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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 1009

Chirality control of tailored one-dimensional polysaccharide nanocrystals

(Georgia Institute of Technology, 2024-04-27) Bukharina, Daria

One-dimensional polysaccharide nanocrystals, derived from living organisms, can self-organize into complex structures that possess long-range hierarchical order making them great candidates for high-performance structural composites with multifunctional capabilities. Their abundance in nature and biodegradability makes them excellent candidates as sustainable materials of the future. However, a greater fundamental understanding of how these nanoscale building blocks organize into functional microstructures is needed to push the boundaries of mechanical and photonic metamaterials for the future. The goal of this thesis is to uncover the intrinsic mechanisms behind self-assembly phenomenon in natural systems, understand the critical forces and parameters required for their successful hierarchical organization into chiral nematic structure and with those insights manipulate the surface chemistry to create self-assembly templates for use in photonic films for optical filters, chiral encryption, smart coatings, or biosensors. In this thesis, we first provide fundamental insight into how chiral interactions in 1D polysaccharide systems emerge, using cellulose nanocrystals (CNCs) as an example. Then, we show how CNCs interactions can be tuned and controlled via their surface modification. By functionalizing them with single stranded DNAs we show the possibility for CNCs chiral complexation through DNA-guided assembly. A nanoscale-controlled strategy to induce stimuli responsiveness and dynamic chirality. The challenges of this process and strategies to overcome them are discussed. Lastly, a top-down 3D printing approach to engineer chiral CNC-based photonic crystals with unique optical activities is developed here. This method shows how thin films capable of controlled pre-programmed circularly polarized absorbance and emission can be constructed from CNC-composites for future smart coatings, optical encryption, or optical filters. Overall, the goal of this work is to inspire applicational implementation of bioderived nanocrystals by demonstrating how their properties can be controlled and tailored based on the application. This work advances fundamental understanding of the assembly of polysaccharides nanocrystals in nature and creates a toolset to aid in the design and engineering of future metamaterials.
Sound sensing B.A.T.S. - Biodegradable acoustic triboelectric nanogenerator sensors

(Georgia Institute of Technology, 2024-04-15) Verma, Harsh Kumar

Two of the prominent challenges facing modern development of electronic devices are minimizing their power requirements and simplifying the disposal processes. Recent developments in self-powered energy harvesters such as triboelectric nanogenerators (TENGs) have been extensively focused on the use of more environmentally sustainable materials to minimize the environmental impact while being able to harvest energy from the environment. In this thesis, I have investigated 3 different versions of biodegradable triboelectric nanogenerators for acoustic energy harvesting, referred to as biodegradable acoustic triboelectric nanogenerator sensors (BATS). To study the negative layer, I use a tribonegative biopolymer, poly (L-lactic acid) and compare the voltage outputs of the device to traditional materials such as FEP. These results indicated good usability of PLLA based BATS, albeit an order of magnitude lower signal than for FEP based BATS. I also examined how the polymer crystallinity and volatile content in the PLLA films impacts the performance. To study the positive layer, I selected silk fibroin as a tribopositive biopolymer. Variation in voltage output was observed and correlated to changes in drying temperature and the water uptake of the polymer, demonstrating the dependence of the TENG performance on the dynamic behavior of the biopolymers and the environmental conditions. Lastly, I used a contact separation mode for the TENG and examined the combined effects of the triboelectric biopolymers PLLA and silk fibroin on the TENG performance. A shift in the resonant frequency was observed in these devices over time due to the hygroscopic property of both PLLA and silk fibroin. As in the previous cases, the reduction in voltage was observed and correlated with the reducing residual solvent in the films over time. Overall, residual solvent after processing and water absorption, measured by changes in the volatile content of the polymer, were found to be a major factor significantly affecting the properties of the triboelectric layers. Optimizing the processing conditions and the solvents used for these polymers is crucial, as this not only affects their triboelectric property but also, the changes in TENG performance over time.
Characterization and effects of heterogeneities on shock compression properties in high-solids loaded additively manufactured polymer composites

(Georgia Institute of Technology, 2024-01-11) Wagner, Karla Brooke

High-solids loaded polymer composites contain several hierarchies of heterogeneities and are of interest for use as ceramic green bodies and energetic crystals embedded in a polymer matrix. The recent and rapid growth of additive manufacturing (AM) and the engineering need for more complex geometries and individualized products has led to a surge of interest in fabricating high loading particle composites via AM. In particular, Direct Ink Write (DIW) extrusion involving layer-by-layer deposition of a composite paste made of a high loading of solids and a curable polymer binder is used to fabricate such composites in different geometries and forms. However, DIW-AM introduces further complexity in composites due to formation of process-inherent heterogeneities such as particle aggregation or porosities, which can be random, directional, or stochastic. The structure and composition of such materials vary across several length scales, resulting in processing and mechanical behavior that is difficult to predict or understand. Shock-compression of heterogeneous particle-filled polymer composites often involves complex interactions, which can make it difficult to predict their dynamic mechanical properties. The shock compression behavior is often dominated by mesoscale defects (including porosity) or interactions of the shock wave with interfaces and particulates. Traditional diagnostic methods, such as velocity interferometry, enable temporally resolved measurements, but are limited in spatial resolution and generally provide volume averaged responses. Spatially resolved measurements are therefore also necessary to provide sufficient information regarding the mesoscale processes which dominate performance of such materials. X-ray phase contrast imaging, a spatially and temporally resolved technique, in conjunction with traditional velocimetry, can enable observation of the effects of hierarchical heterogeneities on shock compression response. In this work, the effect of print geometry and porosity (process-inherent heterogeneities) on the shock compression response of an additively manufactured high-solids loaded composite is studied. The composite contains three reinforcing phases: two inorganic particles and one organic particle, all with differing size distributions and morphologies. They are surrounded by a UV curable polymer binder. In order to investigate the effect of these process inherent heterogeneities on shock response, the high-solids loaded composite’s microstructure is first quantitatively characterized via microcomputed tomography imaging and computational analysis in three dimensions. Next, the composite undergoes plate-impact experiments at Argonne National Laboratory’s Advanced Photon Source’s Dynamic Compression Sector, with X-ray PCI used as an in-situ and in-material diagnostic. This is combined with PDV for validation. The phase contrast images are analyzed in order to measure shock and particle velocities directly from the translation of the shock wave and particles over time. Finally, the effects of print geometry, impact direction relative to print orientation, and porosity are studied by combining the aforementioned structural characterization with the shock response of the material determined via X-ray PCI. This reveals that print geometry does result in differing macroscale shock response (quantified with EOS), and that print geometry, impact orientation, and pore morphology all have an effect on microscale shock response (quantified with pore collapse velocity). We expect that these factors, only studied on a relatively small scale in this work, will become more exaggerated as sample size and therefore quantity of heterogeneities grows.
Surface passivation for enhanced stability and performance in perovskite solar cells

(Georgia Institute of Technology, 2023-12-13) Sharma, Sakshi

Lead halide perovskite solar cells (PSC) have emerged as promising next generation photovoltaics. Their unique ABX3 stoichiometry- where ‘A’ is a monovalent cation, ‘B’ is a divalent metal cation and ‘X’ is a halogen- provides tremendous potential for composition and bandgap engineering to obtain desired optoelectronic properties, enabling high power conversion efficiencies exceeding 25%. Despite their growing appeal, commercialization of PSC technology faces challenges due to device instabilities in ambient conditions. Particularly, device interfaces between the active perovskite layer and adjacent charge transport layers are vulnerable to defects which can accelerate perovskite degradation under environmental stressors such as heat, moisture, or oxygen, limiting their long-term viability. Interfaces also significantly impact charge transport, collection and recombination mechanisms in devices and thus require optimization. To address these challenges, research has concentrated on interface modification to passivate surface defects, protect the bulk of perovskite from external environment, and tune the charge transfer properties at the surface. Conjugated organic ammonium salts have been used at interfaces to introduce hydrophobicity on the perovskite film and promote charge delocalization brought on by conjugation. However, most surface treatment strategies relying on organic molecules introduce an electrically insulating spacer layer under thermal stress. Heat induced diffusion of molecules can reconstruct the interface into lower dimensional phases, which impedes charge extraction and affects photo-conversion efficiency (PCE) of devices. This brings a tradeoff between the benefits of passivation and charge extraction. For proper interface design, it is essential to study the thermal behavior of these passivation layers and establish their relationship with the optoelectronic properties of solar cells. This work explores the thermal behavior of passivation agents, specifically employing long-chain thiophene-functionalized π-conjugated molecules (2TI and 4TmI, with two and four thiophene rings, respectively) on interfacial structural stability and charge extraction. Tailoring the steric hindrance of the bulky cations used to treat perovskite surfaces presents an opportunity to control cation mobility, and consequently any phase changes resulting at elevated temperatures. Structural studies reveal that the length of the cation backbone regulates the rate of interfacial perovskite structure reconstruction on prolonged heating. Consequently, faster phase conversion is observed in 2TI compared to larger 4TmI, with the formation of a n=1 A’PbI4 two- dimensional phase which consists of inorganic PbI6 octahedra monolayers separated by an organic spacer layer, A’ being either 2T or 4Tm. The oligothiophene tail in these molecules further contributes to spacer layer conductivity, prompting distinct charge extraction and recombination behaviors in 2TI versus 4TmI passivated devices, confirmed by synchrotron-based X-ray measurements. Results show that despite the observed phase changes, 2TI treated devices can tune the surface potential to promote efficient hole extraction to the overlying hole transport layer and reduce carrier recombination. This interfacial steric engineering translates to high performing passivated solar cells, with 2TI/CsFAPbI3 devices exhibiting efficiency exceeding 20%, an open-circuit voltage of 1.07 V and minimal changes under continuous thermal exposure. By identifying the nature and impact of heat induced dynamical structural changes at passivated perovskite interfaces, this work highlights the key to surface functionalization so that solar cell performances can be maintained at high operating temperatures.
Sintering methodologies for silicon carbide ceramics

(Georgia Institute of Technology, 2023-12-11) Wang, Annie Wei Chyi

Silicon carbide (SiC) ceramics are known for their high hardness, light weight, high strength, high oxidation resistance, high thermal shock resistance, low elevated temperature creep, and chemical inertness. Sintering of powder compacts has been via both eutectic liquid-phase and solid-state processes; both were investigated in this study. Solid-state sintering, following the method of Prochazka, requires both carbon and boron (or B4C) sintering aids. In this work the use of C additives alone was shown to be necessary but insufficient for sintering. The mechanical properties of SiC with varying B4C and C were studied with results of 98.31 to 99.66% relative density, 22.76 to 27.66 GPa for Vickers hardness and 3.0 to 4.18 MPa⋅m1/2 for Vickers indentation fracture toughness. The work showed that the merits of increasing B4C addition stopped at the solid solubility limit of B4C in SiC, demonstrated to be at ~0.26 wt%. To investigate the liquid-phase sintering methodologies for silicon carbide, 10 wt% of AlN and Y2O3 were added with a molar ratio of 3:2. The effect of different powder beds for the specimens to be immersed in, and different sintering atmospheres were studied. Four types of powder beds were investigated: pure SiC, 1:1 (wt%) SiC and AlN, the same composition used to make the samples, and pure AlN. It was found that the pure AlN powder bed yielded the highest relative density and finest grain size. This indicated that without the powder bed, the relatively high vapor pressure of AlN (or its vapor decomposition products) in the compact favored either evaporation/condensation particle coarsening or grain growth over sintering; the overpressure provided by the AlN powder bed surroundings thus improved sintering conditions. Four different atmospheres were then studied with the use of a 1:1 SiC and AlN powder bed. The results showed that different sintering dwell temperatures were required for optimum relative density using these different atmospheres. Flowing He requires the lowest sintering dwell temperature (around 1700°C), followed by Ar, static vacuum, and then N2 requiring the highest temperature (~1950°C). These higher dwell temperatures were required from the more difficult diffusivity of larger molecular/atomic sized trapped gases out of sintered bodies of closed porosity. Significant grain growth was observed for temperatures higher than their optimum temperatures, with associated decreasing sintered relative density. The highest relative density (96.37%) was achieved with an atmosphere created by pulling vacuum at room temperature, and then maintaining a static atmosphere during sintering. For optimally sintered specimens exposed to these atmospheres, lower Vickers hardness (15.03-18.35 GPa) were measured compared to solid-state sintered SiC, but very high Vickers indentation fracture toughness (2.92-7.85 MPa⋅m1/2) were obtained. This is associated with the relatively weak grain boundary phase deflecting/branching propagating cracks. This work then investigated the sintering of SiC with lower additive concentrations: 1-4 wt% of AlN and 0-2 wt% of Y2O3, using a flow-through He atmosphere, with the compacts immersed in a pure AlN powder bed. Relative densities were inferior to the previous study; it increased with increasing Y2O3 content. In the absence of Y2O3, AlN acted as a grain growth inhibitor, and points toward the potential merit of a Prochazka composition with AlN additions. A 2-D computer model of sintering was constructed using MATLAB. Green microstructures were represented in a 2-D view. The filled circles representing particles were generated with random number generator and a fall-and-roll algorithm. The sintering process was simulated with sequential algorithms of the initial, intermediate, and final stages of sintering. Each stage with controlling factors that could be input depicts microstructures that would result under differing conditions. The simulation depicted particle neck formation, particle re-shaping, pore elimination (densification), and grain growth, forming microstructures generally consistent with those observed after sintering.
Characterization of Commercial Dielectric Zaristo-700 as a Redistribution Layer Material for Advanced Packaging

(Georgia Institute of Technology, 2023-12-06) Madelone, Sophia Marie

This body of work, in detail, outlines the fundamental steps taken to characterize a material for novel use in RDL build-up layers for advanced packaging. The material (Zaristo-700) discussed in this thesis was only used in RF applications, and now we are exploring its use in the wiring layers. In the PRC, research into thin films, spin-on films, and many other dielectrics have been published before. It is essential to understand that this work is necessary to establish a “library” or catalog of information on all the materials we use to provide the correct material, depending on the goals of future projects. The material and electrical properties of Zaristo-700 are characterized through JEDEC adhesion testing (Peel test), leakage current measurements on ITO glass slides before and after Highly Accelerated Stress Testing (HAST) treatment, a series of dose tests to document the most optimized pitch-scalability at 8.0 m L/S, and lastly Shadow-Moire warpage studies of one layer and three-layer RDL samples. Leakage current measurements taken before and after HAST stayed at or below 2.0 nA. As we will explore, the CTE and adhesion of Zaristo-700 are excellent and contribute to making a great material for the RDL wiring layers. Taiyo Ink. has stated that this version of the dielectric film accounts for issues such as stability in how long it can sit, delamination during or after curing, delamination during fabrication processes, and so on. Whereas some of the dielectric films of other companies still have these problems. This research is working towards answering the unknowns about this dielectric and how well it will function as a future RDL build-up material through characterization and analysis of its properties. These results are a positive indication for use as an RDL dielectric in advanced packaging.
Atomic Layer Deposition As A Method Of Fine Tuning The Surface Chemistry Of Oxide Materials

(Georgia Institute of Technology, 2023-12-05) Yom, Typher

Atomic layer deposition (ALD) is a vapor-phase synthesis method in which a material is deposited onto a surface with precise atomic thickness. Through ALD, ultra-thin monolayers of oxide materials can be deposited onto powders, creating mixed oxide surfaces with tunable surface chemistries, enabling their usefulness towards catalytic processes in the petrochemical and fine chemical industries. ALD holds an advantage over typical solution-phase methods of creating mixed oxide materials due to the latter’s difficulty in controlling the surface composition, making analysis difficult. However, if we can better understand the interactions of the surface in solution, it can be used to design more effective catalysts. One way to observe this is by studying the zeta potential of the surface, which is directly correlated with surface charge and is a product of these acid-base interactions at the interface. Each material can be identified using the isoelectric point, which is the point at which the zeta potential/net surface charge is zero. For mixed metal oxides, their isoelectric points were calculated in the literature to be the summation of each individual component’s isoelectric point multiplied by its surface coverage. However, this calculation assumes that the components do not interact with each other when mixed. In order to investigate this discrepancy, we used ALD to deposit thin layers of titanium oxide onto silicon oxide powders. If we were to assume the equation used in the literature, we can assume that one single monolayer over the surface would be sufficient to convert the isoelectric point from that of silicon oxide to that of titanium oxide. However, our results have indicated that the isoelectric point did not reach that of titanium oxide until multiple monolayers were deposited, indicating that a different model/equation must be utilized to better elucidate the surface behavior. Additionally, during these studies of the isoelectric point, we have formulated an equation that can correlate the thickness of ALD-deposited films with the material’s relative atomic percent. This equation was created by assuming that the shape of the particle + film retains its shape, and therefore its volume formula, allowing it to work for ultra-thin films, but not for much thicker films. Finally, this thesis highlights the importance of being mindful of the precursor used for powder ALD: precursors like TiCl4 can create byproducts like HCl from the reactor walls and the powder itself. These byproducts can then adsorb onto the powder surface, which can block film growth or affect the pH of the resulting solution when the powder is dispersed in water. Extra measures, such as a double dose or a post-process washing step, were implemented, and should be used when performing powder ALD.
Design of Organic-inorganic Hybrid Membranes Using Density Functional Theory and Machine Learning

(Georgia Institute of Technology, 2023-08-25) Liu, Yifan

Novel organic-inorganic hybrid membranes processed through vapor phase infiltration (VPI) incorporate the advantages of both organic and inorganic materials. Compared to conventional organic membranes, these hybrid materials offer significant improvements in stability when exposed to organic solvents while retaining desirable membrane properties such as high permeability and selectivity. However, the extensive design space involved in developing such membranes, which encompasses polymer chemistry, inorganic chemistry, and hybrid microstructures, poses challenges to traditional trial and error methods. To surmount these obstacles, this work develops a more efficient and systematic approach. It involves three steps that leverage density functional theory (DFT) and machine learning (ML) to develop the knowledge and tools necessary to predict and explore novel VPI organic-inorganic membranes: 1. This research entails an in-depth investigation into the interactions between three metal precursors and the prototype polymer of intrinsic microporosity 1 (PIM-1) during the VPI process. Our primary objective was to identify crucial characteristics of polymer-inorganic interactions, decipher structure-property relationships, and unveil significant properties that could contribute to ML model predictions for future materials selection. Our work uncovered two atomic-level mechanisms for solvent stability. 2. An ML-based tool predicting sublimation enthalpy was developed to aid chemistry selection and experimental design for precursors. Initial training used a comprehensive DFT dataset of organic molecules constructed in this work due to a lack of metal precursor parameters in the literature. As new data emerged, an active learning algorithm incorporated new chemical species into the model, dynamically improving its accuracy and expanding its applicability. 3. An ML model, incorporating multi-task learning and meta-learning, was trained on a new DFT dataset to predict binding energy between metal precursors and polymers. This enhanced the understanding of polymer-inorganic interactions’ strength and stability, aiding in the selection of potential precursors. The model provides a promising route for informed precursor selection, VPI process optimization, and the design of hybrid materials with custom properties. This foundational work provides automated and effective tools for the design and development of VPI organic-inorganic hybrid membranes, leveraging the combined capabilities of DFT and ML. The predictive models developed here can be employed alongside the insights derived from our atomic-level mechanistic studies in the selection of suitable polymers and metal precursors for designing energy-efficient organic-inorganic hybrid membranes for chemical separation. In addition, the DFT database and ML models developed in this project serve as valuable instruments to be utilized by researchers for future studies on the sublimation enthalpy and binding energy of organic-inorganic systems, facilitating further advancements in the field of material science. This thesis presents and executes a methodical framework through which future models can be developed for the exploration of novel material spaces.
In situ functionalization of anion-conducting solid polymer electrolyte membranes

(Georgia Institute of Technology, 2023-08-16) Shah, Parin Nitin

Hydrogen is a viable option for storage and on-spot generation of energy. Alkaline electrolyzers and fuel cells have several advantages over acidic counterparts such as simple fabrication, non-precious metal catalysts and low crossover. It has been shown that crosslinked anion–exchange membranes synthesized by vinyl addition polymerization of norbornene show excellent performance in alkaline electrochemical devices. However, a long reaction time is needed for converting the tethered bromoalkyl moiety in the polymer to a quaternary ammonium head-group because a tertiary amine has to diffuse into the polymer. This amination process is not compatible with the roll-to-roll membrane formation process. In this study, anion exchange membranes have been prepared by in situ amination of the functionalized polymer during membrane casting. The scope of three different tertiary amine: Trimethyl amine, Triethyl amine and N-methyl piperidine was investigated for the in situ membrane casting process. The polymers used in this study were also in situ crosslinked with N,N,N’ ,N’ - tetramethyl-1,6-hexanediamine during membrane casting to prevent excessive water uptake. By changing the reaction solvent, temperature, and concentration, it was possible to balance the reaction kinetics while still maintaining polymer solubility to cast membranes. The conversion was monitored as a function of reaction time (using NMR) and the reaction conditions were optimized to develop a novel process of producing pre-functionalized membranes that is compatible with the current roll-to-roll infrastructure. Membranes having high ion exchange capacity (upto 3.4 meq g-1 ) and high ionic conductivity (upto 56 mS cm-1 at room temperature) were prepared using this process. Precisely controlling the reaction time made it possible to directly cast quaternized membranes on a roll-to-roll timescale, thus avoiding the need for the long-duration, ex situ amination step. Alkaline electrolyzer performance with these in situ aminated membranes showed comparable performance to membranes prepared by the conventional, ex situ amination method.
Structure-Property Relationships in Lead Halide Perovskites for Solar Cells

(Georgia Institute of Technology, 2023-07-14) Hidalgo, Juanita

Lead halide perovskites (LHPs) solar cells, particularly FA-based, have made impressive advancements in solar energy conversion, achieving high power conversion efficiencies exceeding 26% for a single junction device. However, the limited long-term stability of these devices has hindered their commercialization. The instability issues are influenced by both internal and external factors, leading to rapid degradation of the perovskite phase and overall device performance. Various strategies have been used to stabilize the perovskite phase, but it is crucial to investigate the underlying mechanisms that govern the structural characteristics of the polycrystalline thin films. This dissertation tackles the challenges associated with the instability of LHPs by investigating the complex relationship between structure, properties, and performance in perovskite solar cells. Advanced X-ray characterization techniques are employed to examine the structural properties of FA-based compositions. Understanding the mechanisms and establishing correlations between structure and properties is possible to lay the foundation of a more robust, stable, and efficient material. This dissertation presents a first step toward the design and optimization of LHPs. The first part of this dissertation explores the crystallographic orientation in lead bromide perovskites, demonstrating that the solvent and organic cation used in the precursor solution significantly affect the preferred orientation of the deposited perovskite thin films. The solvent affects the early stages of crystallization and induces preferred orientation by preventing colloidal particle interactions. The choice of organic cation influences the degree of crystallographic orientation, with methylammonium-based perovskites showing a higher degree of orientation than formamidinium-based ones due to a lower surface energy of a specific perovskite facet. These findings identify the importance of understanding (1) the precursor solution chemistry, (2) the facet properties and their correlation with the structural properties of the polycrystalline LHP film, and (3) the effect of crystallographic orientation on charge carrier transport in perovskite solar cells. The second part of this dissertation studies the mechanisms causing FA-based lead iodide perovskites to degrade under water and oxygen exposure. Contrary to common knowledge on humidity-induced degradation, this dissertation reveals the synergistic role of water and oxygen in accelerating phase instability of LHPs. The study uncovers a surface reaction pathway involving the dissolution of formamidinium iodide (FAI) by water followed by the oxidation of iodide, playing a crucial role in causing the subsequent and irreversible undesired phase transformations from perovskite into non-perovskite phases. The interplay of in-situ experimental techniques with theoretical calculations provides a detailed understanding of the degradation mechanisms, establishing a foundation to design more durable and efficient materials. Finally, this dissertation delves into strategies for stabilizing the perovskite phase. A hydrophobic molecule, phenethylammonium iodide (PEAI), stabilizes FA-based perovskites. Adding PEAI hinders undesired phase transformations and leads to a more stable material with improved solar cell power conversion efficiency and enhanced charge carrier mobilities and lifetimes. Further, adding Br to mixed cation lead iodide perovskites improves their phase stability at low temperatures. Overall, understanding structure-property-performance relationships in lead halide perovskites is key for resolving the main challenge of instability in perovskite solar cells. This dissertation lays the groundwork for future research efforts to investigate the fundamentals of LHPs, improve their stability, and broaden their applications in solar cells and beyond.