Brian’s paper, “Spectral phonon scattering effects on the thermal conductivity of nano-grained barium titanate,” has been published in Applied Physics Letters (Appl. Phys. Lett. 105, 082907 (2014)). In this work, we show that grain sizes as large as 50 nm can still lead to reductions in the thermal conductivity of BaTiO3 thin films. For films with grain sizes of 36 nm (on average), the thermal conductivity of the BaTiO3 thin film can reach as low as 1.0 W/m/K. We attribute this reduction in thermal conductivity to the spectral nature of phonon mean free paths in BaTiO3, with some phonons having mean free paths larger than the grain sizes (which, in our samples, range from 36-63 nm). This is in contrast to the typically assumed view of phonon transport in ABO3 perovskite ferroelectrics in which gray phonon mean free paths are assumed with length scales less than 5 nm. Congrats Brian!!!
Abstract
We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of BariumTitanate (BaTiO3) decreases with decreasing grain size as a result of increased phononscattering from grain boundaries. We analyze our results with a model for thermal conductivitythat incorporates a spectrum of mean free paths in BaTiO3. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths.
We appreciate funding from the National Science Foundation (CBET-1339436), the Army Research Office, Grant No. W911NF-13-1-0378, and the Air Force Office of Scientific Research under AFOSR Award No. FA9550-14-1-0067 (Subaward No. 5010-UV-AFOSR-0067). This work was also supported, in part, by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories.
patrickehopkins.com 
Great finding Brian, congratulations! We are so proud of you son. Imagine, so small and yet very meaningful. Keep up the great work!
Esperanza Donovan-Pendzic (MOM)