A characteristic feature of nonevaporable getters is growth of their activity with an increase in intrinsic temperature. The optimum temperature of adsorption of the majority of nonevaporable compositions for monoxide and dioxide of carbon and nitrogen is in the order of 400°C or higher; hydrogen and water vapor are beginning to be adsorbed by them at much lower temperatures (of about 25°C). The adsorption of residual gases except hydrogen by the nonevaporable getters is irreversible.
The main factors determining the activity of nonevaporable getters are the components of the composition, the amount of its surface open for access of gas and porosity determining the true value of the surface.
The advantages of nonevaporable getters include a possibility of reactivation by means of heat treatment which removes gases from the saturated surface of the getter for recovery of their activity.
The scope of application of the nonevaporable getters is determined by their activity at high temperatures and by impossibility of using evaporable getters under certain conditions.
The nonevaporable getters are made in the form of bulk porous bodies in a container and without any container and with a self-contained heater and without a heater. The active composition is a powder of titanium. titanium-vanadium and a zirconium-aluminum alloy as well as compositions formulated as a mechanical mixture of powders.
The active material of the getter composition determines sorption abd vacuum properties. as well as the activation temperature and optimum operating temperature of the getter.
The getter active surface characterizes the rate of adsorption of residual gases in the device. which also depends on the active material composition.
The hydrogen adsorption capacity (by carbon monoxide) is the basic characteristic of the getter activity indicating the amount of gas irreversibly adsorbed by the getter (measured at room temperature within 30 seconds after preactivation).
The porosity provides a large area of contact of the gas composition with the active molecules resulting in a high adsorption capacity and sorption rate at working temperatures.
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