Low Loss Polymer Nanoparticle Composites for Radio Frequency (RF) Applications
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The aim of this research is to develop novel polymer nanocomposites with desired magneto-dielectric properties including high dielectric permittivity, high magnetic permeability, and low energy loss at radio frequencies. Block copolymer-templated and surface-modified magneto-dielectric nanoparticles were utilized because of their ability of uniform dispersion and ordering within the polymer matrices. The influence of intrinsic chemical composition and characteristic lengths (shape and size) of the doped nanoparticles on the resultant composites' magneto-dielectric properties was investigated. Well-dispersed high-dielectric-permittivity titanium dioxide nanoparticles were synthesized utilizing a block copolymer as a template. The nanoparticles were confined within microphase separated domains of sulfonated styrene-b-(ethylene-ran-butylene)-b-styrene block copolymers. A crosslinker (vinyltrimethoxysilane) was incorporated into the block copolymer matrices in order to decrease the dielectric loss from the free sulfonic acid groups. Dynamic mechanical analysis experiments confirmed that nanoparticles and crosslinker were confined within the crosslinked sulfonated styrene blocks and had no effect on the chain relaxation behavior of [ethylene-ran-butylene] blocks. Dielectric experiments showed that higher dielectric permittivity composites can thus be obtained with a significant decrease in loss tan delta ( <0.01 ) when crosslinked with vinyltrimethoxysilane. Surfactant-modified iron oxide and nickel zinc ferrite nanoparticles of various sizes were successfully synthesized by a seed-mediated growth method. The nucleation and growth of surface-modified nanoparticles was controlled by changing the concentration ratio of surfactant to iron-precursor. The free iron ions present during synthesis are the major factor contributing to the growth of larger particles. The dielectric permittivity and magnetic permeability of the resultant block copolymer (styrene-b-ethylene/butylene-b-styrene, SEBS) nanocomposites increased with increasing amount of doped nanoparticles. However, nickel zinc ferrite polymer composites exhibited lower dielectric loss compared to iron oxide composites due to the high electrical conductivity of iron oxide particles. Furthermore, the magnetic permeability of the composites was significantly influenced by the size of the doped nanoparticles. Magneto-dielectric nanoparticles with multi-domain walls lacked the ability to respond to applied alternating field (GHz), thus leading to lower magnetization and magnetic permeability. Superparamagnetic nanopartilces also exhibited lower magnetic permeability due to demagnetization effects from the thermal energy provided by the nanoparticle surroundings.