Transparent magnesium aluminate spinel: Effect of critical temperature in two-stage spark plasma sintering

Con en s lis s a ailable a ScienceDi ec Jou nal o he Eu opean Ce amic Socie y jou nal homepage: www.else ie .com/loca e/jeu ce amsoc 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. Re e ences [1] K. Izumi, S. Miyazaki, S. Yoshida, T. Mizokawa, E. Hanamu a, Op ical p ope ies o 3d ansi ion-me al-doped MgAl2O4 spinels, Physical Re iew B 76 (7) (2007). [2] M. Ruba du Me ac, H.-J. Kleebe, M.M. Mülle , I.E. Reimanis, Fi y yea s o esea ch and de elopmen coming o ui ion; un a eling he complex in e ac ions du ing p ocessing o anspa en magnesium alumina e (MgAl2O4) spinel, J. Am. Ce am. Soc. 96 (11) (2013) 3341–3365. [3] S.F. Wang, J. Zhang, D.W. Luo, F. Gu, D.Y. Tang, Z.L. Dong, G.E.B. Tan, W.X. Que, T.S. Zhang, S. Li, L.B. Kong, T anspa en ce amics: p ocessing, ma e ials and ap- plica ions, P og. Solid S a e Chem. 41 (1–2) (2013) 20–54. [4] A. Golds ein, Co ela ion be ween MgAl2O4-spinel s uc u e, p ocessing ac o s and unc ional p ope ies o anspa en pa s (p og ess e iew), J. Eu . Ce am. Soc. 32 (11) (2012) 2869–2886. [5] K.E. Sicka us, J.M. Wills, N.W. G imes, S uc u e o spinel, J. Am. Ce am. Soc. 82 (12) (2004) 3279–3292. [6] L. Esposi o, A. Piancas elli, S. Ma elli, P oduc ion and cha ac e iza ion o ans- pa en MgAl2O4 p epa ed by ho p essing, J. Eu . Ce am. Soc. 33 (4) (2013) 737–747. [7] K. Mo i a, B.-N. Kim, H. Yoshida, K. Hi aga, Densifica ion beha io o a fine-g ained MgAl2O4 spinel du ing spa k plasma sin e ing (SPS), Sc ip a Ma e . 63 (6) (2010) 565–568. [8] M. Sokol, M. Halabi, S. Kalabukho , N. F age, Nano-s uc u ed MgAl2O4 spinel consolida ed by high p essu e spa k plasma sin e ing (HPSPS), J. Eu . Ce am. Soc. 37 (2) (2017) 755–762. [9] S.M. Hosseini, S uc u al, elec onic and op ical p ope ies o spinel MgAl2O4 oxide, Phys. S a us Solidi B 245 (12) (2008) 2800–2807. [10] G. Gilde, P. Pa el, P. Pa e son, D. Blodge , D. Duncan, D. Hahn, E alua ion o ho p essing and ho isos as ic p essing pa ame e s on he op ical p ope ies o spinel, J. Am. Ce am. Soc. 88 (10) (2005) 2747–2751. [11] J.M. Kim, H.N. Kim, Y.J. Pa k, J.W. Ko, J.W. Lee, H.D. Kim, Fab ica ion o ans- pa en MgAl2O4 spinel h ough homogenous g een compac ion by mic ofluidiza- ion and slip cas ing, Ce am. In . 41 (10) (2015) 13354–13360. [12] C. Gajdowski, J. Bohmle , Y. Lo gouilloux, S. Lemonnie , S. d’As o g, E. Ba aud, A. Le iche, Influence o pos -HIP empe a u e on mic os uc u al and op ical p ope ies o pu e MgAl2O4 spinel: om opaque o anspa en ce amics, J. Eu . Ce am. Soc. 37 (16) (2017) 5347–5351. [13] I. Ganesh, G.J. Reddy, G. Sunda a ajan, S.M. Olhe o, P.M.C. To es, J.M.F. Fe ei a, Influence o p ocessing ou e on mic os uc u e and mechanical p ope ies o MgAl2O4 spinel, Ce am. In . 36 (2) (2010) 473–482. [14] S. Bhadu i, S.B. Bhadu i, Mic os uc u al and mechanical p ope ies o nanoc ys- alline spinel and ela ed composi es, Ce am. In . 28 (2) (2002) 153–158. [15] C. Aksel, B. Rand, F.L. Riley, P.D. Wa en, Mechanical p ope ies o magnesia-spinel composi es, J. Eu . Ce am. Soc. 22 (5) (2002) 745–754. [16] G. Bonne on , G. Fan ozzi, S. T ombe , L. Bonneau, Fine-g ained anspa en MgAl2O4 spinel ob ained by spa k plasma sin e ing o comme cially a ailable nanopowde s, Ce am. In . 38 (1) (2012) 131–140. [17] N. F age, S. Cohen, S. Mei , S. Kalabukho , M.P. Da iel, Spa k plasma sin e ing (SPS) o anspa en magnesium-alumina e spinel, J. Ma e . Sci. 42 (9) (2007) 3273–3275. [18] M. Sokol, S. Kalabukho , M.P. Da iel, N. F age, High-p essu e spa k plasma sin- e ing (SPS) o anspa en polyc ys alline magnesium alumina e spinel (PMAS), J. Eu . Ce am. Soc. 34 (16) (2014) 4305–4310. [19] S. Cohen, B. Ra zke , M. Sokol, S. Kalabukho , N. F age, Polyc ys alline anspa en magnesium alumina e spinel p ocessed by a combina ion o spa k plasma sin e ing (SPS) and ho isos a ic p essing (HIP), J. Eu . Ce am. Soc. 38 (15) (2018) 5153–5159. [20] K. Mo i a, B.N. Kim, K. Hi aga, H. Yoshida, Fab ica ion o anspa en MgAl2O4 spinel polyc ys al by spa k plasma sin e ing p ocessing, Sc ip a Ma e . 58 (12) (2008) 1114–1117. [21] M.M. Mulle , H.J. Kleebe, Sin e ing mechanisms o LiF-Doped Mg-Al-Spinel, J. Am. Ce am. Soc. 95 (10) (2012) 3022–3024. [22] L. Ramond, G. Be na d-G ange , A. Addad, C. Guiza d, T. Rouxel, Sin e ing o soda- lime glass mic osphe es using spa k plasma sin e ing, J. Am. Ce am. Soc. 94 (9) (2011) 2926–2932. [23] A. Be and, J. Ca eaud, G. Delaizi , J.R. Ducle e, M. Colas, J. Co ne e, M. Vandenhende, V. Coude c, P. Thomas, A comp ehensi e s udy o he ca bon con amina ion in ellu i e glasses and glass-ce amics sin e ed by spa k plasma sin e ing (SPS), J. Am. Ce am. Soc. 97 (1) (2014) 163–172. [24] K. Rozenbu g, I.E. Reimanis, H.J. Kleebe, R.L. Cook, Sin e ing kine ics o a MgAl2O4 spinel doped wi h LiF, J. Am. Ce am. Soc. 91 (2) (2008) 444–450. [25] I.E. Reimanis, H.J. Kleebe, Reac ions in he sin e ing o MgAl2O4 spinel doped wi h LiF, In . J. Ma e . Res. 98 (12) (2007) 1273–1278. [26] A. Golds ein, J. Rae hel, M. Ka z, M. Be lin, E. Galun, T anspa en MgAl 2 O 4 /LiF ce amics by ho -p essing: hos –addi i e in e ac ion mechanisms issue e isi ed, J. Eu . Ce am. Soc. 36 (7) (2016) 1731–1742. [27] G.R. Villalobos, J.S. Sanghe a, I.D. Agga wal, Deg ada ion o magnesium aluminum spinel by li hium fluo ide sin e ing aid, J. Am. Ce am. Soc. 88 (5) (2005) 1321–1322. [28] C. Wang, Z. Zhao, T anspa en MgAl2O4 ce amic p oduced by spa k plasma sin- e ing, Sc ip a Ma e . 61 (2) (2009) 193–196. [29] K. Mo i a, B.N. Kim, H. Yoshida, K. Hi aga, Y. Sakka, Influence o spa k plasma sin e ing (SPS) condi ions on ansmission o MgAl2O4 spinel, J. Am. Ce am. Soc. 98 (2) (2015) 378–385. [30] K. Mo i a, B.-N. Kim, H. Yoshida, K. Hi aga, Y. Sakka, Dis ibu ion o ca bon con- amina ion in MgAl 2 O 4 spinel occu ing du ing spa k-plasma-sin e ing (SPS) p ocessing: I –effec o hea ing a e and pos -annealing, J. Eu . Ce am. Soc. 38 (6) (2018) 2588–2595. [31] K. Mo i a, B.N. Kim, H. Yoshida, K. Hi aga, Y. Sakka, Dis ibu ion o ca bon con- amina ion in oxide ce amics occu ing du ing spa k-plasma-sin e ing (SPS) p o- cessing: II - Effec o SPS and loading empe a u es, J. Eu . Ce am. Soc. 38 (6) (2018) 2596–2604. [32] L.M. F aas, J.E. Moo e, J.B. Salzbe g, Raman cha ac e iza ion s udies o syn he ic and na u al Mgal2o4 c ys als, J. Chem. Phys. 58 (9) (1973) 3585–3592. [33] M.P. O’Ho o, A.L. F isillo, W.B. Whi e, La ice ib a ions o MgAl2O4 spinel, J. Phys. Chem. Solids 34 (1) (1973) 23–28. [34] R. Ca acas, E.J. Banigan, Elas ici y and Raman and in a ed spec a o MgAl2O4 spinel om densi y unc ional pe u ba ion heo y, Phys. Ea h Plane . In e . 174 (1–4) (2009) 113–121. [35] M. Lazze i, P. Thibaudeau, Ab ini io Raman spec um o he no mal and diso de ed MgAl2O4 spinel, Phys. Re . B 74 (14) (2006). [36] S. Gunaseka an, G. Anbalagan, S. Pandi, Raman and in a ed spec a o ca bona es o calci e s uc u e, J. Raman Spec osc. 37 (9) (2006) 892–899. [37] M.I. Bu ueco, M. Mo a, J.-S.-C. O Molecula …, Raman mic ospec oscopy o hyd o alci e-like compounds modified wi h sulpha e and sulphona e o ganic an- ions, Jou nal o Molecula (2013). [38] N. Benameu , G. Be na d-G ange , A. Addad, S. Raffy, C. Guiza d, Sin e ing analysis o a fine-g ained alumina-magnesia spinel powde , J. Am. Ce am. Soc. 94 (5) (2011) 1388–1396. [39] A. Talimian, V. Pouchly, H.F. El-Magh aby, K. Maca, D. Galusek, Impac o high ene gy ball milling on densifica ion beha iou o magnesium alumina e spinel e alua ed by mas e sin e ing cu e and cons an a e o hea ing app oach, Ce am. In . 45 (17, Pa B) (2019) 23467–23474. [40] G. Be na d-G ange , N. Benameu , A. Addad, M. Nyg en, C. Guiza d, S. De ille, Phenomenological analysis o densifica ion mechanism du ing spa k plasma sin- e ing o MgAl2O4, J. Ma e . Res. 24 (6) (2011) 2011–2020. [41] B. Ra zke , M. Sokol, S. Kalabukho , N. F age, C eep o polyc ys alline magnesium alumina e spinel s udied by an SPS appa a us, Ma e ials Basel (Basel) 9 (6) (2016) 493. [42] H. Palmou , D.M. Choi, L.D. Ba nes, R.D. McB aye , W.W. K iegel, De o ma ion in Ho -p essed polyc ys alline spinel, in: H.H. S adelmaie , W.W. Aus in (Eds.), Ma e ials Science Resea ch, Sp inge US, Bos on, MA, 1963, pp. 158–197. [43] J.-H. Lee, B.-N. Kim, B.-K. Jang, Non-uni o m sin e ing beha io du ing spa k plasma sin e ing o Y2O3, Ce am. In . 46 (3) (2020) 4030–4034. [44] B. Ra zke , A. Wagne , M. Sokol, S. Kalabukho , M.P. Da iel, N. F age, Op ical and mechanical p ope ies o anspa en alumina ab ica ed by high-p essu e spa k plasma sin e ing, J. Eu . Ce am. Soc. 39 (8) (2019) 2712–2719. [45] P. Mondalek, L. Sil a, M. Belle , A nume ical model o powde densifica ion by SPS echnique, Ad . Eng. Ma e . 13 (7) (2011) 587–593. [46] J. Ra hel, M. He mann, W. Becke , Tempe a u e dis ibu ion o elec ically conduc i e and non-conduc i e ma e ials du ing Field Assis ed Sin e ing (FAST), J. Eu . Ce am. Soc. 29 (8) (2009) 1419–1425. [47] P.S. Colasuonno, Me hod o C ea ing Func ionally G aded Ma e ials Wi h Spa k Plasma Sin e ing and a Con inuous Machine o Fu u e Scalabili y, Colo ado S a e Uni e si y, 2017. [48] B.N. Kim, K. Mo i a, J.H. Lim, K. Hi aga, H. Yoshida, Effec s o p ehea ing o powde be o e spa k plasma sin e ing o anspa en MgAl2O4 spinel, J. Am. Ce am. Soc. 93 (8) (2010) 2158–2160. [49] A.C. Fe a i, J. Robe son, In e p e a ion o Raman spec a o diso de ed and amo phous ca bon, Phys. Re . B 61 (20) (2000) 14095–14107. A. Talimian, e al. Jou nal o he Eu opean Ce amic Socie y 40 (2020) 2417–2425 2425