  Thermoelectric materials can be used to convert heat to electric power
through the Seebeck effect. We study magneto-thermoelectric figure of merit
(ZT) in three-dimensional Dirac semimetal Cd$_3$As$_2$ crystal. It is found
that enhancement of power factor and reduction of thermal conductivity can be
realized at the same time through magnetic field although magnetoresistivity is
greatly increased. ZT can be highly enhanced from 0.17 to 1.1 by more than six
times around 350 K under a perpendicular magnetic field of 7 Tesla. The huge
enhancement of ZT by magnetic field arises from the linear Dirac band with
large Fermi velocity and the large electric thermal conductivity in
Cd$_3$As$_2$. Our work paves a new way to greatly enhance the thermoelectric
performance in the quantum topological materials.
