Do thermoelectric generator modules degrade due to nickel diffusion

Abstract

The author builds a strict diffusion theory of electrical contact resistance by studying the process of non-stationary diffusion of metal into the thermoelectric material, as a result of which a transition contact layer is formed. The distribution of metal atoms in the transition layer and the law of its growth are strictly determined from the condition of the balance of the substance quantity. It is shown that the relative distribution of nickel in the transition layer is described by a function erfc(6,771x), where erfc(…) is the so-called complementary error integral, and x is the normalized depth measured from the metal-thermoelectric material separation boundary. At the same time, the transition contact layer grows with time t according to the law x0 = 13,552(Dt)1/2, where D is the diffusion coefficient of the metal in the thermoelectric material, and its thickness is related to the thickness of the spent metal layer by the ratio x0 = 12h, where h the thickness of the spent nickel layer. Based on the obtained regularities, the temperature dependence of the electrical contact resistance of the transition layer and its time dependence (degradation law) were strictly calculated for the first time. It is shown that for the thickness of the spent nickel layer in the range of 5–20 μm and in the temperature range of 300–500 K, the electrical contact resistance varies in the range from 3,8•10–6 to 3,4•10–5 Ω•cm2 and increases over time according to the square root law. It is also shown that there is no reason to fear a significant decrease in the thermoelectric figure of merit of the generator material as a result of nickel concentration leveling through the thickness of the thermoelectric leg after its surface layer is depleted.