porosity. Thermal diffusivity of nanowires varies from 1.76 to 5.08 × 10-6 m2/s, while thermal conductivity alters from 1.38 to 6.01 W/m*K. SEM image of TiO2 nanowire shows secondary porous surface structure. In addition, nonlinear effects are also observed with experimental data. Two methods, generalized function analysis and direct capacitance derivation, are developed to suppress nonlinear effects. Effective thermal diffusivities from both modified analysis agree well with each other

Surface Engineered Catalysts for Effective Photo-Conversion and Utilization of Greenhouse Gases

Global warming and climate change are among the most critical issues in the 21st century. Emission control of CO2 and CH4, the top two greenhouse gases, is the key to dealing with those issues. As one of the most promising greenhouse gas control techniques, direct greenhouse gas conversion/utilization can be an economically-friendly option to mitigate greenhouse gas emissions. In this dissertation, innovative catalyst designs in two greenhouse gas conversion/utilization processes, namely, CO2 photoreduction on TiO2-based catalysts and photo-thermal-chemical dry reforming of methane (PTC-DRM) on Pt/CeO2-based catalysts, for performance enhancements are demonstrated and discussed. In the CO2 photoreduction process, the low catalyst surface-CO2 affinity is one of the major factors that limit the CO2 photoreduction performance on TiO2 catalysts. In this dissertation, a highly porous TiO2 material derived from MOF material MIL-125 was prepared, which showed a CO2 photoreduction performance that is 4.2 times as high as commercialized P25 material under a 400 W Xe-lamp irradiation. To further enhance the CO2 photoreduction performance, three types of alkali surface modifications with MgO, namely, (1) MgO ALD coating, (2) MgO ALD coating/Ag co-modifications, and (3) MgO doping, were applied on the porous TiO2. All of the modifications were found to substantially enhance the CO2 photoreduction performance up to ~60 times performance improvements compared with P25 materials. In the PTC-DRM process, the occurrence of side reaction reverse water-gas shift reaction has long been an issue that affects the performance, especially the low H2/CO production ratio, of DRM catalysts. This dissertation applied Pt/CeO2 catalyst as an example and demonstrated a simple approach to improve both H2/CO production ratio and PTC-DRM reactivity by applying acidic metal oxide Al2O3 in catalyst preparation. Thanks to the favorable Al2O3-CeO2 synergetic effects, under a reaction temperature of 700 ℃ with 30-sun equivalent solar irradiation, the Pt/Al2O3-CeO2 catalyst exhibits a near-unity H2/CO production ratio and 39.6% and 80.0% improvements in CO and H2 generation efficiencies, respectively, compared with Pt/CeO2 catalyst. The demonstrated innovations should be directly transferable to advance catalytic greenhouse conversions/utilizations in mitigating greenhouse gas emissions and provide guidelines in catalyst design in other photo-driven and/or thermal-driven catalytic processes

Land – Atmosphere – Meteorological Coupling Associated with the 2015 Gorkha (M 7.8) and Dolakha (M 7.3) Nepal Earthquakes

Multiple parameters (brightness temperature, soil moisture, surface latent heat flux, surface air temperature and carbon monoxide) before and after the 2015 Nepal M7.8 Gorkha main earthquake and M7.3 Dolakha aftershock were analysed using satellite observation data. The thermal anomalies from optical and microwave data appear about two months prior to the 2015 Gorkha earthquake. Some of the parameters show anomalous changes at different altitudes about 20 days prior to the main earthquake event and 10 days prior to the strong aftershock. Our results show that pre-earthquake anomalous signals propagate from the in situ to the top of atmosphere, and the anomalies in the atmosphere often observed prior to an impending earthquake. The changes on the land surface and corresponding changes in meteorological and atmospheric parameters show existence of strong coupling during the seismogenic period, although the transfer mechanism of seismic/electromagnetic is still has to be investigated and understood

Backward-Forward Reachable Set Splitting for State-Constrained Differential Games

This paper is about a set-based computing method for solving a general class of two-player zero-sum Stackelberg differential games. We assume that the game is modeled by a set of coupled nonlinear differential equations, which can be influenced by the control inputs of the players. Here, each of the players has to satisfy their respective state and control constraints or loses the game. The main contribution is a backward-forward reachable set splitting scheme, which can be used to derive numerically tractable conservative approximations of such two player games. In detail, we introduce a novel class of differential inequalities that can be used to find convex outer approximations of these backward and forward reachable sets. This approach is worked out in detail for ellipsoidal set parameterizations. Our numerical examples illustrate not only the effectiveness of the approach, but also the subtle differences between standard robust optimal control problems and more general constrained two-player zero-sum Stackelberg differential games