  Asymmetric heat transfer systems, often referred to as thermal diodes or
thermal rectifiers, have garnered increasing interest due to their wide range
of application possibilities. Most of those previous macroscopic thermal diodes
either resort to nonlinear thermal conductivities with strong temperature
dependence that may be quite limited by or fixed in natural materials or rely
on active modulation that necessitated auxiliary energy payloads. Here, we
establish a straightforward strategy of passively realizing asymmetric heat
transfer with linear conductive materials. The strategy also introduces a new
interrogative perspective on the design of asymmetric heat transfer utilizing
nonlinear thermal conductivity, correcting the misconception that thermal
rectification is impossible with separable nonlinear thermal conductivity. The
nonlinear perturbation mode can be versatilely engineered to produce an
effective and wide-ranging perturbation in the heat conduction, which imitates
and bypasses intrinsic thermal nonlinearity constraints set by naturally
occurring counterparts. Independent experimental characterizations of surface
thermal radiation and thermal convection verified that the heat exchange
between a graded linear thermal metamaterial and the ambient can be tailored to
achieve macroscopic asymmetric heat transfer. Our work is envisaged to inspire
conceptual models for heat transfer control, serving as a robust and convenient
platform for advanced thermal management, thermal computation, and heat
transport.
