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Introduction
Getter materials are reactive metals able to chemically sorb active gases in a vacuum environment. They can be classified in two big families: evaporable and non-evaporable getters. The former have been extensively adopted in many industrial applications, like CRTs and lamps, while the latter have drastically changed the way of improving the vacuum levels achievable in vacuum system from medium to extreme high vacuum.
SAES Getters is the repository of the long-standing advanced technology nowadays available on getter materials. In particular in Ultra High Vacuum (UHV) systems compact NEG-based pumps with high pumping speed for hydrogen and active gases have been developed. The list of applications in which SAES NEG Pumps are employed is continuously increasing for the wide range of products offered and their flexibility in adapting to several UHV applications, like Particle Accelerators, Cold Atomic Trap or Surface Science systems.
High vacuum equipment, i.e. systems working in the 1e-7÷1e-9 Torr range, can require getter materials able to pump large gas loads in unbaked or viton sealed systems. A typical example is represented by accelerator facilities or deposition systems where NEG pumps characterized by high pumping speed for hydrogen, water, air and carbon contaminants are requested.
Up to now high vacuum conditions have represented an unsurmountable limit for all the different NEG alloys, since any getter material would get rapidly saturated and would completely lose its pumping speed and capacity. This is the reason why NEG materials have been relegated to the UHV segment.
In this context, SAES studied and designed a NEG alloy able to guarantee the above mentioned characteristics. The result of the long research activity has brought to the development of a new NEG sintered alloy: the ZAO®. In the following sections a presentation of the SAES NEG pumps based on standard NEG alloy for UHV applications and ZAO getter alloy for high vacuum systems will be given.
Old but gold: the St707 compressed alloy
The SAES Getters Group has been the developer NEG since the early 1960s. One of the most used widely applied getter alloy is the St707®, a ternary Zirconium-based alloy with minor percentages of Vanadium and Iron. The alloy has been – and is currently – furnished in forms of compressed pills or washers or deposited and firmly fixed onto thin metallic strips. This manufacturing technique guided the development of the SORB AC® Cartridge NEG pumps, which found wide use in a variety of applications, in combination with traditional UHV pumps. They are particularly efficient in pumping all active gases in UHV systems, and especially Hydrogen and its isotopes. SORB AC® pumps have been and are still used for reaching UHV conditions in the field of particle accelerators, synchrotron radiation sources and related equipment, surface analysis systems and other R&D vacuum systems. A picture of a SORB® AC pump and a real installation in a vacuum undulator (Spring-8 synchrotron radiation facility, Harima Science Park City, Japan) is shown in Fig. 1.
Figure 1: A SORB AC® GP 500 pump (nominal pumping speed of 2000 l/s for H2) and a real installation in the SPring8 synchrotron radiation facility, Harima Science Park City, Japan. The red circle indicates the nipple in which the GP pumps are installed.
As already stated, irrespective of the final form, the St707 is a compressed alloy. Compression does not mean simply putting together some powder in order to get the final product, but implies a dedicated complex procedure. Despite their characteristics, compressed getter alloys still remain the first generation of NEG materials. Their use in UHV systems particularly sensitive to the outgassing during the NEG activation stage and to the particle release is not recommended. Such requirements can be fulfilled only with the advent of a new technology in NEG production mechanism: the sintering process.
From compression to sintering: a new generation alloy
The increased market demand for NEGs with enhanced sorption capacities at lower temperatures, reduced outgassing of the physisorbed species during the activation stage and negligible loss of getter particles during operation triggered SAES Getters’ development of a new NEG alloy through a more advanced production mechanism: the sintering process. Sintered alloys are intrinsically more robust structures, with the getter particles strongly bound together for an excellent mechanical stability and a reduced particle loss even under the most severe environmental conditions. The sintering process allow to achieve overall better NEG alloys, showing the following enhanced characteristics with respect to the compressed NEG:
More UHV compatible design (the material is pre-treated at high temperature)
Higher surface structure.
Compressed pills or disks are much more dusty than sintered ones. SAES pumps with sintered disks are virtually particle free for most applications.
Among the different sintered alloys developed by SAES Getters, the St172 can be considered the one of greatest success. It is a ternary alloy containing Zirconium, Vanadium and Iron in different percentages. This alloy is currently used, in the form of sintered disks, in the CapaciTorr® and NEXTorr® SAES pump series for UHV systems. The systems in which SAES St172-based pumps have been applied encompass a large variety of UHV applications: Surface Science (STM, XPS, SPM, ARPES, etc) and Cold Atom Trap Systems, Atomic Clocks and Gravimeters, particle accelerators, synchrotrons radiation sources and related equipment, portable vacuum instrumentation, and so on. A typical example of application of a NEXTorr® pump (NEG element St172-based plus a compact ion pump for removing the non getterable gases, like Ar and methane) is shown in Fig. 2.
Figure 2: NEXTorr® pump installation in a Cold Atom Trap System. The compact ion elements of the NEXTorr® pumps are visible (red boxes). Courtesy of group of Dr. Rudolf Grimm, University of Innsbruck.
The big innovation that SAES brought in the getter technology with the introduction of sintered non evaporable getters, has fulfilled the market request for NEG pumps with high pumping speeds and sorption capacities, excellent mechanical properties, low particle release and compact dimensions. Anyhow, all NEG pumps currently available on the market are limited to UHV systems. They cannot cope with High Vacuum systems, characterized by high gas pressures (i.e. 1e-7 Torr) and huge loads (outgassing from viton o-rings, unbaked systems, etc.). This is what we were used to know, at least up to know…
Breaking the scheme: the birth of the ZAO® alloy
High gas loads of hydrogen or active gases(e.g., 1e-7 Torr l /s) in high vacuum systems can be faced letting a NEG pump working warm, typically at temperatures around 200 °C (power required few watts). In this way, both getter surface and bulk will be involved in the chemical sorption mechanism, achieving high pumping performances for removing the active gases present in the system under analysis. In these working conditions, standard sintered and compressed getter alloy are not suitable for this purpose due to the loss of performances after some absorption cycles, the higher equilibrium pressure for hydrogen and the possibility to get strong reaction with air at atmospheric pressure, while hot.
The goal to use NEG pumps in high vacuum can be achieved with the innovative ZAO® alloy, able to digest heavy gas loads in high vacuum conditions, maintaining practically unchanged the pumping performances through several regeneration cycles and working warm. The ZAO® getter alloy is integrated in the compact design of CapaciTorr® HV pumps. In Figs 3 and 4 are reported a picture of CapaciTorr® HV200 and its pumping characteristics respectively.
The CapaciTorr® HV pumps can typically work for 1 year at 1e-7 ÷1e-8 Torr. After this period of time, the pump can be reactivated. The total number of reactivations are >20.
Possible applications are:
The CapaciTorr® HV pumps can be effectively combined with other pumping technologies like ion pumps, TMP and cryo pumps.
For more information, please contact :
neg_technology@saes-group.com