Formation of stabilized zirconia with rare earth fluorides

Journal of Fluorine Chemistry, 40 (1988)

FommIo~

0F

375-385

375

ZIRCONIA WITH RARE'EARTH FLUORIDES*

STABILIZED

Masayuki TAKASHIMA and Gentaro KANO

Department of Industrial Chemistry, Faculty of Engineering, Fukui University, 3-9-l Bunkyo, Fukui-shi, 910 (Japan)

SUMMARY

New

reactions

to

prepare stabilized zirconia using rare

earth fluorides as the solid electrolyte

have been examined by

means of X-ray diffraction, DTA and EPMA methods.

The

rare earth fluorides of yttrium and samarium through reacted with zrO2 to form new (LnFSZ)consisting of (2x+3y)Zr02

the

of stabilized

types

ternary

system

eleven lutetium zirconias

of zr02-Ln203-LnF3.

+ (4y+2z)LnF3 = 2(zro2)x(Ln203)y(LnF3)s + 3~zrF4

where x, y and z represent the stabilizing composition at which the homogeneous

solid solution with the fluorite structure is

formed, and x + y + z = 1.

This reaction begins to take place

at about 600 C and is completed

by firing at temperatures

ranging from 1000 to 1300 OC for a few hours in an argon atmosphere. INTRODUCTION Zirconium

oxide,

zr02,

is

one

of the most

interesting

materials in technology. So-called stabilized ceramic zirconia is well-known as the oxide ion conducting solid elecused for fuel cells trolyte (II and oxygen sensors[2], and *

also

has

attracted

special

interest -recently

in

Dedicated to Emeritus Professor W.K.R.Musgrave on the occasion of his 70th birthday.

0022-1139/88/$3.50

0 Elsevier Sequoia/Printed in The Netherlands

376

zirconium-toughened phase it

changes

from

Therefore, or rare

and

earth

earth

So,we

wish

oxide

and

above

studying

fluorides

fluoride

the solid

earth

earth

react

between

examined

analysis

and

eleven

atmosphere,

structure,

reaction

rare

or

with

that

zirconias,LnFSZ,are phase

fluorides

thermal

reaction

in an argon

the cubic

stabilized

to report

differential

with

earth

[5].

phase

could

at up to 13OO'C

[4].

alkaline

alkaline

of

a

4% when

tetragonal

with

1600°C

found that zirconia

exhibits

of about

the

the solid

solutions

rare

to

stabilized

by firing

fluorides

zirconia

contraction

monoclinic

various and

and that solid

Pure

[3].

a volume

is usually

been

elements

rare

the

oxides

have

zirconia earth

with

zirconia

earth

We

rare

ceramics

transformation

formed.

between

by X-ray

is, the

zirconium

diffraction,

electron

probe

micro

analysis.

RESULTS

AND DISCUSSION

Fig.1

shows

systems, endothermic From X-ray marked

stabilized

steeply

even

ionic

at 1400

about

this

radii

than

crystal crystal

size.

of the fluorite

type

that the

formation

or rare earth

For rare

for the

earth

ionic

is surrounded

zirconia

the high

[4].

the

formation earth

ions

0.92 i and

elements does

not

with proceed

point

site is

is to be little

in

peaks of

is at

size of the cation

the octa-coordinate

1.27 i

OC.

fluorides.In

that the break

the possible

three

due to the transformat-

1.0 i , this reaction

The site is

were

or

1400

the tripositive rare radius of Ln 3+ up to about

0.93 i if there

radius

the

temperatures

OC. It is reasonable

structure.

Two up to

confirmed

with

of Zr02-Lx-G3

Lu.

against

the crystal

substitutionally

to be about

wa6

of zirconia

the DTA peak

0.92 i., because

occupy

it

and the others

zirconias

over

through

in each DTA curve

data,

structure

are plotted

for the reaction Sm

corresponded

zirconia

by increasing

Y and

appeared

arrows

of stabilized

larger

are

diffraction

ion of crystal

more

Ln

peaks

by

Fig.2

the DTA curves

where

calculated

disordering

with

to

of the

oxide

ions whose

temperature

structure

Temperature

('C)

1300

1200

1000

1100

v

w

Zr02-YbF3

P

f

I

\

V-Y

V

v

I

ZrOq-TmF34p.

ZrO2-ErF37;

Zr02-YF3

ZrOZ-DyF3

ZrOq-TbF3

ZrO2-GdF3

ZrO2-EuF3

Zr02-SmF3

1400

II

(Heating rate; 15'C/min, In argon stream)

Fig.l.DTA curves for the reaction between Zr02 and various LnF3

r

0.80

1000 0.85

Ln3+ radius

0.90

(i)

0.95

Fig.2.Relationship between the DTA peak temperature and Ln3+ radius.

9 PI

E 1 < 1200 1 2 2 y 1100

E; 0 1300

1400

1.00

378

During rare earth

the

solid

fluorides

phase. reaction of

Y

between

and Sm through Lu,

zirconia and the weight of

sample mixture decreased about 15 wt% at the most. Fig. 3 shows the change in weight

for zirconia-YF3,

-EuF3 and -HoF3,after Any systems

firing for three hours at various temperatures. begin to decrease "C, the weight

in weight at about 600°C , and at around 900

decrease

becomes

great ,and then , above about

1100 'C, it reaches almost the limit decrease of about 15 wt%. The

plot

of

the

weight

decrease

s

time reveals that the

greater part of the weight decrease is over within

a

half

hour, as shown in Fig. 4.

600

800

1000

1200

1300

Reaction temperature ('C) Fig.3.Relationship between the weight change and the reaction temperature for (Zr02)0_75(LnF3)0.25 Reaction time; 3 h, In argon stream O:Zr02-EuF3, l :ZrO2-HoF3, @::r02-YF3,

an

379

L 0

I

I

I

I

I

1

2

3

4

5

Reaction time (h) Fig.4.Weight decrease-time curves for the reaction of Zr02-LnF3 systems at 12OO'C in an argon stream

l :ZrO2-HoF9,

0 :ZrO2-EuF9,

Fig.5

shows

the

X-ray

@:Zr02-YF3.

diffraction

powder

patterns

products for zirconia-holmium fluoride mixtures HoF3 which were prepared by firing at various three hours in an argon atmosphere.

of 25

with

the mol%

temperatures for

In the pattern for 500 "C,

all diffraction lines are due to only the starting materials of the monoclinic

zirconia and hexagonal

is, at this temperature

no reaction

holmium

fluoride, that

is found to take place.

At 700 OC, the diffraction lines of HoF3 have disappeared

and

some new lines begin to appear. These are identified to be

due

to the holmium fluoride oxide, HoFO. In the pattern for 900 'C, the lines assigned to the cubic-zirconia

are

dominant.

The

pattern for 1100 'C shows that this product is the single phase of the cubic with the crystal lattice constant a,:5.16 i.

HOW-

ever, even at 1100

tube

'C,

in case of using a sealed alumina

for a reaction vessel, as the bottom profile in Fig. 5

shows,

the product is not a single phase but consists of three

phases

of the cubic zirconia and two of starting materials.

This fact

suggests that the solid phase reaction cannot be driven to completion if the volatile products cannot escape from the tion system.

reac-

380

Open boat w/ i.

Seal tubing

lloo"c

30

40

50

60

Diffraction angle (29'), CuKs Fig.5.X-Ray diffraction patterns of products from (Zr02)Q.75(HoF3)Q.25 at various temperatures; 0: Monoclinic Zr02, 0: Rhombohedral HoFO,

Fig.6

shows

the

0: Hexagonal HoF3, 0: Cubic zirconia.

change

in weight

plotted

against

content of LnF3 in a starting mixture in the case of at 12QO'C for three hours in an argon stream.

the

reaction

The dotted area

shows the contents of LnF3, at which the homogeneous

phase of

cubic zirconia is obtained. The weight decreases almost linearly with increasing LnF3 content and the weight change is found to stop at around the content of fluoride rich end of the range in which the homogeneous cubic phase is formed. The limit of the weight decrease is between 13 and 15 wt%, and seems depend to a limited extent on the species of rare earth.

to

381

Monoclinic Cubic ZrO24

0

10

20

Cubic / / L

30

Content of LnF3

Cubic + LnF3

40

50

(mol%)

Fig.6.Relationship between weight change and the content of LnF in a start mixture (Reaction at 1a00°C for 3 h); 0: EuF3, 0: HoF3, c): YF3.

Next,we

attempted

to analyze

the products

by electron

probe micro analysis. Fig.7 is the spectrum of the sublimate trapped in a filter placed at the outlet of the argon line and the

solid

product remaining in a alumina boat for a Zr02-HoF3

hours. reaction at llOO‘°C for three after system sublimate consists of aluminum, zirconium and fluorine.

The On

the other hand, the components of the solid product are zircopresent in the nium, holmium, fluorine and oxygen. Aluminium sublimate sublimes

is assumed to come from aluminum fluoride which above about 700 'C [71, resulting from the reaction

between zirconium tetrafluoride and the sample boat or reaction tubing made of aluminum oxide.

382

Sublimate

20.0

15.0

10.0 . Ho-La

I

I

I

10.0

15.0

20.0

Wave

length

(A)

spectrum of the sublimate and the Fig.7.E.P.M.A. cubic product for (ZrO2)0,75(HoF3)0.25.

Putting

together

the above

phase

reaction

process

earth

fluoride

begin

and

zirconium

fluoride anionic zirconia

oxide

(LnF-SZ)

rare

place

earth

fluoride

goes

the reaction

the

zirconium

sublimes

tetrafluoride

estimated

the

Zirconia

8.

with

each

rare

earth

are formed.

which

from

and

takes

tetrafluoride, out

in Fig.

to react

tetrafluoride

diffusion and

as shown

results,we

other

system. is formed,

or rare

earth

that mutual

interface

,and at that above

rare 6OO'C

means

This

easily

and

at about

oxide

on the solid

600

Above

solid

time

between zirconium

'C,is formed

900°C,

it sublimes

as soon out

and as

, and

383

many

vacant

crystal tive

sites

lattice

rare earth

and can readily stabilized

of tetrapositive

ions begin occupy

zirconia

as the ternary

ion are born

with

system

sites.

the cubic

consisting

y

Consequently,

structure

of ZrO2,

is easily

Ln203

800% yLn203

+ 1.5yZrF4

+ zLnF3

r---7 ZrF4 1

_!

900%

L---’

Subl.

Y Cubic-(Zr02)x(Ln203)y(LnF3)z LnF-SZ ZrF4

Solid

interface Ln3+

Ln3+ Zr4+

Zr4+

I

Ln3+F-

02Zr4+02-

02;r4+02-

F‘Zr4+p2-

02-

02-

02-

02-

02-

ZrO2 lattice

phase

reaction

F-

FLn3+

.+LnF3

process

between

the created

and LnF3.

LnFO

Fig.8.Solid LnF3.

in a

On the other hand, the triposito migrate into the zirconia lattice

the vacant

600 xZr02

zirconium

of zirconia.

Zr02 and

384

In Table and

the

decrease

during

conducting found

1 are summarized

composition

they

Details

lytes.

subsequent

TABLE

reaction

properties

that

the optimum

of products for

three

of LnF-SZs

function

as oxide

of their

reaction

calculated hours.

have

been

the

The

electrical

weight

examined,

ion conducting

characteristics

temperature

from

will

and

solid

we

electro-

be reported

in

a

paper.

1

Optimum

reaction

Reaction

condition

and the composition

Composition

mixture

Reaction temp. -time LnFS(mol%)('C) (h)

System

of LnF-SZ

of LnF-SZ

(Zr02)x(Ln203)y(LnF3)z

Zr02-SmF3

25

1250

3

0.80

0.13

0.07

ZrOZ-EuFS

25

1200

3

0.79

0.10

0.11

ZrOZ-GdFS

25

1200

3

0.78

0.09

0.13

ZrOZ-TbFS

25

1200

3

0.78

0.10

0.12

ZrOZ-DyFS

25

1200

3

0.78

0.10

0.12

ZrOZ-HoFS

25

1200

3

0.78

0.10

0.12

Zr02-YF3

25

1200

3

0.78

0.09

0.13

ZrOZ-ErF9

25

1200

3

0.78

0.09

0.13

Zr02-TmFS

25

1200

3

0.79

0.10

0.11

Zr02-YbFS

25

1100

3

0.78

0.10

0.12

Zr02-LuF9

25

1100

3

0.78

0.10

0.12

EXPERIMENTAL

The rare earth cially

obtained

Fluorides (passed with of

and

and

were

zirconium

400 mesh

each other the

fluorides

mixture

sieve)

and zirconium

guaranteed oxides,

in the appropriate were

which

and thoroughly

oxide

were

to be 99.9-99.99 were dried,

finely were

% pure. powdered

fully

compositions.About

put into an alumina

commer-

boat made

mixed

two grams of

99.5%

385

pure A1203, and heated at a temperature 3 h in an argon temperature, diffraction

atmosphere.

the products , electron-probe

thermal analysis.

After were

from 200to1400 cooling

analyzed

slowly

by means

microanalysis

OC for to room

of X-ray

and differential

The details of these analysis methods were

according to our previous reports [81.

ACKNOWLEDGMENTS Acknowledgment

is made to the Iwatani Naoji Foundation's

Research Grant for support of this research.

We are grateful

to Prof. N Watanabe for helpful advice and to Mr. H. Uno for Xray analysis.

REFERENCES T. Takahashi, in J. Hladik (ed.) 'Physics of Electrolytes',Vol.2, Academic Press, London (1972) 989. K.S. Goto and W. Pluschkell, in J. Hladik (ed.) 'Physics of Electrolytes', Vo1.2, Academic Press, London (1972) 539. R.C.Garvie, R.H. Hannink and R.T. Pascoe, Nature (London), 258, (1975) 703. E.M. Levin, C.R. Robbins, H.F.McMurdie and M.K. Reser Ed., 'Phase Diagrams for Ceramists', Am. Chem. Sot. (1964). A.F. Wells, 'Structural Inorganic Chemistry', Clarendon Press, Oxford (1975) 448. T. Moeller, in F.H. Spedding and A.H. Daane (eds.), 'The Rare Earths', Krieger, New York (1971) 9. W. Klemm and E. Voss, Z. anorg. allgem. Chem., 251 (1943) 233. M. Takashima, G. Kano and H. Konishi, Nippon Kagaku Kaishi, m,1896.