Joining of Lanthanum chromite and zirconia ceramics
2
134 CERAMURGIA I,NTERNATIONAL, Vol. 1, n. 3, 1975 JOINING OF LANTHANUM CHROMITE AND ZIRCONIA CERAMICS Vu Tien, L. F. Cabannes and A.M. Anthony Centre de Recherches sur /a Physique des Hautes Temp6ratures, Orldans, France Received, September 15, 1975; revised copy received October, 30, 1975 An experimental procedure allowing the joining LaCr03 ceramic and stabilized Zr02 ceramic is described. Some microstructural characteristics of the joints are presented. In the thin ioining area, no chromium is observed, the porosity is low and the lanthanum concentrations are high, correspon- ding to the compound La2Zr207. 1 - INTRODUCTION The possible uses of oxides as heating elements have been well established by a great number of resear- chers. Recently, LaCrO3 has been used 1,2 in order to attain 1800°C in ceramic furnaces with an oxidized atmosphere. Another oxide, stabilized zirconia 3,4 used in the same conditions, attained even a higher tempe- rature of 2300°C. Compared to LaCrO3, the electrical conductivity of ZrO2 is much lower s, thus an intere- sting project is to design a furnace with ZrO2 as hea- ting element and LaCrO3 as electrical connection. The main problem to be solved is the joining of the two oxides and this paper is devoted to the experi- mental procedure that allows the joining of ZrO2 and LaCrO3. Some microstructural characteristics of the joint are also presented. 2 - EXPERIMENTAL PROCEDURE In the first experimental apparatus, polycrystalline oxi- des rods of 5mm diameter, 10mm length were pressed together and heated up to a temperature of 1550°C. The pressing apparatus consisted merely of a vertical alumina rod. Pressure was applied on the rod to the desired value and the temperature of 1550°C was obtained in a SiC or in an induction furnace. The alumina rod was sepa- rated from the ceramics by a graphite cube so that the alumina would not stick the zirconia or lanthanum chromite. The stress exerted was about IMN/m z and should be less than 1,5MN/m 2 to avoid buckles and cracks. About one hour was needed to reach the temperature of 1550°C. When reaching this maximum temperature the pressure ~Nas increased up to 1MN/m 2 and main- tained at this value for 30 minutes. The specimen was then cooled under the same pressure to room tempe- rature. To test the solidity of the joints,, some speci- mens were broken and it was observed that the broken surface did not correspond to the surface of joining. This means that the joints are stronger than the LaCrO3 ceramics which are porous. During service abo- ve 2300°C, fractured LaCrO3 was often observed and an increasingly plastic deformation appeared in ZrO2, but the joints were suitable enough. 3 - STRUCTURAL OBSERVATIONS The study of the joints by means of microprobe, opti- cal microscopy and scanning electron microscopy led to the following observations. JOINING AREA _ ~ ~11 M v Zr02 LaCrO3 FIGURE 1 - Zirconium distribution showing the Joining area between the two oxides. Sections for microscopy examinations were cut per- pendicular to the plane of the joining. Fig. 1 shows a vertical band, between 25 and 50 ~m wide, across the center of the micrograph. This band represent the joining area between the ZrO2 and the LaCrO3 ce- ramics. Using scanning electron microscopy, it was observed that mean grain size was 25/~m Csee Fig. 2), so the joining thickness between the two oxides was FIGURE 2 - Broken surfaces across the joints, x200.
Transcript of Joining of Lanthanum chromite and zirconia ceramics
134 CERAMURGIA I,NTERNATIONAL, Vol. 1, n. 3, 1975
JOINING OF LANTHANUM CHROMITE
AND ZIRCONIA CERAMICS
Vu Tien, L. F. C a b a n n e s and A . M . A n t h o n y
Centre de Recherches sur /a Physique des Hautes Temp6ratures, Orldans, France
Received, September 15, 1975; revised copy received October, 30, 1975
An experimental procedure allowing the joining LaCr03 ceramic and stabilized Zr02 ceramic is described. Some microstructural characteristics of the joints are presented. In the thin ioining area, no chromium is observed, the porosity is low and the lanthanum concentrations are high, correspon- ding to the compound La2Zr207.
1 - I N T R O D U C T I O N
The possible uses of oxides as heating elements have been well established by a great number of resear- chers. Recently, LaCrO3 has been used 1,2 in order to attain 1800°C in ceramic furnaces with an oxidized atmosphere. Another oxide, stabilized zirconia 3,4 used in the same conditions, attained even a higher tempe- rature of 2300°C. Compared to LaCrO3, the electrical conductivity of ZrO2 is much lower s, thus an intere- sting project is to design a furnace with ZrO2 as hea- ting element and LaCrO3 as electrical connection. The main problem to be solved is the joining of the two oxides and this paper is devoted to the experi- mental procedure that allows the joining of ZrO2 and LaCrO3. Some microstructural characteristics of the joint are also presented.
2 - E X P E R I M E N T A L P R O C E D U R E
In the first experimental apparatus, polycrystalline oxi- des rods of 5mm diameter, 10mm length were pressed together and heated up to a temperature of 1550°C. The pressing apparatus consisted merely of a vertical alumina rod. Pressure was applied on the rod to the desired value and the temperature of 1550°C was obtained in a SiC or in an induction furnace. The alumina rod was sepa- rated from the ceramics by a graphite cube so that the alumina would not stick the zirconia or lanthanum chromite. The stress exerted was about IMN/m z and should be less than 1,5MN/m 2 to avoid buckles and cracks. About one hour was needed to reach the temperature of 1550°C. When reaching this maximum temperature the pressure ~Nas increased up to 1MN/m 2 and main- tained at this value for 30 minutes. The specimen was then cooled under the same pressure to room tempe- rature. To test the solidity of the joints,, some speci- mens were broken and it was observed that the broken surface did not correspond to the surface of joining. This means that the joints are stronger than the LaCrO3 ceramics which are porous. During service abo-
ve 2300°C, fractured LaCrO3 was often observed and an increasingly plastic deformation appeared in ZrO2, but the joints were suitable enough.
3 - S T R U C T U R A L O B S E R V A T I O N S
The study of the joints by means of microprobe, opti- cal microscopy and scanning electron microscopy led to the following observations.
JOINING AREA
_ ~ ~ 1 1
M v
Zr02 L a C r O 3
FIGURE 1 - Zirconium distribution showing the Joining area between the two oxides.
Sections for microscopy examinations were cut per- pendicular to the plane of the joining. Fig. 1 shows a vertical band, between 25 and 50 ~m wide, across the center of the micrograph. This band represent the joining area between the ZrO2 and the LaCrO3 ce- ramics. Using scanning electron microscopy, it was observed that mean grain size was 25/~m Csee Fig. 2), so the joining thickness between the two oxides was
FIGURE 2 - Broken surfaces across the joints, x200.
T E C H N I C A L A N D ENGINEEPJNG NOTES 135
J O I N I N G A R E A
i I l l
ZrOz " " LaCr03
FIGURE 3 - Microstructure of the ceramics and the Joint. The lowest porosity density is observed in the Joining area.
1OO%
0 ~
f
| # I
l l l ! J l
La
Cr
t l l I t t
l I I . 50 p 1OO p 150 p
FIGURE 4 - Zr, La and Cr concentrations across the joining area, obtained by electronic microprobe.
about two or three grain size. This phenomenon has been observed in alumina joints 6 Figure 3 shows the lower porosity level in the joining zone. The decreasing porosity precludes some plastic deformation as one predominant mechanism responsi- ble for the joining process. Another feature of the joining is the large penetration
of lanthanum into zirconia whereas chromium was sur- prisingly low, as it may be seen by microprobe ana- lysis (Fig. 4). The microprobe allowed a quantitative analysis of lan- thanum and zirconium concentrations and they were found to be roughly equal. This may correspond to a decomposition of LaCrO3 when it reaches the zirconia ceramic and the probable formation of the compound La2ZrzOT. The low concentration of chromium can be understood from the high volat i l i ty of chromium oxide which is consistent with the green deposits observed on the insulating shells of the furnace.
4 - CONCLUSIONS It has been shown that two pieces of ceramics can be joined by hot pressing. The boundary between these two pieces was observed and microstructural features of the ZrO2 - LaCrO3 are: low chromium concentration, large lanthanum location and decreasing porosity level. Since the connection between these two oxides has been succesfully achieved, it is possible to use LaCrO3 in furnace with zirconia as heating element instead of more expensive metal as platinum (see Fig. 5}.
FIGURE 5 - ZrO2 heating element and LaCrO3 end connection, with insulating shells.
This type of furnace has already been proved as a fine tool for high temperature technology. Such a furnace is used at this t ime to carry out measurements of fixed points; for instance, the zirconia-yttria eutectic point has been observed at 2250°C. It is intended to be applicable in the long term to higher melting points.
REFERENCES
1. A.M. ANTHONY, G. BENEZECH, F. CABANNES, M. FAUCHER, M. FOEX, L. VUTIEN and D. YEROUCHALMI, Le chromite de lanthane, propri6t6s nouvelles. Third International Symposium on High temperature Technology Asilomar, High temperature Technology, Ed. Butterworths, London, 1969, p. 214.
2. R. BERJOAN, A. MOISE, M. RIVOT and J.P. TRAVERSE, Science of Ceramics 7 (1973) 341.
3. A.M. ANTHONY and M. FAUCHER, C.R. Acad. Sci. 267 (1968) 444.
4. A.M. ANTHONY, K. DEMBINSKY and L. DUPONT, Science of ceramics 7 (1973) 137.
5. B.M. BARYKIN, V.G. GORDON, B.M. LEVINOV, A.I. REKOV and E.G. SPIRINOV, High Temperature-High Pressure 7 (1974) 47.
6. D. BUDWORTH, J.P. ROBERTS and W. SCOTT, Trans. Brit. Ceram. 62 (1963) 949.
