Diffusion of hydrogen and deuterium in ZrVFe

1.553

Journal of Nuclear Materials 122& 123(1984)1553-1557 North-Holland. Amsterdam

DIFFUSION

OF HYDROGEN

AND DEUTERIUN

IN ZrVFe

Knize, J.L. Stanton, and J.L. Cecchi

R.J.

Plasma Physics Laboratory,

Princeton

University,

Princeton,

08544, USA

New Jersey

we have examined the desorption and pumping behavior of a ZrVFe alloy bulk getter. Fr20m lthese for cm Swas calculated to be DN = exp [13.6(0.9) - 18,700(600)/T] data, the diffusivity cm2s-' for deuterium. Using this value of hydrogen and DD = exp [8.7(2.1) - 14,600(1400)/T] DH' the regime where the hydrogen pumping speed becomes pressure dependent was calculated.

and

1. INTRODUCTION evaporable

Non

relatively per

high For

these

in

tokamak and

hydrogenic

recycling

ST101g3 activated which into

for

have

(84% for

temperature

Al)

pumping

PumP

to to

an

to

700°C

operation

is maintained

by

to pump isotopes

depending

exponentially problem

radiative

heating

especially inside

a

alternative

in

of

a

One

it

Optimization necessitates

a

getter operation parameters,

operates

and

of

promising

detailed

regenerates

at

lower.

getter

including pressure,

in

performance of

the

of experimental getter mass,

0022-3115/84/$03.00 @ ElsevierSciencePublishersH.V. (North-Holland Physics Publishing Division)

first of

ZrVFe

II we summarize

interpret

discussed

in

presented

and

summery follows

analyze

we

of

the

deuterium

from

a

detailed

high

pressure

the theory

measurements.

our apparatus

and

Section

III.

discussed

in

The

procedures

are

Results Sec.

used

IV,

are and

a

in Section V.

2. Theory of Pumping We

and

and

been

paper

measurements

determined

an

kinetic have

this

(DR)

at

play

surface

In

hydrogen

as

may

ZrVFe

for

desorption

of

In Section

performance

understanding

for a variety

(DD)

experimental

to ZrAl

of _ 100-200°C

the

diffusivities

be

kinetics.

surface

The K

and

at low

or during pumping diffusion

5

present

the

as

bulk

elsewhere.8'g

is

such

on

reported

to

Zr, 24.6% V, 5.4% Fe) which is similar that

alloy

ki

on

cB4 (70%

temperatures

ZrVFe

role.

the

ST707

except

is the

important

known.

will

operation

desorption6,

parameters

with

structures,

geometries

limiter.

pump

getter

surrounding

constricted

pressures'

pumping.

potential as

higher

the

One

dependent

rapid

are

D

or for pumping

getter

During

analysis

with

desorption,

for

equilibrium

ki,

diffusivity

the

primarily

the

rate

ZrAl

slow

elevating

550-7oooc,

and

K

operation

once the material

speed

pumping

pressures

of

temperature.

of

During

This

temperature.

can be predicted5

constant

(desorption

regeneration

utilization

is

diffuse

and hydrogen

accomplished

alloy

ZrAl

heating (C,N,O)

Subsequently,

temperature

for

the ZrAl

Regeneration is

have been

getter.

by

irreversibly

hydrogen) getter

and

impurities

reversibly.

parameters:

control of 2 impurities. These

of 200-400°C

impurities

they

devices

incorporated

bulk.

any regime

speeds

in-torus

Zr-16%

allows the

reasons,

getter

possess

and pumping

fusion

limiters'

applications

getters

capacities

area.

used

bulk

and Desorption

our

using

data a

and

theory

operation

described

in detail

summarize

here

relevant

the

predict of

bulk

getter getter

in Ref. 5. features

of

We the

theory. The surface reaction of a hydrogen

isotope

1554

R.J. Knize et al. /Diffusion

(such as H) and ZrVFe is

and the boundary

condition

is

TD = ro - r.*

(7)

1

ki

i

H2 + 2CZrVFe)

ofhydrogen and deuterium in ZrVFe

(1)

2(H-ZrVFe),

The

final

equation

necessary

to

specify

the

0 problem where

ki

and

ko

adsorption

and

desorption

area.

?he adsorbed

ri =

are,

respectively, rates

per

determines

the chamber pressure

the unit

flux (ri) is

V g

= IF - A jI'i-I'o)- PSI,

with V the chamber P

is

the

pressure,

and

the

desorption

(r,) is

flux

chamber, back-up

and ko jc(L,t)j2,

(3)

volume,

A the

getter

pumping

speed.

experiments,

r0 =

(8)

(2)

ki P,

where

P,

area

To analyze

we require

(8) for the

F the flux into the

surface

cases

solutions

and desorption

pumping

speed.

the present to

of pumping

pressure

and S the

Qs.

at constant

at a constant

These

(5)

back-up

solutions

are

as

follows. where

c(L,t)

is the hydrogen

the surface.

concentration

at

In equilibrium,

P = Kco2,

The

and

K

=

that

ko/ki

the

of thickness there

is

the

equilibrium

equilibrium getter

L mounted

of hydrogen

by the diffusion

concentration.

on a substrate face,

in the bulk

We

geometry

Po is given by the usual

particle

conservation,

constant

equation

= F/Pa.

for

behavior

by the dimensionless

x=0.

The

is described

of SG is governed

parameter

H,5

s P /A H=-"" D co/L

(10)

equation,

So is the

ratio

diffusion is diffusivity.

The

flux

at

the

surface r, is

initial

getter

and co is the equilibrium the

the

(9)

The characteristic

where

D

at

so that

(5)

with

SG

constant

is of planar

is no flux at the back

behavior

speed

(4)

co is the

assume

pumping

pressure

SG

where

getter

of the limited

surface

In

speed is independent

limited

this

diffusion

ac(x*,t) rD = -D ax-

Ix' =L

(6)

decay

case,

of pressure

as the getter fills toward exponential

rate

case,

after with

an

initial

pressure

H

1s

to the

the

pumping

and decreases

equilibrium

SoPo/co.

is the rate limiting

varies

flux

speed

For H << 1, diffusion

flux.

unimportant.

pumping

concentration.

with an

For H >> step.

1,

In this

time (- .lL2/D). SG -l/2 and exhibits a as P

R.J. Knize et al. /Diffusion

behavior

temporal exponential

decreases

which

with

of hydrogen and deuterium in Zr VFe

an

3. EXPERIMENTAL

apparatus

The

experiment A

Hydrogen

regeneration

increasing

equilibrium

ZrVFe

pressure.

by the backup

behavior for

the

of

increase

concomitant

pumped

ZrVFe

in

the

The

desorbed

gas

is

to that

For

hydrogenic

is described

a

hydrogenic

is similar

solid.

the

concentration

with

The initial time

system.

semi-infinite

condition,

is effected

temperature

of the degorption

an

this

surface

by

is

C(L,Jc) = C(L,O)

chamber

of

166

pumped

by

a

during

diffusion

and the desorption

gradient

becomes primarily

stabilizes dependent

on surface kinetics,

c(L,t) = (SKt +

cc

)

,

(13)

with

three

type

temperature c'

depends

[sa c(L,O)]

on the initial

diffusion-limited

is

a

constant

which

Prior

to

flow

gas admitted

the

into the

"as

thermocouples

flow

D21

allowed

measured

which

showed

over the getter surface

a

the

run,

getter

in order

to activate

thermally

desorbed

concentration

as

The

it.

then

gas

flow "as measured

9,

introduced

speed

limited

[Eq. so

(4, = co/p,

ZrVFe)

"as

hydrogen

into

H2

the

(or Da)

chamber

The

(9)1. the

where

10

and

by the turbomolecular

flow

concentration

Torr-e/q The

gas the

the getter

time

final

and

p is the density

than

embrittlement.

desorbed

the pressure

to determine

that

less

hydrogenic

the

from

(and K) "as -. 0.1 Torr-Q/g.

pumping

heated

getter "as also

that

so

determined

"as

were

"as

to _ 700°C for at least 45 minutes

resistively

TestChamber

K

conductance

(or

area of less than i- 10°C.

loading and the initial

desorption.

a heated valve and

this

H2

temperature

variations

an

Differential

of this

getter

off The

with

through

the

of the total The

a

(or D2) gas "as

across

infer

Integration

chamber.

with

and two Bayard-

H2

chamber

measurements

rates.

monitored

conductance.

to

be

valved

by a piezoelectric

calibrated

also

This pump

was

manometer

the

followed

PumP

speed.

gauges.

into

used

steel

can

speed measurements.

capacitance ionization

pressure

getter

and

WSS

1.

stainless

which

desorption

pressure

calculation

_, -1

B volume,

getter pumping

absolute

were

a

present

in Fig.

wafer

turbomolecular

for

chamber

Pd leak

(12) the

in

300 a/s H2 pumping

used

was

the

alloy

ZrVFe

contained

measured

in

schematically

is

Alpert

11

SAES

standard

a

where

AND PROCEDURES

utilized

shown

module

introduced

Eventually,

APPARATUS

decay rate y, where

(11)

by

1555

of the

to

avoid

getter

"as

pump

while

then the

FP

time FIGURE

Schematic

diagram

of

for getter measurements.

1

the

dependence

recorded. apparatus

util.ized

during

of

Measurements

desorption

temperature

the

in

were

the

of

was

the

diffusivity

for

the

getter

228O-776OC.

The

made

range

pressure

1556

R.J. Knize et al. /Diffusion

range

of temperature

could

be

measured

was

severely

>

1

(P

reasons high

for which the diffusivity from

limited

and

(jSGPdt

the

the

pumping

the

behavior of H

avoidance

limited).

diffusivity

pressure

pumping

by the requirements

large)

embrittlement

of hydrogen and deuterium in Zr VFe

For

determined

of these

from

speed was measured

the

\

IO-S-

only lo+

for hydrogen

at one temperature. _- lo‘5“? NE lo+” 2

lo-710-a-

I/T(10-3K-') FIGURE 4 Deuterium

diffusivity

alloy temperature

DD as a function

of the

T.

4. RESULTS AND DISCUSSION Figure Desorption time. was

of

The

the

getter

initially P1/2

h2' concentration

getter

temperature

loaded

is

as

a

function

was

with

3.1

proportional

to

of

531°C

and

Torr-R/g

of

the

surface

c(L,t).

2

hydrogen

pressure

c(L,t)l

during

initial dependence

(S,

predicted

calculated.

verify

of

these

pumping

speed

.i_lyo 1.0

1.2

FIGURE 3 Hydrogen

diffusivity

alloy temperature

T.

DH as a function

of the

results,

exponential

decay

This

diffusivity 3

and

to Arrenhius

functions

=

to

measured

rate

at 288°C H -

can be found measurement

is shown as an open circle

The

[13.6(0.9)

order

the

independent

agrees

measurements.

e~p

was

For these parameters

using

Fig.

the

the hydrogen

from

the diffusivity

in

I/TL10-3K-‘l

In

5.9 and therefore

of the

1.4

From

variables

(DD) diffusivities

calculated

(111.

t1/2

diffusivity

temperature.

and 2.6 x 10m4 Torr.

Eq.

to

3 and 4 show the measured

desorption was

the

The

the

F.q. (12).

the

sG)

Figures

diffusivity

proportional

exhibits

by

of

desorption.

CDS) and deuterium

functions

as

root

slope and the measured

co,

hydrogen

is

typical

behavior

t112

K,

square

[which a

time

measured

the

shows

with

with

desorption

the

diffusivities

were

fitted

the results;

- 18,700(600)/T]

exp [8.7(2.1) - 14,600(1400)/T]

cm2s-’ cm2s-‘.

and

DH = DD

R.J. Knize et al. /Diffusion

pumping verified T("Cl

1557

of hydrogen and deuterium in Zr VFe

from where

measurement

speed

the diffusivity

the desorption the pumping

dependent

independently

measurements

results. speed

begins

is is calculated

obtained

The pressure

PD

to be pressure

for ZrVFe.

ACKNOWLEDGMENTS We

would

technical U.S.

like

to

Department

thank

This

support. of

E.

Pinelli

for

work is supported

Energy

Contract

No.

by DE-

ACOZ-76-CH03073.

REFERENCES 5

FIGURE Pressure temperature the

where

PD T.

pumping

H=l

as

a

For pressures

speed

is

function

greater

of

Using

these

characteristics predicted

of

for

parameters. pressure

One

the

K,81g

the

2. J. L. Cecchi, R. J. Knize, H. F. Dylla, R. J. Fonck, D. K. Dwens, and J.J. Sredniawski, J. Nucl. Mater. 111&112 (19821 305.

diffusivity

and

3. The St 101 alloy is manufactured Getters, S.p.A., Milan, Italy.

by

SAES

4. The St 707 alloy is manufactured Getters, S.p.A., Milan, Italy.

by

SAES

the

ZrVFe

important the

operational

alloy

can

of

be

external

prediction speed

pumping

dependent.

is the becomes

This will occur when H -

pD where

lorP-

5. R. J. Knize and J. Phys. 54 (1983) 3183.

L.

Cecchi,

J.

APP~.

6. R. J. Knize and J. L. Cecchi, Mater. 111&112 (1982) 645.

J.

Nucl.

7. J. L. Cecchi and R. J. Knize, Technol. Al (1983) 1276. D2A2 P D =2'

(14)

KSO

5 shows that PD is strongly

temperature

dependent.

5. SUMMARY We have measured diffusivities dependence

measured pumping

in ZrVFe

of

diffusivity

the hydrogen

the at

using speed

the initial

desorption. one

the at

using

and deuterium

The

temperature exponential

a

high

time

deuterium was

decay

pressure.

also of

the This

J. Vac.Sci.

8. C. Boffito, B. Ferrario, and D. Martelli, J. Vat. Sci. Technol. Al (1983) 1279. 9.

Figure

111&112

Mater.

and

range

any

where

pressure

of

and

ki

Nucl.

limited

diffusion

values

of

J.

than PD,

falls as P-1'2.

measurements

1. P. Mioduszewski, (1982) 253.

R. J. Knize, J. Cecchi, in print.

L.

Stanton,

and

J.

L.