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Application of titanium materials to vacuum chambers and components

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2008 J. Phys.: Conf. Ser. 100 092002(http://iopscience.iop.org/1742-6596/100/9/092002)
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IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002

IOP Publishingdoi:10.1088/1742-6596/100/9/092002

Application of Titanium Materials to Vacuum Chambers andComponents
H Kurisu1, K Ishizawa1,2, S Yamamoto1, M Hesaka3 and Y Saito4
1

Graduate School of Science and Engineering, Yamaguchi University, Ube,
Yamaguchi 755-8611, Japan2
CT division San-ai Plant Industries, Ltd., Kawasaki, Kanagawa 210-0821, Japan
3
Shinko Industries Co., Ltd., Ube, Yamaguchi 759-0207, Japan
4
KEK-High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801,
Japan
E-mail: kurisu@yamaguchi-u.ac.jp
Abstract. This paper describes the outgassing properties of titanium materials, anddevelopment of vacuum chambers and components for practical UHV/XHV systems. The
mechano-chemically polished and the chemically polished titanium materials have a smoothsurface and a thin (≤ 10 nm) oxide surface layer, which showed extremely low outgassing rate
below 10-12 Pams-1 after baking process. In order to fabricate practical vacuum systems
welding, metallizing and brazing processes were optimized, and complex shaped vacuum
chambers and various vacuum components such as a bellows, valve, electric feedthrough andceramic duct with titanium sleeve were fabricated. Sufficient mechanical properties anddurability were obtained for practical use.

1. Introduction
A reduction in the outgassing rate of vacuum materials is important for the ultrahigh vacuum (UHV)and the extremely high vacuum (XHV) systems. Stainless steels and aluminum alloys have outgassing
rates of the order from 10-9 to 10-10 Pams-1 under the conventional pre-treatments such as electrolytic
or chemical polishing and the baking process with temperature below 500 K. In order to reduce theoutgassing rate for standard materials, the additional surface finishing, e.g. vacuum-firing [1,2],oxidization [3,4] and thin film coating [5, 6], and use of a very thin wall [7] are necessary.
A titanium material is well known as an evaporated and a non-evaporated getter material for the
UHV/XHV pumps. However, titanium materials have been hardly applied to vacuum chambers andcomponents except for some parts of a magnetron ionization gauge [8] until 1980’s, since titanium
was not so machinable comparing to the stainless steels or aluminium alloys, and had difficulty in
welding. The outgassing property for a titanium alloy similar to those for standard vacuum materials
was reported [9], although the outgassing property for the titanium material was not intensively
studied in the past.
After 1990’s, researches on the vacuum characteristics of titanium materials and development ofvacuum chambers and components have began. The vacuum characteristics have been reported thatthe outgassing rate of the chemically polished pure titanium (JIS grade 2) was less than 1/5 of that for
the electrolytic polished stainless steel [10,11].

c 2008 IOP Publishing Ltd

1

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002

The authors have commenced intensive
research and developments for titaniummaterials after year 2000. Some kinds ofsurface finishing were developed andevaluated, in which very low outgassing
properties were clarified [12-14]. And thetitanium materials were successfully applied tovacuum chambers and components forpractical vacuum systems such as UHV
systems for fabrication of nano-devices and theproton beam accelerator in the Japan Proton
Accelerator Research Complex (J-PARC)[15,16].
In this paper, the outgassing properties andthe surface finishing were mainly discussed forpure titanium materials JIS grade 2. And
development of vacuum chambers andcomponents utilizing some kinds of titaniummaterials were described.

IOP Publishingdoi:10.1088/1742-6596/100/9/092002

Table 1 Physical properties of titanium, stainlesssteel and aluminum alloy
Titanium（JIS - 2）

Atomic Number
Specific Gravity(103 kg･m-3)
Hardness(Hv)
Young’s Modulus(G Pa)
Thermal Expansion
Coefficient (10-6K-1)
Thermal Conductivity(W･m-1･K-1)
Specific Heat
(J･kg-1･K-1)
Electric Resistance(10-9Ωm)

Stainless Aluminium
Steel
Alloy
（SUS304）(A5052)

22

Fe: 26
Cr: 24
Ni: 28

13

4.5

7.9

2.7

〜150

〜190

〜60

106

200

69

8.4

17

24

17

16

137

520

500

900

480

720

49

2. Physical Properties of Titanium
NonLittleNonTable 1 shows the physical properties of pure
Magnetism
magnetic
magnetic
Magnetic
titanium (JIS grade 2, purity 99.7 %), stainless
Cost (US$)
steel (SUS304, Cr: 18 wt% Ni: 8 wt%) and
658
134
77
(1m×1m×1mm)
aluminium alloy (A5052, Mg: 2.8 wt%). The
features of the titanium are as follows: (1)lightweight and high hardness are desirable for a chamber and a transfer system; (2) low-Z number isadvantageous for installation and maintenance under high-radiation level, compared to stainless steel;(3) low Yong’s modulus is suitable for a bellows component; (4) low thermal expansion coefficient isadvantageous for a heating component and renders titanium less prone to develop leaks during
bakeout; (6) non-magnetic property is suitable for a duct and the component used in the strong
magnetic field; (7) the high corrosion resistance, since the surface oxide layer is a chemically stable.
Therefore, titanium is considered to be preferable for the vacuum applications.
3. Surface Finishing and Outgassing Property
3.1. Surface Finishing
The surface polishing is beneficial to vacuum materials, because a smooth and passive surface
influences the outgassing property. For stainless steels and aluminium alloys, various surface polishingsuch as the electrolytic polishing and the chemical polishing have been developed so far, and the low
outgassing property was obtained. On the other hand, the surface polishing for titanium materials werehardly evaluated. Although the mechano-chemical polishing was found to be effective to improve theoutgassing rate, this polishing method is not easily applied to complex shaped chambers andcomponents.
In this study, the chemical polishing effects for the titanium material were evaluated. Chemical
polishing using a nitric acid solution was found to be the most suitable for the pure titanium material
among the tested polishing solutions, since a smooth surface and a thin and uniform surface layer wereobtained. Since the cleaning process is important, the surface finishing which combines the chemical
polishing and the precision cleaning was developed. Here, the precision cleaning is composed of adegreasing, a nitric acid soaking and a pure water soaking processes.

2

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002

IOP Publishingdoi:10.1088/1742-6596/100/9/092002

3.2. Surface Analysis and Outgassing Property
3.2.1. Experimental Procedure
Pure titanium of JIS grade 2 was employed as sample material. Unpolished basis metal Ti (Ti(BM)),buffed Ti (Ti(BP)), mechano-chemically polished Ti (Ti(MCP)) and chemically polished Ti (Ti(CP)) wereprepared. The sample size for surface analysis was 10 mm×10 mm×1 mm. The surface roughness (Ra)was estimated by an atomic force microscopy (AFM) in 10 µm2 and 1 µm2 area. In order to measure thesurface oxide layer thickness, depth profiles were analyzed by an auger electron spectrometer (AES)
with Ar etching gas, and transmitting electron microscope (TEM) images were observed. The
distribution of hydrogen atoms around the surface layer was analyzed by a time of flight - secondary ion
mass spectrometry (TOF-SIMS). The outgassing rate was estimated by a modified orifice method
switching between two pumping paths (SPP) [17]. The detection limit of the system was 7×10-13 Pams-1
since the upstream and downstream pressures attained to 10-9 Pa and below, the orifice conductance C0
was set to 6.1x10-3 m3･s-1 and the sample area was 8.9×10-1 m2 [13]. The outgassing rates were measured
under the condition of non-baking or after baking at 393K for 20 hours.
3.2.2. Results and Discussions
Table 2 shows the Ra and thickness of surface oxide layer for Ti(BM), Ti(BP), Ti(MCP) and Ti(CP). In themacroscopic area of 10 µm2, Ra for Ti(CP) is small comparing to those of Ti(BM) and Ti(BP), while it is 10
times larger than that of Ti(MCP). However, the surface of Ti(CP) in microscopic area of 1 µm2 is assmooth as that of Ti(MCP). This implies the difference in Ra is due to the macroscopic undulation. Ra for
Ti(CP) is comparable to that for Ti(MCP) in microscopic area. Since the amount of the adsorption gas isinfluenced by the microscopic roughness, the surfaces of Ti(CP) and Ti(MCP) are considered to be smoothenough.
The thickness of the surface oxide layer in Ti(CP) is smaller than those of the other polished samples
as seen in Table 2, where the thickness was estimated by the half maximum value of the AES depth
profile of oxygen. The surface oxide layer is confirmed to be an amorphous by the cross sectional
TEM observation. The thin oxide surface layer is considered to act as barrier for bulk gas, mainlyhydrogen, diffusion, resulting in low outgassing.
Table 3 shows the outgassing rates measured at 5 and 50 hours after the pumping without baking
process, and those with the baking process. Ti(CP) and Ti(MCP) without baking showed 1/5 of theoutgassing rate comparing to the chemically polished stainless steel (SUS(CP)), and the outgassing rateof Ti(BP) is comparable to that for SUS(CP). The outgassing rates for Ti(CP) and Ti(MCP) with baking
process reach to 7×10-13 Pams-1, which is two orders magnitude smaller than that of SUS(CP) under thesame pre-baking process. This means that the chemical polishing and the mechano-chemical polishingare suitable as the surface treatment for titanium materials.
Table 2 Surface roughness and thickness of
surface oxide layer of polished pure titanium
Surface Roughness
(Ra)
2
10 µm
1.0 µm2
range (nm) range (nm)Table 3 Outgassing rates for polishedtitanium and chemical polished stainlesssteel

Thicknessof Surface
Oxide
Layer (nm)Non-Baking(Pams-1)
5h
50 h

Ti(BM)

108−

13

Ti(BP)

3.3×10-8

Ti(BP)

44−

10

Ti(MCP)

9.3×10-9 4.6×10-10

7×10-13Ti(MCP)

2.5

1.03

11

Ti(CP)

7.0×10-9 5.1×10-10

7×10-13

Ti(CP)

25

1.80

8

SUS(CP)

4.7×10-8

1.0×10-10

3

2.8×10-9

With
Baking(Pams-1)

3.1×10-9−

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002

Intensity (arb. u.)

10

Smooth surface

3

IOP Publishingdoi:10.1088/1742-6596/100/9/092002(a)

Weld point(c)

CF flangeThin surface oxide layer
Distribution ofhydrogen atoms
Bulk titanium

10

(b)

Knife edge

GasketVirgin

KS100

2

0

10
Depth (nm)

Vacuum chamber

20

JIS 2

Fig. 2 (a) Vacuum chamber made of titaniummaterial, (b) weld point between CF flange andpipe, (c) diagram of CF flange, and images ofknife edge of virgin CF flange and knife edge ofthe pure titanium of JIS grade 2 and KS100 after
50 times tightening.

Fig. 1 Depth profile for the negative hydrogenions (H-) in Ti(CP) and schematic diagram forthe low outgassing titanium material (insert
figure).

Figure 1 shows the TOF-SIMS depth profile of negative hydrogen ions (H-) in Ti(CP). Here the depth
was estimated by an etching rate of SiO2. The hydrogen atoms localize at the boundary between thesurface oxide layer and the bulk titanium. This region is considered to prevent the hydrogen diffusion,
which brings the excellent outgassing property. In addition, the thin oxide surface layer is considered
to be important for the lower outgassing because the thick oxide layer has many defects anddislocations which adsorb and dissolve gasses. Consequently smooth surface, thin oxide surface layer
and barrier region preventing the bulk hydrogen diffusion give the extremely low outgassing rate forthe titanium materials.
4. Development of Vacuum Chambers and Components
4.1 Vacuum Chambers and Components
Vacuum chambers and components for the UHV/XHV systems were fabricated using titaniummaterials as shown in Figs. 2 and 3. The machinability of the titanium materials is similar to that of thestainless steels. Titanium, however, has difficulty in welding, because the melting point is higher than1800 K and titanium is easy to oxidize. In order to avoid this problem, the titanium was welded using
an atmospheric shield in a semi-clean room, and the titanium material was successfully welded
without the oxidation scale as shown in Fig. 2 (b). Thus, the complex vacuum chamber with many
ConFlat (CF) flanges can be fabricated as shown in Fig. 2 (a).
A durability of the knife edge for the CF flange made of the titanium materials was evaluated.
Figure 2 (c) shows the top view of the knife edges of CF flanges, virgin flange (left), pure titanium of
JIS grade 2 (middle) and a hard titanium alloy, KS100 (right) which is product of KOBE Steel, Ltd..
Middle and right images of Fig. 2(c) are knife edges after 50 times tightening. The knife edge of the
CF flange made of the pure titanium of JIS grade 2 was worn out, and vacuum leak occurred. On theother hand, the knife edge made of KS100 showed durable property, and vacuum leak did not occur.
Therefore, hard titanium alloy of KS100 (Vickers hardness, 250 Hv), is considered to be suitable for
CF flange.
A titanium bellows is expected to have a low stiffness, because 106 GPa of Young’s modulus fortitanium is the almost half of stainless steel. Mechanical properties for a hydro-formed bellows madeof titanium were evaluated. The stiffness per convolution was 25 Nmm-1 which is 64 % of that for4

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002

IOP Publishingdoi:10.1088/1742-6596/100/9/092002

Ti
flangeback-up ringduct
Ti sleeve(a) Hydro-formed bellows(c) Alumina ceramic duct(e) Gate valve(b) Angle valve(d) Electrical feedthrough(f) AFM stage

Fig. 3 Vacuum components made of titanium materials.
stainless steels. The displacement endurance was similar to that of stainless steel. Many hydro-formed
titanium bellows as shown in Fig. 3 (a) were installed in the 3- and 50-GeV synchrotron and in the
beam-transport line in J-PARC. A welded bellows made of titanium was also fabricated, and the
durability was verified. The bellows was installed in an angle valve as shown in Fig. 3 (b).
Alumina ceramic duct jointed with Ti sleeve (see Fig. 3 (c)) and an electrical feedthrough (see Fig.
3 (d)) were fabricated by the metallizing (Mo-Mn) and the brazing (Ag-Cu). Rupture strength for a
joint part of the alumina ceramic and the titanium was obtained to be greater than 100 MPa which isenough for practical use. The alumina ceramic duct was used in a rapidly varying magnetic field of 3GeV synchrotron. The electric feedthrough was installed in a Kundsen cell for a molecular beam
epitaxy (MBE) system and in an UHV-AFM system. In addition other vacuum components, e.g. a gate
valve (see Fig. 3 (e)), an AFM stage (see Fig. 3 (f)) and a magnetic transporter were fabricated andinstalled in UHV-MBE and AFM systems.

5

10

−4

10

−5

10

−6

10

−7

10

−8

BakingPressure (Pa)

4.2 Vacuum Characteristics of UHV Systems
Figure 4 shows the pump-down curve of the MBE
system (main chamber: φ 200 mm × 440 mm, innerarea: 3.6×10-1 m2) made of the titanium material
measured from the beginning of the evacuation using
the turbo-molecular pump (pumping speed: 250×10-3
m3/s, base pressure: 2×10-8 Pa). The pressure showed
1×10-6 Pa at 3 hours after the pumping. The pressure
showed 5×10-8 Pa at 12 hours after the pumping
through the modest baking process at 393 K for 6hours, and the ultimate pressure at 25 hours later was
2×10-8 Pa which corresponds to the base pressure of
the vacuum pump. Thus, UHV pressure is quickly
obtained for the vacuum system without / withbaking process. Moreover simple test chamber (φ 250
mm × 420 mm, inner area: 4.2×10-1 m2) reached to
10-10 Pa evacuated by an ion pump.
Figure 5 shows the pressure rise measured by a
B-A gauge during the build up measurement of the

0

5

10

15

20

25

Time (hours)
Fig. 4 Pump-down curve of MBE systemcomposed of titanium material.

IVC-17/ICSS-13 and ICN+T2007
Journal of Physics: Conference Series 100 (2008) 092002[× 10-5]

10

Pressure (Pa)simple test chamber (φ 250 mm×420 mm, innerarea 4.3×10-1 m2) composed of titanium material.
The test chamber retained the pressure lower than
1×10-6 Pa for the first 6 months, and the pressurerose to 8×10-5 Pa at 12 months later. The
outgassing rate is estimated to be 4×10-13 Pams-1 at
the rapid increasing region after 8 months. This is
close to 7×10-13 Pams-1 obtained by SPP method.
This indicates that the titanium materials haveexcellent outgassing property.

IOP Publishingdoi:10.1088/1742-6596/100/9/092002

0

2

Month
4
6
8

10

12

5

5. Summary
This paper described the outgassing property of thetitanium materials, and development of the vacuum
07chambers and components for the practical
0
1.0
2.0
3.0 [× 10 ]UHV/XHV systems. The mechano-chemicalTime (s)polishing and the chemical polishing give titaniumFig. 5 Pressure rise during the build up
the smooth surface and the surface oxide layer
measurement of test chamber made ofthinner than 10 nm which provides excellent
titanium material.
outgassing property. Outgassing rates are lower
than 10-12 Pams-1 through baking process, and theoutgassing rate without baking process is about 1/5 of that for the stainless steel.
Vacuum chambers and components have been fabricated for UHV/XHV systems utilizing thetitanium materials. The titanium was successfully welded by the atmospheric shielding without
formation of the oxidation scale. Hard titanium material was found to be preferable for the knife edgeof the CF flange. Vacuum equipments were also fabricated using titanium materials, which showed
excellent vacuum property.
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