1
Blocking o g ain eo ien a ion in sel -doped alumina ma e ials
M. Suá eza, b, A. Fe nándeza, b, J.L. Menéndeza, J. Ramí ez-Ricoc, R. To ecillasa
a Depa men o Nanos uc u ed Ma e ials, Cen o de In es igación en Nanoma e iales y
Nano ecnología (CINN), P incipado de As u ias, Consejo Supe io de In es igaciones Cien í icas
(CSIC), Uni e sidad de O iedo (UO), Pa que Tecnológico de As u ias, 33428 Llane a, As u ias,
Spain
b Fundación ITMA, Pa que Tecnológico de As u ias, 33428 Llane a, Spain
c Dp o. Física de la Ma e ia Condensada-ICMSE, Uni e sidad de Se illa-CSIC, A da. Reina
Me cedes s/n, 41012 Se illa, Spain
Alumina nanopa icles 10–20 nm in diame e we e nuclea ed on alumina pa icles, 150 nm
a e age diame e , by a colloidal ou e ollowed by calcina ion. I is shown ha a e sin e ing,
he inal g ain size is up o 20% smalle due o he addi ion o he alumina nanopa icles.
Elec on backsca e ed di ac ion analysis shows ha whe eas a co ela ion in he ela i e
c ys alline o ien a ions be ween neighbou ing g ains exis s in he pu e ma e ials, he addi ion
o alumina nanopa icles esul s in a andom c ys alline o ien a ion.
Keywo ds Ce amics; G ain g ow h; Sel -doping; Elec on backsca e ing di ac ion (EBSD)
Nanos uc u ed ma e ials a e a ela i ely new kind o ma e ial in which a g ain size in he
nanome e ange, o nanome e -sized second phases dispe sed in a ma ix, con e di e en
p ope ies compa ed o hei bulk coun e pa s. Imp o emen s in physical p ope ies ela ed
o he exis ence o nanome e -sized g ains a e gi en by he Hall–Pe ch law [1] and [2] and
a ious examples ha e been epo ed [3], [4] and [5]. The e ha e been many a emp s
epo ed in he li e a u e o make ully dense monoli hic alumina wi h a g ain size in he
nanome e /submic on ange. Va ious app oaches ha e been de eloped, including wo-s ep
sin e ing [6], [7] and [8], non-con en ional sin e ing [9] and [10], changing he sin e ing
a mosphe e [11] and [12] o using dopan s such as MgO, Y2O3, SiC, e c., which we e shown o
pe o m as e ec i e g ain g ow h inhibi o s o Al2O3[13], [14], [15], [16], [17], [18], [19], [20]
and [21].
Howe e , in he la e case, he in oduc ion o hese addi i es, e en a low concen a ions,
can esul in he o ma ion o seconda y phases wi h a di e en e ac i e index compa ed o
ha o he alumina ma ix, and he e o e ligh sca e ing akes place, impeding he p oduc ion
o anspa en alumina ma e ials. In addi ion, doping wi h silica lea es a glass esidue ha
educes he mechanical pe o mance o he ma e ial.
Alumina-doped alumina powde s ha e been shown o be an e ec i e way o minimize g ain
g ow h in sin e ed ma e ials [22]. I is he aim o his wo k o show ha a sel -doping p ocess
makes i possible o block g ain eo ien a ion du ing sin e ing and hence ob ain a ine
mic os uc u e in doped alumina.
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A colloidal me hod was used [23] o dope pu e comme cial nanoc ys alline alumina powde
TM-DAR (a e age g ain size ∼160 nm, speci ic su ace a ea 14.5 m2 g–1 and 99.99% pu i y)
wi h aluminium e hoxide (97% Sigma–Ald ich). The doping was ca ied ou unde an a gon
a mosphe e by dissol ing an app op ia e quan i y o aluminium e hoxide in anhyd ous e hanol
(99.97%) o o m alumina doped wi h 10 w .% alumina coming om he aluminium e hoxide
(i.e. sel -doping), and hen adding he solu ion d opwise o an alumina/e hanol slu y. The
slu y was i s d ied unde magne ic s i ing a 60–70 °C and subsequen ly in ai a 120 °C in
o de o elimina e any aces o alcohol. The d ied powde s we e subsequen ly c ushed in a
high-pu i y alumina mo a o emo e he agglome a es esul ing om he d ying p ocess, and
sie ed using a 63 μm mesh. The powde s we e he mally e ched a 800 °C o 2 h in o de o
emo e he o ganic esiduals and sie ed again o ob ain deagglome a ed powde s. These
powde s we e cold isos a ically p essed (CIP) a 200 MPa, and sin e ed in ai a di e en
empe a u es (1160–1500 °C) and o di e en holding imes (0.5–2 h).
The densi y o he specimens was measu ed using picnome y powde equipmen (GeoPyc
1360, MICROMERITICS®) and by he A chimedes me hod, and no malized using a heo e ical
densi y o 3.987 g cm−3*24+. Finally, he mic os uc u e and he g ain size o he sin e ed
samples we e cha ac e ized by scanning elec on mic oscopy (SEM; Zeiss DSM 950) o he
ac u e su aces.
The c ys al o ien a ions o he g ains in he sin e ed ma e ials we e assessed by elec on
backsca e ed di ac ion (EBSD) measu emen s. Samples o bo h pu e alumina and 10%
aluminium e hoxide-doped alumina con en ionally sin e ed o 1 h a 1235 °C we e p epa ed
using me allog aphic p ocedu es ha in ol ed polishing up o 1 μm in a diamond slu y and a
inal polishing/e ching s ep wi h 20 nm colloidal silica. Di ac ion pa e ns we e acqui ed using
an EBSD CCD came a (Ox o d Ins umen s C ys al-300) a ached o a ield emission gun
scanning elec on mic oscope (LEO Gemini-1530). Due o cha ging unde he elec on beam, i
was no possible o ob ain de ailed o ien a ion maps, bu ins ead e enly spaced poin s we e
measu ed and indexed o e a ∼300 × 300 μm2 a ea. This was su icien o econs uc he
o ien a ion dis ibu ion unc ions (ODFs) o alumina in bo h samples. Raw EBSD da a was
impo ed in o he MTEX [25] so wa e oolbox o MATLAB, whe e o ien a ion dis ibu ion
unc ion (ODF) econs uc ion was pe o med using a Fou ie echnique wi h a 10° con olu ion
ke nel. Da ase s we e o a ed o make (0 0 0 1) poles pa allel o he no mal di ec ion o ease
o isualiza ion and (0 0 0 1) and pole igu es we e calcula ed.
Figu e 1 shows TEM images o he pu e (Fig. 1a) and doped alumina (Fig. 1b) powde s a e
calcina ion a 800 °C. Figu e 1a shows pu e TM-DAR alumina pa icles wi h clean and ace ed
su aces. On he o he hand, nanopa icles wi h diame e s a ound 10–20 nm ha e nuclea ed
on op o he TM-DAR alumina pa icles a e calcina ion a 800 °C o 2 h o he alumina
doped wi h aluminium e hoxide (Fig. 1b). The nuclea ed pa icles also show a hexagonal
shape, sugges ing ha hey ha e al eady ans o med o α-Al2O3 a e calcina ion a 800 °C,
a below he α phase o ma ion empe a u e (∼1200 °C). The e o e, doping he alumina
powde s wi h an aluminium p ecu so ollowed by calcina ion can modi y he su ace o he
aw alumina powde s. BET su ace a ea analysis shows an inc ease in he su ace o he sel -
doped powde s om 13.7 (Taimei, TM-DAR) o 22.3 m2 g−1.
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Figu e 2 shows SEM images co esponding o ac u e su aces o pu e and doped alumina,
con en ionally sin e ed a di e en empe a u es, 1300 and 1400 °C, o 2 h. No ele an
changes in he mic os uc u e be ween pu e and doped samples a e obse ed a low sin e ing
empe a u es (Fig. 2a and b). The mic og aphs also show ounded g ain bounda ies, indica ing
ha sin e ing is s ill a an ea ly s age. The si ua ion is di e en a he highe sin e ing
empe a u e, 1400 °C: he g ain size eaches he mic on size (1.76 ± 0.49 μm) o pu e alumina
(Fig. 2c), whe eas i emains in he submic on ange (0.48 ± 0.22 μm) in he doped ma e ial
(Fig. 2d).
In o de o gain a deepe unde s anding o he mic os uc u al ea u es, EBSD analysis was
ca ied ou on con en ionally sin e ed powde s. Pole igu es we e ob ained om EBSD
measu emen s and a e shown in Figu e 3a o pu e alumina and in Figu e 3b o 10%
aluminium e hoxide-doped alumina sin e ed o 1 h a 1235 °C. I is clea ha , while he
ex u e o he e hoxide-doped alumina is mos ly andom and closely esembles ha o a pu ely
polyc ys alline ma e ial, he c ys allog aphic ex u e o pu e alumina shows a g ea e deg ee o
p e e ed o ien a ion, as is e idenced by he in ensi y maxima in he pole igu es shown in
Figu e 3a. This obse a ion was con i med by he calcula ion o he ex u e index o he ODF,
which was 2.38 o pu e alumina and 1.22 o e hoxide-doped alumina. The ex u e index is a
global measu e o he deg ee o ex u e in a ma e ial and is equal o 1 o a ma e ial consis ing
o andomly o ien ed c ys alli es.
Addi ionally, he 6- old mul iplici y o he e lec ions (as opposed o 3- old symme y o he
Al2O3 la ice due o lack o cen osymme y) p obably e lec s he p esence o basal wins in
he pu e alumina, cha ac e ized by a 60° deg ee o a ion a ound he [0 0 0 1] di ec ion. In
basal wins he oxygen subla ice emains almos unal e ed ac oss he in e ace [26]. Due o
he high numbe o coinciden si es, basal wins cons i u e low-ene gy g ain bounda ies, as
o en obse ed in sin e ed pu e alumina [27]. Thei absence in e hoxide-doped alumina can
he e o e be explained i g ain eo ien a ion in he ea ly sin e ing s age is supp essed o he
sel -doped alumina.
This di e ence be ween he a angemen o he alumina g ains can be explained by assuming
ha he alumina pa icles unde go a eo ien a ion du ing sin e ing [28], [29], [30] and [31]: in
he case o pu e alumina, a educ ion o ene gy a he g ain bounda ies akes place due o he
o ma ion o solid–solid in e aces h ough planes wi h simila o ien a ion. A highe
empe a u es, di usion h ough g ain bounda ies is p omo ed and g ain g ow h is accele a ed
conside ably. Ini ially, he g een body may be conside ed as a h ee-dimensional
homogeneous dis ibu ion o c ys als wi h andom o ien a ion and andom dis ibu ion o
con ac poin s be ween c ys als (pa icles). In his case, he o al ene gy o he sys em will be
he olume ene gy plus he ee su ace a ea plus he ene gy o he solid–solid in e aces. A
low empe a u es, he only con ibu ion o a o al ene gy educ ion o he sys em will come
om he educ ion in he solid–gas ee ene gy associa ed wi h he di e en alumina su aces
by inc easing he numbe o solid–solid con ac s be ween c ys als. This educ ion will be
a ou ed when he su aces coming in o con ac ha e he same o ien a ion. In ac , con ac
be ween wo planes wi h he same o ien a ion is he mos e ec i e way o educe he ee
ene gy o he sys em when he empe a u e is so low ha he mally ac i a ed di usion
mechanisms canno ake place. The e o e, a low empe a u es, alumina pa icles slip o
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educe he o al ene gy o he sys em by o ming g ain clus e s wi h he same o ien a ion.
These g ains wi h he same plane o ien a ion will o m low-ene gy g ain bounda ies. Du ing
sin e ing, hese in e aces eadily disappea , as only e y low ion mobili y (only sho di usion
pa hs) is equi ed o his o happen, leading o g ain g ow h. Once hese agglome a es a e
o med, sin e ing con inues ia no mal g ow h mechanisms.
Howe e , he si ua ion is di e en in he doped alumina, whe e he p esence o he α-alumina
nanoc ys als a low empe a u es inhibi s g ain eo ien a ion, as opposed o he pu e alumina
case, du ing he i s s eps o sin e ing. The e o e, no ene gy educ ion can ake place as
pa icles a e no able o join h ough simila aces. Consequen ly, his implies ha du ing he
ea ly s ages o sin e ing, a gi en g ain keeps a andom c ys alline o ien a ion wi h espec o
i s neighbou s. When he empe a u e is high enough o allow a omic di usion mechanisms o
become ac i e, sin e ing akes place and he inal mic os uc u e is kep ine . Al hough shown
o a pa icula sys em, such as alumina, he me hod p oposed is gene al and may ha e
implica ions o o he sys ems in which con ol o g ain g ow h du ing sin e ing is equi ed.
This ou e explains he ine mic os uc u e in he doped alumina.
We ha e he e o e shown ha doping wi h aluminium e hoxide is an e icien way o minimize
alumina g ain g ow h du ing sin e ing.
In conclusion, i has been shown ha by modi ying he su ace o he alumina pa icles a e a
sel -doping p ocess, i is possible o block g ain eo ien a ion du ing sin e ing. Finally, his sel -
doping me hod has been ound o be an e ec i e means o educe he a e age g ain size
wi hou in oducing seconda y phases wi h a e ac i e index di e en o ha o he ma ix.
Acknowledgemen s
The au ho s wan o acknowledge he Spanish Minis y o Educa ion and Science and UE o
unding h ough p ojec s MAT2006-01783 and NMP3-CT-2005-515784. M.S. wan s o
acknowledge he I3P p og am o a PhD g an . J.R.-R. acknowledges inancial suppo om he
Jun a de Andalucía. EBSD was pe o med a he CIC o he Uni e si y o G anada, Spain.
5
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Figu e cap ions
Figu e 1. TEM images showing pu e alumina g ains (a) and alumina g ains doped wi h
aluminium e hoxide (b) a e calcina ion a 800 °C o 2 h
Figu e 2. Pu e (le column) and doped ( igh column) alumina con en ionally sin e ed a 1300
(a and b) and 1400 °C (c and d) wi h 2 h holding ime
Figu e 3. Rep esen a i e pole igu es econs uc ed om EBSD da a. Two e lec ions o α-Al2O3
a e shown: (a) pu e alumina and (b) alumina doped wi h 10% aluminium e hoxide and
con en ionally sin e ed o 1 h a 1235 °C. Whi e egions co espond o o ien a ions whe e
densi y is equal o ha o andom dis ibu ion o o ien a ions. The scale is he same in bo h
cases o ease o compa ison.
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Figu e 1