Abstract
This thesis involves two diverse areas of research—microwave absorbers and carbon nanostructures. An ultra-broadband microwave absorber was designed, fabricated, and experimentally measured in the microwave absorber part. My involvement in the carbon nanostructure project represents a leg in Prof. Sheng’s 20-year search for superconductivity in ultra-thin carbon nanotubes. My efforts focused on using CVD to fabricate a network of (3,0) carbon nanotubes in the pores of the ZSM-5 template and dope it with Mg.
The advent of 5G technology, which means quicker communication but with higher microwave power penetrating the 5G functional space, prompted the ultra-broadband microwave absorber project. To alleviate health concerns from the order of magnitude higher microwave power and balance necessary monitoring and privacy, an ultra-broadband metamaterial absorber was designed based on simple elements and cross-combined arrays with hierarchical self-similar layers. This absorber, which is polarization-independent and insensitive to oblique incidents up to 45°, can exhibit an average of 19.4 dB reflection loss from 3-40 GHz with an overall thickness of 14.2 mm, which is only 5% more than the ultimate thickness limit imposed by natural law—the causality principle. The absorber is both electrically and magnetically excited, with two-dimensional capacitively coupled (electrical) dipolar resonances in the lateral plane. By placing a metallic boundary close to the resonant array, the electrical dipolar resonances generate two magnetic resonances that can be coupled to the standing wave along the incident direction. Optimizing the resistance value of the soldered resistors achieves impedance matching and microwave dissipation. With its simple basic structure of printed metallic rings on PCB board, this microwave absorber implies low production cost and, therefore, can be easily mass-produced for diverse applications.
Inspired by Peierls-type metal-insulator transition in carbon nanostructures grown within the pore network of ZSM-5 zeolite, which implies strong electron-phonon interaction, we prepared Mg-doped carbon nanostructures grown within ZSM-5 zeolite by chemical vapor deposition and induction-heating methods, respectively, to further explore their superconducting properties. The carbon nanostructures show a distinct radial breathing mode (RBM) of (3,0) carbon nanotubes under Raman spectroscopy, and they formed a three-dimensional network within the pores of ZSM-5 zeolite with 15.6wt% carbon content. In the SQUID measurements of the magnetic properties, the samples displayed a significant negative magnetoresistance under 50 Oe, which resembles the Meissner effect at 800 Oe. A near 18 K one-dimensional and roughly 20 K three-dimensional superconductive-like signals were observed in the transport measurements. The 3D superconducting signal is composed of a semiconductor-type signal in series with a superconducting signal. However, the SQUID and transport results didn’t correlate, which might be attributed to the sample being inhomogeneous, prone to oxidization, and requiring sophisticated nanofabrication before transport measurement. The underlying mechanism of the phenomenon remains mysterious.
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