MATERIALS DESIGN OF DIELECTRIC POLYMERS FOR ENERGY STORAGE, ELECTROCALORIC COOLING, AND ELECTRO-ACTUATORS
Restricted (Penn State Only)
- Author:
- Chen, Xin
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 17, 2021
- Committee Members:
- Qiming Zhang, Co-Chair of Committee
Michael Lanagan, Outside Unit & Field Member
Qing Wang, Co-Chair & Dissertation Advisor
Weihua Guan, Major Field Member
Michael Hickner, Major Field Member
John Mauro, Program Head/Chair
Qiming Zhang, Co-Chair & Dissertation Advisor - Keywords:
- Dielectric polymer
PVDF-based polymers
Electrocaloric cooling
electromechanical coupling
capacitors
energy storage - Abstract:
- The objective of this research aims at developing dielectric polymers for improved performance in applications of energy storage, electrocaloric cooling, and electro-actuators. In dielectrics for electric energy storage, dielectric constant, dielectric loss, electrical breakdown strength, charge-discharge efficiency (loss at high electric fields), and operation temperature are the key parameters. Compared with inorganic counterpart, dielectric polymers possess low dielectric loss, low cost, and high breakdown strength. Biaxially oriented polypropylene (BOPP), the state of art dielectric polymer, possesses high breakdown strength (Eb > 600 MV/m) and low dielectric loss (<0.02%).. However, the low dielectric constant (K = 2.2) limits the energy density of BOPP capacitors to < 2 J/cm3, since the energy density of capacitors Ue = ½ K0E2, where 0 is the vacuum permittivity. The low working temperature (< 80 oC) of BOPP capacitors also limits their applications and often requires additional cooling loops to maintain safe operation. Hence, recent efforts on new high-performance dielectric polymers focus on high glass transition temperature polymers (Tg > 200 oC), for example, how to improve the performance of polyimide (PI) and polyetherimide (PEI). Polymer nanocomposites have been investigated for decades in raising K and Ue. However, the traditional approach of adding high dielectric constant (K >1000) inorganic nanomaterials, which usually needs the fillers to be > 15 vol%, has achieved limited success. The large dielectric contrast between the nanofillers and polymer matrix results in intensification of local electric fields in the polymer matrix, leading to a large reduction of the dielectric breakdown strength in polymer composites with high-volume loading of nanofillers. In recent years, Zhang’s group discovered and developed a class of dilute nanocomposites. For example, it has been shown that in polyetherimide (PEI) (K~3.2), very low volume loading (< 0.5 vol%) of nanofillers can lead to more than 50% increase in the dielectric constant K while retaining the high breakdown strength and low dielectric loss. The enhancement of dielectric constant does not depend on the dielectric constant of the fillers, but depends on the geometry size of the fillers, which suggests a strong interfacial effect. In this thesis, I will present the in-depth study on the change of polymer morphologies in the presence of ultra-low nanoparticles. The studies will focus on 1) the influence of nanoparticle surface, 2) solvents induced change of polymer morphologies, and 3) in-situ structural analysis of polymer matrix around nanoparticle surface. The thesis also studied the topological effect of nanofillers in the dilute nanocomposites. The results show that 1-D nanofillers (nanorods) at ultralow volume loading (< 1 vol%) generate larger dielectric enhancement of the dielectric response of PEI (from 3.2 to 6.1), compared with 0-D nanofillers (nanoparticles). Different from a spherical shell interface nano-topology of 0-D nanofillers, the cylindrical shell nanostructures generated by 1-D nanofillers are much more efficient in raising the dipolar response in terms of extending the high K in the interfacial region and reducing the influence of low K polymer regions. One driving force for the dielectric enhancement in the dilute nanocomposites is the increased local free-volumes. In this thesis, the approach of polymer blending will also be used control and tailor the free-volumes in high Tg polymers. It was observed that the chain packing in the blends can be tuned by the electrostatic interactions between polymer chains. Consequently, by properly matching the two polymers in the blends, one can achieve enhanced breakdown strength or enhanced dielectric constant. PVDF based ferroelectric polymers have been used for electromechanical (EM) energy conversion applications. On the other hand, there is a great need to improve the EM performance of ferroelectric polymers (due to their low EM performance compared with the inorganic counterpart). This thesis studied “defect modifications” of the relaxor ferroelectric P(VDF-TrFE-CFE) terpolymers and show that small amount of FA (fluorinated alkynes) units (< 2 mol%) in the relaxor polymers can effectively suppress the polarizations which do not contribute much to the EM response while enhancing the polarizations which have a strong EM coupling. As a result, the FA modified terpolymers exhibit marked enhancement of EM responses at low electric fields (< 50 MV/m) in the terpolymer. For example, under a DC bias field of 40 MV/m, a tetrapolymer of P(VDF-TrFE-CFE-FA) generates an delta S/deltaE (piezocoefficient d33) > 1000 pm/V and EM coupling factor k33 of 88%, far exceeding the best performed and widely used piezoceramic PZT. The past decade has witnessed the discovery and advancements in electrocaloric (EC) materials, for example, P(VDF-TrFE-CFE), which exhibits large electric field-induced adiabatic temperature and isothermal entropy changes. However, the large EC response in current materials needs a very high electric field, for example, ΔT > 15 K at 150 MV/m for P(VDF-TrFE-CFE), while at low field, ΔT < 2 K at 50MV/m. On the demand of practicable EC devices, exploring EC materials generating large EC response at low fields is highly desirable. In this thesis, I will introduce a preliminary work on giant electrocaloric effect in molecular organic dielectrics. We demonstrate a > 200 Jkg-1K-1 isothermal entropy change at ultra-low electric field (~ 0.5 MV/m) in a plastic crystal (HOCH2)3CNO / P(VDF-TrFE-CFE) composite, which far exceeds the best performance of PVDF based polymers (entropy change ~ 77 Jkg-1K-1). On the other hand, there are many researchers using Maxwell relations (indirect methods) to deduce ECE from temperature dependent polarizations, which usually claims a giant ECE or even a negative ECE. However, no direct EC measurement can demonstrate a similar performance, not to say develop EC devices based on those materials. I used a normal ferroelectric, P(VDF-TrFE), as an example to interpret the thermodynamics understanding of using Maxwell relations to deduce large ECE, thus demonstrating the irrationality of this indirect measurement.