## 3 Results and Discussion  
A metallization process is performed to form a metallic layer on the SnSe thermoelectric material and the formed metallic layer should act as a high electrical and thermal conductive layer as well as an inter-diffusion barrier layer. The Ag, Ni, or Ti metal layers were selected to make a single metallic layer each on SnSe because Ag has high electrical conductivity, and both Ni and Ti are well-known diffusion barrier materials [9; 10; 11; 12; 13]. Because the sintering temperature of SnSe was already determined according to the optimized material properties, the metallization in this work was also conducted using SPS at the similar sintering condition. The bonding interfaces and element distributions were analyzed, as shown in Figure 2.  
The Ni single-layer metallization formed a thick intermetallic compound (IMC) composed of Ni\({}_{5,63}\)SnSe\({}_{2}\) and Ni\({}_{3}\)Sn alloys, shown in Figure 2a. These alloys are different in their crystal structures and cell sizes, causing severe cracks across the interfaces between the Ni and IMC layers. This indicates clearly that Ni is not suitable for SnSe metallization. On the other hand, both Ag and Ti layers were bonded robustly with SnSe without any cracks or delamination at the interfaces. However, the Sn element of SnSe was diffused into the Ag layer, as shown in Figure 2b. This means that Sn could diffuse continuously into the Ag layer at the operating temperature, possibly degrading the thermoelectric property of SnSe. The Ti layer could prevent the Sn diffusion effectively, as shown in Figure 2c. Figure 2: SEM (Scanning Electron Microscopy) images and EDS (Energy Dispersive Spectrometer) line scanning of single-layer metallization: (**a**) Ni; (**b**) Ag; and (**c**) Ti. The effect of the metallization on electrical contact property was also investigated by the specific contact resistance (SCR). In general, the SCR is defined by R\({}_{\rm sc}\) = R\({}_{\rm c}\).A where A is an interfacial area and R\({}_{\rm c}\) is the resistance difference between thermoelectric materials and metallization layers. The metallized SnSe ingots were cut into rectangular-shaped segments to measure their interfacial areas easily. Figure 3a,b present the resistance versus distance relations for the Ag and Ti metallization layers. The SCR of the Ni metallization layer could not be measured mainly due to the severe cracks. The SCR measurement results for Ag and Ti were 1.19 and 927 m\(\Omega\cdot\)cm\({}^{2}\), respectively. The Ti metallization layer was bonded mechanically well with SnSe, but the electrical contact property was poor, causing a degradation of the thermoelectric power output. In general, the SCRs of the intermediate-temperature thermoelectric legs need to be lower than 10\({}^{-5}\)\(\Omega\cdot\)cm\({}^{2}\) in order to prevent any significant degradation of power output by any contact loss. However, this should be considered as the ratio of the resistance of thermoelectric materials to the contact resistance. The SnSe material shows a high ZT value due to its high Seebeck coefficient and low thermal conductivity, but its electrical conductivity is much lower than those of other intermediate-temperature materials, as shown in Table 1.  
Figure 3: The graphs of SCR (Specific Contact Resistance) of (**a**) Ag and (**b**) Ti single-layer metallization. The electrical conductivities of skutterudite and half-Heusler, both well-known intermediate-temperature thermoelectric materials, are above 1000 S/cm in overall temperature ranges [1; 14]. The electrical conductivity of SnSe is only 18.1 S/cm, indicating that it is not necessary to have low SCR as in other materials [15]. As a result, several 10\({}^{-3}\)\(\Omega\cdot\)cm\({}^{2}\) SCR of SnSe thermoelectric material could be approximated to be the same as the 10\({}^{-4}\)-10\({}^{-5}\)\(\Omega\cdot\)cm\({}^{2}\) SCR of skutterudite and half-Heusler thermoelectric materials.