The water outgassing rate of internal surfaces of vacuum systems

Transcription

1 Journal of Physics: Conference Series PAPER OPEN ACCESS The water outgassing rate of internal surfaces of vacuu systes Recent citations - Soe probles of the vacuu technique developent L N Rozanov and A N Sokolov To cite this article: L N Rozanov 2016 J. Phys.: Conf. Ser View the article online for updates and enhanceents. This content was downloaded fro IP address on 04/03/2018 at 16:14

2 23rd International Conference on Vacuu Technique and Technology The water outgassing rate of internal surfaces of vacuu systes L N Rozanov St. Petersburg Polytechnic University, , St. Petersburg, Polytechnicheskaya st. 29, Russia E-ail: lnrozanov@ail.ru Abstract. On the basis of experiental adsorption isother the ratio between the real and geoetrical surfaces was calculated and the aount of gas required to for a onolayer was defined. Siultaneous usage of Henry and Frendlih equations allowed to deterine the dependence of the heat of adsorption on the logarith of the absorbed gas aount A atheatical odel of puping of the vacuu systes with adsorbing walls is presented. This odel uses the paraeters of the vacuu syste and the dependence of the adsorption heat on the aount of the adsorbed gas.the conditions of the existence of regular puping regie are discussed. The structure database vacuu adsorption properties of aterials was proposed. The experiental data on the deterination of the adsorption outgassing rate were released. 1. Introduction Adsorption outgassing fro internal surfaces of vacuu systes is the ain coponent of the gas load in unheated vacuu systes. It is associated with desorption of water vapor adsorbed fro the atosphere of vacuu chaber during depressurization. There are diffusion [1] and the adsorption [2] odels of outgassing in vacuu systes. This work is devoted to the discussion of the adsorption odel. The work [2] is proposed to use the Tekin s isother equation with the linear dependence of the heat of adsorption on the aount of adsorbed gas. This work describes adsorption odel based on Frendlih s isother equation that describe the experiental curves well. Experiental curves of outgassing published in the works [1 3, 9] for the sae aterials, differ within an order of agnitude and are suitable for the calculation of gasload for vacuu systes, whose paraeters are siilar to the experiental units, in which the coefficients of outgassing were deterined. In this work for calculating of outgassing rates we are taken into account the gas adsorption properties of the surface and the paraeters of the vacuu syste. This odel is the base of ethod of foring the database to deterine the rates of adsorption outgassing of different vacuu settings. 2. The coposition of the surface The basic aterial for anufacturing of vacuu chabers is a non-agnetic austenitic stainless steel. Aluinu alloys are used less and titaniu alloys are used seldo. The eleental and quantitative coposition of the stainless 12X18Н10 surface before and after ion etching was analyzed by the photoelectron spectroeter Therofisher Scientific Escalab 250Xi.The paraeters of first etching cycle of surface were: energy of argon ions 3000 ev, spot s diaeter: 1, etching tie: 10 s, the Content fro this work ay be used under the ters of the Creative Coons Attribution 3.0 licence. Any further distribution of this work ust aintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 23rd International Conference on Vacuu Technique and Technology angle between the surface noral and the direction of the incident ions 40 ; in second cycle of etching the tie was increased to 60 s. The results of the eleental coposition of the surface are shown in table 1. Eleent Table 1. Eleental coposition of the surface. Before etching % of atos After 1 cycle of etching After 2 cycles of etching C O Cr Ni Ca 1.51 Fe A large aount of carbon before the first etching cycle indicates saple containation. The coposition of the ain coponent of steel 12X18H10T in table after the second cycle of etching, in general, coincides with the voluetric coposition of the steel. A noticeable aount of oxygen on the surface ay indicate the oxide presence. The results iply that for untreated stainless steel the adsorption of water vapor occurs on the surface polluted by carbon. After cleaning the surface adsorption takes place largely on the oxides. The results are confired by the data in work [9]. 3. Adsorption isothers The adsorption isother on untreated stainless steel (R a = 17 μ, T = 298 K) in the pressure range Pa deterined by voluetric ethod as in work [6] is well described by Frendlih equation: а(р) = р 0.34, (1) where a the specific aount of adsorbed gas, 3 Pa/ 2 ; p pressure of water vapor, Pa. The adsorption isother on electropolished stainless steel at T = 310 K deterined by dynaic ethod [4], in the pressure range of Pa is well-described by Frendlih equation: а(р) = р (2) Electropolishing leads to decreasing the pressure exponent. 4. The surface structure The real surface of the vacuu chaber is not sooth. It has roughness, cracks and other defects. The relation between the real and the geoetric surface ay be deterined using BET adsorption isother [5]. Considering that at the sooth surface can be placed / 2 of water olecules, we found that the real surface of the raw stainless steel is bigger in 5.8 ties than geoetric surface. We assue that the water condensation probability s = The heat of adsorption Henry and Frendlih isother equation gave siilar results over a wide range of pressures if a linear dependence of the heat of adsorption on the logarith of the aount of absorbed gas exists: where Q 1 1 d RT ln ; K K 0 Q RT Qа Q1 KQlg( a), (3) 2

4 23rd International Conference on Vacuu Technique and Technology According to equation (1) for water on the stainless steel: d = ; = At τ 0 = s and K = we receive that Q 1 = J/kol and K Q = J/kol. 6. The odel of puping of vacuu chaber with adsorbing walls Without puping of vacuu chaber (q p = 0) (figure 1) processes of evaporation q e and condensation q c of gas olecules occur at the free surface siultaneously. Figure 1. The odel of puping of the vacuu chaber with absorbing walls: 1 vacuu chaber; 2 the vacuu pup. For puping water vapor at 298 K in vacuu systes equation a = dp of Frendlih s adsorption isother can be used. Adsorption outgassing rate will be deterined by the forula q a q c T, V, F, S, p q e q p da dp d S p dp dt V F d p. 1 The odel of puping vacuu chaber with adsorbing walls can be presented: dp S p a d p ; 0; dt V V 1 d S p Vd F d p ; qa ; 1 V F d p p S Ke S1. Kp e The results of calculations on the (5) at Т = 298 K; М = 18 kg/kol; s = 1; d = ; = 0.34; V = 1 3 ; F = 10 2 ; S = /s; p f = 10 8 Pa; р s =10 3 Pa and K e = 1 give the graphs on the tie (figure 2). д (4) (5) q a, 3 Pa/s 2 p, Pa Figure 2. Dependence of pressure (1) and outgassing rate (2) of vacuu syste on puping tie. t, s 3

5 23rd International Conference on Vacuu Technique and Technology 7. Dependence of the adsorption outgassing rate on puping tie and paraeters of vacuu syste Adsorption outgassing rate during puping of the vacuu chaber in accordance with equations (5) depends on coefficients of the adsorption isother and paraeters of the vacuu syste. The equations for regular ode depend only on the surface area and the effective puping speed of chaber. They can be obtained by integrating (4) and neglecting the factors depending on the volue of the chaber, initial and ultiate pressures. Regular ode in figure 2 is executed at puping ties in range s.the dependence of the pressure on the puping tie in the regular ode is as follows: 1 1 d F pt ( ). S 1 t The dependence of the adsorption outgassing rate on puping tie in regular ode has the for: S p() t F d qa ( t). F S 1 t 8. The database of adsorption properties of vacuu aterials The database of adsorption properties of vacuu aterials ay be coposed of the coefficients d and of Frendlih s isother equation (table 2). Table 2. The adsorption outgassing rates of water vapor at T = 298 K. (6) (7) Material Stainless steel Х18Н9Т Preliinary processing d,[ 3 Pа] 2 Pа Reference Without processing This work Electropolishing [4] The calculations in regular ode (7) except d and fro table 2 should be copleted by the paraeters of vacuu syste (F, S). For exaple, when F c = 10 2, S = /s after 1 hour of puping the specific outgassing rate according the equation (7) is equal to Pа/(s 2 ). The value = 0.34 (table 2) is confired by our experients and works [1, 9]. 5. Conclusions Untreated stainless steel surface is polluted by carbon. Real surface is 5.8 ties bigger than geoetrical one. Water adsorption isother is described by Frendlih equation with paraeters d = and = The proposed atheatical odel in regular regie coplies experiental results. References [1] Li M and Dylla H F 1994 JVST A [2] Redhead P A 1995 JVST A [3] Nuss H E 1996 Vacuu [4] Jilnin V S, Jilnina L N and Kuzin А А 1974 Electronic Engineering. Ser. Electric vacuu and gas appliances [5] Rozanov L N 1994 Vacuu technique and technology [6] Rozanov L N and Akiov Y D 1985 Journal of Applied Cheistry

6 23rd International Conference on Vacuu Technique and Technology [7] Rozanov L N 2002 Vacuu Technique (London, New York: Taylor and Francis) [8] Garker B, Edellan Chr and Ehert M 1996 Vacuu [9] Chen J R, Huang J R, Hsiung G Y, Wu T Y and Liu Y C 1994 JVST A