Dielectric, AC conductivity, and EMI Shielding studies of PEDOT-PSS nanocomposites with carbon based fillers /

Includes bibliographical references

PhD;2024;Physics

Polymer nanocomposites (PNCs) have emerged as a significant material for addressing two critical contemporary challenges: the escalating global energy demand and the pervasive electromagnetic interference (EMI) issues. The energy stored within a dielectric material is related to its dielectric polarization, dielectric constant, conductivity, and breakdown strength. The dielectric loss, quantified in terms of loss tangent (tan δ), must be minimized to reduce the energy dissipation in the form of heat and to achieve high energy efficiency and device performance. One can tailor different polarization mechanisms to achieve high dielectric constant / and low dielectric loss. In this work, we have developed PNCs based on intrinsically conducting polymer (PEDOT-PSS) with graphene oxide, functionalized carbon nanotubes, and boron nitride as fillers via drop casting technique. The addition of a co-solvent, ethylene glycol, helped to improve the delocalization of electrons in the conjugated π orbitals, thus enhancing the electronic polarization. Conductivity in PEDOT-PSS arises from the delocalized π electrons in their conjugated backbone, and the presence of permanent dipoles that align along the external field contributes significantly to its high dielectric constant. Incorporating nanoparticles with different dielectric permittivity compared to the polymer matrix helped in improving the interfacial polarization and, thus, ultrahigh permittivity (order of 105 at 1 kHz) with minimal loss in the composite material is achieved. AC conductivity study revealed that these composites exhibit NSPT (non-overlapping polaron tunnelling) and CBH (correlated barrier hopping) mechanisms of conduction. For harvesting the energy density properties, we have tried to tune the filler loading and optimize the filler to get high permittivity, breakdown strength, and appreciable energy density for practical applications. Strong attenuation capacity and well-matched impedance are two requirements for excellent EM wave absorbers. A trade-off between impedance matching characteristics and strong attenuation, must be compromised to reach an excellent EM wave absorbing performance. Dielectric loss and magnetic loss are the underlying causes of EM wave dissipation, and the extent of energy lost greatly depends on the permittivity (ϵ′) and permeability (μ′) of the material. Functionalized carbon nanotubes, carbon-coated iron nanoparticles (CCFeNPs), and purified CCFeNPs were subsequently used to fabricate composite films based on PEDOT-PSS. The EMI shielding properties of the composite films are evaluated in the Ku and X band range. All the composites exhibited a high permittivity due to the intrinsic permittivity of the matrix and the interfacial polarization due to filler addition. An interplay always existed between better impedance matching and longer relaxation time of the electric dipoles along with the natural resonance of the magnetic dipoles to give maximum shielding effectiveness. We were able to achieve a shielding effectiveness of 38.6 dB at 12.5 GHz, which corresponds to 99.98 % EM wave attenuation in one of the composite films with micron thickness.