Dehydrofluorination Induced High Piezoelectric Poly(Vinylidene Fluoride) and Applications

The piezoelectric effect in poly (vinylidene fluoride) (PVDF) and its copolymers allows the exchange of mechanical and electrical energy. Combined with their unique properties as soft materials, piezoelectric polymers have attracted great interest in actuating, sensing and energy harvesting applications. The electromechanical coupling capacity of the piezoelectric polymers is governed by their crystalline phase composition. The β-phase of PVDF, which has an all-trans chain conformation, is desired because it has the highest aligned dipole density and piezoelectricity. However, the production and application of β-phase PVDF has been restricted by complex processing methods (i.e. in situ mechanical stretching) and limited thermal stability. Developing a method to prepare a thermally stable high β-phase PVDF homopolymer that does not require mechanical stretching is of great importance for high-performance PVDF devices and has been a long-lasting challenge in the PVDF industry. This dissertation details investigations into the structure-property relationships in piezoelectric fluoropolymers and the development of a novel and versatile method to synthesize high β-phase PVDF with improved piezoelectricity and thermal stability. Initially, the effects of molecular structural defects on the crystallization behavior of PVDF are studied through molecular simulations. It is demonstrated that defects consisting of carbon-carbon double bonds on the PVDF backbone can effectively reduce the relative conformational energy of β-phase thus leading to preferential β-phase crystallization. Secondly, a novel dehydrofluorination method is developed and optimized in this work to induce the discussed molecular defects into PVDF in order to promote β-phase formation without mechanical drawing. It is demonstrated that after chemical modification by the developed method, PVDF with β-phase fraction of over 80% can be directly produced through solution casting. Further research is then performed to thoroughly investigate both the direct and inverse piezoelectric effects of this dehydrofluorinated PVDF. Record-breaking piezoelectric strain coefficients are observed from dehydrofluorinated PVDF which achieves as much as a 107% increase in the piezoelectric strain coefficient d33 and a 40% increase in d31, compared with conventional drawn PVDF. Furthermore, the dehydrofluorination method removes the restrictions encountered during high-temperature processing of conventional drawn PVDF. The improved thermal stability of dehydrofluorinated PVDF allows it to recrystallize in the β-phase from any temperature below 210 ˚C which allows a variety of polymer processing methods to be used for PVDF device fabrication. Lastly, this work evaluates the performance of dehydrofluorinated PVDF in piezoelectric applications. Two novel additive manufacturing methods are developed to fully utilize the unique properties of dehydrofluorinated PVDF: a direct-writing method and an electrospin-assisted 3D printing method. Based on these methods, energy harvesters and actuators are fabricated using dehydrofluorinated PVDF and their performance is evaluated. The power density of a dehydrofluorinated PVDF-based stretching mode energy harvester is shown to reach 34.80 mW/cc, which exceeds previously reported PVDF-based energy harvesters and is almost five times higher than the power density of similar devices based on conventional drawn PVDF. By integrating dehydrofluorinated PVDF using novel additive manufacturing methods, the advances described in this dissertation provide new approaches to the development of future piezoelectric devices. This dissertation will serve to disseminate a novel and versatile method of preparing high β-phase PVDF with excellent piezoelectricity and improved thermal stability for the future development of high-performance piezoelectric devices.PHDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149843/1/jjlin_1.pd

Crosstalk Impacts on Homogeneous Weakly-Coupled Multicore Fiber Based IM/DD System

We numerically discussed crosstalk impacts on homogeneous weakly-coupled multicore fiber based intensity modulation/direct-detection (IM/DD) systems taking into account mean crosstalk power fluctuation, walk-off between cores, laser frequency offset, and laser linewidth.Comment: 3 pages, 11 figures

An interpretability framework for Similar case matching

Similar Case Matching (SCM) plays a pivotal role in the legal system by facilitating the efficient identification of similar cases for legal professionals. While previous research has primarily concentrated on enhancing the performance of SCM models, the aspect of interpretability has been neglected. To bridge the gap, this study proposes an integrated pipeline framework for interpretable SCM. The framework comprises four modules: judicial feature sentence identification, case matching, feature sentence alignment, and conflict resolution. In contrast to current SCM methods, our framework first extracts feature sentences within a legal case that contain essential information. Then it conducts case matching based on these extracted features. Subsequently, our framework aligns the corresponding sentences in two legal cases to provide evidence of similarity. In instances where the results of case matching and feature sentence alignment exhibit conflicts, the conflict resolution module resolves these inconsistencies. The experimental results show the effectiveness of our proposed framework, establishing a new benchmark for interpretable SCM

Boosting Generalization with Adaptive Style Techniques for Fingerprint Liveness Detection

We introduce a high-performance fingerprint liveness feature extraction technique that secured first place in LivDet 2023 Fingerprint Representation Challenge. Additionally, we developed a practical fingerprint recognition system with 94.68% accuracy, earning second place in LivDet 2023 Liveness Detection in Action. By investigating various methods, particularly style transfer, we demonstrate improvements in accuracy and generalization when faced with limited training data. As a result, our approach achieved state-of-the-art performance in LivDet 2023 Challenges.Comment: 1st Place in LivDet2023 Fingerprint Representation Challeng