Non-Evaporable Getters · = 100 l/s Duration of ... 5 10 1.4 5 3 H 2 sorption regeneration cycle ....

Non-Evaporable Getters

Dr. Oleg B. Malyshev

Senior Vacuum Scientist

ASTeC Vacuum Science Group,

STFC Daresbury Laboratory, UK

1 VS-2, 18-19 October 2011, Ricoh Arena , Coventry

Sorption and diffusion

Gas atoms or molecules

(adsorbate)

Solid surface (adsorbent)

Sticking probability s1

Physisorption (dipole or

van der Waals forces)

Chemisorption (covalent

linkage)

Binding energy

Reflecting probability (1s)

Handbook of vacuum technology. Ed.

K. Jousten, Weley-VCH, Weinheim,

2008, Chapters 6,11

VS-2, 18-19 October 2011, Ricoh Arena , Coventry 2/40

Solid

adsorbent

Gas,

adsorbate

absorbed molecules diffusion

surface diffusion adsorbate desorption

Classification of sorption pumps


Sorption pumps

Adsorption pumps (physi- and cryosorption )

Getter pumps (physi- and chemisorption)

With ionisation Without ionisation

Orbitron

pumps

Ion getter

pump

Evaporation

pumps Bulk getter

pumps

NEG coating

NEG

pumps

Getters

Evaporation pumps

Adsorption mainly

Chemical binding at surface

Covering with a fresh material

after saturation

Bulk (non-evaporable) getter pumps

Bulk getter not only adsorb gases at the

surface

but also employs an effect of gas

diffusion into a getter material

Re-activation by heating to an activation

temperature

Gas accumulation in getter materials due to:

Adsorption at the surface

Absorption (i.e. solvation) in the bulk

Due to diffusion, reversible process

Chemical binding

Irreversible process

VS-2, 18-19 October 2011, Ricoh Arena , Coventry 4

Complications in studying and using NEG

Complexity of getter material and processes:

Oxide layers

Surface roughness

Bulk structure and morphology

Temperature (activation and operation)

History of using the getter

Gas-solid combinations

Optimum condition of operation depends on

applications


NEG structure

VS-2, 18-19 October 2011, Ricoh Arena , Coventry 6/40/

Metal substrate

NEG powder

• Zr, V, Ti as active metals mixed with Fe, Al

• Developed surface area and boundaries between

grains

• Protective oxide layer before activation

How adsorption works

Sticking coefficient

Local parameter, depends

on incident angle and

surface orientation

Sticking probability

Statistically averaged value

for the surface

Capture probability β

Statistical value for a pump

with a given geometry


Cannel with sorbing walls L/d=10

α

α

α β

Q

1 104

1 103

0.01 0.1 1

1 104

1 103

0.01

0.1

1

α

β

Types of NEGs (SAES getters)


St707 Strip

The thickness of

the getter layer is

about 70

micrometers on

each side of the

strip

Getter Wafer

Modules (based on

St101 or St707

getter strips) are

specifically designed

to maximize

hydrogen pumping

speed.

Cartridge

pumps can

be mounted

on CF

flanges

Compressed

getter pills

and washers

for various

applications

Pumping

What can be pumped by NEG

M + O2 MO

M + N2 MN

M + CO2 CO + MO

MC + MO

M + CO MC + MO

M + H2O H + MO

MO + H (bulk)

M + H2 M + H (bulk)

M + CxHy MC + H (bulk)

at T > 500C

What can’t be pumped

Hydrocarbons, CxHy, etc.

at T < 500C

He, Ne, Ar, Kr, Xe (inert

gases)


Activation

First activation consiredations:

If capacity Q=5000 mbarl

Activation pressure Pa= 10-4

mbar

Pumping speed during

activation Seff = 100 l/s

Duration of activation t:

Pa= 10-2 mbar and Seff = 100 l/s

Pa= 10-3 mbar and Seff = 10 l/s


dayshrst

hrsSP

Qt

effact

6140105

4.1105

5

3

H2 sorption

regeneration cycle

Activation temperature and duration (SAES Getters)

Short (tens minutes)

activation used for

independent activation

T 300C allows

activation during vacuum

system bakeout (without

special arrangement)

NEG partially activated

during bakeout

at T 250C


Problems and limitations

Limited capacity for pumping

Aging (limited number of

activation cycles before full

saturation)

Hydrogen

enbrittlement (2 x

pumping capacity)

Thermal fatigue

(number of activation

cycles before initial

peel-off)

Storage (in vacuum,

N2 or inert gas

atmosphere)


Operation at high temperature

Below activation

temperature

High temperature

increases the diffusion

Continues re-activation

Operation at activation

temperature for high

pumping speed and

capacity.

Requires another pump

for H2 pumping


Use of NEGs

High gas load

UHV/XHV application

In combination with a pump for noble gases and

hydrocarbons, ex. such as TMP and SIP

Purification of noble gases

The only gases that can’t be pumped by NEG

Low power consumption

Power used for activation only

Hydrogen pressure regulator in UHV system

By changing the NEG temperature

14/40 VS-2, 18-19 October 2011, Ricoh Arena , Coventry

NEG application in accelerators


Usual accelerator vacuum chamber

Long tube with length L >> a, where a - transversal dimension

Average pressure depends on vacuum conductance u(L,a) of

the beam vacuum chamber

16 WS-63, 14-19 September 2010, Ávila, Spain

1

12 2B

eff

LP qL k T

u S

P

z

L

Vacuum chamber cross sections

WS-63, 14-19 September 2010, Ávila, Spain 17/

Beam pipe

Circular or elliptical

4 mm d, a, b 200 mm

Vacuum chamber with an antechamber

for larger vacuum conductance, U

Distributed pumping In dipole magnetic field With NEG strips

(LEP in CERN)

d a

b

Two concepts of the ideal vacuum chamber

Traditional:

surface which outgasses as little as

possible (‘nil’ ideally)

surface which does not pump

otherwise that surface is

contaminated over time

Results in

Surface cleaning, conditioning,

coatings

Vacuum firing, ex-situ baling

Baking in-situ to up to 300C

Separate pumps

18/40

New (C. Benvenuti, CERN, ~1998):

surface which outgasses as little as

possible (‘nil’ ideally)

a surface which does pump,

however, will not be contaminated

due to a very low outgassing rate

Results in

NEG coated surface

There should be no un-coated parts

Activating (baking) in-situ at 150-

180C

Small pumps for CxHy and noble

gases

VS-2, 18-19 October 2011, Ricoh Arena , Coventry

Stainless steel vs NEG coated vacuum chamber under SR

19/40 EVC-11, 20-24 September 2010, Salamanca, Spain

V.V. Anashin et al. Vacuum 75 (2004), p. 155.

NEG coating for accelerators

First used in the ESRF (France);

ELETTRA (Italy), Diamond LS (UK);

Soleil (France) – first fully NEG coated;

LHC (Switzerland) – longest NEG coated vacuum chamber;

SIS-18 (Germany) and many others.

NEG film capacity for CO and CO2 is ~1ML:

If P = 10-9 mbar then 1 ML can be sorbed just in ~103 -104 s;

Lab measurements of different NEG coatings often don’t repeat

CERN’s data on sticking probability and capacity;

However, NEG coated parts of accelerators work well.


NEG coating for accelerators (2)

What is required:

Input data for accelerator design:

(D,E,Ta), (M, Ta), pumping capacity;

Better understanding:

what and why;

practical ‘do’s and ‘don’t’s;

Further development of this coating:

lower , Ta, SEY;

higher (M), pumping capacity;

optimising for an application.


What NEG coating does

Reduces gas desorption:

A pure metal film ~1-m thick

without contaminants.

A barrier for molecules from

the bulk of vacuum chamber.

Increases distributed

pumping speed, S:

A sorbing surface on whole

vacuum chamber surface

S = Av/4;

where – sticking probability,

A – surface area,

v – mean molecular velocity

EVC-11, 20-24 September 2010, Salamanca, Spain 22/40

Vacuum NEG Subsurface Bulk

Coating Layers

Deposition method

Planar magnetron deposition

Cylindrical magnetron deposition


Details are in the talk A3.TM.OR.13 by Dr. R. Valizadeh

ASTeC activation procedure


O.B. Malyshev, K.J. Middleman, J.S. Colligon and R. Valizadeh. J. Vac. Sci. Technol. A 27 (2009), p. 321.

NEG pumping properties


Pressure ratio P1/P2 measured during

gas injection is used to estimate:

initial sticking probability and sorption capacity

Films deposited on Si test sample from a single metal wire

Ti film

Average grain size

100 – 150 nm.


V film

Average grain size

100 – 150 nm.

Hexagonal lattice structure

Zr film.

Average grain size

100 – 150 nm. Hexagonal

lattice structure

27/40 WS-63, 14-19 September 2010, Ávila, Spain

Single metal pumping properties

140 160 180 200 220 240 260 280 300 3201 10

3

0.01

0.1

1

Ti

Zr

V

Hf

CO

stick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti

Zr

V

Hf

Activation temperature [ C]

H2 s

tick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3201 10

3

0.01

0.1

1

10

Ti

Zr

V

Hf

CO

pum

pin

g c

apac

ity [

ML

] Zr is best:

Lowest activation Temp. and

highest capacity

Hf

Ti

V has highest activation

temperature 27

Binary films deposited on Si test sample from twisted wires.

Ti-V film.

Average grain size

50 – 100 nm.

Hexagonal lattice

structure.

WS-63, 14-19 September 2010, Ávila, Spain 28

Ti-Zr film.

Average grain size

50 – 100 nm.

Hexagonal lattice

structure.

Zr-V

Average grain size

10 – 20 nm

29/40 WS-63, 14-19 September 2010, Ávila, Spain

Binary alloy pumping properties

140 160 180 200 220 240 260 280 300 3201 10

3

0.01

0.1

1

Ti-Zr

Ti-V

Zr-V

CO

stick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr

Ti-V

Zr-V


H2

stick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3201 10

3

0.01

0.1

1

10

Ti-Zr

Ti-V

Zr-V

CO

pu

mp

ing c

apac

ity

[M

L]

Zr-V is best

Ti-Zr activation temperature

is lower than for Ti-V

Zr-Hf was not studied

Ternary NEG film deposited on Si test sample from

twisted Ti, V, Zr, and Hf wires and TiZrV alloy wire

Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.

Average grain size 5 nm. Hexagonal lattice structure.

Ti-Hf-Zr twisted wire V-Hf-Zr twisted wire

Ti-Zr-V alloy wire Ti-Zr-V twisted wire

30


Ternary alloy pumping properties

140 160 180 200 220 240 260 280 300 3201 10

3

0.01

0.1

1

CO

stick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr-V

Hf-Zr-V

Ti-Zr-Hf

Ti-Hf-V


H2

stick

ing p

robab

ility

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

10

CO

pu

mp

ing c

apac

ity

Hf-Zr-V, Ti-Zr-Hf and

Ti-Hf-V are comparable

Ti-Zr-V has the highest

activation temperature

Twisted wires vs. alloy target: reducing Ta

1.E-02

1.E-01

1.E+00

140 160 180 200 220 240 260 280 300 320

Activation temperature, oC

CO

sti

ck

ing

pro

ba

bil

ity

TiZrV(twisted wires)

TiZrV (alloy wire)

TiZrV (alloy wire)


R. Valizadeh, O.B. Malyshev, J.S. Colligon, V. Vishnyakov. Accepted by J. Vac. Sci. Technol. Aug. 2010.

Quaternary NEG alloy film deposited on Si test sample from twisted Ti, V, Zr, and Hf wires.

Cylindrical Magnetron: Power = 60 W, PKr = 10-2 mbar, deposition rate = 0.12 nm/s, T = 120°C.

Very glassy structure.

33


Quaternary alloy pumping properties

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

CO

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3201 10

4

1 103

0.01

0.1

Ti-Zr-Hf-V

Hf-Zr-V

Ti-Zr-Hf

Ti-Hf-V

Ti-Zr-V

Ti-Zr

Zr-V

Zr


H2

stic

king

pro

babi

lity

140 160 180 200 220 240 260 280 300 3200.01

0.1

1

10

CO

pum

ping

cap

acity

Ti-Zr-Hf-V is the best

Hf-Zr-V, Ti-Zr-Hf, Ti-Hf-V and

Zr are comparable

Ti-Zr-V is lower

Zr-V (best binary alloy) has the

lowest activation temperature

Pressure in the accelerator vacuum chamber

Improving pumping

properties is limited:

1.

0.005 < H2 < 0.01

0.1 < CO < 0.5

0.4 < CO2 < 0.6

Reducing the

desorption yields

. in orders of magnitude

is a realistic task


P

where

- desorption yield

- sticking probability

Reducing the gas desorption from the NEG coatings

Main gases in the NEG coated vacuum

chamber are H2 and CH4

Only H2 can diffuse through the NEG

film under bombardment or heat

CH4 is most likely created on the NEG

surface from diffused H2 and C

(originally from sorbed CO and CO2)

Therefore the H2 diffusion must be

suppressed

Where H2 come from?



Gas molecules are contained

on the NEG coating surface

after exposure to air

inside the NEG coating

trapped during deposition

in subsurface substrate layer

in the substrate bulk

Vacuum

NEG

Coating

Subsurface

Layers

Bulk



Gas molecules are contained

on the NEG coating surface

after exposure to air

minimise exposure to air

inside the NEG coating

trapped during deposition

purity of discharge gas

background pressure

in subsurface substrate layer

substrate bakeout before NEG deposition

in the substrate bulk

vacuum firing

Vacuum

NEG

Coating

Subsurface

Layers

Bulk



SEM images of films (film morphology )

columnar dense

Best for pumping A first candidate for a barrier

O.B. Malyshev, R. Valizadeh, J.S. Colligon et al. J. Vac. Sci. Technol. A 27 (2009), p. 521.

Modified NEG pumping properties

evaluation rig:

To measure sticking probability

To measure electron stimulated gas

desorption as a function of

Electron energy

Dose

Wall temperature (20-100C)

Activation/bakeout temperature

Can be used for samples with:

NEG coating

Low desorption coating

No coatings


Electron stimulated desorption

Electron Stimulated Desorption (ESD) studies programme

ESD as a function of

Activation/bakeout temperature

Electron energy

Electron dose

Coating density, morphology and structure

Deposition conditions

Substrate


ESD: stainless steel vs activated NEG coated vacuum chamber


1 1020

1 1021

1 1022

1 1023

1 108

1 107

1 106

1 105

1 104

1 103

0.01

0.1

H2

CH4

H2O

CO

O2

Ar

CO2

dose [electrons/m2]

yie

ld [

mole

cule

s/e

lectr

on]

1 1020

1 1021

1 1022

1 1023

1 108

1 107

1 106

1 105

1 104

1 103

0.01

0.1

H2

CH4

CO

dose [electrons/m2]

yie

ld [

mole

cule

s/e

lectr

on]

O.B. Malyshev, A. Smith, R. Valizadeh, A. Hannah.

Accepted by J. Vac. Sci. Technol., Aug. 2010.

Baked to 250°C for 24 hrs Activated to 180°C for 24 hrs

1 103

0.01 0.1 1

1

10

100

1 103

G1

G4

Surface coverage [monolayers ]

Rat

io [

P1

/P2]

Electron

bombardment 1

Electron

bombardment 2

Electron stimulated NEG activation


Non-activated NEG

The electron stimulated NEG activation efficiency

estimated as 7.9×10-4 < 1 < 2.4×10-3 [CO/e-] 1

COCO

e D

Activated at 180°C P1

P2

(Ee-) for different gases for NEG coating

44 EVC-11, 20-24 September 2010, Salamanca, Spain

NEG cartridges vs films


Bulk NEG NEG film

Thickness 50 m – 5 mm 0.5 – 3

Materials Zr-V-Fe (St 707)

Zr-Al (St 101)

Ti-Zr-V (CERN)

Ti-Zr-Hf-V (ASTeC)

Activation T 300 – 700 C 150 – 300 C

Pumping capacity Large 1 ML for CO, CO2

No. activation cycles

before full staturations

A few hundreds < 100

Working range HV, UHV, XHV UHV, XHV

Activation by photon or

electron bombamdment

n/a possible

Main purpose pumping Barrier coating to reduce

outgassing + pumping

Conclusions

NEGs pay and increasingly important role in vacuum

technology including UHV/XHV

They have wide range of applications, including particle accelerators

NEG can be activated in vacuum by heat,

NEG films can also be activated by photon or electron bombardment

Ones activated no power or controllers required for

operation

No noise and cooling channels

In some applications no cables, feedthroughs

To use them some special knowledge of NEG installation,

activation operation are essential

46/40 VS-2, 18-19 October 2011, Ricoh Arena , Coventry

Non-Evaporable Getters · = 100 l/s Duration of ... 5 10 1.4 5 3 H 2 sorption regeneration cycle ....

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