Vacuum characteristics of titanium

Abstract

Little attention has been paid to metals other than stainless steels and aluminum alloys as materials for ultrahigh vacuum (UHV) and extremely high vacuum (XHV) chambers. Titanium is another candidate material for use in UHV and XHV applications. To obtain basic data for the application of titanium to UHV and XHV, an apparatus employing the conductance modulation method was fabricated for the measurement of the outgassing rate. In the measurement of the outgassing rate of small samples, it was found that the outgassing rate of titanium decreased 1.5 times faster than that of stainless steel. Furthermore, a TiN coating was found to reduce the total amount outgassed from 1.78×10−2 to 1.17×10−2 Pa m3 in the measurement of titanium and from 1.02×10−1 to 9.22×10−3 Pa m3 in the measurement of stainless steel. In a mass analysis of the gases from the samples, the ion current of hydrogen from chemically polished titanium, chemically polished and TiN-coated titanium, and TiN-coated stainless steel decreased after attaining a maximum, while that from buffed and electropolished stainless steel increased continuously during the baking at 200 °C. Considering these results, a vacuum chamber made of titanium coated with TiN and a vacuum chamber made of electropolished stainless steel were prepared and pumped down into XHV region. The pressure of the titanium chamber reached 6×10−11 Pa after a 312 h baking at 250 °C, while the stainless steel chamber required a 434 h baking at 250 °C to attain the ultimate pressure of 7×10−11 Pa. The pressure of the titanium chamber decreased more rapidly than that of the stainless steel chamber during the baking at 250 °C.