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THE FORMATION AND CHARACTERIZATION OF A CERAMIC-CERAMIC INTERFACE BETWEEN
STABILIZED ZIRCONIA AND LANTHANUM CHROMITE
D. Smith, M. Sayer, P. Odier
To cite this version:
D. Smith, M. Sayer, P. Odier. THE FORMATION AND CHARACTERIZATION OF A CERAMIC- CERAMIC INTERFACE BETWEEN STABILIZED ZIRCONIA AND LANTHANUM CHROMITE.
Journal de Physique Colloques, 1986, 47 (C1), pp.C1-153-C1-157. �10.1051/jphyscol:1986122�. �jpa-
00225550�
JOURNAL DE PHYSIQUE
Colloque Cl, supplément au n°2, Tome 47, février 1986 page cl-153
THE FORMATION AND CHARACTERIZATION OF A CERAMIC-CERAMIC INTERFACE BETWEEN STABILIZED ZIRCONIA AND LANTHANUM CHROMITE
D.S. SMITH, M. SAYER and P. ODIER*
Department of Physics, Queen's University, Kingston, Ontario, K7L 3N6, Canada
'Centre de Recherches sur la Physique des Hautes Températures, C.N.R.S., F-45045 Orléans Cedex, France
Résumé - Il est possible d'obtenir par compression à 1500°C la formation de contact entre la zircone stabilisée et le chromlte de lanthane. La caractérisation de l'interface ainsi créé a été faite par microscopie électronique à balayage et microanalyse X. Elle a révélé la présence d'une couche intermédiaire qui joue un rôle important dans l'obtention d'une liaison solide entre les deux matériaux. L'utilisation de ces jonctions en tant qu'électrodes céramiques pour des fours haute température est également discutée.
Abstract - Interfaces have been formed between stabilized zirconia and calcia doped lanthanum chromite by pressing at 1500°C. Characterisation of the interfaces with scanning electron microscopy and X-ray microanalysis shows the presence of an intermediate layer at the junction. This layer has an important role in the formation of a mechanically strong bond.
The application of ceramic electrodes in high temperature furnaces is discussed.
I - INTRODUCTION
Simple techniques which join one material to another are important in the development of ceramic devices. The formation of strong joints permits the manufacture of ceramic bodies with complex shapes and functions. Stabilized zirconia is an oxygen ion conductor which is used for heating elements /l/. Present designs employ platinum electrodes but there is strong motivation to use a cheaper ceramic alternative such as lanthanum chromite. Lanthanum chromite is an electronic conductor wih a melting point above 2300°C and a co-efficient of thermal expansion well matched to zirconia. The application requires a completely ceramic ionic - electronic interface with good electrical contact and mechanical strength.
Ceramic bodies can be joined by the choice of a suitable intermediate. For example, high silica content oxide glasses are used to join pieces of silicon nitride / 2 / . In special cases the formation of an intermediate occurs directly between the materials at sufficiently high temperatures. Thus sintered zirconia has been joined to sintered lanthanum chromite by simply pressing at 1550°C with 1 MPa / 3 / . This temperature and pressure leads to considerable deformation of the lanthanum chromite due to thermal creep. Vu Tien and Odier have proposed joining the ceramic pieces during sintering
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986122
J O U R N A L D E PHYSIQUE
in a process termed 'CO-sintering' 1 4 1 . Because green compacts are more reactive than the final sintered body it is hoped to achieve a strong joint with reduced temperature and less applied pressure.
I n this paper we examine the CO-sintering technique for zirconia and lanthanum chromite and the formation of a n intermediate bonding layer. This layer has an important effect on the electrical behaviour of the interface and we report a preliminary electrical characterisation. The consequences of these results are discussed with respect to the potential utilization of ceramic electrodes in high temperature furnaces.
I1 - EXPERIMENTAL
Green compacts of 12 wt% yttria stabilized zirconia (YSZ) and calcia doped lanthanum chromite (CaLC) were isostatically pressed at 480 MPa in rubber moulds to form cylinders between 4 and 8 mm in diameter. The CO-sintering experiment consisted of a gravity loaded press which applied a light pressure of 25 KPa to keep the contact faces of the cylinders together during isothermal firing. The green cylinder ends were ground "flat" and discs of dense zirconia were used to separate the press from the sample. Firing was carried out in a furnace with Superkanthal elements which allowed 1500°C to be achieved in 15 minutes.
The sintering of the individual pressed samples was studied in a dynamic manner using an automated high temperature vertical differential dilatometer. Scanning electron microscopy (S.E.M.) was used to examine the morphology of the interfaces and X-ray microanalysis to obtain a profile of the chemical composition across the junction. The electrical behaviour of the interfaces was studied by a.c. impedance measurements. These were carried out between 100 KHz and 3 Hz using a Princeton Lock-in amplifier controlled by a microcomputer. The samples were mounted in an alumina cell which was then placed inside a furnace with a temperature range of 400 to 1000°C.
Platinum e l e c t r o d e s were used f o r e l e c t r i c a l c o n t a c tt o
t h e sample.I11 - RESULTS AND DISCUSSION
a) CO-sintering and Differential Shrinkage
Zirconia and lanthanum chromite have thermal expansions which are approximately matched but in commercially available powder there i s a considerable difference in the time and temperature dependence of linear shrinkage during sintering (Fig.1). The resulting differential shrinkage will exert two sets of stresses on the junction during CO-sintering a s first one material densifies and then the other (Fig.2).
The following conclusions have been made in a separate study on this aspect (submitted for publication). The detrimental effect of these stresses on the strength of bonding depends on the relaxation processes. The visco-elastic nature of a pressed compact during sintering permits relaxation of stress by viscous deformation /S/
and the formation of an intermediate layer may also play a role. In the zirconia/lanthanum chromite system the differential shrinkage is thought to be sufficiently relaxed that bonding is not seriously affected but it can cause cracking of the main bodies in long firing cycles. Choice of sintering characteristics and fast firing are suggested a s methods which can be employed to reduce these stresses.
With these considerations zirconia was successfully joined to
lanthanum chromite by firing for 4 hours at 1450'~ or above. Below
this temperature no bonding was achieved.
Fig.1 Linear shrinkage during sintering of (A) yttria stabilized zirconia (Zircar) and (B) lanthanum chromite (Pyrox).
Fig.2 Stresses which result a s first material A densifies and then material B. Subsequent deformation is not illustrated in this schematic.
b) Microstructural Characterisation
Zirconia is joined to lanthanum chromite by a phase which is different from either bulk material (Fig.3). The profile of chemical composition shows that this intermediate layer of approximately 2 0 microns is well defined and contains a high proportion of silicon (Fig.4). This suggests there has been considerable transport to the junction because silica is only an impurity in both zirconia and lanthanum chromite,0.3 and 0.17 wt% respectively. The silicon of heat intermediate layer is associate with an equal proportion of calcium which come from the lanthanum chromite. The lanthanum chromite is doped with calcia to enhance its conductivity. The strong affinity of silica and calcia for each other, the affinity of zirconia and calcia, and the lack of solubility of silica in
CsaChsare factors which are important in the formation of the intermediate layer.
The presence of silicon and calcium in pores at the junction
suggests the formation of a liquid phase. A variety of liquid phases
can occur between silica and calcia around 1450-1550°C depending on
the composition and may explain why 1450°C is a critical temperature
for bonding. This i s a n attractive explanation because a liquid phase
would relax stresses due to differential shrinkage and then could
provide bonding on cooling.
C l - 1 5 6
JOURNAL DE PHYSIQUE
5 0 Atomic Concentration 40
(%l
'
,'I-
20-10
0 25 Distance (microns) 50 75
Fig.4 Profile of chemical composition across the interface.
c) Electrical Characterisation
A.c. impedance measurements were carried out on zirconia joined to lanthanum chromite electrodes (Fig.5). Despite the high conductivity of lanthanum chromite (approximately 3 (ohm-cmTTat 700°C) the bulk resistance of zirconia cannot be resolved in the spectrum. This suggests there is an extra resistance in the system. The equivalent circuit models the situation if the junction layer has a significant resistance (Fig.6), and produces a satisfactory fit to the experimental data when physically reasonable values of the layer resistance and capacitance are chosen, 5 K s h e end 5 x I Q - ~ '
F.Fig.5 Impedance spectrum of zirconia/lanthanum chromite (Pyrox).
5
z
"-
K ahmsL C l p I YSZ I LClp 713'~
SIMULATION
- -
I I I t l I----'
=.
m = m2 0 K H z . , m m m m =
W , C g Clayer (h
. . . ' * . .
. . . .
Fig.6 Equivalent circuit.
O o
z' 5 Kohrns
The microstructural characterisation suggests that the levels of Si and Ca control the intermediate layer formation and therefore calcia doping was reduced in the lanthanum chromite. The resulting interface exhibited an intermediate layer of less than 5 microns. The corresponding impedance spectrum suggests that the resistance of the layer has also been reduced (Fig.7).
LCl. I YSZ I
m.
812Oc *P
