  The phonon and electron transport in single-walled carbon nanotubes (SWCNT)
are investigated using the nonequilibrium Green's function approach. In zigzag
SWCNT ($n$, 0) with $mod(n,3)\not=0$, the thermal conductance is mainly
attributed to the phonon transport, while the electron only has few percentage
contribution. The maximum value of the figure of merit ($ZT$) is about 0.2 in
this type of SWCNT. The $ZT$ is considerably larger in narrower SWCNT because
of enhanced Seebeck coefficient. $ZT$ is smaller in the armchair SWCNT, where
Seebeck coefficient is small due to zero band gap. It is found that the cluster
isotopic doping can reduce the phonon thermal conductance obviously and enhance
the value of $ZT$. The uniaxial elongation and compress strain depresses
phonons in whole frequency region, leading to the reduction of the phonon
thermal conductance in whole temperature range. Interestingly, the elongation
strain can affect the phonon transport more seriously than the compress strain,
because the high frequency $G$ mode is completely filtered out under elongation
strain $\epsilon >0.05$. The strain also has important effect on the subband
edges of the electron band structure by smoothing the steps in the electron
transmission function. The $ZT$ is decreased by strain as the reduction in the
electronic conductance overcomes the reduction in the thermal conductance.
