Interface Density Effects on Cross-Plane Thermal Conductance of Nanolaminate Thin Films

Thermal resistance arises at the interface of two different materials. However, developers of sensors, electronics, and power conversion devices often ignore this effect. The additional thermal resistance imposed due to interface can enhance sensitivity of thermal sensors, and efficiency of thermal energy harvesting devices such as thermoelectrics. This work demonstrates that the effect of interfaces cannot, and should not, be ignored when dealing with many interfaces in a system. We use frequency-domain thermoreflectance to demonstrate that nanolaminate thin films significantly reduce the overall thermal conductance of the film stack. As an example, with 500nm total film thickness, a repeated period of 10nm each of high thermal conductivity aluminum (k bulk,Al = 212 Wm-1K-1) and silicon dioxide (k bulk,SiO2 = 1.4 Wm-1K-1) have measured effective thermal conductivity less than that of silicon dioxide alone (1.20 Wm-1K-1 vs 1.38 Wm-1K-1). While this is substantial, the diffuse mismatch model (which often over-predicts thermal conductance of single interfaces) predicts an even lower value of effective thermal conductivity (0.56 Wm-1K-1), meaning much lower nanolaminate thermal conductivity could be realized with appropriate treatment of the surface where an interface will form.