Abstract
As the quest for green energy continues, thermoelectric materials that are clean, safe, and highly-efficient become available for, as are the subsequent thermoelectric generators and coolers on the market. N- and P-type pellets are fixed by soldering to copper conductor and sandwiched between two ceramic plates for structural protection. Sn-3Ag-05Cu is often the solder used for commercial thermoelectric coolers. This research discusses in depth the reaction between solder and thermoelectric material. Since thermoelectric devices are subjected to high electric current, the presence of uneven temperature or in other words the hot and cold regions caused by Peltier effect will make the integrity of thermoelectrics/metal junctions extremely important as the lifetime and performance of the device rely heavily on it. When the device is subjected to a current density of 700A/cm2, the creation of SnTe and the sequential formation of extensive voids at the interface of solder and P-type thermoelectric elements could lead to device failure. At such interface, nickel diffusion barrier is deposited to prevent the formation of SnTe by reaction between Sn and Te. The structural integrity of this nickel barrier remains intact at the cold regions after electrical current stressing. However, at the hot regions, whether it is at N-type junction where the electron current passes from nickel barrier to solder, or it is at P-type junction where the electron current passes from the solder to nickel barrier, the nickel barrier is depleted as Sn reacts with (Bi,Sb)2Te3 to form SnTe intermetallic compounds (IMCs). After the reaction couples were subjected to electric sintering, extrusion of SbSn was found on SnTe. Since the reaction layer was a mixture of SnTe and Sn, Sb an Bi elements were dissolved in Sn by forming a solid solution. Other researchers have shown that Te and Sb elements are susceptible to the influence of a hole wind effect and Sn element is influenced by the electron wind force. Experimental results have shown that the formation of SnTe was not caused by concentration gradient, but rather by EM-induced atomic diffusion. The calculated DZ* at 200 oC was 10-8–10-9 cm2/s, which was very closed to the value 1.5×10-8 cm2/s found for the reaction between Sn and Te under electrical stressing. Electrical stressing of Sn/P-type/Sn reaction couple resulted in much thicker IMC layer compared to the thickness obtained with thermal annealing. The rapid reaction between Sn and Te was facilitated by electrical current. From the mark-line experiments, it was found that Sn was the dominant diffusing species in SnTe for both electrical stressing and thermal annealing. At the interface of Sn and P-type thermoelectric element, a thin SbSn layer was found, followed by SnTe IMCs. After electrical stressing, large quantity of Sb2Sn3 IMCs was found at cathode; such quantity was drastically reduced with thermal annealing. On the contrary, Sb2Sn3 was not observed at anode for both electrical and thermal treatments. The difference in IMC thickness was that Te and Sb were pushed toward solder by the hole wind effect, which coincided with Sn element being pushed by the electron wind effect. The calculated activation energy for the thermal formation of SnTe was 138 KJ/mole and the flux was 9*1014 (atoms/cm2□s) at 140 oC. The flux under electrical stressing was 9.8*1014 (atoms/cm2□s), and the resulting calculated DZ* obtained was 4.27*10-8 cm2/s. This value was slightly higher than the literature reported value of 3.5×10-9 cm2/s, which was obtained for the diffusion of Sn into SnTe-Te compound under electrical stressing at 142°C.