Thermoelectric figure-of-merit barrier at minimum lattice thermal conductivity?

An analysis of the transport properties data on established thermoelectric materials indicates that the ratio of the electrical power factor to optimum electrical conductivity appears constant. This result, when combined with the previously proposed concept of a minimum thermal conductivity, enables realistic maximum dimensionless figures-of-merit to be calculated. It is concluded that a barrier to increasing the dimensionless figure-of-merit appears to exist which is around 2 at room temperature and that overcoming this barrier will require the development of materials with radically different transport properties from those currently available.