Interfacial Tuning of Multifunctional Composites and Bio-Inspired Materials with Atomic Layer Deposition

Abstract

Surface and interfacial modification processes have the potential to address several of the performance limitations in the fields of structural materials, energy conversion, and energy storage, among others. Hence, there has been a need to integrate reliable surface and interfacial modification techniques into materials processing. In this dissertation, atomic layer deposition (ALD) was utilized to impart multifunctionality onto composites and biotemplate surfaces and demonstrate its potential to enable the development of transformative materials.

Accurate temperature monitoring and control of temperature distribution is necessary in order to obtain ideal and uniform ALD growth. The work in Chapter 3 assessed how inaccuracies in temperature monitoring and temperature control of the reactor can lead to nonuniform deposition. The implementation of an improved heater and thermal monitoring design drastically improved uniform ALD thickness across the reaction chamber. This study highlighted the need of properly designed ALD systems that can control and monitor the temperature of the reactor with accuracy and precision. This is especially important as new ALD precursors emerge that have narrower temperature windows for ideal deposition conditions.

To explore the capabilities of ALD to impart multifunctional properties into structural composites, we first studied the effects these coatings have on their interfacial properties, which directly affect the bulk strength and toughness of the composite. In Chapter 4, we utilized ALD as a method to conformally coat structural fibers with various materials and studied the impact of these interlayers on their mechanical adhesion to polymer matrix materials. It was observed that the mechanical force required for debonding of the polymer droplet from the coated fiber surfaces depended on the composition and thickness of the coating. The ability to tune the interfacial properties presents an opportunity to develop engineered structural composite materials designed with specific macroscopic mechanical properties. Additionally, ALD of ZnO was demonstrated to prevent fiber degradation from ultraviolet radiation and high-temperature thermal treatments, demonstrating a pathway towards multifunctional composite interphase engineering by ALD.

Expanding upon this work, ALD coatings were utilized to impart multifunctionality. Imparting electrical conductivity into polymer-matrix composites (PMCs) is an important step in enabling multifunctionality, while maintaining mechanical stiffness and strength. In the work presented in Chapter 5, electrically conductive PMCs were fabricated by conformally coating Kevlar® 49 woven fabrics with aluminum-doped zinc oxide using ALD. This work demonstrates a new pathway for scalable and tunable incorporation of electrical conductivity into fiber-reinforced composites without significantly changing their density or load-bearing capabilities.

Lastly, the versatility of ALD was demonstrated in Chapter 6 by imparting multifunctional properties onto Morpho sulkowskyi butterfly wings. The wings contain high aspect ratio nanostructures, which make them ideal templates for applications in solar energy and photocatalysis. This study demonstrated the ability to precisely tune the natural structural coloration while also integrating multifunctionality by imparting photocatalytic activity onto fully intact Morpho wings. These structurally colored photocatalysts exhibited an optimal coating thickness to maximize photocatalytic activity, which was attributed to trade-offs between light absorption and catalytic quantum yield with increasing coating thickness. These multifunctional photocatalysts present a new approach to integrating solar energy harvesting into visually attractive surfaces that can be integrated into building facades or other macroscopic structures to impart aesthetic appeal.

In summary, ALD has been demonstrated as a method to impart multifunctional properties onto a variety of materials that require precise control of thickness and uniformity.