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The sorption of hydrogen by barium getters
The Sorption of Hydrogen by Barium Getters P. DELLA PORTA
and S. ORIGLIO
S.A.E.S. Getters, Research Laboratory, Milan, Italy Sorption of hydrogen by barium films is shown to be made up of surface adsorption of the gas followed by its diflusion to the interior. The velocity of surface adsorption is studied at low pressures and its dependence on surface area, pressure and temperature is considered. The velocity of diflision is examined for pressures and film thicknesses which are near to those in current industrial use. In this case too, particular attention is paid to the influence of temperature, as well as to the influence of parameters suck as temperature of condensing walls, residual pressure at evaporation and time of flashing of the getter, all of which are of considerable practical interest. Introduction
In fact if the pressure is sufficiently high and the temperature sufficiently low, so that the hydrogen molecules which are adsorbed on the surface are in excess of those which can diffuse to the interior, then the latter will control the sorption velocity. On the other hand if the pressure is sufficiently low and the temperature sufficiently high so that the molecules sorbed on the surface may quickly diffuse towards the interior, the sorption velocity will depend only on the velocity with which fresh molecules can be adsorbed. Typical results for these different conditions of sorption are shown in Fig. 1. Curve (a) is distinctly decreasing with time, this is typical of a diffusion process, whilst curve (b) describes the behaviour of the velocity of surface adsorption. In both cases the sorption velocity was measured by keeping constant the chosen hydrogen pressure on the getter. This point having been elucidated, in the present work the previously published results will be integrated and completed according to the following scheme : 1. Study the velocity of surface adsorption under those conditions which make it constant, that is to say on the basis of curves of type (b) in Fig. 1. 2. Study the diffusion phenomenon (curve (a) ) of Fig. 1, on films whose thickness is of practical interest, to control if the results precedently obtained for thinner films are also applicable in this case. 3. Study the influence on diffusion (which is of prominent practical interest) of parameters of industrial importance, such as film thickness, temperature of condensing walls, residual pressure at evaporation and time of flashing of the getter.
In a recent communicationl, data concerning the sorption of hydrogen by barium films were presented. These data made it possible to furnish a model of the sorption analogous to that previously described for nitrogen and carbon monoxidezV 3, 4. Through a kinetic study of the sorption it was in fact possible to distinguish a surface adsorption phenomenon from one of diffusion to the interior of the getter film, due to the differences in shape of the velocity of sorption curves depending on the pressure and temperature region investigated. These differences in behaviour are due to the fact that the overall phenomenon (made up of surface adsorption of the gas and of its subsequent diffusion to free barium atoms) proceeds with the velocity of the slowest one.
i :
Q
2 I
c
FIG. 1. Velocity of sorption
for Hz as a function pressures.
I = time in set pm = manifold pressure in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure on getter
of time at two diRerent
Experimental
The apparatus employed in the present experiments is shown in Fig. 2. The method of measurement is that described in a number It is based on the evaluation of previous communications. of the flow of gas, under Knudsen conditions, through a
100 cm2 2mg 298°K 298°K (a) 2.5 x 10-4 Torr (b) 3.7 x 10-6Torr 26
.:. 7
27
The Sorption of Hydrogen by Barium Getters
!, :,, :I
To vo&um
FIG. 2. Sorption apparatus. capillary of known conductance. The dimensions of the capillary used in the present experiments are length 9.5 cm and diameter 2.02 mm. All experiments were carried out at a constant pressure over the getter deposit. The film was evaporated from a stabilized barium-aluminium alloy getter (St2/7 x 7) which is capable of liberating 2 mg of barium. Getters of different types were only used when greater quantities of barium had to be evaporated, such as when studying the influence of surface area and thickness of the barium film. Hydrogen pressure measurements with an ionization gauge are rather misleading due to the appreciable pumping by the gauge. It is well known that a tungsten filaments can bring about the dissociation of hydrogen molecules and that the atomic hydrogen thus formed is easily sorbed on the enclosing walls. The ionizing electron current also produces an appreciable effect6. 7. Nevertheless, by using very low electron ionizing currents the magnitude of the chemical pumping by the electron current may be kept to very low values. In fact all measurements previously carried out and those which will now be considered were obtained using an electron ionizing current of 0.01 mA ; the temperature of the filament under such conditions never rose above 1500°K. The measurements at various temperatures were carried out by employing suitable thermostatic baths. The hydrogen employed was supplied by British Oxygen Gases Ltd., London, and its impurity content was less than 2 p.p.m.
1x10'
FIG.
3. Velocity of sorption for H2 as a function of time.
t = time in set pm = manifold pressure in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure on getter
100 cm2 2mg 298 “K 298°K 3.7 x lo-6 Torr
evidently not very mobile, and in all probability the initial decrease in velocity shows the dependence of the initial chemisorption on the degree of coverage of the metal surface exposed to the gas. One can deduce that a really constant velocity can only be achieved when a first layer has been almost completely formed. If one accepts such an hypothesis the constant velocity should be referred to the adsorption not on the metal but on the hydride already formed. An accurate investigation of the initial decreasing region must be carried out at much lower pressures. Nevertheless it is quite clear that the surface adsorption velocity which is effectively important in gettering is that of adsorption on the product of reaction already formed. Therefore, all the data presented in this section will refer to the velocity of surface adsorption which will be determined from the value corresponding to the horizontal region of the curve rather than from the initial value. The velocity of surface adsorption as a function of the apparent area of the barium film is shown in Fig. 4. The 7. 5-
5.0% 0 x >
2.5-
Results and discussion Surface adsorption
velocity
If the behaviour shown in curve (b) of Fig. 1 is closely analyzed, the existence of a small initial decrease in velocity is observed. This is followed, Fig. 3, by a stabilization to a constant velocity value. The quantity of gas adsorbed during this initial decreasing region is of the order of 1016 molecules. This may probably be related to the formation of an initial hydride layer. The hydrogen molecules sorbed in such a manner are
e 0
FIG. 4. Initial sorption velocity for Hz as a function of the barium film. = velocity in molecules see-1 k = surface area in cm2 Weight of evaporated barium per cm* Temperature of walls at flashing Temperature of sorption Constant pressure on getter
of the surface area
0.02 mg 298 “K 298 “K 1 x l&5 Torr
28
P. DELLA PORTA AND S. ORIGLIO
measurements were carried out at a pressure of 1 x 10-s Torr at room temperature. The quantities of evaporated barium were made to vary in such a manner as to maintain a nearly constant film thickness of 2 x 10-s g cm-z. Since, as is to be expected, the velocity of adsorption is found to be directly proportional to surface area, it may be referred to unit area. At the pressure considered, 1 x 10-s Torr, the velocity of surface adsorption is 6 x 1011 molecules set-1 cm-? The velocity of surface adsorption as a function of pressure is shown in Fig. 5. The measurements were carried out at
/
j
I
I
2
I
I
3
4.
A
5
ps x10-s FIG. 5. Initial Y
sorption
velocity for HZ as a function on the getter film.
= velocity in molecules
of the pressure
set-1
pe = constant pressure on getter in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption
:II /
93%
1
I
I
I
of sorption for HZ as a function of time at different increasing temperatures of sorption.
Apparent surface area Weight of evaporated Temperature of walls Constant pressure on
V
473aKi
,423’K \ ;
t = time in set pm = manifold pressure in Torr
-I-
0
(
: :
FIG. 6. Velocity
0
~
100 cm* 2mg 298 “K 298 “K
room temperature on a film of 2 mg which had an apparent surface area of 100 ems. The solid circle indicates the velocity referred to unit area, as calculated from Fig. 4. Since the velocity of surface adsorption is found to be directly proportional to the pressure, the mechanism appears to be one of molecular adsorption. Although the hydrogen molecules sorbed may subsequently dissociate into atoms, this does not seem to be the rate determining process. Since the adsorption velocity is directly proportional to the pressure, all measurements may be referred to constant pressure. Thus the velocity becomes 6 x 1016 molecules see-icm-2 Torr-1. Bearing in mind the fact that the number of hydrogen molecules which strike unit area of the barium film in unit time at room temperature is 1.99 x 1021 molecules XCicm-2 Torr-1, the sticking probability is : s = 4.17 x 10-s. This value appears rather low ; it is nevertheless of the same order of magnitude of that previously published by Perdjik* who finds s = 11.25 x 10-s at room temperature. A comparison with the values published by Wagener is rather more difficult as they vary between about 1 x 10-3 [9,*01 and 2 x lo-6.1111 The influence of temperature on the velocity of surface adsorption is shown in Figs. 6 and 7. The curve in Fig. 6 was obtained with a film of 2 mg having an apparent surface area of 100 cm2 and with a
of film barium at flashing getter
100 cm2 2mg 298 “K 3.7 x 10-e Torr
constant pressure of 3.7 x lo-6Torr above the getter. The temperature of the getter was varied by a thermostatic bath in which the sorption vessel was immersed. The first horizontal region of the curve corresponds to the velocity of surface adsorption for hydrogen at a temperature of 293°K. After the consistency of this velocity had been ascertained for a sufficiently long period of time the temperature was increased to 373°K. Initially, whilst the temperature was increasing, the velocity apparently decreased, in all probability due to the release of gas, This indicates that an appreciable quantity of hydrogen is lightly held on the surface. However, after a given time, the velocity increased once again to a value above the original one and then remained constant. The same behaviour was subsequently found during the process of increasing the temperature to 423 and 473°K. At all these temperatures a This constant velocity of surface adsorption was attained. is shown by the various horizontal tracts of the diagram.
0
FIG. 7. Velocity
of sorption for Hz as a function of time at different decreasing temperatures of sorption.
t = time in set pm = manifold pressure in Torr Apparent surface area Weight of evaporated Temperature of walk Constant pressure on
of film barium at flashing getter
100 cm2 2mg 298 “K 3.7 x IO-6 Torr
The Sorption of Hydrogen by Barium Getters
29’
Furthermore if the temperature is slowly decreased back to 293°K the velocity also falls to its original value, which confirms its constant value at a given temperature. The curve in Fig. 7 was obtained under similar conditions to the above but the temperature was made to decrease from its highest to its lowest value. The previously observed degassing does not occur in this case and a stepwise decrease in the curve is obtained, each step corresponding to a constant velocity of surface adsorption at the given temperature. From the sorption velocity values given in Figs. 6 and 7, it is possible to calculate the energy of activation for the process from the slope of the curve in Fig. 8 relating the
2.5
t x103 2.001
I
I.5
2.0
2.5
30
I 35
I
4.0
velocity of hydrogen on barium temperature. Y = velocity in molecules se-t = constant pressure on getter in Torr P T = sorption temperature in “K Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
films as a function
of
100 cm* 2mg 298 “K 3.7 x l&6 Torr
logarithm of the sorption velocity to the reciprocal of the absolute temperature. This is found to be 1880 cal mole-i. This value is slightly higher than that previously found, 1500 cal mole-1,111 which was calculated from the initial velocity of sorption curves obtained at higher pressures and of the type shown in Fig. l(a). From the comments made in the introduction the present result should be considered as the more accurate. Dijiuion
velocity of hydrogen as a function films at various temperatures.
velocity
As pointed out in the introduction, this section is essentially concerned with the verification of the applicability of our previous results to thicknesses and pressure conditions comparable to those used in industry. Hence the results in this section refer to barium films of 2 mg having an apparent surface area of 15 cma, and the velocity of sorption was measured from curves of the type shown in Fig. l(a). The influence of temperature on diffusion is shown in Fig. 9, which gives the velocity of sorption at various temperatures as a function of time. By graphical integration it is possible from such curves to evaluate the quantities of hydrogen sorbed at various given t times, Qt = V(t)dt, and by plotting these quantities s 0 against the square root of the corresponding times to obtain curves such as shown in Fig. 10. These curves present, after
= velocity in cm3 Torr secrl = timein set kpparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
of time by barium
v
I/TxIO-~
FIG. 8. Sorption
FIG. 9. Sorption
15 cm2 2mg 298 “K 2.5 x 10-4 Torr
It is well known the initial region, an almost linear portion. that such linearity characterizes the quantity of gas which diffuses through a surface, on which the concentration of gas remains constant, when diffusion is assumed to occur in a semi-intinite medium. At constant pressure and with this approximation the slopes of the straight lines (dQt/dt) are, through the diffusion It is therefore possible coefficient, a function of temperature. to evaluate the energy of activation for the diffusion process by considering the dependence of D = DO e - j% on temperature. The logarithms of dQt/dt as a function of temperature are plotted in Fig. 11. The solid circles are the values as obtained from Fig. 10, whilst the open circles are the results previously published1 for thinner films (2 mg/lOO cm2). By comparing the two sets of points one draws confirming evidence for the hypothesis put forward concerning the kinetics of sorption, and in particular for the unique nature of the process even for getter films of very different thicknesses. The activation energy has a value of about 3000 cal mole-i. A discussion concerning the consequences of such a low potential barrier against diffusion of hydrogen towards the interior of the film has been previously put forwardr. Parameters gettering
of industrial
consequence
and their influence on
The velocity of sorption is shown as a function barium film thickness in Fig. 12.
of the
30
S. ORIGLIO
P. DELLA PORTA AND
O-
5I !
~
I
I
2
/ I
3
4
tx103 FIG. 12. Sorption
o-
”
= velocity in cm3 Torr sea-1
t
= time in set
Apparent surface area of film Temperature of walls at flashing Temperature of sorption Constant pressure on getter Weight of evaporated barium
.5-
0
velocity for HZ, for different quantities barium, as a function of time.
of evaporated
15 cm2 298 “K 298 “K 2.5 x lo-4Torr (a) 18.7 mg (b) 13.8 mg (c) 8.3 mg (d) 2.0 mg
1
FIG. 10. Time
dependence of the quantity of hydrogen barium films at various temperatures.
= timein set Qt = quantity sorbed in cm3 Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
sorbed
by
t
15 cm2 2mg 298°K 2.5 x IO-4 Torr
L-
110-!
2
10. I
2
FIG. 11. Temperature
3
4 I/TxIO-~ dependence of hydrogen films.
Q, = quantity sorbed in cm3 Torr t = timein set T = sorption temperature in “K Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
6
5 diffusion
into barium
2mg 298 “K 2.5 x 10-4 Torr
It illustrates how the velocity of diffusion (not only the capacity) increases with fIm thickness. Bearing in mind the granular structure of the !ilm this result may be attributed to the different surface areas from which diffusion originates.
FIG. 13. Sorption
velocity condensing
3
4
5
1X103 for Hz, for different temperatures walls, as a function of time.
= velocity in cm3 Torr set-t = time in :ec apparent surface area of film Weight of evaporated barium Temperature of sorption Constant pressure on getter Temperature of walls at flashing
of
”
15 cm2 2mg 298 “K 2.5 x IO-4 Torr (a) 293°K (b) 473°K
This could imply an extremely rapid surface diffusion such as to guarantee a high degree of coverage of the real total surface area of the getter film. It would seem to be an advantage, therefore, for hydrogen
The Sorption of Hydrogen by Barium Getters sorption, to employ a film of an appreciable thickness, contrary to what had been previously found for other gases. Similar observations may also be made on the influence of other factors which are able to affect the structure of the film. Thus the influence of the temperature of the condensing walls on the velocity of diffusion is shown in Fig. 13. A decrease of the velocity of sorption may be produced by increasing the substrate temperature during film condensation, so decreasing the real total surface area of the film (formation
31
of larger crystals and sintering of the film). The curves referred to above were obtained at room temperature and at a pressure of 2.5 x 10-4 Torr on 15 cm2 getter films. The curves in Fig. 14 were produced under the same conditions as those in Fig. 13 except the substrate was held at room temperature during evaporation and the evaporation rate was varied. In this case the effect produced arises from the difference in the internal surface area of the films obtained using different times of evaporation. Finally the influence of the residual pressure during barium evaporation is shown in Fig. 15. These curves too were obtained on films of 2 mg, having an apparent surface area of 15 cm2, at room temperature and at a constant pressure over the getter of 2.5 x 10-4 Torr It appears that the influence of the residual pressure is primarily due to a preliminary poisoning of the getter, in the same manner as had been previously found for nitrogen and carbon monoxide. Conclusions
t x103 FIG. 14. Sorption Y =
t
velocity
for Hz, for different function of time.
times of flashing,
as a
velocity in cm3 Torr set+
= time in set
Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure over getter Time of flashing
(a) (b) (c)
I5 cm2 2mg 298 “K 298 “K 2.5 x 10-4 Torr 5 set 12 set 30 set
The velocity of surface adsorption and the velocity of diffusion within a barium film have been studied independently. With reference to surface adsorption it has been possible to show two distinct phases, in the first of which the velocity decreases whilst in the second it remains constant. Since the first phase corresponds to the adsorption of a quantity of gas comparable to that required for the formation of a monolayer, it has been deduced that the constant velocity of surface adsorption characterizes the adsorption of gas on the reaction product already formed. This is in agreement with the relatively high mobility of the adsorbed phase which would be inexplicable if one assumed direct chemisorption on the metal ; it also justifies the partial degassing occurring when increasing the temperature of the film. Concerning the diffusion to the interior of the film, the picture previously put forward for thinner films has been verified for films of greater thickness. Finally all the measurements carried out to study the influence of various parameters on the efficiency of gettering have contributed to show the importance of a barium film of high porosity. References
0
I.5
3.0
4.5
6.0
7.5
t xl03 FIG.
15. Sorption
velocity for Hz, for different residual flashing, as a function of time.
Y = velocity in cm3 Torr secrl I = time in set Apparent surface area Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure over getter Residual pressure during flashing
pressures
15 cm* 2mg 298 “K 298 “K 2.5 x IO-4 Torr (a) 1 x 10-S Torr (b) 1 x lo-4 Torr (c) 2 x IO-3 Torr
at
1 P. della Porta and S. Origlio ; Trans. Znt. Symp. on Residual Gases ; Coma, Vacuum 10, 227 (1960). 2 P. della Porta and F. Ricca ; 5th Nat. Symp. A. VS., S. Francisco (1958), Pergamon Press (1959), p. 25. 3 P. della Porta and F. Ricca ; Trans. Znt. Symp. on Residual Gases ; Como, Vacuum 10, 215 (1960). 4 P. della Porta and F. Ricca ; Le Vide 85, 1 (1960). 5 I. Langmuir ; J. Amer. Chem. Sot. 37, 417 (1915). 6 P. della Porta, S. Origlio and E. Argano ; Trans. Znt. Symp. on Residual Gases, Coma, Vacuum 10, 194 (1960). 7 L. Holland ; J. Sci. Znstrum. 36, 105 (1959). * J. J. B. Fransen and H. J. R. Perdjik ; Philips Tech. Rev. 19, 290 (1957/58). 9 S. Wagener ; J. Phys. Chem. 60, 567 (1956). 9 S. Wagener ; J. Phys. Chem. 60, 267 (1957) ; J. Phys. Chem. 61, 267 (1957). 10 S. Wagener ; Brit. J. AppI. Phys. 2, 132 (1951). 11 S. Wagener ; Proc. 4th NatI. Con. on Tube Techniques (1958), p. 1 (1958), New York University Press (1959).
and S. ORIGLIO
S.A.E.S. Getters, Research Laboratory, Milan, Italy Sorption of hydrogen by barium films is shown to be made up of surface adsorption of the gas followed by its diflusion to the interior. The velocity of surface adsorption is studied at low pressures and its dependence on surface area, pressure and temperature is considered. The velocity of diflision is examined for pressures and film thicknesses which are near to those in current industrial use. In this case too, particular attention is paid to the influence of temperature, as well as to the influence of parameters suck as temperature of condensing walls, residual pressure at evaporation and time of flashing of the getter, all of which are of considerable practical interest. Introduction
In fact if the pressure is sufficiently high and the temperature sufficiently low, so that the hydrogen molecules which are adsorbed on the surface are in excess of those which can diffuse to the interior, then the latter will control the sorption velocity. On the other hand if the pressure is sufficiently low and the temperature sufficiently high so that the molecules sorbed on the surface may quickly diffuse towards the interior, the sorption velocity will depend only on the velocity with which fresh molecules can be adsorbed. Typical results for these different conditions of sorption are shown in Fig. 1. Curve (a) is distinctly decreasing with time, this is typical of a diffusion process, whilst curve (b) describes the behaviour of the velocity of surface adsorption. In both cases the sorption velocity was measured by keeping constant the chosen hydrogen pressure on the getter. This point having been elucidated, in the present work the previously published results will be integrated and completed according to the following scheme : 1. Study the velocity of surface adsorption under those conditions which make it constant, that is to say on the basis of curves of type (b) in Fig. 1. 2. Study the diffusion phenomenon (curve (a) ) of Fig. 1, on films whose thickness is of practical interest, to control if the results precedently obtained for thinner films are also applicable in this case. 3. Study the influence on diffusion (which is of prominent practical interest) of parameters of industrial importance, such as film thickness, temperature of condensing walls, residual pressure at evaporation and time of flashing of the getter.
In a recent communicationl, data concerning the sorption of hydrogen by barium films were presented. These data made it possible to furnish a model of the sorption analogous to that previously described for nitrogen and carbon monoxidezV 3, 4. Through a kinetic study of the sorption it was in fact possible to distinguish a surface adsorption phenomenon from one of diffusion to the interior of the getter film, due to the differences in shape of the velocity of sorption curves depending on the pressure and temperature region investigated. These differences in behaviour are due to the fact that the overall phenomenon (made up of surface adsorption of the gas and of its subsequent diffusion to free barium atoms) proceeds with the velocity of the slowest one.
i :
Q
2 I
c
FIG. 1. Velocity of sorption
for Hz as a function pressures.
I = time in set pm = manifold pressure in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure on getter
of time at two diRerent
Experimental
The apparatus employed in the present experiments is shown in Fig. 2. The method of measurement is that described in a number It is based on the evaluation of previous communications. of the flow of gas, under Knudsen conditions, through a
100 cm2 2mg 298°K 298°K (a) 2.5 x 10-4 Torr (b) 3.7 x 10-6Torr 26
.:. 7
27
The Sorption of Hydrogen by Barium Getters
!, :,, :I
To vo&um
FIG. 2. Sorption apparatus. capillary of known conductance. The dimensions of the capillary used in the present experiments are length 9.5 cm and diameter 2.02 mm. All experiments were carried out at a constant pressure over the getter deposit. The film was evaporated from a stabilized barium-aluminium alloy getter (St2/7 x 7) which is capable of liberating 2 mg of barium. Getters of different types were only used when greater quantities of barium had to be evaporated, such as when studying the influence of surface area and thickness of the barium film. Hydrogen pressure measurements with an ionization gauge are rather misleading due to the appreciable pumping by the gauge. It is well known that a tungsten filaments can bring about the dissociation of hydrogen molecules and that the atomic hydrogen thus formed is easily sorbed on the enclosing walls. The ionizing electron current also produces an appreciable effect6. 7. Nevertheless, by using very low electron ionizing currents the magnitude of the chemical pumping by the electron current may be kept to very low values. In fact all measurements previously carried out and those which will now be considered were obtained using an electron ionizing current of 0.01 mA ; the temperature of the filament under such conditions never rose above 1500°K. The measurements at various temperatures were carried out by employing suitable thermostatic baths. The hydrogen employed was supplied by British Oxygen Gases Ltd., London, and its impurity content was less than 2 p.p.m.
1x10'
FIG.
3. Velocity of sorption for H2 as a function of time.
t = time in set pm = manifold pressure in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure on getter
100 cm2 2mg 298 “K 298°K 3.7 x lo-6 Torr
evidently not very mobile, and in all probability the initial decrease in velocity shows the dependence of the initial chemisorption on the degree of coverage of the metal surface exposed to the gas. One can deduce that a really constant velocity can only be achieved when a first layer has been almost completely formed. If one accepts such an hypothesis the constant velocity should be referred to the adsorption not on the metal but on the hydride already formed. An accurate investigation of the initial decreasing region must be carried out at much lower pressures. Nevertheless it is quite clear that the surface adsorption velocity which is effectively important in gettering is that of adsorption on the product of reaction already formed. Therefore, all the data presented in this section will refer to the velocity of surface adsorption which will be determined from the value corresponding to the horizontal region of the curve rather than from the initial value. The velocity of surface adsorption as a function of the apparent area of the barium film is shown in Fig. 4. The 7. 5-
5.0% 0 x >
2.5-
Results and discussion Surface adsorption
velocity
If the behaviour shown in curve (b) of Fig. 1 is closely analyzed, the existence of a small initial decrease in velocity is observed. This is followed, Fig. 3, by a stabilization to a constant velocity value. The quantity of gas adsorbed during this initial decreasing region is of the order of 1016 molecules. This may probably be related to the formation of an initial hydride layer. The hydrogen molecules sorbed in such a manner are
e 0
FIG. 4. Initial sorption velocity for Hz as a function of the barium film. = velocity in molecules see-1 k = surface area in cm2 Weight of evaporated barium per cm* Temperature of walls at flashing Temperature of sorption Constant pressure on getter
of the surface area
0.02 mg 298 “K 298 “K 1 x l&5 Torr
28
P. DELLA PORTA AND S. ORIGLIO
measurements were carried out at a pressure of 1 x 10-s Torr at room temperature. The quantities of evaporated barium were made to vary in such a manner as to maintain a nearly constant film thickness of 2 x 10-s g cm-z. Since, as is to be expected, the velocity of adsorption is found to be directly proportional to surface area, it may be referred to unit area. At the pressure considered, 1 x 10-s Torr, the velocity of surface adsorption is 6 x 1011 molecules set-1 cm-? The velocity of surface adsorption as a function of pressure is shown in Fig. 5. The measurements were carried out at
/
j
I
I
2
I
I
3
4.
A
5
ps x10-s FIG. 5. Initial Y
sorption
velocity for HZ as a function on the getter film.
= velocity in molecules
of the pressure
set-1
pe = constant pressure on getter in Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption
:II /
93%
1
I
I
I
of sorption for HZ as a function of time at different increasing temperatures of sorption.
Apparent surface area Weight of evaporated Temperature of walls Constant pressure on
V
473aKi
,423’K \ ;
t = time in set pm = manifold pressure in Torr
-I-
0
(
: :
FIG. 6. Velocity
0
~
100 cm* 2mg 298 “K 298 “K
room temperature on a film of 2 mg which had an apparent surface area of 100 ems. The solid circle indicates the velocity referred to unit area, as calculated from Fig. 4. Since the velocity of surface adsorption is found to be directly proportional to the pressure, the mechanism appears to be one of molecular adsorption. Although the hydrogen molecules sorbed may subsequently dissociate into atoms, this does not seem to be the rate determining process. Since the adsorption velocity is directly proportional to the pressure, all measurements may be referred to constant pressure. Thus the velocity becomes 6 x 1016 molecules see-icm-2 Torr-1. Bearing in mind the fact that the number of hydrogen molecules which strike unit area of the barium film in unit time at room temperature is 1.99 x 1021 molecules XCicm-2 Torr-1, the sticking probability is : s = 4.17 x 10-s. This value appears rather low ; it is nevertheless of the same order of magnitude of that previously published by Perdjik* who finds s = 11.25 x 10-s at room temperature. A comparison with the values published by Wagener is rather more difficult as they vary between about 1 x 10-3 [9,*01 and 2 x lo-6.1111 The influence of temperature on the velocity of surface adsorption is shown in Figs. 6 and 7. The curve in Fig. 6 was obtained with a film of 2 mg having an apparent surface area of 100 cm2 and with a
of film barium at flashing getter
100 cm2 2mg 298 “K 3.7 x 10-e Torr
constant pressure of 3.7 x lo-6Torr above the getter. The temperature of the getter was varied by a thermostatic bath in which the sorption vessel was immersed. The first horizontal region of the curve corresponds to the velocity of surface adsorption for hydrogen at a temperature of 293°K. After the consistency of this velocity had been ascertained for a sufficiently long period of time the temperature was increased to 373°K. Initially, whilst the temperature was increasing, the velocity apparently decreased, in all probability due to the release of gas, This indicates that an appreciable quantity of hydrogen is lightly held on the surface. However, after a given time, the velocity increased once again to a value above the original one and then remained constant. The same behaviour was subsequently found during the process of increasing the temperature to 423 and 473°K. At all these temperatures a This constant velocity of surface adsorption was attained. is shown by the various horizontal tracts of the diagram.
0
FIG. 7. Velocity
of sorption for Hz as a function of time at different decreasing temperatures of sorption.
t = time in set pm = manifold pressure in Torr Apparent surface area Weight of evaporated Temperature of walk Constant pressure on
of film barium at flashing getter
100 cm2 2mg 298 “K 3.7 x IO-6 Torr
The Sorption of Hydrogen by Barium Getters
29’
Furthermore if the temperature is slowly decreased back to 293°K the velocity also falls to its original value, which confirms its constant value at a given temperature. The curve in Fig. 7 was obtained under similar conditions to the above but the temperature was made to decrease from its highest to its lowest value. The previously observed degassing does not occur in this case and a stepwise decrease in the curve is obtained, each step corresponding to a constant velocity of surface adsorption at the given temperature. From the sorption velocity values given in Figs. 6 and 7, it is possible to calculate the energy of activation for the process from the slope of the curve in Fig. 8 relating the
2.5
t x103 2.001
I
I.5
2.0
2.5
30
I 35
I
4.0
velocity of hydrogen on barium temperature. Y = velocity in molecules se-t = constant pressure on getter in Torr P T = sorption temperature in “K Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
films as a function
of
100 cm* 2mg 298 “K 3.7 x l&6 Torr
logarithm of the sorption velocity to the reciprocal of the absolute temperature. This is found to be 1880 cal mole-i. This value is slightly higher than that previously found, 1500 cal mole-1,111 which was calculated from the initial velocity of sorption curves obtained at higher pressures and of the type shown in Fig. l(a). From the comments made in the introduction the present result should be considered as the more accurate. Dijiuion
velocity of hydrogen as a function films at various temperatures.
velocity
As pointed out in the introduction, this section is essentially concerned with the verification of the applicability of our previous results to thicknesses and pressure conditions comparable to those used in industry. Hence the results in this section refer to barium films of 2 mg having an apparent surface area of 15 cma, and the velocity of sorption was measured from curves of the type shown in Fig. l(a). The influence of temperature on diffusion is shown in Fig. 9, which gives the velocity of sorption at various temperatures as a function of time. By graphical integration it is possible from such curves to evaluate the quantities of hydrogen sorbed at various given t times, Qt = V(t)dt, and by plotting these quantities s 0 against the square root of the corresponding times to obtain curves such as shown in Fig. 10. These curves present, after
= velocity in cm3 Torr secrl = timein set kpparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
of time by barium
v
I/TxIO-~
FIG. 8. Sorption
FIG. 9. Sorption
15 cm2 2mg 298 “K 2.5 x 10-4 Torr
It is well known the initial region, an almost linear portion. that such linearity characterizes the quantity of gas which diffuses through a surface, on which the concentration of gas remains constant, when diffusion is assumed to occur in a semi-intinite medium. At constant pressure and with this approximation the slopes of the straight lines (dQt/dt) are, through the diffusion It is therefore possible coefficient, a function of temperature. to evaluate the energy of activation for the diffusion process by considering the dependence of D = DO e - j% on temperature. The logarithms of dQt/dt as a function of temperature are plotted in Fig. 11. The solid circles are the values as obtained from Fig. 10, whilst the open circles are the results previously published1 for thinner films (2 mg/lOO cm2). By comparing the two sets of points one draws confirming evidence for the hypothesis put forward concerning the kinetics of sorption, and in particular for the unique nature of the process even for getter films of very different thicknesses. The activation energy has a value of about 3000 cal mole-i. A discussion concerning the consequences of such a low potential barrier against diffusion of hydrogen towards the interior of the film has been previously put forwardr. Parameters gettering
of industrial
consequence
and their influence on
The velocity of sorption is shown as a function barium film thickness in Fig. 12.
of the
30
S. ORIGLIO
P. DELLA PORTA AND
O-
5I !
~
I
I
2
/ I
3
4
tx103 FIG. 12. Sorption
o-
”
= velocity in cm3 Torr sea-1
t
= time in set
Apparent surface area of film Temperature of walls at flashing Temperature of sorption Constant pressure on getter Weight of evaporated barium
.5-
0
velocity for HZ, for different quantities barium, as a function of time.
of evaporated
15 cm2 298 “K 298 “K 2.5 x lo-4Torr (a) 18.7 mg (b) 13.8 mg (c) 8.3 mg (d) 2.0 mg
1
FIG. 10. Time
dependence of the quantity of hydrogen barium films at various temperatures.
= timein set Qt = quantity sorbed in cm3 Torr Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
sorbed
by
t
15 cm2 2mg 298°K 2.5 x IO-4 Torr
L-
110-!
2
10. I
2
FIG. 11. Temperature
3
4 I/TxIO-~ dependence of hydrogen films.
Q, = quantity sorbed in cm3 Torr t = timein set T = sorption temperature in “K Weight of evaporated barium Temperature of walls at flashing Constant pressure on getter
6
5 diffusion
into barium
2mg 298 “K 2.5 x 10-4 Torr
It illustrates how the velocity of diffusion (not only the capacity) increases with fIm thickness. Bearing in mind the granular structure of the !ilm this result may be attributed to the different surface areas from which diffusion originates.
FIG. 13. Sorption
velocity condensing
3
4
5
1X103 for Hz, for different temperatures walls, as a function of time.
= velocity in cm3 Torr set-t = time in :ec apparent surface area of film Weight of evaporated barium Temperature of sorption Constant pressure on getter Temperature of walls at flashing
of
”
15 cm2 2mg 298 “K 2.5 x IO-4 Torr (a) 293°K (b) 473°K
This could imply an extremely rapid surface diffusion such as to guarantee a high degree of coverage of the real total surface area of the getter film. It would seem to be an advantage, therefore, for hydrogen
The Sorption of Hydrogen by Barium Getters sorption, to employ a film of an appreciable thickness, contrary to what had been previously found for other gases. Similar observations may also be made on the influence of other factors which are able to affect the structure of the film. Thus the influence of the temperature of the condensing walls on the velocity of diffusion is shown in Fig. 13. A decrease of the velocity of sorption may be produced by increasing the substrate temperature during film condensation, so decreasing the real total surface area of the film (formation
31
of larger crystals and sintering of the film). The curves referred to above were obtained at room temperature and at a pressure of 2.5 x 10-4 Torr on 15 cm2 getter films. The curves in Fig. 14 were produced under the same conditions as those in Fig. 13 except the substrate was held at room temperature during evaporation and the evaporation rate was varied. In this case the effect produced arises from the difference in the internal surface area of the films obtained using different times of evaporation. Finally the influence of the residual pressure during barium evaporation is shown in Fig. 15. These curves too were obtained on films of 2 mg, having an apparent surface area of 15 cm2, at room temperature and at a constant pressure over the getter of 2.5 x 10-4 Torr It appears that the influence of the residual pressure is primarily due to a preliminary poisoning of the getter, in the same manner as had been previously found for nitrogen and carbon monoxide. Conclusions
t x103 FIG. 14. Sorption Y =
t
velocity
for Hz, for different function of time.
times of flashing,
as a
velocity in cm3 Torr set+
= time in set
Apparent surface area of film Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure over getter Time of flashing
(a) (b) (c)
I5 cm2 2mg 298 “K 298 “K 2.5 x 10-4 Torr 5 set 12 set 30 set
The velocity of surface adsorption and the velocity of diffusion within a barium film have been studied independently. With reference to surface adsorption it has been possible to show two distinct phases, in the first of which the velocity decreases whilst in the second it remains constant. Since the first phase corresponds to the adsorption of a quantity of gas comparable to that required for the formation of a monolayer, it has been deduced that the constant velocity of surface adsorption characterizes the adsorption of gas on the reaction product already formed. This is in agreement with the relatively high mobility of the adsorbed phase which would be inexplicable if one assumed direct chemisorption on the metal ; it also justifies the partial degassing occurring when increasing the temperature of the film. Concerning the diffusion to the interior of the film, the picture previously put forward for thinner films has been verified for films of greater thickness. Finally all the measurements carried out to study the influence of various parameters on the efficiency of gettering have contributed to show the importance of a barium film of high porosity. References
0
I.5
3.0
4.5
6.0
7.5
t xl03 FIG.
15. Sorption
velocity for Hz, for different residual flashing, as a function of time.
Y = velocity in cm3 Torr secrl I = time in set Apparent surface area Weight of evaporated barium Temperature of walls at flashing Temperature of sorption Constant pressure over getter Residual pressure during flashing
pressures
15 cm* 2mg 298 “K 298 “K 2.5 x IO-4 Torr (a) 1 x 10-S Torr (b) 1 x lo-4 Torr (c) 2 x IO-3 Torr
at
1 P. della Porta and S. Origlio ; Trans. Znt. Symp. on Residual Gases ; Coma, Vacuum 10, 227 (1960). 2 P. della Porta and F. Ricca ; 5th Nat. Symp. A. VS., S. Francisco (1958), Pergamon Press (1959), p. 25. 3 P. della Porta and F. Ricca ; Trans. Znt. Symp. on Residual Gases ; Como, Vacuum 10, 215 (1960). 4 P. della Porta and F. Ricca ; Le Vide 85, 1 (1960). 5 I. Langmuir ; J. Amer. Chem. Sot. 37, 417 (1915). 6 P. della Porta, S. Origlio and E. Argano ; Trans. Znt. Symp. on Residual Gases, Coma, Vacuum 10, 194 (1960). 7 L. Holland ; J. Sci. Znstrum. 36, 105 (1959). * J. J. B. Fransen and H. J. R. Perdjik ; Philips Tech. Rev. 19, 290 (1957/58). 9 S. Wagener ; J. Phys. Chem. 60, 567 (1956). 9 S. Wagener ; J. Phys. Chem. 60, 267 (1957) ; J. Phys. Chem. 61, 267 (1957). 10 S. Wagener ; Brit. J. AppI. Phys. 2, 132 (1951). 11 S. Wagener ; Proc. 4th NatI. Con. on Tube Techniques (1958), p. 1 (1958), New York University Press (1959).