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School of Materials Science and Engineering

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https://hdl.handle.net/1853/70762

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College of Engineering

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Publication Search Results

Now showing 1 - 10 of 161

Micellar and liquid crystalline phases of surfactant/Pluronic mixtures studied by SANS

(Georgia Institute of Technology, 2019-10-04) Zhou, Boyang

The phase behaviours of Pluronic L62 in aqueous solutions in the presence of Aerosol-OT(AOT) molecules was investigated by small angle neutron scattering (SANS). The presence of AOT was found to significantly change the micellization phenomenon of L62 micelles in aqueous solutions, including their critical micelle temperature (CMT), global size, and asphericity. The origin of these observations is attributed to the complexation between the neutral L62 surfactants and the ionic AOT molecules: The ionic groups of AOT renders the molecular charge to the aggregates of L62/AOT. On the context on molecular charge, we address the phase properties of L62/AOT complexation such as the critical micelle temperature, global size, asphericity revealed by SANS at different controlled thermodynamic conditions.
Moving towards stable metal halide perovskite solar cells for use in low-earth orbit

(Georgia Institute of Technology, 2019-08-21) Rager, Matthew Scott

Perovskite solar cells have recently emerged as a new leader in the third-generation of photovoltaics. Additionally, this new technology has the potential for application in several areas, including aerospace. The light-absorbing material in perovskite solar cells is an organometal halide compound with the perovskite structure (ABX3) where various atoms can be combined and interchanged to tune the optoelectronic properties. Typically, the A site is filled by organic, small- molecule cations (e.g. methylammonium and formamdinium) and/or inorganic atoms (e.g. Cs or Rb), the B site is filled by metal atoms (e.g. Pb2+ or Sn2+), and halide anions (e.g. I- and Br+) fill the X site. In this study, I fabricated organic-inorganic (MAPbI3 and Cs0.05(MA0.17FA0.83)Pb(I0.83Br0.17)3 and all-inorganic (CsPbBr3) perovskite solar cells to improve the efficiency and stability with the goal of creating devices to operate in the low-Earth orbit environment. The harsh environment of space requires materials with good thermal stability due to large variations in temperature. The organic-inorganic solar cells are more efficient than all-inorganic, but the organic cation places limitations on the thermal stability of the material. Thus, all-inorganic perovskite solar cells (e.g. CsPbBr3) were fabricated and studied as the best candidates to survive the extreme conditions in low-Earth orbit.
Epoxy/triazine based high performance molding compound for next generation power electronics packaging

(Georgia Institute of Technology, 2019-07-26) Li, Jiaxiong

The power electronics industry has been actively seeking encapsulant materials that can serve in harsher environments. For example, with the power semiconductors leading into SiC era, the higher operation temperature (250 ºC) have proposed great challenges on the packaging materials especially on epoxy molding compound (EMC) technologies, since the temperature exceeds the stability limit of typical epoxy (EP) chemistry. In this thesis, EP/triazine system was selected to develop high temperature stable resin system that can meet the temperature requirement of next generation power electronics packaging. In the first part of the thesis, different approaches were discussed to enhance the high temperature performance of a previously studied cyanate ester (CE)/ biphenyl EP blend which is impaired by the hydrolysis degradation of remaining cyanate groups. Firstly, the effects of different metal catalyst on the CE properties were discussed. Secondly, a triazine containing molecule triglycidyl isocyanurate (TGIC) was employed to increase the triazine content without increasing CE feed ratio to circumstance problem of unreacted cyanate groups. Finally, the high heat resistant novolac type CE was employed to form the NCE/EP blend, and their blends with different feed ratio were systematically evaluated. In the second part, a detailed characterization of a high heat resistant CE/novolac type EP blends and the investigation on their degradation under long-term high temperature storage were summarized. The effects of the CE concentration on the thermomechanical properties of the copolymer were explored, where a tradeoff behavior between the triazine content and crosslink density was accounted for the property change. In addition, the distinguished thermal degradation mechanisms in copolymer with different compositions were identified and illustrated. The knowledge obtained in this work can serve as a reference in the future to formulate EP/triazine based resin system for high temperature applications.
Rheology and characterization of high-solids suspensions for direct ink writing of energetic materials

(Georgia Institute of Technology, 2019-07-23) Woods, Hannah Kathryn

Direct ink writing is a promising approach for preparing energetic materials with unique geometries that are of great interest in military and civil engineering fields due to their potential to control shock wave propagation and energy focus or dissipation. However, there are significant challenges to overcome in using additive manufacturing to produce energetics, particularly in using inks with high particle content (>60 vol% particles) while maintaining both extrusion capability and print quality. Voids and interfaces in energetics are areas of high risk for hot spot formation, and with the layer-by-layer additive manufacturing process, voids can manifest both between and within the extruded filaments as well as between printed layers. Concerns associated with the challenges of printing high-solids suspensions make understanding the flow and print capabilities of these materials of great importance. The binder used in suspensions for direct ink writing plays an important role in overall flow characteristics of the ink, and therefore has significant impact on final print quality. In this work, glass microspheres in polymer-solvent and photocurable monomer binders are examined as model systems to provide an in-depth study of polymer binder design. This work aims to understand how binder characteristics affect the viscosity and printability of such high-solids suspensions. We show that the suspension viscosity is primarily controlled by the particle volume fraction for the photocurable binder system, while both the particle volume fraction and polymer molecular weight influence the viscosity in the case of the polymer-solvent binder system. Both binder types can be tuned to make printable suspensions that result in lines of consistent width and 3D disc-shaped objects, indicating that both paths show promise for future direct ink writing formulations of energetic materials.
Integration of optical structures with chiral nanocellulose films

(Georgia Institute of Technology, 2019-07-23) Yu, Shengtao

Generation of circularly polarized (CP) light via cellulose nanocrystals (CNC) has demonstrated great potential for next generation of chiroptical materials, thanks to the abundance, cost-effective preparation, as well as the retained chiral liquid crystal ordering in solid form. To enhance the chiroptical properties of CNC film, a convenient strategy capable of integrating both diffractive and refractive optical structures is developed based on top-down lithographic techniques and bottom-up evaporation-induced self-assembly. The feasibility of such strategy is proved by a few optical structures including photonic gratings, photonic crystals and micro-lenses. The successful integration of these extrinsic structures with intrinsic chiral structure of CNC is evidenced by preservation of helical stacking structure of CNC as well as a good registration of features size down to sub-micron level with uniformity across large area on the film surface. As a result, a combination of more accurate and sharper structural color, light focusing capability as well as enhanced circular dichroism is observed, indicating great potential in advanced optical systems based-on CP light. This is the first study to manipulate and enhance the chiroptical properties of CNC with artificial photonic structures, while the unconventional hybrid strategy demonstrates a fledgling, yet promising method for development of hierarchical biophotonic materials in general.
Stability of double perovskite cathodes under high humidity for solid oxide fuel cells

(Georgia Institute of Technology, 2019-05-06) Liu, Yuchen

Solid Oxide Fuel Cells (SOFCs) can directly convert a wide variety of fuels to electricity efficiently. They can also be run in reverse as Solid Oxide Electrolysis Cells (SOECs) to produce hydrogen (and carbon-containing fuels) from electrolysis of water (and carbon dioxide). However, the kinetics of oxygen reduction reaction (ORR) on the cathode is often hindered by various contaminants, which may react with the cathode to form insulating phases and degrade fuel cell performance. The stability and performance of the cathode in moisture is critical to the cell performance as SOFCs and SOECs. Several state-of-the-art cathode materials are investigated in a high moisture environment to uncover their performance and degradation mechanism. First, powders of electrode materials were analyzed for any degradation before and after long-term moisture exposure using XRD to probe the bulk and Raman Spectroscopy to probe the surface. SEM was also used to characterize any morphological changes during the exposure. Second, electrochemical impedance spectroscopy (EIS) was used to monitor the long-term performance of symmetric cells under various conditions. Finally, current-voltage relationships of symmetric cells were acquired under typical operating conditions for SOFCs and SOECs to determine the polarization resistance, stability and durability of the cathode materials.
Encapsulation and design of scalable packaging materials for thin film perovskite solar cell applications

(Georgia Institute of Technology, 2019-04-25) Hah, Jinho

There have been many attempts to improve the stability of the environmentally-sensitive perovskite solar cells (PSCs) from adverse environments. The next generation encapsulation method should be compatible with roll-to-roll (R2R) processing, which can manufacture thin-film PSC modules at large scale and make solar electricity economically competitive with conventional electricity generation. This work investigates the interface chemistry between the polymer backsheet and the polymer encapsulants to understand the moisture, thermal, and UV stability of the packaging materials for PSCs. First, surface modification on the commercially available PET backsheets was done using various types of silane-based coupling agents, and their adhesion profiles were studied upon damp-heat exposure on these samples. Second, thorough XPS analysis was conducted on the delaminated PET surface from the PET/EVA/PET encapsulation architecture upon the UV, thermal, and moisture aging to understand the degradation mechanism at the interface. Moreover, this work also includes encapsulant design by combining the polymer blends to improve the mechanical and chemical bulk properties of a PV encapsulant. In short, this work serves to investigate on the encapsulation methods to improve the reliability and lifetime of PSCs.
Immobilization of adhesive protein domains in PEG hydrogels

(Georgia Institute of Technology, 2018-08-13) Hyland, Kelly Elise

The fundamental goal of biomaterials design for regenerative medicine is to promote the restoration of functional tissue. In wound healing research, one strategy is to introduce space-filling materials, or scaffolds, to intervene and prevent scarring. The scaffold must be nontoxic, permit high rates of oxygen and small molecule diffusion, and offer tissue-matching stiffness. Critically, they must also promote attachment of wound healing cells. A class of materials called synthetic hydrogels meet the first three criteria, but must be functionalized with bioactive ligands to promote cell attachment. Synthetic hydrogels, most numerously poly(ethylene glycol) (PEG) hydrogels, offer a modular platform for biomaterials design because the bioactive ligand identity and density, as well as hydrogel stiffness, can be precisely and independently controlled. However, PEG hydrogels have seldom been used as a 3D platform for investigating differences in cell behavior when in contact with different extracellular matrix protein domains. Using recombinant protein design, expression, and characterization, this study compares cell behavior when cultured on PEG hydrogels presenting structured protein domains and minimum sequence peptides. We observe differences in cell morphology, protease production and attachment force when cultured on hydrogels with different adhesive protein domains.
Study on epoxy based composites for high temperature molding compounds

(Georgia Institute of Technology, 2018-07-19) Wu, Fan

Epoxy molding compound (EMC) is one of the most widely used encapsulation materials for electronic packaging. To provide substantial protection for the electronic packages, EMCs are frequently required to work at elevated temperature, especially when high power and high density devices are developing rapidly and more heat is generated during operation. This thesis discussed the study on epoxy based composites for high temperature molding compounds by investigating two components most important in EMC system, namely the polymer resin and the filler system. In order to increase the glass transition temperature (Tg) and thermal stability of epoxy resin, cyanate ester was incorporated into the polymer matrix. The copolymer network formed by epoxy and cyanate ester (CE/EP) exhibited excellent thermal stability and high Tg above 270 ℃ because of the thermally stable s-triazine structures formed by cyanate ester trimerization. However, cyanate ester was affected by the hydrolysis reaction and too much cyanate ester in the system led to blistering and Tg drops in high temperature and high humidity tests. The cyanate ester amount in this copolymer was optimized to be 33-50 %. Polyimide was incorporated into CE/EP system as an additive (CE/EP-PI) to further improve the thermal stability of this epoxy-based resin. Aromatic polyimide exhibited good compatibility with CE/EP for their structural similarity. Improvements in Tg, storage modulus, fracture toughness and long term high temperature performance were observed at 5-10 % polyimide loading. At high polyimide loading level (> 10 %), a secondary phase emerged which deteriorated the resin properties such as storage modulus. The second part of this thesis investigated a modified filler system with surface coated silicon carbide (SiC) for thermal conductivity enhancement. In this part, SiC with high thermal conductivity was adopted as a replacement for conventional silica fillers. After surface treatment by silane coupling agent (SiC-GPTMS) and reactive silicon rubber (SiC-A15), modified SiC increased the thermal conductivity of the composites from 0.11 W/mK to 0.28 W/mK at 30 % filler loading.
The synthesis of atomically thin PT monolayer to multilayer PT films on graphene and MOS2

(Georgia Institute of Technology, 2018-04-26) Arnold, Chris

In the area of catalysis, substantial research is aimed at exploring the influence of the size and morphology of precious metal nanoparticles as well as catalyst supports on reaction kinetics. Recently, this has evolved into depositing highly wetted monolayers/multilayers (ML) of precious metal onto various catalyst supports. Similar to nanoparticles, ML growth significantly reduces precious metal loading (therefore improving cost) but additionally enhances catalytic activity with a better surface area to mass ratio and encouraging the substrate to tune the catalyst’s electronic properties via ligand effects and strained lattice geometry. This work explores the deposition of Pt-ML on graphene in full 3D and 2D MoS2 as well as the influence of the Ni substrate ligand effect in electrocatalysis. Commercially available, Ni foam was utilized as a substrate for the 3D graphene growth before Pt was deposited layer-by-layer in a surface-limited electrochemical scheme to achieve highly wetted and atomically thin growth. A similar surface-limited technique was utilized for the deposition of Pt on both vertically and horizontally aligned MoS2. High resolution TEM, XPS, SEM/EDX, and electroanalytical techniques were used to characterize these samples and reveal the evolution of the atomic/electronic structure as a function of Pt-ML thickness. It is demonstrated that the synthesis scheme outlined here allows for the design of dimensionally-controlled and ligand-flexible catalyst architectures that can have a wide range of chemical applications.