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O iginal A icle
T anspa en magnesium alumina e spinel: Effec o c i ical empe a u e in
wo-s age spa k plasma sin e ing
A. Talimian
a,
*, V. Pouchly
b,c
, H.F. El-Magh aby
a,d,e
, K. Maca
b,c
, D. Galusek
a,d
a
Cen e o Func ional and Su ace Func ionalised Glass, Alexande Dubcek Uni e si y o T encin, T encin, Slo akia
b
CEITEC BUT, B no Uni e si y o Technology, Pu kyno a 123, B no, Czech Republic
c
Facul y o Mechanical Enginee ing, B no Uni e si y o Technology, Technicka 2, B no, Czech Republic
d
Join Glass Cen e o he IIC SAS, TnUAD and FChPT STU, T encin, Slo akia
e
Re ac o ies,Ce amics, and Building Ma e ials Depa men , Na ional Resea ch Cen e, 33 El-Bohous S ., 12622, Cai o, Egyp
ARTICLE INFO
Keywo ds:
Magnesium alumina e spinel
Spa k plasma sin e ing
Ca bon con amina ion
Op ical p ope ies
ABSTRACT
The discolou a ion o magnesium alumina e spinel caused by ca bon con amina ion is a main d awback o
ab ica ing anspa en bodies by spa k plasma sin e ing (SPS). In his s udy, a wo-s age hea ing a e p ofile was
used o p oduce anspa en MgAl
2
O
4
wi hou using sin e ing aids by SPS a 1250°C. The effec o c i ical
empe a u e (Tc), a which he hea ing a e is dec eased, on anspa ency and ca bon con amina ion was in-
es iga ed: highe c i ical empe a u e esul ed in highe con amina ion. Non-uni o m densifica ion indica ed
ha as hea ing esul s in a ho -zone o ma ion in he cen e o sin e ed pelle s; he highe empe a u e o cen e
a ou ed eac ion o g aphi e die wi h spinel and o ma ion o diso de ed ca bon s uc u es in esidual po es.
1. In oduc ion
Magnesium alumina e spinel is a p omising op ical ce amics due o
i s high anspa ency o e a wide window o elec omagne ic adia ion,
om ul a iole o mid-in a ed (0.2–5.5 μm). I s c ys al s uc u e is
capable o hos ing a a ie y o op ically ac i e ions and, hence, he
op ical p ope ies can be modified [1–4]. Ha ing an op ically iso opic
s uc u e, highly anspa en bodies a e ob ained by p oducing highly
dense g een bodies and emo ing he po es, ac ing as ligh sca e ing
cen es, by a sui able sin e ing p ocess [5–9]. Fab ica ing a highly
dense magnesium alumina e spinel is, howe e , a difficul ask. Due o
slow diffusion o a omic species, pa icula ly oxygen, anspa en
magnesium alumina e spinel is usually ab ica ed ei he in wo s eps by
p essu e-less sin e ing ollowed by ho isos a ic p essing o a leng hy
ho p essing p ocess (HP) [10–13]. Subjec ing magnesium alumina e
spinel o high empe a u es o a long ime esul s in g ain g ow h,
changes he mic os uc u e and de e io a es mechanical p ope ies
[13–16].
Spa k plasma sin e ing (SPS) is a p ac ical me hod o ab ica ing
highly dense fine-g ained ce amics. Howe e , he ca bon con amina-
ion o ma e ials p epa ed by SPS is ine i able [17–23]. Addi ion o
li hium fluo ide acili a es densifica ion o spinel ce amics and cleanses
he ca bon con amina ion. Howe e , he in e ac ions be ween LiF and
MgAl
2
O
4
yield de imen al phases, in oduce s uc u al de ec s (i.e. F
cen es) and de e io a e op ical p ope ies o he final body [21,24–27].
Ca bon con amina ion can be also minimised by op imising he spa k
plasma sin e ing pa ame e s. Se e al a emp s ha e been made o s udy
he ca bon con amina ion and o ab ica e anspa en spinel wi hou
sin e ing aids [20,28,29]. Mo i a e al. ha e epo ed ha he le el o
ca bon con amina ion is sensi i e o he hea ing a e du ing SPS: slow
hea ing imp o es he anspa ency o MgAl
2
O
4
[30]. While slow
hea ing is beneficial o anspa ency, i ex ends he du a ion o he
p ocess and he eby elimina es one o he main ad an ages o SPS, i.e
as sin e ing.
In he p esen s udy, he ab ica ion ime o anspa en magnesium
alumina e spinel is educed by pe o ming a wo-s age spa k plasma
sin e ing schedule comp ising o ini ial as hea ing ollowed by slow
hea ing. The effec o c i ical empe a u e, a which he hea ing a e is
changed, on anspa ency and ca bon con amina ion was in es iga ed
using a Raman spec oscopy and desc ibed ollowing he modynamic
app oach.
2. Expe imen al p ocedu es
Comme cial magnesium alumina e spinel powde , S30CR
(Baikowski, F ance), was used as he s a ing ma e ial. The powde was
dispe sed in isop opanol using an ul asonic mixe (Sonopuls HD 3400,
BANDELIN, Ge many). A e wa ds, he mix u e was ans e ed o a
h ps://doi.o g/10.1016/j.jeu ce amsoc.2020.02.012
Recei ed 15 Oc obe 2019; Recei ed in e ised o m 4 Feb ua y 2020; Accep ed 5 Feb ua y 2020
⁎
Co esponding au ho .
E-mail add ess: [email p o ec ed] (A. Talimian).
Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
A ailable online 06 Feb ua y 2020
0955-2219/ © 2020 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license
(h p://c ea i ecommons.o g/licenses/BY-NC-ND/4.0/).
T
o a y e apo a o and he liquid emo ed; Ready- o-P ess (RTP)
powde was p epa ed by passing he d ied powde h ough a sie e wi h
0.5 mm mesh wid h.
The samples we e ab ica ed by spa k plasma sin e ing (SPS) (DR.
SINTER SPS-625, FUJI, Japan). The g anula ed powde was filled in a
g aphi e die wi h an inne diame e o 12 mm. G aphi e pape was used
o sepa a e he powde om he die and he punches. Then, he die was
w apped in ca bon el insula o . Sin e ing was pe o med unde a-
cuum (5–9 Pa). The empe a u e was measu ed using an op ical py-
ome e ocused on he hole d illed in o a die wall. A cons an uniaxial
p essu e o 75 MPa was applied abo e 800 °C.
P elimina y in o ma ion on he densifica ion beha iou o mixed
spinel powde was ob ained by pe o ming a single-s age SPS. The ex-
pe imen s we e pe o med by inc easing he empe a u e o samples o
600 °C in 3 min; hen, he samples we e hea ed a 100 °C min
−1
o 1300
°C. A e wa ds, he wo-s age SPS was ca ied ou by inc easing he
empe a u e o 600 °C in 3 min and, a e wa ds, he samples we e he-
a ed o a empe a u e be ween 1100 °C–1200 °C wi h he cons an
hea ing a e o 100 °C.min
−1
. Then, he hea ing a e was dec eased o
2.5 °C.min
−1
and hea ing con inued up o 1250 °C. The a ia ions o
empe a u e and applied p essu e agains ime du ing sin e ing p ofiles
a e shown in Fig. 1.
Sin e ed pelle s we e subsequen ly hea ea ed in a muffle u nace
a 800 °C (hea ing a e: 2.5 °C min
−1
) o 60 min in ai in o de o
emo e esidual ca bon om he su aces. The samples we e ca e ully
mi o polished on bo h sides using diamond ab asi e pape s down o
0.5 μm o u he op ical cha ac e isa ion. The final hickness o he
samples a e polishing was ∼1 mm.
The densi y o sin e ed bodies was measu ed using A chimedes’
me hod in deionised wa e .
The In-line ansmission was measu ed in he wa eleng h ange
be ween 200−2000 nm using a UV– is-NIR spec opho ome e (Ca y
5000, Agilen , USA) wi h g a ing and sli change a 800 nm.
Raman spec oscopy was conduc ed on he su ace and he polished
c oss-sec ion o sin e ed samples by using a Raman mic oscope spec-
ome e (inVia Qon o , Renishaw, UK) using 532 nm exci a ion wa-
eleng h a oom empe a u e.
The mic os uc u e was examined using scanning elec on mic o-
scopy (JSM-7600 F, JEOL, Japan). The c oss-sec ion o samples was
mi o -polished using 1 μm diamond pas e. The ac u e su aces and
polished c oss-sec ions we e he mally e ched a 1150 °C o 30 min.
The e ched samples we e fixed a aluminium sample holde s using he
conduc i e adhesi e ape and, a e wa ds, coa ed wi h ca bon; he
g ain size was de e mined om he SEM images using linea
in e sec ion me hod.
3. Resul s
Fig. 2 shows he punch displacemen (no malised by weigh o
samples) and he displacemen a e as a unc ion o empe a u e du ing
single-s age SPS o magnesium alumina e spinel a he cons an hea ing
a e o 100 °C min
−1
. The p essu e applied by he punches plo ed
agains sin e ing empe a u e is also shown. The displacemen o pun-
ches consis s o h ee main egions: he fi s s age be ween 800−950 °C
whe e he sh inkage is he as es , and he main peak in he sh inkage
a e is obse ed. The sh inkage is a ibu ed o applica ion o p essu e
a 800 °C esul ing in pa icle ea angemen . This is ollowed by a
con inuous sh inkage, wi h a cons an a e be ween 950−1100 °C. Fi-
nally, he densifica ion slows down, and he sh inkage a e dec eases
significan ly. The sh inkage e en ually s ops abo e 1250 °C whe e he
displacemen cu e eaches a pla eau, and a densi y o > 99.0 % is
achie ed. The e o e, he maximum sin e ing empe a u e was limi ed
o 1250 °C o elimina e phenomena such as g ain g ow h and po e
coalescence.
A se o a ious sin e ing egimes was applied: he hea ing a e was
dec eased o 2.5 °C.min
−1
a e a specific empe a u e in he in e al
be ween 1100−1200 °C (namely 1100, 1150, 1175 and 1200 °C) was
achie ed. In he la e ex , his empe a u e is deno ed as c i ical
empe a u e, Tc.Fig. 3 shows he appea ance, and he alues o in-line
ansmissions measu ed a 550 nm o samples p oduced by SPS, using
diffe en Tc be ween 1100 °C–1200 °C. A sample p oduced by slow
hea ing om 1100 °C o 1200 °C is also shown. Samples p oduced wi h
he Tc = 1100 and 1150 °C a e almos anspa en and exhibi a
maximum ansmission o 65 and 67 %. The sample ab ica ed using Tc
= 1175 °C has a la ge da k spo in he cen e, and he ansmission
dec eases o 51 %. The sample sin e ed wi h a Tc o 1200 °C is almos
en i ely black wi h a limi ed ansmission o 10 %. Howe e , he pe -
iphe y o he pelle is anspa en . The sample sin e ed by slow hea ing
om 1100−1200 °C, is anspa en in he cen e, bu i is opaque and
whi ish a he edges, indica ing he p esence o la ge po es and in-
comple e sin e ing.
Fig. 4 shows he in-line ansmission, ILT, o he samples as a
unc ion o he inciden ligh wa eleng h measu ed in he ange be-
ween 200−2000 nm. The measu ed ILT is ecalcula ed o he same
hickness o d
2
= 1.0 mm, using Eq. 1[16]:
=− −
ILT d R ILT d
R
() (1 )( ()
1)
s
s
d
d
221
2
1
(1)
Fig. 1. Tempe a u e and p essu e agains ime o wo-s age spa k plasma sin e ing p ofiles.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2418
whe e R
S
is he o al su ace eflec ance (≈0.14), and ILT(d
1
) is he
alue ob ained o he sample wi h he hickness d
1
. A discon inui y a
he ILT cu e ha is obse ed a 800 nm, and eco ded o all samples,
is ela ed o ins umen al limi a ions and he de ec o and g a ing
change-o e . The e o e, he measu ed spec um is di ided in o wo
egions: 200−795 nm, and 805–2000 nm. Samples p oduced using Tc
o 1100 and 1150 °C exhibi simila ansmission in bo h IR and UV
anges wi h a sha p dec ease in ansmission a λ= 300 nm. Con-
e sely, he ansmission o samples p oduced wi h a Tc = 1175 °C,
dec eases wi h a shallow slope as he wa eleng h dec eases. The
blackening o anspa en bodies ab ica ed by spa k plasma sin e ing is
a ibu ed o ca bon con amina ion [29–31].
The dis ibu ion o ca bon in samples was examined by Raman
spec oscopy. Fig. 5 shows mic o-Raman spec a collec ed o e he
su ace o samples ab ica ed using a sin e ing egime wi h he c i ical
empe a u e o 1150 °C ( he highes anspa ency) and 1200 °C ( he
lowes anspa ency); Raman spec a o he used ca bon pape and RTP
spinel powde a e also shown.
The Raman spec um o he as- ecei ed powde con ains fi e majo
peaks a ibu ed o ib a ion modes o magnesium alumina e spinel and
h ee o he peaks co esponding o sulpha e, chlo ide and ca bona e
species. The s ong peaks loca ed a ound 410, 672 and 770 cm
−1
a e
associa ed wi h E
g
,T
2g
and A
1g
ib a ions o MgAl
2
O
4
, espec i ely. The
weak band cen ed a ound 311 cm
−1
is a ibu ed o T
2g
ib a ion o
MgAl
2
O
4
. The peak loca ed a 727 cm
-1
is associa ed wi h he b ea hing
mode o aluminium ca ions loca ed in e ahed al si es implying ha
in e se spinel s uc u e exis s in he powde [5,32–35]. The peak
a ound 980 cm
-1
is a ibu ed o he ib a ion o SO
x
species. The e a e
also wo mo e peaks assigned o he ib a ion modes o ace Cl
2
and
CO
32-
a 560 and 1064 cm
-1
, espec i ely [36]. The chlo ide, sulpha e
and ca bona e species o igina e om he p ocess o powde syn hesis
[37].
The peaks o impu i ies a 560, 980 and 1064 cm
−1
disappea ed
en i ely in he samples subjec ed o spa k plasma sin e ing. Ins ead, he
Raman spec um o he sample p oduced a he T
c
= 1200 °C is dis-
inc i ely diffe en om he spec um o he sample ab ica ed a he T
c
= 1150 °C: i exhibi s a s ong peak associa ed wi h he D-band mode o
diso de ed g aphi e (1350 cm
-1
). In con as , he Raman spec um o
ca bon pape exhibi s a s ong peak a a ound 1580 cm
-1
ha is asso-
cia ed wi h he G-band mode o g aphi e and a highly o de ed s uc u e.
The p esence o ca bon in he sin e ed bodies wi h a diffe en s uc u e
om he ca bon pape ules ou he possibili y ha he obse ed
blackening o samples is caused by ca bon inclusions in oduced om
he deb is o he die o ca bon pape . Blackening o samples can be hus
a ibu ed o deposi ion o ca bon o med h ough he eac ion o
ca bon om g aphi e pape /die wi h MgAl
2
O
4
a high empe a u es.
Fig. 6 shows he Raman spec a collec ed nea he cen e o he
pelle s and close o he edge o he pelle s’c oss-sec ions. The spec a
collec ed a he cen e and he edge o he samples p epa ed a he T
c
=
1100 °C a e basically he same, implying ha ca bon con amina ion is
Fig. 2. (a) Punch displacemen (no malised by weigh ) and p essu e agains empe a u e as a unc ion o empe a u e; (b) fi s de i a i e o punch displacemen .
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2419
e y limi ed. In con as , he peaks associa ed wi h ca bon D-band
(1350 cm
−1
) and G-band (1600 cm
−1
) s a o be isible in he spec-
um collec ed in he cen e o he samples ab ica ed a he T
c
= 1150
°C. The in ensi ies o peaks associa ed wi h ca bon inc eased sig-
nifican ly in he sample sin e ed using he Tc = 1175 °C. Ca bon- e-
la ed peaks ha e become he main peaks o he samples p oduced a
he T
c
= 1200 °C. The Raman spec a collec ed nea he edge a e es-
sen ially simila o all samples, and no peaks associa ed wi h ca bon
appea in he spec a. Howe e , low-in ensi y peaks associa ed wi h he
ca bon D-band and G-band a e obse ed in he spec um o he sample
p epa ed wi h a c i ical empe a u e o 1200 °C.
4. Discussion
4.1. Densifica ion
The densifica ion cu e shown in Fig. 2 consis s o h ee main s eps:
he fi s s ep co esponds o he powde compac ion, c ushing o
g anules and/o pa icle ea angemen , which appea s as a as dis-
placemen o he punch a e he p essu e is applied a T=800 °C. The
displacemen a e is almos cons an in he nex empe a u e in e al
be ween 900−1100 °C. The s a ing empe a u e o his s ep is simila
o he densifica ion h eshold o con en ional sin e ing o he used
spinel powde (S30CR, Baikowski) [38,39].
The densifica ion p oduces a solid body con aining isola ed closed
po es. The de o ma ion o he solid body subjec ed o he p essu e is
limi ed and, hence, he punch displacemen du ing he final s age o
sin e ing (> 1100 °C) is caused only by sin e ing. A he same ime, he
gas p essu e inc eases inside he closed po es, which sh ink as he esul
o applied empe a u e and p essu e. The inc ease o in e nal p essu e
o gasses also esul s in he dec ease o sh inkage a e.
In addi ion o con en ional sin e ing mechanism, i.e. g ain
bounda y diffusion o la ice diffusion, densifica ion du ing spa k
plasma sin e ing is a esul o ce amics’c eep unde la ge comp essi e
s esses [40]. Inc easing concen a ion o la ice de ec s such as a-
cancies and disloca ions in oduces ligh sca e ing cen es and de e-
io a es he anspa ency o ce amics [28]. Howe e , he slip sys ems o
magnesium alumina e spinel a e no ac i a ed below 1300 °C [41,42].
G ange e al. s udied he sin e ing beha iou o magnesium alumina e
spinel du ing SPS and concluded ha g ain bounda ies sliding, ac-
commoda ed by he in-se ies eac ion o in e aces and la ice diffusion
o oxygen, go e ns he densifica ion p ocess [41]. The e o e, he
anspa ency o samples is unlikely o be affec ed by he changes in
disloca ion densi y a he empe a u es applied in his s udy.
Fig. 7 isualises a sample ab ica ed by as hea ing (100 °C min
−1
)
o T
C
= 1100 °C ollowed by slow hea ing (2.5 °C min
−1
) o 1200 °C.
While he cen al egion o he sample is anspa en (a anspa en
egion ca. 6 mm in diame e is p esen ), he edge o he sample is
opaque. This indica es ha du ing he sin e ing he samples ha e been
Fig. 3. Op ical pho og aphs o samples p o-
duced wi h he use o a slow hea ing egime
(2.5 °C/min) be ween Tc and 1250 °C; he
samples ha e a hickness o ca 1 mm and a e
placed di ec ly a he backg ound. The alues
o ILT we e measu ed a a wa eleng h o 550
nm. (a ows poin ou he spo o ansmission
measu emen s (Fig. 4) amd collec ing Raman
spec a ((Fig. 5)).
Fig. 4. In-line ansmi ance agains he wa eleng h o samples ab ica ed by
wo-s age spa k plasma sin e ing using diffe en c i ical empe a u es Tc: 1100,
1150, 1175 and 1200 °C. The ansmission was e alua ed o a hickness o 1
mm using Eq. 2.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2420
densified in-homogenously: he cen e is dense han he im and ee
om la ge po es.
Fig. 8 shows he g ain size dis ibu ion in he cen e and a he
pe iphe y o samples sin e ed a a ious Tc. While he g ain size o
samples is independen o Tc, he cen e o all samples is cha ac e ised
by la ge g ains han he pe iphe ies. Simila ly, such non-uni o m dis-
ibu ion o g ain size in ce amic bodies, e.g. Al
2
O
3
and Y
2
O
3
, has been
epo ed in o he wo ks [43,44]. Th ee ac o s can be held esponsible
o la ge g ains in he cen e: lowe ca bon con amina ion, a highe
concen a ion o de ec s, and empe a u e g adien . As shown in Fig. 3,
sin e ed samples exhibi la ge amoun s o ca bon in he cen al a ea.
Thus ca bon con amina ion canno explain he non-uni o m dis ibu-
ion o g ain size. The accumula ion o de ec s, i.e. acancies, migh be
esponsible o he g ain g ow h; howe e , i e en ually esul s in he
inc ease o samples’po osi y, pa icula ly in he cen e. In con as ,
samples show a diffe en dis ibu ion o po es. Fig. 9 shows he ac u e
su ace o pelle s sin e ed a Tc = 1100 °C. All obse ed po es a e
isola ed and loca ed a he g ain bounda ies. Mo eo e , he edge o he
Fig. 5. Raman spec a collec ed om he as- ecei ed spinel
powde ( ed line), and he samples p oduced by spa k plasma
sin e ing a 1250 °C using c i ical empe a u es o 1150 °C and
1200 °C. The spec a collec ed om g aphi e pape used in SPS is
shown o compa ison. (Fo in e p e a ion o he e e ences o
colou in his figu e legend, he eade is e e ed o he web
e sion o his a icle).
Fig. 6. Raman spec a collec ed om polished c oss-sec ion o samples a he edges and in he cen e o samples SPSed a 1250 °C using he c i ical empe a u e o : (a)
1100 °C, (b) 1150 °C, (c) 1175 °C and (d) 1200 °C.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2421

pelle con ains mo e po es han he cen e (Fig. 9a sFig. 9b) due o
inhomogeneous densifica ion (compa e o Fig. 7) and exp essed in in-
c eased opaci y o he sample edge. The e o e, he inc easing po osi y
in he pe iphe y canno be explained by he inc ease o de ec con-
cen a ion only.
The non-uni o m densifica ion o elec ically non-conduc i e
ma e ials is mainly a ibu ed o he empe a u e g adien du ing SPS,
i.e. o he hi d p oposed mechanism [45,46]. Ce amic compac s a e
hea ed up by passing an elec ic cu en h ough he g aphi e die.
Consequen ly, one expec s ha he edge o non-conduc i e samples is
ho e han he cen e. As onishingly, he isual examina ion o samples
p epa ed in his s udy sugges s he opposi e beha iou (Fig. 7). Cola-
suonno epo ed on he inhomogeneous empe a u e dis ibu ion and
he o ma ion o local ho -zones in ce amics due o he elec ic cu en
flow pa hs shi du ing spa k plasma sin e ing [47]. Pa o fine g an-
ules o he RTP powde escapes om he gap be ween punches and die
wall du ing he ini ial powde compac ion and p oduces a ing su -
ounding he punches. This esul s in a change o he cu en pa hways
and o ma ion o a ho spo in he cen e o he pelle . Unde s anding
he eason behind such peculia densifica ion inhomogenei y in samples
and he empe a u e dis ibu ion, howe e , equi es mo e s udies.
None heless, he esul s ob ained in his s udy indica e he pelle s’
cen e migh ha e expe ienced a significan ly highe empe a u e
du ing sin e ing, esul ing in anspa en cen es o sin e ed pelle s; he
effec s and esul s o ho -zone o ma ion du ing SPS will be discussed
u he la e on.
Al hough highly dense bodies we e p oduced by spa k plasma sin-
e ing, small po es emain a he g ain bounda ies. Po es’diame e is
smalle han 100 nm; al hough such po es ha e limi ed sca e ing effec
a highe wa eleng hs (> 1000 nm), hei effec becomes p ominen in
he ul a iole egion o he spec um (< 400 nm) whe e he ligh
wa eleng h is compa able wi h he effec i e diame e o he esidual
po e.
4.2. Ca bon con amina ion
Ca bon con amina ion becomes significan and is esponsible o he
ligh ansmission dec ease in samples ab ica ed using Tc > 1150 °C.
Al hough g aphi e pa icles can be in oduced di ec ly o he ce amic
bodies p oduced by SPS, [23] Raman analyses e ealed highly dis-
o de ed o glassy ca bon in he samples, which is en i ely diffe en om
possible ca bon con amina ion sou ce (g aphi e pape ). This ules ou
di ec con amina ion by deb is om g aphi e pape . Mo eo e , all
samples we e subjec ed o he same cooling p ocess and, hus, he
ca bon con amina ion is unlikely o occu du ing cooling.
Kim e al. epo ed ha annealing spinel powde p io o spa k
plasma sin e ing emo es ola ile species and esul s in highe ans-
pa ency, and concluded ha ca bon-con aining species p esen a p io i
in powde cause ca bon con amina ion du ing SPS [48]. In o he wo ks,
he g aphi e die/pape is shown o be esponsible o ca bon con-
amina ion [30,31]. In ou case, he p esence o some ca bona e species
was confi med by Raman analysis (see Fig. 5); i is, he e o e, impo an
o e i y he sou ce o ca bon con amina ion. The o ma ion o solid
ca bon in po es om ola ile species can be exp essed by Boudoua d’s
eac ion:
→+CO g CO g C s2() () ()
2
(2)
Fig. 7. Op ical pho og aphs o a spinel disk p oduced wo-s age SPS: ini ial as
hea ing (100 °C min
−1
) oTc= 1100 °C, ollowed by slow hea ing (2.5 °C
min
−1
) o 1200 °C.
Fig. 8. G ain size dis ibu ion o samples as a unc ion o Tc in he cen e and a
he edge o sin e ed pelle s; median alues, 25 h and 75 h pe cen iles, as well as
maximum and minimum alues a e shown.
Fig. 9. Scanning elec on mic oscopy
images o he ac u e su ace o sam-
ples SPSed a 1250 °C wi h Tc = 1100
°C; he images we e aken om (a) he
cen e o sample and (b) close o he
edge o he sample. The samples we e
subjec ed o a hea ea men a 1150 °C
o 30 min o emo e he esidual
ca bon om he po es. A ows indica e
apped po es.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2422
The Gibbs ee ene gy a ia ions o Boudoua d’s eac ion agains
empe a u e change was calcula ed using Fac Sage®so wa e by as-
suming mos s able ca bon phases a e p oduced and conside ing he
o al p essu e inside closed po es a ying be ween 10 Pa ( acuum ap-
plied in he SPS chambe ) and 75 MPa ( he nominal applied mechanical
p essu e du ing SPS). Immedia ely a e he closu e o po es, he p es-
su e o gasses apped in hem is assumed o be equal o he acuum
p essu e in he SPS chambe . As he po es sh ink by sin e ing, he
p essu e inc eases, un il i eaches he maximum alue o 75 MPa, i.e.
he hyd os a ic p essu e in he po es equilib a es wi h he applied ex-
e nal mechanical p essu e. Fig. 10 shows he a ia ion o Gibbs ee
ene gy o Eq. 2as a unc ion o empe a u e; he empe a u e windows
be ween 1100 °C and 1250 °C is enla ged in Fig. 10b. Unde low
p essu es Gibbs’ ee ene gy o Eq. 2is posi i e indica ing ca bon de-
posi ion is no he modynamically a ou ed. The nega i e ee ene gy
o eac ion below 1150 °C in Fig. 10b, implies ha he deposi ion o
ca bon om ola ile species occu s only a low empe a u es unde
high p essu es. In con as , samples p oduced using Tc o 1100 and
1150 °C, which a e exposed o he ca bon deposi ion condi ions o a
longe ime, show highe anspa ency compa ed o hose p oduced
using Tc= 1175 °C and 1200 °C. Thus, ca bona e species a e unlikely o
be he cause o ca bon con amina ion.
I is also in e es ing o in es iga e how ca bon con amina ion is
dis ibu ed in he samples; Raman spec a we e collec ed in wo di-
ec ions: along he adii o he pelle s, and ac oss he hickness in he
cen e o he pelle s. The spec um collec ed in each poin was ea ed
by emo ing he backg ound and fi ing ca bon’s D, G and D’peaks
using a Gaussian unc ion ollowing leas -squa es cu e fi ing app oach
[49]. The ampli ude o he peaks was used o e alua e he concen a-
ion o espec i e species. Fig. 11 shows he dis ibu ion o ca bon and
he a io o ca bon D-band o G-band in he samples p oduced using he
c i ical empe a u e o 1175 °C. Ca bon con amina ion is dis ibu ed
almos uni o mly ac oss he heigh o samples (Fig. 11a), wi h 3 imes
highe concen a ion o diso de ed ca bon (D-Band) han highly o -
de ed ca bon (G-band). On he con a y, ca bon con amina ion is
concen a ed in he cen e o he sample ollowed by a g adual dec ease
owa ds he edges (Fig. 11b). The diso de ed ca bon (D-Band) dis-
ibu ion esembles ypical e o unc ion om he cen e o he edge
(dashed line in he g aph). The con amina ion o samples due o ca bon
diffusion om he ca bon pape should p oduce a concen a ion
g adien dec easing om he edge owa ds he cen e o plo s; his is
opposi e o he measu ed concen a ion p ofiles. The e o e, ca bon
dis ibu ion seems o be a esul o he empe a u e g adien in he
samples. Mo eo e , he uniaxial dis ibu ion o ca bon sugges s ha i is
in oduced in o samples h ough a deposi ion p ocess om he a mo-
sphe e wi h inc eased pa ial p essu e o ca bon in he u nace. Ac-
co ding o Mo i a e al., ca bon con amina ion du ing SPS o MgAl
2
O
4
occu s as a esul o eac ions be ween g aphi e die/pape and mag-
nesium alumina e spinel powde , especially when high hea ing a es a e
applied [29–31]. Wang e al. epo ed on he ab ica ion o anspa en
magnesium alumina e spinel ce amics using wo-s ep p essu e p ofile
[28]. The sin e ing p og amme consis ed o an ini ial low-p essu e s ep
ollowed by he applica ion o high p essu e in he final s ep; he esul s
showed ha applying a lowe p essu e o a sho e ime esul s in
lowe ca bon con amina ion du ing spa k plasma sin e ing.
Magnesium alumina e migh eac wi h ca bon esul ing in he de-
posi ion o glassy ca bon in samples; he e o e, i is wo h in es iga ing
u he he impac o applied p essu e on ca bon con amina ion. The
eac ion o magnesium alumina e spinel wi h he ca bon impu i y
p esen in he po es, which is coming om g aphi e die/pape , can be
simplified and w i en as [49]:
+→++
M
gAlO s Cs g Mgg AlOg CO g() 3 (, ) () () 3 (
)
24 2
(3)
Fig. 12 shows he a ia ion o Gibbs’ ee ene gy o Eq. 3as a
unc ion o he empe a u e unde diffe en p essu es be ween he
p essu e co esponding o he acuum (10 Pa) and p essu e ela ed o
he p essu e applied by punches (75 MPa), which is conside ed o be
equal o he equilib ium p essu e o gasses apped in closed po es. The
Gibbs’ ee ene gy dec eases con inuously wi h he empe a u e; how-
e e , in he whole ele an empe a u e in e al, he ee ene gy has
always a posi i e alue.
The ee ene gy o he eac ion is nega i e abo e 1300 °C and only
unde he eac ion p essu e below 50 Pa. The o ma ion o gases and
he magnesium alumina e spinel deg ada ion is he e o e ene ge ically
a ou ed a empe a u es below 1300 °C unde acuum (10 Pa). The
posi i e eac ion ee ene gy a he highe p essu es, i.e. abo e 50 Pa,
implies ha solid phases, ca bon, MgAl
2
O
4
, and possibly o he oxides
such as MgO and Al
2
O
3
, a e s able.
Taking in o accoun he changes o he s abili y o phases upon he
applica ion o p essu e, he ollowing mechanism can be p oposed o
Fig. 10. Changes o he Gibbs ee en-
e gy as a unc ion o empe a u e o
he o ma ion o solid ca bon om he
eac ion o oxides apped in he closed
po es unde hyd os a ic p essu es be-
ween 10 Pa o 75 MPa (equal o he
applied p essu e du ing SPS p ocess).
Gibbs ee ene gy s. empe a u e: (a)
1100−1400 °C-P=10 Pa- 75 MPa, (b)
1100−1250 °C-P = 50 MPa and 75
MPa.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2423
he gene a ion o ca bon con amina ion in he anspa en spinel bodies
p oduced du ing he wo-s age spa k plasma sin e ing.
Du ing he ini ial s ages o sin e ing, he po es a e connec ed, and,
he e o e, he gas p essu e is equal o he acuum applied du ing SPS.
The as hea ing in he fi s s age o sin e ing p og amme (100 °C
min
−1
) and he changes in elec ical pa hways esul in he o ma ion
o ho zones close o he pelle ’s cen e. Consequen ly, he local em-
pe a u e could exceed he c i ical empe a u e o magnesium alumina e
deg ada ion esul ing in he o ma ion o ca bon-con aining gases.
Fu he densifica ion p oduces closed po es, in which he hyd os a ic
p essu e o en apped gasses inc eases, e en ually equilib a ing he
mechanical p essu e applied he punches. The inc ease o p essu e and
he o ma ion o ho zones p o ides he he modynamically a ou able
condi ions o he deposi ion o ca bon and MgAl
2
O
4
. The local em-
pe a u e and size o ho zones inc eases when a highe c i ical em-
pe a u e is applied (i.e. 1100 s 1200 °C), which is eflec ed in he
o ma ion o la ge black a eas o samples p oduced a Tc = 1200 °C.
5. Summa y and conclusions
T anspa en magnesium alumina e spinel was ab ica ed using wo-
s age spa k plasma sin e ing o a comme cial MgAl
2
O
4
powde : a high
hea ing a e, 100 °C min
−1
, was used up o a Tc, c i ical empe a u e,
be ween 1100 and 1200 °C. Subsequen ly, he hea ing a e was slowed
down o 2.5 °C min
−1
, un il he maximum empe a u e o 1250 °C was
eached. T anspa en bodies a e ob ained a a c i ical empe a u e
lowe han 1150 °C. Discolou a ion and lack o anspa ency obse ed
in he samples sin e ed a he applied Tc > 1150 °C was caused by
ca bon con amina ion.
The c i ical empe a u e has a decisi e ole in achie ing anspa -
ency. Samples p oduced using a c i ical empe a u e lowe han 1150 °C
exhibi a highe in-line ansmission compa ed o hose p epa ed using
a highe c i ical empe a u e. The le el o ca bon con amina ion de-
c eases wi h dec easing c i ical empe a u e. Visual inspec ion and
analysis o Raman spec a confi med ha ca bon con amina ion is
dis ibu ed inhomogeneously. The o ma ion o ho zones in he cen e
o sin e ed compac s is esponsible o inhomogeneous densifica ion.
The modynamic calcula ions indica e ha local inc ease o empe a u e
can cause eac ions be ween spinel and g aphi e pape /die, p oducing
ola ile gasses. Inc ease o p essu e in closed po es changes he he -
modynamics condi ions a ou ing he o ma ion o diso de ed ca bon
Fig. 11. Ca bon concen a ion p ofile o e polished c oss-sec ion o samples
SPSed a 1250 °C using Tc = 1175 °C. The in ensi ies o ca bon D-band and G-
bands, in ela i e uni s ( .u.), a e shown o ep esen he concen a ion o
ca bon o e : (a) he sample hickness a he cen e and (b) along he adial axis
o he disks.
Fig. 12. Compa ison o he Gibbs ee ene gy change o he eac ion o MgAl
2
O
4
wi h g aphi e and o ma ion o gasses acco ding o Eq. 4 unde acuum (10 Pa) and
a hyd os a ic p essu e applied du ing he SPS (75 MPa). The dashed line poin s ou ΔG=0.
A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425
2424
p ecipi a es, which ac as ligh sca e ing cen es.
Decla a ion o Compe ing In e es
The au ho s decla e ha hey ha e no known compe ing financial
in e es s o pe sonal ela ionships ha could ha e appea ed o influ-
ence he wo k epo ed in his pape .
Acknowledgemen
This pape is a pa o dissemina ion ac i i ies o he p ojec
FunGlass. This p ojec has ecei ed unding om he Eu opean Union´s
Ho izon 2020 esea ch and inno a ion p og amme unde g an ag ee-
men No 739566. Financial suppo o his wo k by he g an s SAS-
MOST JRP 2015/6, and VEGA 2/0026/17 is g a e ully acknowledged.
We app ecia e he suppo p o ided by he Czech Minis y o Educa ion
unde g an LTT18013 In e -T ans e .
We app ecia e help ul discussions and use ul commen s o D K.
G iebenow on Raman Analyses o samples. We also app ecia e he help
o P o M. Liska and P o R. Klemen wi h he modynamic calcula ions
and op ical measu emen s.
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