For grain sizes of 10 nm and 20 nm the transport properties follow the expected trends across all temperatures and charge carrier concentrations. At charge carrier concentrations below 4 × 10 19 cm -3 , the predicted decrease in the power factor is less than 30%. The greatest percentage reductions are seen for high carrier concentrations and at low temperatures where the power factors are generally low. In absolute terms, the maximum reduction of over 3200 µW m -1 K -2 is observed for carrier concentrations between 2.37 × 10 20 -4.22 × 10 20 cm -3 and tem-peratures between 350-625 K for a grain size of 10 nm. A smaller reduction is predicted for a grain size of 20 nm, for which the power factor decreases by at most by 2400 µW m -1 K -2 between n = 2.37 × 10 20 -4.22 × 10 20 cm -3 and T = 350-575 K. Table : Maximum predicted in-plane and out-of-plane ZT at 1000 K and the corresponding lattice (κ l ) and electronic (κ e ) thermal conductivity , charge carrier concentration (n) and power factor (PF). Also shown are the values obtained with simulated grain sizes of 10 nm and 20 nm . The question then becomes whether the reduction in the lattice thermal conductivity can offset the decreased power factor to increase the ZT. While a 10% increase in the ZT is observed along the z direction, the very low conductivity and power factor mean the ZT remains around 0.01. In the xy plane, the maximum ZT is in fact decreased on limiting the MFPs, as can be seen in Table : the more favourable (i.e. lower) thermal conductivity obtained by nanostructuring is mitigated by the reduced power factor.