Impact of grain boundaries on properties of mullite as a solid electrolyte

Science of Ceramic Interfaces II J. Nowotny (Editor) 9 1994 Elsevier Science B.V. All rights reserved.

571

IMPACT OF GRAIN BOUNDARIES ON PROPERTIES OF MULLITE AS A SOLID ELECTROLYTE Kazuo Yamana ~, Masald Miyamoto ~, Kenji Doi Nowotny d

b,

Industrial Research Institute of Ishikawa, Tomizu-machi, Kanazawa, Ishikawa 920-02, Japan

Tadahisa Arahori ~ and Janusz

Ceramic

Section,

Ro-1,

b Sumitomo Metal Industries, Ltd., Advanced Materials Division, l - 3 , Ote-machi, 1-chome, Chiyoda-ku, Tokyo 100, Japan c Sumitomo Metal Industries, Ltd., Advanced Technology Research Laboratories, 16, Sunayama, Hasaki, Ibaraki, 314-02, Japan d Australian Nuclear Science and Technology Research Laboratories, Menai, NSW 2234, Australia

Organisation,

Lucas

Heights

Abstract Basic properties of mullite such as chemical composition, phase stability, crystalline structure and microstructure are briefly reviewed. Effect of impurities, resulting in the formation of grain boundary glassy precipitates on high temperature properties such as mechanical properties and ionic conductivity is considered. Properties of mullite which is free of the glassy phase are analyzed as an ionic conductor for high temperature oxygen sensors. Electromotive force (EMF) of an oxygen concentration cell, based on mullite (which is free of impurities or involving an excess of silica) as an oxygen conductor, indicates that the electronic conductivity component at high temperatures assumes negligible values especially at very low oxygen activities (below 5ppm).

572 1. INTRODUCTION MuUite is a ceramic material with interesting properties and a wide range of applications. Recent studies indicate that muUite can also be applied as a sensor-type material with interesting properties as a solid electrolyte. The purpose of the present paper is to overview basic properties of muUite. Impact of grain boundaries, in particular the grain boundary glassy phase, on mechanical properties and ionic conductivity will be discussed in more detail.

2. GENERAL CONSIDERATIONS Mullite is the only stable phase of Al 2 0 3 - S i O z under atmospheric pressure. Its chemical composition varies between 3AI 2 0 3 9 2SiO 2 and 2A1203" Si02. MuUite is fabricated from clay mineral-based raw materials such as kaolinite and by adding A12 0 3. The resulting product involves a glassy intergranular phase which has negative effect on mechanical properties lowering its strength of the material above 800~ Mullite involving the glassy phase has limited applications. It has been applied as a refractory materials for fabrication of potteries [1-4]. The objective of this paper is to analyze several properties of the synthetic muUite, free of the glassy phase, as a solid electrolyte for oxygen sensors operating at high temperatures. Dokko et al.[5] have reported that the glass free mullite exhibits enhanced mechanical properties. The synthesis is based on so called molecular or/and colloidal method [6]. Thus produced material exhibits very high creep and thermal shock resistance as well as much lower thermal expansion and dielectric constant than those of the traditional material [7]. Several properties of mullite are summarized in Table 1. Schuh et al.[8] reported that silica rich muUite exhibits an ionic conductivity. According to Ficher and Janke [9], the mullite phase becomes a pure ionic conductor above 1000 ~ This property make mullite an excellent candidate for oxygen sensors to special applications such as the determination of oxygen activity in steel melts during steel making processes instead of

573

Table 1. P r o p e r t i e s of commercial m u l l i t e alumina[2].

unit

in comparision with those of

Mullite 3A1203 9 2Si02

Alumina AI20a

S i02

~

38

O. 1

A1203

%

60

99.5

CaO

%

O. 4

MgO

%

-

O. 15

NauO

%

O. 2

O. 05

Specific gravity

-

2.7

3.9

Water absorpt i on

%

O. 0

O. 0

Compress ive s t r e n g t h

blPa

1,400

3, 000

F I exural s t r e n g t h

MPa

180

300

Refractoriness

~

1,850

2, 000

Chemical compos i t i on

Thermal c o n d u c t i v i t y

kcal/mh~

2, 7

7.2

xlO-6/~

5, 6

8. 1

(400 ~ Coef. of thermal expans i on (20-1000 ~ Serv ice temperature

Electrical resistivity

max i mum

~

1,650

1,930

common

~

l, 600

I, 850 > 10 '3

20 ~

ohm 9 cm

> 10 '3

500 ~

ohm 9 cm

2xlO 8

109

574 zirconia-based sensors. The oxygen sensor based on mullite will be described in more detail.

3. BASIC PROPERTIES 3.1. Phase diagram in the A1 2 0 3 -SiO 2 system There have been many conflicting data and interpretations on the stability of the mullite phase. One may expect that all these apparent conflicts can be explained by differences in both chemical omposifion of starting materials and the experimental procedure applied to faboficate muUite. A detailed phase diagram for the AI 2 0 3 - S i O 2 system was reported by Bowen and Greig [10]. They claimed that mullite melts incongruently at 1810 ~ Aramaki and Roy [11] reported that mullite melts congruently at 1850 ~ involving a solid solution varying between 71.8 wt% of Al 2 0 3 & 28.2 wt% of Si02 (3:2 ratio) and 74.3 wt% of A 1 2 0 3 & 25.6 wt% of SiO~(Fig.1). A metastable solid solution may reach up to 77.3 wt% of A1 2 0 3 (2:1 ratio). According to the phase diagram of Aksay and Pask [12], determined by using a diffusion couple technique, mullite melts incongruenfly at 1828 ~ (Fig.2). 3.2.

Crystal structure The x - r a y diffraction pattern of muUite is similar to that of sillimanite which is a stable phase under high pressures and at elevated temperatures. Thus mullite structure can be derived from the sillimanite structure by substituting alumina by silicon in the tetrahedral sites and by removing appropriate oxygen atoms as required by the neutrality condition [13]. Basically, the mullite structure is an orthorhombic one. It consists of octahedral AlO 8 chains parallel to the c-axis that are cross-linked by tetrahedral (A1Si)O4 chains (Fig.3)[14]. Variation of its composition from 3 A 1 2 0 3 . 2Si02 to 2 A 1 2 0 3 . SiO2 results in changes of the lattice parameters (increase of a and c and decrease of b ) [15]. Thus the following formula for mullite can be proposed 9 IV AI I(AI IVz,zx Si2-2x) 010-x

(1)

575

A 1,0~ [mo] %] 20 i

40

i

i

2000 -

60

i'

i

i

80 "

i

i

-

100 i

Mullite(ss) ) liquid

Liquid

l==---i

l=_..a

.

,

1800

1800

1400

,

I

I

,

,I

a

:o

i

4o

eo

so

1-oo

Al ~O~ [wt%] Fig. 1. The phase diagram of the hl2Oa-Si02 and Roy [ l l ] .

A1 0

20 9

"

system a c c o r d i n g

Iwt%J

40

60

51

w

80 9

9.

/

~c3

//."

.~/ I [

!-

I ]400L 0

100 .

.

.

.

Liquid

2000-

]6oo

to Aramaki

Hu]lite(ss)+Lzquld

.-7--,-. -"7 ~'~,. / i ~. ", I ! ',:.

Si I icaOlul I i t e ( s s ) ; i i . ',. 20 40

Alumina+Liquid

I1: JJ. /

i

I~

i Alu, ina

]

!

Mullite(ss) i

I

I:

i

.ti

~

60

]

,

. 80

I

I

j l O0

A1 ~.03 [mo 1%] Fig. 2. The phase diagram of the A1203-Si02 system according to Aksay and Pask [12].

576

(a)

(b)

Fig. 3. Crystal s t r u c t u r e of m u l l i t e projected on the (001) plane : ( a ) h y p o t h e t i c a l " p e r f e c t " m u l l i t e (dark atoms are located at z=O) and (b) average unit cell of m u l l i t e (heavily outlined s i t e s have p a r t i c a l occupancies and r e p r e s e n t displaced atoms due to the presence of oxygen in 03 c o n f i g u r a t i o n s ) [ 1 6 ] .

9 II

0 II,Si

0

|

Fig. 4. Model for l a t t i c e r e c o n s t r u c t i o n around an oxygen vacancy (dashed c i r c I e).

577 where x is the number of oxygen atom missing in average unit cell, and 1V and VI represent coordination states of the cations [16]. Both x - m y and electron diffraction studies indicate a tendency of oxygen vacancies to order resulting in the formation of ordered structure within the mullite composition range between 3:2 and 2:1 of AI 2 0 a to SiO, ratio. Fig.4 shows the structure model illustrating a reconstruction around oxygen vacant site (dashed circle). The AI and Si cations, which lose their tetrahedral coordination because of oxygen removal, are shifted by about 0.12 nm to new lattice sites (indicated by T * in Fig.3). These displaced atoms become fourtly recoordinated through an associated motion of an adjacent oxygen atom into the position indicated as 0 * in Fig.3. 3.3.

Microstructure Mullite, fabricated traditionally from raw materials, involves a glassy phase in the form of an intergranular precipitate. Chemical compositions of the glassy phase in both sintered and fused mullite phases are shown in Table 2. The amount of the glassy phase in refractory materials depends on the content of its impurity (Fig.5). During growing a mullite single crystal by using both Czochralski method and the floating zone method, the glassy phase is located in spaces between the muUite crystals (Fig.6) [17-19]. If alumina and silica are mixed on a molecular level, then the muUite formation becomes controlled by a nucleation process. Then the glassy phase is not present [20-23]. Therefore, the mullite powders is an important source of a glass-free mullite phase [2]. Fig.7 illustrates the SEM photograph of a polished and then thermal-etched muUite specimen which was sintered from synthetic materials at 1650 *C. 3.4.

Mechanical properties Mullite has commonly been applied as a refractory material in construction of heating tin'races. It exhibits an excellent resistance to creep, thermal shock, abrasion, spalling and corrosion of acid slags. However, mullite has never been regarded as a suitable material for high-strength applications at elevated temperatures. The preparation of muUite, which is free of glassy phases, results in a wide range of application as a structural material of good mechanical properties. Fig.8 illustrates a relationship between the bending strength and temperature for the synthetic mullite in

578

T a b l e 2. Chemical c o m p o s i t i o n s of r e f r a c t o r i e s (made of s i n t e r e d and f u s e d mul I i t e ) and t h e g l a s s y p h a s e s i n v o l v e d in t h e refractory material.

BULK

GLASSY PHASE

Sintered mul I i te

Fused mull i te

Sintered mul 1i te

Fused mul I i te

Si02

27. 98

21.39

56.95

21.39

AlcOa

68.81

74.25

28.32

33.71

Fe~03

1.54

O. 54

5.66

5.13

TiO~

O. 40

2.81

1.75

16.96

CaO

O. 18

O. 31

O. 21

2. 46

MgO

O. 08

O. 10

O. 10

O. 45

Na20

O. 16

O. 02

O. 62

O. 45

K~O

O. 75

O. 02

6.39

O. 45

(unit : wt~;)

Amount of glassy phase

12. 5

5.2

20

10

0

0

I

I

0.4

i, 8

I

J

. .,I

,

2,0

I

2,4

2,8

IHPURITIES[Fe20~+TiO~CaO+~gO+Na20*K~O](wt%)

Fig. 5. R e l a t i o n s h i p refractories.

between g l a s s y

p h a s e s and i m p u r i t i e s

in m u l l i t e

579

Fig. 7.

SEM micrograph of the thermally etched surface of the synthetic mullite.

580

~

400 ~-

99Z Altmina

~a

'

n

'

'

-

~

1

~

|

0

300

100

0

200

400

600

800

I000

1200

1400

Fig. 8. Relationship between bending strengthes and temperature for synthetic mullite, t r a d i t i o n a l mullite, and 99% alumina.

68A

600

500

J,;'

I

400 Z

.-~\ --- -

300

~" 200

RT

60A /~" 99%Alumina I000

71.8A\\

~

II00

TEMPERATURE

7'

". 1200

1300

1400

[~3]

Fig. 9. Relationship between the bending strength and the temperature for d i f f e r e n t alumina contents 9 60A, 68A, 71.8A (mullite + s i l i c e o u s glassy phase), 74A (mullite + alumina) and 99A (alumina).

581 comparision with those of alumina and natural mullite [3]. The natural muUite has considerably smaller strength than that of alumina, and nearly the same strength as the synthetic mullite. However, the strength of alumina decreases gradually above 1000 ~ On the other hand the synthetic mullite still exhibits its strength at room temperature. Moreover the strength even increases at 1300 ~ The more detailed data of strengthes on synthetic mullite at different alumina content in the Al, 0 3 - S i O , system were reported by Kanzaki [24]. As shown in Fig.9, only silica-rich specimens results in an increase of strength, while alumina-rich specimens exhibit a degradation effect. It is thought that the observed increase in the bending strength above 1200 ~ is due to the presence of siliceous glass phases [25]. 3.5.

Thermal shock resistance Data on the thermal shock resistance of the commercial muUite, collected in Table 3, correspond to the temperature difference between the specimen and water, if the water quenching procedure is applicable. Table 3 also gives the amount of the glassy phase [2]. Fig.10 shows the relationship between the temperature difference of the quenching procedure ( t l T ) and the bending strength after the water quenching. As seen A T gradually increases with the amount of the glassy phase although this difference is relatively small. One may thus assume that the glassy phase, inbetween the mullite grains, has a tendency to lower the crack propagation which apparently affects the strength. The thermal shock resistance of a mullite tube, from the viewpoint of its application for construction of a high temperature oxygen sensor, was discussed in ref. [26]. The analysis of the thermal stress distribution was performed for a mullite tube immersed into iron melt at 1600 ~ by using a finite element method [26]. As shown in Fig.11 the temperature of the tube increases with the elapsed time. After 0.15 seconds the temperature difference between the inner and the outer part of the tube is 300 ~ The stress distribution on the cross section in tube after 0.15 seconds is shown in Fig.12. The maximum stress results in a center of the inner part of the tube. Its value is 257 MPa. As it was already reported muUite shows the tensile stress about 130-140 MPa [26]. Thus it has been suggested that the thermal shock fracture occurrs in the muUite tube. In order to protect the thermal shock fracture, plastic-coating and porous tubes have been employed [26].

582

Table 3. P r o p e r t i e s of commercial m u l l i t e s employed in thermal shock tests.

Sample

Composition A120a/Si02 (wt%)

Density

Grain structure

Glassy phase (wt%)

I

I

I

I

0

10

20

30

A

47 / 49

2.50

Bullite

B

55 / 41

2.65

Mullite

C

60 / 38

2.70

Mullite

D

67 / 31

2.75

Mullite

E

76 / 23

2.98

Mul I i te+A120a - - - -

'G

200 ;=I:2

D ~

m

o . e .... .c........... 0"0

B

~E}--

...... A

"E~-'I

~

E--,

100

z Z

200

2 0 AT [~]

-3i0

Fig. lO. Thermal shock r e s i s t a n c e of the commercial m u l l i t e corresponding to water quenching. Symboles A, B, C and D correspond to Table 3.

583 --

,

,..

1500

A lO00

~--~

500

._,

,

,

,

,

I 0.5

1

I

I

1.0

1.5

2.0

ELAPSED TIME[SEC, ] Fig. ll. The outer and the inner surface-temperatures of the mullite tube in relation to theelapsed time a f t e r immersing into steel melts.

300

200I '-"2 101

~ N

-~oo[

~ -2oo Fig. 12. Relationship between the calculated s t r e s s (after O. 15 sec.) and the

~ -300 FI

wall thickness on cross section L

J

..,

i

waI l thickness ins ide outside B A

(A-B) in the tube shown in Fig. ll.

584 4. MULLITE AS AN OXYGEN CONDUCTOR Oxygen conduction in solids requres that predominant lattice defects are oxygen vacancies which exhibit high mobility such as the defects in yttria-doped zirconia. As mentioned above muUite is a very good oxygen conductor at high temperature. The oxygen vacancies in mullite may be formed as a result of the following reaction 9 A 120a-[-28 i si+Oo

=

2A I si+V 6 + S

i 02

(2)

where Sis~ indicates Si on Si sites, O o indicates 0 on 0 sites, Alsi indicates A1 on Si sites and V 6 indicates vacancy on 0 sites. Fischer and Janke [9] have reported the electromotive force (EMF) of the oxygen concentration cell of the following compositions 9 air / mullite / H 2 0, H 2

(3)

air / muUite / iron melt (1600 ~ )

(4)

air / mullite / AI melt saturated with A1 2 0 3 (1000-1600 ~ )

(5)

where the mullite phase involved an excess of the silic acid. It has been observed that the mullite phase enriched with Si02 exhibits good ionic conductivity at 1600 ~ at p(O 2 ) value above 10-8 atm (Fig.13). The studies indicate that mullite exhibits much better properties for construction of an oxygen sensor in steelmaking than sensors based on zirconia. Similar studies were also reported by Koller [27]. At high temperatures and at low oxygen activities zirconia exhibits mixed (both electronic and ionic) conductivity. The EMF can be expressed as

E- R T P'(o2)1". PH I/4 F I n P(oz)V4+ p~11/4 ....

(6)

where P(O~.) and P'(O~.) are the partial pressures of oxygen at two electrolyte interfaces, P . is defined as the oxygen partial pressure at which both ionic and electronic conductivity components are equal, R is the gas constant, F is the Faraday's constant, and T is the absolute temperature. The tube-type mullite probe was manufactured from high purity materials

585

2500 ! : ~

I / ~

2000 -

L..__j

"

\%,\\%\% ~'

\

~.

\

\

\

\

1500

,%\ ~,<..,.'-~.,, -...:-,~\.~\

1000

500

-30

j

,,

I

--20

i

I

....

I

--10

J

\

0

Log P(O~o) [P(Oz) in Pal

Fig. 13. Relationship between the electromotive force (EMF) and the oxygen partial pressure for the mullite e l e c t r o l y t e .

586 coprecipitated by a sol-gel method [28]. Studies on oxygen sensor by using a mullite tube were also performed by Kawai et a1.[29]. Extensive studies were performed by Suito et al. [30]. Their measurements were carded out by using the equipment shown in Fig.14. A mullite probe was dipped into an iron melt and the EMF values were measured at 1600 *C. The response of the mullite probe remained within 10 seconds. Suito et al.[30] have reported the measurements of oxygen activity in the iron melt at different content of A1 as deoxidiser. They have assumed that (1) the mullite solid electrolyte exhibits pure ionic conduction, and that (2) the thermodynamic equilibrium at the elecrolyte/electrode interface is reached. As shown in Fig.15, oxygen activities ( a o ) obtained from EMF measurements are plotted against the Al content, which corresponds to the dissolved Al in the melt as well as the Al ~ O a inclusions. As seen the a o values for the A1 contents below 0.5 wt% are in a good agreement with the Nemst dependence. Fig.16 shows that the values on the zirconia-plug probe corrected by the electronic conductivity component are in an excellent agreement with those of the mullite probe obtained by using the Nemst equation. Suito et al.[30] have concluded that the electrical conductivity of mullite is lower than that of zirconia (by about three orders of magnitude) mainly due to the electronic leakage of zirconia at elevated temperatures. This may indicate that muUite-type oxygen sensor exhibits superior properties to those of zirconia-type sensor especially at very low oxygen activities. Fig.17 illustrates the oxygen activity values determined by using the stoichiometric mullite probe vs. the EMF obtained by using the mullite probe with an excess of either Al ~ O a or SiO ~. As seen the a o values obtained with the mullite probe containing an excess of A1 ~ O a are slightly higher than those determined by using the probe based on muUite of stoichiometric composition. On the other hand the oxygen activity values of mullite involving an excess of SiO ~ are lower than those corresponding to the stoichiometric mullite phase. In order to evaluate the performance of mullite-based oxygen sensor at very low oxygen activities. Nagatani et al. [ 3 1 ] have determined the parameter P n in equation (6) and illustrates the relationship between the EMF determined for the mullite-based sensor and the oxygen activity for the zirconia sensor. As seen the EMF obtained by using the muUite probe is in a good agreement with the Nemst law at very low oxygen activities. On the other hand the EMF values for the zirconia-based sensors exhibit substantial

587

Deoxidized Ar (CO,Ar-5~H2)

I

Mo rod Alumina tube

L-I

m

V-

Water-coo led cap

Alumina reaction tube

Crucible (alumina, graphite) Z i r c o n i a sensor

r-711 J

ldullite sensor

. .I.I. I

nN

Iron melt

"

Zirconia tube

! v.

L~r "

Fig. 14.

~

Deoxidized Ar

Thermocouple

S c h e m a t i c d i a g r a m of e x p e r i m e n t a l oxygen a c t i v i t y

in i r o n m e l t .

apparatus

for

measuring

588 t

-2 -

t

I

I

1873K _

9 9

r-----i

9

Q

-3

_

0

0

_

9

-4

AI/A120.~ c~. -4

I

I

i

I

-3

-2

-1

0

1

Log[mass-% A1]obs. Fig. 15. Oxygen a c t i v i t y of iron melt measured by the mul I ite-based sensor vs. Al content (deoxidizer) [30].

-2

I

I

r--"-'-I

c~

[mass-% AI_] _g O. 5

9, r - - , i

o r

-3 -,---4

--4 o

-5

I -5

-4

-3

-2

Log ao[EMF, M u l l i t e ] Fig. 16. Comparision between the oxygen a c t i v i t i e s measured by the zirconia plug-type probe and the mullite probe.

589 --2

-

I

;:~ - 3 0 o~ t:uO

o

--4

/ -5 . . . . -5

Al20.-rich @ Si02-rich 0 l .... -3

I

-4

L o g a o [EMF,

-2

Stoichio~etric]

Fig. 17. Comparision between oxygen a c t i v i t y measured by using mullite with an excess of Al20a and Si02 on one side and the s t o i c h i o m e t r i c mullite.

600

,

,

t

"!

1873K

,

PH

. . . . 15.29(Zr-7M)[32] ~f. =1 ...... 14.04(Zr-9M)[33] 400 . . ~ ~ o n u I O n x -----13.51(Zr-9M)[34] t_.____s

200

-~--~sH82(~ullite) "----13.15(Zr-9M)[31] -200 0.1

| 1

- - ~ .... ! 10

O0

ao [ppm] Fig. 18. Relationship between electromotive force (EbtF) and oxygen a c t i v i t y at various P. values.

590 deviations from the theoretical dependence at very low oxygen activities (below 5ppm). These experimental data indicate that the muUite-based oxygen sensors exhibit much better characteristics at lower oxygen activities than that of zirconia sensors.

5. CONCLUSIONS

It has been documented that mechanical & electrical properties of the mullite phase are in a large extent determined by its grain boundary composition. It has been shown that mullite fabricated from clay-type raw materials involves an intergranular glassy phase precipitate which has negative effect on mechanical properties. Elimination of this glassy phase by either powder processing or by using alumina and silica free of impurities results in substantial improvement of its mechanical properties. Analysis of electrochemical properties of muUite indicates that muUite-based oxygen sensors exhibits much better characteristics than that of sensors based on zirconia. It has been documented that at 1600 ~ muUite exhibits pure ionic conductivity especially at very low oxygen activities. ACKNOWLEDGEMENTS This work was supported by the Ishikawa Prefecture. gratefully acknowledged.

This support is

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