In situ study of the sintering process of yttria stabilized zirconia by impedance spectroscopy

Solid State lonics 9 & 10 (1983) 989-996 North-Holland PublishingCompany

989

IN SITU STUDY OF THE SINTERING PROCESS OF YTTRIA STABILIZED ZIRCONIA BY IMPEDANCE SPECTROSCOPY E.J.L. SCHOULER, N. MESBAHI, G. VITTER

Laboratoire d'Energ~tique Electrochimique ENSEEG - BP 75 38402 Saint Martin d'H~res cedex (FRANCE) In this paper the authors analyse the merits of the impedance spectroscopy technique for studying the sintering process of yttria stabilized zirconia. The great advantage of this technique with respect to classical ones is the possibility of separating the behavior of the grains from the grain boundaries during thermal cycles. Because of large conductivities at high temperatures, measurements must be performed in the range 300 - 600°C after a rapid cooling of the sample down from the firing temperature. In this temperature range both intragrain and grain boundary contributions are observable. The results give valuable informations on : - the solid state reaction of the dopant dissolution into the ZrO 2 matrix at the initial stage of the process - the grain boundary formation and evolution for different firing conditions.

i.

INTRODUCTION

The performances of electrochemical cells involving solid oxide electrolytes aredirectly related to the quality of the electrolyte. (i.e. conductivity structure and purity). This has been clearly demonstrated in a recent study on the behavior of oxygen sensors made of yttria stabilized zirconia electrolytes (I)0 The main factors which determine the electrolyte conductivity are the nature and concentration of the dopant oxide. Other factors such as the porosity, the grain size and the purity also strongly influence the ionic conductivity. These parameters a r e directly related to the preparation procedure of the electrolyte and more precisely to the sintering process itself. In a recent paper, Brook reviews the merits of different firing procedures in terms of the electrical performances and properties of zirconia ceramics (2). The sintering process can be simply described as the elimination of the porosity of compacted powders with associated grain growth and formation of well defined grain boundaries. The impedance spectroscopy technique has been demonstrated to be a powerfull tool for measuring accurately the conductivity of solid electrolytes. Its major advantage with respect to d.c. conductivity measurements is to separate out from the total conductivity the contribution of the grains and grain boundaries, without any perturbation due to electrode polarization effects (3-4). A typical impedance spectrum for yttria stabilized zirconia is shown on fig. I. Three relaxation phenomena a r e obtained in the frequency range 500 kHz - IO-2Hz. They are identified as : - intragranular relaxation at high frequencies - grain boundary effects at intermediate

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ZrO~ -Y.O~ "(0.91) " ~(OD9) T=490"C P 0 2 = 1 ( ) 2 a t m.

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4 (&).2 5 10 o ~ O ~f -e- - -

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*

"~. •" *l.=troa. o,. , polarization • / poll~lzllion • --~

~."a,,,

,

2

=.

~,~.

,

3

Z ~/krL

Fig. I : example of impedance diagram obtained with a s i n t e r e d sample a t low temperature (ref 5) (frequencies i n Hz) frequencies - electrode polarization at low frequencies Such a complete diagram is only observable in a rather narrow temperature range. At high temperature - the relaxation frequency of the intragranular effect becomes too high to be measured with conventional instrumentation the grain boundary effect vanishes in the range 600 to 9OO°C depending on the purity of the samples (5). At low temperatures, the impedance of solid electrolyte cells becomes very large at low frequencies and only a small portion of the diagram is observable.

E.Z L. Schouler et al. / In situ study of the sintering process

990

des are pressed against the planar surface of the samples. The temperature is monitored by a Pt - Pt, Rh ]0 % thermocouple located next to the sample. All the measurements are performed under air except for separating out electrode effects from bulk polarizations when argon was used.

\ (~)o1

104 102

(1400 "C-7h)

/ ~/i

F

(1100"C-lh)

~1/1/

M+T+~F

(900"C-Oh)

1

',,.

100 ~62

1

1.5

Fig. 2 : variation of the relaxation frequencie~ with temp~at~re (ref 5 ) , ~ r ~ r a g r a i n semicircle, ~ grain boundary semicircle ~; a~d J4

8'o 'rb

low frequency e l e ~ o d e semicircles. Fig. 2 shows the relaxation frequencies ranges versus the temperature for bulk and interfacial phenomena (5). Since the impedance spectroscopy technique permits a separate characterization of bulk and grain boundary effects, it was used to studying the evolution of these parameters durin~ the course of the sintering processes and was compared with conventional geometrical and optical measurements. |0 mole % yttria - zirconia has been chosen because of its interest as solid electrolyte.

2.

EXPERIMENTAL

All the tested samples are prepared under the same conditions. They are machined out of a rod of 10. mole % Y 2 0 3 - ZrO2 , prepared by "humid . ,, cogrzndlng of powders , pressed at |t cm -2 and fired at 900°C. Their initial diameter and thickness were 1.510 cm and 0.605 cm ± O.Olcm respectively, with a 52 % initial density. X-rays analysis showed a two phase domain (monoclinic + tetragonal) and no fluorite phase (Fig. 3). The samples prepared this way are then fired separately between 950°C and |850°C. The experimental set up has been described in details in a previous paper (4). Table I summarizes the firin~ conditions. The impedance measurements are performed in a Pt resistor furnace. Two platinum foils electro-

8"o ~ o

M+T

4b '3"0 2 b '

2~i~ ~

Fig. 3 : X-rays p a t t e r ~ for d i f f e r e n t samples fired at low temperature (M = ~,Ionoc~nic, T = Tetragonal, F = Fluo~te) the different samples will be identified by there firing temperature (°C) and firing time. For example, a sample fired at 1400°C for 20 hours is noted <1400 - 20 h>. The impedance measurements are carried out with an Alcatel impedancemeter, in the frequency range 500 kHz - 5 Hz (4). Because of the high relaxation frequencies of the bulk phenomena at high temperatures and too high resistances at low temperatures, the measurements must be done in the range 300 600°C in which both intragrain and grain boundary effects are observable (Fig. 2). The samples were also studied using - density measurements - dilatometry ~ - X-rays analysis - scanning electron microscopy

~ZrO 2 Y203

provided by MERCK provided by Pechiney St Gobain

~NETZCH

dilatometer

991

E.J.L. Schouler et al. / In situ study of the sintering process

Temperature range

sintering equipment

°C 950 - 1400

1400 - 1700

treatment conditions

heating rate AT/At

- electric - Pt resistor

air

constant - 120°C/h

- electric - MoSi2resistor

air

constant - 120°C/h

25 - 1850

gas furnace

25 1200°C-400°C/h 1200 - 1500°C-200°C/h 1500 - 1800°C-300°C/h

heating cycle : reducing conditions

cooling cycle : 1850 - 13OO°C-400°C/h oxydizing condictions 1300 - 25 free

Table I : fi~ing c o n d ~ o ~

Y203 (0.9) (0.1) s~,~ples a f t e r presir~ering a t 900°C.

3.

of ZrO2

RESULTS T

r

T

T

Fig. 4 shows the variation of the ratio of the thickness (e) and the area (A) of the pellets as a function of the firing temperature. This curve is similar to direct dilatometry measurements of a single sample heated at a constant rate AT/At = ]20 ° h-l, up to 1500°C (fig. 5),

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Electrical characterization

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1300

Fig. 5 : v ~ o n of t h e shinkage of a sample up to 7500°C at constant r a t e when heated at a 120°C/h.

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1200

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1100

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I 1.4

I

I

1.6

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10 3 T'C

Fig. 4 : v a r i a t i o n of t h e r a t i o of t h i c k n e s s and area (A) of t h e t e s t samples v e r s ~ t h e sintering temperat~e.

(e)

a) Low temperature (T < IOOO°C) Impedance measurements (between 300 - 600°C) of sample fired from 900°C to I]OO°C exhibit very high values related to very high capacitive impedance. This insulatin~ behavior is consistant with X-rays analysis showing that the reaction between the ZrO 2 and Y203 powders has not yet started. According to X-rays measurements the onset of the fluorite conductivite phase appears at 1100°C (Fig. 3). b) Medium temperature (12OO - 14OO°C) Measurements were performed on a sample sucessively fired at 1280°C for 20 h, 1330°C for 5 h and 1400°C for 7 h. After each cycle, impedance measurements were done at 380°C. The evolution of the diagrams is shown on fig. 6.

992

E.J.L. Schouler et al. / In situ study of the sintering process

The first spectrum after heating the sample up to 1280°C for 20 h evidences a complex shape characteristic of an overlapping of two semi-circles with close relaxation frequencies (5). A good separation of the constituting elementary semicircles is obtainable when the relaxation frequencies differ from a factor about IOO. A tentative resolution of the curve (1280 - 20 h) would give a ratio of about 16. After the next cycle (1330 - 5 h) a major decrease of the impedance is observed with a clear separation between the intra grain and grain boundary semi-circles (,~OlI~o2 = 150). This effect is even more pronounced after firing at 1400°C for 7 hours. From these results one can conclude that the impedance technique is fully appropriate for monitoring the formation and evolution of intergranular (IG) and ~rain boundary (GB) effects. Both IG and GB resistances are found to decrease with higher thermal treatments. The relative variations of R and K = e/A are reported on table II.

~(1280-2Oh)

(AR-1)

(1330-5h)

+ 68 %

(14OO-5h)

AK K

+ 72 %

+ 2,7 %

+ 7,5 %

ratio), the higher the sintering temperature. The same result is obtained for long isothermal firing conditions at 14OO°C (Fig. 8). the relative variation of intragranular resistances (RIG) and grain boundary resistances (RGB) strongly depends upon the thermal treatment : the higher the firing temperature and the longer the times the smaller the RIG/RGB ratio. On fig. (9) and (10) are reported the Arrhenius plots for the IG and GB conductivities. From these diagrams one can see that : are very close

to each other for similar short sintering times (I-2h) and firing temperatures ranging from 14OO to 1850°C. On the contrary, larger variations are observable on the grain boundary conductivities (fig. ]O).

.AR-1" + 61%

c) High temperature region (T > 14OO°C) Several identically prepared samples were sintered at 14OO-15OO-17OO-1850°C. Fig. (7) and (8) show examples of diagrams plotted in the 3OO-350°C range. These diagrams evidence that : - the better separation between the intra and inter granular semi-circles (larger i~o]/U~o2

- the OiG for each sample

+ 24 %

R-I IG ~--~ZT)GB

In this temperature region, the evolution of the GB resistance is due to the modification of the geometrical area of contact between the grains in accordance with the elimination of the intergranular porosity.

~1

I (ms°-2~) 300°C

.5t Table II - Relative variation of the IG and GB resistances and of the geometrical factor k = e/A. i

A first conclusion is that the decrease of the IG resistance in this temperature range must be attributed tothe onset and increase of the ionic conductivity in connection with the stabilization of the fluorite phase, in other words to the complete dissolution of the dopant and confirmed by the X-rays patterns of a (1400 - 7 h) sample which only show the fluorite structure.

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Fig. 7 : impedance diagram~ for sample,~ f i r e d at d i f f ~ e n t t e m p e r a ~ e s (frequencies en kHz)

E.J.L. Schouler et al. / In situ study of the sintering process

(1051~)

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This confirms the previous conclusion on the larger influence of the sintering conditions on the grain boundaries rather than on the intragranular behaviors. One can also notice that better conductivities are obtained for long isothermal treatments (2Oh) at rather low firing temperatures (]4OO°C). The corresponding activitation energies are reported on table III. Values obtained for (2OOO-2h) samples in a previous studies (5) are given for comparison. Metallographic characterization shown in Fig. ii has been analyzed. The porosity, grain size and pore size are shown in Table IV for various sint er ing conditions.

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Fig. 10 : Arrhenius plots of the grain boundary conductivity for d i f f e r e n t sample~

E.g'.L. Schouler et al. / In situ study o f the sin tering process

994

T

(2000-2h)

(1850-2h)

(1700-Ih)

(1500-ih)

(1400-I~)

(1400-20h)

Ea(IG) ± 0,0 2 eV

1.13

1.08

1.12

1.06

1.05

1.13

Ea(GB) ± O,0 2 eV

1.23

1.12

1.13

1.17

1.12

1.14

Ta~e III:

a c t i v a t i o n energy v ~ u e s

f o r ~IG and ~GB

for diff~entsamples

a)

c)

(1400 - lh)

(1400 - 2Oh)

e)

(1850 - 2h)

Fig. 77 : SEM picture of ZrO 2(0. 90)-Y203(0.10) surfaces

Porosity

sample

%

grain size(~m) I

pore size(Dm) 1.5

1400-lh

a

20

1500-Ih

b

15

3 to 9

0.3 - i

1400-2Oh

c

11

3 to 4

0.7 - I

1700-1h

d

I0

1850-2h

e

9

7 to I0

0,5

Table If! : c o m p a ~ o n o f t h e pore s i z e , g r a i n s i z e and t o t a l p o r o s i t y f o r s e v e r a l YSZ s a m p l e

4.

DISCUSSION

In this preliminary study we confirm that the impedance spectroscopy technique is well adapted for following the evolution of the morphology of a conductive ceramic material during the sintering process. The evolution of the electrical properties of the grain and of the grain boundaries give valuable informations on the whole process from the appearance of the conductive phase to the final stage of densification. Several factors play an important part in sintering . They deal with the elimination of porosity, grain growth and the nature, concentration and location of impurities. All of them affect the conduction properties in the grains and the grain boundaries.

Three classical stages of the evolution of a ceramic surfng sintering can be deduced from Figs 4 and 5 : - in the low temperature region (900 - II00°C) a very weak shrinkage effect is observed corresponding to a sticking effect of the powder grains• - the intermediate temperature region (IIOo 1450°C) is characterized by a large shrinkage of the samples associated with the progressive elimination of the interconnected pores between the grains. According to theory the densification process is controlled by : • either a diffusion of the atoms • or by intergranular diffusion•

in volume

- the high temperature region (145o - 1850°C) is characterized by an asymptotically stabilization of the shrinkage. In this temperature range, the grains grow up to their final size and the open porosity is fully eliminated. The initial rapid increase of the conductivity, for firing temperature ranging from 12OO ° C to 1300°C, arises from the formation of the charge carriers (oxygen vacancies) during the dissolution reaction of the dopant oxide into the lattice according to : Y203 + 2 Y'Zr

+

V"o

+

3 0 xo

At this stage the distinction between the intra and inter grain effects is not yet experimentally observable because of the close relaxation frequencies. Their separation clearly appears for f i

995

E.J.L, Schouler et aL / In situ study o f the sintering process

ring temperatures of about 1300°C. For higher firing temperatures (15OO=C < T F < 1850°C) the intragrain characteristics do not alter markedly, whereas the grain boundary effects are more affected. This confirms their determinant role in the sinter ing process. Impurities affect both the IG and GB conductivities. Within the grain they can have either a favorable or detrimental effect (6). Their dissolution into the matrix can in fact lead to an additionnal stabilizing effect, with an increase of the oxygen vacancy concentration like Y203 or CaO dopants, or a consumption of the oxygen vacancies when the cation M dissolves in interstitial position according to : MO 2

+

2V''o ~

M4"I +

2OXo

Such a reaction is very likely for silica (which is one of the major impurity into zirconia) because of the much lower ionic radii of Si 4+ (O,41 A) than that of Zr 4+ (0,80 A). This effect has been observed for YSZ electrolytes containing few per cents of SiOp (5) for which the IG conductivity was much smaller than for pure YSZ. The effect of the impurities is even more drastic on the GB conductivity. Several studies have shown that they precipitate at the grain boundary leading to a blocking effect of the oxygen ions. It is attributed to either a change of the ionic conductivity in the segregated phase or a change in the area of contact between grains. Pores initially exist within the grains and at the grain boundary. They contribute in changing the actual path of oxygen ions within the grains and decreasing the effective area of contact between grains. They therefore lead to an additional blocking effect which is proportional to their volume (iO). Their elimination during sintering is related to the heating rate AT/At. High heating rates favor their trapping within the grains. Pores or inclusions with occluded gases have been studied by Francois (14). A consequence of this effect is the stopping of the densification process before attaintment of high densities (15). In contrast low heating rates and long isothermal exposures favor their elimination. In related studies on the nature of the contact between polished samples, Fabry et al. (18) evidence a pure o|~ic contact resistance between grains only affected by the presence of a gap across which the oxygen ions can move by simpie direct exchange of ions. Similar conclusions are reported by Dragoo et el. in single grain boundary studies (19). The influence of the grain size has also been mentioned for accounting for the variation of the total conductivity. Previous results evidenced a large effect on the grain boundary resistance whereas the intragrain resistance is less affected by the grain size (iO) (17).

Bernard (IO) found that the blocking effect at the grain boundary is proportional to the area of contact at the grain boundary and therefore to the reciprocal of the grain size. During the course of the sintering electrical measurements neeesseraly mation about the variations of all tioned effects and are therefore a to the study of the phenomenon.

process the supply inforthe above menglobal approach

Nevertheless the separation between intra and inter granular conductivities enables us to evaluate the relative importance of such or such parameter on their evolution. For example, the intragrain conductivity is little affected by the firing conditions. Only long isothermal treatments give rise to an increase of OTG. This leads to assume a complete elimination ~ of intragranular pores and/or impurities. On the other hand the grain boundary conductivity evidences a continuous improvment of the contact between the grains due to the elimination of the intergranular porosity or the grain growth. This was shown by Table IV. To summarize, this study using impedance spectroscopy technique offer a way of characterizing the evolution of a conductive ceramic during sintering Further investigations are in progress for which this technique should be usefull. These are : i) the role of aging phenomena at low temperatures and the influence of different thermal cycles on the segregation mechanism of impurities at grain boundary. ii) the influence of sintering aids. An interesting development of this technique would also be to study the sintering of less conductive ceramics as impedance measurements could be performed directly at the sintering temperature.

REFERENCES (]) ANTHONY, A.M., BAUMARD, J.F., and CORISH, J. Collaborative Study on ZrO2-based oxygen Gauges - Second International Conference on the Science and Technology of Zirconia - Stuttgart FRG, June 21-23 (1983) To be published in "Advances in Ceramics" (2) BROOK, R.J. in "Advances in Ceramics" - Vol. 3 - Edited by HEUER, A.H. and HOBBS, L.W. - The American Ceramic Society (1981) p 272 - 285 (3) BAUERLE, J.E. J. Phys. Chem. Solids, 30 (1969) 2657 (4) SCHOULER, E.J,L,, KLEITZ, M., DEPORTES, C. J. Chim. Phys, 70, (1973) 923

996

E.ZL. Schouler et al. / In situ study o f the sintering process

(5) SCHOULER, E.J.L., Thesis Crenoble

(1979)

(6) INOZEMTZEV~ M.V., PERFIL'EV, M.V. Elektrokhimiya, ]1 (]975) iO31 (7) SCHOULER, E.J.L~, GIROUD, M., KLEiTZ, M., J. Chim. Phys. 70 (1973) 1309

(12) NOWICK, A.S., DA YU WAN£, PARK, D.S., GRIFFITH, J., in "Fast Ion Transport in Solids" Edited by Vashista, P.,Mundy, J.N. and Shenoy, G.K. North Holland, Amsterdam (]979) D.673

(13) SCHOULER, E.J.L., HAMMOU, A°, KLEITZ, M. Mater. Res. Bull.

(8) BEEKMANS, N.M., HEYNE, L. Electrochim. Acta 21 (1976) 303 (9) CHIANG, C.K., BETHIN, J.R°, DRAGOO, A.L., FRANKLIN, A.D., YOUNG, F.K. J. Electrocem. Soc. 129 (1982) 2113

l] (1976)

I137

(14) FRANCOIS, B., Thesis Paris (1971) (]5) COBLE, R.L., BURKE, J.E., Prog. Ceram. Sci. 3 (1963)

197

(16) DRAGOO, A.L., CHIANG, C.K., FRANKLIN, A.D. (10) BERNARD, H., Thesis Grenoble

(]980)

(11) KLEITZ, M., BERNARD, H., FERNANDEZ, E., SCHOULER, E.J.L., in "Advances in Ceramics" - Vol. 3. Edited by A.H. HEUER, L.N. HOBBS Ed. - The American Ceramic Soc. (198]) - 310

Solid State lonics 7 (1982) 249

(17) IOFFE, A.I., INOZEMTSVEV, M.V., LILIPIN, A.S PERFIL'EV, M.V., KARPACHOV, S.V. Phys. Stat. Sol. 30 (1975) 87

(18) FABRY,P., SCHOULER, E.J.L., KLEITZ, M. Electrochim.

Acta 23 (1978) 539