Thermal metamaterials have made significant advancements in the past few
decades. However, the concept of thermal metamaterials is primarily rooted in
the thermal conduction mechanism, which has consequently restricted their
application scope. It is imperative to consider thermal radiation, another
crucial thermal transport mechanism, particularly in high-temperature regimes,
when designing thermal devices. In this review paper, we present the
advancements in this area, with a specific focus on research conducted using
the effective-medium theory. Additionally, we explore the potential
applications of radiative thermal metamaterials and discuss prospective
research directions from a microscopic perspective for future investigations