Grain size-induced thermo-mechanical coupling in zirconium thin films

At the bulk scale, thermal conductivity of metals is not coupled with mechanical strain. However, this may not be the same for grain sizes below the electron mean free path, for which mechanical deformation mechanism and volume fraction of grain boundaries are drastically different from the bulk. To investigate this hypothesis of grain size-induced thermo-mechanical coupling, we developed an experimental setup to measure the thermal conductivity of 100-nm-thick freestanding nanocrystalline zirconium (Zr) films as a function of externally applied mechanical strain. Experimental results show strong mechanical strain-thermal conductivity coupling, thermal conductivity of Zr film dropped from 20 to 13 W m^-1 K^-1 with an increasing strain from 0 to 1.24 %. Thermal conductivity of Zr film was also measured as a function of average grain size ranging from as-deposited 10 nm to as-grown 250 nm. The results clearly show that for grain sizes above 10 nm, mechanical strain has no influence over thermal conductivity, which supports the proposed hypothesis.