## 3 Results and Discussion
The relation between the SCR and power output for the SnSe is calculated by the finite element method, as shown in Figure 4.  
The 8-couple SnSe thermoelectric module was designed to be 20 \(\times\) 20 mm\({}^{2}\) in size. The hot and cold side temperatures were 873 K and 293 K, respectively. When we define the SCR of 10\({}^{-7}\)\(\Omega\cdot\)cm\({}^{2}\) as making no significant electrical contact loss, the power output is decreased about 1.9% for Ag and 86.6% for Ti metallization layer for the respective SCRs. Therefore, the Ag metallization, where the SCR is 1.15 m\(\Omega\cdot\)cm\({}^{2}\), is acceptable in contact resistance for SnSe legs and devices. The analysis results show that it is difficult to apply a single metallization layer on the SnSe thermoelectric material due to its interface cracks, element diffusion, or high electrical contact resistivity. In order to complement these drawbacks, a multi-layer metallization technique was used in this work. Based on the single-layer metallization experiments, it is found that Ag can be used as a metallic contact layer and Ti can be used as a diffusion barrier. The Ag/Ti bi-layer was formed on SnSe and the interfacial cracks were found to exist at the interface between Ag and Ti layers, as shown in Figure 5. \begin{table}
\begin{tabular}{c c c c} \hline
**Thermoelectric properties** & **Skutterudite [1]** & **Half-heusler [14]** & **Polycrystalline SnSe [15]** \\  & **(776K)** & **(776K)** & **(773K)** \\ \hline Seebeck & & & \\ coefficient (\(\mu\)V/K) & 144 & 161 & 425 \\ Electrical & & & \\ conductivity (S/cm) & 1600 & 1700 & 18.1 \\ Thermal & & & \\ conductivity (W/mK) & 3.33 & 5.1 & 0.23 \\ ZT (Figure of merit) & 0.77 & 0.67 & 0.96 \\ \hline \end{tabular}
\end{table} 
Table 1: Thermoelectric property comparison between well-known thermoelectric materials which are utilized in an intermediate temperature range. Figure 4: The simulation result of relation between power output and SCR depending on the metallization. The bi-layer metallization seems unsuitable mainly because the difference in the coefficient of thermal expansion (CTE) between Ag (18.9 \(\times\) 10\({}^{-6}\) K\({}^{-1}\)) and Ti (8.6 \(\times\) 10\({}^{-6}\) K\({}^{-1}\)) is so large that an interfacial thermal stress is induced. To resolve this issue, as a buffer layer, we used the Co layer where CTE is 13 \(\times\) 10\({}^{-6}\) K\({}^{-1}\). Finally, the interface characteristics of the Ag/Co/Ti multi-layer metallization were analyzed by EDS line scanning and SCR, as shown in Figure 6a,b. The metallization layers and SnSe are bonded robustly and their interfaces are clearly distinguished by EDS line scanning. The Sn and Se elements are not diffused into the Co and Ti layers. The SCR of the Ag/Co/Ti multi-layer is 1.53 m\(\Omega\cdot\)cm\({}^{2}\) which is similar to that of the Ag single-layer. The multi-layer metallization can simultaneously achieve both high electrical contact and good diffusion barrier properties. In order to verify the thermal stability of the multi-layer metallization, heat treatments were conducted at 723 K for 20 h under an Ar atmosphere. The Ag/Co/Ti multi-layer structure was  
Figure 5: SEM image of Ag/Ti bi-layer metallization on SnSe. Figure 6: (**a**) SEM image and EDS line scanning of the Ag/Co/Ti multi-layer metallization; and (**b**) the graph of SCR before heat treatment. maintained robustly and the IMC of each interface did not increase even after heat treatment, as shown in Figure 7a. However, the EDS line scanning analysis revealed that the Sn and Se elements were diffused into the Ag layer which could lead to a slight increase in SCR of 2.27 m\(\Omega\cdot\)cm\({}^{2}\) in Figure 7b. As shown in Figure 4, the 20 h heat treatments may cause a negligible reduction in power output of about 0.5%.