Development of Ag/CNT Films As Electrical Connectors Using a Cyanide-Free Electroplating Bath
Authors: Kikuhara Taishi , Shimizu Masahiro , Arai Susumu 
Published: December 2020

Ag films as an electrical contact material have been applied to electrical devices because of the best electrical conductivity and is relatively low cost compared to that of Au. However, Ag is easily sulfurized in the air. In addition, Ag connectors with extremely soft are easy to wear. These are responsible for the increasing in resistance between electrical connectors. To overcome these issues, Ag/graphite1 with wear resistant property and Ag/CNTs2,3 with excellent mechanical strength have been studied. In the later composite, CNTs function as electrical pass even in the degradation derived from sulfidation and friction. This is the motivation of the development of Ag composite as a connector. The Ag/CNT composites have been prepared by using toxic cyanide electroplating baths4-6 which suppress dendrite growth of Ag by stable complex with cyanide ions. We focused on the iodide aqueous solutions instead of toxic cyanide electroplating baths for realizing eco-friendly approach. In the present study, the pH effect in iodide aqueous solutions on current efficiency of Ag electroplating was investigated. Then, we tried to produce Ag/CNT composite with high electrical contact properties and remarkable wear resistance.
The pH in iodide-electroplating solutions was adjusted using H2SO4 or KOH. CNTs (VGCF, SHOWA DENKO K.K.) were used without further purification, and then the CNTs were dispersed into the iodide-electroplating solutions. Co-electrodeposition of Ag/CNT was performed under galvanostatic conditions using a pure Ag plate as the anode and a pure Cu plate as the cathode at temperature of 298 K. The concentration of I3
− causing poor current efficiency during the electroplating was determined by iodometric titration of I2 (I3
− ⇄ I− + I2) using Na2S2O3-dissolved aqueous solution (I2 + 2S2O3
2− → S4O6
2− + 2I−).7

Under the condition of low pH at atmosphere, I− is generally oxidized to I2, and then the I2 reacts with I− to form I3
− (I2 + I− ⇄ I3
−). Considering the equilibrium potentials of Ag+/Ag = +0.799 V vs. SHE and I3
−/I− = +0.536 V vs. SHE, the reduction of Ag+ (Ag electrodeposition) proceeds preferentially. However, at the presence of [Ag
m
I
n
]
m

−

n
complex ions with a relatively high stability constant, the equilibrium potential of Ag+/Ag shifts to negative and thereby consumes electrons (currents) by the reduction of I3
− during the electroplating. Therefore, the I3
− formation should be suppressed. In the solution adjusted to pH=1, the I3
− concentration was suddenly increased, whereas in the solution of pH=12, the concentration was maintained below detection limit even after 30 days (Fig. 1a). The current efficiency was significantly improved, and 100% efficiency was achieved only when adjusting to pH=12. In the composite electroplating-film prepared using the solution (pH=12), we can clearly observe that CNTs are uniformly immobilized by Ag film and incorporated inside the film (Fig. 1b). The structure with large amount of CNTs should offer not only the high electrical contact property but also the improved wear resistance.
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Figure 1