Synthesis of monophase Al4O4C and the effect of Al4O4C addition to MgO–C refractory

Synthesis of monophase Al 4O 4C and the effect of Al 4O 4C addition to MgO–C

refractory

Chao Yu,Hongxi Zhu,Wenjie Yuan,Chengji Deng ?,Peng Cui,Shimin Zhou

The State Key Laboratory Breeding Base of Refractories and Ceramics,Wuhan University of Science and Technology,Wuhan 430081,PR China

a r t i c l e i n f o Article history:

Received 10April 2013

Received in revised form 13June 2013Accepted 14June 2013

Available online 22June 2013Keywords:Al 4O 4C Synthesis

Aluminum content Antioxidant Oxidation

a b s t r a c t

Monophase Al 4O 4C ceramic powder was synthesized using a mixture of a -alumina,carbon black and alu-minum powders as raw materials.The effects of different starting compositions and raw materials ratio on the phase transformation and micromorphology of Al 4O 4C were investigated.The results indicated that a -alumina,carbon black and aluminum powders was more appropriate as raw materials,and 20mol%excess of aluminum resulted in single phase Al 4O 4C.Al 2OC would occur if excessive carbon black was used.The Al content was found to have a signi?cant effect on the microstructure of synthesized Al 4-O 4C.Al 4O 4C added to the MgO–C refractory initially reacts with CO to form Al 2O 3and C.After the reaction,the formed Al 2O 3will further react with MgO to form MgAl 2O 4.The above reaction processes can form protective layers on the surfaces of the refractories,which inhibit the oxidation of the refractories.

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1.Introduction

Magnesia–carbon bricks are established high duty refractory products with unique mechanical,thermal and chemical properties for applications in converters,electric arc furnaces and steel treat-ment ladles,and carbon presents many of the desired properties for the MgO–C bricks,however the main drawback is its high sus-ceptibility to oxidation.In order to improve the resistance to oxida-tion of MgO–C bricks,the so-called antioxidants have often been added.Among the antioxidants,Aluminum (Al)is one of the most extensively used antioxidants in refractory compositions.So far,the behavior of Al in the carbon-containing refractories has been extensively studied [1].However,its by-product can be Al 4C 3,which may lead to disintegration of castables due to the poor hydration resistance of this phase [2].In addition,in the presence of cement,the Al–H 2O reaction during the castables mixing step takes place much earlier and faster,negatively affecting the perfor-mance of this additive to further decrease the carbon oxidation [3].In order to overcome such effects,some efforts have been made to develop alternative antioxidants presenting suitable properties but with less drawbacks.In this sense,aluminum oxycarbide (Al 4O 4C)is a promising antioxidants due to its low density (2.684g/cm 3)[4],high stable temperature (1890°C)[5]and good oxidation and hydration resistances [6,7].

Due to the desirable properties of Al 4O 4C as described above,numerous studies have been performed to investigate its synthesis [6–9].Zhang and Yamaguchi [7]synthesized Al 4O 4C,by heating a mixture of Al 2O 3(0.23l m),Al and C in an argon atmosphere at 1700°C for 6h.Zhao et al.[6]synthesized Al 4O 4C compound at 1700°C for 9h using Al 2O 3(0.1l m)and C as raw materials.The above-mentioned methods require ultra?ne powders and high temperature and prolonged sintering time,which would increase the costs and limit the industrial production of Al 4O 4C.The reports on the synthesis of Al 4O 4C powder have not been abundant,and the synthesis of monophase and plate like Al 4O 4C powder has never been obtained so far.In our experiments,the starting particle size of alumina was $20l m and the keeping time was 2h,the lar-ger particle size of Al 2O 3and shorter keeping time can decelerate the reaction rate of reduction,but the cost savings might bene?t.On the other hand,to the best of our knowledge not much research has focused on evaluating this antioxidant performance in refrac-tory compositions [6].Therefore,there is a lack of studies concern-ing the analysis of this aluminum oxycarbide behavior in MgO–C bricks.

The present investigation reports the synthesis of monophase Al 4O 4C powders using a mixture of a -alumina,carbon black and aluminum powders as raw materials and discuss the major syn-thetic mechanism,and evaluate the Al 4O 4C performance in MgO–C refractory.

2.Experimental

a -Alumina (99%,20l m),carbon black (99.9%,<800mesh size),and aluminum (99.5%,20l m)powder were used as starting materials.The compositions and syn-thesis temperatures of Al 4O 4C samples,which are shown in Table 1,were designed according to the theoretical reactions (1)and (2).In the ?rst place,samples S1and

0925-8388/$-see front matter ó2013Elsevier B.V.All rights reserved.https://www.360docs.net/doc/c817497607.html,/10.1016/j.jallcom.2013.06.088

Corresponding author.Tel.:+862768862041;fax:+862768862085.

E-mail address:cjdeng@https://www.360docs.net/doc/c817497607.html, (C.Deng).

S2were employed to determine which compositions was more suitable for this study.Then,in order to improve the purity of Al4O4C powder and explore the syn-thesis mechanism,10mol%–20mol%excess of aluminum were added to samples A1and A2,then10mol%–20mol%excess of carbon black were added to samples B1,B2,respectively.

2Al2O3esTt3CesT?Al4O4CesTt2COegTe1T4Al2O3esTt4AlesTt3CesT?3Al4O4CesTe2T

The powder mixtures according to Table1were ball milled for24h and dried at 100°C for12h in a vacuum dryer.The samples were then placed in a graphite cru-cible and sintered in?owing argon atmosphere(purity99.995%)in electric furnace with a graphite heater at1200,1400°C for1h and1600,1700°C for2h, respectively.

The crystalline phases of the synthesized products were examined by X-ray dif-fraction(XRD,X’Pert Pro MPD),and the scanning electron microscopy(SEM,Nova 400Nano)with an energy-dispersive spectroscopy detector(EDX,Penta FETx3,Ox-ford)was used to observe the micromorphologies of samples.The crystallinities (formation yields)of the Al4O4C phase in the?nal products were calculated from the obtained XRD spectra,according to the following relation[10]:

Crystallinitye%T?I Al

4

O4C I all

where I Al

4O4C

and I all are the intensities of the Al4O4C peak and all peaks,respectively.

The content and particle distribution of the fused magnesia particles,?ake graphite,and5mass%phenolic resin bonding agent used as raw materials for the MgO–C bricks are shown in Table2.Cylindrical samples(d=60mm and h=60mm)were prepared and dried at110°C for24h,followed by?ring at 1500°C for1h in an electrical furnace in oxidizing atmosphere(oxygen partial pressure=0.21atm,heating rate=5°C minà1)for the oxidation resistance tests.

The mass changes of graphite,Al4O4C and a mixture of Al4O4C and graphite(in a weight ratio of1:1)were recorded as a function of temperature(200up to1500°C) by thermogravimetric experiments(NETZSCH STA449C),using a heating rate of 10°C minà1with a synthetic air(80%N2–20%O2)?ow of50cm3minà1and a-Al2O3as a correction standard.The aim of these tests was to detect the beginning of the carbon oxidation and the Al4O4C reactions with O2and CO.

3.Results and discussion

3.1.Synthesis of Al4O4C

In order to study the high temperature phase reactions during heating and the synthesis mechanism,the samples with stoichiom-etric ratio were synthesized at different temperatures(1200,1400 for1h and1600,1700°C for2h).Fig.1shows XRD patterns of samples S1and S2synthesized at the different temperatures in ?owing Ar.As is indicated in the?gure,no new phase was found at1200°C in samples S1and S2.With increasing the synthesis temperature,the diffraction peaks of Al almost disappeared and a little Al2OC formed in sample S2.Besides,the diffraction peaks cor-responding to Al2OC were found in sample S1at1600°C while Al4-O4C began to form in sample S2at same heating temperature. Carbon existed until$1600°C in both cases while the diffraction peaks corresponding to carbon were weak.It is seen from the?g-ure that the relative intensities of aluminum oxycarbide in the powder sample synthesized using a-alumina,carbon black and aluminum powders as raw materials are much higher than using a-alumina and carbon black powders as raw materials at 1700°C,the crystallinities(formation yields)of the Al4O4C phase in sample S1was36.5wt%and that sample S2was73.1wt%.These data demonstrated that powder sample synthesized using a-alu-mina,carbon black and aluminum powders as raw materials was favorable for the formation of Al4O4C.Considering all the above, a-Al2O3+Al+C was more appropriate for preparing Al4O4C mate-rial in this study.

The in?uence of aluminum content on the products was studied in a-Al2O3+Al+C system at1700°C for2h.XRD patterns of sam-ples S2,A1and A2with different contents of aluminum are shown in Fig.2.It can be seen that Al4O4C was the main phase in these samples.Besides,the stoichiometric content of aluminum was not suf?cient for the reaction which was indicated by a small amount of Al2O3and Al2OC existing in sample S2.As the addition of aluminum was10mol%excess in sample A1,Al2O3and Al2OC phase reduced remarkably.Monophase Al4O4C powder was de-tected when20mol%excess of aluminum was added in sample A2.

Previous work by Zhao et al.[11]and Zhang and Yamaguchi[7] revealed that the formation of Al4O4C from different compositions (S1and S2)proceeds mainly via the solid–solid mechanism(keep-ing time52h),and the?nal product exhibit a irregular morphol-ogy.However,in this work,unlike a-Al2O3+C system,in the a-Al2O3+Al+C system,the major reaction mechanism was consid-ered to not only a solid–solid reaction but solid–liquid and solid–gas process for the preparation of Al4O4C powder.In the solid–solid reaction the initial materials are a-Al2O3and carbon black pow-ders.In the solid–liquid reaction process,liquid phase is formed by Al to the a-Al2O3,carbon black and Al powder mixtures while Al remained in sample S2until1400°C,which melted at elevated temperatures.The solid–liquid reaction processes are considered to promote chemical reactions which is in favor to form products. On the other hand,the Al2O3surface occurred at the temperature ranging from1000°C to1400°C with extremely low oxygen partial pressure,and Al2O gases has been postulated as a high-tempera-ture form[12].French et al.[12]reported that the transformation temperature decreased if Al2O3was treated with metals of high oxygen af?nity,such as Al,and at temperatures higher than 900°C the Al2O3layer is known to be disrupted most probably by the gaseous phase formed by reaction(3)[13,14].In a-Al2O3+-Al+C system(S2),when heating temperature was1200–1600°C, Al reduced but the formation of aluminum oxycarbide did not in-creased.This can be explained by partial losses of Al2O gases vola-tilized by reaction(3)while the formation of Al4O4C and Al2OC reaction velocity was low in that case and the reaction velocity be-came rapid at=1700°C.Because of the high heating rate (10°C minà1),metallic Al(=700°C)can possibly react with Al2O3 and C,and Al2O gases generated above900°C can possibly react with Al2O3and C,according to reactions(4)and(5).The theoretical ratio of constituents for forming single-phase Al4O4C is a-Al2O3:-

Table1

Composition of starting powders(wt%).

Sample Proportion Temperature

(°C)

Composition

S1S.R.11200,1400

1h

a-Alumina+carbon black

1600,1700

2h

S2S.R.21200,1400

1h a-Alumina+carbon black+aluminum

1600,1700

2h

A1S.R.2+10%mol%Al17002h a-Alumina+carbon

black+aluminum A2S.R.2+20%mol%Al17002h a-Alumina+carbon

black+aluminum

B1S.R.2+20%mol%

Al+10%mol%C 17002h a-Alumina+carbon

black+aluminum

B2S.R.2+20%mol%

Al+20%mol%C 17002h a-Alumina+carbon

black+aluminum

S.R.=Stoichiometric ratio.

Table2

Composition of MgO–C refractory(wt%).

MgO Flake graphite Al4O4C Al

3–1mm1–0mm<0.088mm

M3037258––

M–AOC30372583–

M–A3037258–3

C.Yu et al./Journal of Alloys and Compounds579(2013)348–354349

2OC es Tt2CO eg Te6T

photographs of cross sections of the MgO–C heated at 1500°C for 1h in air.A carbon rich area)and a partially oxidized region (white be identi?ed.Fig.6shows the average thick-layers of the samples shown in Fig.5.It is thickness of decarbonized layers of the samples con-almost the same as that of the samples containing thinner than those of the samples without patterns of oxidized region (white or light refractory sample with Al 4O 4C.MgO was iden-phases,and traces of MgAl 2O 4was also detected.simulate a reducing atmosphere,TG tests of the a mixture of Al 4O 4C +graphite were carried shown in Fig.8,where the oxygen compris-reacted with C to form CO(g)and CO 2(g)dur-exposing the antioxidant to these gases.reactivity of the used carbon sources,graphite approximately 560°C (Fig.8a).Conversely,Al 4-809°C in the presence of air giving rise to Al 2O 3results in an expressive mass gain up to Based on Fig.8c,graphite oxidation was identi-1000°C which is obvious that the starting Al 4O 4C +graphite sample were much higher graphite,demonstrating that the oxidation of and the oxidation resistance was improved Al 4O 4C,under reducing conditions,some vari-in the Al 4O 4C behavior and its transformation approximately 1000°C (Reaction (8)and Fig.8c).

2O 3es TtC es Te7T2Al 2O 3es Tt3C es T

e8T

Fig.3.SEM images of samples synthesized at 1700°C for 2h in ?owing Ar:(a)S1,(b)S2,(c)A1and (d)A2.

Fig.4.XRD patterns of samples A2,B1and B2synthesized

domains of the condensed phases in thermochemical data of Al4O4C reported the calculation[17].Those of Al2O3 NIST-JANAF Thermochemical Tables[18].As under condition of P CO=1.013?105Pa

below$2256K,Al4O4C will initially react C.

noted that the reactions shown in Reac-

gases formed in the system.There are

Al–O–C system,while Al and CO gases

O4C and CO reacted with each other to Reactions(9)and(10),respectively.The formation of MgAl2O4was considered nisms.One is the solid–gas mechanism according to Reactions(9)and(11)

and O2(g)formed diffuse to the surfaces MgO to form MgAl2O4according to

the solid–solid mechanism in which Al rectly at the interface between them, (13).

3Al4O4CesT?4Al2O3esTt4AlegTt3CesT

2AlegTt3COegT?Al2O3esTt3CesT

cross sections of the Magnesia–carbon Bricks samples heated at1400°C for2h in air:(a)no addition, decarbonized layer of the Magnesia–carbon Bricks samples

Fig.7.XRD patterns of oxidized region of the

Al4O4C.

Equilibrium partial pressures of Al(g)corresponding to Reactions

(9)and (11)is so low that Reaction (12)might not occur to a sig-ni?cant extent within the temperature range in this study.Based on the above results,it is considered that the solid–gas mechanism is not the main one responsible for the formation of MgAl 2O 4.In other words,the formation of MgAl 2O 4from Al 4O 4C proceeds mainly via the solid–solid mechanism (Reaction (13)).

As shown in Figs.6–8,Al 4O 4C inhibits the oxidation of the car-bon-containing refractories.The mechanisms can be given as fol-lows.Under the oxidizing conditions,as long as carbon exists,the partial pressure of CO(g)around and inside the refractories can be considered to be about 1.013?105Pa (1atm)[19].Under these conditions,Al 4O 4C react with CO to form Al 2O 3and C accord-ing to Reaction (8).After these reactions,Al 2O 3react with MgO to form MgAl 2O 4on the surfaces,according to Reaction (13).In the above manner,the MgAl 2O 4protective layers are formed on the surfaces of MgO–C refractory,which ?lls in the pores,and the pro-tective layers can inhibit the further oxidation of the carbon,that is,they enhance the oxidation resistance of the refractories.

Aside from the effect of inhibiting the oxidation of carbon-con-taining refractories stated above,the addition of Al 4O 4C has the fol-lowing advantages.The addition of aluminum,as mentioned already,may cause hydration and deterioration of a material.Al 4-O 4C has been reported to show an excellent hydration resistance [7].If it is added to the carbon containing refractories as an antiox-idant,the hydration of the refractories to which Al is added could be prevented.Furthermore,as Al 4O 4C itself is a carbide,when Al 2O 3from it according to Reactions (8),C is left behind,thus the cavity structure does not form in the refractories.4.Conclusions

Monophase aluminum oxycarbide (Al 4O 4C)ceramic powder was synthesized using a -alumina,carbon black and aluminum powders as raw materials.The behavior and the effect of Al 4O 4C

as an antioxidant for the carbon-containing refractories have been investigated and the corresponding mechanisms have been dis-cussed.The conclusions are as follows:

(1)a -Alumina,carbon black and aluminum powders was more

appropriate as raw materials.The crystallinity (formation yield)of Al 4O 4C was much more dependent on the ratio of Al than on the ratio of carbon black,and 20mol%excess of aluminum resulted in single phase Al 4O 4C.Al 2OC would occur if excessive carbon black was used.The reasonable mechanism for aluminum oxycarbide formation consists of a solid–solid reaction (nucleation reaction)during the initial stage and solid–liquid reaction and solid–gas reaction (growth reaction)in the later stage.

(2)Al 4O 4C added to the MgO–C refractory initially reacts with

CO to form Al 2O 3and C.After the reaction,the formed Al 2O 3further react with MgO to form MgAl 2O 4.In the reac-tion processes,MgAl 2O 4protective layers were formed on the surfaces of the refractories,and thus the oxidation of the refractories is inhibited.

Acknowledgements

This work was ?nancially supported by National Key Basic Re-search Program of China (973)(No.2012CB722702)and National Natural Science Foundation of China (NSFC,Grant No.51274156).References

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Compounds 579(2013)348–354