Sintering mechanisms and dielectric properties of cold sintered (1-x) SiO₂ - x PTFE composites

This study reports the sintering mechanisms of SiO₂ and the relationship between structure, dielectric properties and processing of 1-xSiO₂-xPTFE composites with volume fractions up to x = 0.25, prepared by the cold sintering process (270 °C, 430 MPa, 60 min). The importance of transient liquids was evaluated by using TEOS and 5 M NaOH and the densification behavior was investigated by using a semiautomated uniaxial press equipped with a dilatometer. The shrinkage behavior observed when 5 M NaOH was used highlighted the kinetics of nucleation and growth during the phase transition from amorphous silica to α-quartz. Dielectric properties (ε’ and tan δ) of cold sintered composites were studied at frequencies between 10⁻¹ Hz and 10⁷ Hz. For composites cold sintered with 5 M NaOH, investigations allowed to confirm the presence of PTFE polymer at the interface between two α-quartz grains by STEM EDS analysis and the presence of a dielectric relaxation at 10³-10⁴ Hz.