Jason B. Baxter

Achieving a balance between density and strength in synthetic sintered lightweight aggregates (LWA) is crucial for their effective performance in various applications such as lightweight concrete. LWA density and strength are governed by the properties and morphology of their solid and pore components. This study explores engineering solid and pore components during LWA sintering by balancing the viscosity of the melt and gas release. Two types of waste coal combustion ash (WCCA), with different iron oxide ( Fe 2 O 3 ) and carbon (C) contents, were utilized, with aqueous NaOH added as a fluxing agent. Fe 2 O 3 powder was added to WCCA to modulate gas generation during sintering. The development of pore structures in LWA was evaluated after rapid sintering at temperatures of 1,000°C–1,100°C. Pore structures were analyzed using X-ray computed tomography, while mineralogical composition was assessed through quantitative X-ray diffractometry. Key properties such as specific gravity and crushing strength were evaluated. The findings reveal that the porosity, strength, and density of LWA can be controlled by modifying the Fe 2 O 3 and C content in WCCA/NaOH blends. WCCA with lower Fe 2 O 3 and C contents yielded LWA with reduced porosity and higher strength, whereas those with higher Fe 2 O 3 and C contents resulted in higher porosity, suitable for applications where strength is less critical.

This thesis is a culmination of efforts to study and identify optoelectronic effects in cadmium telluride (CdTe)-based photovoltaic (PV) absorbers. Specifically, it elaborates on characterization of films alloyed with selenium (Se) in Chapter 3 or doped with antimony (Sb) in Chapter 4. The goal of this research is to investigate fundamental impacts of processing conditions on absorber properties from alloying and doping using non-contact optical probes. Polycrystalline thin film PV are desirable for their short time and low energy intensity to manufacture compared to single-crystal silicon PV. Thus far, CdTe-based absorbers are the only successful thin film technology on the global PV market, comprising ~4% of global module production. The power conversion efficiency from the detailed balance limit for CdTe, is ~31%, however CdTe-based technologies to date have only reached 23.1% efficiency. Despite successful commercialization, CdTe shows a large deficit in open circuit voltage (V_[OC]), which represents a viable direction for technological improvement. Research efforts to support CdTe manufacturing have improved efficiency to 23.1% at lab scale and ~19.5% at the commercial scale in the last 10 years. Improvements are largely the result of incorporation and optimization of: 1) selenium alloy gradients and 2) the transition from copper doping to arsenic doping. Both changes have been implemented commercially; however, the full extent of microstructural and electronic effects of the processing is not fully understood. Both Se-alloying and As-doping were critical to enhance performance. However, there are known problems and knowledge gaps with both Se and As in CdTe, and both can be correlated to harmful defect chemistry. Selenium alloying improved performance greatly, but it is induces defects in quantities that can limit V_[OC]. The identities of Se-related defects are not known precisely, and their behaviors have only recently been studied. Films containing selenium show also show native n-type conductivity, which reduces p-type doping ability and can result in compensation. Arsenic-doped CdSeTe are the current state-of-the-art devices; however, dopants show low activation, leading to inactive impurities. Inactive impurities can form new defects of varying impact, in the worst cases causing extreme electronic disorder, but in general reducing or limiting performance in devices. The exact contributions of each impurity or defect complex on performance limitations require detailed study. Optical spectroscopies are non-destructive and non-contact methods that can be applied to absorber films or full devices. Photoluminescence (PL) and Time Resolved Photoluminescence (TRPL), as well as Optical Pump Terahertz-Probe (OPTP) and terahertz Time Domain Scans (TDS) are valuable and complementary experimental that are used in this work to inform on band and defect structures, as well as carrier transport and recombination, with regard to processing. In general, developing a deeper understanding of how chemistry and processing affect thin film electronic behavior can help improve performance. Se-alloying appears to increase carrier lifetimes, yet estimates from TPRL do not match performance expectations from device models. In Chapter 3, we report the results extracted from fitting a photocarrier transport and recombination model to TRPL and OPTP transients. We quantify effects of Se concentration on carrier dynamics and recombination in CdSe_[x]Te_[1-x]. Bulk carrier and surface lifetimes are improved with Se alloying, however, carrier mobilities are reduced. We show that carrier trapping in shallow defects can complicate interpretations of carrier lifetime from TRPL decays. We show that a model incorporating trapping physics results in good description of dynamics of Se alloyed CdTe films. These results demonstrate the need for both careful parameter estimation for devices and controlled processing with Se. Additionally, using terahertz TDS analysis, we show that the incorporation of Se does not fundamentally change the carrier mobility, and there are instead other causes for mobility reduction. Incorporation of group V dopants in place of traditional copper led to a boost in device V_[OC] and efficiencies; however, low activation and defect chemistry of state-of-the-art dopants such as As or P can lead to unintended V_[OC] deficits. Sb was proposed for its size match with the Te-anion site and demonstrated high activation in CdTe. In Chapter 4, we show that Sb-doped films achieve high carrier concentrations (>2x10¹⁶ cm⁻³) in graded CdSeTe devices with aggressive CdCl₂, at the cost of inducing defect luminescence ~350 meV below the gap energy, which can limit V_[OC]. In samples with mild CdCl₂, Sb doping has no deleterious effects on carrier dynamics or emission spectrum in CdTe or CdSeTe up to 3x10¹⁷ cm⁻³. High dopant heater temperature during processing results in complex chemistry and transport physics in newly identified radiative defects. While it has yet to be demonstrated, this work suggests the combination of high doping and sufficient optoelectronic performance is possible. Ultimately, many material properties of relevance to device performance can be probed with optical spectroscopies, which can be used to enhance understanding of processing-structure-property-performance relationships. The methods used in this thesis are well suited for initial analyses and screening of experimental thin films as well as deeper correlation between processing variables and their potential impacts on device physics and performance.

In this study, we developed a new method for in situ Sb doping of CdTe thin films combining vapor transport deposition with a Group V pyrolyzer to address Sb doping concentration and doping efficiency. The Sb doped CdSeTe (CdSeTe:Sb) films were deposited in solar cell structures under variations of Sb dopant source heater, vapor pyrolyzer temperature, and Cd vapor excess. Results indicate that although these parameters do not affect the CdTe morphology or crystal structure, they critically influence doping efficiency and trap concentration. Capacitance–voltage measurements show that a higher dopant heater (T_D) or pyrolyzer (T_P) temperature leads to higher net carrier concentration, achieving a net carrier concentration of 10¹⁶ cm⁻³ and 20% doping efficiency with a T_D/T_P combination of 600 °C/1100 °C. By tuning the Cd/Sb flux ratio during CdSeTe:Sb deposition, the lowest defect concentration is achieved at Cd/Sb of 1.4:1, which produced the best V_OC CdSeTe:Sb cell. This demonstrates a path to produce high net carrier concentration polycrystalline CdTe thin film with a low concentration of dopant-induced defects.

The wind energy sector is a growing contributor to global electricity generation. The increasing deployment of wind turbines also creates significant waste when turbine materials reach their end-of-life. Glass fiber reinforced polymer composites, which comprise the majority of a wind turbine blade's mass, are difficult to separate into their component parts for recycling. This study employs a cradle-to-gate life cycle assessment to evaluate the environmental impacts of utilizing waste wind turbine blade material in cement clinker and fiberglass production. We find that incorporating waste blades as 15% of the feedstock in a cement clinker production plant reduces global warming and human health-respiratory impacts by 9 and 34%, respectively, compared to using virgin materials only. For a fiberglass plant, this substitution increases global warming impacts by 11% but decreases respiratory health impacts by 3%. Each kilogram of secondary product diverts approximately 0.25-0.32 kg of WTB waste from landfills. The projected rate of blade decommissioning of ∼800,000 tonnes per year would replace less than 1% of the overall virgin material demand for the cement clinker industry and up to 8% for the fiberglass industry, indicating plenty of capacity for these industries to accommodate this waste blade material in their feedstocks.

The wind industry is expanding to meet net zero energy emission goals set by the International Energy Agency. Cumulative US wind energy generation exceeded 150 gigawatts in 2023, accounting for 12% of all US electricity generation and 15% of the cumulative global wind capacity. Wind turbine blades (WTBs) are increasing in length to help meet the expanding wind energy demand. This growth has been enabled by maturing material selections. Glass fiber reinforced polymer composites (FRPC) decrease the weight and increase the strength and stiffness of the exterior shell of a blade. However, these composite materials do not readily break down and tend to be landfilled in 7-10 meter segments at their end-of-life (EOL). Blades pack less efficiently in the landfill compared to other waste streams because of their hollow design, which can be mitigated by shredding, though this isn't common practice in the US. In this work, a multi-criteria decision-making framework is developed to compare alternative EOL pathways that may be used to maximize the returned value from the materials in retired blades. This framework suggests that cement co-processing is an active pathway to handle the current stream of retired WTBs. In cement co-processing, waste materials are input to a cement kiln as alternative sources of virgin raw materials and fuels for cement clinker production. However, present and future advancements in material choices for WTBs - especially the increasing use of carbon fibers instead of fiberglass - will make this process unsuitable to manage this material stream in the future. Pyrolysis is a near-term technology that may be more suitable for carbon fiber-reinforced blades and provides a pathway toward recovery and reuse of the glass fibers in the current wind blade waste stream. In pyrolysis, the polymers in the WTB are thermally decomposed and converted into high-energy hydrocarbons and the reinforcing fibers are recovered. The hydrocarbons can be used as an alternative fuel source and the recovered fibers can be reused, remanufactured, or downcycled into other applications. A life cycle assessment (LCA) is performed to compare the environmental impacts of producing cement clinker and fiberglass with conventional raw material feedstocks or a blended feedstock of raw materials and retired blades. Key findings suggest that cement co-processing may provide environmental benefits to the cement industry. However, the added burden to recover glass fibers from the blades using pyrolysis and remanufacture them into new fiberglass increases the environmental burden of fiberglass manufacturing. Monte Carlo simulations are performed to explore the uncertainty in these results based upon variability of input parameters in the literature. This study suggests that the energy demands for high-energy process steps contribute the greatest level of uncertainty in LCA results, compared to transportation distances, material compositions, and low-energy process steps. Further study of the system boundaries and crediting strategies show that these methodological choices critically shape the outcomes of LCA results in a comparison of the EOL pathways. Overall, this work provides a quantitative representation of the environmental impacts for current and near-term technologies to manage retired wind turbine blades. The insights from this work can support more informed decision-making across the wind, cement, and glass industries as they respond to the growing volume of decommissioned blades.

Halide perovskite thin films can be the centerpiece of high-performance solar cells, light-emitting diodes, and other optoelectronic devices if the films are of high uniformity and relatively free of pinholes and other defects. A common strategy to form dense films from solution has been to generate a high density of nuclei by rapidly increasing supersaturation, for example, by timely application of an antisolvent or forced convection. In this work, we examine the role of retrograde solubility, wherein solubility decreases with increasing temperature, as a means of increasing the nucleation density and film coverage of slot-die-coated methylammonium lead iodide (MAPbI₃) from γ-butyrolactone (GBL) solution. Coverage was investigated as a function of the substrate temperature and the presence and temperature of an air knife. Results were considered within the framework of the dimensionless modified Biot number, which quantifies the interplay between evaporation and horizontal diffusion. Moderate temperatures and a heated air knife improved film coverage and morphology by enhanced nucleation up to ∼80 °C. However, despite the dense nucleation enabled by retrograde solubility, slow evaporation as a result of the low vapor pressure of GBL, combined with Ostwald ripening at high temperatures, prevented the deposition of void-free, device-quality films. This work has provided a more detailed understanding of the interplay between perovskite processing, solvent parameters, and film morphology and ultimately indicates the obstacles to forming dense, uniform films from solvents with high boiling points even in the presence of rapid nucleation.