ZrO2 interfaces

Vol. 40, Suppl., pp. $85-$93, 1992 Printed in Great Britain. All rights reserved

0956-7151/92 $5.00 + 0.00 Copyright © 1992 Pergamon Press Ltd

Acta metall, mater.

ELECTROCHEMICALLY-INDUCED REACTIONS AT Ni/ZrO2 INTERFACES T. W A G N E R , R. K I R C H H E I M and M. R t ~ [ L E Max-Planck-Institut fiir Metallforschung, Institut ffir Werkstoffwissenschaft, Seestrassc 92, D-7000 Stuttgart 1, Germany Abstract--A symmetrical solid state galvanic cell, NiIZrO2 + 9.5 mol% Y203[Ni, was used as a model system to study and modify chemical reactions at Ni-ZrO 2 interfaces. The cell was produced by diffusion bonding Ni on either side of a single crystal of yttria-doped cubic zirconia. Different oxygen activities were established at the interfaces by applying an electrical potential across the galvanic cell. When the dectrechemical potential was greater than a critical value, of the order of I V, the intermetallic compound Ni~Zr formed at the interface with low oxygen activity and NiO formed at the interface with high oxygen activity. Under these experimental conditions, the ionic transference number of the electrolyte was ~ 0.03. In order to avoid internal, electrical short circuiting of the cell, a voltage had to be applied during cooling. In a different experiment, after applying the electrical current, the open circuit voltage of the cell was measured. During this period, the celi was short circuited internally, which caused the oxidation of the NisZr layer to Ni and monoclinic ZrO 2 and the reduction of the NiO layer to Ni. The microstructure, chemistry and morphology of the phases, grown under different conditions, were investigated using scanning and transmission electron microscopy. In addition, the measured and calculated thicknesses of the reaction products were compared. R/mm:~ On utilise une pile galvanique sym~trique ~ l'ttat solide, NilZrO 2 + 9,5 mol.% Y2031Ni, comme systtme modtle pour ~tudier et modifier ls r~actions chimiques aux interfaces Ni/ZrO2. La pile est produite en liant par diffusion du Ni sur chaque face d'un monocristal de zircone cubique dopte ~ l'oxyde d'yttrium. Diff~rentes activitts de roxy#ne sont rtalistes aux interfaces en appliquant un potentiel 61ectrique ~ la pile galvanique. Quand le potentie161ectrochimique est plus grand qu'une valeur critique de l'ordre de 1 V, le compost interm~tallique NisZr se forme fi l'interface avec une faible activit~ d'oxy#ne tandis que NiO s'y forme avec une forte activit~ d'oxy#ne. Pour ces conditions exptrimentales, le nombre de transfert ionique de r~lectrolyte est d'environ 0,03. Afm d'~viter un court circuit 61ectrique interne de la pile, une tension dolt 6ire appliqute pendant le refroidissement. Dans une exptrience diff~rente, aprts appfication du courant 61ectrique, la tension en circuit ouvert de la pile est mesurte. Pendant cette ptriode, la pile est en court-circuit interne, ce qui provoque l'oxydation de la couche de NisZr pour former du Ni et du ZrO 2 monociinique et la rtduction de la couche de NiO pour former du Ni. On 6tudie la microstructure, la chimie et la morphologie des phases dtvelopptes dans diff~rentes conditions en utilisant la microscopie 61ectronique fi balayage et en transmission. De plus, les ~paisseurs mesurte et calculte des produits de rtaction sont compartes. Zesanmtenfassung--Eine symmetrische galvanische Festk6rperzelle, NilZrO2 + 9,5 mol% Y203INi, wird als Modeli benutzt, um die chemischen Reaktionen an Ni/ZrO2-Grenzfl/ichen zu studieren und zu modifizieren. Die Zelle wird mittels Diffusionsbonden yon Ni auf beide Seiten eines mit Yttfiumoxid dotierten kubischen Zirkonoxid-Einkfistalles hergestelit. Verschiedene Sauerstoffaktivi~ten werden an der Grenzfl~ehe eingestellt, indem ein elektrisches Potential fiber der galvaniscben ZeUe angelegt wird. Ober~chreitet dieses Potential einen kritischen Wert in der Grote yon 1 V, dann bilden sich an der Grenzfl~che mit niedriger Sauerstoffaktivit~t die intermetallische Leglerung NisZr, an der mit hoher Sauerstoffaktivitiit NiO. Unter diesen expefimentellen Bedingungen ist die Ionen-Obertrittszahl ~0,03. Um einen inneren elektrischen Kurzschlui3 tier Zelle zu vermeiden, muBte w/ihrend des AbkiJhlens eine elektrische Spannung angelegt werden. In einem anderen Experiment wird nach dem Anlegen des elektfischen Stromes die Leerlaufspannung der Zelie gemessen. Wghrend dieser Periode wird die Zelle intern km-zgeschlossen, wedurch die NisZr-Schicht zu Ni und monoklinem ZrO 2 oxidiert und die NiO-Schicht zu Ni reduziert wird. Mikrostruktur, Chemie und Morphologle der Phasen, die unter verschiedenen Bedingungen gewachsen sind, werden im Raster- und im Durchstrahlungselektronenrnikroskop untersucht. AuBerdem werden die gemesscnen und berechneten Dicken der Reaktiosprodukte miteinander verglichen.

1. INTRODUCTION The structure and chemistry o f metal/ceramic interfaces play an important role in metal/ceramic joints, composites and microelectronic packaging. Due to the chemical reactions during processing or service,

the mechanical and electrical properties o f the interfaces can change, thereby altering the performance o f the devices. Therefore a detailed study of metal/ ceramic interface reactions with the possibility of controlling the reaction conditions would be useful for both the fundamental study of the reactions and $85

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the engineering of interface properties.~" In order to realize this goal, a solid state galvanic cell using ZrO 2 as an oxygen conducting electrolyte with metal electrodes was used in this work. Galvanic cells are widely used in studies of thermodynamic and kinetic properties of materials, as oxygen pumps and probes, and as fuel cells [1, 2]. By applying an electrical potential across the cell, the oxygen activity at the electrode/electrolyte interfaces can be varied, usually more precisely than by thermochemical means (e.g. degasing of the metal with oxygen before and after bonding to the ceramic, annealing of the bond in vacuum, etc. [3]). Starting with a symmetrical galvanic cell Me l ZrO2-electrolyte IMe and applying a voltage across the cell, the oxygen activity decreases at the cathode and it increases at the anode. When the oxygen activity is sufficiently low at the cathode, the ZrO 2 electrolyte decomposes and an intermetallic phase of zirconium and the electrode metal forms [4]. When the oxygen activity at the anode is sufficiently high, the metal electrode is oxidized [5]. If both reactions occur the overall cell reaction is given by (z + y ) M e + x Z r O 2 - , ZrxMey + Me, Oex.

(1)

The change in Gibbs free energy for the cell reaction is given by the difference between the free energies of formation of the reactants and products

AG = AGz~xMCy+ AGMc,oz, - xAGzro2.

(2)

Inserting the relation AG = - n F E into equation (2), where F is the Faraday constant and n the number of transferred Faradays in equation (1), the corresponding electrical potential E for the cell reaction can be calculated. In the present study a symmetrical solid state galvanic cell NilZrO2 + 9.5 mol% Y2031Ni was used as a model system to study and modify reactions at the Ni/ZrO2 interface, e.g. oxidation, reduction and formation of intermetallic phases. The microstructure, morphology and chemistry of the reaction products were studied using scanning electron microscopy (SEM) and conventional and analytical transmission electron microscopy (TEM). 2. EXPERIMENTAL PROCEDURES

2.1. Cell fabrication A single crystal of yttria-stabilized cubic zirconia (9.5 tool% Y203, Ceres Corporation, U.S.A.) was cut tThe idea of enhancing or suppressing interface reactions at metal/ceramic interfaces by an imposed voltage was developed independently by Prof. H. Schmalzried, University of Hannov©r, Germany.

into 2 x 17 x 20mm pieces with a diamond saw. The crystals were oriented such that the broad faces were parallel to (100). A nickel sheet (99.98% Ni, GoodfeUow, U.K.) of 1 mm thickness was cut into 17 x 20 mm plates. The surfaces of both materials were polished on an automatic polishing machine with diamond paste through 1 ~m and then cleaned ultrasonically in acetone. The cells were produced by simultaneously diffusion bonding two nickel plates on either side of the zirconia crystal. Bonding was performed in a vacuum furnace at a residual pressure of ,,,10 -3 Pa at 1573 K for 2 h with an applied pressure of 2 MPa. After bonding, the sandwich specimens were cut with a diamond wire saw into four pieces of equal size (cross sectional area 0.75 cm2). Finally, electrical contacts were made by spot welding Pt wires to the Ni using a capacitative arc discharge. 2 2 Electrochemical experiments Two electrochemical experiments (henceforth referred to as experiments A and B) were conducted, both in a quartz tube inside a vacuum furnace with a residual pressure of ~ 10 -3 Pa at 1200 K. In both experiments the cell was first heated to temperature and a constant current density of 40 mA/cm 2 (galvanostatic mode) was imposed. The voltage drop across a calibrated decade resistance box provided a measure of the current through the cell. After ~ 2 0 min, the voltage drop across the cell reached a stable value of ~ 2 V. The voltage drop across the resistance and cell were monitored by a chart recorder. In experiment A the current was applied for 130 min, after which the cell was cooled down to room temperature in ~ 10 min. During cooling, the cell voltage was continually adjusted manually to the final value obtained at the end of the galvanostatic treatment. In experiment B the current was applied for 1 h. Then the current was switched off and the open circuit voltage of the cell was recorded as a function of time. When the open circuit voltage reached a steady state value of approximately zero volts ( ~ 1 5 m i n ) , the cell was cooled slowly to room temperature in 4 h without applying an external, electrical potential.

2.3. TEA1 specimen preparation and characterization For the preparation of cross sectional TEM specimens of the regions around the Ni/ZrOz interfaces, the following technique was used. The galvanic cell was cut into strips 1.6 x 1.2 x 10 mm containing the Ni/ZrO2 interface in the 1.6 x 1.2 mm crosssectioned area. The strips were glued inside an alumina tube with an inner diameter of 2 mm. Before gluing, spaces between the wall of the tube and the strip were filled with smaller alumina strips. The tube was then cut into thin discs, which were mechanically polished to a thickness of ~ 100/~m, dimpled to

WAGNER et al.: ELECTROCHEMICALLY-INDUCED REACTIONS ,--20#m, and thinned with argon ions at 6kV. T o avoid preferential ion milling parallel to the interface and to account for the higher milling rate of the nickel, shields were placed around the interface and the nickel. Thus, only ions incident from the zireonia side of the discs contributed to the milling. All TEM studies were performed using a JEOL 2000FX operated at 200 kV with a Traeor Northern EDS system. 3. EXPERIMENTAL OBSERVATIONS

3.1. As-bonded interfaces During bonding the colour of the ZrO2 single crystal changed from colourless, transparent to yellow. Such a colour change of the crystal was attributed to partial oxygen loss, the formation of colour centres [6, 7], and Fe impurity valence changes [8]. First investigations of the Ni/ZrO2 interface with optical microscopy by looking through the ZrO 2 crystal revealed that some pores were present at the interface. A typical region containing the Ni/ZrO2 interface of a galvanic cell after diffusion bonding is shown in Fig. 1. The interface is flat and exhibits no reaction layer to a resolution of 0.5 nm. The absence of a reaction layer after bonding under the present conditions is in agreement with the observations of other investigators [9]. The grains of the polycrystalline Ni foils were randomly oriented to the single crystal zirconia and had an average size of ~0.5 mm.

/

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\ 0 2.

Q

i . 40 mA/cm t Q

!

i

o

8 Fig. 2. Schematic of the galvanic cell for experiment A.

The galvanic cell is shown schematically in Fig. 2. During current flow, a gradient in colour developed from brown to yellow between cathode and anode. It was suggested that this colour change might be due to the formation of colloidal particles, possibly of metallic zirconium [10]. The polarisation of the cell was so strong that the oxygen activity at the cathode (Fig. 2, interface I) reached a low enough value to

reduce the Zr02 electrolyte. The resulting Zr reacted with the Ni forming the intermetallic compound NisZr between the Ni and the zirconia (Fig. 3). The Ni5Zr/ZrO2 interface boundary had a planar morphology in contrast to the unstable morphology of the Ni5Zr/Ni interface boundary. As can be seen in Fig. 4(A), the grains of the Ni5Zr layer had a size of ~200 nm and the morphology of the NisZr/Zr02 interface was crystallographically rough on the nanometer scale. The dopant yttria was not reduced, as expected, and instead we observed small particles randomly dispersed in the Ni5Zr matrix [Fig. 4(B)]. These particles showed no preferred orientation relationship to the Ni5Zr, ZrO2 or Ni. As shown by semi-quantitative EDS analysis, the yttria particles contained some Zr (less than 20 at.% Zr). After a reaction time of 130 min the average thickness of the reaction layer was ~ 10/~m. A ~500 nm thick layer of the Ni5Zr layer in contact with the Ni, however, contained no yttria particles. No pores were observed in the reaction layer or at the Ni5Zr/ZrO2 interface. As can be seen in Fig. 3, some pores were present at the Ni5 Zr/Ni interface. The formation of pores at the nickel-rich side has also been reported for binary Ni/Zr diffusion couples, indicating that the necessary diffusion occurs by the vacancy mechanism [11].

Fig. 1. Typical TEM micrograph of the Ni[ZrO2lNi sample after bonding containing one of the Ni/ZrO 2 interfaces. The interface boundary is fiat and no interracial reaction layer is present.

Fig. 3. SEM micrograph of the intermetallic compound NisZr formed at the Ni/ZrO 2interface during experiment A. (a) NisZr layer containing yttria particles, (b) Ni5Zr layer without yttria particles.

3.2. Experiment A

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At the other side of the cell (Fig. 2, interface II) the oxygen activity was high enough to form a layer of NiO between the Ni and the ZrO2 (Fig. 5). The NiO layer contained pores in the NiO grains and at the grain boundaries. The NiO layer was composed of two sublayers of different thicknesses (Fig. 6); this was attributed to the formation of duplex NiO scales which has also been observed by other investigators [12, 13]. Furthermore, pores were present at the NiO/ZrO2 interface. The thick NiO layer in contact with the zirconia electrolyte showed a columnar grain morphology. About 50% of the grains had approximately the following well defined orientation relationship with zirconia (Fig. 7)

[1I0]NiO II[001]ZrO2

q,

Fig. 5. SEM view of the NiO layer which formed between the Ni and ZrO2. Many pores are enclosed in the NiO scale.

(111)NiO II(100)ZrO2 as also observed after eutectic solidification in the NiO/ZrO2 system [14, 15]. The grains of the thinner NiO layer in contact with Ni were randomly oriented with Ni. Furthermore, using electron diffraction in the TEM, some small Ni particles were detected between the NiO layer and ZrO2 (Fig. 8). These Ni particles probably formed during cooling the cell. The formation of the Ni particles was accompanied by a volume decrease, which contributed to the pore formation at the NiO/ZrO2 interface. 3.3. Experiment B

As shown earlier in experiment A, by applying a constant current to the cell the intermetallic compound NisZr and NiO formed at interface I and II, respectively. In experiment B, we applied the same

Fig. 4. (A) TEM micrograph of a typical region containing the ZrO2/NisZr interface. The interface boundary is crystalIographically rough on the nanometer scale. (B) TEM micrograph of the intermetallic phase NisZr. The yttria particles are randomly distributed in the Ni5Zr matrix.

Fig. 6. TEM micrograph of the NiO layer. The layer can be divided into two sublayers with different thicknesses. The thicker sublayer shows a columnar grain morphology.

WAGNER et al.: ELECTROCHEMICALLY-INDUCED REACTIONS

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Fig. 7. Diffraction pattern of a columnar NiO grain and the ZrO 2 which satisfy approximately the following orientation relationship between these two materials: [1T0]NiO II[001]ZrO2; (111)NiO I[(100)ZrO2.

,

B

0 0

constant current to form the reaction products as in experiment A. However, in this experiment, after a certain time t = tl, the current was switched off, and the open circuit voltage (o.c.v.) of the cell was measured several times as a function of time (Fig. 9). After the current was switched off, the cell voltage dropped to a reproducible plateau value E l and remained constant during time At [Fig. 9(part A)] and then it decreased to approximately zero [Fig. 9(part B)]. For an ideal ionic conductor and a cell not interacting with the environment, the measured plateau value of the open circuit voltage should never decrease to zero. Hence, the decay of the open circuit voltage with time can be attributed to the transport of oxygen ions through the electrolyte, increasing the oxygen activity at interface I and decreasing it at interface II. This will result in the oxidation of the intermetallic compound NisZr at interface I and a reduction of the NiO layer at interface II (Fig. 10). Such a transport of oxygen ions through the electrolyte during open circuit conditions is possible, if the electrolyte is internally short-circuited by an electronic current. Due to the defect structure of the zirconia electrolyte, the electronic conductivity ~e contributes at very low oxygen partial pressures to

,

m

tl

10

20

(rain)

Fig. 9. Measured open circuit voltage of the cell as a function of time. the total conduction and is proportional to p~/4 [1]. Therefore, a very low oxygen partial pressure at interface I is expected, in order to short-circuit the electrolyte internally. This is discussed in more detail in Section 4.2. As shown by EDS and electron diffraction in the TEM, the oxidized intermetallic compound NisZr (Fig. 11) consisted of a mixture of interconnected columnar Ni and monoclinic ZrO 2 (m-ZrO2) grains. The formation of such an aggregate product morphology has also been observed in other systems [16]. On the electrolyte surface, a continuous layer of monoclinic ZrO2 formed (Fig. 12). This layer was ~ 1 0 0 n m thick and contained a high density of twins. The yttria particles which formed during the electrolysis of the zirconia electrolyte were distributed in both the Ni and ZrO 2 grains. However, a small part of the NisZr layer, in contact with Ni, has not been oxidized (Fig. 13). This was probably due to the loss of oxygen atoms corresponding to edge effects, e.g. NiO dissolution in the Ni during formation of

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Fig. 8. TEM micrograph of the region around the NiO/ZrO2 interface revealing the presence of some Ni particles directly at the interface. AMM ~/S--G

Fig. 10. Schematic of the galvanic cell for experiment B. During the measurement of the open circuit voltage the cell was internally short circuited (ie +/i -- 0,/i -- ionic current, ic = electronic current).

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the NiO layer and incomplete reduction of the NiO layer. The interface boundary of the oxidized Nis Zr showed almost the same unstable morphology as the NisZr/Ni interface boundary. As can be seen in Fig. 13, the oxidized NisZr layer can be divided into two layers of different grain sizes. The layer in contact with the unoxidized part of the NisZr layer contained the larger grains. This difference in grain size probably resulted from a change of the oxidation rates during the oxidation of the NisZr layer. It is likely that such a change appears when the complete NiO layer is more or less reduced and the transport of the oxygen ions through the electrolyte is then controlled by oxygen diffusion in Ni. The region containing the reduced NiO layer had typically three different features (Fig. 14). In some regions many large pores had formed whereas other regions contained no pores. The formation of these pores was due to the large volume decrease (41%) accompanying the reduction of the NiO layer. Furthermore the reduction of the NiO layer was not complete and parts of the initially formed NiO layer were enclosed in the Ni. 4. ANALYSIS AND DISCUSSION 4.1. Experiment A

The Ni-Zr binary system consists of eight intermediate phases [17]. The formation of the most Ni-rich intermetallic NisZr at the cathode indicates that the rate of reduction of the electrolyte controlled the rate of intermetallic growth; in other words, the reaction at interface I was interface-controlled instead of diffusion-controlled. As is known from binary Ni-Zr diffusion couples [11], the formation of the NisZr compound is also expected because of the

Fig. 12. TEM micrograph revealing the presence of a continuous layer of monoclinic ZrO2 between the interconnected Ni and monoclinic ZrO2 (m-ZrO2) grains and the electrolyte. much faster diffusion of Ni in Zr rather than vice versa. The oxygen activity was not low enough, however, to reduce the yttria. This result is consistent with both the higher stability of Y203 compared to ZrO2 [18] and the lower stability of Ni-Y phases compared to Ni-Zr phases [19]. Most of the ~ 500 nm thick NisZr layer in contact with the Ni which contained no yttria particles formed by the diffusion of Zr into Ni. As known from an experiment where the current was imposed for a much smaller time, however, no yttria particles were present in the NisZr layer as long as the thickness of the layer was smaller than ~ 100 nm. This shows that a small part of the Ni5Zr layer without yttria particles formed by diffusion of Ni into Zr. Thus, for sufficiently long reaction times (t > 2 h), the yttria particles can be used as markers for the initial Ni/ZrO2 interface, confirming that Ni diffuses faster than Zr. Neglecting the yttria, the interfacial reactions (Fig. 2) can be written as 5 Ni + ZrO2 + 4e- --. 2 02- + NisZr interface I, cathode

Fig. 11. TEM micrograph showing the aggregate product morphology of the oxidized NisZr layer.

(3)

Fig. 13. SEM view of the oxidized NisZr layer. (a) ZrO2, (b) oxidized NisZr layer, (c) rest of the NisZr layer which was not oxidized, (d) Ni.

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Fig. 14. SEM micrographs of the reduced NiO layer. (A) Porous region, (B) region without pores, (C) NiO embedded in Ni. 2Ni+202-

~ 2NiO+4einterface II, anode

(4)

4.2. Experiment B

and the total cell reaction is 7 Ni + ZrOz --* 2 NiO + NisZr.

of the oxygen partial pressure at the NisZr side of the cell [20].

(5)

After applying the current for 130 min the average thicknesses of the reaction products were measured as dr,liszt = 10/~m and dNio=4/~m. Using these thicknesses and the corresponding densities of the compounds an equivalent electric charge of 6.6 C for NisZr and 6.9C for NiO can be calculated. The values agree within experimental error of the thickness measurements. Therefore, we conclude that the interaction between any residual oxygen in the vacuum furnace and electrolyte can be neglected. Thus the ionic current transported through the electrolyte was carded almost exclusively by the oxygen ions released in the reduction of the electrolyte. The total electric charge passed through the cell was 234 C (i = 40 m_A/cm2, t = 130 min). Compared with the charge calculated above, only the fraction 0.03 was consumed for the formation of NiO or NisZr, respectively. If this reduced current yield is due to the electronic conductivity, the overall transference number of the oxygen ions would be ti = 0.03, which requires a very low but not unreasonable value

In Section 3.3 it was claimed that the short-circuited by an internal electronic Thus, the initially formed NisZr layer was and the NiO layer at the other side of the reduced (Fig. 10)

cell was current. oxidized cell was

02 + Ni5 Zr --* 5 Ni + ZrO 2 interface I 2 NiO ~ 2 Ni + 02

interface II

(6) (7)

The overall cell reaction [reverse to equation (5)] is 2 NiO + NisZr ~ 7 Ni + ZrO 2.

(8)

In the following section this point will be discussed in more detail: as long as the open circuit voltage of the cell exhibits a stable plateau value [Fig. 9(part A)], the ratio of the oxygen activities of interface I and II was constant. Therefore, it is reasonable to assume, that the oxygen activities at the interfaces I and II were fixed and the equilibrium oxygen partial pressures at both interfaces were determined by the interfacial reactions. Using the standard Gibbs free energy of formation of the compounds NisZr and ZrO2 [18, 19],

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the equilibrium oxygen partial pressure corresponding to reaction (6) can be calculated as 2 x 10-27 atm. At this pressure the electronic conductivity of the electrolyte is 2 x 10-3 (flcm) -1 [20]. Using literature data for the ionic conductivity [20, 21], the electronic transference number, te, of the electrolyte is calculated as 0.02. Therefore the electrolyte cannot be treated any longer as a pure ionic conductor. During the measurement of the open circuit voltage, the cell was then internally short-circuited by a significant electronic current flow. Due to the requirement of electroneutrality, the total cell current is zero and the electronic current has to be balanced by a counter current of oxygen ions through the cell. The latter current resulted in the oxidation of the intermetallic compound NisZr at interface I and a reduction of the NiO layer at interface II (Fig. 10). During the time At, the NisZr compound was not completely oxidized and the NiO layer was not completely reduced and the overall chemical reaction was controlled by the rate of transport of electrons through the electrolyte. After complete consumption of one of the reaction layers, the oxygen activity difference across the cell decreased, accompanied by the decrease of the open circuit voltage with time [Fig. 9(part B)]. The average thickness dsi_z~ of the oxidized NisZr layer was measured as 9/zm. This thickness can also be calculated with the assumptions given above by first calculating the electronic current density i, ( = ionic current density) using equation (9) [22] ie = t e a i E ° / A x .

(9)

In equation (9), E ° is the open circuit voltage of the cell according to the Nernst equation, Ax the electrolyte thickness, and t7a the ionic conductivity. Inserting E°w, El = 0.97 V [Fig. 9(part A)] (i.e. the electronic conductivity is small compared to the ionic conductivity), Ax = 0.2 cm, ai = 0.1 (f/cm) -1 [20, 21], and te = 0.02 (calculated in Section 3.3) into equation (9) yields i, = 9.7 mA/cm 2. Once the value of i, is known, one can use the stoichiometric relation given by equation (10) to calculate the layer thickness dNi_ZrO2= ieMzro2At(1 4- 5pzr%MNi/PNiMzr02)

(10)

4Fpz~o2

The symbols M z ~ and MNi in equation (10) denote the molecular weights of ZrO2 and Ni, respectively, P z ~ and p)~i are the corresponding densities, and At is 8.5 min [Fig. 9(part A)]. Inserting these values into equation (11) yields a value of dNi_ZrO2=7/zm for the oxidized NisZr layer thickness. This result is in good agreement with the measured thickness of 9 #m, revealing that most of the oxygen ions set free in the reduction of the NiO layer were transported through the electrolyte and consumed in the oxidation of the NisZr layer. In addition, the good agreement between measured and calculated thickness indicates, that the assumptions which were made for the calculation of the electronic transference number were reasonable

and justified, as long as the open circuit voltage showed the stable plateau value. Furthermore, the open circuit voltage of the cell can be calculated assuming that the electrolyte is an ideal ionic conductor. The standard Gibbs free energy change AG ° of the overall cell reaction (8) is then given by equation (11) o

o

o

AG ° - AGz~o2- 2AGNio -- AGNbzr - -

(11)

where A G ~ , AG~io and AG~isZ~ are the standard Gibbs free energies of formation of the compounds ZrO 2, NiO and NisZr [18, 19]. Using equation (12) [1] E ° = -AG°/4F

(12)

the open circuit voltage E ° of the cell can be calculated as 0.93 V. This value is in good agreement with the measured open circuit voltage E~ =0.97 V [Fig. 9(part A)]. Therefore, we are convinced that our original assumption is correct, namely, that the equilibrium oxygen partial pressures at both interfaces were fixed by the corresponding inteffacial reactions (6) and (7) during open circuit conditions. 5. SUMMARY A symmetrical galvanic cell • NilZrO2 + 9.5 tool% Y203[Ni has been used to study and control interracial reactions at metal/ceramic interfaces. Whilst a constant current was applied across the cell at T = 1200 K, chemical reactions were induced at both ZrO2/Ni interfaces. At the cathode the intermetallic compound Ni 5Zr formed. The dopant yttria clustered to small particles which were randomly distributed in the NisZr matrix. The formation of the most Ni-rich intermetallic phase NisZr revealed that the chemical reaction at the cathode was interface rather than diffusion controlled. However, by applying the electronic current for longer times, the increasing thickness of the NisZr compound should slow down the Ni diffusion and finally the formation of Ni-Zr compounds with higher Zr content should occur. Further work is in progress to study the formation of other intermetaUic compounds. At the anode, the nickel electrode was oxidized, forming a NiO layer in contact with the electrolyte. In order to avoid the decomposition of the reaction products during cooling by an internal short-circuit in the electrolyte, an electronic current had to be applied through the cell. A calculation of the equivalent charge for the formation of the NiO was in good agreement with the calculation of the equivalent charge for the formation of the NisZr layer. This result reveals that almost all of the oxygen ions which contributed to the oxidation of the Ni originated from the reduced ZrO 2. After removing the applied electrical current, the cell was internally short circuited by an electronic current. Thus, the NisZr which formed initially was

WAGNER et al.: ELECTROCHEMICALLY-INDUCED REACTIONS oxidized and a layer of monoclinic ZrO 2 and Ni grains formed, including the yttria particles. Furthermore, a continuous layer of monoelinic ZrO2 formed on the electrolyte surface. The oxidized NisZr was an aggregate of columnar Ni and monoclinic ZrO2 grains. At the other interface the NiO layer was reduced. The rate of oxidation or reduction was determined by the transport of electrons through the electrolyte. The good agreement of calculated and measured thicknesses of the oxidized NisZr layer confirmed that in a first estimate, the interaction between residual oxygen pressure in the furnace and the galvanic cell can be neglected. Acknowledgements--This work was supported by the Bundesministerium fiir Forschung und Teehnologie (BMFT) through contract NT SO 230/0. The authors are indebted to K. P. Trumble for helpful discussions and suggestions during the experiments and preparation of the manuscript. REFERENCES

I. H. Rickert, Electrochemistry of Solids. Springer, Berlin (1982). 2. E. C. Subbarao, Solid Electrolytes and Their Applications. Plenum Press, New York (1980). 3. K. P. Trumble and M. Riihle, Acta metall. 39, 1915 (1991). 4. W. Weppner, J. Electroanal. Chem. 84, 339 (1977).

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