Vacuum firing

Transcription

Vacuum firing

Thermal outgassing of
hydrogen: models and methods
for reduction.
Paolo Chiggiato
CERN
TS-MME
Coatings, Chemistry and Surfaces
CH-1211 Geneva 23
05 Novembre 2003
Thursday, May 18, 2006, CAS, Spain
1
1. Introduction
2. Decreasing the H concentration:
a. the thermodynamic frame
b. kinetics models: diffusion or recombination?
c. efficiency of bakeout
d. vacuum firing
3. Hindering H desorption from the surface
a. air bakeout
b. passive coatings
c. active coatings
4. Internal trapping
2
Introduction
Hydrogen is released in the gas phase after migration in the metal lattice
and recombination on the surface.
It follows that hydrogen outgassing could be reduced in essentially
three ways:
1. decreasing the concentration of this gas in the solid,
2. hindering the desorption from the surface
3. reducing the mobility of atomic hydrogen in the lattice.
by adding by
internal
bycoating
heating
H traps
the
in inner
vacuum
(internal
wall pumping)
3
Decreasing the concentration
Heating of UHV components is applied:
¾ in-situ: bakeout of the vacuum system already assembled
¾ ex-situ: vacuum firing of the components or of the rough materials
The Significant
heating temperature
only for austenitic
is limitedstainless
by the materials:
steel;
in general applied in a vacuum
furnace
at temperatures in the range
Al alloys
Ö 200°C
Cu450°C÷1050°C
alloys Ö 250°C
or by the kind of flange adopted:
Standard Conflat® Ö 400°C
4
Decreasing the concentration:
the thermodynamic frame
H depletion is possible during the thermal treatment only if the H2
pressure is lower than the dissociation pressure.
Exothermic absorption
xH = B ⋅ e
−
ΔH s 0
2 k BT
⋅ PH 2
Sieverts’ law
Endothermic absorption
ΔΗs/2
Most of the metals used for the
construction of UHV vessels absorb H2
endothermically (ΔHs>0).
For these materials, for a constant applied
pressure, increasing the temperature
results in an increase of the equilibrium
concentration of H in the solid.
5
the thermodynamic frame
P [Torr]
1 wt. ppm
Solubility of H2 in stainless steel:
10000
xH [at.ppm] = 71.8 ⋅ P[Torr ] ⋅ e
0.114 [eV ]
k B ⋅T
The H content in standard austenitic
stainless steels is about 1 ppm in
weight (≈50 at. ppm).
1000
H2 pressures lower than 6 Torr are
necessary to reduce the H content
of as received stainless steels at
950°C.
100
10
−
H depletion is possible
Treatments at lower temperatures
are much more efficient from a
thermodynamic point of view.
20
50
100
200
500
1000 But time…
T [°C]
6
the kinetic frame
The efficiency of the degassing treatment can be calculated once the
limiting mechanism of the outgassing process is identified.
Two mechanisms could cause an obstruction to the degassing process:
∂c
1. diffusion in the metalÆ q (t ) ∝ −
∂x
2
2. recombination on the surfaceÆ q (t ) ∝ cs
Concentration gradient
Square of the
concentration on the
surface
7
the kinetic frame
After one bakeout at the temperature Tbo for a duration tbo the outgassing rate is:
Diffusion model
4 ⋅ c0 ⋅ D(TRT )
n ⋅ D(Tbo ) ⋅ tbo ⎞
⎛
qn (t ) =
exp⎜ − π 2 ⋅
⎟
L
L2
⎠
⎝
Recombination model
⎛ c0 ⎞
⎟⎟
q1 = K [TRT ] ⋅ ⎜⎜
⎝ 1 + B ⋅ c0 ⎠
2
where B =
K R ⋅ tbo
L
supposing Fo>5x10-2.
It can be shown that after n identical bakeout:
qn = A ⋅ exp(− B ⋅ n )
where A =
4 ⋅ c0 ⋅ D(TRT )
D(Tbo ) ⋅ tbo
B =π2⋅
L
L2
2
⎛
⎞
c0
⎟⎟ ∝ n − 2
qn = K [TRT ] ⋅ ⎜⎜
⎝ 1 + n ⋅ B ⋅ c0 ⎠
Æ Linearity in Log(q)-n plot
qn +1 (t )
D(Tbo ) ⋅ tbo ⎞
⎛
= exp⎜ π 2 ⋅
⎟
qn (t )
L2
⎝
⎠
Æ Each bakeout reduces the
outgassing rate at room
temperature by the same factor.
Æ Additional bakeout are less
efficient.
Experimental verification Ö
8
efficiency of in-situ bakeout
Stainless steel: CERN unpublished results (P.C.)
Case study 1:
stainless steel sheets 2 mm thick
y = 1.7456e-12
* e^(-0.47082x)
bakeout
at 300°C
for 20R=h0.97304
-11
10
Each bakeout reduces the outgassing rate
by a factor of ≈ 1.6.
-2
cm ]
A=1.75x10-12
B=0.471
From B:
10
If the diffusion rate at room temperature
reported in the literature is assumed, from
the value of A:
-13
10
Co= 0.05 Torr l/cm3=0.75 wt. ppm
This is a very reasonable quantity for
austenitic stainless steels.
2
H Outgassing [Torr
l
s
-1
D(300°C)=2.2x10-8 cm2 s-1
-12
-14
10
0
Ö
2
4
6
8
10
12
14
bakeout cycles (300°C x 24 h)
9
Case study 2:
316 LN Stainless steel: CERN
AT-VAC int. note
J-P Bojon, N. Hilleret, B.
Versolatto
1.00E-12
H2 outgassing rate [Torr l s-1 cm-2]
bakeout at 300°C, 24 h
stainless steel sheets 1.5 mm thick
1.00E-13
As received
1.00E-14
After vacuum firing
1.00E-15
Each bakeout reduces the outgassing rate
by a factor of ≈ 1.8
1.00E-16
1
2
3
4
5
Bakeout cycles
10
Case study 3:
stainless steel sample
2.0 mm thick
Bakeout @ 300°x25h
ESCHBACH, H. L.,
et al. 1963,
Vacuum, 13, 543-7.
“…a logarithmic decrease with successive bakeouts, behaviour to be
expected from the analysis, though the slope is somewhat smaller than
expected (-0.43 instead of -0.75 from equation. This could be attributed
to a slightly different diffusion coefficient…“
11
The experimental results indicate that diffusion could be the rate
limiting process for bakeout carried out at 300°C and for ≈ mm thick
sheets.
Other experimental results shows that, for samples of similar
thickness, a unique energy is associated to the desorption process: the
diffusion energy
Other experimental cases Ö
12
Stainless steel: CERN unpublished results
(Géraldine Chuste)
Case of study 4:
-7
10
Desorption energy: stainless steel
-1
-2
Outgassing rate [Torr.l.s .cm ]
235
-8
10
Vacuum pipe dimensions
Not fired : baked @ 200°C
20h
200
150
Length
200 cm
Diameter
3.4 cm
Thickness
2 mm
-9
10
Literature:
100°C
0.5 eV/ at. ≈ diffusion
energy of hydrogen in
austenitic stainless steel
-10
10
-11
10
Ed = 11 kCal/mol ~ 0.5 eV/at.
50
RT
OK!
-12
10
3. 10-12 torr.l.s-1.cm-2
-13
10
300°C
0.0023
0.0028
-1
1/T [K ] - T sample
0.0034
18°C
13
OFS copper bakes at 200°C for 20h:
CERN unpublished results (Géraldine Chuste)
Case of study 5:
10
-1
-2
Outgassing [ Torr.l.s .cm ]
Desorption energy: surface etched OFS copper
-12
10
-13
10
-14
10
-15
10
-16
Vacuum pipe dimensions
Ed=9 Kcal mol-1
700 cm
Diameter
8 cm
Thickness
2 mm
Literature:
0.39 eV/ at. ≈ diffusion
energy of hydrogen in
copper
2
2.2
2.4
2.6
2.8
3
o
250 C
⎡ 0.39 ⎤
D = 8.34 ⋅10 exp ⎢−
⎥
⎣ kB ⋅T ⎦
−3
Length
3.2
3.4
OK!
o
-1
1000/T [K ] - T sample
20 C
2. 10-15 Torr.l.s-1.cm-2
14
Stainless steel: CERN unpublished results
(Géraldine Chuste)
Case of study 6:
Bakeout at low temperature (stainless steel)
-7
-1
-2
10
200
150
-8
10
-9
100°C
10
10
RT-100°C:
The outgassing rate is 2
times lower when a
bakeout at 80°C is applied
-10
50
Between 100 and 150°C:
The outgassing rates after
bakeout at 200°C and 80°C
converge.
RT
10
-11
10
-12
10
-13
Not fired :
baked @ 200°Cx20h
baked @ 80°Cx20h
300°C
0.0023
Ed = 0.51 eV/at.
Hydrogen is blocked or
converted to water.
0.0028
-1
1/T [K ] - T sample
0.0034
18°C
1 eV/at = 23 kCal.mol-1
15
Vacuum firing
Vacuum firing
16
Vacuum firing
Vacuum firing
T < 500°C
diffusion is too slow
500°C (600°C)< T < 900°C (depending on the steel grade)
carbide and carbo-nitride precipitation
residual δ-ferrite transformation into σ- phase (very brittle)
T > 1050°C
Solution annealing, abnormal grain growth, recrystallisation, excessive
nitrogen loss
17
Vacuum firing
Vacuum firing
2
24 h
Ldif = D(TF ) ⋅ t F
Diffusion length [cm]
1.5
12 h
6h
1
4h
2h
0.5
1h
0.5 h
0
0
200
400
600
800
1000
1200
1400
Temperature [°C]
18
Vacuum firing
Vacuum firing
The CERN large furnace
Length:
6m
Diameter:
1m
Maximum charge weight:
1000 Kg
Ultimate pressure:
8x10-8 Torr
Pressure at the end of the
treatment:
high 10-6 Torr range
19
Part 6: Methods for the reduction of H2 outgassing
Melting in electric Arc furnace (ARC) + AOD
Preliminary
considerations:
production of austenitic
stainless steels for UHV
applications
Refinement: Electro-Slag Remelting (ESR):
Final ingot
Rolling
Thermo-Mechanical treatments
Forging
Thickness ≥ 5mm: Hot-rolling
Thickness < 5mm: Cold-rolling
Sheets (vacuum chambers)
Plates (for flanges)
Solution annealing (1050-1100°C) + water quenching
Acid pickling (HNO3 solution)
Skin-passing (for very thin sheets)
20
Vacuum firing
Modification of Mechanical and Metallurgical properties after vacuum firing
Hardness HB (ISO 6506)
As received
Fired 950° C
Fired 1050° C
304L
150
128
126
316L
130
121
109
316LN
155
151
139
No additional precipitates have been detected after vacuum firing at 950° C
No significant variation of ”rupture strength” and “stretch at break”: less than 5%
21
Vacuum firing
Modification of the surface roughness induced by vacuum firing
recrystallization
R
as cleaned
Vacuum fired
t
as cleaned
0.1
0
R values [μm]
a
0.2
t
Vacuum fired
3
0.4
0.3
R
3.5
Vacuum fired
0.5
R values [μm]
a
2.5
as cleaned
2
1.5
Vacuum fired
as cleaned
1
0.5
304L
316L
0
304L
316L
22
Vacuum firing
Sublimation of metallic elements during vacuum firing
Vapor Pressure of the pure elements
Vapour Pressure [Torr]
10
10
0
[R.K. Wild, Corrosion Science, 14(1974)575]
[A.F. Smith, R. Hales, Metals Science
Journal, 9(1975)181]
950°C
-2
Cr
Mn
10
DCr= 7 x 10-15 cm2 s-1
-4
Ni
10
-6
10
-8
Diffusion coefficients at 950°C
in austenite:
DMn= 6 x 10-15 cm2 s-1
DFe= 2 x 10-15 cm2 s-1
Fe
600
800
1000
1200
Temperature [°C]
1400
DNi= 5 x 10-16 cm2 s-1
Because of the higher sublimation rate of Cr, the surface of stainless steels
is expected to be enriched with Fe after firing .
23
Vacuum firing
Surface composition after vacuum firing
cleaned
Cr 2p3/2
cleaned
fired
fired
585
580
575
binding energy [eV
570
536
534
OX
O1s
OH
532
530
528
binding energy [eV]
¾ After vacuum firing the oxide layer is strongly enriched with Fe: Cr/Fe= 0.33
for 316L and 0.22 for 304L (0.75 for cleaned); oxide thickness as for cleaned.
¾ Cr2p2/3 and O1s lines indicate the presence of less hydroxides than on cleaned
samples (Cr2O3 and Fe2O3)
J. Gavillet and M. Taborelli, unpublished results
24
Vacuum firing
10
0
10
316LN
Vacuum fired
-1
H
10
As cleaned
-2
Vacuum fired
2
CO
Desorption yields
[molecules/electron]
Desorption yields
10
10
0
-1
10
-2
10
-3
-3
150
300
24 hours bakeout temperature [°C]
Vacuum fired
CO
As cleaned
CH
Vacuum fired
2
10
316LN
10
4
-4
150
300
24 hours bakeout temperature [°C]
The desorption yields of vacuum fired stainless steels are similar to those
of cleaned samples Ö vacuum firing does not reduce electron stimulated
desorption.
25
Vacuum firing
BN surface segregation
¾ At temperature higher than 700°C, boron segregates to the surface and, in
N added stainless steels (316LN), can form h-BN. Heating temperatures
higher than 1150°C are needed to dissolve the h-BN layer.
¾ BN does not form for B concentration lower than 9 ppm.
¾ When the concentration is equal or larger than 9 ppm BN forms only when B
is free to move, namely not blocked in BN precipitates already existing in the
steel bulk.
¾ The BN layer strongly reduces the surface wettability and may produce peeloff of thin film coatings.
¾ The BN layer can be effectively removed by electropolishing.
26
Vacuum firing
Vacuum firing
The outgassing rate after the vacuum firing treatment can be calculated in the frame of
the diffusion limited model.
Two asymptotic values are identified.
For thin sheets the initial gas content is fully emptied. In this case the H2
pressure in the furnace can’t be neglected: it defines the final concentration
through the Sieverts’ law.
2650
−
⎡ Torr l (H 2 ) ⎤
−2
1.99⋅T [ K ]
c⎢
=
8
.
21
⋅
10
P
[
Torr
]
⋅
e
3
⎥⎦
⎣ cm
An ultimate minimum concentration of about 6x1015 atoms H cm-3 could be
attained after the treatment when the pressure in the furnace is about 10-5 Torr.
For thick slab, the model has to converge to the semi-infinite solid
approximation. Actually, in this case the pressure in the furnace has a very
limited influence
27
Vacuum firing
After firing the concentration in the solid is given by:
Vacuum firing
(2n + 1)π ⋅ x −( 2 n +1) 2 π 2 Fo (T f ,t f )
(−1) n
c( x, t ) = cw + (co − cw ) ∑
e
cos
π n = 0 2n − 1
L
4
∞
Fo(Tf, tf) is the Fourier
number for the firing
treatment
D(T f ) ⋅ t f
F0 (T f , t f ) =
L2
In the consecutive bakeout it evolves
as the concentration in a plane sheet
with uniform initial concentration cw
and close to zero surface concentration
In the consecutive bakeout the
Fourier number is increased
accordingly.
4 ⋅ cw ⋅ D(T ) ∞ −( 2 n +1) 2 π 2 ⋅Fo (Tbo ,tbo ) 4 ⋅ (c0 − cw ) ⋅ D(T ) ∞ −( 2 n +1) 2 π 2 ⋅[ Fo (T f ,t f ) + Fo (Tbo ,tbo )]
+
q(t ) =
e
e
∑
∑
L
L
n =0
n =0
where T is the temperature of the measurement.
28
Vacuum firing
D(T ) ⋅ c0
π ⋅ (D(T f )t f + D(Tbo )tbo )
2. ´ 10- 13
Calculation performed for 1 wt. ppm
initial concentration and in-situ
bakeout at 150° C for 24 h.
semi-infinite model
q [Torr l s-1 cm-2]
1. ´ 10- 13
5. ´ 10- 14
Values for diffusivity and solubility
taken from P. Tison, Thesis
Université Pierre et Marie Curie, Paris
6 CEA-R-5240(1) 1984
2. ´ 10- 14
1. ´ 10- 14
10-5 Torr pressure
in the furnace
5. ´ 10- 15
Outgassing rates in the 10-15 Torr l s-1
cm-2 range are expected. For thick
plates (flanges) the rate is 20 times
larger
For zero pressure
in the furnace
2. ´ 10- 15
1. ´ 10- 15
Vacuum firing
0.5
1
1.5
2
2.5
3
3.5
4
L [cm]
29
Vacuum firing
Case study of 1:
CERN unpublished results (Géraldine Chuste)
Material:
316LN
Vacuum firing:
950° C x 2 h, 10-5 Torr H2
For the fired chambers,
the outgassing rate is
limited by the background
signal. The results were
confirmed in a second
system. On both systems,
the upper limit at RT is
10-14 Torr.l.s-1.cm-2.
Not Fired: (baked @ 200°C)
-2
-1
2 mm
Wall thickness:
-8
10
-9
10
10
-10
10
10
-11
250
Fired: (baked @ 200°C)
200
-12
150
10
-13
10
-14
100
50
RT
background
300°C
0.0023
0.0028
-1
1/T [K ] - T sample
0.0034
18°C
30
Vacuum firing
Case study of 2:
R Calder and G. Lewin, Brit. J. Appl. Phys., 1967, Vol. 18, p 1459
In 1967, R. Calder and G. Lewin reported several data for vacuum fired stainless steels
(firing:1000° C for 3 h in 2x10-6 Torr residual H2 pressure, in situ bakeout: 360°C for 25
h, dimensions: 1.1x103 cm2 of 2 mm thick vacuum chamber + 104 cm2 of 0.25 mm thick
strip). Their main results, obtained by the throughput method, were:
1. the measured outgassing rates at room temperature of fired and in-situ baked
stainless steels were between 6.9x10-15 and 1.3x10-14 Torr l s-1 cm-2.
2. the value of the outgassing rate does not increase by heating the sample up to 100°
C, and a small increase can be record only at 200° C (1.9x10-14 Torr l s-1 cm-2). They
explained this unexpected behavior by arguing that the untreated section of the
system (1.4% of the total area) could be responsible for all the H2 observed up to
100° C.
The implication of the results is that “the specimen outgassing rate was very much less
than 10-14 Torr l s-1 cm-2 “.
Calculations performed with the diffusion model show that the outgassing rate should
be lower than 10-16 Torr l s-1 cm-2
31
Vacuum firing
Case study of 3:
G. Grosse and G. Messer, Proc. of the 7th Int. Vac. Cong., Vienna, 1977, p. 223
G. Grosse and G. Messer, Proc. of the 8th Int. Vac. Cong., Cannes, 1980, p. 399
G. Grosse and G. Messer measured the outgassing rate of several materials by
accumulation and selective molecular beam methods. The detection limit of such a
method was extremely low: 10-17 Torr l s-1 cm-2 .
Stainless steel was heated at 550° C for 3 days in an excellent vacuum of 10-8 Torr. The
accumulated Fo was very high (7.9) and the final in-situ bakeout was done at 280° C for
24 h. From the diffusion limited model:
⎛ 2 2.6 ⋅10 −8 ⋅ 3600 ⋅ 24 ⎞
4 ⋅ (1.6 ⋅10 −6 ) ⋅1.9 ⋅10 −12
H 2 molecules
−17 Torr ⋅ l
⎜
⎟
q=
exp
π
3
10
≅
1000
−
⋅
≅
⋅
⎜
⎟
0.3
0.32
s ⋅ cm 2
s cm 2
⎝
⎠
The lowest value reported by the authors is 9x10-17 Torr l s-1 cm-2 .
The implication of this result is that the diffusion limited model can provide estimation
of the outgassing rate for concentration as low as 1 at. H ppb.
32
Vacuum firing
Case study of 4:
316 LN Stainless steel: CERN
AT-VAC int. note
J-P Bojon, N. Hilleret, B.
Versolatto
1.00E-12
H2 outgassing rate [Torr l s-1 cm-2]
bakeout at 300°C, 24 h
stainless steel sheets 1.5 mm thick
1.00E-13
After vacuum firing
1.00E-14
For mm thick vacuum chambers, the
outgassing of H after vacuum firing
can be reasonably described by a
diffusion model only if the pressure of
H during the treatment is taken into
account
1.00E-15
1.00E-16
1
2
3
4
5
Bakeout cycles
33
Hindering the desorption from the surface
This approach consists in covering the surface with a thin layer of a
material having either:
¾
very low hydrogen permeability (passive barriers) or
¾
high hydrogen solubility (active barriers).
The surface layer can be produced by:
¾
deposition techniques,
¾
segregation of elements contained in the alloy
¾
or oxidation.
34
Passive barriers: air bakeout
Air bakeout
This method, originally proposed by Petermann (French Patent, No 1,
405, 264), consists in forming a thick oxide layer on the metal by
heating in air.
D.G. Bills, J. Vac. Sci. Technol. 6, 166 (1969):
“Such processing is reported to decrease the hydrogen diffusion rate
[he means outgassing rate] by 103 times if the oxidized surface is not
subsequently baked above about 200° C”
35
Case of study 1:
V. Brisson et al., Vacuum 60(2001)9. and
M. Bernardini et al., J. Vac. Sci. Technol. A16(1998)188
Air bakeout
After air bakeout at 450° C for 38 h and in-situ bakeout at 150° C for 7
days:
q≈10-15 Torr l s-1 cm-2
The result confirm the indication given by the Bills’ paper in ‘69.
It would be worthwhile to understand whether the benefit of the treatment
is due to hydrogen depletion or not.
A dedicated experiment was performed at CERN by thermal desorption. The
1 mm thick 316 LN samples were air-baked at 450°C for 24 h and 100 h.
The in-situ baking was at 200°C for 12 h. The heating ramp was 5 K/min.
36
Test Chamber
TDS system
Test Chamber
Feedthrough
RGA
Gate valve
BA Gauge
Conductance
Water cooled
double wall
Sample
Variable leak valve
TMP
Penning & Pirani
Gauges
Sample: 60 cm long, 1 cm wide, 1 mm thick
Thermocouple: 0.1 mm diameter S type
37
Air bakeout
2 10
-6
50% remaining
24 h air bakeout
-6
1.5 10
no air bakeout
-1
-2
Q [Torr l s cm ]
100 h air bakeout
80% remaining
-6
1 10
5 10
Heating rate
5 K min-1
-7
0 10
0
100
200
300
400
500
600
700
800
900
Temperature [°C]
38
The quantity of hydrogen extracted by TDS from air baked samples is of the
same order of that from untreated stainless steels.
Consequence: air bakeout decreases the outgassing rate without depleting
the residual hydrogen significantly.
However, the main hydrogen peak is shifted to higher temperatures
(650°C).
Consequence: hydrogen is blocked or trapped by the thick oxide layer.
How thick is the oxide?
Which is the nature of this oxide?
39
The oxide layer of air baked austenitic stainless steels
O1s
cleaned
cleaned
air baked
Fe2O3
air baked
Fe2p
Met
536
534
532
530
binding energy [eV]
528
735
730
725
720
715
710
binding energy [eV]
705
700
¾ Cr/Fe= 0.01 (0.75 for cleaned) Æ very high Fe concentration
¾ Oxide thickness 10 times larger than for as cleaned samples.
J. Gavillet and M. Taborelli, unpublished results
40
0.35
0.25
0.2
as cleaned
heated
400°C in air
0.15
0.1
0.05
0
2
heated
as cleaned 400°C in air
1.5
t
heated
400°C in air
as cleaned
heated
400°C in air
as cleaned
1
t
a
R values [μm]
0.3
R
a
R values [μm]
R
304L
316L
0.5
0
304L
316L
41
Electron induced desorption yields of an air baked sample
(400°Cx24h) normalized to those of the cleaned samples:
150°C
300°C
Normalized desorption
yields
1
0.8
0.6
0.4
0.2
0
H
2
CH
4
CO
CO
2
CH
2
6
CH
3
8
42
Air baked at 400°c for 24h
H
CH
CH
CH
CO
CO
2
-1
10
As cleaned
2
4
3
H
6
-1
10
8
6
CH
CH
CO
CO
3
8
2
-2
-2
10
η [molecules/electron]
-3
10
-4
10
-5
10
η
2
4
2
10
-3
10
-4
10
-5
10
-6
-6
10
-7
10
10
10
CH
2
-7
1
10
Q [Coulombs]
100
1
10
100
Q [Coulombs]
43
Passive barriers by deposition
¾A variety of compounds (Al2O3, BN, TiN, ZrO2, etc…) deposited as a thin film should
entirely block H2 outgassing and permeation from the metallic substrate since their
permeability is negligible for H.
¾However, experimental results have shown that only a partial reduction of the flux is
attained.
¾This could be attributed to defects on the coating (pinholes or scratches) that cause
discontinuity on the surface coverage.
¾Pinholes are produced during the deposition process and they are presumably due to
atmospheric dust particles.
44
Passive barriers by deposition
Normalised uncoated surface areas of the order of 10-4 are usually
measured for sputter coated film (due to atmospheric dust). The diameters
of the pinholes is of the order of some μm.
But the residual H2 flux is much higher than 10-4 because lateral diffusion
around the pinhole dominates.
qH
Barrier improvement factor:
R0
ρ
metal
L
*
⎛
1.18
ρ =1Θ × ⎜⎜ 1 +1.18
1
BIF = = ⎝
coating
LL ⎞⎟
R00 ⎟
R
⎠
⎛
L ⎞
⎟⎟
Θ × ⎜⎜ 1 + 1 .18
R0 ⎠
⎝
Amplification factor
qC
q⊥
qC
* After W.Prins and J.J.Hermans, J.Phys.Chem., 63 (1959) 716.
C. Bellachioma, Ph.D thesis, Università di Perugia
45
Active barriers
Active barriers absorb H exothermically. The solution enthalpy is negative.
They should absorb the H atoms coming from the substrate.
Possible candidates: transition metals of the first groups.
For the elements of the 4th group
and their alloys, surface activation
is also possible by dissolution of the
native oxide (Non Evaporable
Getters) .
Endothermic
metal
Exothermic
metal
Vacuum
The lowest activation temperature
has been found for the Ti-Zr-V
sputter-deposited alloys:
180°C for a 24 h heating
46
Active barriers
Efficiency of Ti-Zr-V as H barrier CERN unpublished results: (Géraldine Chuste)
o
-2
-1
OFS Copper
Outgassing rates [Torr.l.s .cm ]
150 125 C
10
-10
After the first
activation cycle
100
-11
10
10
-12
10
-13
10
-14
10
-15
10
-16
75
50
After the third
activation cycle
175°C
0.0025
-1
-10
10
-11
10
-12
10
-13
10
-14
10
-15
10
-16
RT
After the second
activation cycle
0.002
10
0.003
1/T [K ] - T sample
18°C
47
Reducing the mobility of atomic H
Internal trapping
Trapping sites are generated in the metal with the purpose of blocking
hydrogen migration to the surface, hence providing a sort of internal
pumping.
pumping This technique, which is not applied intentionally at present,
requires a modification of the material production process.
The trapping effect can be taken into
account by introducing an effective diffusion
coefficient Deff .
The outgassing rate is then calculated with
the usual diffusion equations.
Because Deff << D, a much lower
outgassing rate is expected.
cL, cTrap: H atoms in the regular interstitial site, trapping sites
48
Conclusions
1. H outgassing rates of the order of 10-15 Torr l s-1 cm-2 are obtained
by vacuum thermal treatment for 1 ÷ 2 mm thick vacuum
chambers made of copper or austenitic stainless steel.
2. In this respect stainless steel is worst than copper because its H
diffusivity is very low and, as a consequence, it needs higher
temperature for degassing.
3. Diffusion theory provides the mathematical tools to describe and
predict outgassing rates for 1 ÷ 2 mm thick vacuum chambers.
Recombination theory could be useful for thinner walls or for very
low H content (less than 10 at. ppb?).
4. When vacuum firing is considered, the H pressure in the furnace is
the crucial parameter for sheets thinner than 5 mm.
5. Air bakeout at 400°C for 24h reduced the H outgassing rate by at
least two order of magnitude.
49
Conclusions
6. The efficiency of passive barriers is limited by the enhanced
diffusion gradient around pinholes.
7. The potentiality of active barriers is obtained only after surface
activation.
8. Internal pumping is for the future.
50

Vacuum firing

Transcription

Documents pareils

ironing board

Catalog S Version_rev-01

SIG2SG - Sigarms SigTac Range Master Marque

E-PAK units - ksu

balayette brosse

ManforaDay in Paris

G7167 - Gerber Trendy Marque :GERBER Prix :33.00 € Descriptif

PDF - Boss Cleaning Equipment Company