Nonevaporable Getters: Properties and Applications
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oys to bind gaseous molécules to their surfaces, through the formation of highly stable chemical compounds. The effect of this process is to remove the ambient active gases, providing a pumping action which can be exploited to maintain the required vacuum in evacuated devices and Systems. This sorptibn action can be exerted once the getter material has been submitted to an activation process, performed at high température for a certain rime in vacuum, causing the diffusion of the stable (protective) surface oxide and nitride film naturally formed upon initial exposure of the material to air. The gettering mechanism is essentially related to the physics and chemistry of the surfaces, since the initial and often dominant process of the overall gettering mechanism is the gas-surface interaction. The real surface composition of the material, which is often quite différent from the bulk composition as a resuit of ségrégation effects, represents therefore an important parameter in the dynamics of the gettering process. Modification of the surface composition can either promote or inhibit the gas-surface interaction.13 In some cases adsorption is accompanied by catalytic effects. In instances where, for example, the adsorbed species is dissociated as a resuit of the adsorption, the gettering process is enhanced;4 in other cases, limited to evaporable getters such as barium and titanium, superficially catalyzed reactions can lead to the formation of new gaseous species such as méthane, which are not easily adsorbed by the getter, creating a residual atmosphère of such gases.5 The importance of the chemistry and physics of the surface in gettering is demonstrated by the large number of papers which hâve been published on the subject; nevertheless further theoretical and expérimental work is neces-
sary to achieve a deeper understanding of the gettering mechanisms. To prevent poisoning of the surface of a bulk getter by adsorbed gases, the getter material should be preferably operated at an elevated température in order to promote diffusion of the adsorbed gas into the bulk. In this way it is possible to keep the surface coverage (namely the degree of occupancy) sufficiently low so that enough surface sites are available to keep the gettering speed high and reasonably constant. The effëct of the température increase in enhancing the sorption performance is particùlarly évident for gases such as 0 2 , N2, CO, C0 2 , containing atomic species with low inhérent diffusivity; while for H2, which diffuses far faster, high sorption capacities are achieved also at room température or lower. Gettering Materials and Structures The care put into the development of advanced gettering materials and related structures has been motivated by différent application requirements. One of the most important is linked to the limitation in the température conditions attainable during activation and opération of the getter. For this reason strong efforts hâve been directed toward the development of new nonevaporable gettering alloys with better diff
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