Oxygen diffusivity in MoMoO2 reference electrode of oxygen probe from electrochemical measurements

Solid State lonics 18 & 19 (1986) 865-870 North-Holland, Amsterdam

865

O X Y G E N DIFFUSIVITY IN Mo-MoO 2 REFERENCE ELECTRODE OF OXYGEN

ELECTROCHEMICAL

PROBE FROM

MEASUREMENTS

Wang Nanmeng Shanghai Metallugical Instruments and Metrological Factory, China. The oxygen diffusivity in Mo-MoO2 Reference mixture of oxygen probe was measured by Electrochemical method at 1600°C. Do=7.08+1:885 X I0 -4 cm2/s. Compared the value of DoM° obtained with the values of DoFe , the oxygen diffusivity in steel melt, reported in the literature, we found that DoMo is as 3-5 times as DoFe • It indicates that the oxygen diffusion in molten steel is main rate-controlling step in the Kinetics ' processes of oxygen cells.

i. INTRODUCTION

method has been successfully used to determine

In recent years, oxygen probes h a v e widely

the diffusion coefficient of oxygen in Mo-MoO2

been used in steel industry. The Kinetics study

mixture at 1600°C. This type of experiment has

of the oxygen diffusion at the reference electrode

not so far been reported in the literature.

of oxygen probe is lacking now. Particularly, the

2. PRINCIPLE

oxygen diffusivity in Mo-MoO2 electrode have not

The electrochemical cell used in the experiment

been reported in the literature so that we have

is shown in Fig.l and described schmatically by

not

determined

diffusions

at

w h i c h process the

interfaces

the

oxygen

between

of

molten

steel/electrolyte and electrode/electrolyte is main

Mo contact[}Mo-MoO2[IZrO2(7wtO6CaO)l [Pt-Argon

(P°2 (t))

(P~)2)

where, P°2 =the oxygen partial pressure in Argon gas,

rate-controlling step.

is e o n s t a n t value. P~2 =the oxygen partial pressure in Mo-MoO 2

Some authors studed oxygen diffusion in liquid steel. Oberg et al I , Kawakami and GoTo2 measured

mixture.

the oxygen diffusivity in molten steel, respectively.

when the cell is subjected to an appllied voltage

Hartog and Slanger3 reported that the e.m.f from

E 1. The ionic c u r r e n t will vanish at s t e a d y s t a t e

oxygen cell could be affected by stirring steel

(only

bath. The resulte indicates that the oxygen diffusion

electrolyte).

at the liquid steel/electrolyte interface can influence

chemical potential

the value measured by oxygen probe. These studies

mixture is given by

electronic

help us understand the polarization phenomenon at the interface between liquid steel and electrolyte.

El=

current

flows

As a consequence, for p' 02 P~2(1)

RTIn 4F

through

the

the s t e a d y - s t a t e

oxygen in the

Mo-MoO 2

(1)

Igami et al 4 , Wang Nanmen5 studed polarization phenomenon at the reference electrode/electrolyte interface and determined the value of polarization

The oxygen pressure Po2(1) in whole Mo-MoO2 mixture is kept

uniform corresponding to the

symmetrical

Mo-MoO2 equilibrium mixture. Whenapplied voltage

cell: Mo-MoO2 l}electrolyte]]Mo-MoO2,respectively,

is changed to E2 sundlly, the P'o2(1) will be changed

However, because h a v e not the data of oxygen

to Po2(II) at the Mo-MoO2 electrode /electrolyte

diffusivity in Mo-MoO2 mixture, we have lacked

interface and the oxygen pressure in Mo-MoO2 body

essential

will be changed according to the direction and

overvoltage

at

1550°C, using the

understanding

for

w h o l e electrode

processes. In the present paper the potentiostatic

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

magnitude of

E2.

Thd following expression is

N. Wang / Oxygen difJusivitv in Mo-MoO 2 reference electrode o f oxygen probe

866

measured

obtained

(~IpI )(~e

i>n(t)" £2i~,~= R T

E,,

where, the

(2)

4 b" :i=,(t) is the ionic resistance of the celL

(~2

It's value depends on the ionic conductivity thickness

of

the

s o l i d electrolyte

used

and

in

the

experiment.

The ii,,, ~ i o n product is very small

relative

E2,

to

approximete

so

the

and

can

satisfy

a true potentiostatic condition.

i~,n(t)

transient ionic current time

experiment

vanish

sufficiently

long

at

a

will decrease with

new

times.

The

steady-state

As a

after

consequence,

steady-state chemical potential

the

for oxygen in the

In i > ,

4 rate

of

oxygen

(3)

I'(;~(II)

'

experiment,

molecules

from

of o x y g e n ions t h r o u g h

recording

the

current

the diffusion

Mo-MoO 2

body

the

through

electrolyte.

a

external

By

circuit

as a f u n c t i o n of t i m e , t h e d i f f u s i o n r a t e of o x y g e n in

Mo-MoO 2 m i x t u r e

use in

of the

in t h e

a

can

closed-end,

present

be

computed.

cylindrical

experiment,

Mo-MoO 2 e l e c t r o d e

the

For

electrolyte oxygen

is d e s c r i b e d

the tube

diffusion by Fick's

1

~t

,-

,

C//I;

(6)

a2 13

and

(7)

I). 8,"-,/'} • [ " ' ] ) ( , [ C ' , (

1 )

In Eq.(6) 2.405 is the f i r s t

D (rI),, 3C°) ar

('.(

(4)

mixture filled in the cylindrical tube. From Eq.(6), a curve of (HJi.,~ VS.t should be linear with a slope of -Do'2.405=/a 2 except at short times. Because a is known, Do can be calculated i --

by recording the

t curve. Mo-MoO2 mixture

was connected to tine negative electrode of power source. When E2 > E1 oxygen ion will migrate from Mo-MoO2

electrode

experiment. added

to

the

Ar2gas,

Mo-MoO2

in experiment. determine

to

Likewise, when El

electrode,

oxygen content

in steel

a

pumpused to

bath,

oxygen

is pumped out from Mo-MoO2 reference electrode, so all

the

present

experiments

The Do calculated from i - accurate

for experimental

were

pump-out.

t relation was quite times greater than

3.

5

EXPERIMENTAL PROCEDURES solid

electrolyte

used

was

a

tube

containing

7wt%

of

CaO

at the e l e c t r o l y t e / M o - M o O 2 e l e c t r o d e i n t e r f a c e ,

by S H E N Y A N G C E R A M I C F A C T O R Y .

of oxygen in

stabilised supplied

The zirconia

t u b e had t h e f o l l o w i n g c h e m i c a l a n a l y s i s (in wt%): ZrO 2 91.07, C a O 7.14, SiO 2 0.16, A120 3 0.26, F e 2 0 3

the Mo-MoO2 electrode, in cm2/s. t = the time in seconds

0.07,

r = the inner radius of the tube used, in Cm.

I.D., 1 0 m m in O.D.. T h e o u t s i d e of e a c h t u b e was

Co= Co(1), () " r Co = Co (2) ,

a,

electrode.

t>0

Solving Eq. (4) with above boundary conditions, then 6,

C.(1)

Co(r, t)

C0(1)

C,,('2)

w h e r e ),n a r e order. change

(:x> k~ ,1 exp ( i)oA~t).. 1 32, (a~

H ,

a'-'

t h e r o o t s of t h e Depending of

the

0.16

and

had

the

dimensions

8mm

in

rod w a s c o n n e c t e d to t h e p l a t i n u m - - A r g o n r e f e r e n c e

a , t=0

r

MgO

c o a t e d with a p o r o u s p l a t i n u m e l e c t r o d e . A p l a t i n u m

For the boundary conditions,

the

pump-out

When oxygen probes are

zirconia

zero

a

> E2 oxygen is

where, Co = the oxygen c o n t e n t expressed as Wt Pct

and

(8)

function of zero order, h is the height of Mo-MoO2

The

Sr

Do ---=the diffusion coefficient

2 )]

root, /~z, of the Besse]

minutes after applying E2.

s e c o n d law for c y l i n d r i c a l c o o d i n a t e s :

(()C,,/

current i~,,, through

the

In the present experiments

to m i x t u r e / e l e c t r o l y t e i n t e r f a c e e q u a l s t h e m i q r a t i o n rate

and

T

where

Mo-MoO2 mixture is given by

In t h e p o t e n t i o s t a t i c

electrode

the cell, for long times we obtain: 2. 1()3~ t

on

oxygen

the

B e s s e l f u n c t i o n of relation

concentration

between in

the

Because

MoO 3 c a n

a t high t e m p e r a t u r e ,

change

into

MoO 2

in t h e e x p e r i m e n t t h e p o w d e r

m i x t u r e of Mo-MoO 3 was u s e d i n s t e a d of Mo-MoO 2 mixture.

The

molybdenum

powder

of

99.9wt%

p u r i t y was t h r o u g h t l y m i x e d with t h e MoO 3 p o w d e r of s a m e p u r i t y (Mo/MoO 3 = Mo,

MoO 3

particles

were

95/5). -160,

The -300

sizes

of

meshes,

867

N. Wang / Oxygen diffusivity in Mo-MoO 2 reference electrode of oxygen probe respectively. The mixture was pressed into a ziconia

potentiostat for 15-20 minutes. Then the voltage

tube by hand. The volume density pressed was 2.3-

was changed suddenly to E2 (250 or 300mv), while

2.4 g/cm 3. Filled

of pure A1203

the changed curve of the voltage across a standard

above Mo+MoO3 mixtures, the top of cell was sealled

resistance (accuracy 0.1%) VS. time was recorded

some powders

by A1203 cement. A ~ I m m

Mo rod was served

as the external lead wire.

by a XWX2042 recorder. When the voltage recorded did not change with time, the cell reached a new

The experiment cell shown schematically in Fig.l

steadystate and the current flow passing through the cell was entirely electronic (ii,n=0). The time reached a new steady-state generally was 15-20

~Pt contact

min.

M__or%

transformed the curve of voltage VS. time

into the current-time curve and plotted the curve

-~

of {17iron--t, the oxygen diffusivity Do could calculated

SiO2 tube

by Eq.(6). The experimental block diagram is presented

~ flowingstrea m argon

celnent

I

Ar2gas - =

Pt-etRhlO , '~[ ~ . ~ ', ~"1'. . . .

(reference gas]

;~l:-,!t

Zro2/c~o)

f

~,~,~'.~}

in

Fig.2.

oo3 n.xturo Pained Pt el-ectrode

1

0.2

=

[cell

,[

[ ] Voltmeter,]

[ ez-8

]w.-]

[×w>2042 I

[recorder I FIGURE 1 Arrangement of experimental electrochemical cell FIGURE 2

for diffusion experiments

Experimental block diagram was positioned in a sx-8-16 resistance-heated furnace at 1600°C such that the portion was entirely within a

constant

controlled controller.

temperature to A

_+5°C by Pt/PtRhl0

zone

which

KSY-8D-18

could

kept

in

contact with the solid electrolyte tube was used to measure the temperature of solid cell.

A representative plot of cell current VS. time

be

temperature

thermocouple

4. RESULTSAND DISCUSSION

A argon

for a

typical potentiostatic diffusion experiment

is presented in Fig.3. The curve designated i~,)u,1 was reproduced directly from

the

recorder, included

Both Parts of the ionic current and the electric

stream flowed slowly over the external surface

current •

of the electrolyte tube to ensure a constant oxygen

out experimences.

The ie was constant during entire pump-

potential at the reference electrode (flux: 4L/min).

the

For an experimental run, the cell inserted into

ionic current, iion , can be calculated by substraction

the furnace was initially brought to steady-state

of

with a voltage (150 or 200mv) E1 applied by the

current value.

At a steady-state the ie was

the

electronic

The transient values of the

current

ie

from it,,tm . " The

iron computed in this manner is also plotted in Fig.3.

:V. Wang / Oxygen dij]'usivity in Mo-MoO 2 reference electrode oJ'oxygen probe

868

1( m A ) El=lS0mv E2=250mv 20

15

10

1

I 5

t5

I0

c)

t(min)

FIGURE 3

I

I

5

i0 t~ min)

FIGURE 4

Cell current VS. time for determination of Do

Log of ionic current VS. time for

determination

of Do at 1600°C A typical plot of/~/i,,~VS.t is presented in Fig.4. It

is clear

that

the

relation

of

(~H ii~n with

Kawakami et al 2. reported that Do=(l.9+0.7)-10~4cm2/s at

1550°C.

Obviously,

the

oxygen

diffusion

t is linear. From the 25 successfull runs at 1600°C,

coefficient

the oxygen diffusivity was calculated to be

determined in the present experiment is as 3-

in

Mo-MoO2

mixture

which

was

5 times as the diffusivity of oxygen in liquid iron.

DoMO~ ~ n~+1.85 x 10-4 cm2/s • .u~_2.80

i~ indicates that the diffusion of oxygen in molten Literature in

Mo-MoO2

values

for

Mixture

is

the

oxygen diffusivity

quite

rare

and

show

considerable variation. Ullman and Madix7 reported Do=10-4±lexp (10_+ 5 kcal mole-I/RT) cm2/s temperature range

15000 to 2100°C.

computed by the resultat 1600°C Do is

consistent with

the

in the

steel is a main rate-controlling step in the electrode processes of oxygen probes and oxygen diffusion in reference electrode is a minor. Used the cell of zirconia tube with Mo-MoO2 referer.ce, llartog

The value

et al 3. Carrietl out the experiment of stirring liquid

10-4-+1 cm2/s

steel. The results obtained are plotted in Fig.5.

present result.

Baranova

The result indicates that the e.m.f from the cell

et al 8.reported Do M ° = 3.0 x 10-2 exp (-31000/RT)

increases sharply after stirring. The account for

and the DoM° calculated is 4.1 x I0 -~ cm2/s at

the occurrence is that the stirring accelerats

1600°C. However, Kashing et al. reported

mass

DoM° -~

transmission process

of

oxygen

at

The steel

0.55 x I0 -~ - 1.83xi0-3 c m 2 / s a t 2780 ° to 4000°C 9 ,

Bath/electrolyte interface. The line of dashes in

DoM o = 1 . 3 - 2.3 x I0 -~ cm2/s at 3727°C. The value

the Fig.5 is added by present author. It stands for

reported by Baranova appears to be too high.

the

Oberg et al 1. reported that the oxygen diffusivity in liquid iron at 1620°C equals (1.5+_0.1)- 10-4 cm2/s

steel bath.

state probably produced by strongly stirring ']'he inclined line indicates that the

N. Wang / Oxygen diffusivity in Mo-MoO 2 reference electrode of oxygen probe polarization is produced by the oxygen diffusion

are

in Mo-MoO2 electrode, too. From Fig.5, it is clear

laboratory,

used to

determine

oxygen content

869

in the

However, in the industrial tests the

that the polarization produced by oxygen diffusion

splashing produced by the protective paper of oxygen

in the liquid steel is larger than in Mo-MoO2

probe promote the mass transmission process greatly,

electrode.

so the oxygendiffusion in the reference electrode

oxygen

The fact supports this conclude that diffusion

rate-controlling

in

step.

melten

steel

is

main

Through above discussion,

it is suggested that stirring melten steel be essential

appear to be a

mainrate-controlling step.

REFERENCES 1.

K.E. O b e r g e t al. : JISI, 210 (1972) , 359.

2.

M . K A W A K A M I e t al. : T. ISIJ, 16 (1976), 204.

3.

H.W. d e n H a r t o g e t al. : I r o n m a k i n g a n d s t e e l m a k i n g , (1976), 2, 64.

4.

E. Igami et al. : Iron and s t e e l , 66(1980), 306.

5.

Wang N a n m e n g : P r o c e e d i n g s of t h e s e c o n d s y m p o s i u m on K i n e t i c s of M e t a l l u g i c a l R e a c t i o n s , C h i n a , (1984), 473.

to overcome the polarization effect when oxygen cells

o

I

I

I

I

-50 ;1~-

- 100

is(

435ppm

-20C

-250 -300

--.-d I'---

-350 -400 450

3spp~,

7. A.Z. U l l m a n e t a h High. T e m p . S c i . , 6(1974), 342.

stirring I l

I 2

6. J. C r a n k : The M a t h e m a t i c s of D i f f u s i o n , o x f o r d , (1957).

I 3

I 4

t (rain)

FIGURE 5 The result of stirring steel Bath by Hartog et al 3.

8.

V.I. B a r a n o v a e t al. : Fig. K h i m . obrab. M a t e r . , 2(1968), 61.

9.

V.E. K a s h i n g e t al. : M e t a l , 2(1971), 39.

10. V.E. K a s h i n g e t al. : F i g . - k h i m . A s n . Mat. Pro., (1973), 90.