desorption and materials stability

Vacuum/volume 41/numbers 7-9/pages 2089 to 2092/1990

0042-207X/9053.00 + .00 Pergamon Press plc

Printed in Great Britain

Gas adsorption/desorption and materials stability M B e r g o g l i o , A C a l c a t e l l i , M Plassa and P C h e n * , Istituto di Metrologia "G Colonnetti', Torino, Italy

The desorption of substances from austenitic s t a i n l e s s steel (25% Cr, 20% Ni) is considered from the standpoint of the stability of this material as the constituent of artifacts used as mass standards. The out-gassing rate was measured on samples subjected to thermal treatments in vacuo or in nitrogen atmosphere at a lO00mbar pressure in the 300-600"C temperature range. The out-gassing rate was determined on treated samples also after re-exposure to atmospheric air. In addition, some samples, having clean surfaces owing to previous out-gassing, were exposed to nitrogen at room temperature. Their total out-gassed amount was far lower than in the other cases.

1. Introduction The development of metrology and the increased precision of measurements have brought into evidence the connection between the attainable accuracy and the stability of the materials used for standards construction. In the different metrological sectors, the stability of different properties is significant; in a number of instances, for example in the case of solid density standards and of low pressure gauges, the stability of materials connected with gas-solid interactions is of significance. It is especially important in mass metrology, since the primary standard of mass itself is an artifact (the International Prototype Kilogram) from which there are derived the reference standards (kilograms, multiples and submultiples) on which all mass measurements in science, technology and commerce are based. The mass of such standards may undergo, in the course of years, variations caused by gas adsorption and desorption on the surface or by gas diffusion from the bulk interior 1"2. Thanks to recent developments in weighing techniques, measurement repeatability is now of the order of I0 -~°, so that variations in mass standards, which are of the order of 0.1 # g e m -2, are easily detected. As a rule, reference mass standards are made of austenitic stainless steel, whose adsorption/desorption properties have been extensively investigated3 6 in the field of vacuum technology, mainly to reduce the out-gassing rate and thus obtain increasingly low residual pressures. As the stainless steels studied had nickel-chromium contents lower than those mostly used in the metrological field, it was judged advisable to determine the out-gassing characteristics of stainless steel with 25% chromium and 20% nickel (AISI 310). Previous work 7 had brought into evidence a permanent mass decrease caused by heat treatments in vacuo. In the first treatment, such reduction may amount to 1 #g cm -2, whereas it is smaller in the following cycles, owing to the previous elimination of contamin*Guest scientist from Tsinghua University, Beijing (People's Republic of China) under a fellowship by the International Centre for Theoretical Physics (ICTP), Trieste (Italy).

ants from the surface and hydrogen out-gassing from the bulk. In this work, the amount of substance out-gassed from the surface of the thin samples during vacuum heat treatments at various temperatures was determined; subsequently, the effect of exposure to nitrogen atmosphere at high temperatures and at room temperature was investigated, and the effect of the nitrogen treatment was compared with that of the thermal treatment in vacuo. The nitrogen atmosphere treatment was considered a possible alternative to the present mass standards maintenance in air from the standpoint of better stability in time, and the reason for choosing nitrogen as a possible maintenance atmosphere was to be found in the order of gas replacement on the surface, the usual sequence being N 2, CO, H2, and Ar.

2. Experimental apparatus and procedure The adopted procedure applied the dynamic method s for measuring gas flow by pressure difference across a cylindrical conductance (C = 0.2 L s -~ for nitrogen). The flowrate was determined by means of the formula dP(t)

q(t) = C P ( t ) + ( V + a) - dt

where P(t) is the pressure measured at time t in the sample chamber, whose geometrical volume is V, while (V + a) is the equivalent volume. For the measurement of total pressure, Bayard-Alpert vacuum gauges were used, which were calibrated in situ by means of a spinning rotor gauge previously calibrated against a primary device9. The evolved gases were analysed by means of a quadrupole mass spectrometer, which was itself calibrated against Bayard-Alpert gauges. The samples (cylindrical tubes of 6 mm in diameter, 15 mm length and 0.1 mm thickness) were obtained by rolling up a metal sheet and were de-greased with ethyl ether. Some of the samples were directly subjected to out-gassing measurements in vacuo; others were previously treated in vacuo or in nitrogen atomsphere at 1000 mbar at various temperatures for about 24 h. The samples attained the test temperature in a couple of minutes, and total and partial pressures were 2089

M Bergoglio et al: Gas a d s o r p t i o n / d e s o r p t i o n and materials stability

recorded for about l h. Some samples previously baked in vacu o or in nitrogen were re-exposed to room air for about one month to determine re-adsorption. During total pressure measurements, the quadrupole analyser was isolated. So as to avoid the effect of its selective pumping speed on measurement results. The effect of gas adsorption/desorption of the chamber wall surface on measurement results was taken into account by using an equivalent volume (V + a), under the hypothesis that Henry's law is obeyed. The equivalent volume was determined for some gases (H2, N2, CO) by filling the sample chamber with the testing gas to a pressure in the 10 4 mbar range and by recording pressure values vs time when the system was being pumped down.

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3. Experimental results For N 2 and CO the equivalent volume resulted to be nearly the same as the geometric volume of the chamber. As the interaction of H 2 with a chamber surface is complex, different equivalent volumes were determined in different pressure ranges (from 14 L in the 10 - 6 mbar range to 100-1300 L in the 10 - 7 mbar range). The evolved gas flowrate values were calculated from total pressure measurements, and the flowrate values of the main components were determined from partial pressure measurements. Some typical results are summarized in Figures 1-3. At temperatures ranging from 300 to 600°C, the total outgassed amount ranged from 2.0 to 5.3 x 10-4mbar L cm -2 [Figure l(a)]. As expected, in a second cycle the out-gassing rate was considerably lower, as shown by curve 5. The results in Figure 3 (curves 1 and 2) show that the decrease in the out-gassing rate after the first heat treatment is not only due to the removal of the hydrocarbon contaminants, but also to the decrease of all the main components. Another series of samples were heated in nitrogen, at a pressure of 1000mbar at various temperatures, before outgassing rate measurements at the same temperatures as the treatments• Also in this case [Figure l(b)] the flowrate increased with increasing temperature, but the total out-gassed amount was lower than for non-treated samples, as it ranged from 4.6 x i0 -5 to 2.3 x l0 -4 mbar L cm -2. Some of the samples used for out-gassing rate determinations, either subjected to previous treatments or not, were maintained in room air at least for one month, after which time desorption rates were re-determined at 400 and 500°C [Figure 2(a)]. This long exposure to the atmosphere made the surfaces similar; the total out-gassed amount at 500°C for samples (either N 2 treated or not) re-exposed to atmosphere was !.5 x 10-4 mbar L cm -2, i.e. midway between the amounts in a first and in a second heat treatment cycle. Figure 2(b) shows the out-gassing rate for vacuum heattreated samples after exposure to pure nitrogen at room temperature for about four days. Nitrogen treatment not only keeps the surface clean, but makes it cleaner, as is shown by the fact that the total out-gassed amounts at the temperatures considered lay between 4.5 and 9.3 x 10 -6 mbar L em -2. In Figure 3 the out-gassing rates of different molecules in two successive out-gassing cycles are compared with the results of samples treated in nitrogen at the same temperature as that of out-gassing determinations. The curves of Figure 3 show clearly that sample exposure to pure nitrogen at high temperatures is 2090

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Figure 1. Out-gassing rate for 25/20 stainless steel under different conditions. (I) T=300°C; (2) T=400°C; (3) T=500°C; (4) T = 600°C; (5) T = 300°C, 2nd cycle• (a) Vacuum bake-out of untreated samples; (b) vacuum bake-out of samples previously treated in nitrogen atmosphere at the same temperatures as vacuum bake-out.

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almost as effective as vacuum bakeout, and that the composition of the desorbed gas is very similar after vacuum and nitrogen treatment.

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Figure 2. Out-gassing rate values. (a) Samples previously subjected to various treatments, then to out-gassing rate determination, and finally exposed to room air for one month (N 2 treated samples, (1) T=400°C, (2) T=500°C; untreated samples, (3) T=500°C); (b) samples of Figure 2(a), after exposure to nitrogen at 1000mbar at room temperature (samples previously N 2 treated, (l) T = 300°C, (2) T = 400°C; untreated samples, (3) T = 400°C).

Surface cleaning by means of a thermal treatment in v a c u o is of interest in mass metrology, as this technique is effective without being destructive• In fact, with artifacts to be used as measurement standards, one must not apply techniques that, though more powerful (e.g. glow discharge), cause the removal of metal atoms. The result of the gas release measurements given in the present paper are collected in Figure 4, which shows also the corresponding mass variations calculated under the assumption of an average molar mass of 28 g m o l - 1. The nitrogen treatment at 300-400°C at the atmospheric pressure (curve 3) appears to be a satisfactory alternative to heat treatments in v a c u o . Still more promising is the result concerning the maintenance in nitrogen of samples previously vacuum treated, as their surfaces are the cleanest of all those examined (curve 5). The results obtained provide the necessary support for an experiment on the cleaning and maintenance of AISI 310 2091

M Bergoglio et al." Gas adsorption/desorption and materials stability

stainless-steel mass standards according to new methods, by which both vacuum and nitrogen treatments should be used and out-gassing rates should be related to permanent mass changes detectable by gravimetric techniques.

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The authors wish to thank G Coggiola, L Marzola and G Rumiano for their contribution in the construction o f the experimental apparatus. P Chen gratefully acknowledges the finanacial assistance given by ICTP.

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2092

M Plassa, M Tolomelli and A Torino, Proc 10 Int ConflMEKO TC3 Meas Force Mass, Kobe, p 85 (1984). 2 M Plassa, Bull Bur Nat Metrol, 76-77, 27 (1989). 3 G Moraw and R Dobrozemsky, Japan J Appl Phys, Suppl 2, 1, 261 (1974). 4 R Calder and G Lewin, Br J Appl Phys, 18, 1459 (1967). 5 R S Barton and R P Govier, Proc 4th Int Vacuum Congr, Manchester, The Institute of Physics and the Physical Society, London, UK, 775 (1968). 6 R Nuvolone, Le Vide, 193, 171 (1978). 7 M Bergoglio, A Catcatelli, M Plassa and A Torino, Vuoto, 17, 272 (1987). 8 p Chen, Technical report IMGC R292, Torino, Italy (1989). 9 M Bergoglio, A Calcatelli, L Marzola and G Ruminao, Vacuum, 38, 887 (1988).