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Sorption of nitrogen by continuously deposited titanium films
Vacuum Congress solids, especially of glasses and silica, differs greatly from that of the bulk, depending on its thermal history, weathering and corrosion process. A surface really free of any contaminant can only be obtained if created in ultrahigh vacuum environments. Therefore the surfaces under investigation are of two different types: (a) The natural or "passivated" surface of the vacuum chamber itself. (b) The surface created in ultrahigh vacuum environments called "fresh" surface and equally the fresh surface submitted to a thermal treatment, so that the influence of the state of the surface on the adsorption process can be determined. The fresh surfaces are obtained by the breaking of rods or by abrasion of the natural surface of a tube with a tool, consisting of small diamonds embedded in a glass bead. The experiments are carried out in a vacuum chamber essentially of Pyrex glass whose volume is about 1.5 litre. The associated pumping system is conventional--comprising a mechanical pump, a three-stage oil diffusion pump and an activated alumina trap. The lowest total pressure attainable after bakeout is below I:,: 10 '° torr; the partial pressures are measured by means of an omegatron type mass spectrometer. The pure gases employed are admitted inlo the vacuum chamber via a variable leak valve. The adsorption and thermal desorption measurements allow the determination of the following characteristic parameters: the thermal desorption energy and related mean time of adsorption, the surface coverage, the sticking probability and the mobility in the adsorbed state. For instance, in the case of the COUfresh silica system, the following values are found: thermal desorption energy E 11 Kcal/mole, pre exponential factor (according to Frenkel's formula): 10-~s, fractional surface coverage 0 at ambiant temperature: 0.04, initial sticking probability s o = s (0=0) 0.12. The variation of the sticking probability s as a function of coverage 0 predicts a virtually zero mobility at ambiant temperature. The variations of 0 and s o with temperature are also given. The values for the same parameters are also given for CO., on the different states of the silica surface and on various glasses. For instance the thermal desorption energy spectrum of CO2 on a fresh surface of a borosilicate glass has two maxima corresponding to 6 and 11 Kcal/mole respectively. The behaviour of Oz is also investigated and compared to that of CO~ under the same experimental conditions.
however, of measured adsorption heats it was felt necessary to obtain other evidence. The chemisorption criterion chosen for consideration is the Elovich relation, q - - a log t i b; where q is a quantity adsorbed; t, the time of exposure to the sorbing gas; and a and b are constants. It is noted that this relation yields a pressure change rate which merely approximates the power relation, dp :atb, which is occasionally found in the literature. The pressure range studied was between 5 . 10 :' and 3 • 10 1, torr at two temperatures, 30'C and 180C. The sample was a bakeable steel system of apparent surface area, 22,000 cm 2. Water purity was maintained to greater than 98 per cent at exposures above 2 x 10 ~ torr. The data were found to fit the Elovich relation well.
Sorption of nitrogen by continuously deposited titanium films
N E Birjukova, M 1 Vinogradov, M N Yefimov, USSR Sticking probabilities as functions of film temperature and surface coverage for chemisorption of nitrogen on continuously deposited titanium films have been measured in the pressure range of 10 J°-10 8 mm Hg. At small surface coverage the sticking probability rises monotonously with lowering of film temperature from i 200 to - - 196 C and at 196 C approaches 1. For the whole temperature range the sticking probability is constant for surface coverage up to about 0.6 and then sharply diminishes with the further surface coverage rise. The maximum surface coverage by chemisorbed nitrogen atoms corresponds to atomic ratio N-Ti 1:2. The theoretical formula is deduced which is in close agreement with the experimental curves. Basing on the experimental results the following mechanism of chemisorption of nitrogen on titanium films is adopted: (1) physical adsorption of nitrogen molecules on titanium film surface; (2) transition of physisorbed nitrogen molecules into a chemisorbed molecular state and forming of a movable adsorbed layer; (3) transition from chemisorbed molecular state into a chemisorbed atomic state in which an unmovable adsorbed layer is probably formed.
S E S S I O N 1D Water chemisorption on stainless steel
R A Strehlow and D M Richardson, Reactor Chemistry Div&ion Oak Ridge Nathgnal Laboratory, Oak Ridge, Tennessee, USA Water vapor sorption processes are well known to affect vacuum system characteristics. Both the achieved base pressure and the pumping time are often determined by the desorption rate of water. In an effort to understand better the persistance of water in a controlled thermonuclear research device, a study was undertaken to establish the sorption kinetics of water vapor. Water vapor sorption has not been studied extensively in the pressure range less than 10 a torr. Evidence is ample that processes other than a simple physical adsorption is involved. A diffusional model leads to an exhaust rate relation which has not been found to apply to the water-steel system. Temptation is strong to invoke a chemisorption model interaction simply because of the sluggishness of water exhaust. In the absence,
128
Surface Phenomena Proton scattering as a tool for the study of crystal structures ([nvited paper)
R S Nelson, Metallurgy Division, ,4ERE, Harwell, Berkshire, England The regular nature of the crystal lattice has a marked influence on the trajectories of protons travelling through crystalline solids, and on the spatial distribution of protons elastically scattered from their surfaces. The periodic potentials existing within the crystal cause the protons to undergo correllated collisions with the lattice atoms so that their trajectories are steered either along or away from certain well defined crystallographic directions. Such steering effects have been termed "Channelling" and "Blocking" respectively.
however, of measured adsorption heats it was felt necessary to obtain other evidence. The chemisorption criterion chosen for consideration is the Elovich relation, q - - a log t i b; where q is a quantity adsorbed; t, the time of exposure to the sorbing gas; and a and b are constants. It is noted that this relation yields a pressure change rate which merely approximates the power relation, dp :atb, which is occasionally found in the literature. The pressure range studied was between 5 . 10 :' and 3 • 10 1, torr at two temperatures, 30'C and 180C. The sample was a bakeable steel system of apparent surface area, 22,000 cm 2. Water purity was maintained to greater than 98 per cent at exposures above 2 x 10 ~ torr. The data were found to fit the Elovich relation well.
Sorption of nitrogen by continuously deposited titanium films
N E Birjukova, M 1 Vinogradov, M N Yefimov, USSR Sticking probabilities as functions of film temperature and surface coverage for chemisorption of nitrogen on continuously deposited titanium films have been measured in the pressure range of 10 J°-10 8 mm Hg. At small surface coverage the sticking probability rises monotonously with lowering of film temperature from i 200 to - - 196 C and at 196 C approaches 1. For the whole temperature range the sticking probability is constant for surface coverage up to about 0.6 and then sharply diminishes with the further surface coverage rise. The maximum surface coverage by chemisorbed nitrogen atoms corresponds to atomic ratio N-Ti 1:2. The theoretical formula is deduced which is in close agreement with the experimental curves. Basing on the experimental results the following mechanism of chemisorption of nitrogen on titanium films is adopted: (1) physical adsorption of nitrogen molecules on titanium film surface; (2) transition of physisorbed nitrogen molecules into a chemisorbed molecular state and forming of a movable adsorbed layer; (3) transition from chemisorbed molecular state into a chemisorbed atomic state in which an unmovable adsorbed layer is probably formed.
S E S S I O N 1D Water chemisorption on stainless steel
R A Strehlow and D M Richardson, Reactor Chemistry Div&ion Oak Ridge Nathgnal Laboratory, Oak Ridge, Tennessee, USA Water vapor sorption processes are well known to affect vacuum system characteristics. Both the achieved base pressure and the pumping time are often determined by the desorption rate of water. In an effort to understand better the persistance of water in a controlled thermonuclear research device, a study was undertaken to establish the sorption kinetics of water vapor. Water vapor sorption has not been studied extensively in the pressure range less than 10 a torr. Evidence is ample that processes other than a simple physical adsorption is involved. A diffusional model leads to an exhaust rate relation which has not been found to apply to the water-steel system. Temptation is strong to invoke a chemisorption model interaction simply because of the sluggishness of water exhaust. In the absence,
128
Surface Phenomena Proton scattering as a tool for the study of crystal structures ([nvited paper)
R S Nelson, Metallurgy Division, ,4ERE, Harwell, Berkshire, England The regular nature of the crystal lattice has a marked influence on the trajectories of protons travelling through crystalline solids, and on the spatial distribution of protons elastically scattered from their surfaces. The periodic potentials existing within the crystal cause the protons to undergo correllated collisions with the lattice atoms so that their trajectories are steered either along or away from certain well defined crystallographic directions. Such steering effects have been termed "Channelling" and "Blocking" respectively.