A reduction in the outgassing rate of vacuum materials is important for the ultrahigh vacuum (UHV) and extremely high vacuum (XHV) systems. Standard vacuum materials have outgassing rates of the order from 10 -9to 10-10 Pam/s under the conventional pre-treatments such as electrolytic or chemical polishing and the baking process with temperature below 500 K. In order to reduce the outgassing rate for standard materials, the special treatments, e.g. vacuum-firing processes with high temperatures, oxidization and thin film coating, are necessary. However, the decrease in the outgassing rates of these materials is already facing the limit.
A titanium has a passivation surface which is expected to provide low outgassing, since the surface oxide layer is chemically stable. Furthermore, titanium material has advantageous properties for a high vacuum material, lightweight, a low Yong's modulus, a small thermal expansion coefficient, high-corrosion resistance, non-magnetism, and low-Z number. Recent years, vacuum characteristics of titanium materials have been investigated, and vacuum chambers and components made of titanium were developed by some research groups so far. In this paper, our studies and developments utilizing industrial pure titanium materials, mainly JIS class 2, are discussed.
Outgassing properties and surface characteristics for pure titanium material of JIS class 2 was evaluated in terms of the effects of polishing method. The smooth surface and about 10 nm-thick surface oxide layer are obtained by a mechno-chemical polishing and a chemical polishing. These surface conditions provide an excellent outgasing property. Outgassing rate of this material is lower than 7×10-13 Pam/s through the modest baking process, and the outgassing rate of this material without baking treatment is about 1/4 of that for chemically polished stainless steel. A vacuum chamber and vacuum components, a bellows, an electric feedthrough and etc. for UHV/XHV systems have been developed using titanium materials. The developed vacuum chamber and components have been applied to UHV/XHV systems, molecular beam epitaxy and high-intensity proton beam accelerator systems, which showed excellent vacuum properties.
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