Process-Structure-Properties Relationship of Electrodeposited Au Thin Film Used in Thermoelectric Power Generation Device

Abstract

An effective thermoelectric (TE) power source consists of TE modules and thermal management device. The TE modules must be connected electrically in series on both the top and bottom surface with bridging metal interconnects that must maintain low contact resistance with high mechanical strength while operating at high current densities and temperature gradients. Due to its low emissivity, excellent electronic, and mechanical properties, Au coatings can be used for interconnect metallization and heat mitigation in TE modules and thermal management devices respectively. In this study, we report on the successful synthesis of perfectly coherent and atomically sharp nanoscale growth twin boundaries (figure 1(e&f)) and the introduction of highly crystalline atomic structure (figure 1(f)) in the interior of each grain of Au electrodeposits using pulsed electrodeposition from an additive stable cyanide gold bath. The increase in sharpness of nanoscale twin boundaries resulted in improved mechanical and electronic properties in Au electrodeposits1. During the study of nucleation and growth mechanism of Au electrodeposition process, we established that both instantaneous and progressive nucleation modes could be achieved depending on the overpotential used during deposition. Instantaneous nucleation was observed at lower overpotentials (≥ -0.75 V vs Ag/AgCl) while progressive nucleation occurred at larger overpotentials (< -0.75 V vs Ag/AgCl). We then compared direct (DC) and pulse current (PC) electrodeposition modes and their effects on mechanical and electronic properties of the electrodeposits. We determined that the use of large overpotentials (≤ -0.9 V vs Ag/AgCl) successfully produced films with enhanced properties. Compared to films generated using DC electrodeposition, pulsed Au coatings had improved physical and electronic properties with emissivity as low as 0.006, contact resistance five times less, hardness and electrical conductivity twice as much. We also found that long pulse on-times (≥ 2ms) and a small duty cycle (10%) were needed to produce smooth, porous free(figure 1(b&c)), highly crystalline, strongly textured, low emissivity, reduced contact resistance, enhanced electrical conductivity, and mechanically strong Au electrodeposits. Reference 1. L. Lu, Y. F. Shen, X. H. Chen, L. H. Qian, and K. Lu, Science, 304 (5669), 422-426 (2004). Figure 1