Study of physisorption isotherm of water on technical nickel surface with a wide pressure range

During the pumping process of unbaked all-metal vacuum systems after air exposure, water dominates in a time of ∼105 s (>10−6 Pa). The pumping rates of vacuum systems are difficult to be calculated because of the term of the re-adsorption rates. Under the assumption of quasi-equilibrium state between the adsorption and the gas phases, theoretically, the isotherms of water can be used to solve the pumping equation without the re-adsorption term. Thus, it is important to measure the isotherms of water on technical metal surfaces. In this work, the physisorption isotherm of water on the nickel surface with a wide pressure range (from 1.59 Pa to saturated vapor pressure Ps) is measured using an all-metal apparatus. The results show that the experimental data agree quite well with the empirical isotherm of Redhead within 0.16 ≤ ψ ≤ 0.95 (ψ = P/Ps, P is the pressure; 1.0 ≤ θ ≤ 4.4, and θ is the surface coverage), and the BET isotherm within 0.04 ≤ ψ ≤ 0.40 (0.7 ≤ θ ≤ 1.5). When 1.59 ≤ P ≤ 250 Pa (θ ≤ 0.85), there is a linear relationship between θ and ln(P). This follows the Temkin isotherm, and it confirms the common experimental phenomenon that parts of the outgassing curves of vacuum systems are approximately P = constant × t−1. The results also indicate that some water is weakly trapped in the pores and the grain boundaries of technical metals, and it diffuses and desorbs very slowly.