Graduation Year

2016

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Physics

Major Professor

Hariharan Srikanth, Ph.D.

Co-Major Professor

Manh-Huong Phan, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Committee Member

Gerald Woods, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Carbon nanotubes, magnetic nanoparticles, polymer nanocomposites, confinement, nanostructures

Abstract

Multifunctional nanocomposites are promising for a variety of applications ranging from microwave devices to biomedicine. High demand exists for magnetically tunable nanocomposite materials. My thesis focuses on synthesis and characterization of novel nanomaterials such as polymer nanocomposites (PNCs) and multi-walled carbon nanotubes (MWCNTs) with magnetic nanoparticle (NP) fillers.

Magnetite (Fe3O4) and cobalt ferrite (CoFe2O4) NPs with controlled shape, size, and crystallinity were successfully synthesized and used as PNC fillers in a commercial polymer provided by the Rogers Corporation and poly(vinylidene fluoride). Magnetic and microwave experiments were conducted under frequencies of 1-6 GHz in the presence of transverse external magnetic fields of up to 4.5 kOe. Experiments confirm strong magnetic field dependence across all samples. When incorporated in to a cavity resonator device, tangent losses were reduced, quality factor increased by 5.6 times, and tunability of the resonance frequency was demonstrated, regardless of NP-loading.

Work on PNC materials revealed the importance of NP interactions in confined spaces and motivated the study of confinement effects of magnetic NPs in more controlled environments, such as MWCNTs with varying diameters. MWCNTs were synthesized with diameters of 60 nm, 100 nm, 250 nm, and 450 nm to contain magnetic NP fillers (~10 nm) consisting of ferrites of the form MFe2O4, where M = Co2+, Ni2+, or Fe2+. All confined samples exhibit superparamagnetic-like behavior with stronger magnetic response with respect to increasing MWCNT diameter up to 250 nm due to the enhancement of interparticle interactions.

This thesis provides the first systematic study of this class of nanocomposites, which paves the way to inclusion of novel nanostructured materials in real-world applications.

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