Electrical conductivity study of defects in zirconium doped
yttrium aluminium garnet ceramics
Citation for published version (APA):
Schuh, L. H., Metselaar, R., & With, de, G. (1988). Electrical conductivity study of defects in zirconium doped yttrium aluminium garnet ceramics. In D. Taylor (Ed.), Science of ceramics : proceedings of the ... international conference, 14th, Canterbury, England, 7-9 September, 1987 (pp. 973-978). (Science of Ceramics; Vol. 14). The Institute of Ceramics.
Document status and date: Published: 01/01/1988
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ELECTRICAL CONDUCTIVITY STUDY OF DEFECTS IN ZIRCONIUM DOPED YTTRIUM ALUMINIUM GARNET CERAMICS
L. Schuh, R. Metselaar, G. de With
fU..gh t:empVla.:tUlte. me.a.-ÓuJteme.n.t:-ó
06
c.ondtic.tivUy a-ó 6(T}06
YjAtS012 (YAG) dop~ "."-lUh 50 ppm ZJt .i..n. CO/C02 and H2/H20 ga-ó m.ut:UIt~-ó -óhow a -ó-i.gYLL6Lc.aYLt: cü..6tVlenc.e
.i..n. t:he. ac.tiva.:tLon €J'tVlgy (3.54 Jte-óp. 7.97 eV) .a.:t e.on-ót:aYLt: oxygen paJr.ti.al PJtUl-6ul!-e.
The. -ó-f..gl'l. o~ t:he Se.ebec.k c.oe66Lc.J...e.n.t: .i..n.dLe.a.:te-ó t:ha.:t a H2/H 20 a.:tmo-óphVle. .ur.t:JtOdUe.Ul pJtot:on-ó a-6 c.o~duc.t:.i..n.g -ópec.J...e..6 J...nt:o t:he. gaJtnd tat:Uc.e. ' lmpedanc.e -ópe.áJtO.&c.opy
Jteve.a.t~ t:ha.:t t:hJ...-6 1...-6 a bulk e6~e.c.t:, wille t:he gJta.f..n. boundaJt+e.-6 aJte n~Jtty un-, . af.le.c.t:~ by ambLe.n.t: atmo-óphVle. IoYLLe. and e.te.c.t:JtonJ...c. e.onduc.tJ...vUy a-ó 6(p02} -óhow
t:ha.:t LoYLLc. c.onduc.:tJ...vUy dom.i..n.a.:te.-ó .i..n. t:he,. hLgh p02 1té.gJ...me;:i'whll.'e a.:t tow p02
e.te.c.-t:Jton.J...c. c.onduc.t:LvUy .i..n.c.!te.a.-óe.-ó wUh pO;l/S.i..n.cü..e.a:t.i,Jr.g t:ha.:t eUhVl V·· and V· oJt
de.le.c.t: C.tu-ót:Vl-ó appeaJt a-ó pJtevaJ...tJ...ng LonJ...c. de6ect:-ó. 0 o ·
.'.
INTRODUCTION
Aluminium oxide ceraroics are well
stu~ied
and have many applications. Thism~leri-1 .
rial can be sintered to translucency, usually with MgO as dopant. Recently it was shown that Y
3Al5012 (yttrium aluminium garnet or YAG) can be sintered to a quite
bet ter translucency with Si0
2 or MgO as sintering
additi~.e.
than Al203~
Moreov'er it is known that YAG has a higher resi~ta~ce against aggressive environments (viz. hot alkali vapour) than A12o.
3
~
Thecom~ination
of the;epr~~erties
make YÁGcera-mic a candidate material, that may be used to produce"high durability envelopes
.,'
for sodium discharg~ lamps.
Till now the mechanical, microstructural, and optical properties of YAG cera-mies we re investigated to some extent4
-T
The defect chemistry is, however, also- ,
quite important for the understanding of sintering and corrosion beHavlour. Some r
authors determined the high-temperature defect properties of single crystal
Y Al 03 5 12 8-11,ecauseb th'~s mat · 1 .er~a ~s an ~mpor. tant 501 . d~ St ta e 1as er host crys at 1 (Nd-YAG laser). All investigators came to the resul t, tha·t YAG is a mixed ionic-electronic conductor at high temperatures. The domihating- ionic-defects areoxy-gen-vacancies'. The subject of 'this pa.per- is to give sbme inforrnatioÎ'l about high-" temperature defect properties of zirconià~dopedpolycrystalline YAG-ceramics.
EXPERIMENTAL Materials
In this work we present an lnvestigation of YAG ceramic doped with 55 wt.-ppm Zr. The YAG powder was prepared by using the wet-chemical route (sulphate process) as described in ref. 7. Doping with zirconium was done by adding a proper amount of the nitrate. The samples were sintered at 2020 K for 4 hours in a wet H2 -atmo-sphere. All samples reached a relative density higher than 98%. Microstructure of various materials was deterrnined by SEM/EDX. The grainsize ranged from 4 - 7 ~.
SCIENCE OF CERAMICS 14
Zirconium-ions have a relative high solubility in the YAG lattice (up to 700 ppm without any
second,pha~e
segregation 12) • SEM/EDX observ?-tions and X-ray diffrac-tometry further showed that up to 6% Al-rich inclusions (presumably Al203) with 0.5 - 1 ~ diameter are present. The presence of these inclusions was confirmed by TEM
investigations~
MethodÈf"'"
The 'défect-chémicaï investigat10n wàS done by impedance spe'ct·roscopy, iOl11c
trans-~~" ~~
port and Seebeck Cgeffici~~t measurem~nts. Our samplesj.for.el~ctrl~al conduc~ivity measurements'were disc-shaped with a diameter of 10mm and a th1ckn~ss of 0.7mm.
~ ~., ~ . ,~
-The,two faces of a spec~men wer~. poli.sb~d down to 1~ with diamopd paste, cleaned and patnted wl th platÜiuin paste (Emet.ron·· Leitplat.in) .': 4f~ -electrode. surface .was
~;~ ,~' " ' - , . ~ . rq".
kept small (about 3mm in diameter) in order to minimize the contribution of
sur-'.
.tivitywasdetermined as a function of temperature (up to 1700 K) and oxygen
parti-. 5 -14 '
al pressure .p02 (10 -10 Pa). Usually a vbltage of 100mV rros was applied across the sample. Aft~~ a change of pOZ the sy~te~ was equilibrated about 12 hours
be-f~re the cond~ctivi~ywas roeasured. We used H2/H20, CO/C02 and 0Z/N
Z
gas mixtures, 1 : .
to 7stablish a controlled p02~ Every measurement of the frequency dispersion was
che~ked
b;~ deter~i~ation
of thed.c.-resist~nce.
Complete impedancè'plots in theface and air conduction paths.
A Solartron 1174 frequency response analyser (0.1 mHz - 1 MHz) interfaced to a
.t.'~. .
Hewlett Packard HP 86 computer was used for impedance
measurementSh.A.c.-conduc-;
Z'-Z" representation allow to get information about bulk (grain interior) arid
fC\',
grain.boundary conductivity.
The investigation of the Seebeck coefficient and transport number was done in
"rr ""V, ." "13
an appara~us described elsewhere •
RESULTS AND DISCUSSION
Fig., gives the temperature depepdence of the total conductivity measured at different,p02's. Each point was determined by analysing the appertaining Z'-Z" plot. Because (J)·f' the difficulty to separate bulk-CRb) and grain boundary (R
gb) conductivity at all temperatures we plotted (Rb+Rgb) vs. aT. Tha activation ener-gies E
Awere calculated by using the Arrhenius relation: 0' = ao/T .exp(-EA/kT) with a=conductivity, T=temperature, k:Boltzmann-constant.
TABLE 1. E
Afor YAG:Zr in different gas atmospheres
Curve Atmosphere pOZ (Pa) El>.:(eV)
H/H2O 10- 14 1.91
2 CO/C02 10- 11 3.54
3 CO/CO
z
10-7 3.38, ,~{ ," in a YAG specimen +
P02=105:~4(pure
02)to3.5,eV:~
(pOZ=10 Pa) E Ais significantly enter the YAG-lattice 14formingSign of the Seebeck~~;efficient
""
-
":;,'}' I -e ,-!:,.
E ~ ti) .......
~ -7 Ol 0 -NZ/OZ CO/CO Z H Z/H20OH-bonds. So we expect a
contribution~~f
protonicconducti~n
FIG.1 Temperature dependence of the volume conductivity in Zr-doped YAG.Acti-vation energies dërlvéd-from 'curv;e 1-4 are g:iveii Tri table L ,
-e.~1l1---~---1:---J.7 .8 .8
E~for the conduction process varies from 3 eV at
. ";11 . . " "\: ' : pOZ=10 Pa (CO/COzmixture). In a HZ/HZO mixture smaller. Recently it was shown that hydrogen can
1000 KIT
-4r--~"""",,:--- ....
-s
. , . ',_ . ' . ift!.' '. î .. ' ~ 1 •
t,hat iS,equilibrated in a HZ/H
20 atmosphere. In order to clarify this point the
thermop~wer in diff~r~nt gas mixture~ wa~ measured. The sign of the thermopowéi'or , "J Seebeck coefficient gives information about the sign of the dominating mobile charge carriers. Negative charge carriers are dominati~g in NZ/O
Z and CO/COZ mix-tures,while positive carriers are prevailing in H
z/H20. NZ/OZ'and CO/C02
mixtu-red affect the conduction mechanism by'a change in ,poz~ The EA'in both atmoshere~
is a~p;óximatelY ~qual.'A ,HZ/HZO mixture influences t'he conduction mechani'smby'
1n-troducing new positive charge carrier>sü~to'the latt1ce-.This. is confirmed 'by tR-14
spectroscopic detection of OH stretch-bonds aftel' an anneal 1n H
Z-'
TABLE Z. Sign of the Seebeck-coefficient in different gas-mixtures
Fig. Zand 3 give two 105 Pa) at 1650 Kand
impedance spectra of Zr-doped YAG"~asuredin N2/OZ (pOZ= -13
in HZ/HZO (pOZ=10. Pa) at .1450K: Fig.Z shows a wide semi-circle that could ,be extrapolated to ze~e.resistance.This arc is caused by bulk conduct ion . The interse'ction of tha tó'éircle owi th the Zi.:.axis gives the bulk-resis-tance as 360 kil. At frequencies lower taan ·20 k]jz begins a distortion of the arc which is,caused by an influence of t~e grain boundaries. The resistance of the grain boundaries can be estimated as ZO kO • That means that highly resistive YAG
fY16 SCIENCE OF CERAMICS 14
grains are covered by a much better conducting bo~ndary laye~.
1.00
-
•
::I: Cl 10ft Cl -•....
2.00 17t UIl•
",-+- - - ... "..'...
....,
.... .... 'Ot '(HllIl,
,
.
."
,
.
.
\ " •• lUIIz .-\,
.
.
.'",'
-
\~.,-. 1.00 I.OG. Y.QO Z'(105 0RM ) ".00FIG. 2 Impedance.plot for Zr~doped YAG at 1650 K and pOZ =
Fig. 3 gives the impedance plot for the same sort of sample, measured in a,H 2/H20 atmosphere. The relaxation times of the two conduct i on processes are now well se-pa~ated. The bulk resistance can be ~erived as 40.5 kn and the grain boundary re-sistance is 18 kQ. A low frequency dispersion, which is typically observed, is of
r; ,T l~ 1 r
no further interest here.
~,l:"'
. Comparing both spectra one can conclude tbat the grain boundary resistance is
. , .
~ardly influenced, by ambient gas atmosphere (note that fig. 3 is recorded at a
~. f • : .~',
lower temperature than fig. 2 to get well formed arcs). The bulk resistance
r •.~ç '~
ct),a.n~,e~s significaI1tly wi.th the gas mixture. An interpretation of the impedance
15.00 ".00
•
-
•
11: Ct 3.00 oor!!
... 2.00 - - - " ..lIS...
Uz.
1.00 "",.,
.
.
• ".IlUIIi •".Ir
\ ~...
..:.-..
•
',~"'".1,
•
•
•
I,Ul 2.00 3.00 ".00 15.00 1.00 7.00 B.OOla 4 -~-:---+. ". < -6;ç--~t----j:---t--+--+--+---~ ~ ~ ~ ~ ~ 0 2
...
=-...
- J -4-
... I < E Ol ... .!:!. -5 Cl 0 ......
109fp02/Pal
FIG. 4 Total electr.lcal conductivity, ionié transpor:;t numb~r ti' electro,~.~C:.)'ei and ionic qêond~ctiv~ty;in YAG dqped with,55ppm Zr .at 161Q.K.
I0Il
spectra indicates that a H2-containing gas atmosphere changes the bulk propertie~
while leaving the grain boundaries nearly unaffected.
The p02 dependenee of ccnductivity at 1610 K is shown in fig. 4 together with the ionic transport number ti and the ionic and electronic contribution to the total conductivity. Zr-doped YAG shows a red colour after treatment in a reducing atmo-sphere and decolours upon annealing in an oxidizing atmoatmo-sphere.
An interpretation of the isotherms shown in fig. 4 combined with the fact, that the sign of the Seebeck-coefficient is negative at low p02 (except in a H2-90n-taining atmosphere), indicate that a=oel in this p02 "regime. oeIincreasescpo~ .with n=-1/5. In the high oxygen pressure region conductivity is independent of p02. This is the regime of ionic conductivity. Recently it was shown by several workers that the dominating defects in YAG are oxygen vancancies v··8-11. The
o
electronic conduct ion increases with a slope of -1/5. This_dependence_is_inter~
mediate between the intrinsic conduction mechanism with a slope of -1/6 and a simple impurity controlled mechanism exhibiting a slqpe of -1/4. Several workers interpreted an analogous 1/5 power oxygen pressure dependenee in NiO by assuming the simultaneously presence of singly and doubly ionised nickel vacancies16• Taking over this proposition we can interprete the 1/5 slope found in the present work by assuming singly and doubly ionised oxygen vacancies V· ando . ~'••
0
Recently it was shown by computer simulation for NiO, CoO, MnO and FeO that also cluster formation can explain a slope of 1/511•
918 SCIENCE OF CERAMICS 14
FINAL REMARKS
Further investigation of an influence of other doping elements on defect related
'~f-:
propertie$ in YAG may enable us to decide whether V~ and V~' or cluster formation
dominat~s"
.-'''-' REFERENCES
1. PEELEN, J.G.J., Ceramurgj;á Int. 5,70 (1979) and 5, 115:(197'9),
2. WITH,G. de, DIJK,
H.,.r;K.,
Mater: Res. Bull.21,
1699 (1~84):'3. WITH, :G. de, VRUGHT·, J.P., VEN, A.J.C. v.d., J. Mater. Sci.~O, 1215, (1985).
4. MULDE&, C.A.M.; \U:rH, G. de, Solid State Ionics 16, 81 (1985):"
5. WITH,G. de, .PAIÜ~EN, J.E.D., Solid State Ionics,-16, 87' (1985).
6. WITH, iG. :<ie,. Philips J. ·Res. 42, 119 (1987).
-7. TOOLEN;AAR, F.~.C.,WITH, G. de, Proc. Br. Ceram. Soc. 37, 241, (1986).
8. NEIMA~, A.J~iwTKECHENKO, E.V., Z~UKOWSKII, V.M., ~okl. Akad. Nauk. SSSR
240, B7·6( 1918.): -'. ., " _ ,- ~:" . ,~_.-- u'
9. BATEB., J.L., G.A:RNIER, J.E.-, J. Am. Ceram. Soc., 64, C-138 (1981).
10. ROTMAN, S.R., TANDON, R.P., TUL1.-ER~,}a'~L~,-:J. AppT:" Phys. 57, 1951 (1985).
11. ROTMAN, S.R., TULLER, H.L., Mater. Re~. 'Soc. Symp. Proc. 60, 413 (1986).
12. Private Gommunicatlon, J.Vreeswijk. .
l!". , , :,I'r ' ~~-\;~'" j ,- .• (.". ' r " ' ' 1 ,.,.", ~..., , . . - ; :" , ' J ,~ï
13; HOgPSLOOT,'
A.
M., THIJSSEN ,P. H. F., METSELÁAR, R., Silic. Ind. 3~4, 35 (1985).14. DEVOR ID. P., PASTORi R.e.,· DeSHAZÉR I 1:.
.-'G.
IJ. Chèm.·Pnys:
!l,
41(~_~, (1984).15. CHOI, S.C., KOOMOTO, K., YANAGIDA, H., J. Mater. Sci. 21, 1947 (19'86).
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:fr I ' ii"' " 1,', ,'.;",.~ "\".
