## 3 Results and Discussion
With the content of nano-TiO\({}_{2}\) increasing from 0.6 vol % to 1.8 vol %, the grain size of Cu\({}_{2}\)SnSe\({}_{3}\) decreases from 2.2 \(\upmu\)m to 0.9 \(\upmu\)m. The nano-TiO\({}_{2}\) particles in the sintering process effectively inhibited the growth of grains, and the inhibitory effect is more obvious with the increase of nanoparticles [20; 21; 22]. In contrast, it can be found in Figure 5e that most TiO\({}_{2}\) particles in the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites were distributed in the internal grain. This is possibly related to the surfactivity of powder, SPS process, and so on. This issue will be investigated in future work. In addition, when the amount of nano-TiO\({}_{2}\) reached 1.8 vol %, the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite kept more holes at the grain boundary. The microstructure of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites could influence the electrical and thermal transport properties of the materials. The relative density of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites is shown in Table 1. It can be seen that the addition of nano-TiO\({}_{2}\) reduced the relative density of TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites. \begin{table}
\begin{tabular}{c c c c c c} \hline \(x\)**TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\)** & \(x\) = 0 & \(x\) = 0.6 & \(x\) = 1.0 & \(x\) = 1.4 & \(x\) = 1.8 \\ \hline Density (g/cm\({}^{3}\)) & 6.042 & 6.017 & 5.833 & 5.686 & 5.343 \\ Relative density & 98\% & 97.8\% & 97.2\% & 96.5\% & 95.6\% \\ \hline \end{tabular}
\end{table} 
Table 1: Density of the \(x\) vol % TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites with different nano-TiO\({}_{2}\) content. Figure 3: The XRD patterns of Cu\({}_{2}\)SnSe\({}_{3}\) powder before and after ball milling. Figure 6 shows the temperature dependence of electrical conductivity (\(\sigma\)) for the TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites in the range of 300-700 K. The electrical conductivity of all samples decreased with increasing temperature in all of the investigated temperatures, which indicates that all samples demonstrated degenerate semiconductor behavior. The composites samples showed lower \(\sigma\) than the Cu\({}_{2}\)SnSe\({}_{3}\) matrix except for the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample, which suggests that the addition of nano-TiO\({}_{2}\) into the Cu\({}_{2}\)SnSe\({}_{3}\) matrix can reduce the electrical conductivity. Nano-TiO\({}_{2}\) particles affect the carrier transport, leading to a reduction in the conductivity of the material. On the other hand, the nano-TiO\({}_{2}\) particles which were distributed at the grain boundary effectively suppressed the grain growth, resulting in a large number of grain boundaries during the sintering process. These grain boundaries have great effects on the transport of the carriers, which reduce the conductivity of the material. However, the conductivity of the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) sample was effectively improved. As shown in Figure 5e, the nano-TiO\({}_{2}\) particles in the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites were mainly  
Figure 4: The XRD patterns of the \(x\) vol % TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites. Figure 5: (**a**) TEM image of the initial TiO\({}_{2}\) nanoparticles. (**b**–**f**) SEM images of the \(x\) vol % TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composites: (**b**) \(x=0\); (**c**) \(x=0.6\); (**d**) \(x=1.0\); (**e**) \(x=1.4\); (**f**) \(x=1.8\). distributed in the internal grain. Therefore the effect of suppressing the grain growth was not as obvious as that of other composites. Meanwhile, the carrier concentration of 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite at room temperature is listed in Table 2. The increase in the conductivity of the 1.4%TiO\({}_{2}\)/Cu\({}_{2}\)SnSe\({}_{3}\) composite is attributed to the high carrier mobility. These nanoparticles distributed in the grain provide a special channel and contribute to the transport for the carrier.