Effects of APTES-coated SiO₂ nano-fillers on electrical, mechanical, and thermal properties of PAA-based composites for HTS cable insulation at cryogenic conditions

Polyamic acid (PAA) is a promising cryogenic dielectric for high-temperature superconductor (HTS) power transmission cables. However, its relatively low dielectric and tensile strengths, along with high thermal contraction, limit its effectiveness. This study develops PAA-based nanocomposites with 3-aminopropyl triethoxysilane (APTES)-coated SiO₂ nano-fillers to enhance both dielectric and mechanical properties. Dimethyl sulfoxide (DMSO) was used as a solvent to disperse SiO₂, and five compositions were synthesized: pure PAA and PAA with 2–8 wt.% SiO₂. The dielectric and tensile strengths of these nanocomposites were evaluated at both room (RT) and cryogenic temperatures. The dielctric constant (ε') and loss tangent (tan δ) were measured in a frequency range of 20 Hz – 2 MHz using an LCR meter at RT. Scanning electron microscopy (SEM) analyzed failure structures, while Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) characterized their chemical and thermal properties. Results indicate that increasing SiO₂ content significantly influences material properties. Among the compositions studied, the 6 wt.% SiO₂ nanocomposite exhibited the highest dielectric strength at 93 K, 292.8–373.8 kV/mm, compared to 147.6–193.6 kV/mm at RT. It showed the lowest dielctric contant of 2.61 and loss tangent of 0.015. Additionally, it demonstrated strong mechanical performance at 193 K. The results can be attributed to enhanced interfacial interactions between the surface-coated SiO₂ particles and the PAA matrix, which contributes to the improvement in both dielectric and mechanical strength as the filler content increases. The findings highlight the potential of these nanocomposites to improve insulation performance and mechanical reliability in HTS power transmission systems.