MICROSTRUCTURE AND TRANSPORT PROPERTIES OF Y2O3 DOPED OR UNDOPED POLYCRYSTALLINE ALUMINA IN RELATION WITH ITS ELABORATION

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MICROSTRUCTURE AND TRANSPORT PROPERTIES OF Y2O3 DOPED OR UNDOPED POLYCRYSTALLINE ALUMINA IN RELATION

WITH ITS ELABORATION

M. Loudjani, A. Huntz, G. Petot-Ervas

To cite this version:

M. Loudjani, A. Huntz, G. Petot-Ervas. MICROSTRUCTURE AND TRANSPORT PROP- ERTIES OF Y2O3 DOPED OR UNDOPED POLYCRYSTALLINE ALUMINA IN RELATION WITH ITS ELABORATION. Journal de Physique Colloques, 1986, 47 (C1), pp.C1-323-C1-328.

�10.1051/jphyscol:1986147�. �jpa-00225577�

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JOURNAL D E PHYSIQUE

Colloque C1, suppliiment au n 0 2 , Tome 47, fgvrier 1986 page cl-323

MICROSTRUCTURE AND TRANSPORT PROPERTIES OF Y,O, DOPED OR UNDOPED POLYCRYSTALLINE ALUMINA IN RELATION WITH ITS ELABORATION

M. LOUDJANI, A.M. HUNTZ and G. PETOT-ERVAS*

Laboratoire d e MBtallurgie Structurale, U A 1107, Universite Paris X I , F-91405 Orsay Cedex, France

* ~ a b o r a t o i r e P.M.T.M., Avenue J.B. Clkment, F-93430 Villetaneuse, France

_R4sum4: Peu d'4tudes ont eu trait d la caract6risation des propri4t4s de transport de I'alumine polycristalline. L'objet de nos travaux a consist6 b determiner les propri4t6s de conductivit6, de diffusion chlmiquo ot de diffusion de trapeurs cationiques dans I'alumine polycristalline "pure" et dopOc b I'oxyde d'yttrlum, en rotation avec la microstructure fine resultant de la quanlltb do dopant et du mode d'Olaboratlon, ot co,_grace I'utllisation conjointo do techniques trhs diverses t mesures de o i T ) , D ( T I , analyses X, XPS, EXAFS) .

Les resultats obtenus, compar4s B ceux caract4ristiques de I'alumine monocris- talline, permettent de mettre en 4vidence le role des joints de gralns et celui de sbgregations ou pr4cipltations de seconde phase, sur les propri6t6s de transport.

En particulier, les joints de gralns s'averent Otre caract6ris6s par des propri4tes de transport qui different de celles du volume, et, la temperature de dissolution de grenats d'Y s t Al varie selon que ces pr6cipit4s sont en p0SItiOn inter ou intragranulaires.

Abstract: Few studies have been carried out on the transport properties of polycfystalllno alumina. Thus, our aim consisted in determining the conductivity, chemical diffusion and cationic tracer diffusion values in polycrystalllne undoped or Y203 doped alumina. These properties were related to the alumina microstructure due to processing and to the dopant distributio_n. The results, obtained by comblning various experimental technics a and D measurements, X, XPS and EXAFS analyses.. . , are compared to those relative to monocrystalline aluminas and allow to point out the grain boundaries effects and the segregation or precipitation phenomena on the alumina transport properties. Particularly, the grain boundaries are characterized by transport properties differing from those of the matrix and the dissolution temperature of yttrium garnet precipitates is different according to their localization in the lattice or along grain boundaries.

I - INTRODUCTION

In most cases, alumina applications are relative to polycrystalline doped alumina and depend on their transport and mechanical properties. During these last years, a lot of data were collected on monocrystalline, doped or undoped alumina. But data relative to polycrystalline aluminas are not numerous

(

1, 2 ) . Moreover, when transport or mechanical properties of doped aluminas are studied, it is rare that their behavlour is related to the localization and the chemical state of the doping element.

The aim of this work consists in relating transport properties of Y203 doped polycrystalilne alumina to its microstructure,,

1.8.

to the yttrlum localization and chemical state, taklng Into account processing and the specimen heat treatment: a particular attention was pald on the grain boundary influence.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986147

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30URNAL DE PHYSIQUE

Ii - MATERIALS AND EXPERIMENTAL PROCEDURE

The a aiumina powder (A1251 was provided by "CRICERAM" and is characterized by a 5N purity and granulometry of - 50nm. The Y2O3, provided by "Rh6ne Pouienc" has a 4N purlty and a granuiometry of - ^1OOnm.

Two types of mixture were performed: a first one, made by dry mixing the two powders for 6 hours and the second, carried out by mixing alumina in a (Y(N03)6H20) 2 voi. % solution.

This last technique, described in ( 3 ) , offers the advantage, as seen later, to obtain homogeneous doped aiumina samples, but has the prejudice t o induce incorporation of impurities. As the Y2O3 doping ratio was chosen equal to 1 and 0. 1 mole 96, the impurity amount for the aiuminas prepared by solubilization, is not negiigibie. Indeed, for wet mixtures, the impurity amount is equal or higher to 1000 ppm that Is a value equal to the lowest (0. 1 mole %I doping element amount. These considerations explain why the yttrium content cannot be decreasedbelow 0. 1 mole %, value which is higher than the yttrium solubility in aiumina at temperatures lower than - 1600°C (4.9). The choice of 1 mole % Y203 doping was done in order to be sure that the doping element amount will be preponderant when compared to other impurities.

These mixed powders were sintered under load at high temperature by M. P. CARRY (Ecoie Polytechnique de Lausanne) (3) so that the alumina voiumic mass was about 3.989. cm-3 (relative density: - ^98%) .

The microstructurai aspects were studied on thin foils, prepared by ionic thinning (Laboratoire de Chimie Appiiqu68, UPS, Orsay) and observed by TEM or STEM (imphy S. A. 1 . Some more informations were obtained by XPS analyses and EXAFS technlque ( 5 ) . Various heat treatments were previously performed in order to modify the yttrium distribution. The transport properties were determined by conductivity and chemical diffusion measurements (6) .

Eiectrlcal conductivity measurements were performed with alternating current (1592 Hz), using a three-probe arrangement in order to eliminate surface and gaseous phase conduction. Electrical connections were made by platinum wires and folis and the three leads were connected to a Wayne-Kerr bridge. Equilibrium atmospheres were established with air, argon or CO-C02 mixture. Oxygen partial pressures were measured in the vicinity of the sample with a zirconia electrochemical gauge. The voltages were measured with an high impedance digital voltmeter. Measurements were carried out at thermodynamic equilibrium i n order to determine the conductivity values versus Po2 or T.

When the thermodynamical equilibrium conditions are changed, by modifying Pop value at constant temperature, a change in non-stoichiometry and/or in charge state of impurity atoms can be observed. A relaxation process for electrical conduction can result from this variation and the re-equilibrium kinetics depends on the mobility of the relevant defect species in particular and occurs by a so-called chemical diffusion process. The variation of the electrical conductivity as a function of time during-the equilibration of the gas-oxide system is related to the chemical diffusion coefficient D by the relation:

where h is the thickness of the sample, uo the conductivity at time zero, when the Pop is changed, and om the conductivity when the new equilibrium is reached. This relation.

available for monocrystaliine cylindrical samples, will be extended to polycrystais case ( i n a rough approximation) .

Up to now, microstructurai studies were carried out on all type of polycrystals samples described here, whereas the transport properties were only determined on the Y2O3 1 mole

% doped A1203 prepared by the dry way. Other measurements are carried out by now. The comparison with monocrystals samples is limited to a Y2O3 0. 1 mole % doped A1203 in account of the lack of results with higher amounts.

Ill - EXPERIMENTAL RESULTS 1

)

Microstructural results

For the 1 mole % Y2O3 doped aluminas, it clearly appears that mlxing the two powders

(Y2O3 and A12031 in solution increases the homogenelty of the samples (fig. l a ) While,

with a dry mixing, heterogeneities in the grain size and in the yttrium distribution Occur

(fig. l b ) : large areas of small grain size are particularly enriched in yttrium, whereas such

zones are not observed in the first case. Nevertheless, the powder nitrates wet way induces

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a nitrogen pollution whose level is not accurately known, up to now, and an hydrogen contamination is suspected as observed by (7).

Fla. 1 l%Y203 doped alumina microstructure: samples annealed at 1500°C: a- sample prepared by mixing in solution; b- samples prepared by dry mixing.

Notwithstanding mixture processing, yttrium enriched precipitates are observed in the 1 mole 96 Y2O3 doped aiumina when the annealing temperature Is equal to ISOO'C ( 3 ) . The precipitates are made of yttrium garnot Y3A15012 ( 3 , 5). if the doped aiumina is heat treated at T

2

1600*C, the garnet intergranular precipitates dissolve, whereas the transgranular ones persist (fig. 2). Ttiis obsorvation reveals that the graln boundary thermodynamic properties differ from the iattico ones.

F i g . 2 l%Y2O3 doped alumina ( s o l u t i o n m i - Fig.3 0.1%Y203 doped A1203 ( s o l u t i o n x i n g ) , h e a t t r e a t e d a t 1600°C; i n t e r - m i x i n g ) , h e a t t r e a t e d a t 1350°C g r a n u l a r p r e c i p i t a t e s a r e d i s s o l v i n g . and showing an heterogeneous

g r a i n s i z e .

When the alumina is doped only with 0. 1 mole % Y2O3, yttrium garnet precipitates are still observed. After an heat treatment at T

3

l60O0C, the intergranular precipitates dissolve while transgranular do not, similarly to the 1 moie % Y2O3 doped Al2O3. Thus, the same difference between the graln boundary and the lattice behaviour is noted. Nevertheless.

yttrium garnet precipitates are fewer in the 0. 1 moie % doped aiumina than in the 1 mole %

doped, Moreover, due to the low yttrium amount (0. 1 mole %), heterogeneities i n the

grain size are observed (Fig. 3) : the yttrium garnet precipitates induce a small grain size

by iocklng the grain boundaries ( F l g . 4 ) . At 1350°C, qualitative EDAX analyses performed

on graln boundaries without precipitates and especially on triple points show an yttrium

segregation associated to silicium enrichment. After an heat treatment at 165O8C, the

yttrium precipitate dissolution induces the formation of vitreous phase along grain

boundaries, especially on the trlpie points ( F l g . 5 ) .

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~ 1 - 3 2 6 JOURNAL DE PHYSIQUE

Fig.Q 0. 1%Y203 doped A1203 ( s o l u t i o n m i - Fig.5 A1203

:

0.1%Y203 ( s o l u t i o n mixing) x i n g ) , h e a t t r e a t e d a t 1350°C; h e a t t r e a t e d a t 1650°C: v i t r e o u s y t t r o g a r n e t p r e c i p i t a t e s lock t h e phase a l o n g t r i p l e p o i n t s . g r a i n boundaries.

Fig.6 l o g o

=

1 ( l / T ) , polycrys-

- t a l l i n e aluminas.

OAl2O3: l%y203 ( d r y m i x i n g ) , an- nealed under a i r .

A1203: 15Y203 ( d r y mixing) , an- n e a l e d under A r .

undoped A1203, annealed under a i r

Figure 6 shows the variation of the electrical conducti- vity versus the temperature (under 1 atm. gas) . ^First,

It can be noted that the yttrium doping increases the alumina conductivity. Secondly, by comparing these

D

values with those obtained on monocrystalline undoped or Y2O3 doped aluminas (8-91, it clearly appears that the grain boundaries induce an Increase of the conductivity. In both cases, doped o r undoped A1203, a breakdown in the curve log u vs. l / T is observed at about 1400eC. In undoped alumina, below 1400*C, a decrease of the activation energy is observed. The opposite effect is observed with the 1 mole % Y203 doped alumina. In the case of undoped alumina, for temperature lower than 1400°C, Impurity segregation and/or precipitation phenomena In the grain bounda- ries are supposed to take account for the unexpoctedly high conductivity observed in this temperature range.

Indeed, chemical analyses Indicate that the undoped alumina contains impurities, mainly SiO2

( -

850ppm) and such a beakdown is not observed in a rnonocrys- talllne alumir~a of the same purity ( 9 ) . Again, this result shows the particular role of the grain boun- daries. In the case of the 1% Y2O3 doped alumina, the fast decrease of u, for temperatures lower than

1400°C, cannot be associated to the conductivity through yttrium garnet precipitates in account of the high conductivity values in such phases ( 1 0 ) . Thus, the unexpoctcdiy low conductivlty values observed in

this tsmpsrature range, can be due to the low mobility of ths moving species along the

interfaces between alumina grains and yttrium garnet precipitates, interfaces which look as

coherent. But, an effect of the simultaneous intergranular segregation of yttrium and

olilcium cannot be excluded.

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The shape of the curves log o vs. Log Po2

A

tog o[W1cm-1)

(fig. 7 ) Is similar to that obtained on mono-

-

4- P O L ~ . A L ~ O ~ + 1%

3 0 ~

^(drymixing

I crystalline alumina (91, with a n-p transi-

1600°C

tlon. It Is not easy to deduce from the slope

of the u curve vs Pop, the nature of the defect responsible for the transport. The only thing which can be mentlonned is that the time necessary for reaching the equillbrlum after modifying the pressure Is long, sugges-

-

6-

tlng that transport phenomena are not only

due to electrons or holes but also to atomic point defects and to heterogeneities of mtcrostructure as observed on Fig. l b .

tog %!at'$

These assumptlons have to be verified by

-

¹⁵

-

¹⁰

-

⁵ ⁵

Ionic transport number determinations, but agree results obtained by Krdger ( 8 ) who F i g . 7 loglO a = f ( l o g 1 0 P 0 2 )

;

showed that, for Po2 - ^latm, ^undoped

~ 1

:

1%y203 ~ 0

(dry

~ mixing) monocrystalllne alumlna Is preferentially an electronic conductor, whereas Y2O3 doped alumlna acts as an ionic conductor.

At l40O0C, anomalies are observed, In an Intermediate pressure domain, related to yttrium garnet precipitations (extrinsic domaln) .

1 7 ~

. rspo

1~x7

%PO 13PO

T'C

Flgure 8, relative to the variations of 6 vs temperature, also lndlcatos that the yttrium ynrrtet precipitation modlfles the transport properties: botween I t 5 0 and 1400eC, a great dlsperslon in the D values is observed for polycrystaillne Y2O3 1 mole % doped alumina. For monocr~stalllne Ye03 doped alumina (91, a transition is noted i n the curve Log D vs 1/T, ketween - ^1550-

1650'G, leading to low D values at high temperature, whereas, _for undoped rnono- cryslallino Ai203, the D variation with T Is linear i n all the temperature range. These differences are related to the previous mi- crostructurai observations:

- For undoped monocrystalline Al2O3, preci- pitation phenomena do not occur and defects

-6-

- -

0: 3

Q

2

-7-

royrtx) such as grain boundaries are not present,

- For Y203 doped monocrystalllne A1203 (0. 1 F i g . 8

:

loglO D= f

(

I/T) mole % since no study was carried out on a

- 1 mole % doped alumina), the yttrium garnet

E l p o l y c r i s t a l l i n e A1203

:

1% Y2O3, precipitation occurs in the lattice, between

annealed under

A r .

1650 and 1550°C, and induces an Increase of

m o n o c r y s t a l l i n e A1203

:

O.l%Y203, D value, In account of the high mobility of annealed under a i r ( 7 ) . the moving species i n such garnet phases ---monocrystalline undoped ~ 1 2 0 3 (4,7). ( 1 0 ) . It was previously observed that garnets do not completely dissolve for temperatures lower than 1650°C.

- For Y2O3 doped polycrystalflne At2O3, two simultarreous effects occur between 1550 and 1400°C: intergranular garnets precipitate and such phases induce an lncrease in D vaiues, but-coherent interfaces between garnets and a l u r n l ~ a grains appear and induce a decrease in D values. This two opposite effects lead to a D values dlsperslon.

Thus, It is again observed that graln boundaries and lattice do riot present tho same transport properties. Flnaliy, grain buundariea and precipitation or segregation phenomena also affect the activatior~ energy of the chemical diffusion (which was determined with more expermental values than thase appearing on fIg,8) :

- for Y doped polycrystalllne alumlna: Q- 2. 12eV

- for Y doped rnonocrystailine alumlna: Q = 3.47eV

These vaiues are relative to the low temperaturo domain. in case of monocrystalllne alumina

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