Electrochemical capacitors (ECs) serve as promising electrical energy storage systems due to their potential to achieve both high energy and high power densities. They can be classified as either electric double layer capacitors (EDLCs) or pseudocapacitors depending on the charge storage mechanism. EDLCs store charge in the electric double layer (EDL) forming at the electrode/electrolyte interface. Pseudocapacitors store energy both in the EDL and in the redox reactions occurring at or near the electrode surface along with ion intercalation into the electrodes. However, the interpretations of experimental results and the electrode optimization of such systems are made difficult by the coupling effect of EDL formation, redox reactions, and ion intercalation in the multidimensional porous electrode structures. This dissertation presents rigorous physical interpretations of conventional experimental characterization methods and provides design rules for EC electrodes using a multidimensional physicochemical model.

First, electrochemical impedance spectroscopy (EIS) measurements (Nyquist plots) of planar EDLC electrodes and devices were reproduced numerically for different electrode conductivity and thickness, electrolyte domain thickness, as well as ion diameter, diffusion coefficient, and concentrations. The electrode resistance, electrolyte resistance, and the equilibrium differential capacitance were identified from Nyquist plots without relying on equivalent RC circuits. These results and interpretations were then confirmed experimentally for EDLC devices consisting of two identical activated-carbon electrodes in both aqueous and non-aqueous electrolytes. Similarly, EIS measurements of planar redox active electrodes were reproduced numerically for a wide range of electrode electrical conductivity, electrolyte thickness, redox reaction rate constant, and bias potential. The electrode, bulk electrolyte, charge transfer, and mass transfer resistances could be identified from the Nyquist plots. The results were then confirmed experimentally for LiNi0.6Co0.2Mn0.2O2 and MoS2 electrodes in organic electrolytes.

Moreover, multidimensional simulations under cyclic voltammetry were performed for EDLC electrodes of different thicknesses consisting of spherical nanoparticles arranged in either simple cubic (SC) or face-centered cubic (FCC) packing structure. The capacitance under quasi-equilibrium (low charging/discharging rate) and rate-dependent (high charging/discharging rate) conditions were compared for different electrode nanoarchitectures and design suggestions were drawn. Moreover, multidimensional simulations were carried out for pseudocapacitive electrodes consisting of ordered conducting nanorods coated with a thin film of pseudocapacitive material. The contributions of EDL formation and redox reactions were discriminated and an optimum pseudocapacitive layer thickness that maximized total areal capacitance was identified as a function of scan rate and confirmed by scaling analysis.

Finally, commonly used methods to calculate energy and power densities of EDLC and hybrid pseudocapacitors were compared and evaluated. Energy conservation law was applied to the devices to identify the most appropriate method to calculate energy and power densities. The findings were confirmed by experimental measurements on EDLCs made of activated carbon electrodes and organic electrolyte and on hybrid pseudocapacitors made of MnO2-graphene and activated carbon electrodes in aqueous electrolyte.

Depleting fossil fuels and climate change have necessitated development of novel energy technologies for a sustainable future. Many of these technologies involve interaction of solar radiation with scattering particles embedded in an absorbing medium, albeit at different length scales. For example, photoelectrochemical water splitting involves direct conversion of water into H2 and O2 gases using semiconductor photoelectrodes exposed to sunlight. However, the gases are released in the form of bubbles of about 1 mm in diameter that scatter the incident light. Moreover, some of the near-infrared radiation is absorbed in the aqueous electrolyte, resulting in optical losses. Similarly, geoengineering entails large-scale modification of Earth's natural systems to reflect enough solar energy to counter the energy imbalance (known as ``radiative forcing") caused by greenhouse gases. Notably, microbubbles entrained in ship wakes scatter sunlight, and thus can be harnessed to reflect a significant amount of solar radiation, thereby mitigating global warming. Likewise, radiative cooling is a passive cooling technology that involves radiating heat to the outer space (at 3 K) via thermal emission primarily in the long-wavelength infrared (LWIR) atmospheric transparency window between 8 to 13 μm. Mesoporous aerogels made up of silica nanoparticles around 4 nm in diameter could be promising radiative cooling materials because of their high transmittance (> 90%) in the visible and near-unity LWIR emittance.

This dissertation aims to (i) systematically quantify the optical losses caused by gas bubbles during photoelectrochemical water splitting, (ii) critically review and validate different radiative transfer models for semitransparent media containing scatterers and use the most accurate model to predict albedo of ship wakes containing microbubbles, and (iii) demonstrate the use of mesoporous silica aerogels as optically transparent radiative cooling materials.

First, state-of-the-art Monte Carlo ray-tracing models were developed to simulate interaction of solar radiation with gas bubbles attached to a large horizontal Si photoelectrode surface as well as with those dispersed in the semitransparent aqueous electrolyte. The optical losses were quantified by comparing the photoelectrode absorptance with and without the presence of bubbles for a wide range of bubble diameters, volume fractions, plume thicknesses, and contact angles. Overall, the results suggested that in order to minimize the optical losses, the bubble departure diameter D should be large, while the bubble volume fraction fv and plume thickness H should be as small as possible. The effect of bubble contact angle θc on the optical losses was explained by identifying three different optical regimes based on the interplay of total internal reflection at the electrolyte/bubble interface and reflection at the bubble/photoelectrode interface. Accordingly, design guidelines were provided, such as using hydrophilic photoelectrodes to minimize the bubble contact surface area coverage and using convection to decrease bubble plume thickness.

Second, the relevant models predicting radiation transfer through semitransparent media containing scatterers were critically reviewed. A new hybrid model was proposed that predicts the effective scattering coefficient and asymmetry factor using the Lorenz–Mie theory and the effective absorption coefficient as the volume-weighted sum of the bubbles and medium absorption coefficients and solves the radiative transfer equation using the Monte Carlo method. Its predictions showed excellent agreement with those by the Monte Carlo ray-tracing method based on geometric optics for a wide range of bubble volume fractions, slab thicknesses, and medium absorption coefficients. For experimental validation, microcomputed X-ray tomography scans were performed on a fused silica sample containing bubbles to retrieve the exact locations, diameters, and total volume fraction of bubbles. Here also, predictions of the hybrid model using the retrieved data agreed well with experimental measurements of the spectral normal-hemispherical reflectance and transmittance of the sample for wavelengths between 0.4 and 3 μm when silica ranges from weakly absorbing to absorbing. Finally, simulations were performed over solar spectrum throughout the day using hybrid model to predict the albedo of ship wakes containing microbubbles for a wide range of bubble diameters and seafoam thicknesses. The results indicated that albedo as high as 0.9 could be achieved with bubble volume fraction fv = 74%, diameter D = 20 μm, and seafoam thickness H = 20 mm.

Lastly, mesoporous aerogels composed of silica nanoparticles and having porosities φ = 72.5%, 80.9%, or 87.5% and thickness around 1 mm were prepared and characterized for passive radiative cooling applications with or without aluminum substrates. The samples’ normal-hemispherical reflectance over the solar spectrum and normal emittance over mid-IR wavelengths were measured experimentally using UV-Visible and FTIR spectrophotometers. Their radiative cooling power was quantified across different aerogel thicknesses by performing spectral simulations using the Transfer Matrix method. Overall, it was found that increasing the porosity of silica aerogels eliminated the characteristic decrease in emittance of silica around 9 μm, enabling significantly larger radiative cooling power.

The present study investigates the complex relationship between nanostructures and microscale thermal transport in nanoporous thin films for energy applications. It experimentally and numerically demonstrates that the effective thermal conductivity of nanoporous materials can be tuned by controlling their nanoscale architectures including porosity, pore diameter, wall thickness, nanocrystal size, and crystallinity as well as surface passivation.

This study reports measurements of the cross-plane thermal conductivity of nanoporous thin films with various architectures between 25 and 315 K. Physics-based models combining phonon transport theory and effective medium approximations were developed to interpret the experimental data. Ordered mesoporous titania and silicon thin films were prepared based on evaporation-induced self-assembly method. Pure silica zeolite films were produced by either in-situ growth or by spin coating a zeolite nanoparticle suspension followed by crystal growth upon heating. These synthesized thin films were systematically and fully characterized. They featured ordered nanopores with porosity, pore diameter, and film thickness ranging from 30% to 59%, 0.5 to 25 nm, and 120 to 370 nm, respectively. Their dense matrix was amorphous, polycrystalline, or consisted of an aggregate of nanocrystals. The thermal conductivity of all synthesized nanoporous films increased monotonically with temperature within the temperature range considered. At low temperatures, the nanoporous films behaved like amorphous or strongly disordered materials and their thermal conductivity was proportional to Tⁿ with n varied between 1 and 2.3. At high temperatures, the thermal conductivity increased slowly with temperature or reached a plateau due to strong phonon Umklapp scattering and the saturation of phonon modes. The presence of pores in amorphous mesoporous thin films had a purely geometrical effect by reducing the cross-sectional area through which heat can diffuse. By contrast, in crystalline mesoporous thin films the presence of pores also increased phonon scattering. In addition, the film thickness generally did not affect the measured thermal conductivity. Indeed, phonon scattering by pores and by nanocrystal grain boundary dominated over boundary scattering and were identified as the dominant scattering mechanisms for nanoscale energy transport in the synthesized nanoporous films.

This study further establishes that the effective thermal conductivity k_eff of crystalline nanoporous silicon was strongly affected not only by the porosity f_v and the system's length L_z but also by the pore interfacial area concentration A_i. A modified effective medium approximation combining kinetic theory and the coherent potential approximation suggested that k_eff was proportional to (1-1.5f_v) and inversely proportional to the sum (A_i/4+1/L_z). This scaling law was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by molecular dynamics simulations for spherical pores as well as for cylindrical pores and vacancy defects.

Finally, this study demonstrated, using equilibrium molecular dynamics simulations, that surface passivation added another parameter for reducing the thermal conductivity of nanostructured materials. To do so, there should be strong acoustic vibrational modes coupling between surface and passivation atoms. For example, oxygen passivation reduced the thermal conductivity of nanoporous crystalline silicon. In addition, the effect of passivation reduced with temperature because of increasing contribution of Umklapp scattering.

These results could help establish new strategies to control the thermal conductivity of nanoporous materials for a wide range of applications including thermoelectric devices, supercapacitors, dye-sensitized solar cells, and hydrogen storage devices.

Value-added products derived from photosynthetic microalgae could serve as a useful renewable resource in the face of mounting pressures on food, energy, and water systems from global climate change. In addition to acting as a carbon sink, microalgae are fast-growing organisms whose rich biodiversity is reflected in their variety of potential applications, ranging from cosmetics and pharmaceuticals to biofuels, food products, and animal feed. Light transfer plays a vital role in the productivity of outdoor microalgae cultivation systems. Indeed, in the case of optimal operating conditions such as temperature, pH, and nutrient availability, microalgae growth depends entirely on the rate of light absorption by the cells. However, large-scale microalgae cultivation typically takes place in outdoor raceway ponds where light transfer can be impacted by a variety of factors. For instance, outdoor raceway ponds may feature a transparent cover to achieve better control of the growth conditions. However, evaporation from the culture results in condensate droplets on the underside of the cover, potentially reducing the window transmittance and solar energy input to the culture. Furthermore, a variety of species of interest for value-added products readily form colonies either in the forms of aggregate-like clusters or of ordered spherical shells. Such a change in the arrangement of the cells may impact their ability to absorb the incoming photons. Finally, outdoor ponds are subject to low solar intensities and large angles of incidence in winter, mornings, and evenings. These phenomena may result in limiting light transfer conditions and low biomass productivity and remain major barriers to unlocking the potential of microalgae as a sustainable and inexpensive source of value-added products. Therefore, a comprehensive understanding of light transfer in microalgae cultivation systems is necessary to optimize their performance.

This dissertation aims (1) to quantify the impact of small and large condensate droplets on the transmittance of transparent windows and on the performance of outdoor raceway ponds, (2) to assess the impact of colony formation on light absorption by microalgae cells, and (3) to investigate the use of external reflecting surfaces to increase light availability in dense cultures and increase raceway pond productivity. First, light transmittance through horizontal and tilted windows supporting large pendant droplets was predicted for various droplet volumes, contact angles, and window tilt angles. Compared to windows supporting small droplets, the transmittance of windows supporting large droplets was up to 37% smaller for horizontal windows and up to 14% larger for tilted windows. Then, light transfer through a window supporting small, cap-shaped droplets was coupled with a growth kinetics model to elucidate the impact of condensate droplets on the biomass productivity of an outdoor raceway pond. Biomass productivity was predicted to decrease by up to 18% when condensate droplets were present. Second, light transfer in aggregate-like colonies of Botryococcus braunii was also investigated both experimentally and numerically. Good agreement was found between the experimental and predicted absorption cross-sections. This approach was also applied to study light absorption in larger, ordered colonies like those observed in species of the Volvocaceae family. In both cases, mutual shading between the cells in the colonies decreases light absorption by up to 23% compared to single cells, which may decrease the algae growth rate. Third, the performance of a novel reflecting outdoor raceway pond design was predicted throughout the year for two locations and several different design configurations. A single south-facing mirror was predicted to increase biomass productivity by as much as 73% in the winter months. Overall, the biomass productivity was found to improve throughout the year thanks to the increased solar energy input provided by the additional sunlight reflected onto the culture surface. This approach could extend the growing season for outdoor cultivation of microalgae.

Electrochemical capacitors (ECs) have drawn significant attention as electrical energy storage systems for high power applications thanks to their high cycle efficiency and long cycle life. However, under high current cycling, they can experience a significant amount of heat generation resulting in excessively high cell temperatures. Elevated temperatures, in turn, can lead to (i) increased self-discharge rates, (ii) accelerated aging of the device, and (iii) electrolyte decomposition and evaporation causing deterioration of their performance and lifetime. In this context, ionic liquid (IL) electrolytes are promising due to their excellent thermal stability over large operating temperature windows. However, most recent calorimetric studies have measured irreversible and reversible heat generation rates in electric double layer capacitors (EDLCs) consisting of activated carbon (AC) electrodes with organic and aqueous electrolytes at room temperature. This doctoral thesis investigates experimentally the heat generation rate in AC electrodes with ILs neat or diluted in an organic solvent electrolytes in the temperature range between 5 and 80 �C. First, a potential window of 1 V was used to compare with past calorimetric studies using aqueous or organic electrolytes. Then a more realistic potential window of 2.5 V was tested for the same temperature range. Endothermic dips were observed in the instantaneous heat generation rate at the negative electrode in diluted IL and grew with increasing temperature due to overscreening effects, ion desolvation, and/or decomposition of PC. The irreversible heat generation was similar in each half-cell and decreased with increasing temperature due to the increase in the electrolyte conductivity with temperature. The total irreversible heat generation was in good agreement with Joule heating for potential window of 1 V, as also observed with aqueous and organic electrolytes. However, the total irreversible heat generation exceeded Joule heating for potential window of 2.5 V, especially at high temperature and low current. This was attributed to ion desorption and charge redistribution in the porous electrodes. Finally, the reversible heat generation increased with temperature and was larger at the positive than at the negative electrode due to the difference in anion and cation sizes.

Moreover, elevated temperatures associated with high heat generation rates are very concerning for flexible and wearable all-solid-state supercapacitors with gel electrolytes placed in direct contact with the users. Therefore, quantifying the amount of heat generation in such devices is essential for developing thermal management strategy in order to ensure user's comfort and safety. This thesis also investigates heat generation in flexible all-solid-state supercapacitor devices consisting of graphene petals grown on buckypaper electrodes with either conventional (non-redox) or redox-active gel electrolyte. The total irreversible heat generation was equal to Joule heating for both types of devices but was larger in the device with redox-active gel electrolyte due to its larger internal resistance. In addition, the reversible heat generation rate was different at the positive and negative electrodes in each device due to asymmetry in the charging mechanisms caused by a combination of electric double layer (EDL) formation, overscreening effect, and additional redox reactions in the redox-active gel electrolyte. This study further illustrates how in operando calorimetry can not only quantify the heat generation rate necessary to device thermal management but also provide insights into the electrochemical phenomena occurring during cycling of ECs.

Increased demand for electrical energy storage to support the deployment of renewable energy sources and to decarbonize the transportation sector has established electrochemical capacitor (EC) and rechargeable batteries as the current frontier of science. Accurate understanding of the phenomena occurring during cycling of ECs and batteries can assist in determining their limitations, in optimizing their design and the material selection and in developing novel electrode materials. Recently, isothermal operando calorimetry has been successfully developed to identify the charging mechanisms occurring in ECs and batteries. It has identified the thermal signatures of (i) Joule heating, (ii) EDL formation/dissolution, (iii) redox reactions, (iv) overscreening effect, (v) electrolyte decomposition, (vi) irreversible ion intercalation into the electrode, and (vii) insulator to semiconductor transition.

Heat generation rates in hybrid supercapacitors consisting of positive α-MnO2 cryptomelane electrodes and AC counter electrodes with different aqueous electrolytes were measured experimentally via isothermal operando calorimetry under galvanostatic cycling at 20 �C. First, two devices with 0.5 M K2SO4 or Cs2SO4 aqueous electrolytes were investigated for their different solvation shell thickness and bare ion size. The measured heat generation rate at the AC electrode was attributed to irreversible Joule heating and reversible EDL formation/dissolution. On the other hand, the heat generation rate at the α-MnO2 electrode was caused by Joule heating and redox reactions. Moreover, for large potential windows, an endothermic dip was observed at the α-MnO2 at the end of the charging step and was attributed to the onset of hydrolysis. Hydrolysis was observed for potential window of 2 V in 0.5 M Cs2SO4 aqueous electrolyte and for 1.8 V for 0.5 M K2SO4. The wider potential window than the theoretical 1.23 V was attributed to the thinner solvation shell of Cs+ compared to K+ which limited the amount of water present near the electrode.

Heat generation rate measured in hybrid supercapacitors is often dominated by ion adsorption/desorption and redox reactions. However, during adsorption/desorption and redox reactions, ions fully or partially shed or form their solvation shells. Here, hybrid supercapacitors consisting of an α-MnO2 and an AC electrode with MgSO4 aqueous electrolytes with different concentrations were investigated via isothermal operando calorimetry. MgSO4 salt was chosen for its high solubility in water (≤ 2.9 M at 20 �C) near neutral pH and its large enthalpy of solvation compared to previously considered K2SO4 and Cs2SO4. The measured heat generation rates at the AC electrodes were similar to those previously measured in devices with K2SO4 and Cs2SO4 aqueous electrolytes. However, the heat generation rate measured at the α-MnO2 electrodes was significantly different from those observed for K2SO4 and Cs2SO4 aqueous electrolytes. This was attributed to the heat generation due to solvation/desolvation of Mg2+ cations. However, the thermal signature of solvation was not observed at the AC electrode as the solvated Mg2+ cations were small enough to enter the pores in the AC electrodes without becoming partially desolvated. This interpretation was confirmed by a simple thermal model.

Ion size can also affect the charging mechanisms and capacity of hybrid supercapacitors. To assess the effect of ion size on the charging mechanisms and heat generation in hybrid supercapacitors three devices were tested and consisted of an α-MnO2 and an AC electrode with either (i) 0.5 M Li2SO4, (ii) 0.5 M Na2SO4, or (iii) 0.5 M Cs2SO4 aqueous electrolyte. The thermal signatures measured at the AC electrodes in 0.5 M Na2SO4 and 0.5 M Cs2SO4 aqueous electrolyte were qualitatively similar and attributed to EDL formation/dissolution. However, they were different from the thermal signature measured in 0.5 M Li2SO4 aqueous electrolyte when Li+ possibly participated in surface redox reactions with the AC electrode due to its small size and high electronegativity. Then, the heat generation rate was endothermic during charging due to the non-spontaneous Li+ surface redox reactions and exothermic during discharging. Interestingly, Na+ and Cs+ cations did not participate in surface redox reactions into AC as their compounds with carbon are not stable.

The irreversible heat generation in the redox active α-MnO2 electrode of hybrid supercapacitors exceeded Joule heating due to the contributions of polarization heating and the hysteretic ion concentration profile evolution at the electrode surface. This dissertation formulates expressions and performs numerical simulations of the irreversible heat generation rates associated with charge and mass transfer resistances based on the modified Poisson-Nernst-Planck model. These contributions to the total heat generation rate were expressed as resistive losses through the charge transfer or the mass transfer resistances by analogy with Joule heating. These resistances were not constant during cycling and instead depended on the state of charge of the electrode.

Finally, while batteries feature high energy density, their power density is often limited. LixNa1.5-xVOPV4F0.5 (LNVOPF) has been identified as a promising high rate cathode material whose rate performance can rival that of pseudocapacitive electrodes such as α-MnO2. This dissertation investigates the structural evolution of LNVOPF during lithiation and delithiation to elucidate the origin of its excellent rate performance. Open circuit voltage and entropic potential were measured in three different coin cells with LNVOPF cathodes and Li metal anodes in 1 M LiPF6 in EC:DMC electrolyte. The cathodes consisted of either LNVOPF micronbricks or nanoparticles and were manufactured with either P3HT or PVDF as binders. The evolutions of open circuit voltage and entropic potential indicated that LNVOPF exhibits solid solution behavior with ion ordering. The device with the LNVOPF nanoparticles featured faster kinetics and larger apparent diffusion coefficient of Li+ than that with LNVOPF micronbricks. This was attributed to the larger particle size and the large electrode resistance of the coin cell with LNVOPF micronbricks which retained more Na than LNVOPF nanoparticles after Li exchange. This interpretation was corroborated with thermodynamic calculation of the entropic potential evolution.

This thesis investigates the effects of adding microencapsulated phase change materials (PCM) on the thermal conductivity of cement paste and cement mortar composites. Embedding cementitious composites with microencapsulated PCM has been considered a promising method for increasing the thermal mass of buildings to achieve greater energy efficiency. Cement paste and cement mortar samples were synthesized with a constant water to cement ratio of 0.45. Both contained microencapsulated PCM with diameter ranging from 17-20 microns, volume fraction up to 30%, and a melting temperature around 24C. The cement mortar also contained quartz grains 150-600 microns in diameter such that the sum of the volume fractions of quartz and microencapsulated PCM was fixed at 55\%. All samples were aged for more than 28 days. Their effective density and free moisture content were systematically measured. A guarded hot plate apparatus was designed, assembled, and validated according to the ASTM C177 to measure the effective thermal conductivity of the aged specimens of cement paste and cement mortar without and with microencapsulated PCM. Measurements were performed between 10 and 40C, encompassing the entire PCM phase change temperature window. The effective thermal conductivity of both the cement paste and the cement mortar composites was found to be nearly independent of temperature in the range considered. It also decreased as the volume fraction of microencapsulated PCM increased. Finally, excellent agreement was obtained between experimental data and the effective medium approximation derived by Felske (2004) for core-shell-matrix composites.

Microalgae are tipped as the feedstock for next generation transportation fuels due to their significantly higher photosynthetic efficiency compared to higher plants. These microorganisms can contain large amounts of triglyceride fatty-acids (TG-FAs) that can be converted to biodiesel by transesterification. However, microalgae cultivation in photobioreactors (PBRs) typically suffers from low productivity due to light transfer limitations. To optimize microalgae growth rate and productivity, radiation transfer analysis must be performed in order to optimize light availability in PBRs. Nitrogen starvation coupled with strong illumination has been used to induce large amounts of TG-FA accumulation in microalgae. However, the role of light absorption rate by the microalgae cells on TG-FA productivity is not well understood. This study aims (1) to study the interaction between light and photosynthetic microorganisms and (2) to optimize light transfer conditions in PBRs to maximize microalgal biomass and lipid productivity.

First, the complete set of radiation characteristics and optical properties of the eustegmatophycea Nannochloropsis oculata, a promising marine microalgae for biodiesel production, was obtained for cells grown under various light spectra and irradiances. Second, the radiation characteristics of aggregates and colonies of microalgae were studied theoretically. Significant differences were observed in the average absorption and scattering cross-sections of cells either free-floating or aggregated in colonies. Third, a feed-forward inversion control scheme was designed and experimentally demonstrated for maintaining an optimum incident irradiance on PBRs during batch cultivation. A data-based model-free optimization was utilized to rapidly estimate the optimum average fluence rate set-point value leading to maximum microalgae growth rate. This control scheme increased biomass productivity and reduced lag time compared to batch cultures exposed to constant irradiance throughout the cultivation process. Finally, N. oculata were grown under nitrogen starvation and were characterized in terms of their biomass, pigment, and TG-FA concentrations as well as their absorption and scattering cross-sections as a function of time for several batch cultures. The TG-FA production rate correlated to the rate of photon absorption by cells suggesting that the TG-FA production process was limited by light.

Mesoporous materials could serve as promising thermally insulating materials for window applications due to their low thermal conductivity at ambient conditions. However, they tend to be translucent and hazy due to their nanoscale porous architecture controlling their optical and radiative properties. This dissertation aims to (i) computationally generate and characterize realistic mesoporous materials, (ii) explore how their nanoscale structure affect their light scattering characteristics, and (iii) apply the knowledge gained to understand light transfer through drying mesoporous monoliths and optical characterization of thin films.

First, three-dimensional mesoporous materials consisting of a network of touching or overlapping spheres were computationally generated using the diffusion-limited cluster-cluster aggregation (DLCCA) method. A new algorithm was developed to reproduce nitrogen adsorption porosimetry and retrieve their pore size distributions. The numerically computed specific surface areas and pore size distributions were in good agreement with experimental data reported for mesoporous silica. Second, unpolarized electromagnetic wave scattering by the computer-generated mesoporous structures, described as porous fractal aggregates, were investigated using the T-matrix and discrete-dipole approximation (DDA) methods. The transition between the independent and dependent scattering regimes was studied in systems with up to 8 particles. Independent scattering refers to situations when particles are sufficiently distant that some radiation characteristics of the ensemble can be determined by adding the contributions of each particle. When particles are in close proximity, however, dependent scattering prevails and is affected by near-field interactions and far-field interferences among scattered waves from nearby particles. Here, the dimensionless parameters governing the scattering cross-section and asymmetry factor of non-absorbing particle suspensions and aggregates were found to be (i) the particle size parameter, (ii) the relative index of refraction, (iii) the interparticle distance-to-wavelength ratio, and (iv) the number of particles. Different transition criteria were derived for the scattering cross-section and the asymmetry factor. Dependent scattering effects prevailed in all aggregates and increased with decreasing particle size parameter. In addition, particle overlapping had a negligible effect on the scattering cross-section and asymmetry factor. Furthermore, predictions of the integral scattering characteristics of non-absorbing aggregates with relatively small particle size parameter were found to be accurately predicted by the equivalent effective property (EEP) approximation treating the aggregates as homogeneous spheres with the same volume and an effective refractive index. The EEP approximation was then combined with the Monte Carlo method to predict the transmittance and haze of ambiently drying mesoporous monoliths. The temporary decrease in transmittance and increase in haze observed experimentally during drying of the monoliths could be explained by light scattering by growing dry domains forming within the monolith. Finally, optical interferometry was demonstrated as a robust and simple alternative to ellipsometry for measuring the film thickness, effective refractive index, and total porosity of non-absorbing multicomponent mesoporous thin films.

This study is concerned with the direct conversion of thermal and mechanical energy into electricity using ferroelectric materials. These materials possess a spontaneous polarization and can undergo solid-state phase transitions as a result of a change in applied electric field. All ferroelectric materials are pyroelectric, i.e., phase transitions can also be induced by a change in temperature. The Olsen cycle takes advantage of the thermally-induced phase transitions to directly convert thermal energy into electrical energy. It consists of two isoelectric field and two isothermal processes in the electric displacement versus electric field diagram. This study aims to improve the understanding and performance of the Olsen cycle by exploring the use of relaxor ferroelectric lead zirconate niobate-lead titanate (PZN-5.5PT) and lead magnesium niobate-lead titanate (PMN-28PT) single crystals. These materials were chosen for the fact that their phase transitions take place at low temperatures. The performance of the cycle was improved by varying the operating frequency to maximize changes in polarization due to phase transition taking place during the cycle.

Moreover, all pyroelectric materials are also piezoelectric, i.e., phase transitions can occur as a result of a change in applied compressive stress. This study aims to explore new methods to simultaneously convert mechanical and thermal energy directly into electrical energy by combining electric field, compressive stress, and/or thermal cycling to induce phase transitions. Two new energy conversion cycles were conceived and demonstrated on PMN-28PT for its outstanding piezoelectric response over a broad temperature range. First, a thermomechanical cycle converting thermal and mechanical energy into electricity was performed. It proved to be more versatile than the Olsen cycle by generating electrical energy at all temperatures. It also achieved larger power densities thanks to larger operating frequency. In addition, it improved upon the material efficiency of the Olsen cycle by reducing the heat input during the cycle. Second, a thermally-biased mechanical cycle converting mechanical energy into electricity under a thermal bias was performed. It improved upon the power generation and material efficiency of the thermomechanical cycle by substituting the thermal cycling with a thermal bias to approach the desirable phases. In addition, it was capable of generating larger power per unit volume of material than existing direct mechanical to electrical energy conversion methods.