Radio-frequency negative permittivity of carbon nanotube/copper calcium titanate ceramic nanocomposites fabricated by spark plasma sintering

Ceramic nanocomposites have been developed to provide a feasible and effective strategy in realizing extraordinary electromagnetic properties, such as negative permittivity. Here, copper calcium titanate CaCu3Ti4O12 (CCTO) nanocomposites incorporated with multiwall carbon nanotube (MWCNT) were prepared by spark plasma sintering (SPS) technology. It was found that MWCNT clusters were randomly embedded in CCTO matrix, which destroyed the microstructures of internal barrier layer capacitors in CCTO. The composites with low MWCNT content presented a hopping conduction behavior, while a metal-like conduction behavior was observed in the ceramic with 18 wt% MWCNT content. Interestingly, weakly negative permittivity (similar to -10(3)) was obtained in the ceramic consisting of interconnected MWCNT networks, as a great number of free electrons in the MWCNTs formed significantly collective oscillation state over 10 MHz-1 GHz range. Macroscopically, leakage current among MWCNT clusters or networks caused strong conduction loss at low frequencies. Equivalent circuit analysis manifested the correlation between low-frequency plasmonic state and inductive character in the composite with negative permittivity. This work could extend the potential applications of ceramic nanocomposites to metamaterials.