## 3 Results and Discussion
The electrical conductivity of 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample was 2.91 \(\times\) 10\({}^{3}\)\(\Omega^{-1}\)/cm at room temperature. Compared with the electrical conductivity of doped-Cu\({}_{2}\)SnSe\({}_{3}\) sample in the previous studies [8; 9; 10; 11], the electrical conductivity of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample was higher, which is possibly due to high carrier concentration (about 10\({}^{21}\) cm\({}^{-3}\)), as shown in Table 2.  
Figure 7 shows the temperature dependence of Seebeck coefficient (\(\alpha\)) of the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite samples. All samples kept positive values of \(\alpha\) in the whole temperature range, indicating that they are \(p\)-type semiconductors. The Seebeck coefficient of all samples increased with increasing temperature. The Seebeck coefficient of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites was higher than that of the matrix material, except for the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample. The introduction of nano-TiO\({}_{2}\) brings a large number of phase interfaces, which can filter out the low-energy carriers so as to increase the density of states of the material. As a result, the Seebeck coefficient of composites was improved. As for the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample, the lower Seebeck coefficient should be related with the different distribution of nano-TiO\({}_{2}\) particles in the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite. Combined with the above analysis of the microstructure and conductivity, there is an effective channel in the composite which allows a large number of low-energy carriers to pass through, so the composite maintains a lower Seebeck coefficient. The detailed reason will be further investigated. Figure 8 shows the temperature dependence of the power factor (PF) of the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites. The PF of 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) reached 5.27 \(\times\) 10\({}^{-4}\) Wm\({}^{-1}\) K\({}^{-2}\) at 700 K, which is 20% higher than that of Cu\({}_{2}\)SnSe\({}_{3}\) matrix material (4.59 \(\times\) 10\({}^{-4}\) Wm\({}^{-1}\) K\({}^{-2}\)). For the other TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites, since the increase in the Seebeck  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \(x\)TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) & \(R_{H}\) (10\({}^{-3}\) cm\({}^{3}\)/C) & \(p\) (cm\({}^{-3}\)) & \(\mu\) (cm\({}^{2}\)/Vs) & \(\sigma\) (10\({}^{3}\)\(\Omega^{-1}\)/cm) \\ \hline \(x\) = 0 & 3.368 & 1.86 \(\times\) 10\({}^{21}\) & 6.58 & 1.96 \\ \(x\) = 0.6 & 3.621 & 1.71 \(\times\) 10\({}^{21}\) & 4.61 & 1.25 \\ \(x\) = 1.0 & 3.262 & 1.92 \(\times\) 10\({}^{21}\) & 5.38 & 1.64 \\ \(x\) = 1.4 & 3.570 & 1.75 \(\times\) 10\({}^{21}\) & 10.4 & 2.91 \\ \(x\) = 1.8 & 3.31 & 1 coefficient is not sufficient to compensate for the decrease in conductivity, the PF of the composite samples is maintained at a lower level than that of the Cu\({}_{2}\)SnSe\({}_{3}\) matrix. Figure 9 shows the temperature dependence of thermal conductivity (\(\kappa\)) of the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite. Compared with the \(\kappa\) of Cu\({}_{2}\)SnSe\({}_{3}\) matrix, it can be seen in Figure 9a that the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites maintained the lower \(\kappa\), except for the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample. The 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite kept higher \(\kappa\) than that of the Cu\({}_{2}\)SnSe\({}_{3}\) matrix. The lattice thermal conductivity (\(\kappa_{\rm L}\)) of the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite can be calculated from the total thermal conductivity (\(\kappa\)) minus the carrier thermal conductivity \(\kappa_{\rm e}\) (\(\kappa_{e}=L\sigma T\)), as shown in Figure 9b.