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Dissertation_Alex Adaka.pdf (4.91 MB)

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Electrical and Optical Properties of Ferroelectric Nematic Liquid Crystals

Adaka, Alex Oniovuogha
http://orcid.org/0000-0003-2337-7620
http://rave.ohiolink.edu/etdc/view?acc_num=kent1728671131477664

Abstract Details

2024, PHD, Kent State University, College of Arts and Sciences / Materials Science Graduate Program.
The discovery of ferroelectric nematic liquid crystal (FNLC) was envisioned by Born already in 1916, but it was only 100 years later that Nishikawa et. al. and Chen et. al reported macroscopic polarization of 5-6 μC/cm^2 on newly synthesized highly polar (dipole moment of ~ 10 Debye) rod-shape molecules called DIO and RM734. Large number of researchers also reported very large ε~10^4 dielectric permittivity at 1 kHz. Our group began investigating FNLCs in 2020 on chiral RM734 and RT10011 synthesized by Prof Twieg in the Department of Chemistry and Biochemistry at KSU. Since then, supported by an NSF grant Twieg and his students synthesized over 100 FNLC materials that we have been characterizing by measuring their novel physical properties. My role in this group was to measure the ferroelectric polarization, dielectric permittivity , electro-optical properties and electric field-induced shift of phase transitions of newly synthesized FNLC materials. For this I have explored several ways to deduce the magnitude of the ferroelectric polarization using several geometries, learned dielectric spectroscopy, characterized the phase sequences of the materials using polarized optical microscopy (POM). After introduction and description of the experimental techniques I used, I will describe my experimental results in four chapters. Firstly, I will give a detailed analysis of about fifty FNLC materials that I have received to determine their phase sequences and the temperature dependencies of the ferroelectric polarization. I found that the measured polarization of materials with isotropic-nematic-ferroelectric nematic phase sequence has polarization that can be explained by their molecular dipoles, a large number of FNLC materials with direct isotropic-ferroelectric nematic transition, the measured polarization values are too high to explain based on the magnitude of their dipole moments. I will discuss possible reasons for these discrepancies. Secondly, I will discuss the electrocaloric effect of a new ferroelectric nematic material with direct isotropic to ferroelectric nematic phase. Here we show that with an enthalpy of around 4.5J/g we can induce ~2K change in the transition temperature by as small electric field as ~1 V/µm. Thirdly, I will discuss the phase characterization and measurement of various physical properties of a new N_F material that vitrifies at room temperature. This material is a single compound that has N_F phase down to room temperature and the rotational viscosity of the materials increases dramatically below 45°C. This temperature dependence indicates this material has a glassy nature at a glass transition temperature of ~19°C. In chapter 6, I will discuss dielectric measurements on an FNLC material in films with various thicknesses of the insulating alignment layer. With such measurements we were able to test the so-called polarization-capacitance Goldstone (PCG) mode model proposed by Clark et al Within the measurement error we were able to prove that what people measure and interpret as due to a very large dielectric constant of FNLC is in fact, the capacitance of the insulating layer at LC/electrode surface and not that of the liquid crystal. We also gave a crude estimate that the dielectric permittivity measured perpendicular to the director is ε⊥~10^2 in the N_F phase. Finally, I will highlight challenges and areas to do in the future for the different synthesized ferroelectric nematic materials to pave ways for their deeper understanding and their possible technological applications.
Antal Jákli (Advisor)
129 p.
Materials Science
Liquid crystals; electrocaloric, vitrified, dielectric constant, ferroelectricity

Recommended Citations

Citations

  • Adaka, A. O. (2024). Electrical and Optical Properties of Ferroelectric Nematic Liquid Crystals [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1728671131477664

    APA Style (7th edition)

  • Adaka, Alex. Electrical and Optical Properties of Ferroelectric Nematic Liquid Crystals. 2024. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1728671131477664.

    MLA Style (8th edition)

  • Adaka, Alex. "Electrical and Optical Properties of Ferroelectric Nematic Liquid Crystals." Doctoral dissertation, Kent State University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=kent1728671131477664

    Chicago Manual of Style (17th edition)

kent1728671131477664
84
© 2024, all rights reserved.
This open access ETD is published by Kent State University and OhioLINK.