Ultrathin millimeter-wave–absorbing film for automotive radars based on an epsilon iron oxide/carbon nanotube composite material

Abstract

Millimeter waves are used in automotive radars to detect surrounding objects such as vehicles and pedestrians. In millimeter-wave automotive radar systems, unnecessary millimeter waves arising from the radar system itself and other radar sources should be suppressed. Herein, we report an ultrathin millimeter-wave–absorbing film for the automotive radar frequency band (79 GHz) based on a composite of gallium-substituted epsilon iron oxide (ε-Ga_0.45Fe_1.55O₃) nanomagnets, which exhibit a high magnetic permeability, and carbon nanotubes (CNTs), which exhibit a high dielectric constant. The composite exhibits a complex dielectric constant (ε = ε′ − iε′′) of ε′ = 19.4 and ε′′ = 3.3 and a complex magnetic permeability (μ = μ′ − iμ′′) of μ′ = 0.87 and μ′′ = 0.13 at 79 GHz. Using this composite, we prepared an ultrathin millimeter-wave–absorbing film which shows a reflection loss of −21.4 dB (99.3% suppression) at 79 GHz with a thickness of 213 ± 8 μm. This ultrathin millimeter-wave–absorbing film can be easily coated on radar device covers, automotive parts such as bumpers, and infrastructure components like guardrails and traffic lights.