A Survey on Industrial Applications of Oxygen Concentration Cells with Solid Electrolytes*

UDC 543.272.1 : 541.136.8 : 546.21: 669 .14

By Kaz uhiro S . GOTO**

1. A H istory of Investigations on Oxygen Con Then, h e tried to determine rapidly the oxygen centration Cells at Elevated Temperatures concentration in molten steels from the electromotive The oxygen concentration cell can b e defined as force of the following cell; 7) a type of galvanic cells, whose electromotive force is Si02 Slag generated by the difference in the chemical potential Liquid I f + 'd Ag, H 2 or O 2 gas Fe- O A rom aCl open (reference elect'd.) of oxygen, or in other words, in the oxygen concen - , g hearth furnace tration, at the two electrodes under isothermal and ...... (3) isobaric conditions. As the electrolyte, used are the solid or liquid oxides Figure 1 shows the structure of this cell and Fig. 2 with predominant ionic conductivity a t elevated tem the relation between the electromotive force and the peratures. This category of galvanic cells can be ex oxygen content in steels determined by the conven pressed by the following generic cell formula ; tional vacuum fusion method. A similal' experiment was made by Sanbongi and ,:) , Solid or liquid I °2' 'd ' h P ~ .; , Ohtani supported by Fuji Iron a nd Steel Corp., using Electrode I OXI es WIt Eh:: ctrode 11 IOmc conduction solid magnesia as the electrolyteS) in 1958 . In J apan, these two investigations played a n im ...... ( I ) pOl·tant role in the development of the oxygen concen In 1908, H aber' ) m easured the electromotive force tration cel ls for the rapid oxygen a nalys is of molten of this type of cell, using Thuringer hard glass as the steels. The cells are now widely used in Japanese electrolyte. Eight years la ter, Treadwell2) tried to steel plants as discribed in the following section. All use fused borates, silicates, or so lid porcelain as the electrolyte and to control the oxygen pressure at the 1-1 2(- 1-1 20) gas electrodes by using the two phase mixtures of Fe and t FeO , or Cu20 and C uO. A similar study was in dependently reported by Baur, et al,3) After the extensive study by Treadwell,2) the oxygen concentration cells have been investigated in many 4 coun tries. Among them , Esin ) in Russia has con structed various types of the oxygen concentration cells. In 1957, Sanbongi, Ohtani a nd Omori5 ) reported Fe-O 550°C a review including all reports after Treadwell, and melt o hence the details will not be repeated here, except for o o the industrial applications. The first industrial applica tion of the oxygen con l centration cells was reported by Ona ka ) in 1944. To control the refining operation of steels, he tried to determine the oxidizing power of the slag of a n acid open hearth furnace from the electromotive force of the cell:

Pt (1), Slag I Si02 Slag 1 Si0 2 Slag II Pt (II), o o Po, tube tube P ~ , (f) (f) ...... (2) where the electrolyte was composed of solid silica containing iron oxide absorbed from slag 1. EMF Onaka showed that the electromotive force is ex Fig. I. Schematic diagram of galvanic cell to determine clusively generated by the difference in the oxygen oxygen pressure in liquid iron used by Onaka." potential of the two slags or in the ratio of Fe2 +- to The electrolyte is made of silica containing iron Fe3+ -content. oxide.

* Originally published in Tetsu-to-Hagane, 62 ( 1976), 1265, inJapanese. English version received J anuar y 14, 1976. ** Department of M etallurgy, Tokyo J ns titute of Technology, O okayama, M egul'o-ku , Tokyo 152.

Rev iew C469 1 ( 470 ) Transactions ISIJ, Vol. 16, 197 6

--I tions of the oxygen con centration cells and thus the present report has been m ade to supplement them. 160 /' The scope of the present survey is confined to the • following four types of applications; (l ) present state 140 of commercial oxygen concentration cells designed for 120 the a nalysis of waste gas from boilers and various in / dustrial furnaces, (2) d etermina tion of total oxygen 100 demands of the waste water from industrial plants .1· a nd NOx control of the exhaust gas of gasoline • 80 •• engines, (3) rapid oxygen analysis in copper a nd steels :--- at their refin eries, a nd (4) continuous oxygen analysis E 60 • in liq uid sodium for fast bleeder reactors. • 40 • • II. The Conduction M echanism in the Oxide 20 •• Electrolytes and the Electro-motive Force ./. 1. For the Case oj the Oxide Electrolytes oj Uniform Com O~~ __~ __~ __L-~L--L __~ o 100 200 300 400 500 600 700 position %QX 10' ----> The cell can be described as follows. Fig. 2. R elation between the EMF of the ce ll of Fig. I An oxide, and the oxygen content determined by the conven P ~ o' f . C P:;" E lectrode I MO, 0 UTIllorm Electrode II ... (4) tional vacuum fus ion method') composition

The charge carriers in the oxide, MO, would be the investigations cited above are quite unique, but 2 M2 +, 0 - , electrons or positive holes. Using the the fo llowing reasons limited their wider applications. assumption of the local equilibrium in the electrolyte, ( I ) The electric cond uction mechanism in the Wagner9l obtained the Gibbs fr ee energy change of electrolytes was not cl early known. Virtual cell re the thin slice of the electrolyte, when the infinitesimal actions were not definite accordingly. current was supplied to the cell under isobaric con (2) Because solid oxides with large ionic conduc ditions. Then, this infinitesimal Gibbs free energy tivity were not yet found, the internal resistance of cha nge was integrated from the leftmost to the reight the cells was too la rge to have a reproducible electro most side of the electrolyte to obtain the total free motive force. energy cha nge of the virtual cell reaction. Using the (3) When liquid oxides are u sed instead of solid relation, J G = - nFE, one gets the fo llowing expression ones, the interna l resistance of the cell can be reduced, for the electromotive force: but in many cases, th e liquid electrolytes react with the electrode m aterials. Therefore, the stability of RT P" E = 4F f p~ ' (t~t , .+ t u '- ) d l nPo , the electromotive force was not acceptable. 0 , These difficulties could be covered to some extent RT f a~; by the a rt of the construction of the galvanic cells and = - , (tM " + to,-)d In aM 2F aM the careful measurements of the electromotive forces. RT aM However, the difficultes limited the applications of = - f (l - te)dlnaM ··· ...... (5) the oxygen concentration cells only to specific experi 2F aM m ents. where, E: the electromotive force 9 To solve these theoretical problems, C. Wagner ,lOl F: the Faraday constant reported two theoretical works in 1933 and 1966, Pb, and P(f, : the oxygen pressures at the two which strictly related the conduction mechanism of electrodes the oxide electrolytes to the electromotive force at a:u and a~: the chemical activities of the elevated temperatures. metal at the two electrodes Furthermore, Kiukkola and Wagnerlll discovered t i : means the transference number of the that zirconia or thoria base solid solutions can be used charge carriers as the electrolyte to have very stable and reproducible For zi rconia base solid electrolytes , the transference electromotive forces, simply b ecause of their very number of oxygen is known to be unity at a higher large oxygen ion conductivity a bove 500°C. oxygen pressure, a nd thus all other transference num Tha nks to this celebrated publication, high tem bers become zero . Therefore, Eq. (5) gives simply perature electrochemistry has been extensively d e veloped a ll over the world in the 1960's. E = RT I P :;, Two review papers have been published on this ...... (6) 4F n P'0 , development in 1970 by R app and Shores 12l and in 1972 by Goto and Pluschkell 13l documenting 242 a nd At a n oxygen pressure lower th an tha t for chromium 295 publications, respectively. However, these re chromium oxide equilibrium, the transference num vi ew papers did not deal with the industrial a pplica- ber of electrons can not be neglected and is given as a

R eview Transac tions ISIJ, Vol. 16, 1976 ( 471 ) function of oxygen press ure. In this case, Eq. (5) even at lower tempera tures. H owever, its use is very can be integrated to yield Eg . (12 ) in the following limited, because the second term can not be neglected section. d ue to ea y vaporization of a lkali metal oxides .16 )

2. For the Case oj the B inary Oxide E lectrolytes of Non III. Industrial Production oj Zirconia Base Solid uniform Composition Electrolytes and Thier Impurity Contents Using AO- B02 as the binary oxides, the cell can In la boratory zirconia - lime solid solution can be be described by made from the aqueous solution of zirconium nitra te, (ZrO)(N0 3)2 a nd calcium carbonate, CaC0 3. Slow AO- B02 of El d II P~ " non-uniform ectro e , .. . (7) ly drying the solution, the compounds deposit in very E lectrode I composition P;;, fin e powders. The decomposition of the nitra te a nd carbonate a t 700 0 to 800 0 e gives the mixture of zir where Po, denotes the chemical potential of oxygen , conia a nd lime. After p ressing the powd ers in a a nd the possible cha rge carriers would be ionic species, desired shape, sintering at I 500 0 e gives the solid solu A2+, B4+ and 0 2-, a nd positive holes or electrons. tion of zirconia and lime with the fluori te type crystal If the local equilibrium condition applies through structure with white color. H owever, commercial out the electrolyte, the Gibbs free energy change due zirconia base solid electroly tes have yellowish color to the migration of the carriers induced by the ex due to the impurities. ternally supplied current can be calculated, a nd the T a ble I is the fl ow sheet of the industrial production electromotive force can be ex pressed by the following of the solid electrolyte reported 17) from a J apa nese equa tion : 10) company. T a ble 2 shows the impurity contents in the commercial electrolytes a nd their physical proper ties. 17) The users' claims a re m ostly the spalling of the electrolytes, the precision in size and sha pe of the elec trolytes, a nd the effects of the impurities upon the elec tromotive force a nd the life time of the oxygen where, t i means the tra nsference number of charge conce ntra tion cells for ind ustrial uses. As T able 2 carrier i, a nd Pi means the chemical potential of °2, shows, the major impurities a re Si02, Ti02 , AI 20 AO or B0 . 3 2 a nd Fe 0 . H owever, the impurities would ha rdly The local equili brium being assumed , the Bibbs 2 3 affect the electromotive force in a short time because Duhen rela ti on will be sati sfi ed throughout the el ec of the a bsence of the cationic conduction as explained trolyte: in the a bove. In a long time, silica a nd ferric oxide would react with the electrod e materials, decreasing ...... (9) the life time of the galvanic cell s. Electromotive force observed a t extremely low oxygen press ures would be where, N Ao and N Ro, are the mole fraction of oxides, influenced by ferric ox ide im purity in the electrolyte, AO a nd B02 • Inserting Eg . (9) and /l g,+ RT In Pu, because ferric oxide decom poses to yield oxygen a t for Po, in Eg. (8), one has such pressures and, if existing at grain boundaries, the ferric oxide provide the p ath for oxygen transfer. RT p " f [I I Figure 3 shows the specific resistivity of com E = 4F In p?' - 2 tA"--4 0 , mercial electrolytes. 17l The electrolyte sta bilized by - ~; f t,d ln Po, ...... (10) Y20 3 has la rge conductivity even a t lower tempera-

Table I. A Row sheet of manufacturing of Zr02-base solid The first term in the right hand side of this equa tion solutions for the 'oli d electrolyte is the same as E in E g . (6), a nd the second term is (1) Mixing of raw materi a ls the electromotive force induced by the difference Chemical analysis, for a fixed composition between the diffusivities of the cation species. The Grain size measurement X· ray ana lysis third term, arising from the internal current, caused by the migration of electrons or positive holes in the Testing 'of size di stribu tion and sli p character electrolyte, means the decrease of the measura ble istics electromoti ve force. (3) Screening to have an In the case of zirconia or thoria base solid solution, un ifo r m pa rticle the tra nsference numbers of the cation pecies a re size zero. Thus the second term becomes zero rega rdless (4) of non-uniforming of the electrolyte compoSltlOn . This is one of the reasons why these electrolytes give Using a standard heating very stable and reproducible electromotive force. cooling program with a 14, 1 5) , In contrast, beta-alumina phases (A 20 - nM 20 3 controll ed atmosphere where, A is Na, Rb, Ag, K , Li or T I a nd M is AI, T esting of gas tightness, 3 Chemical and X·ray analyses, G a or Fe +) have a very large cati onic conductivity T esting of physical properties

Review' ( 472 ) Transactions ISIJ, Vol. 16, 1976

Table 2. Chem ical composition a nd physical properties of a com mercial stabilized zirco ni a for electrolyte use

Stabilizers CaO MgO Y2O . l 5mol% 9mol% Il mol% 15mol% 15mol%

Chem ical composition (% )

Si02 1.63 1.58 1.66 0 .6 0 .49

T i02 0 .28 0.24 0.27 0 . 24 0. 26

AI 20 3 0 .69 0 .71 0 .67 0 .29 0 . 27

Fe20 . 0 . 16 0 . 17 0 . 18 CaO 4 .7 5.8 7.7 0 .36 0 .04 MgO 0 .89 0.92 0 .99 5 . 2 Y20 . 13.2

Na20 0 .03 0.02 0 .04 0 .03 0 .03

K 20 tr. tr. tr. tr. tr.

Zr02 91.62 90. 55 88 .5 93. 27 85.67

Physical properties Max. water absorption (% ) 0 .29 0 . 1 0 0 .05 0 .02 Apparent porosity (%) 1.56 0 .06 0.01 0 . 24 0.07 Apparent density 5.5 5.36 5. 13 5 .60 5 .41 Degree of stabili za ti on (% ) 74 93 100 100 100

IV. On the Meters for Oxygen Analysis in the 10' r-_l,30_0 ,12,OO_Ir-:-1OO.:...... :..r1OO..:..:O--=9cr OO,,---=-80T-'0,-----,7'-T0..:...O_----=;60;..:.0 __CC_ )__ Waste Gases of Boilers and Various Furnaces

/ The first try to use the oxygen concentration cell 5 // with the solid electrolytes for gas a nalysis has been l 8 3 reported independently by Weissba rt a nd Ruka ) m 1 9 1961 a nd b y Goto a nd St. Pierre ) in 1962 . 2 1/ I P ,I // The ce1l 19 ) can be expressed by 10 3 r------~------~~~----~r/------ Gases for zoe 0 Fe, FeO mixture, I a nalysis, Pt r 2- a reference electrode ' " ( II ) I ; /0 Figure 4 shows the structure of thi s ceU. 19 ) In steel 0 / 5 /-1" 1 20 industry, this cell was used by M a tsushita, et al. ) to determine continuously the oxygen content in the j waste gas of the open hearth furnace. As shown in 10:t------1-I-'rf-i7L--~~//.-~-I-I-II-I------ Fig. 5, the waste gas was continuously sampled by the steam ejector and sent to the oxygen concentra tion n I em / 0 cell. Figure 6 shows the cha nge of the electromotive 5 / 0 ---- CaO I5 ~IOI.% force with respect to time, when the standard el ec .6------CaOllMOI.% trode is m ade of a mixture of nickel a nd its oxide '" 3 "c:: x ---- CaO 9 ~ I OI.% powders. During the use of No. I burner, the elec ~ 2 en o ------Y0 I.5 15 ~ l ol . % tromotive force gave the oxygen pressure in the waste en E] ------M g O I5 ~lol.% gas bu t during the use of TO . 2 burner, it gave the 0::'" oxygen pressure in air used for the combustion of the fu el oil. From thi s measurement, a n optima l amount

'"0-" of the secondary a ir for the combustion was deter en mined. H owever, the life time of the cell was only 3 to 4 d ays for its continuous use, probably due to iron oxide powders genera ted by oxygen la ncing. 21 After this study, Fischer a nd J anke ) m ade a n 103/ T (KG) extensive study on the behaviors of the ga lvanic cell Fig . 3. Reciprocal temperature a ncl the specific res isti vity as the oxygen analyzer for hi gh tempera ture gases. of commercial zirconi a base solid electrolyte") T owa rd the end of the 1960's, ma ny compa nies manufacturing industrial instruments have sta rted to tures. make commercial oxygen m eters using the solid elec Beca use chemical and physi cal properties are di f trolytes . Table 322 ) includes fiv e co mmercial oxygen ferent for various electrolytes, carefu l selection is neces meters. For the sakes of easy ha ndling a nd long life sary to have long life a nd stable electromotive force time, air a nd pla tinum are used as the reference elec of the galvanic cells for various industrial uses. trode in a ll meters in T a ble 3. The deteriora tion by

Review Transactions ISH, Vol. 16, 1976 ( 473 )

SiO, Tube for Argon A rgon G as I ' t' Gas sampling appa ratus I nJ ec Ion ~ ~ Rubber Stopper To Potenti<> meter ------CO- CO,-- Gas No.2 I ZrO,-CaO Tube

~ _____ Brass Cap

recuperator P ylex Glass Tube for CO-CO, Gas F lu e Stack ____ Porcelain Reaction ..--- Chamber

Insulating Tube for Lead Wires Fig. 5. Schema ti c diagram of the open hearth furnace and the galvanic cell to make the continuous analys is of its waste gas by Matsushita, et aU")

I Tap / Pt Sheet (13' 30 ') / ~ Fe-FeO Mixture 1 Date Nov.6 1964 T 13 ' 26 ' T / ~ Pt Windings 2 Melt down 11 ' 40 ' No.2 3 Chart speed 300mm / mi n ner 4 Ce ll temp. 1193'1< I-" 81 Pt-Rh Pt Thermo c:::- 13 ' 18' couple T I> .~ No. 1 Burner +13' 10' No . 2 '"E BUler f- s: 13 ' 04 ' No.\ Fi g . 4. A prototype of the oxygen concentra ti on cell to Burner a nalize the gas compositi on by Goto and St. Pierret.) ~ 1/+12' 55 ' ~ NO.2 ha rmful elements of the pla tinum electrode during Burne r 1 12' 49' the gas a nalysis is always the problem of these meter , T so that platinum is fix ed to the electrolyte with various No .1 special techniques developed by each ma nufacturing c: r---,~ Burner companies. The oxygen meters in Table 3 can be '==h..12·30' 300 400 500 600 700 800 used for (h e foll owing pUl-poses: EMF (mV) I ) Continuous oxygen a nalysis of waste gases of boilers a nd various industria l furnaces Fi g. G. The sheet with the recorded EMF as the function of 2) O xygen a nalysis in various heat treatment fur time: obtained by the experime ntal se t-up of Fig. 5 naces in ferrous and nonferrous industry 3) Oxygen a nalys is in heating and decomposition furnaces in petroleum industry no buffering capacity with resp ect to the leaked-in 4) Oxygen press ure determination of sintering bed oxygen gas. in iron m aking industry23) The fl ow ra te dependency of the electromotive force 5) Permeability determination of moisture through for inert gas-oxygen mixtures has been reported by the coating a nd mold materials for electronic Etsell and F lengas,25 ) I wase, Fujimura and Mori,26) parts. 24 ) a nd Sunayama, et at. m Figures 7 and 8 show27 ) the Manufacturing of the oxygen m eters was very much relation between the determined oxygen press ure in accelerated in J apan by a new law to prevent th e argon and the linear veloci ty of the gas flow in the pollution of air, effective since June, 1971. In this vicinity of the gas electrode m ade of a pla tinum wire. law, equations are proposed to estima te NOx content When the oxygen content was lowered, the larger from the oxygen pressure in the industri al waste gases. flow rate was needed to have a constant and repro The lower limit of oxygen a nalysi was reported13 ) ducible oxygen content in argon. The larger flow rate to be close to the equilibrium oxygen pressure of Cr seems to he needed to exclude the effect of the oxygen and Cr20 3 or about 10- 28 atm a t 800°C. H owever, leaked in from various pa rts of the gas train and in reality it is not so, particula rly when the gas has through the solid electrolyte.

Review ( 474 ) Transactions ISIJ, Vol. 16, 1976

Table 3. The comparison of fiv e commercial oxygen meters

Specification A B C D E

Cell design* OBE COE OBE COE COE

Electrolyte Zr02 , CaO Zr02 , CaO Zr02 , CaO ZrO,CaO

Cell tempera ture 850°C 81 5 .6°C 850°C 850°C 81 5.6°C

Sample gas flow rate 200-300 ml/min 0.97-42.5l/min 500 ml/m in 250 ml/min 0.24-42.5l/min

Less than 0.1 % Less than 0.01 % Flow sensitivity per 50 ml/min per 470 ml/min

1- 100 ppm 0 .8- 150 ppm 0-0 .5% , 0-1 % 1-100 ppm 0-1 % , 0-5% O input ranges 0.1 -20%, 1-20% 80-1 50000 ppm 0-5% , 0-10% 2 0 . 1-10% 0-10% , 0-25% 98- 100% o 8-150% 0-20% , 0-100%

I sec for 63% 1 sec for 63 % I sec for 63 % I sec for 63 % Speed 01 response 3 sec for 90% 3 sec for 90 % 3 sec for 63% (9 .44 ml/min) (250 ml/min) (250 ml/min) (5 00 ml/min)

T e mpera ture of 0_50°C 150°C - 45-120°C sample gas

Power requirements AC 100 V , 300 W AC 11 5V, 300W AC 100 \ ', 130 W AC 118 V, 120W

* OBE means" open on both ends" and COE means" closed one end ".

o --0-- Present results ---0- Present resul ts - --0--- Etsell and Flengas ----<>---- Etsell and Flengas

- 1

"0- - ______0..- - ______~ - ______

Range to obtain a constant oxygen pressure

Range to obtain a co nstant / 11/ . / OD o -----Ij/ oxygen pressure -l -"' ~------_ ~ ______-

- 5 - 1

- 18

-6 ~----~-----L--____L- ____~ ______~ o 2 3 4 - 20L-______~ ______L ______~ ~ Gas Flow Veloc ity (em/sec at 25"C ) o 5 10 15 Fig. 7. The linear ve loc ity of a rgon gas and logarithm of Gas fl ow ve loc ity (em/sec at 25"C) oxygen press ure obtained f"om tir e EMF of th,. Fi g. 8. The linear veloci ty of argon gas a t the vicinity of galvanic cell by Sunayama, et al.'7) the' gas electrode conve rted to 25°C a nd the obtained oxygen press ure'" V, Determination oj Total Oxygen Demand in Waste Water and NOx Control oj Gasoline cedure of its determination is sampling a fix ed quan Engines tity of poll uted water, heating it to a high temperature Two special applications of the above oxygen with a catalyst and with excess of oxygen gas, and meters are the determination of total oxygen demand then, the quantity of oxygen consumed to combust of polluted water and NOx control of vehicle engines. organic compounds contained in the sampled water The total oxygen demand, TOD, is recently used can be measured by an oxygen concentra tion cell as an index of magnitude of pollution of water. Pro- wiLh the solid electrolyte. Furthermore, the quantity

Re v iew Transactions lSIJ, Vol. 16, 1976 ( 475 ) of carbon dioxide produced is determined by a n in the electric signal at 300° to 900°C with a response fra red a bsorption gas analyzer to have total carbon, time of 5 X 10- 3 sec and its in ternal resistance is I05.D, TC. Thus, the combination of an a utomatic water at 400°C. sampler, the oxygen concentration cell , a nd the in DUker, et al.ll) reported that the electri cal conduc frared a bsorption gas a nalyzer makes it possible to Li viLY was large even at lower temperatures with check continuously TOD and TC in the industrial zirconia stabilized by 8mol % Y 203' and that the waste wa ter. This combined set is now commercially m echanical strength of th e el ectrolyte increases by the availa ble in J apan. The sample water of 10 g is addition of 3% Si02 but at the sacrifice of the con intermittently ta ken, and TOD and T C can be deter d uctivi ty. m ined in the range of 10 to 10 000 ppm a nd 5 to 500 ppm, respectively. VI. Rapid Determination of Oxygen Content in Another special appli cation of the oxygen concen Copper and Steels at Their Refining Plants tration cell is its use for vehicle engines to control the In 1965, Pluschkell and Enge1l31 ) first reported on NOx content in the exhaust gas by m easuring the the oxygen content determination in molten copp er oxygen content in the exhaust gas and feeding it back by the electromotive force method usi ng zirconia to the carburetor to control the ratio of air to fuel to lime solid electrolyte. Recently, J a nke and Fischer32 ,33) minimize the NOx content. This unique study was studied on many copper alloys with this method. reported in 1973 independently by Fleming, et al. 30 ) and by DUker and Neidhard. 29 ) 30 Later in 1975, DUker, et al. ) reported a detailed mV study on the relation of the life time to the quantities co ~O, HC 1000 of stabilizers of CaO , Y20 3' a nd MgO, a nd to the g/ kll"h g/ kll"h g/ kwh structure of the oxygen concenLration cell. The life 400 13 time was reportedly corresponding to that for the 12 9 350 vehicle running distance of 24 000 km. Figure 9 11 8 750 shows Lh e relation between the contenLs of CO , NO , 300 10 x 7 a nd hydrocarbon in the exhaust gas a nd the ratio of 9 air Lo fuel supplied to a reciprocaLing engine. In the 250 8 6 figure, the curve of a n isothermal electromotive force 7 500 200 5 of the oxygen meter shown in Fig. II is al 0 given to 6 4 show its stepwise cha nge at the quantity of air needed 150 5 theoretically for a complete combustion. Figure 10 4 3 250 shows the principle how to automaticall y control the 100 3 2 ra tio of air to fu el to minimize the NOx content with 2 the aid of the catalyst. Figure II shows the construc 50 tion of the oxygen concentration cell used. T he inner o~~~ __~ __~==~==~==== __ ~o=-~o pla tinum electrode is fixed to the solid electrolyte with 10 11. 5 13 14.5 16 17.5 19 - Air/ F ue l t the coating technique of thick fi lm . The outer one Theoretica ll ,' needed ai r is made with the evaporation m ethod and then cover Fig. 9. Contents of CO, NOx a nd h ydrocarbons in waste ed by a magnesium spinel oxide layer sprayed by gas of a gasoline engine as a function of ra tio air to plasma Aa m e. This spinel layer would protect the fue l. The EMF curve is the values obtained b y the platinum electrode from compounds of P, S a nd Pb oxygen concentration cell shown in Fig. I I b y conLa ined in the ex ha ust gas. The galvanic cell gives Diike r, el al .'O)

EX HAUST GAS ENG INE -- A-SENSO R (Oxygen Concentrati on Cell)

FUE~L~ ____l- ______~~~~1

FUEL QUA 'TITY

AIR QUAN TITY ELECTRON IC A/F CONTROL UN IT

Fig. 10. Principle to contral the ra tio of a ir to fu el based on the EMF of galvanic cell to minimize the contents of harmful gases in the waste gasao)

Review ( 476 ) Transactions ISIJ, Vol. 16, 1976

EMF

HO USING

AIR _ GASTIGHT ELECTRICALLY CONDUCTIVE SEAL

EX HA US T GA S THR EE PHA SE BOUNDARY

" " }----->po , ) ','"'~ SO LI D ',' EL ECTROLYTE : ,',' ,----A----. CO CO, HC H,O OU T E R NO, ELECT RODE RES IDUA L OX YGEN EQUILIBRIUM OX YGEN INNE R EL ECTRODE (po,) Fig. II . Schematic diagra m of the oxygen concentrati on ce ll to m a ke the continuous a na lys is of the waste gas of the engine"O)

K a metani a nd Yamauchi 34 ,35) have measured the oxygen potential change during vacuum treatm ent of molten copper a nd molten m a tte. In pa rallel with the e basic studies, Dompas and Lockyer have studied for 7 years on the applica ti on of the galvanic cell to copper refining industry a nd reported their results in 1972.36 ) They have continuously dipped the oxygen concentration cell in molten copper in the la unders of wirebar cast ing, of a n a node reverbera tory furnace, a nd of con tinuous casting of tough pi tch billets. In Fig. 12, the cell is dipped in copper a t the conn ec tion of the / la under a nd the continuous casting machine. From Oxy~e r Concentration Ce ll the obtained electromoti ve force, the quantities of the air to the burner to heat the la under and of the adding deoxidizer a re controll ed to maintain a consta nt oxygen content in co pper for continuous casting. Fig. 12. Backfeed control system for continuous casting of tough pitch copper billet using the oxyge n conce n Figure 13 shows the measured electromotive force a nd trati on cell a t the exit of the launder, by Dompas the oxygen content determined by the conventional and Lockyer 3S ) method . Until 19 72, this n ew monitoring m ethod has been used as the standa rd o pera tion of the wirebar 700 700 production of 300 000 t. The life time of the galvanic cell was a bout 70 hr in average. The details of the structure of the cells are given in R efs. 37) and 38). 650 650 On the a pplications of the galvanic cells to steel . ... industry, more than twenty p a pers have been pub ... ;;- lished13) between 1960 a nd 19 70, because of the intense . .. .5 600 600 industrial needs of the rapid d etermina tion of oxygen . . E 1'. . content in teels. w The first try to use thi s m e thod in steel pla nts in J a pa n was m ade by Ihida a nd Kawai,39) which 550 stimula ted very much further development in other steel compa nies in J apan. Two groups of the steel pla nt metallurgists, the one represented by Saeki a nd 125 Hiraoka 40,41 ) a nd the other by Suzuki42) have m ade o very extensive investigations to u se the oxygen concen ppm (0 ) i n C u tra tion cells on lines of steel refining, with co-opera ti on Fig. 13. The EMF o bta ined by the method of Fig. 12 and the oxygen content in copper determined by with compa nies manufacturing the instruments for vacuum fu sio n a nalys is'6I steel industry. Because of these investigations, the galvanic cell as shown in Fig. 14 is now commercially trod e m ade of a mixture of Cr a nd Cr 20 3 and can availa ble in J apan. This cell has the reference el ec- determine tempera ture a nd the oxygen content of 3

Review Transactions ISIJ, Vol. 16, 1976 ( 477 )

r 188 @ 25 15 / ~ 60 i / /

o 00..... \ \

( a) ------63 , 140 A '

370 ' \

Review ( 478 ) Transactions ISIJ, Vol. 16, 1976

iron- oxygen melt with respect to a ny small cha nge . ~ 15 ~ ~ of oxygen potential would be very small. ~ ~ 10 ~;:;: This means that the oxygen pressure at the inter ~~ 5 face between the solid electrolyte a nd molten iron 0 would so easily devia te from the average oxygen poten 0- 4 ;;;:" ti al in the bulk of the m olten iron . ~ 2 Further, when one considers the contents of ferri c --;;; 0 oxide and silica in the electrolyte given in Table 2, ~ 2 CO it is a very na tural a nd sound question to ask a bout the * 0 m agnitude of the deviation of the oxygen pressure due to the reduction of these unsta ble impurities. In CO·"i 0 008 this respect, Fischer a nd J a nke45 ) suggested a n intense ~ 0.004 stirring of the m olten iron to minimize the devia tion.

VII. Continuous Determination oj Oxygen Con ~ 0' 1 tent in Liquid Sodium Used as Coolant oj 0...... I ~ 0. 005 Fast Bleeder Reactors \ 'Q, " /1) \ ,,- _.. .. /" For the safe operation of the a tomic reactors, It IS '. /'. -0-_ ~~~_.a (% OJ I • '0...... ----0-" very important to check continuously the oxygen con 0 ~0 --~2~0 ----4~0 --~6~0 --~80~--1~0~0 tent in liquid sodium used as the coolan t. Three papers in 1965 a nd 1968 have been already document Cast ing ti me ( % ) ed in R ef. 13). R ecent two pa pers by Berkey a nd Fig. 17. Change of the Aux composition of the tundish a nd R ead ,50) and by R oy5l) reported the wide use of the of oxygen content steel during its continuous foll owing cell s in the world for the fas t bleeder re casting"l actors; vanic cells to steel pla nts during September, 1974 air Na- Q (liquid) Th02- Y 20 3 (Pt'r ) ... (1 3) 43 reJerence to M a rch, 1974. ) The reproducibility is said very good, when the cell s are used for the plain carbon E(volts) = K, - K210g(pp m Q ) ...... (14) steels containing more tha n 50 ppm oxygen . H ow ever, the results are not impressive, when they are Kl a nd K2 a re constants at a given tempera ture a nd used for steels with less tha n 50 ppm oxygen or for a re 1. 828 a nd 0.075, res pectively, a t 483°C. Yttria special steels. is used as the sta bilizer to have a good conductivity T herefore, the recent studies a re concerned to the a t its service tempera ture of 350° to 500°C. Because determina ti on of oxygen less than 50 ppm or in of the pa rtial positive hole conducti on at the oxygen 6 special steels.44 -49) When the oxygen content be pressure of higher tha n 10- a tm, the electromotive comes extremely small , the electronic conduction in force measured would be sma ll er by 5 to 10 % tha n the zirconia - lime solution can not be neglected . For tha t calcul a ted according to Eq. (14). such a case, Pluschkell49 ) integra ted Eq. (5) a nd ob When the thoria- yttria solid electrolyte was highl y tained the foll owing express ion for the sta nda rd elec purified a nd then isostati call y pressed, followed by complete homogenizing by sintering, its corrosion trode of C r a nd Cr 0 : 2 3 resista nce against liquid sodium increased very much a nd the life time became 5 000 hr under the con Go = K {[( P;)I /4 + (P('r/" r,O,)1 /4] tinuous se rvice. lJ ) The lower li mit of the oxygen a nalysis was reported 0.1 ppm Q in liquid sodium . . exp ( - ;~) _ ( P ; )1 / 4 } ...... ( 12) VIII. Concluding Remarks where, 00 IS the H enrian activity of oxygen in unit Since a very celebra ted pa per by Kiukkola a nd of wt%, K is the equilibrium consta nt of 1/202 = W agner in 1957, several hundreds of papers have been Q (% in iron ), a nd P: is the oxygen pressure, a t published throughout the world, a ll using zirconi a which the tra nsference number of ions becom es equal or thori a base solid electrolytes. T he reasons of this to that of electrons. J n zirconia- lime solid solution, wide uses of the special soli d oxides seem simply be P: is abou t 10-15.8 a rm a t 1 600°C. Pel'/(' r,O, means cause of their excell ent oxygen ion conducti vity a nd the equilibrium oxygen p ressure with chromium a nd their thermodynamic stability with high melting its oxide. Inserting these consta nts, Pluschkell point points. T he former reason gives the ve ry small inter ed out that no correcti on is necessary down to 10 nal resistance of the galvanic cells, which is very ppm Q in plain carbon steel, bu t it is needed for the necessary to have a stable a nd reproducible electro range of 1 to 0.01 ppm Q . Therefore, the galvanic m otive force. In ma ny colleges inJa pa n, the galvanic cell can be u sed, in principle, at I 600°C down to cell s are constructed by students to learn one of the 0.01 ppm Q according to E q . (12 ). However, in m ethods to determine the Gibbs free energy cha nge of reality, it would not be so. When the content be reactions a t high tempera tures. The latter reason is comes so low as 10 ppm, the buffering capacity of the as well important to exclude a ny solid state reacti ons

Review Transactions ISIJ, Vol. 16, 1976 ( 479 )

between the electrolyte a nd the electrode materials 56 1. even at high temperatures. This means that the 5) K. Sanbongi , M. Ohtani and Y. Omori: Report of No. wide variation of combinations of electrode materials 19th Committee of J apan Society for the Promotion of is possible to make very different types of galvanic Science, No. 19-4674, ( 1957),July. cells with miscella neou objectives. 6) T. Onaka: Tetsu-to-Hagane, 30 ( 1944), 62. 7) T. Onaka: Report of No. 19th Committee ofJ apan Society Because two review papers12,13) have been publish for the Promotion of Science, No. 19-2023, ( 1949), Septem ed on the applications of the solid electrolytes to ber and ( 1950), J anuary. basic studies, the present survey was made on their 8) M. Ohtani and K . Sanbongi: Tetsu-to-Hagane, 49 ( 1963), industrial applications. The industri al applications 22. have ma ny difficulties as follows, which are quite 9) C. Wagner: Z.Phys.Chem., B21 ( 1933),25. different from the case of the a pplications to basic 10) C. Wagner: Advances in Electrochemistry and E lectro studies; chemical Engineering, I V, lnterscience Publi shers, ( 1966), (I ) The electrolytes must be made by mass pro I. duction with constant and good qualities in chemical, II ) K. Kiukkola and C. Wagner: j. Eleetrochem. Seo., 104 phys ical a nd mechanical properties. ( 1957),379. 12) R . A. Rapp and D. A. Shores: Physicochemical M easure (2) The oxygen concentration cells are some ments in M etals Research, II, I nterscience Publishers, times used continuously in the gas or liquid metals ( 1970), 123. containing m a ny harmful elements. 13 ) K. S. Goto and W. Plusc hkell: Phys ics of Electrol ytes, 11 , Thus, to have a long life time, special ways should Academic Press, ( 1972),539. be developed for each case to prevent the deteriora 14) W. R . Roth: General Electri c I nc., Technical I nformation tion of the electrode a nd electrolyte. Series, Report No. 74CRD054, ( 1975), March. (3) For the use of the galva nic cells in molten 15) A . Jmai and M. H arata: j. Appl. Phys. , 11 ( 1972), 184. steels, a stable electromotive force should be ob 16) K . S. Goto: Symposium on Solid E lectrolytes, T he Japan tained in less than 10 sec, because the molybdenum Institute of M eta ls, Tokyo, ( 1974), J anuary 28. electrode dissolves quickly. Thus, the cells must be 17) M. Takeuchi: Private Communication, Manager of Nip pon Kagaku Togyo Co., Ltd., ( 1975), May. constructed in very small size but with good precision 18) J . Weiss bart a nd R. R uka: Scientific Instruments , 593 in many aspects. ( 196 1), 32. (4) In thc ind ustri al appli cations, untrained em 19) K . S. GolO: Ph. D. Thesis, ( 1962), O hi o State University, ployees use the cells in rough ways. Thus, the hi s Adviser was Professor Sl. Pierre. galvanic cells m ust be mechanically and thermall y :l0) Y. M atsushita, K . S. Goto, A. Cho, K. Igaras i, M. Tate very strong. a nd H . Sasao: Tetsu-to-Hagalle, 52 ( 1966),393. The above difficulties will be eventually solved with 2 1) W. A. Fisc her and D.Janke: Arch. Eisellhllttenw, 39 ( 1968), the help of the art of peoples with different origins a nd 89. traditions, because the principle of the galva nic cell 22) Y. Endo: Private Communication, Mitaka Instrument is simple a nd universal. Co., Ltd., ( 1975), M ay 10. 23) H . Yosikoshi: Private Communication , Nippon K okan Acknowledgements K. K., ( 1975), J une 13. The present a uthor would like Lo cxprcss sincere 24) R . K ano: Private Communication, Elcctrotechnical and deep a ppreciation Lo the following gentlemen, Laboratory, The Ministry of I nterna tiona l Trade and J n who have vel'y positively helped him to collect the dustry, ( 1975), June 20. information about many Lypes of the industrial ap 25) T . H . Etsell a nd S. N. Flengas : Al e/. Trans. , 3 (1972), 27. plications of the oxygen concentration cells; Dr. 26) M. Iwase, S. Fujimura and T . Mori : j. japan h zst. K oichi Asano, Hirohata W orks, Nippon Steel Corp., M etals, 39 ( 1975), 952. Mr. Kiyoo Ono, Central R esearch Laboratory, Daido 27) H . Sunayama, K . T suda, K. H ori and K . S. Goto: Tetsu Steel Co., Ltd., Mr. Minoru T a keuchi a nd Mr. to-Hagan!, 61 ( 1975) S52 . Noboru Miyamoto, Technical Development Section, 28) W .J . Fleming, D. S. Howarth and D. S. Eddy: 2nd Nippon K agaku Togyo Co., Ltd., Mr. Akira Masui Automotive Engrs. Automobile Eng. M eeting, Paper 730575, and Mr. Hideyuki Yoshikoshi, Technical R esearch ( 1973), M ay 14-18. Center, Nippon K oka n K .K ., Mr. Masao M atsuoka, 29) H . Duker and H . Neidhard: 2nd Automotive Emiss ion Conf. An n Arbor ( 1973). Yamari-Electronite Co., Ltd., Mr. Yasuo Endo, Mi 30) H . Duker, K-H. Friese and W-D. H acker: Automotive taka Instrument Co., Ltd., Mr. Sumio Shiomi, De Engrs. Automobile Eng. Meeting, Paper 750223, ( 1975), partment of Metallurgy, The University of Tokyo, February 24-28. Dr. Hiroyuki Sato a nd Dr. R yoichi K ano, Electro 3 1) W . Plusc hkell a nd H -J. Engell: Z. MetaUk., 56 ( 1965), technical L abora tory, The Ministry of International 450. Trade and Industry, and Dr. Hiroshi K a metani, 32) D.Janke a nd W. A. Fisc her : Arch . Eiseniziittenw . , 44 ( 1973), National R esearch Institute for Metals. 15. 33) W. A. Fischer and D. J anke: M etaUwissellsehaft und Teclmik, R EFER ENCES 26 ( 1972), 11 23. I) F. H a ber: Z. Anorg. Chern., 57 ( 1908), 154. 34) H. K ametani and C. Yamauchi: Trans. JIM, 13 ( 1972), 2) W.D.Treadwell: Z .Elektroehern., 22 ( 19 16), 4 11. 13 . 3) E. Baur, A. Peterson and G. f"Lill eman : Z. Elektroehem., 35) C. Yamauchi a nd H . Kameta ni: Trails. JIM , 14 ( 1973), 22 ( 19 16),409. 26 1. 4) O. A. Esin: lzu. Akad. Nauk SSSR, Metally, OT.. ( 1948), 36) J . M. Dompas a nd P . C. Lockyer: Met. Trans., 3 ( 1972).

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2597. 44) K. Ono: The 50th M eeting of I nstrumentation Commit 37) J. Dompas and J . van M ell e: J. Insl. Metals, London, 98 tee, J oint R esearch Society, ISl.J , ( 1972), March. ( 1970), 304. 45) V;. A . Fischer a nd D. J anke: Arch. EisenhIWenw., 41 38) .J . Dompas and 1. Hens: BNFMRA Conference, Liege, ( 1970), 1027. Paper No.4, ( 197 1), October. 46) W. Plusc hkell , U. Puckoff, C . G atellier and M . Olette : 39) M. thida and Y. K awai: Tetsu-to-Hagane, 58 ( 1972 ), 1477, Stahl u. Eisel!, 94 ( 1974), 539. and Trans. J.SI) , 12 ( 1972), 269. 47) S. M aruhas hi a nd T. Yamauchi: No. 19th Committee of 40) H. Kumai, Y. Arima, T. Saeki , '\". Hiraoka and K. l sikura: J apan, Society for the Promoti on of Science, ( 1975), Sep Tetsu-to-Hagane, 59 ( 1973) S438. tember . 4 1) K. Hiromoto, T. Saeki , S. Nisugi , Y. Arima, T. Hiraoka, 48) P . .1. K reyger, B. Slangen and H. W. den H artog: Stahl u. K . T ama and S. Igaki: R eport of No. 19th Committee of EiSel!, 95 ( 1975), 393. .1 apan Society for the Promotion of Science, ( 1974), Feb 49) W. Pluschkell: Arch. Eisenhiittenw., 46 ( 1975), II . ruary. 50) E. Berkey and W. H . R ead: Proc. of lnt. Symp. on M etal 42) K . Suzuki , M. Kato, A. Ejima and H . Nakamura: Kawa Gas-Slag Reactions and Processes, Toronto, ( 1975), 92 1. saki Steel Technical Rellort, 7 ( I 975), 137. 5 1) P . Roy: Proc. of lnt. Symp. on M etal-Gas-Slag R eactions 43 ) M . M atsuoka: Private Communication, Manager of and Processes, Toronto, ( 1975), 9 19. Yamari-Electronite Co. Ltd., ( 1975), May.

Review