UC Berkeley

Various modern devices involve highly anisotropic materials. For example, Bi₂Te₃ is used in thermoelectrics, and graphene finds broad applications ranging from microelectronics to optoelectronics. The heat transfer in these materials can deviate significantly from classical isotropic transport theory. Nonlinear thermal devices have also drawn a great deal of attention for such applications as thermal regulation of building envelopes, and thermal protection of delicate components in electrical hardware, spacecraft thermal shielding, and satellite radiators.

In this thesis, heat transfer in nonlinear devices and anisotropic materials, in particular graphene, is investigated using both experimental and theoretical methods. Measurements on graphene sheets encased by silicon dioxide layers show the strong effect of the encasing oxide in disrupting the thermal conductivity of adjacent graphene layers, leading to more than one order of magnitude suppression as compared to the freely-suspended graphene experiment reported in literature. Modeling thermal properties of anisotropic materials reveals an unexpected guideline to engineer heat transport: due to phonon focusing effects, in many cases the heat transfer can be enhanced by reducing a phonon velocity component perpendicular to the transport direction. Finally, a nonlinear thermal diode, based on a new mechanism exploiting asymmetric scattering of ballistic energy carriers by pyramidal reflectors, is demonstrated experimentally. Experiments underline that all thermal rectifiers require nonlinearity in addition to asymmetry.