Combustion Synthesis of Aluminum Oxynitride in Loose Powder Beds

Home , Aluminium oxynitride, Ammonium nitrite

materials

Article Combustion Synthesis of Aluminum Oxynitride in Loose Powder Beds

Alan Wilma ´nski*, Magdalena Zarzecka-Napierała and Zbigniew P˛edzich*

Faculty of Materials Science and Ceramics, AGH University of Science and Technology, 30 Mickiewicz Av., 30-059 Kraków, Poland; [email protected] * Correspondence: [email protected] (A.W.); [email protected] (Z.P.)

Abstract: This paper describes combusting loose powder beds of mixtures of aluminum metal powders and aluminum oxide powders with various grain sizes under various nitrogen pressure. The synthesis conditions required at least 20/80 weight ratio of aluminum metal powder to alumina powder in the mix to reach approximately 80 wt% of γ-AlON in the products. Finely ground fused white alumina with a mean grain size of 5 µm was sufficient to achieve results similar to very fine alumina with 0.3 µm grains. A lower nitrogen pressure of 1 MPa provided good results, allowing a less robust apparatus to be used. The salt-assisted combustion synthesis upon addition of 10 wt% of ammonium nitrite resulted in a slight increase in product yield and allowed lower aluminum metal powder content in mixes to be ignited. Increasing the charge mass five times resulted in a very similar γ-AlON yield, providing a promising technology for scaling up. Synthesis in loose powder beds could be utilized for effective production of relatively cheap and uniform AlON powder, which could be easily prepared for forming and sintering without intensive grounding and milling, which usually introduce serious contamination. Keywords: aluminum oxynitride; combustion synthesis; salt-assisted; SHS; AlON Citation: Wilmański,A.; Zarzecka-Napierała, M.; P˛edzich,Z. Combustion Synthesis of Aluminum Oxynitride in Loose Powder Beds. 1. Introduction Materials 2021, 14, 4182. https:// The binary Al2O3–AlN system contains a solid solution known as aluminum oxyni- doi.org/10.3390/ma14154182 tride with a cubic structure usually abbreviated as γ-AlON [1]. The compound was first synthesized in 1959 by Yamaguchi and Yanagida but gained more interest after the first Academic Editor: Andres Sotelo successful sintering of a transparent specimen by McCauley and Corbin [2]. The material can achieve excellent transmittance from ultraviolet to mid-infrared wavelengths [3]. These Received: 2 June 2021 ceramics have various favorable properties, such as good mechanical strength, temperature Accepted: 15 July 2021 Published: 27 July 2021 stability, and chemical resistance, that allow them to be applied as transparent armor, missile domes, refractory materials, optoelectronics, and high temperature windows [4–6].

Publisher’s Note: MDPI stays neutral Manufacturing of aluminum oxynitride ceramics usually starts with the synthesis of γ with regard to jurisdictional claims in -AlON powder with one of the well-known methods, i.e., carbothermal reduction and published maps and institutional afﬁl- nitridation (CRN) [7–11] or solid-state synthesis (SS) [12–15], or one less frequently used, iations. such as aluminothermic reduction and nitridation (ARN) [16,17] or self-propagating high temperature synthesis (SHS) [18–22]. The main drawbacks of the most popular processes are high temperatures of over 1650 ◦C and prolonged soaking times. Dealing with these problems by employing the sol-gel method results in other disadvantages, such as high cost of chemical reactants and long gel preparation times, producing, in return, extremely Copyright: © 2021 by the authors. γ Licensee MDPI, Basel, Switzerland. ﬁne -AlON powders after synthesis [23,24]. Another concern of the CRN method relates This article is an open access article to oxidizing the remaining carbon, present in the powder batch post reaction, in open ◦ distributed under the terms and air furnaces at 600–700 C for a few hours that can lead to a change in the aluminum conditions of the Creative Commons oxynitride stoichiometry, which has not been addressed in any of the publications on CRN. Attribution (CC BY) license (https:// An additional disadvantage is a necessity of a secondary operation before the powders can creativecommons.org/licenses/by/ be used to form green bodies. A recent paper describes the oxidation behavior of coarse ◦ 4.0/). (30 µm) γ-AlON powders in air at temperatures above 800 C[25]. The powders were

Materials 2021, 14, 4182. https://doi.org/10.3390/ma14154182 https://www.mdpi.com/journal/materials Materials 2021, 14, 4182 2 of 10

prepared by the CRN technique and the referred publication, on the basis of which the powders were prepared, does not provide any data on the procedure. What it does provide is a DTA plot ranging from 400 ◦C and presents a small constant increase in exothermic reaction taking place that cannot be attributed only to residual carbon oxidation, as the fine grains of γ-AlON may also oxidize. The SHS has the potential of being a cheap and fast alternative of producing less pure γ-AlON. Several papers describe different reaction environments with the key component being a mixture of Al2O3 and Al. The first attempts were conducted in air at ambient pressure, giving the best results within 40–50 wt% of aluminum [18]. The obtained powders contained a significant number of impurities consisting of Al2O3, AlN, and melted aluminum. Zientara et al. investigated igniting mixtures of 15–50 wt% Al with Al2O3 under 2.5 MPa nitrogen [19]. At low aluminum contents (15, 20%), γ-AlON yield was low (c.a. 30%); when increasing the proportion of Al, the synthesis resulted in a higher, i.e., 40–50 wt%, γ phase and a significant decrease of residual Al2O3, simultaneously having increased amounts of AlN, 21R, and unreacted aluminum. The same year, another paper described mixtures of 30 and 50 wt% Al ignited in nitrogen atmosphere under 1, 3, and 5 MPa pressure, resulting in mixtures of γ-AlON as the main phase and residual AlN, Al2O3, and Al [20]. Samples with 30% Al synthesized under 3 and 5 MPa N2 had no residual aluminum present; the authors do not present number values for phase compositions of the resulting powders. The addition of aluminum nitrite nonahydrate and high nitrogen pressure were examined in [21]. A fixed amount of 40 wt% Al was used, and 10–40 wt% of aluminum nitrite, in exchange for aluminum oxide, was mixed and pressed to 60% relative density. Samples were ignited under 100 MPa nitrogen, forming a porous structure. All samples were composed mostly of γ-AlON with impurities consisting of Al (in low nitrite sample), 21R, and AlN (in high nitrite sample). Another approach with an oxidizer was conducted by Akopdzhanyan et al. [22]. The proportion of Al/Al2O3 was fixed at 21/79 and 25/75 wt% with up to a 17 wt% addition of Mg(ClO4) under 5–100 MPa N2 pressure. Perchlorate acted as an oxidizer leaving MgCl2 as a minor impurity that was washed by boiling in deionized water. The amount of magnesium that can enter the solid solution in aluminum oxynitride was not taken into account and not determined. The best result was given by the lower aluminum mix with 11.4 wt% perchlorate synthesized under 20 MPa nitrogen pressure followed by washing, presenting no detectable aluminum and magnesium compounds on the diffraction pattern. None of the cited papers present the influence of the different grain sizes of powder substrates on the γ-AlON formation and yield. There is also a lack of information on the influence of lower nitrogen pressure on the mentioned process. Most of the available literature describes synthesis in powder compacts rather than loose powders. Synthesis in loose powder beds could be utilized for effective production of relatively cheap and uniform AlON powder which could be easily prepared for forming and sintering without intensive grounding and milling, which usually introduce serious contamination. In this work, we present a broad approach to investigate the influence of the grain size of aluminum metal and oxide powders, nitrogen pressure in the reaction chamber, proportion of Al/Al2O3, and the use of ammonium nitrite in loose powder beds on phase composition and γ-AlON phase yield of SHS-derived powders.

2. Materials and Methods

Commercial powders included Al2O3 (KOS Korund, Koło, Poland, EA-320 d50 = 55 µm, EA-1200 d50 = 5 µm, Almatis CT3000 LS SG d50 = 0.3 µm), Al (Sun Chemicals, Skawina, Polnad, Al-7 d50 = 8 µm, Al-32 d50 = 11 µm, Al-63 d50 = 49 µm, Al-150 d50 = 200 µm), NH4NO3 (Avantor, Gliwice, Poland, <250 µm), and N2 (Air Liquide, Cracow, Poland). Powder purity was over 99.5%, and nitrogen was 99.99%. Powders were weighed and mixed in a rotating vibration mill using 5 mm alumina balls in isopropanol for 1h. The powders were dried in a laboratory oven at 60 ◦C. A 100 g powder batch was placed on a boat-shaped graphite foil and surrounded by high temperature insulating wool. Ends of Materials 2021, 14, x 3 of 10

rotating vibration mill using 5 mm alumina balls in isopropanol for 1h. The powders were dried in a laboratory oven at 60 °C. A 100 g powder batch was placed on a boat-shaped graphite foil and surrounded by high temperature insulating wool. Ends of the foil were connected to copper electrodes. A schematic representation of the SHS reactor setup is presented in Figure 1. This setup was placed in a pressure chamber which was purged with nitrogen for a few seconds; then, the release valve was closed, and the chamber was Materials 2021, 14, x 3 of 10 evacuated. The chamber was filled with nitrogen to reach the designed pressure. The powder ignition was done by passing current through the graphite foil for 15 s. After the reac- torrotating cooled vibration down (0.5–2 mill using h, depending 5 mm alumina on balls nitrog in isopropanolen pressure for used) 1h. The the powders slightly were sintered pow- Materials 2021, 14, 4182 dersdried were in a broken laboratory down oven by at 60hand °C. Aand 100 crushed g powder in batch a laboratory was placed jaw on a crusher. boat-shaped3 of 10 graphitePhase foil composition and surrounded of bythe high powders temperat wasure insulating analyzed wool. by EndsX-ray of diffractionthe foil were (X’Pert Pro, PANalytical,connected to Malvern,copper electrodes. UK), and A schematicthe Rietveld representation refinement of theallowed SHS reactor determining setup is the quanti- presented in Figure 1. This setup was placed in a pressure chamber which was purged tativethe foil phase were connectedcontent. Powder to copper electrodes.morphology A schematic was observed representation using of scanning the SHS reactor electron micros- with nitrogen for a few seconds; then, the release valve was closed, and the chamber was setup is presented in Figure1. This setup was placed in a pressure chamber which was copyevacuated. (FEI Nova The chamber Nano SEM was filled 200, with Thermo nitrog Fisheren to reach Scientific, the designed Hillsboro, pressure. OR, The pow-USA). The time– temperaturepurged with nitrogenprofile forhas a few not seconds; been measured then, the release as valvethe reaction was closed, zone and theis fully chamber insulated with wasder evacuated.ignition was The done chamber by passing was ﬁlledcurrent with throug nitrogenh the tographite reach the foil designed for 15 s. pressure.After the reac- The aluminasilicatepowdertor cooled ignition down waswool,(0.5–2 done h,and depending by passingthe reaction on current nitrog chamber throughen pressure the was used) graphite not the foilequippedslightly for 15 sintered s. Afterwith pow- thean outlet for a thermocouple.reactorders were cooled broken down down (0.5–2 by h,hand depending and crushed on nitrogen in a laboratory pressure jaw used) crusher. the slightly sintered powdersPhase were composition broken down of the by handpowders and crushedwas analyzed in a laboratory by X-ray jawdiffraction crusher. (X’Pert Pro, PANalytical, Malvern, UK), and the Rietveld refinement allowed determining the quantitative phase content. Powder morphology was observed using scanning electron microscopy (FEI Nova Nano SEM 200, Thermo Fisher Scientific, Hillsboro, OR, USA). The time– temperature profile has not been measured as the reaction zone is fully insulated with aluminasilicate wool, and the reaction chamber was not equipped with an outlet for a thermocouple.

Figure 1. Schematic of the reactor setup. (A) powder bed, (B) high temperature brick, (C) graphite foil boat, (D) copper contact electrode, (E) mounting screw and nut, (F) insulating wool. Figure 1. Schematic of the reactor setup. (A) powder bed, (B) high temperature brick, (C) graphite foil boat,Phase (D) composition copper contact of the electrode, powders was (E) analyzedmounting by screw X-ray diffractionand nut, ( (X’PertF) insulating Pro, PAN- wool. alytical, Malvern, UK), and the Rietveld refinement allowed determining the quantitative 3. phaseResults content. Powder morphology was observed using scanning electron microscopy (FEI NovaFigure Nano 1. Schematic SEM 200, of Thermothe reactor Fisher setup. Scientific, (A) powder Hillsboro, bed, (B) OR,high USA).temperature The time–temperature brick, (C) graphite profilefoilThe boat, has first(D not) copper part been contactof measured the electrode, experiment as the (E reaction) mounting was zone design screw isfully anded nut, insulatedto (determineF) insulating with aluminasilicate wool.the optimal grain size of aluminumwool, and theand reaction aluminum chamber oxide was notpowders equipped and with an an optimal outlet for nitrogen a thermocouple. pressure. The weight ratio3. Results of Al/Al 2O3 was fixed at 20/80 as this was known to sustain the reaction self-propa- 3. Results gation.The The first nitrogen part of the pressure experiment was was established designed to as determine 0.1, 1, 2, the and optimal 3 MPa. grain The size outcome of of the The first part of the experiment was designed to determine the optimal grain size of synthesisaluminum was and rated aluminum for the oxide amount powders of γand-AlON, an optimal secondary nitrogen oxynitride, pressure. The and weight residual alumi- aluminumratio of Al/Al and2O aluminum3 was fixed oxide at 20/80 powders as this and was an known optimal to nitrogensustain the pressure. reaction The self-propa- weight num.ratiogation. The of The Al/Al amount nitrogen2O3 wasof pressure γ fixed-AlON atwas 20/80obtained established as this in asthis was 0.1, knownpart 1, 2, andis topresented 3 sustain MPa. The the in outcomereaction Figure of self-2. the propagation.synthesis was The rated nitrogen for the pressure amount of was γ-AlON, established secondary as 0.1, oxynitride, 1, 2, and 3 MPa.and residual The outcome alumi- ofnum. the synthesisThe amount was of rated γ-AlON for theobtained amount in ofthisγ-AlON, part is presented secondary in oxynitride, Figure 2. and residual aluminum. The amount of γ-AlON obtained in this part is presented in Figure2.

Figure 2. Cont.

MaterialsMaterials2021 2021, 14, 14, 4182, x 44 of of 10 10

FigureFigure 2. 2.Aluminum Aluminum oxynitride oxynitride yield yield achieved achieved by by the the means means of of SHS SHS method method from from various various aluminum aluminum metal metal powders: powders: Al-7 Al- (•7), (● Al-32), Al-32 ( ),(○ Al-63), Al-63 ( ),(□ and), and Al-150 Al-150 ( ()■ and) and under under varied varied nitrogen nitrogen pressures pressures for for different different substrates: substrates: ( a(a)) coarse coarse EA-320, EA-320, (b(b)) medium medium# EA-1200, EA-1200, (c ()c fine) fine CT3000 CT3000 alumina. alumina. The coarse alumina substrate presented the lowest tendency for γ-AlON formation. The coarse alumina substrate presented the lowest tendency for γ-AlON formation. The highest yield was achieved with Al-63 under 1 MPa nitrogen (74% γ-AlON). What The highest yield was achieved with Al-63 under 1 MPa nitrogen (74% γ-AlON). What is is worth noticing is that no residual aluminum was detected, and the sum of present worth noticing is that no residual aluminum was detected, and the sum of present oxyni- oxynitrides was calculated to be 87.5%. The medium and fine alumina substrates performed trides was calculated to be 87.5%. The medium and fine alumina substrates performed similar to each other. Both achieved between 70 and 80% γ-AlON at every pressure in combinationsimilar to each with other. Al-7, Both -32, andachieved -63, and between the coarse 70 and aluminum 80% γ-AlON showed at every a steep pressure decrease in withcombination increasing with synthesis Al-7, -32, pressure. and -63, The and highest the coarse amount aluminum of γ-AlON showed phase wasa steep achieved decrease at with increasing synthesis pressure. The highest amount of γ-AlON phase was achieved 1 MPa N2 pressure and Al-63, the same as for the coarse Al2O3 substrate, and additionally withat 1 Al-32MPa N combined2 pressure with and medium Al-63, the alumina; same as however, for the coarse therewas Al2O a3little substrate, residual and aluminum addition- detected.ally with TheAl-32 sum combined of oxynitrides with medium was found alumina; to be slightlyhowever, higher there for was the a mediumlittle residual alumina alu- substrateminum detected. than for the The fine sum alumina of oxynitrides one, reaching was fo 89%und and to be 87.7%, slightly respectively. higher for Both the aluminamedium substratesalumina substrate showed a than high for conversion the fine ratealumina to γ-AlON one, reaching with coarse 89% aluminum and 87.7%, powder respectively. under lowBoth pressure alumina andsubstrates a sharp showed decline a high with conversion increasing rate pressure. to γ-AlON These with results coarse may aluminum prove thatpowder using under ultra low fine aluminapressure andand aluminuma sharp decline metal with powders increasing does notpressure. give any These benefit results in returnmay prove over cheaperthat using and ultra coarser fine alumina resources and when aluminum reacted metal in loose powders beds. The does same not give applies any forbenefit the nitrogenin return pressureover cheaper in the and reaction coarser chamber. resources Judgingwhen reacted from in the loose above beds. results, The same the powdersapplies for of the Al-63 nitrogen and EA-1200 pressure and in athe pressure reaction of chamber. 1 MPa of Judging nitrogen from gas the were above chosen results, for furtherthe powders investigations. of Al-63 and EA-1200 and a pressure of 1 MPa of nitrogen gas were chosen for furtherSubstrate investigations. composition in the ratio of 20/80 for Al/Al2O3 reacting with nitrogen resultedSubstrate in a 50/50 composition mol ratio ofin AlN/Al the ratio2O 3of, which20/80 isfor richer Al/Al in2O nitrogen3 reacting than with any nitrogen aluminum re- oxynitridesulted in a in 50/50 the phase mol ratio diagrams of AlN/Al presented2O3, which in theliterature. is richer in The nitrogen next step than was any to determinealuminum ifoxynitride lowering thein the aluminum phase diagrams ratio could presented be a viable in the solution literature. for γ The-AlON next yield step increase.was to deter- For this,mine the if lowering aluminum/alumina the aluminum ratio ratio was could decremented be a viable insolution 1% steps for γ until-AlON reaching yield increase. 14/86, whichFor this, did the not aluminum/alumina ignite. ratio was decremented in 1% steps until reaching 14/86, whichThe did resulting not ignite. powder compositions are presented in Figure3. The powder mixture with theThe lowest resulting amount powder of aluminumcompositions did are not presented ignite. The in 15Figure and 16%3. The batches powder were mixture able towith ignite, the butlowest the amount reaction of was aluminum unsustainable, did not with ignite. much The of15 the and aluminum 16% batches left were unreacted, able to partiallyignite, but nitrided the reaction to AlN andwas partiallyunsustainable, being able with to much react withof the alumina aluminum to form left 13 unreacted, and 22% γpartially-AlON, respectively.nitrided to AlN From and 17% partially Al, there being is a able noticeable to react linear with increasealumina into theformγ-AlON 13 and phase22% γ yield-AlON, and respectively. consumption From of alumina; 17% Al, the there residual is a AlN noticeable and Al stabilizelinear increase at around in 5the and γ- 0.5%,AlON respectively. phase yield In and the batchconsumption containing of 19%alumina; Al, the the 21R residual phase appears. AlN and This Al experimentstabilize at showedaround no5 and benefit 0.5%, from respectively. decreasing In thethe amountbatch containing of aluminum 19% inAl, the the starting 21R phase powder appears. by evenThis oneexperiment percent. showed no benefit from decreasing the amount of aluminum in the starting powder by even one percent.

Materials 2021, 14, x 5 of 10

MaterialsMaterials 20212021,, 1414,, x 4182 55 of of 10 10

Figure 3. Phase composition of powder mixtures obtained by the means of SHS method from mixes FigureofFigure powders 3. 3. PhasePhase with composition composition14–20 wt% aluminumof of powder powder mixturescontent mixtures an obtained obtainedd medium by by aluminathe the means means EA-1200 of of SHS SHS powdermethod method synthesizedfrom from mixes mixes ofunderof powders powders 1 MPa with with nitrogen, 14–20 14–20 wt%γ wt%-AlON aluminum aluminum (●), Al2 Ocontent content3 (○), AlN an andd (medium□ medium), Al (■ ),alumina alumina and Al 7EA-1200O EA-12003N5 (∆). powder powder synthesized synthesized underunder 1 1 MPa MPa nitrogen, nitrogen, γγ-AlON-AlON ( (●•),), Al Al22OO33 (○( ), ),AlN AlN (□ (), ),Al Al (■ (), and), and Al Al7O73NO35 N(∆5).( ∆). The third part of the experiment was# to investigate the influence of ammonium nitrite on theTheThe system. third third part partThe ofamount of the the experiment experiment of salt used was was was to to in 5–35 investigatevestigate wt%, keepingthe the influence inﬂuence the Al/Al of of ammonium ammonium2O3 ratio fixed nitrite nitrite at on20/80.on the the system.Phase system. compositions The The amount amount of of salt the used resulting was 5–35powders wt%, are keeping presented thethe Al/AlAl/Al in Figure2OO33 ratioratio 4. Afixed ﬁxed slight at at 20/80.increase20/80. Phase Phase in γ -AlONcompositions compositions synthesis of of occurredthe the resulting resulting for samples powders powders cont are areaining presented presented 10 and in in 15% Figure Figure nitrite. 44.. A Adding slightslight γ increasemoreincrease salt, in in γabove-AlON-AlON 20%, synthesis synthesis resulted occurred occurred in a sharp for for samples samplesdecrease cont containing in ainingoxynitride 10 10 and and and 15% 15% a risenitrite. nitrite. in aluminaAdding Adding morecontent.more salt, salt, The above above powder 20%, 20%, bed resulted resulted after the in in reactiona a sharp sharp withdecrease decrease 20% inor in moreoxynitride oxynitride salt was and and visibly a a rise rise meltedin in alumina with content.cavitiescontent. inThe The the powder powder middle, bed bed with after after graphite the the reaction reaction foil st withuck with to 20% 20% the or orbottom, more more salt saltand was wasbeing visibly visibly hard melted meltedto break with with by cavitieshand.cavities in in the the middle, middle, with with graphite graphite foil foil stuck stuck to tothe the bottom, bottom, and and being being hard hard to break to break by hand.by hand.

FigureFigure 4. 4. PhasePhase composition composition of ofpowders powders obtained obtained by bythe themeans means of SHS of SHS method method from from mixtures mixtures con- Figuretainingcontaining 4. different Phase different composition amount amount of ammonium of of powders ammonium nitrite,obtained nitrite, γ-AlON by γth-AlONe (means●), Al (•2 Oof),3 AlSHS(○2),O AlN method3 ( (),□), AlN Alfrom ( (■ ),mixtures), and Al (Al),7 Ocon- and3N5 taining(Al∆).7 O3N different5 (∆). amount of ammonium nitrite, γ-AlON (●), Al2O3 (○), AlN# (□), Al (■), and Al7O3N5 (∆). TheThe experiment experiment with with the the reduced reduced amount amount of aluminum of aluminum was was repeated repeated using using 10% 10%am- moniumammoniumThe experimentnitrite nitrite to determine to with determine the reducedif the if the salt saltamount additi additionon of aluminumcould could help help waswith with repeated the the synthesis. synthesis. using 10%FigureFigure am- 55 moniumshowsshows the the nitrite phase phase to composition composition determine ifof the produced salt additi powders. on could The The help most most with noteworthy the synthesis. fact is Figure that the 5 showsmixmix of the thethe phase lowestlowest composition aluminum aluminum content ofcontent produced not not only onlypowders. ignited ignited butThe but alsomost also allowed noteworthy allowed for thefor fact reactionthe is reactionthat zone the mixzoneto go of to through the go lowestthrough the wholealuminum the whole powder content powder bed, yieldingnot bed, only yielding aroundignited around 46%but γalso-AlON 46% allowed γ and-AlON thefor and samethe thereaction amount same zoneof Al 2toO go3. More through aluminum the whole in the powder mixtures bed, resulted yielding in around increasing 46% the γ-AlON amount and of aluminum the same

Materials 2021, 14, x 6 of 10

Materials 2021, 14, 4182 6 of 10 amount of Al2O3. More aluminum in the mixtures resulted in increasing the amount of amountaluminum of Aloxynitride2O3. More yield aluminum and decreasing in the mixtures the alumina resulted content in increasing in the system; the amount however, of aluminumAlN was found oxynitride to be retainedyield and at decreasingaround 5 to th 8e wt% alumina without content any aluminumin the system; detected. however, AlNoxynitride was found yield to and be retained decreasing at around the alumina 5 to 8 contentwt% without in the any system; aluminum however, detected. AlN was found to be retained at around 5 to 8 wt% without any aluminum detected.

Figure 5. Phase composition of powder mixtures obtained by the means of SHS method from mixes FigureofFigure powders 5. 5. PhasePhase with composition composition 14–20 wt% ofaluminum of powder powder mixtures mixturescontent with obtained medium by the alumina means under of SHS 1 methodMPa nitrogen from mixes with of10%of powders powders ammonium with with nitrite,14–20 14–20 wt% γ-AlON aluminum (●), Al 2content O3 (○), AlN with ( □medium ), Al (■), alumina and Al7O under 3N5 (∆ 1 ). MPa nitrogen with 10% ammonium nitrite, γ-AlON ( ●•), Al2OO33 ((○),), AlN AlN (□ (), ),Al Al (■ (), ),and and Al Al7O73ON35N (∆5).( ∆). Finally, the scalability of the setup #was investigated. Mixes of 20/80 aluminum ratio with Finally,Finally,10% ammonium the scalability nitrate of thewere setupsetup prepared waswas investigated.in andvestigated. weighed MixesMixes in batches ofof 20/8020/80 of aluminum 20, 50, 100, ratio and with500 g 10% to be ammonium ignited in 1nitrate nitrate MPa nitrogen.were were prepared The smaller and weighed batches in were batches less of successful 20, 50, 100, with and γ- 500AlON500 g g to tosynthesis; be be ignited ignited however, in in 1 1MPa MPa the nitrogen. 500 nitrogen. g charge The The presentedsmaller smaller batches less batches than were were a 2%less lessdecrease successful successful in γwith-AlON with γ- AlONformation.γ-AlON synthesis; synthesis; The results however, however, are presentedthe the 500 500 g g chargein charge Figure presented presented 6. less less than than a a 2% 2% decrease decrease in γ-AlON-AlON formation.formation. The The results results are are presented presented in in Figure Figure 66..

Figure 6. PhasePhase composition composition of of powder powder mixtures mixtures obtained by the means of SHS method for different powder mass batches, γ-AlON (•), Al O ( ), AlN ( ), and Al O N (∆). Figurepowder 6. mass Phase batches, composition γ-AlON of powder (●), Al2O mixtures3 (○), AlN obtained (■), and by Al the7O7 3 meansN35 (5∆). of SHS method for different powder mass batches, γ-AlON (●), Al2O3 (○#), AlN (■), and Al7O3N5 (∆). The morphology of the achieved powders is is vi visualizedsualized in in Figure 77.. ItIt presentspresents elon-elongated columnar shapes and whiskers with hexagonal cross sections and layered structures. gatedThe columnar morphology shapes of andthe achievedwhiskers powders with hexagonal is visualized cross in sections Figure and7. It presentslayered struc-elon- Flat platelets can be also seen in the images. gatedtures. Flatcolumnar platelets shapes can be and also whiskers seen in thewith images. hexagonal cross sections and layered structures. Flat platelets can be also seen in the images.

Materials 2021, 14, 4182 7 of 10 Materials 2021, 14, x 7 of 10

(a) (b)

Figure 7. SEM images of powders after SHS, ( a) medium alumina with Al-63 under 1 MPa, ( b) coarse alumina with Al-63 under 1 MPa. 4. Discussion 4. Discussion As expected, the least amount of γ-AlON phase was synthesized with the coarse As expected, the least amount of γ-AlON phase was synthesized with the coarse alu- alumina substrate due to the longest distance that the diffusing atoms have to travel. Addi- mina substrate due to the longest distance that the diffusing atoms have to travel. Addi- tionally, a larger grain poses a higher resistance for reaction heat to raise the temperature tionally, a larger grain poses a higher resistance for reaction heat to raise the temperature inside the grain. The similarity in the synthesis outcome for alumina powders with a inside the grain. The similarity in the synthesis outcome for alumina powders with a mean mean grain size of 5 and 0.3 µm means the threshold for the oxide substrate is at least grain size of 5 and 0.3 μm means the threshold for the oxide substrate is at least 5 μm. The 5 µm. The similarities continue with the proposed optimal reaction pressure as well as the similaritiesaluminum graincontinue size. with Contrary the proposed to cited articles,optimal pressuresreaction pressure lower than as well 5 MPa as werethe aluminum sufﬁcient grainfor nitridation size. Contrary of whole to cited aluminum articles, in pressures the powder lower batch. than Such 5 MPa conditions were sufficient allow most for ofnitri- the dationformed of aluminum whole aluminum nitride toin reactthe powder with the batch. surrounding Such conditions alumina. allow most of the formed aluminumThe basic nitride reaction to react allowing with the for surrounding the reaction alumina. to be self-sustaining is The basic reaction allowing for the reaction to be self-sustaining is

2Al + N2 → 2AlN + Q (1) 2Al + N2 → 2AlN + Q (1)

The heat (Q) released in first ﬁrst reaction heats up adjacent grains and and triggers triggers the the alu- alu- minum ones to also react with nitrogen. Alum Aluminuminum is always covered with a passivation oxide layer that prevents it from further reaction; however, during the initial heating, the grain expands more than the oxide, exposing a clean surface for the nitrogen to react. The amount of heat generated melts the grain and causes partial or complete evaporation. Aluminum vapors are very reactive and bond with nitrogen immediately. The freshly Aluminum vapors are very reactive and bond with nitrogen immediately. The freshly formed AlN condenses on surrounding Al2O3 grains that had already been heated and formed AlN condenses on surrounding Al2O3 grains that had already been heated and starts the secondary reaction: starts the secondary reaction:

xAlNxAlN + yAl + yAl2O23O→3 →Al Alx+2yx+2yOO3y3yNNxx (2)

From the appearance of the reacted powder bed, the reaction did not exceed 2050 °C,◦C, which is the melting temperature of aluminum oxide, as no sign of macroscopic melting could be found. OnlyOnly partsparts ofof the the powder powder near near the the graphite graphite foil foil ends ends that that ignite ignite the the reaction reac- tionare moreare more sintered, sintered, being being heated heated from from thereaction the reaction and resistiveand resistive heating heating simultaneously. simultaneously.Nevertheless, Nevertheless, at temperatures at temperatures below melting, below melting, alumina becomesalumina volatilebecomes and volatile may reactand withmay reactAlN inwith gaseous AlN in phase. gaseous The phase. SEM images The SEM in Figure images7 depict in Figure elongated 7 depict columnar elongated shapes columnar with shapeshexagonal with cross hexagonal sections cross and layeredsections growth and layered structures growth and structures support both and mechanisms support both of mechanismsV-S and V-L-S of asV-S proposed and V-L-S by as Zientara proposed et al.by [Zi19entara]. These et al. structures [19]. These can structures be composed can be of composedAlN and γ -AlONof AlN asand EDS γ-AlON shows as the EDS presence shows ofthe oxygen presence and of nitrogen oxygen and in varying nitrogen amounts, in var- yingbut thisamounts, method but is this not precisemethod enoughis not precise to determine enough the to determine stoichiometry the ofstoichiometry the examined of the examined spot. Both compounds can form such structures, AlN growing in the [0001]

Materials 2021, 14, 4182 8 of 10

spot. Both compounds can form such structures, AlN growing in the [0001] direction and the γ phase in the [111] direction, forming hexagonal crystal forms. There are also more solid grainy structures that suggest a solid-state synthesis with interdiffusion taking place between nitride and oxide. As our apparatus is not equipped with a thermocouple or pyrometer and also does not have a window, we can only estimate the time of the parts of the powder bed remaining at temperatures above 1650 ◦C which promotes γ-AlON synthesis. In general, the reactor cooled down slower during the low-pressure experiments. According to the literature data on aluminum nitriding in the SHS regime at a lower pressure range, an increase in pressure speeds up the reaction front propagation [26]. For γ-AlON synthesis, it was observed that at high pressures above 50 MPa, a further increase of pressure slows down the reaction [23]. The reference also presents a data point at 20 MPa with a reaction front having half the velocity of 50 MPa. What is more, the experiment was conducted on pressed pellets, and the current is run on loose powder beds; consequently, the velocities may differ significantly. The incomplete conversion of substrates may be also attributed to less particle contacts and a greater distance from each other. These conditions hinder the formation of oxynitride, at the same time allowing easier nitrogen penetration into to the bottom of the powder bed. What also has to be taken into consideration is the edge effect; even though the whole batch could be removed from the reactor intact, the conditions prevailing in layers in contact with graphite and the top layer may contribute to an overall lower yield. The use of ammonium nitrite fulfills a number of functions. The salt melts before it decomposes, thus creating capillary action and pulling neighboring grains closer. The decomposition creates active nitrous oxide, oxygen, and nitrogen, aiding in nitriding and oxidizing aluminum, increasing temperature and flame front velocity. Another advantage of using an ammonium salt is the absence of any additional metal cations dissolved in the oxynitride lattice or metal salts that have to be further removed by washing and could still remain in the product. The ammonium nitrite addition has a positive effect on the synthesis yield of up to 10 wt%; a further increase in salt content led to more aluminum being oxidized than nitrided due to the higher affinity of aluminum to oxygen than to nitrogen. The temperature increase above the γ-alon melting temperature of 2150 ◦C can be inferred from strongly sintered and partially melted products with less and less of the desired oxynitride. Experiments on scalability are promising with batches five times larger (500 g) than the standard ones (100 g) used in the presented investigation. The amount produced in the scale up experiment only required a redesign of the graphite boat to the largest size that could fit the current reactor construction. The enlarged powder bed post reaction was very similar to the standard one, and the γ-AlON yield was also very similar. This experiment gives promising insight into the possibility of scaling up to several kilograms of product in one charge. The product obtained from a 20/80 mixture results in an Al2O3:AlN ratio equal to 1:1, which is twice the amount of nitride required to form the γ-AlON phase. This fact can be addressed during preparation for sintering by mixing it with an appropriate amount of alumina to compensate the excess nitrogen in the product. This issue is going to be presented and discussed in further publications.

5. Conclusions The synthesis of γ aluminum oxynitride using the SHS technique on a loose powder bed has been studied. The inﬂuence of alumina and aluminum particle size as well as nitrogen pressure and ammonium nitrite salt on phase composition and γ-AlON phase yield was presented. The tested procedure allowed us to achieve products with over 80% γ-AlON content, having no residual aluminum where obtained. Promising results could be achieved using relatively cheap resources. The substrates used for the experiments have particle sizes larger than those presented in the literature as of yet. The ignition of homogenously mixed alumina powders with a mean grain size of 5 µm or smaller, with Materials 2021, 14, 4182 9 of 10

aluminum powder with a mean grain size of 50 µm or smaller, and with the addition of 10 wt% of ammonium nitrite in 1 MPa pressure nitrogen results in over 80% γ-AlON yield. Presynthesizing γ-AlON using SHS and cheap raw materials followed by a lower nitrogen pressure than that reported in the literature could be a promising route to scaling up the technology and could be beneficial for transparent armor manufacturers. Additionally, low pressures around 1 MPa require a less robust reactor apparatus, which is both cheaper and safer. The conducted experiments were the first reported in the literature to attempt using ammonium nitrite as an aid in γ-AlON synthesis by the means of the SHS method. Increasing the test batch five times resulted in a consistent and very similar outcome as that achieved during the standard tests. Flame front velocity, temperatures reached, cooling rate, and edge conditions are still parameters which should be investigated for process optimization.

Author Contributions: Conceptualization, methodology, investigation, formal analysis, validation, data curation, writing—original draft preparation and editing, A.W.; investigation M.Z.-N.; validation, visualization, supervision, funding acquisition, project administration, writing—review, Z.P. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by subvention of the Polish State Ministry of Education and Science for AGH University of Science and Technology, Faculty of Materials Science and Ceramics (project no. 16.16.160.557). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: The authors are deeply grateful for Mirosław Bućko,who was the guiding spirit of the performed investigations. His suggestions and remarks were invaluable during the realization of the experiments. Unfortunately, he could not finish our shared task. He passed away in December 2020. The authors want to express their thanks to Jakub Domagała for his help with the first steps of the experiments. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Yamaguchi, G.; Yanagida, H. Study on the reductive spinel—A new spinel formula AlN–Al2O3 instead of the previous one Al3O4. B. Chem. Soc. Jpn. 1959, 32, 1264–1265. [CrossRef] 2. McCauley, J.W.; Corbin, N.D. Phase relations and reaction sintering of transparent cubic aluminum oxynitride spinel (ALON). J. Am. Ceram. Soc. 1979, 62, 476–479. [CrossRef] 3. Hartnett, T.M.; Bernstein, S.D.; Maguire, E.A.; Tustison, R.W. Optical properties of ALON (aluminum oxynitride). Infrared. Phys. Techn. 1998, 39, 203–211. [CrossRef] 4. Klement, R.; Rolc, S.; Mikulikova, R.; Krestan, J. Transparent armour materials. J. Eur. Ceram. Soc. 2008, 28, 1091–1095. [CrossRef] 5. McCauley, J.W.; Patel, P.; Chen, M.; Gilde, G.; Strassburger, E.; Paliwal, B.; Dandekar, D.P. AlON: A brief history of its emergence and evolution. J. Eur. Ceram. Soc. 2009, 29, 223–236. [CrossRef] 6. Goldman, L.M.; Kashalikar, U.; Ramisetty, M.; Jha, S.; Sastri, S. Scale up of large ALON and Spinel windows. In Window and Dome Technologies and Materials XV; International Society for Optics and Photonics: Bellingham, WA, USA, 2017; Volume 10179, p. 101790. [CrossRef] 7. Yawei, L.; Nan, L.; Runzhang, Y. The formation and stability of γ-aluminium oxynitride spinel in the carbothermal reduction and reaction sintering processes. J Mater. Sci. 1997, 32, 979–982. [CrossRef] 8. Liu, Q.; Jiang, N.; Li, J.; Sun, K.; Pan, Y.; Guo, J. Highly transparent AlON ceramics sintered from powder synthesized by carbothermal reduction nitridation. Ceram. Int. 2016, 42, 8290–8295. [CrossRef] 9. Wang, Y.; Li, Q.; Huang, S.; Cheng, X.; Hou, P.; Wang, Y.; Yi, S. Preparation and properties of AlON powders. Ceram. Int. 2018, 44, 471–476. [CrossRef] 10. Jin, X.; Gao, L.; Sun, J.; Liu, Y.; Gui, L. Highly transparent AlON pressurelessly sintered from powder synthesized by a novel carbothermal nitridation method. J. Am. Ceram. Soc. 2012, 95, 2801–2807. [CrossRef] 11. Loghman-Estarki, M.R. Effect of carbon black content and particle size on the phase evolution of aluminum oxy nitride (AlON). Ceram. Int. 2016, 42, 17680–17686. [CrossRef] Materials 2021, 14, 4182 10 of 10

12. Bandyopadhyay, S.; Rixecker, G.; Aldinger, F.; Pal, S.; Mukherjee, K.; Maiti, H.S. Effect of reaction parameters on γ-AlON formation from Al2O3 and AlN. J. Am. Ceram. Soc. 2002, 85, 1010–1012. [CrossRef] 13. Qi, J.; Wang, Y.; Lu, T.; Yu, Y.; Pan, L.; Wei, N.; Wang, J. Preparation and light transmission properties of AlON ceramics by the two-step method with nanosized Al2O3 and AlN. Metall. Mater. Trans. A 2011, 42, 4075–4079. [CrossRef] 14. Willems, H.X.; Hendrix MM, R.M.; Metselaar, R.; De With, G. Thermodynamics of Alon I: Stability at lower temperatures. J. Eur. Ceram. Soc. 1992, 10, 327–337. [CrossRef] 15. Willems, H.X.; Hendrix MM, R.M.; De With, G.; Metselaar, R. Thermodynamics of Alon II: Phase relations. J. Eur. Ceram. Soc. 1992, 10, 339–346. [CrossRef] 16. Su, M.; Zhou, Y.; Wang, K.; Yang, Z.; Cao, Y.; Hong, M. Highly transparent AlON sintered from powder synthesized by direct nitridation. J. Eur. Ceram. Soc. 2015, 35, 1173–1178. [CrossRef] 17. Qi, J.; Wang, Y.; Xie, X.; Wang, Y.; Zhou, J.; Wei, N.; Lu, T. Effects of Al2O3 phase composition on AlON powder synthesis via aluminothermic reduction and nitridation. Int. J. Mater. Res. 2014, 105, 409–412. [CrossRef] 18. Gromov, A.; Ilyin, A.; Ditts, A.; Vereshchagin, V. Combustion of Al–Al2O3 mixtures in air. J. Eur. Ceram. Soc. 2005, 25, 1575–1579. [CrossRef] 19. Zientara, D.; Bu´cko,M.M.; Lis, J. Alon-based materials prepared by SHS technique. J. Eur. Ceram. Soc. 2007, 27, 775–779. [CrossRef] 20. Lee, J.R.; Lee, I.; Chung, H.S.; Ahn, J.G.; Kim, D.J.; Kim, B.G. Self-propagating high-temperature synthesis for aluminum oxynitride (AlON). In Materials Science Forum; Trans Tech Publications Ltd.: Stäfa-Zurich, Switzerland, 2006; Volume 510, pp. 662–665. [CrossRef] 21. Zheng, Y.; Li, H.; Zhou, W.; Zhang, X.; Ye, G. Combustion synthesis and characteristics of aluminum oxynitride ceramic foams. Ceram. Int. 2012, 38, 5139–5144. [CrossRef] 22. Akopdzhanyan, T.G.; Borovinskaya, I.P.; Chemagina, E.A. Aluminum oxynitride by SHS under high pressure of nitrogen gas. Int. J. Self Propag. High Temp. Synth. 2017, 26, 110–114. [CrossRef] 23. Naderi-beni, B.; Alizadeh, A. Preparation of single phase AlON powders aided by the nitridation of sol-gel-derived nanoparticles. Ceram. Int. 2019, 45, 7537–7543. [CrossRef] 24. Naderi-beni, B.; Alizadeh, A. Development of a new sol-gel route for the preparation of aluminum oxynitride nano-powders. Ceram. Int. 2020, 46, 913–920. [CrossRef] 25. Yang, S.; Chen, H.; Li, J.; Guo, H.; Mao, X.; Tian, R.; Wang, S. Oxidation behavior and mechanism of aluminum oxynitride (AlON) at elevated temperatures. J. Am. Ceram. Soc. 2021, 104, 1040–1046. [CrossRef] 26. Shin, J.; Ahn, D.H.; Shin, M.S.; Kim, Y.S. Self-propagating high-temperature synthesis of aluminum nitride under lower nitrogen pressures. J. Am. Ceram. Soc. 2000, 83, 1021–1028. [CrossRef]