Isotope Effect in Thermal Conductivity of Polycrystalline CVD-Diamond: Experiment and Theory

We measured the thermal conductivity kappa(T) of polycrystalline diamond with natural (natC) and isotopically enriched (C-12 content up to 99.96 at.%) compositions over a broad temperature T range, from 5 to 410 K. The high quality polycrystalline diamond wafers were produced by microwave plasma chemical vapor deposition in CH4-H-2 mixtures. The thermal conductivity of C-12 diamond along the wafer, as precisely determined using a steady-state longitudinal heat flow method, exceeds much that of the C-nat sample at T>60 K. The enriched sample demonstrates the value of kappa(298 K)=25.1 +/- 0.5 W cm(-1) K-1 that is higher than the ever reported conductivity of natural and synthetic single crystalline diamonds with natural isotopic composition. A phenomenological theoretical model based on the full version of Callaway theory of thermal conductivity is developed which provides a good approximation of the experimental data. The role of different resistive scattering processes, including due to minor isotope C-13 atoms, defects, and grain boundaries, is estimated from the data analysis. The model predicts about a 37% increase of thermal conductivity for impurity and dislocation free polycrystalline chemical vapor deposition (CVD)-diamond with the C-12-enriched isotopic composition at room temperature.