1
High- empe a u e mechanical beha io o polyc ys alline y ium-doped ba ium
ce a e pe o ski e
C. Vaque o-Aguila (a), M.J. López-Robledo (a), J. Ma ínez-Fe nández (a), C. Real
(b), M. Jiménez-Melendo (a)
(a) Depa amen o de Física de la Ma e ia Condensada, Uni e sidad de Se illa, Ap do.
1065, 41080 Se illa, Spain
(b) Ins i u o de Ciencias de Ma e iales de Se illa, Uni e sidad de Se illa-CSIC, A .
Ame ico Vespucio 49, 41092 Se illa, Spain
Abs ac
The high- empe a u e mechanical p ope ies o he mixed ionic-elec onic conduc o
pe o ski e BaCe0.95Y0.05O3−δ wi h a e age g ain size o 0.40 μm ha e been s udied
in comp ession be ween 1100 and 1300 °C in ai a di e en ini ial s ain a es. The ue
s ess– ue s ain cu es display an ini ial s ess d op, ollowed by an ex ended s eady-
s a e s age. As he empe a u e dec eases and/o he s ain a e inc eases, he e is a
ansi ion o a damage- ole an s ain-so ening s age and e en ually o ca as ophic
ailu e. Analysis o mechanical and mic os uc u al da a e ealed ha g ain bounda y
sliding is he p ima y de o ma ion mechanism. The s eng h d op has been co ela ed
wi h he g ow h o ul a ine g ains du ing de o ma ion, al eady p esen a g ain
bounda ies and iple g ain junc ions in he as- ab ica ed ma e ial.
Keywo ds
D. Pe o ski es; C. C eep; A. Sin e ing; B. Mic os uc u e- inal; A. G ain g ow h
1. In oduc ion
Pe o ski e-s uc u ed oxides o gene ic o mula ABO3 (whe e A and B deno e si e
ca ions) ha e ecei ed inc easing a en ion in he las yea s because he selec i e A and
B si e doping p o ides a unique oppo uni y o ailo ing hei p ope ies o impo an
p ac ical applica ions, pa icula ly in he ield o elec ochemical ene ge ics as
elec oly es in solid oxide uel cells (SOFCs) and chemical senso s.1, 2, 3, 4, 5, 6 and 7
Subs i u ion o B4+ si es by i alen ions in hese pe o ski e oxides causes he
o ma ion o oxygen acancies, which allow o p o onic conduc ion (hyd ogen ion
anspo ) a he han he mo e amilia oxygen ion conduc ion. Because o he highe
mobili y o p o ons, he ope a ing empe a u e o SOFCs can be sa ely educed wi hou
deg ading pe o mance.8 and 9
Y ium-doped ba ium ce a e (BCY) is a p o o ype o his class o p o on conduc ing
oxides. I exhibi s one o he highes p o on conduc i i ies a ele a ed empe a u es, as
2
well as an excellen chemical s abili y o e a la ge ange o empe a u es and oxygen
pa ial p essu es.10, 11 and 12 In addi ion, i has been ecen ly disco e ed ha BCY
also exhibi s ema kable s eam-pe mea ion capabili ies unde app op ia e condi ions,13
and 14 ha can be used o imp o e he pe o mance o a a ie y o ene gy con e sion
echnologies including SOFCs, memb ane e o me s and coal gasi ica ion. Mos o he
in es iga ions on BCY a e conce ned wi h i s p ocessing, s uc u e, de ec chemis y
and conduc i i y cha ac e is ics. Only wo s udies, howe e , ha e ocused on he high-
empe a u e mechanical p ope ies o hese high- empe a u e p o on conduc o s.15 and
16 These p ope ies a e o special ele ance in he design o componen s and sys em
a chi ec u es in SOFCs and o he high- empe a u e ope a ing de ices, which a e
ypically submi ed o c eep condi ions: de o ma ion, mechanical deg ada ion and
ailu e de e mine he ma e ial beha io and ul ima e o e all pe o mance. C eep s udies
o pe o ski es a e also impo an o unde s anding he de o ma ion beha io o he
Ea h's lowe man le.17 and 18 In addi ion, high- empe a u e plas ic de o ma ion is
usually con olled by di usion, and can hus p o ide basic in o ma ion abou mass
anspo in he compound. Since sin e ing and g ain g ow h a e go e ned by di usional
p ocesses, such an in o ma ion can be used o de ise op imum p ocessing schedules.
The e o e, he objec i e o his wo k was o in es iga e he high- empe a u e
mechanical beha io o ine-g ained BaCe0.95Y0.05O3−δ polyc ys als ab ica ed by
solid s a e eac ion. De o ma ion cu es we e ob ained in ai a a ious cons an
displacemen a es in he empe a u e ange be ween 1100 and 1300 °C. The mechanical
da a we e co ela ed wi h mic os uc u al obse a ions o deduce he de o ma ion
mechanisms.
2. Expe imen al p ocedu e
2.1. Sample p epa a ion
BaCe0.95Y0.05O3−δ (BC5Y) powde s we e p epa ed by solid-s a e eac ion o BaCO3,
CeO2 and Y2O3 powde s (Ald ich, pu i y >99.0%). The aw ma e ials we e weigh ed
in he equi ed s oichiome ic a io, mixed and g ounded in aga e media o 1 h using a
plane a y ball mill. The esul ing powde was calcined o 10 h a 1200 °C in ai and
hen manually c ushed o educe agglome a es. X- ay di ac ion analysis con i med
ha he esul ing powde was pu e o ho hombic pe o ski e phase. The powde was
uniaxially p essed a 150 MPa in o 20 mm-diame e pelle s and hen isos a ically cold
p essed a 210 MPa. A wo-s ep sin e ing p ocess was pe o med on he pelle s in ai
a mosphe e a low hea ing and cooling a es: a i s s ep a 600 °C o 2 h o elimina e
wa e and hyd oxides o med du ing in e media e s ages, and a second sin e ing s ep a
1550 °C o 10 h. The inal densi y o he samples, de e mined using A chimedes’
me hod, was abou 90% o he heo e ical alue (6310 kg/m3).
2.2. Mechanical es s and mic os uc u al obse a ions
Specimens o 5 × 3 × 3 mm in size we e cu om he sin e ed pelle s wi h a low-speed
diamond saw and used o mechanical es ing. Comp ession es s we e ca ied ou a
3
empe a u es T be ween 1100 and 1300 °C (0.74Tm < T < 0.85Tm, whe e Tm = 1850 K
is he mel ing empe a u e o BaCeO3) in ai a cons an c oss-head speed. The
specimens we e sandwiched be ween CSi pads in o de o educe he ic ion wi h he
alumina push ods o he de o ma ion machine, which ends o cause plas ic
cons ic ions a ei he end o he specimen. The eco ded da a, load s ime, we e
plo ed as σ–ɛ cu es, whe e σ is he ue s ess and ɛ is he ue s ain. Mechanical es s
we e discon inued a e eaching a o al ue s ain o 50% o subsequen
mic os uc u al obse a ions.
The da a we e analyzed using he s anda d high- empe a u e powe law o s eady-s a e
de o ma ion:
(1)
whe e A is a pa ame e depending on he de o ma ion mechanism, d is he g ain size, n
is he s ess exponen , p is he g ain size exponen , Q is he ac i a ion ene gy o low
and R is he gas cons an .
2.3. Mic os uc u al obse a ions
The mic os uc u al cha ac e iza ion o as- ab ica ed and de o med polyc ys als was
ca ied ou using con en ional (SEM) and high- esolu ion (HRSEM) scanning elec on
mic oscopy (Mic oscopy Se ice, Uni e si y o Se illa, Spain). To e eal g ain
bounda ies, longi udinal sec ions we e cu om he samples and mechanically polished
using up o 1 μm g ade diamond pas e, and hen he mally e ched a 1200 °C o 2 h in
ai . The ele an mo phological pa ame e s, g ain size d ( aken as he equi alen plana
diame e d = (4(g ain a ea)/π)1/2) and o m ac o F (de ined as F = 4π(g ain
a ea)/(g ain pe ime e )2) we e measu ed by using a semiau oma ic image analyze .
F ac u e su aces we e also analyzed by HRSEM; in as- ab ica ed and non- ac u ed
s ained specimens, ac u e su aces we e ob ained h ough b eaking he samples by
impac a oom empe a u e.
3. Expe imen al esul s
Fig. 1 shows a HRSEM mic og aph o a polished and he mally e ched c oss-sec ion o
he ab ica ed ma e ial. The g ains appea o ha e an equiaxed shape (F = 0.80), wi h an
a e age g ain size d o 0.40 μm. Po osi y consis ed in ca i ies a mul iple g ain
junc ions, wi h po e sizes o a ew mic ome e s.
Typical ue s ess σ– ue s ain ɛ cu es a e shown in Fig. 2 o di e en de o ma ion
condi ions o s ain a e and empe a u e. As no ed abo e, es s we e usually ended a
50% s ain o a be e obse a ion o he esul ing mic os uc u e, unless p ema u e
ac u e occu ed. The compound exhibi s a ansi ion om duc ile o agile beha io as
he ini ial s ain a e inc eases o he empe a u e dec eases. Se e al cha ac e is ics may
be no iced om his igu e:
4
In he duc ile egion ( ha is, a lowe s ain a es and highe empe a u es), he cu es
exhibi a peak s ess a he beginning o he de o ma ion, ollowed by a con olled s ess
d op and hen by an ex ensi e seconda y c eep egime (cha ac e ized by a posi i e slope
o he σ–ɛ cu e due o he inc ease in specimen sec ion and ue s ain a e du ing
comp ession). The e is a p og essi e ansi ion om duc ile o agile beha io when
inc easing he ini ial s ain a e and/o dec easing he empe a u e. The onse o ma e ial
deg ada ion is cha ac e ized by a slow dec ease in low s ess wi h inc easing s ain, as
can be seen in he σ–ɛ cu e o View he Ma hML sou ce in Fig. 2a. Despi e he
damage, he ma e ial s ill e ains a ela i ely high s eng h a e he comple ion o he
es in his semib i le egime (cu e o T = 1200 °C in Fig. 2b), indica ing he
a ainmen o a damage- ole an egime in such expe imen al condi ions E en ually, a
he wo s es ing condi ions o s ain a e and empe a u e, ca as ophic ailu e occu ed
a e ela i ely small s ains.
The magni ude o he s ess d op ( he di e ence be ween uppe and lowe yield
s esses) dec eases when dec easing he s ain a e and/o inc easing he empe a u e.
The a io be ween he peak s ess and he minimum s ess is, howe e , p ac ically
cons an wi h a alue o 1.10. This esul sugges s ha he mechanism esponsible o
he ini ial s eng h d op is he same a all de o ma ion condi ions.
The c i ical s ain ɛp a which he peak s ess σp and subsequen so ening akes place
also shi s sligh ly o lowe s ains wi h inc easing empe a u e and/o dec easing s ain
a e.
The mechanical cha ac e is ics o BC5Y polyc ys als in he duc ile egion (s eng h
d op ollowed by s eady-s a e de o ma ion, and a ia ion o σp and ɛp wi h he wo
con ol a iables View he Ma hML sou ce and T) a e e y simila o hose epo ed o
a ious me als and alloys a ho de o ma ion condi ions. 19, 20, 21, 22 and 23 On he
con a y, e y ew s udies ha e epo ed he occu ence o yield d op in ce amic
polyc ys als, 24 and 25 pa icula ly a small g ain sizes.26 To ou knowledge, no such a
beha io has been ound on simila ly s uc u ed pe o ski es.
The mac oscopic aspec o he s ained samples is well co ela ed wi h he mechanical
beha io displayed by he co esponding s ess–s ain cu es. Excep o he mos
se e e de o ma ion condi ions whe e ca as ophic ailu e occu ed, he samples
unde wen a a he homogeneous de o ma ion wi hou ba eling. No special ea u es
we e obse ed o samples exhibi ing s eady-s a e low. As he empe a u e dec eased
and/o he ini ial s ain a e inc eased, mac oscopic c acks pa allel o he loading
di ec ion we e isible on he la e al su aces, co esponding o samples wi h con olled
deg ada ion. As no ed p e iously, he ma e ial was ole an o hese mac oc acks e en
a e s ains o 50% in comp ession. Finally, hese c acks de eloped as e a highes
s ain a es and lowes empe a u es, ac u ing he sample in o small pieces.
Fig. 3 shows a polished and he mally e ched longi udinal c oss-sec ion o a sample
de o med in s eady-s a e condi ions (T = 1300 °C, View he Ma hML sou ce, ɛ = 50%).
The g ains emain equiaxed, indica ing ha g ain bounda y sliding is he p ima y
5
de o ma ion mechanism a such expe imen al condi ions, as usually ound in ine-
g ained supe plas ic me als and ce amics. 27 and 28 As T dec eases o View he
Ma hML sou ce inc eases, g ain bounda y sliding is no as enough o accommoda e
he de o ma ion imposed, and c acks de elop along g ain bounda ies leading e en ually
o ailu e. A close inspec ion o Fig. 3 e eals ha he a e age g ain size is sligh ly
smalle han ha o he as- ab ica ed ma e ial due o he p esence o new ine g ains o
abou 100 nm in size. This inding can be be e obse ed in Fig. 4, which shows
HRSEM mic og aphs o e ical ac u e su aces ob ained unde a ious expe imen al
condi ions. In he as- ab ica ed ma e ial (Fig. 4a), he g ain bounda ies and iple
junc ions a e deco a ed wi h ul a ine g ains smalle han 20 nm in size; hese g ains
inc ease up o abou 100 nm in size du ing s eady-s a e de o ma ion (Fig. 4b). A simila
e olu ion o he g ain s uc u e was obse ed in samples ha ailed du ing es ing (Fig.
4c).
4. Discussion
The mechanical cha ac e is ics displayed by he σ–ɛ cu es o BC5Y indica e he
occu ence o a so ening mechanism p io o he es ablishmen o he s eady-s a e low,
leading o a peak in s ess. Mo i a e al.26 ha e ecen ly epo ed yield d op in a ine-
g ained (d < 1 μm) e agonal Z O2–30 ol% spinel composi e, which was a ibu ed o
a sudden inc ease in he mobile disloca ion densi y in spinel g ains. Simila ly, Dokko
and Pask24 asc ibed he yield d op in la ge-g ained MgO (d > 10 μm) o disloca ion
mul iplica ion in di e en slip sys ems. These in ag anula disloca ion- ela ed
mechanisms, also ound in single c ys als, usually esul in a he sha p s ess d ops,
which a e li le a ec ed by s ain a e. In he p esen wo k, all yield cu es a e o
ounded con igu a ion, and he yield s esses s ongly depend on he s ain a e (and
empe a u e). This beha io is e y simila o wha has been epo ed in he ho
de o ma ion o me als and alloys, 19, 20, 21, 22 and 23 in which he peak s ess and
s eady-s a e low esul om a dynamic balance be ween disloca ion eco e y and
dynamic ec ys alliza ion. The mic os uc u al e olu ion obse ed du ing mechanical
es ing o BC5Y (Fig. 4), along wi h he a ia ion in peak s ess and s ain alues wi h
empe a u e and s ain a e condi ions, sugges s ha he g ow h o he ul a ine g ains
unde de o ma ion is esponsible o he peak s ess and subsequen con olled s eng h
d op. A he beginning o he de o ma ion, he ul a ine g ains a e so small ha
“no mal” neighbo g ains a e in con ac wi h each o he ; hese no mal g ains, wi h an
a e age g ain size d, hus will ca y he imposed load a a s ess le el gi en by Eq. (1).
As he de o ma ion p og esses, he ul a ine g ains will g ow and pa icipa e in he
p ocess suppo ing some o he load. The e ec i e a e age g ain size in he ma e ial is,
he e o e, e ined, leading o a dec ease in low s ess wi h s ain acco ding o he
in e se dependence o he s eady-s a e s ain a e wi h g ain size (Eq. (1)), as
expe imen ally obse ed (Fig. 2).
Wi h he p e ious s a emen , he mic os uc u e o he samples can be ega ded as
cons an up o he peak s ess σp, ega dless o es ing condi ions. The s eady-s a e
equa ion o low (Eq. (1)) can be hus w i en as:

6
whe e B is a cons an independen o s ess. The s ess exponen n can be es ima ed
om a plo o log View he Ma hML sou ce e sus peak s ess σp a a gi en
empe a u e, as shown in Fig. 5 o T = 1300 °C. A alue o n = 1.8 was ound, which is
in close ag eemen wi h he alue o 2 sys ema ically epo ed in ine-g ained me als
and ce amics, 27 and 28 whe e g ain bounda y sliding is he dominan de o ma ion
mechanism. A s ess exponen o 2 has been also p edic ed by many heo e ical models
de eloped o explain he supe plas ic beha io o ine-g ained ma e ials based on
di usion and/o disloca ion slip.28 None o hem, howe e , is able o explain
success ully he body o expe imen al da a; his is p esen ly a ma e o in ense deba e.
The alue o n = 1.8 expe imen ally ound in ine-g ained BC5Y subs an ia es he
p e ious conclusion, based on mic os uc u al obse a ions abou he e en ion o
equiaxed g ain shapes du ing c eep, ha g ain bounda y sliding plays a key ole in he
de o ma ion o hese compounds.
When de o ma ion is achie ed by g ain bounda y sliding, he s eady-s a e low
gene ally is con olled by he di usion necessa y o accommoda e he s esses gene a ed
du ing sliding.27 and 28 Thus, he ac i a ion ene gy o low Q (Eq. (1)) mus
co espond o he ac i a ion ene gy o he slowes mo ing species in he compound. A
alue o Q = 390 ± 40 kJ/mol was de e mined om an A henius plo o log σp e sus
ecip ocal empe a u e 1/T a ixed ini ial s ain a es. Un o una ely, he e a e no
enough di usion da a a ailable o BCY o make a p ecise compa ison. Only he
ac i a ion ene gy o oxygen bulk di usion Q ≈ 60 kJ/mol has been epo ed,15 which
is subs an ially smalle han he alue o c eep, and sugges s ha ca ion di usion is he
a e-limi ing p ocess. The lack o comp ehensi e ca ion bulk and g ain bounda y
di usi i ies p ecludes a de ini i e iden i ica ion o he a e-con olling species.
Two p e ious wo ks15 and 16 ocused on he high- empe a u e de o ma ion o
BaCe1−xYxO3−δ, wi h x a ying be ween 0.05 and 0.20, g ain sizes anging om 2 o
9 μm and ela i e densi ies o abou 90% o heo e ical alues. In hese s udies, a s ess
exponen close o uni y and an ac i a ion ene gy o c eep o 360 ± 60 kJ/mol we e
epo ed. These alues, along wi h he ac ha g ains did no change shape du ing
es ing, allowed he au ho s o conclude ha he de o ma ion was achie ed by g ain
bounda y sliding accommoda ed by ca ion la ice di usion. The same conclusion was
d awn in he de o ma ion o o he oxide pe o ski es wi h g ain sizes anging be ween 2
and 50 μm, whe e a s ess exponen o abou 1 was sys ema ically ound:
La0.9S 0.1MnO3,29 La0.8S 0.2Ga0.85Mg0.15O2.825,30 BaTiO3,31
La0.2S 0.8Fe0.8C 0.2O332 and S Co0.8Fe0.2O333. The di e ence in s ess exponen
alues be ween hese s udies (n ≈ 1) and he p esen wo k (n ≈ 2) can be ela ed wi h he
di e ence in g ain sizes. A ansi ion om n = 1 owa ds n = 2 wi h dec easing g ain
size has been no ed o o he ce amic oxides 27 and 34; he c i ical g ain size seems o
be abou 1 μm, in ag eemen wi h he p esen esul s. Ne e heless, di usion is he a e-
limi ing s ep in bo h cases; he alues o Q measu ed in ine- and la ge-g ained BCY
(390 and 360 kJ/mol, espec i ely) compa e e y well and sugges ha ca ion la ice
7
di usion is also he a e-con olling mechanism o g ain bounda y sliding in he n = 2
egion. I is wo h no ing ha he same mechanism was p oposed o he supe plas ic
beha io o ine-g ained e agonal zi conia.27
5. Conclusions
Polyc ys alline BaCe0.95Y0.05O3−δ wi h a e age g ain size o 0.40 μm was s ained in
comp ession be ween 1100 and 1300 °C in ai a a ious cons an ini ial s ain a es.
The ue s ess– ue s ain cu es sys ema ically displayed an ini ial s eng h d op
ollowed by an ex ensi e s eady-s a e egime, excep a he mos se e e de o ma ion
condi ions (lowes empe a u es and highes s ain a es) whe e ca as ophic ac u e
occu ed. The yield s ess showed s ong dependence on empe a u e and on he applied
s ain a e. No change in g ain shape was obse ed a e 50% s ain, indica ing ha
g ain bounda y sliding is he main de o ma ion mechanism in his compound, as also
ound in ine-g ained supe plas ic ma e ials. The s ess d op has been co ela ed wi h
he g ow h o ul a ine g ains p esen p e iously a g ain bounda ies and iple g ain
junc ions in he as- ab ica ed ma e ial. A s ess exponen n = 1.8 and an ac i a ion
ene gy o low Q = 390 kJ/mol we e es ima ed om mechanical. The n alue is
consis en wi h g ain bounda y sliding accommoda ed by di usion. By compa ison wi h
la ge-g ained oxide pe o ski es, i is sugges ed ha de o ma ion is a e-con olled by
bulk ca ion di usion.
Acknowledgmen
This wo k was suppo ed by he P ojec no. MAT2009-13979-C03-01, Minis e io de
Ciencia e Inno ación, Spain.
8
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Disloca ion c eep in MgSiO3 pe o ski e a condi ions o he Ea h's uppe mos lowe
man le
16
Figu e 3

17
Figu e 4
18
Figu e 5