PREPARATION OF Y-DOPED ZIRCONIA TECHNIQUE G

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22I
P R E P A R A T I OONF Y - D O P E DZ I R C O N I A
BY EMULSION
TECHNIQUE
G . R i n na n d H . S c h m i d t
Fraunhofer-lnstitut
fÜrSilicatforschung,
Neunerplau2, D-8700Würzburq,Federal
R e p u b l i co f G e r m a n y
ABSTRACT
sphericar
Y-doped zirconia
powders have been prepared from
water
in oiL-emulsions
of acid stabilized
igudou=
so1s.
The precipitation
of hydroxides
inslde
the droplets
was
achieved
by an ion
excl,ange process between the
aqueous
phase and an anion exchangä resin,
using ; phase transfer
catalyst
as a carrier
for the .nion= ttirougn
the organic
solvent.
A.zeotropic
distillation
of the
water
caused a
densification
to
sphericar
amorphous particres.
Filtra_
tion,
dry-ing and cälcination
reä to irr.
formation
of
a
tetragonal
zirconia
powder.
rsostaticatly
pressed
p
o
w
d
e
r
compacts sintered at 1650 .c to theoretlcär
aenslty.^
r NTRODUCTI OII
ceramic powders with definite
particle
size distributions
and shapes are of increasing
iiterest
for the preparation
of high perfornance
ceramics-.
Many procedures
for the preparation
of fine
sphericar
powders with a narro$/ size-aistribution
h";1"är,
descrited,
e. q. by the synthesis
f rom solution
( l_3 ) or f rorn gas
.
phases
( 4 ) ; but
it
is very
difficult
to achieve
defect
free
microstructures
from ionosized
poräLr=
( 5 ) . serious
disadvantages
of some of these processes
are their
low
yields
because of extremely
low concentrations
of the precursor
in the gas o.
phase , the expens ive ,u"' ,u. I iquld
terials
{e. g. arkoxides- in ärguni"
"orv"nts1
and difficurties
in the preparation
of rülti"orp""""t
Fystems
The use of an emursion tec.hnigue with an aqeous phase
al_
lows the introduction
of inorlanic
salts
or sols,
armost
independent on composition
solid
particles
can be forrned from emulsions by the rapid
evaporation
of the water from salt
sorutions
(6-7) or the
precipitation
of hydroxidesr
€. g. by bubbling
g.=Lor= ammonia through the reaction
rnediurn (B) . nut in this
cases
the thermal. decomposition
of the meLal- o-r ammoniumsarts
in tl"
particles
c-us"= a large weight fo=-= ana a disintegration
of.the
spheres or larie
intiagr;;;lu,
pores.
An extraction
of acids
or .n-ion= of acid salts
f rom the
aqueous phase can be performed by the
additlon
of oil-soLuble amines to the emul.sion (g); lut
in a batch process a
Iarge excess of amines is n""Lr=ury
to achleve a^ complete
extraction.
Furthermore
the
emptoyed weak bases
cannot
transform
neutral
sarts to the correspondi4g
hydroxides.
2 2 2-
route to avoid these disad.vantages seems to be
A possible
the use of oil
soluble
strong bases, €. g. quaternary
alk y l a rn mo n i u m h y d ro x i d e d ' .
A l kyl anunoni um-cati ons
are used as
phase transfer
catalysts
chemistry;
in organic
they
are
able to carry
aniones
through an organic
solvent
and to
exchange them at a oil-water
phase boundary. Therefore
it
should be possible
to use a phase transfer
catalyst
as a
carrier'for
anions between an anion exchange reiin
as a
base reservoir
and the emulsified
aqueous phase. Consequently
catalytic
concentrations
compound should
of this
be sufficient
to achieve a cornplete exchange of the anions
and the precipitation
of the hydroxides.
The airn of Lhis investigations
$/as to work out an experiprocedure
mental
for
the preparation
of cerarnic powders
and to test the process with y-doped zirconia
as a moder.
EXPERIME}{TAL
Agueous zirconia
sors were prepared by the acid hydrolysis
of zirconiurnpropoxide
for these first
investigations,
but
preparation
their
from aqeous solutions
of zirconiunsalts
is possible, too.
Log.2 g of a solution
of zirconium-n-propoxide
in 2-propanol (75 wtt) were dissolved
in ethanol and a nixture
of
22 ml nitric
(65 wtt) and 25 rnl rvater was added. The
acid
morar ratio
of zirconium
was about 1. Double
to nitrate
evaporation
of the solvents at 60 'c and redissolving
of
the residue in water resulted
aqueous zirc-onia
in a clear
sor which is stable
for severar days. The use of nitric
acid as a peptizing
agent enables the preparation
of sors
with a concentration
of up to 50 wtt rvith respect to zir-conia. 5.76 q yttriumnitrate-hexahydrate
were dissolved in
water and added to the zirconia
sol to achieve tetragonal
zirconia,
stabilized
with 3 mol-* yttria
as the final
product.
The totat
vorume of the precursor
was adjusted
to
100 mI by the addition
of water.
The aqueous sor was emul-sified in petroreumether
(boiling
range 50
7a 'c),
using Emulsogen oc (Fa. Hoechst) as an
emulsifier.
The resulting
droplet size distribution
was in
the range of about 0.5
5 pm.
1.5 q didodecyldimethylarnmoniumbromide
were dissolved
in
10 ml l-octanor
and added to the emulsion.
After
stirring
fo! a few minutes,
the emulsion lras put in an ion exchangä
column, f ill-ed with 5oo g Dowex 1x8 (Fa. Dow chernical), a
strongry
basic ion exchange resin in the basic form. The
completion of the exchange process v/as controlled
with pHand nitrate-indicatorsticks.
According to the flow rate of
50
200 rnJ-/min a singre or doubre passage of the emulsion
through the column v/as sufficient
to achieve an approximately complete exchange (pH 7
B,
The removal
of
the
vater
resurttrng
f rom the
hydroxide
droplets
was performed by azeotropic
distillatlon
and its
separation
with
a Dean Stark moiÄture trap.
The addition
of L0 nl l-butanol
increased the concentration
of water in
the gaseous phase from about 5
20 wtt.
223
The water-f:ee
hydroxide
particles
were filtered
off,
re_
dispersed
in petioreurnethär,
firtered
again, dried. at
'
c
10o
and calcined at different
temperatures for z h.
The resurting
powders were exanined.j using inirared
spectroscopy,
thernal anarysi:,-.X;ra|
diffraction
äna =".nning
erectron
microscopy.
rn addition
the specific
surface
are_
as and the densities
of green and sintäreä-""rp""t=
were
investigated
in relation
to different
calcination
temperatures.
R E SU L T S A} { D D T SC U S SION
The data cescribed
in
part show the pos_
!h" experinentar
sibility
tc prepare
zirco,niä. f""a"rr
wi_tn the
proposed
techniquestandärd characterizätions
v/ere carried
out to
demonstrate the processing
properties
of the powders.
Infrared
spectroscopy
and therrnal analysis
The processing of the inorganic
zirconia
sor was expected
to led to a powder almost free of organics
and sar-ts and
therefore
a srnarl weight
loss
duriig
".t.i'ation.
The
therrnar anarysis_ (Fig.
1) shows a tolal
weight
loss
of
about 22 wtt and inrrareä
spectra
inai.at"J_in"
presence
of organlc compounds (peaks ät 2900
3ooo cm-r lqd 12oo
1700 cm ^) and some nitrates
(peak at 1380 cm -) up to
'
C
.
250
The emursifier
.ld th-" phase transfer
catalyst
are surface
active
agents and prolally
=tr"ngry adsorbed at the zirco_
TG
0
5 1 0
a
q
o
tr
c
820
.q)9
o
o
x
o
=
DTA
JO
II
tr
tr
o
4
o
I
'g
40
ü
0
c
o
200
400
600
800
10 0 0
T e m p e r o t u r e( " C )
Fig'
1:
Thermar anarysis
of
the zirconia
powder.
I 200
224
are not
so that
s.i.nple washingr processes
nia particles,
cornplete elimination.
for their
sufficient
and w ei ght
e x o thermal
l oss
at
reacti on
T h e s p o n ta n e o u s
of the
to the decomposition
about 24O 'c can be related
peak
a l k y l a rn mo n i umsal t
and the broad exothermal
a d s o rb e d
of the emul450 'C to the degradation
in the range 3oO
any weight
peak without
The small exothernal
loss
sifier.
of zirconia,
the crystallization
as
at 4 5o 'c indicates
shown later
from X-ray data. At 500 'C the powder was aland no reand hydroxides
nitrates
most free of organics,
'C.
markable weight loss occured up to 1200
X-ray
diffraction
pattern
(F,ig. 2l show an amorphous mal
X-ray diffraction
after
drying
at I0O 'c and tetragonal
zirconia
teriaL
at
very small or distorted
500 'C. The broad peaks indicate
crystals;
heating resulted
in a sharpening and infurther
of the peaks, but no monoclinic
creasing
intensities
zi-rconia occured.
In general pure amorphous zirconia
from gels or hydroxides
crystallizes
metastable
first
high temin the tetragonal
perature
the stable
monoclinic
form, but transforms
into
form below 1000 'C. The lack of phase transforrnation
in
case demonstrates
this
of the
a homogeneous distribution
yttria
in the zirconia,
of the
causing the stabilization
te tra g o n a l
fo rm a t room temperature.
A
'ä
a l
I
c l
o l
- l J l
c l
I
40
2 0 (D"grees)
rig.
2z X-ray diffraction
zirconia
powders
pattern
of
dried
and carcined
225
Scanning
electron
nicroscopy
powder show
of the dried
micrographs
electron
Scanning
of about
a size distribution
spheres with
almost perfect
up to 850 'c led
4). The calcination
3 trü.(Fig. 3
0.3
6). Even in
5
to a linear
shrinkage of about 50 * (fig.
coul d be detected.
Thi s
s a m p l e n o h a rd aggl ornerates
th i s
and the
of the emulsifiers
is attributed
to the adsorption
as desurfaces,
at the particle
phase transfer
catalyst
at temperaEspeciatly
spectroscopy.
by infrared
tected
(< 400 'C) ,
where OH-groups on surfaces disappear
tures,
by conpowders tend to form strong agglomerates
sol-gel
molecules
processes.
of organic
The adsorption
densation
as the reason for the lack of agglomeration
is considered
formation
the
of
this
case.
Sorne f ragments indicate
in
and demonspheres only
frorn very large droplets
hollow
maximum
a smaller
emulsions with
the needs for
strates
growth oca crystal
size.
temperatures
droplet
At higher
surfaces
of the particle
cured and caused a substructure
of some agglomerates.
and the formation
Fig.
3
4:
SEl4-micrographs of the dried
Fig.
5
6:
SEM-micrographs
after
zirconia
calcination
at
powder.
850
'C.
226 -
BET-surface
area,
densification
and sintering
High- green densitiesrof
powder compacts require
the processing
of powders ffee or internai
pores ana hard aglromerates.
Therefore
the specific
surfate
areas after
cäicination
in the range 650
L05o .c were determined.
rne
BE T -s u rfa c e s
.c and
-o f th e pogders decreaged betr+ een 650
'
c
850
frorn about 4.6 m'/g to 1.9 m"/g and remained arrn"=i
constant
at higher temperätures
(Fig. 7) .
6.0
Ol
c{
E 5.0
o
tu
o
o
o
o
L
4.0
J.0
E
:l
v,
2.O
t
I
LLI
m
r?
E
o
1.0
3.6
r-r
BET surfoca oreo
l s o s to ti c pressi ng: S 00 Mpo
N o o d d i ti ves
T, 59
gl
\
'a
J.4
c
o
-o
c
o
J.2
q,
r_
(,
J.0
H
Grean density (geomatric
io
6.2
E
o
(tl
6.0
'ö
4J
c
o
T'
5.8
'lJ
C)
L.
c)
sq
5.6
Sinteringtemperoture:1650 "C
Heoting rote:
2OOK/h
Tmo of T.o*:
I h
r-r
5.4
600
Sintercd dansity (pycnomohic
| 100
Colcinotiontemperoture(.C)
Fig.
7 z BET-surface
areas, green and sintered
densities
a function
of calcination
temperatures.
as
227
p re s s i n g
rs o s ta ti c
o f the pow ders at 5oo H pa w i thout
any
a d d i ti v e s
a n d p ro c e s s
opti ni zati on
in
cpnpacti
' 3 .resurted
2
green densities
with
in the range
(' w
f i ig€rr
.
3.5 g/ca.'
ü easl ct i n edde n s i ty ' eS O
7 ) . T h e h i g-c
(s7 t th.
d. ) w as a" f,i " t' " a
'C.
th e p o w d e r
at
The sintered
densities
of the powders calcined
at diffe'l
'
te n p e ra tu re s
re n t
i s show n i n ri g.
, too . onry- the 8 50 c
powder resulted
calcined
in a sintered
body with
Loo *
th e o re ti c a l
d e n s i ty .
P ow ders cal ci ned
at l ow -er ternperature s l e d to l o w e r g re e n densi ti es
by the standard, dry presp ro c e s s
sing
a n d a s a conseq[uence to remai ni ng
pöräs i n
sintered
the
body.
calcination
above B50 'c caused, the
forznation of aggregates,
which, since no nrirring
vas perw e re tra n s fe rred
fo rm e d ,
the gE een body,
i nto
l eadi ng
to
lower densities
in the sintered
body, too.
High
te m p e ra tu re
d i l atonetry
The sintering
behavior
of the porvder calcined
at g5o . c
was investigated
(Fig. g). A powder compact
by dilatoruetry
with 55 * theoretiäal
density i'ui annealed tä 15so .c with
'
C
/
rnin. Sintering
a heating
rate of l"
started. between LOOO
and 1050 'c and, up to t-s8o 'c (the end point of the dilatometer),
the compact densified
to 97 t theoretical
density .
T h i s c o n f j .rrn s th e observed
f ornati on
of aggl onerates
after
calcination
at lo5o 'c, because it is just
the temperature,
where the sintering
started.
The very broad sintering
interval
demonstrates
on the one side tlre high sintering
activity
of the powder, but on the other
siäe also
the needs for smarrer maximum particle
sizes,
to achieve
theoreticar
densities
of compacts at lower temperatures.
l,^.
l b 3
l -
p - J.J7 9/cm} (ss z th. d.)
l =
t:
-5
ls
I E
-10
C o l c i n o t i o nt o m p a r o t u r o : 8 5 0 ' C
l s o a t o U cp r o s s l n g :
Heoting roto:
500 MPo
1 'C/mln
l H
-15
v5
-20
Temperoture ("C)
Fig - I : High-temperature
dilatometry
of a powder compact
228
CONCLUS
rO N S
The described
comrrined emursion- and ion-exchange_process
for the synthesis
of äphericar
cerarnic powders offers
some
advantages in reration
to other-p..p"r.Lio'
iecr,nigues:
rt is possibre to use aqueous
sors and/or inorganic
salts as precursors
for a sor-ger-rike'pr""ä==
within
droplets.
Multicomoonent
systems can be realized
by the use of
mixture=
s
o
f
u
üf"-p...u.sors.
?f
Droplet sizes and aistriuutions
can be defined by mecha_
nical and chemical parameters;
e. g..shear rate and the
properties
of the sorvent and the emursifier.
The powders show a rerativ"
rriöi-r-sintering
activity
without
intense milli.g.
R E F ER EN C E S
1'
2'
3'
4'
5.
6-
7'
8'
9.
Matijevic,
E-: Monodispersed metar (hydrous)
oxides.
A fascinating
field
of colloid
science.
A c c . c h e m . R e s . 1 4 ( 1 e B1 ) i z _ i g - .
Fegley, B- and nairinger,
E.A.: synthesis,
characteri_
zation and processing of monosized
ceramic powders.
Mat. Res. Soc. proc. 32 (1984)
187_Lgl.
Rinn, c- and. schnidt, H., nSepa.ition'är
monodispersed,
zirconia
powders forrn solutioir.
rn: Messirg, G.L.,
Futler,
E.R. and Hausner, H.,
powder
1eas.'i,";;;;;i"
science rr.
cerarnic Transaetions r, Amer. cer.
soc.,
Westervil"le (1988) 23_3O.
schulz, o- and Haüsner, H.:
Gas-phase synthesis of ce_
ramic powders.
This volume
Brook, R. J.:
powder design
for sintering.
This volume
Reynen, p., Batius, H. and
Fiedler,
I,{.: The use of
emulsions in
preparation
of ceramic powders.
!
h
"
rn: Vincenz
i r nl 1äa.1 : cerarnic powders, Elsevier,
A m s t e r d a m ( 1i l 9
83) agd_soq.
Richardson,
K- and Akinc, M.: preparation
of sphericar
yttriurn
oxide powders_n=i-rg .*rt-rio.,
";;;";;tion.
ceraroics rnrernarionar
13
253-26r.
ii
K a n a i , T - , R h i n e , w . E - a n a i' rnöoäw
en,
preparation of
2Zro- Y^o. by emulsion technigue. H.K.:
I
n
:
M
essing, G.L.,
Full6.,tEinand iu,-,=.,"r, H.
(eds. ) : ceramic powder
' s c i e n c e r r - c e r a m i c T r a n s a c t i ,o n s
r, Amer. cer. soc.,
Westerville
(1988) 119_126.
Cima, !t.J. , Chiu,'R,
Rhine,
: BaryCu_,o"
pow_
der preparation
a
ü v s o 3r -ngde l " ^ " i = i ol .nl . E . i " ; ; R i ö ö s r .
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99 irgse) z4L_244.
Science
CeramicPowder Processing
Proceedingsof the
SecondInternationalConference
Berchtesgaden(Bavaria) FRG
O c t o b e r 1 2 - 1 4 ,1 9 8 8
E d i t e db y
H. Hausner
Technische
UniversitätBerlin, FRG
G.L. Messing
StateUniversity,USA
The Pennsylvania
S. Hirano
Nagoya lJniversity,Japan
Deutsche KeramischeGesellschaft
rsBN 3-925543-03-1
O D e u t s c h eK e r a m i s c h eG e s e l l s c h a fet . V . , 5 0 0 0K ö l n 9 0
Druck: OffsetdruckH. Krannich, 5465Erp.l
| 49
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