  Thermal rectification is of importance not only for fundamental physics, but
also for potential applications in thermal manipulations and thermal
management. However, thermal rectification effect usually decays rapidly with
system size. Here, we show that a mass-graded system, with two diffusive leads
separated by a ballistic spacer, can exhibit large thermal rectification
effect, with the rectification factor independent of system size. The
underlying mechanism is explained in terms of the effective size-independent
thermal gradient and the match/mismatch of the phonon bands. We also show the
robustness of the thermal diode upon variation of the model's parameters. Our
finding suggests a promising way for designing realistic efficient thermal
diodes.
