Solid State Ionics 40/4 1 ( 1990) 3 12-3 15 North-Holland

IMPURITY SEGREGATION STUDY AT THE SURFACE OF YTIRIA-ZIRCONIA ELECTROLYTES BY XPS

A.E. HUGHES and S.P.S. BADWAL CSIRO, Division ofMaterials Science and Technology, Normanby Road, Clayton 3168, Victoria, Australia

The segregation of yttrium as a function of the heat treatment has been studied on the external surface of polycrystalline yttria- zirconia electrolytes with X-ray photoelectron spectroscopy (XPS). Yttrium segregates at the external surface along with a num- ber of impurities dominated by silicon. Generally, silicon segregates in a small region near the surface, whereas the enrichment region for yttrium is considerably deeper. Grain boundary migration appears to be the predominant mechanism for impurity segregation at the external surface.

1. Introduction

Yttria-zirconia (fully or partially stabilized) elec- trolytes are commonly used in fuel cells, oxygen sen- sors and oxygen separators. These materials are sin- tered at temperatures in excess of 1300°C while the cell operating temperatures are much lower (400- 1OOO’C). The surface segregation of yttrium at the grain boundaries can influence the grain boundary resistivity whereas such segregation at the external surface of the solid electrolyte can modify the elec- trode kinetic behaviour. Steele and Butler [ 1 ] have reported a Y/Zr ratio as high as 1: 2 at the external ( 100 ) surface of a yttria-zirconia single crystal with a bulk composition of 0.86(Zr02)0.14(Y01.5). However, no details of experimental conditions were given. Burggraaf et al. [ 21 reported that yttrium en- richment occurs at the grain boundary and external surfaces of yttria-zirconia and that it is temperature and time dependent. Hughes and Sexton [3] also observed enhanced yttrium concentration both at grain boundary and external surfaces as a function of the sintering temperature in fully stabilized ytt- ria-zirconia. The segregation of main components at the surface in oxide ceramics is not uncommon and has been discussed by Nowotny [4]. There is little, if any, information available on the mechanism by which segregation occurs, although the most likely process for yttrium segregation, at least at grain boundaries, appears to be by bulk diffusion and or

0167-2738/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland )

chemical reaction with impurities. In order to un- derstand the reasons for yttrium segregation at the external surface of yttria-zirconia electrolytes, XPS studies have been made on cubic and tetragonal po- lycrystalline specimens as a function of the anneal- ing temperature.

2. Experimental

Yttria-tetragonal zirconia polycrystalline (Y- TZP) specimens were prepared from powders ob- tained from two different sources. The high purity powder supplied by TOSOH Corporation, Japan (TS3Y) contained 5.15 wt% Y203, 20 ppm SiOz, 190 ppm Na,O, and 50 ppm A1203. The other powder, obtained from Daiichi Kigenso Kagaku Co. Ltd., Ja- pan (HS3Y), contained the following impurities: SiOz = 870 ppm, NazO= 300 ppm, Fe203= 30 ppm and TiO*= 920 ppm. The Y203 concentration was 5.44 wtW. Discs were made from both powders and sintered at 1500°C for four hours. Fully stabilized 10 mol% Y203-Zr02 specimens (YSZlOV) were made by sintering at 1700°C ( 15 h, air) discs pre- pared from a high purity powder ( SiOl= 20-60 ppm) supplied by Viking Chemicals Ltd., Denmark. The YSZ 1 OV specimens were polished and given heat treatment in dry air at 1100°C for 60 h or 1500°C

A.E. Hughes, S.P.S. Badwal / Yttrium segregation study 313

for 50 h. Sintered discs of TS3Y and HS3Y were ground, polished and given heat treatment in dry air at800”C (160h), 1000°C (160h), 1200°C (lOOh) or 1500 o C ( 50 h ) . To avoid contamination from ex- ternal sources during annealing most of the discs were enclosed in a high purity platinum foil (cleaned in HF).

As-fired, polished and all heat-treated surfaces were examined with XPS (two specimens from each treatment) in a Vacuum Generators’ ESCALAB util- ising an Al Ka, X-ray source ( 1486.6 eV, 150 W) op- erating at a pressure of 7 x 1 O-lo Torr. A number of specimens were depth profiled using argon-ion sput- tering. Binding energies were referred to the Zr 3d line at 182.2 eV [ 31. The data reduction methods described previously [ 31 were used here.

3. Results and discussion

The data presented in fig. 1 for TS3Y and HS3Y provides strong evidence for impurity phase migra- tion to the external surface being one of the domi- nant mechanism for Y segregation in polycrystalline yttria-zirconia based systems. In this figure the Y/ Zr an1 Si/Zr atom number ratios have been plotted vers:s the annealing temperature. In the as-polished surfa~: ;he Y/Zr ratio correlates well with the bulk coml-.l+l;ion for both ceramics and no other impur- ities were detected. In the annealed specimens the relative ratios of various impurity components at the

T arms Oc

Fig. 1. Plots of Y/Zr and Si/Zr ratios as a function of the anneal- ing temperature (T,.) for TS3Y and HS3Y discs presintered at 1500°C.

external surface were consistent with the impurity levels in the starting powders up to 1000°C. In TS3Y the Y /Zr ratio reached a maximum at 1000 ‘C and then remained constant whereas Si/Zr ratio peaked between 800 and 1000°C and then decreased at higher temperatures. In HS3Y the Y/Zr ratio peaked at 1200°C but the Si/Zr ratio continued increasing with the annealing temperature.

The extent to which various impurities segregate to the external surface during annealing of Y-TZP materials can be seen by depth profiling through the surface layer of the discs. The depth profiles for TS3Y and HS3Y discs annealed at 1200°C are presented in figs. 2 and 3 respectively. All the impurity com- ponents present in the starting powders appear to migrate to the external surface. In fig. 2 depth pro- files for Na, Fe, Si and Y as a function of the argon-

0.16 0

L 0.14

s 2 0.10

n

5 0.06 V-J i -0.02

-Y/Zr A -Si/Zr 0 -Na/Zr 0 -Fe/Zr

00 TS3Y 1200'(

0 0 0

A 0

0

A

0

o A

00

B f A

-0.02 -5 20 45 70 95 120

Sputtering Time, set

Fig. 2. Depth profile for TS3Y annealed at 1200°C.

o 3. 0-Y/Zr A-Si/Zr 0 -Na/Zr q -Ti/Zr

A HS3Y 1200’

0.25 - L

So.20 “4

F do.15- O

Z A0 o o ^

i7i 0.10 - 0

A 0

'C 1

A A

0 54 -10 40 90 140 190 240 290 340 390 440

Sputtering Time, set

Fig. 3. Depth profile for HS3Y annealed at 1200°C.

314 A.E. Hughes, S.P.S. Badwal / Yttrium segregation study

ion sputtering time (60 s = 50 A) show that Na and Fe are removed within the first 30 s of sputtering. The Si is mostly removed after 90 s ( x 75 A) whereas the enrichment factor for Y was still 1.5 even after 120 s of sputtering. Similar features were observed for TS3Y discs annealed at other temperatures. The distinguishing feature between the TS3Y and HS3Y profiles were: (i) the presence of Ti in the near sur- face region in place of Fe; (ii) the depth to which various impurities were segregated (fig. 3 ) ; and (iii ) the persistence of the Si segregation to similar depths as the Y enrichment layers.

In these ceramics the formation of the impurity phase(s) is likely to take place during sintering. It appears that on subsequent heat treatment it is this impurity phase, rich in Y, which migrates to the ex- ternal surface. In addition to minimization of free energy as the driving force for impurity phase seg- regation another possible mechanism may be the squeezing out of the grain boundary phase due to thermal expansion mismatch between the yttria-zir- conia matrix and the grain boundary phase.

and external surfaces of YSZlOV discs sintered at temperatures between 1300 and 1700” C. They have shown that the predominant mechanism for Y seg- regation at the external surface is by transport of an impurity phase rich in Y. The impurity phase ap- peared to form during sintering at lower tempera- tures. With increasing sintering temperature the im- purity phase first wetted the grain boundaries. This was evident from the observed maxima in the Y/Zr and Si/Zr ratios which correlated well with the melt- ing point of sodium-yttrium silicates prepared by Hughes and Sexton according to the stoichiometry measured by XPS. At higher temperatures the grain boundary phase migrated to the external surface. The composition of the segregated material at the exter- nal surface was similar to that of the grain boundary phase. In the present study, in complete agreement with Hughes and Sexton, Na, Y, and Si were ob- served to segregate to the external surface of YSZ 1 OV discs to similar levels, confirming that impurity phase migration is the main mechanism for Y segregation at least in the first few atomic layers.

The above observations indicate that the grain boundary migration is the main mechanism for Y segregation to the external surface. In HS3Y the level of impurities was considerably higher in the starting powder and as a consequence the enrichment region is considerably deeper.

4. Conclusions

For both TS3Y and HS3Y specimens (annealed at 1000°C and above) the Y/Zr ratio decreased more rapidly during the first 60 s of sputtering (figs. 2 and 3) consistent with the segregation region for other impurities (Fe, Na, Ti etc.) but Y segregation per- sisted to much larger depths. It appears that initial Y segregation in the first 50 8, surface layer is as- sociated with the impurity phase but Y segregation in the subsurface region must occur by a different mechanism such as that by bulk diffusion. Other possibilities also exist for yttrium segregation in the subsurface region in Y-TZP materials. For example at the annealing temperatures used in this study sol- ute partitioning is known to occur in materials such as TS3Y and HS3Y leading to regions with high and low yttria content [ 51. This process is further en- hanced by the presence of glassy impurities in these materials [ 6 1.

The Y and impurity segregation behaviour is dif- ferent in all the materials studied. The Y-enrichment factor is a function of annealing temperature, im- purity levels and bulk composition of the ceramic. Besides lattice diffusion it appears that impurities play a major role in Y segregation to the external sur- face. In all the three polycrystalline ceramics studied the segregation behaviour was different as a function of the annealing temperature and in terms of com- position of the segregated material. As the level, type of impurities and consequently the nature of the grain boundary phase is different in each material, the above observations are consistent with impurity phase migration being one of the main contributing mechanisms for yttrium segregation.

References

Impurity phase segregation is not just confined to [ 1] B.C.H. Steele and E.P. Butler, Brit. Ceram. Sot. Proc. 36 Y-TZP. Hughes and Sexton [ 3 ] examined fractured (1985) 45.

A.E. Hughes, S.P.S. Badwal / Yttrium segregation study 315

[2] A.J. Burggraaf, M. Van Hemert, D. Scholten and A.J.A.

Winnubst, in: 10th Proc. Intemat. Symp. Reactivity of Solids,

eds L.C. Dufair and P. Barret (Elsevier, Amsterdam, 1985)

p. 791.

[3] A.E. Hughes and B.A. Sexton, J. Mater. Sci. 24 (1989) 1057.

[4] J. Nowotny, Solid State Ionics 28-30 (1988) 1235.

[ 51 S.P.S. Badwal, ET. Ciacchi and R.H.J. Hannink, Solid State

Ionics 40/41 ( 1990) 882, this volume.

[6] P.J. Whalen, F. Reidinger, S.T. Correale and J. Marti, J.

Mater. Sci. 22 (1987) 4465.