Thermal conductivity of chemical vapor deposition diamond enriched with 13C isotope

Thermal conductivity κ(T) of single-crystal CVD diamond enriched with 13C isotope to 98.16% was measured by the method of steady-state longitudinal heat flow in the temperature range from 6 to 410 K. This crystal with low nitrogen impurity content (<50 ppb) showed thermal conductivity 2010±50 W m−1 K−1 at 300 K (with a maximum of 12 100 W m−1 K−1 at 83 K), which is significantly lower than that of diamond with natural isotopic composition: κ(300K)=2360±50 W m−1 K−1. The measured data were analyzed using first-principles theory and the Callaway model, taking into account phonon scattering in three-phonon processes, scattering at sample boundaries and at isotopes. The first-principles calculations overestimate the thermal conductivity compared to the measured one near and to the right of the κ(T) peak, indicating the presence of significant additional phonon scattering by lattice defects in the studied chemically pure diamond samples. The results of both theoretical approaches for thermal conductivity at 300 K are in good agreement with our measured data and other published experimental data for isotopically modified diamonds. First-principles calculations yield a thermal conductivity ratio of κ12(T)/κ13(T)=1.072 for monoisotopic defect-free 12C and 13C crystals at 300 K. This ratio decreases at high temperatures to a value of 1.041 according to the Leibfried–Schlömann theory and to a value of 0.921 at very low temperatures.