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Physics of Outgassing 99 PHYSICS OF OUTGASSING J.L. de Segovia Instituto de Física Aplicada, CETEF “L. Torres Quevedo”, CSIC, Madrid, Spain Abstract This paper relates the physical bases of the gassing phenomena in ultra-high vacuum. The main phenomena discussed are: diffusion of gases through system walls and solids in vacuum, solubility of gases on solids and permeation as well as the phenomena of adsorption, desorption and absorption. 1. INTRODUCTION VACUUM permeated It has been recognised for many years that specie desorbed gassing and outgassing are processes controlling specie adsorbed the ultimate pressure and gas composition in species adsorption absorbed high- and ultra-high vacuum systems [1, 2]. The species mechanisms to stimulate outgassing are diverse: bulk (i) Thermal desorption. atoms (ii) Desorption induced by electronic diffusion transitions. Diluted (ii) Vaporisation of materials. specie (iv) Gas diffusion from the bulk and subsequent desorption. 30/Aug/2000 (v) Gas permeation through the walls. Perm eation: Solubility +Diffusion open-2000-273 In Fig. 1 the several mechanisms producing ATM OSPHERIC PRESSURE outgassing have been depicted. Fig. 1 Surface and bulk phenomena in vacuum. 2. PERMEATION OF GASES THROUGH MATTER Gases permeate through most materials, so gases present in the ambient air can permeate to vacuum chambers through the system walls. Permeation of gases is a combination of two physical processes: dissolution and diffusion. First, the gas dissolves in the solid and then it diffuses to the inner wall of the system. When the gas arrives at the inner surface, it desorbs to the vacuum volume. The dissolution phenomenon obeys Henry’s law: c = sP n where c is the concentration, P the gas pressure, s the solubility, and n depends on the material and is 1 for non-metals. Diatomic molecules dissociate upon dissolving. The concentration is measured in mbar.l or Pa.m3 and it is the amount of gas expressed in mbar.l or Pa.m3 measured at 293 K that dissolves in 1l or 1 m3 of substance. s is the amount of gas measured in STP that dissolves in 1 l or 1 m3 of substance at a pressure of 1033 mbar or 1.033 × 105 Pa, for n = 1 it is dimensionless and for n = 1/2 it has the dimensions of mbar0.5 or Pa0.5. After dissolving, the gas diffuses to the inner wall of the system according to Fick’s first law of diffusion, for the stationary case. The gas flow, Q, through an area of unit cross section and time unit, is given by: 100 dc Q = −D dx where D is the diffusion coefficient, which depends exponentially on temperature. E − = kT D D0 e where k is the Boltzmann constant; E is the activation energy for diffusion, T is the wall temperature and D0 is a coefficient of proportionality. We consider a wall of thickness d and very large area and at time t = 0, the two lateral surfaces are exposed to pressures P1 and P2 . The concentrations on both faces due to solubility will be: = n = n c1 sP c2 sP In addition, the gas flow will be: Ds()P n − P n Q = 1 d where n = 1 for non-metals, and n = 1/2 for diatomic molecules in metals. Ds = K is the permeation constant. As the equilibrium is reached in many cases after a long period of time, so the transition regime is determined by applying Fick’s second law: ∂ 2c ∂c D = . ∂x 2 ∂t This equation can be solved for specific cases useful in vacuum technology. Let’s consider a wall as before with the following boundary conditions: c = 0 0 < x < dt = 0 P = P1 P2 = 0 c = c1 x = dt > 0 c = 0 x = 0 t > 0 The total amount of gas permeated into the vacuum is given by: 2 = c1D − d Q t d 6D and the time constant: d 2 ta = 6D is the required time where Q is valid. This time constant is important to determine D. By representing Q as a function of t, the value of ta can be determined by extrapolating the straight line and then D can be found. Table 1 summarises the permeation properties of some materials and it can be seen that elastomers should be avoided at pressures lower than 10-6 due to the high permeation rates for most 101 gases, helium permeation is important only in glass systems while gas permeation is not of importance in metal systems at room temperature except for H2 but this gas is not in high concentrations in air. Table 1 Permeation of gases through materials [3] Metals Semiconductors Polymers Glasses Rare gases do not He and H2 through Ge All gases permeate all H2, He, D2, Ne, A, O2 permeate. and Si. polymers. are measurable through SiO2. H2: high rate through Ne and A not Water rate high. palladium, through Fe measurable. Many particularities Vitreous silica. by corrosion and H2 varies as with the material. All rates vary as electrolysis. 0.5 (pressure) . All rates vary as (pressure). 0.5 O2: permeates Ag. (pressure) . Permeation rates vary as (pressure)0.5. Table 2 shows an example of the permeation of an O-ring of standard dimensions: 0.5-m length and 4-mm diameter for the indicated materials. The O-ring when compressed takes a squared form 7-mm thick by 2-mm high so that the area of permeation is 0.5 m × 0.002 m2. Table 2 Permeation of air through an O-ring MATERIAL Permeation constant at Air rate Ultimate pressure 20ºC with a 1000 l/s pump -1 -2 -1 (mbar.l.s .mm.m .bar ) (mbar.l.s-1) (mbar) Perbunam 2 × 10-2 2 × 10-2 × 0.002 × 1/8 = 5.0 × 10-6 5.0 × 10-9 Viton 3 × 10-3 3 × 10-3 × 0.002 × 1/8 =7.7 × 10-7 7.5 × 10-10 Silicone 2 × 10-1 2 × 10-1 × 0.002 × 1/8 =7.5 × 10-5 7.5 × 10-10 3. DIFFUSION FROM A SEMI-INFINITE WALL An important case for vacuum technology is the degassing of a semi-infinite wall with an initial concentration c1 that is exposed at t = 0. The boundary conditions are now: c = c x > 0 t > 0 P = 0 1 x c = 0 x = 0 t > 0 The gassing rate at time t is given by: − 1 = < 2 (()π −1/ 2 ) Q c1 d t . When the vacuum is connected to a pump of speed S, the pressure is given by: 1 1 Q 2 −1 − P = = c D S ()π t 2 . S 1 The total amount of gas removed from the slab is: 102 1 1 t − ∂c 2 2 Q = ∫ D dt = 2π c ()Dt . T ∂ 1 0 t x=0 Note that in a log-log scale the slope is 0.5. The same amount of gas will diffuse from the atmosphere in an initially clean wall with a final concentration of c1. 4. DIFFUSION FROM A FINITE WALL Another important case in vacuum engineering is the degassing from a finite wall, with uniform initial concentration c1 and both faces are exposed to vacuum at time zero. In this case the boundary conditions to solve Fick’s second law d are: c = c1 0 < x < dt = 0 P = 0 P = 0 c = 0 x = 0, x = dt > 0 x The instantaneous gassing rate is given by: ()+ π 2 − 2i 1 Dt ∞ 2 = 2c1D 4d Qx=d ∑e d i=0 with 2 = 4d ta π 2 D which is the characteristic parameter. According to the value of time with respect to this parameter, the instantaneous gassing rate is given by: (1) t > 0.5ta π 2 = 2Dc1 − D Qx=0,d e .t . d 4d 2 (2) t < 0.5ta π = 2Dc1 ta Qx= d . 0, d 16t The degree of outgassing e.g. the remaining gas in the wall with respect to the initial number of molecules, is given by the expression: 103 t t t −9 −25 N(t) 8 e ta e ta f = = e ta + + + ....... 2 N 0 π 9 25 Figure 2 shows the required time to reduce the initial outgassing rate by 108 e for the case of t > 0.5ta. Stainless steel 7 10 time required to Q=Q0/3 2 2 Example ta=4.d /π .D 106 Let’s determine the gas diffusing time, s from a finite nickel wall inside a 105 vacuum system with the following [ ] parameters 4 : 104 Thickness: 2.10-3 m 103 ∆: 8.103 kg.m-3 2 -5 10 400 600 800 1000 1200 1400 1600 1800 2000 H2 concentration: 10 (10 ppm) temperatue, K Fig. 2 Time required to reduce Q0 by a factor of e Determination of the outgassing rates -5 × -3 -2 -3 Mass of dissolved H2 10 8.10 = 8.10 kg.m Molar concentration: 8.10-2 kg.m-3/2 kg.kmol-1 = 4.10-2 kmol.m-3 Particle density number: 4.10-2 kmol.m-1.6.1026 kmol-1 = 2.4.1025 m-3 Table 3 Calculation of the diffusion rate from a nickel wall Temp. Diffusion Time Time to reduce Gas flow Gas flow at t -t/t (K) coefficient constant concentration, at t = 0 Q(t) = Q0.e a -7 (-4350/T) 2 Β 2 D = 2.10 .e ta = 4d / D f = N/N0, by (s) Q0 = (2D/d)n0, (m2.s-1) (s) (kg.m-2.s-1) (kg.m-2.s-1) 0.1 10-3 10-6 -13 7 8 8 15 15 -t/t 300 1.01.10 1.6.10 3.3.
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