Potentiometric sensors for high temperature liquids

PART 2

Jacques FOULETIERGrenoble University, LEPMI, ENSEEG, BP 75, 38402 SAINT MARTIN D’HERES Cedex (France)E-mail: Jacques.Fouletier@lepmi.inpg.fr

Véronique GHETTALPSC, IN2P3-CNRS, 53 Avenue des Martyrs, 38026 GRENOBLE Cedex (France)E-mail: Veronique.Ghetta@lpsc.in2p3.fr

MATGEN-IV: International Advanced School on Materials for Generation-IV Nuclear ReactorsCargèse, Corsica, September 24 - October 6, 2007

ML 4-1 & ML 4-2

Case studies:- Oxide ion activity in molten chlorides- Oxidation potential in molten fluorides- Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)

Sources of errors in potentiometric cells:- Errors ascribed to the reference electrode

- reversibility- reactivity

- Errors due to the porous membrane- concentration modification- diffusion potential

- Errors due to the solid electrolyte membrane- partial electronic conductivity- interferences

- Errors due to the measuring electrode- buffer capacity- mixed potential

Part 2

Sources of errors in potentiometric cells

Sources of errors in potentiometric cells:

(-) Pt / Ag / AgCl / NaCl - KCl / Porous / NaCl - KCl - Na2O / YSZ / Pt, O2 (+) membrane

- Errors ascribed to the reference electrode(s)- reversibility- reactivity

- Errors due to the porous membrane

- concentration modification

- diffusion potentialErrors due to the solid electrolyte membrane

- partial electronic conductivity- interferences

- Errors due to the measuring electrode- buffer capacity- mixed potential

Errors ascribed to the reference electrode (1)

Types of reference electrodes:• 2nd kind electrodes:Ag/AgCl/Cl-, Ni/NiO/YSZ• Gas electrodes: O2/Pt/YSZ, Cl2/Cg/Cl-

Requirements:

- Easy to handle- Long term stability (oxidation, miscibility within the electrolyte, etc.)- No partial reduction of the electrolyte, inducing electronic conductivity- Known thermodynamic data (calibration often necessary), advantage of air or pure oxygen reference electrode- Reversibility (low sensitivity to perturbations)

Errors due to the porous membrane (1)

Porous plugs or frits are used to prevent mixing of the contents of the variouscompartments in a manner analogous to aqueous bridges.

- concentrations modification of the analyzed medium- contamination of the reference salt- diffusion potential

Glue cap

Porous alumina tube(5 % porosity)

Ag Platinum wire

LiCl-KCl+ AgCl 0.75 mol.kg-1

Flux of matter throughthe porous membrane

H+HClC1

HClC2

Ionic membrane: protonic

conductor

C1 > C2

What is “diffusion potential”?

u(H+)HClC1

HClC2

Non-porous mixed

conductor

Porous membrane

u(Cl-)

C1 > C2

u(H+) > u(Cl-)

Jmatter

u(H+)HClC1

HClC2

Porous membrane

Porous membrane

u(Cl-)

C1 > C2

u(H+) > u(Cl-)

Jmatter

Jmatter1

2

Is there only a flux of matter?

Errors due to the porous membrane (2):

Electrochemical Diffusion - Liquid Junction

u(H+)HClC1

HClC2

Liquid Junction

Porous membrane

u(Cl-)

C1 > C2

u(H+) > u(Cl-)

J(H+)

HClC1

HClC2

Transitory StateSPACE CHARGE

J(Cl-)

- +- +- +- +

J(H+)

HClC1

HClC2

J(Cl-)

Stationary StateJ(H+) = J(Cl-)

E

Junction PotentialEj = 2 - 1

1

2

Electric fieldE

Errors due to the porous membrane (2’):Electrochemical Diffusion - Liquid Junction

When the solvent salt is the same in both compartments and the concentrations of solutes are low (less than 0.1 molal), the liquid junction potentials across the compartment separator is at most one or two mV and can be neglected.

Concentratedsolvent salt

E ~ 0

Porous membrane

Concentratedsolvent salt+ diluted solute

Errors due to the solid electrolyte membrane (1)- partial electronic conductivity- interferences

Errors due to mixed conductivity of the electrolyteTwo situations:

- one or both interfaces are outside the electrolytic domainExamples: oxygen monitoring in molten steel or molten sodium

- both interfaces are within the electrolytic domain (theelectronic transport number is smaller than 1%)

Errors due to interferences at the electrolyte interface(s)- exchange of particlesExamples: exchange H+/Li+(pH electrode), K+/Na+

Errors due to the solid electrolyte membrane (2)1. One or both interfaces are outside the electrolytic domain

Oxygen dissolvedin liquid steel

Oxygen dissolvedin liquid sodium

Case of oxygen monitoring in molten steel and sodium

• use of YDT instead of YSZ for oxygen analysis in molten sodium• use of YSZ (or CSZ) in molten steel: correction required

E measured = Eth x ti

Patterson diagram

Temperature

Log P

O2

Domain of predominant

ionic conduction(99%)

Errors due to the solid electrolyte membrane (3)

(1) Nernst(2) Correction of the ionic transport number

0.3

0.2

0.1

010 20 50 100 200

Oxygen content (ppm)500 1000 2000

- E (

V)

(1)(2)

(3)

T = 1600°CLiquid steel

Oxygen concentration gradient at theliquid steel-electrolyte interface

2

CSZ Steel

(% O)interface

(% O)bulk

J(O2)

p(O2)

Mo/MoO

(3) Diffusion polarization correction

1. One or both interfaces are outside the electrolytic domain

Errors due to the solid electrolyte membrane (4)

2. Both interfaces are inside the electrolytic domain

Partial electronic conductivity within the electrolyte

e- flux

Compensated by an identical ionic flux

Electroneutrality

Consequence: oxygen semipermeability flux through the electrolyte without external current

O2 (P1) O2 (P2)

P1 > P2

O2-

e-

J(O2)

METALSOLID ELECTROLYTE

GAS

a

P

Jdes

Jads

Equilibrium conditions Jads = Jdes

measured activity = oxygen pressure ≠

METALSOLID ELECTROLYTE

GAS

a

P

Jdes

Jads

Jsp

Stationary state Jsp + Jads = Jdes

measured activity oxygen pressure

Interference

When a solid electrolyte is in contact with an active species, it can penetrateinto the the bulk by exchange followed by diffusion:

iSE + jsol jSE + isol

Empirical equations for the Galvanipotential have been developed

ibulkC

isurfC

i

j

SE Solution

bulk

surf

solution

meastheor

NASICON (Na3Zr2Si2PO12)

Exchange Li+, Na+, K+, as a functionof the size of the channel

-Alumina (NaAl11O17)

Potassium cation

Examples:

Errors due to the solid electrolyte membrane (6)

Interference

Case of the glass electrode

Protons do not penetrate into the membrane (their mobility is very low).The interfering phenomenon is a surface reaction with the formation of a gelwhich can be viewed as a thin protonic membrane

pH sensor

Errors due to the solid electrolyte membrane (7)

Errors due to the measuring electrode

- The analyzed solutions are often complex and the cell e.m.f. is nota thermodynamic voltage but a mixed potential

Main source of error:

This mixed potential can be due to impurities within the analyzed medium or can be observed after a long term exposition due to deposition of impurities on the measuring electrode.

Mixed potential

If there is more than one redox couple in the analyzed system (solution or gas),the voltage is not a thermodynamic potential.It the case of a M+/M in a solution saturated with oxygen. The following reactions take place:

(2)

(1)

IoxIred I

E2

E1

Em

E

oxidationreduction

Mixed potential: generally, the voltage takes an intermediate value (E2 < Em < E1)

At the mixed potential, Em,Ioxidation = Ireduction

M M++ e-

1/2 O2 + 2 e- O2-

Exhaust gases composition

Mixed-potential type oxygen sensor

Variation of the emf vs. A/F ratio

14,5A/F0 RICHMIXTURE

LEANMIXTURE1E(V)

Lambda sensor

Case studies:

- Oxide ion activity in molten chlorides- Oxidation potential in molten fluorides- Monitoring of oxygen, hydrogen and carbon in molten metals (Pb, Na)

Menasina beach

Media conditions: Solid electrolyte measurements in melts

Sodium Copper Steel Glass

Temperature (°C)

400 1150 1650 1500

Rate of T changes

(K.min-1)

5 50 5000 50

CO (ppm) 0.5 - 50 1 - 1000 1 - 2000

Time of operation

10 000 h 100 h 5 s 50 h

SE material ThO2 - Y2O3

(cub.)

ZrO2 - CaO

(cub.)

ZrO2 - MgO

(cub.-mon.)

ZrO2 - Y2O3

(cub.)

Main difficulties:- thermal shock- wide temperature range (200°C - 1600°C)- time life required- stability domain of the electrolytes- corrosion- reference electrode

Oxide ion activity in molten chlorides (1)

B. Tremillon, G. Picard, Proc. 1st Intern. Symp. onMolten Salt Chem. and Techn. Kyoto (1983), p. 93.

Zirconia sensor

(-) Pt/Ag/AgCl/NaCl-KCl/Pyrex/NaCl-KCl-Na2O/YSZ/Pt,O2 (+)

Ref.1 Ref.2Sensingmembrane

YSZ

YSZ

Pt(-)

Pt(+)

((O )) 2- E

ERef1

ERef2

((O2-))M/Ref.1 Ref.2/M

Emea.

Air

PtMelt

E

Ref 1

YSZ

Pt

Seal

M/MOx

Pd/PdO, Ni/NiO, etc.

€

E = ΔE° −2.3RT2F

.log (O2− )

Oxide ion activity in molten chlorides (2)

Ref / O2- / YSZ / Pt, O2

Theor. Slope: 72 mV/u. p(O2-)

Measurement of oxide solubility in molten chlorides

ZnO

Li2O

-270

-280

-290

-300

-310

-320

-log(O2-)1 1,4 1,8

E(m

V)

/ A

g

LiCl-KCl-ZnCl2T = 723 K

J. Shenin-King, PhD Thesis, Paris 6, 1994

€

E = ΔE° −2.3RT2F

.log (O2− )

Monitoring of oxygen, hydrogen and carbonin molten metals (Na, Pb)

The Dolmen of Paomia

Monitoring of oxygen

Oxygen monitoring in molten sodium (1)

Brookhaven National Lab., USA, 1972Interatom, Germany, 1975Berkeley Nuclear Lab., UK, 1982 Harwell, UK, 1983Nuclear Research Institute, Czechoslovakia, 1984

Oxygen meters have application to both primary and secondary circuits of a fast reactor.When used in a fast reactor primary coolant circuit they have to perform in high-radiationenvironment.The corrosion of metals and alloys increases with high oxygen concentration in sodium.Stability of the electrolytes (n-type or p-type electronic conductivity).Electrode reaction kinetics at low temperatures.

YDT electrolyte

J. Jung, J. Nuclear Mat., 56 (1975) 213.M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125R.G. Taylor, R. Thompson, J. Nuclear Mat., 115 (1983) 25.D. Jakes, J. Kral, J. Burda, M. Fresl, Solid State Ionics, 13 (1984) 165.H. Ullmann, K. Teske, Sensors and Actuators B, 4 (1991) 417.

Oxygen monitoring in molten sodium (2)

R.G. Taylor, R. Thompson, J. Nuclear Mat., 115 (1983) 25

• Reference electrode Sn/SnO2 and In/In2O3.

• Good performance over lifetimes exceeding 400 days.

• Grain-boundary attack under high-oxygen sodium.

• Tests under -radiation

Oxygen monitoring in molten sodium (3)

D. Jakes, J. Kral, J. Burda, M. Fresl, Solid State Ionics,13 (1984) 165.D. Jakes, J. Sebkova, L. Kubicek, J. Nuclear Mat.,132 (1985) 88.

Glass brazing

Electrolyte crucible (ThO2 - Y2O3 10-12 mol.%).Glass brazing.

H. Ullmann, K. Teske, Sensors and Actuators B, 4 (1991) 417.

Thoria-yttria electrolyte solderedin a stainless steel tube.

Temperature dependence of the saturation solubility in liquid sodium.Log CO = 5.64 ± 0.22 - (2120 ± 100)/T (CO in ppm)

Oxygen monitoring in molten lead and lead-bismuth (1)

Reversibility of the electrode reactions

Main challenge: measurement at low temperatures (< 400°C)

YSZ electrolyte

Reference electrode

• Air / Pt / YSZ and Air / La0.7Sr0.3CoO3 / YSZ mixed conductor

V. Ghetta, J. Fouletier, M. Hénault, A. Le Moulec, J. Phys. IV France, 12 (2002) 123

• Bi / Bi2O3 / YSZ J.L. Courouau, P. Deloffre, R. Adriano, J. Phys. IV France, 12 (2002) 141. J. L. Courouau, J. Nucl. Mat., 335 (2004) 254.

YSZ

• In / In2O3 / YSZ or Air / Pt / YSZ J. Konys, H. Muscher, Z. Vo, O. Wedemeyer, J. Nucl. Mat., 296 (2001) 289

Measurement of oxygen activity in saturated molten lead (335°C < T < 530°C)

-14.0

-13.0

-12.0

-11.0

-10.0

-9.00

300 350 400 450 500 550

log

(a

o)

T (Celsius)

Oxygen monitoring in molten lead and lead-bismuth (2)

V. Ghetta et al.

Oxygen sensorwith air reference

E(mV)

LEAD ao

Air

Gas inletGas outlet

Coupling of an oxygen sensor with a zirconia pump

Oxygen sensor

E(mV)

LEAD ao

Air

Gas inlet Gas outlet

IpumpAir

I(mA)

Oxygen monitoring in molten lead and lead-bismuth (4)

Oxygen pumpThe oxygen content in the bath iscontrolled by the oxygen pump

The actual oxygen activity is measured with the oxygen sensor

V. Ghetta, F. Gamaoun, M. Hénault, A. Le Moulec, J. Fouletier, J. Nucl. Materials, 296 (2001) 295-300. V. Ghetta, J. Fouletier, M. Hénault, A. Le Moulec, J. Phys. IV France, 12 (2002) 123-140.

• Zirconia tube• Pt/Air internalelectrodes

• Zirconia tube• Pt/Air internalelectrodes

€

nO =Ipump dt

t0

t1∫

2 F

€

nO =mPbMPb

Pair1/ 2

γO∞

exp2FRT

Esensor ⎡ ⎣ ⎢

⎤ ⎦ ⎥

SENSOR PUMP

E(mV)

ao

Air Air

I(mA)

Closed system

2 independentmeasurements

Oxygen monitoring in molten lead and lead-bismuth (5)

TheoreticalFaraday law

TheoreticalNerst law

Verification of the functioning of the set-up

0.0 100

1.0 10-4

2.0 10-4

3.0 10-4

4.0 10-4

0 10 20 30 40 50 60 70 80

T = 527 °C

nO

vari

ati

on

s (m

ole

s)

Qcumulative (Coulomb)

slope ∞oγ

Oxygen monitoring in molten lead and lead-bismuth (6)

Theoretical straight line

V. Ghetta et al.

Monitoring of hydrogen

Hydrogen monitoring in molten sodium (1)

C.A. Smith, CEGB Technical Disclosure Bulletin, 227 (1974). M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125.T. Gnanasekaran, V. Ganesan, G. Periaswami, C.K. Mathews, H.U. Borgstedt, J. Nuclear Mat. 171 (1990) 198.

Na(H) / Fe / CaH2 - CaCl2 / Fe / Li, LiHSolid electrolyte

Iron diffusion membraneReference electrode

400°500°

Hydrogen monitoring in molten metals (2)

Use of protonic conductors:- Yb, Nd or Gd cerates (BaCeO3)- In doped zirconate (CaZrO3)

Sensors for monitoring of hydrogen in Al (ca. 973 K), Cu (ca. 1423 K)or Zn (ca. 723 K)

Main specificity:Various conductivity domainsas functions of temperatureand atmospheres

-30 -20 -10 00

-10

-20

-30

-40

Log pH2

Log

pO

2

(h•) domain

T =600°

(Hi•)

domain

(VO••) domain

N. Kurita, N. Fukatsu, K. Ito, T. Ohashi J. Electrochem. Soc., 142 (1995) 1552.N. Fukatsu, N. Kurita, T. Yajima, K. Koide, T. Ohashi, J. Alloys and Compounds, 231 (1995) 706.

Hydrogen monitoring in molten metals (3)

N. Kurita, N. Fukatsu, K. Ito, T. Ohashi J. Electrochem. Soc., 142 (1995) 1552. N. Fukatsu, N. Kurita, Ionics, 11 (2005) 54.

T=1473 K

T=973 K

T=773 K

Condition of Almelting

Mixed conduction at lowoxygen activities

No direct contact between(Al) and the electrolyte

Condition of Namelting

Protonic conductor

Three limiting casesCaZr0.9In0.1O3-

Condition of coppermelting

((H))metal // Zirconate // Pt/gaz, fixed % H2

Hydrogen monitoring in molten metals (4)

Sensor for liquid Al

Measurement of the hydrogen activityin the gas phase equilibrated with themolten Al

With Pb, Pb-Li (or Na ?) theelectrolyte could be in contactwith the melt metal.

Monitoring of carbon

The greek church

Carbon monitoring in molten sodium (1)

M.R. Hobdell, C.A. Smith, J. Nuclear Mat., 110 (1982) 125.M.R. Hobdell, E.A. Trevillion, J.R. Gwyther, S.P. Tyfield, J. Electrochem. Soc., 129 (1982) 2746.S. Rajendran Pillai, C.K. Mattews, J. Nuclear Mat., 137 (1986) 107.

An optimum amount of carbon in austenitic and ferritic steels used as structuralmaterials is essential for maintaining good mechanical properties during the life ofthe reactor.

Owing to the solubility of carbon in molten sodium, according to the temperature, carburization or decarburization can take place.

Moreover, accidental ingress of oil from pumps or contamination from carbon dioxidein air will lead to a build up of carbon activity in sodium.

Carbide-chloride electrolytes: not successful

Alkali molten carbonates

Carbon monitoring in molten sodium (2)

Fe3C / Fe / Na2CO3 - Li2CO3 / (( C ))Na

orGraphite / Fe / Na2CO3 - Li2CO3 / (( C ))Na

Hobdell et al.

Rajendral Pillai

Electrode reaction at both electrodes:CO3

2- + 4 e- = C + 3 O2-

€

E =RT4F

lna((C))Na

a<<C>>Ref

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

Main difficulties:- use of a permeable -iron membrane (equilibrium ?)- life time of the reference

Carbon monitoring in molten sodium (3)

Cgraphite/Fe/Na2CO3-Li2CO3/(( C ))Na

Molten Na

Permeable thinFe membrane

GraphiteNa2CO3-Li2CO3

E Ni capsulecontaining Cg

S. Rajendran Pillai, C.K. Mattews, J. Nuclear Mat.,137 (1986) 107.

M.R. Hobdell, C.A. Smith, J. Nuclear Mat.,110 (1982) 125.

Molten Na

Permeable thinFe membranes

Fe3CNa2CO3-Li2CO3

Fe3C/Fe/Na2CO3-Li2CO3/(( C ))Na

E

Discontinuous measurement

Thank you for your attention

BUFFER CAPACITY OF A GAS

Buffer capacity : number of moles of acid(or base) inducing ∆pH = ± 1

Buffer capacity of an acid/base mixture

Titration of a weak acid

Bu

ffer

cap

acit

y

Maximum buffer capacity

Errors due to the measuring electrode

BUFFER CAPACITY: OXYGEN SENSORS

Oxygen pressure domain10-25 10-20 10-15 10-10 110-5

PO2 (bar)

He, Ar or N2 - O2

partial vacuum - O 2

mixtures CO - CO2 or H2 - H2O

- log pO2 (atm)

log

(m

ol.)

161284- 8

0

- 6

- 4

He-O2

1000°C

900°C

800°C- 2CO2-CO-O2

Pressure domainsof correct utilization

of the sensorat 900°C

1 <---------> 10-6 10-10 <---------------> 10-25

: Buffer capacityof the gasNumber of molesof oxygen for changingthe chemical potentialof 1 kJ/mole of gas

Errors due to the measuring electrode (3)

Errors due to the measuring electrode

Monitoring of the oxygen pressure down to 10-25 barprovided the gas exhibits a sufficient buffer capacity

PUMP SENSOR

I E P(O2)

D

CO2 orAr-H2 (5%)or H2

A pressure less than 10-23 bar, is it possible?

P > 10-7 bar

He + O2

P < 10-7 bar

He + O2

+ traces of CO, CO2,H2, H2O

P < 10-7 bar

He + O2

CO, CO2+ H2, H2O

P < 10-7 bar: two situations

No oxygen monitoring Easy oxygen monitoring