(Georgia Institute of Technology, 2021-04-22)
Li, Yi
Cellulosic materials are widely used in our daily lives for paper products and functional polymers. The cellulose molecule has a high density of hydroxyl groups, which causes strong intra-/inter- fiber hydrogen bonding. These abundant hydroxyl groups make cellulose super-hydrophilic and difficult to disperse or dissolve in nonpolar organic solvents or polymers. The traditional methods to functionalize cellulose is either surface modification or regeneration. Vapor phase modification of cellulose has gained interest in recent years. Instead of using liquid phase precursor solutions, vapor phase processing uses gas molecules as precursors to realize surface coatings with better uniformity and consistency amongst batches. Atomic layer deposition (ALD) technique can be conducted at relative lower reaction temperatures (25 – 300 ℃) and realize a conformal coating on substrates with high aspect ratios.
Reported literatures on ALD modified cellulose are more focus on functional coatings, replicas, physicochemical property of new generated materials, while less focus has been given to studying the reaction mechanisms and underlying physics of the physicochemical property changes. This work focuses on the study of the initial cycle’s reaction mechanism and process-structure-property relation for TMA and water reacting with cellulose substrates. Different cellulose products (chromatography paper, cotton ball and cellulose free-standing film made from cellulose nanofibrils) were investigated for their corresponding properties after ALD reaction. Specifically, this work contains three sections, which are (1) Investigating the heat stimulated surface wettability transition after “low” cycle ALD reaction on cellulosic materials, then apply different wetting models to explain this wettability transition. (2) Investigating the reaction mechanism and resulting physicochemical property difference for low cycle of TMA and water ALD reacted cellulose nanofilms. (3) Developing new ALD processing recipes to modify cellulose nanofilms by exploring the effect of the TMA exposure time to alter the cellulose’s chemistry and physical microstructure
This study provides a novel insight of surface property and mechanical property control for cellulosic materials through ALD process parameters development and will offer guideline information for future process development and substrate selection to achieve designed material property.
(Georgia Institute of Technology, 2007-11-02)
Li, Yi
Electrically conductive adhesives (ECAs) are one of the lead-free interconnect materials with the advantages of environmental friendliness, mild processing conditions, fewer processing steps, low stress on the substrates, and fine pitch interconnect capability. However, some challenging issues still exist for the currently available ECAs, including lower electrical conductivity, conductivity fatigue in reliability tests, limited current-carrying capability, poor impact strength, etc. The interfacial properties is one of the major considerations when resolving these challenges and developing high performance conductive adhesives.
Surface functionalization and interface modification are the major approaches used in this thesis. Fundamental understanding and analysis of the interaction between various types of interface modifiers and ECA materials and substrates are the key for the development of high performance ECA for lead-free interconnects. The results of this thesis provide the guideline for the enhancement of interfacial properties of metal-metal and metal-polymer interactions. Systematic investigation of various types of ECAs contributes to a better understanding of materials requirements for different applications, such as surface mount technology (SMT), flip chip applications, flat panel display modules with high resolution, etc. Improvement of the electrical, thermal and reliability of different ECAs make them a potentially ideal candidate for high power and fine pitch microelectronics packaging option.