Highly Efficient Broadband Millimeter‐Wave–Absorbing Ultrathin Films

Abstract

Millimeter waves (30–300 GHz) hold promise as carrier waves for car radars, next‐generation wireless communications, and beyond fifth‐generation technology standards for broadband cellular networks (Beyond 5G). Millimeter‐wave absorbers suppress noise and crosstalk between devices. Herein, a high‐efficiency broadband millimeter‐wave absorber based on a composite of aggregated tetratitanium heptoxide nanocrystals covered by Ga–Ti–Co–substituted ε‐iron oxide nanomagnets (Ti4O7@GTC‐ε‐Fe2O3) is reported. This composite exhibits both high permeability (high‐μ) and high dielectric constant (high‐ε) in the millimeter‐wave range. In addition, millimeter‐wave–absorbing ultrathin films are designed and prepared. The 183‐μm absorber suppresses 99.1% (−20.3 dB) of the reflections at 78 GHz with a broadband absorption of 14.3 GHz. Because it is composed of only inorganic materials, this absorber is weather‐resistant and suitable for outdoor use. Furthermore, a 216‐μm flexible ultrathin film was prepared by mixing Ti4O7@GTC‐ε‐Fe2O3 with an acrylic acid ester polymer. The flexible film is extremely lightweight, only 41 mg cm−2, and strongly suppresses 99.8% (−27.2 dB) of the reflections at 80 GHz with an ultrawide absorption of 16.2 GHz. Such ultrathin and lightweight films exhibiting high efficiency for car radar frequencies are not reported so far and should be useful as absorbers for transportation vehicles.