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Structural and Microstructural Properties of FusedZrO2-MullitesSasmita Prusty a , Gopa Mishra a , B. K. Mohapatra a , Shikha Varma b , S. K. Singh a & DilipKumar Mishra aa Advanced Materials Technology Department , Institute of Minerals and Materials Technology(CSIR) , Bhubaneswar, Orissa, Indiab Institute of Physics , Sachivalaya Marg , Bhubaneswar, Orissa, IndiaPublished online: 24 Mar 2011.

To cite this article: Sasmita Prusty , Gopa Mishra , B. K. Mohapatra , Shikha Varma , S. K. Singh & Dilip Kumar Mishra (2011)Structural and Microstructural Properties of Fused ZrO2-Mullites, Materials and Manufacturing Processes, 26:2, 282-287, DOI:10.1080/10426914.2010.536925

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Materials and Manufacturing Processes, 26: 282–287, 2011Copyright © Taylor & Francis Group, LLCISSN: 1042-6914 print/1532-2475 onlineDOI: 10.1080/10426914.2010.536925

Structural and Microstructural Properties of Fused ZrO2-Mullites

Sasmita Prusty1, Gopa Mishra

1, B. K. Mohapatra

1, Shikha Varma

2, S. K. Singh

1,

and Dilip Kumar Mishra1

1Advanced Materials Technology Department, Institute of Minerals and Materials Technology (CSIR),Bhubaneswar, Orissa, India

2Institute of Physics, Sachivalaya Marg, Bhubaneswar, Orissa, India

Silimanite and zircon rich sea beach sand is taken with alumina powder for the synthesis of fused zirconia mullite (MUZ) composites byextended arc thermal plasma reactor. The bulk density value of fused MUZ (∼3�79 g/cm3) is well comparable to the density value of commercialavailable MUZ (∼3�6 gm/cm3). The electron probe microstructure analysis predicts that alumina is rich in mullite grains, whereas the grainboundary region is rich with zirconia. A small percentage of zirconia is also present in the intragrain region, which reveals that mullite compoundis surrounded by well-structured and compacted zirconia. The X-ray photoelectron spectrum reveals the presence of mullite phase along withcrystalline Si in the composites.

Keywords Composites; Fused; Microstructure; Mullite; Plasma; UV-visible; XPS; XRD.

Introduction

Mullite is an alumina-silicate material �3Al2O3 · 2SiO2�having abundant percentage of alumina and silica. Thismaterial is treated as a high temperature refractory materialin the ceramic industry due to its physical propertieslike good high temperature strength, good thermal shockresistance, excellent thermal stability, resistance to mostchemical attack, excellent stability in acid metal slags,insoluble in most acids, resistance to oxidation andattack by furnace atmospheres, resistance to abrasion, andgood electrical resistivity [1–6]. However, these physicalproperties can be enhanced by adding or doping zirconiain the mullite materials [5–12]. The resultant product isknown as zirconia-mullites (MUZ). Fused MUZ is usedfor producing zirmull bricks and shapes, which are usedin the glass industry as well as an additive in continuouscasting refractories [5, 7, 8]. Mostly arc melting, microwavesintering, and high temperature conventional sintering aretreated as the best sintering technique for synthesizing MUZusing the commercial available powders of Al2O3, SiO2,and ZrO2. This type of synthesis is taking a long time,and it is treated as a more energy consuming processin comparison to the thermal plasma sintering technique.Hence, it is required to adopt a quick sintering technique,which can synthesize the desired product within a shortduration of time. Also the low cost method to get all thesecomponents �Al2O3�SiO2�ZrO2� from sea beach sand andtreated in a plasma reactor is a challenging one. Indiahas a long coastal region in its eastern and western side,endowed with beach sand. These sea beach sands consist

Received July 10, 2010; Accepted October 19, 2010Address correspondence to Dr. Dilip Kumar Mishra, Advanced

Materials Technology Department, Institute of Minerals and MaterialsTechnology (CSIR), Bhubaneswar 751013, Orissa, India; Fax: +91-674-2581637; E-mail: kumardilipm@yahoo.co.in

of alumina, silica-rich sillimanite, and zirconia, a silica-richzircon. The same product to be used for the synthesis ofMUZ composite in one step process using thermal plasmareactor is a novel technique. Especially, zirconia is used inmullite to make the composite toughened, hardened, and forenhancing thermomechanical properties [7, 9, 10].In this paper, fused MUZ composite is synthesized by

fusing silimanite and zircon sand with alumina powderin a thermal plasma reactor within a few minutes (∼5minutes). The structural and microstructural studies werecarried out using X-ray diffraction technique and electron-probe microstructural analyses. X-ray photoelectron studieshave been carried out to know the electronic structure andthe various coordination numbers of all elements present inthe MUZ composite.

Experimental details

Composite of fused MUZ was prepared by extendedarc thermal plasma technique. For the above synthesispurposes, the major component sillimanite �Al2SiO5�and zircon is recovered from the beach sand collectedfrom Chhatrapur, Orissa, India. The detail procedure andflow sheet to recover sillimanite from beach sand isdiscussed elsewhere [13]. Around 100kg of beach sandwas collected from Chhatrapur, Orissa, India. About 1kg ofthis was used for heavy liquid separation using bromoform�specific gravity = 2�89�. Large scale continuous spiralseparation was used to recover the total heavy minerals. Awet, high-intensity magnetic separator (BOXMAG–RAPIDLtd.) was used in the recovery of nonmagnetic minerals.Flotation experiments were conducted by a laboratorymodel Denver D12 subaeration flotation machine. Rougherflotation experiments utilized a 2-liter cell, whereas asmaller 1-liter cell was used in the cleaner flotationexperiments. All the magnetic and nonmagnetic minerals,and flotation products, were subjected to heavy liquid

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PROPERTIES OF FUSED ZrO2-MULLITES 283

separation by bromoform to assess the quality of theproducts. Methylene iodide �specific gravity = 3�332� wasused to assess the sillimanite content in the total heavyminerals. In rougher flotation experiment, the sample wasconditioned for 3min at pH 10. Sodium silicate (0.5 cc/kg)and oleic acid (0.1cc/kg, pH 9) were added duringconditioning time. Methyl Isobutyl Cellulose (MIBC) wasused as a frother. It was observed that during the rougherflotation, smaller amounts of other heavy minerals (zircon,rutile) and quartz were floating with sillimanite. The frothwas cleaned with pH 9 water and subjected to cleanerflotation. In cleaner flotation no additional chemicals wereused except maintaining the pH 9. The cleaner flotationconcentrate is further subjected to a second cleaningstage (re-cleaner flotation), yielding an ultimate productof sillimanite. The sillimanite concentrate obtained afterbeneficiation of the beach sand sample contains 97.1%sillimanite, 0.5% ilmenite, 1.4% garnet, 0.8% quartz, andtrace of rutile and zircon (0.2%).For the preparation of MUZ, the ratio of sillimanite,

alumina, and zircon was taken as 47:38:15, which iscomparable to SiO2�Al2O3, and ZrO2 ratio of 23:67:10.The sillimanite, zircon, and alumina powder in appropriateratio was taken and homogeneously mixed using a mixergrinder. For plasma fused technique, an indigenouslydeveloped 50kW thermal plasma reactor was used. Oxygenas plasmagen gas was used in the reactor, and the flowrate was kept fixed for 1 lit/min. The detail of the reactoris discussed briefly elsewhere [14]. The mixed powders(∼500g) in the required ratio were fed to the plasma reactorfor the synthesis of fused MUZ. The plasma power usedfor the synthesis is of 15kW (50V and 300Amp current).The time of plasma exposed to the sample was kept onlyfor five minutes. The time parameter used in plasma reactorwas optimized by performing a number of experiments. Outof 500g charging powders, 72.4% of the raw specimen wasconverted to MUZ, and around 27.6% of the raw specimenremained unreacted. The properties of fused MUZ havebeen studied by various experimental techniques.The bulk density and porosity of the material is measured

using Archimedes’ principle. Hardness of the fused mulliteis measured using Leco hardness tester (LM247AT). Thestructural characterization and different mineral specieswere identified by X-ray diffraction (XRD) technique usingCuK� radiation. Electron probe microstructure analysis(JEOL EPMA, JXA-8100) was used to study the generalmicrostructure and compositional variation between Al,Zr, and Si. X-ray photoelectron spectroscopy (XPS)experiments were performed in a VG ESCA system. It isequipped with a dual Mg-Al anode and has a base pressureof 1�0 × 10−10 Torr. The system resolution is 0.9eV. XPSmeasurements of surfaces were carried out using the Mg K�X-ray source with pass energy of 20eV. Diffused reflectivespectroscopy (DRS) study has been done to estimate theband gap of MUZ by UV-visible spectrophotometer (Model:Varian Cary 100).

Results and discussions

The bulk density of fusedMUZmeasured by Archimedes’technique is 3�79 g/cm3. The value is fairly better than

the bulk density value of commercially available fusedMUZ (3�6 g/cm3� and more than the bulk density value(3�12 g/cm3) of mullite. The fused MUZ has the porosityaround 1.04%. The MUZ product synthesized by DCextended thermal plasma reactor is done by melt processedtechnique. It is known that plasma temperature is muchhigher than the melting point of alumina, silica, and zirconia,so it favors alumina and silica to react with each otherin the melting state and forms the mullite phase. Zirconiapresent in the specimen rushes to the grain boundaryregion to minimize the intergranular spacing and, hence,decreases the porosity. The hardness value measured usingthe Vickers indentation method is of the order of 1�27 ×104MPa by applying 100gf for 13 seconds. The structuralinformation observed from the XRD spectrum speaks thebetter crystalline ordering and the better conversion of MUZproducts.XRD spectrum of MUZ is recorded and is shown in

Fig. 1. All the peaks corresponding to mullite are observedin the spectrum with major Bragg reflections at (210), (200),(111), (121), and (220), which is well matched with theJCPDS data (#150776) having orthorhombic structure ofPbam space group. Apart from mullite peak, monocliniczirconia peak is observed at the 2� angle of 28.12, 31.35,44.58, and 55.31�, which is in close agreement to theJCPDS data (#830944), and also crystalline Si peak isobserved. No observation of zircon peak is found in theXRD spectrum, which indicates that zircon dissociates intozirconia and silica completely during the reaction processand some minor percentage of silica reacts with carbon(i.e., from graphite crucible and graphite electrode) andforms crystalline Si. But a major percentage of dissociatedsilica takes active part in the formation of mullite materialsreacting with alumina. This prediction supports that plasmasintering is a novel technique to dissociate 100% sillimaniteand zircon to synthesize the desired property of the materialwithin a short duration of time. This improvement in thephysical property of plasma fused material is supportedfrom the EPMA results.The electron micrograph picture of fused MUZ is shown

in Fig. 2. It shows a clear picture of intergrain and intragrainregion of the MUZ specimen. It is very well observed that

Figure 1.—XRD pattern of fused zirconia mullite (MUZ) composite (M =mullite; Zm = monoclinic zirconia, and Si = silicon�.

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284 S. PRUSTY ET AL.

Figure 2.—Electron micrograph of fused zirconia mullite (MUZ) composite.

the surface area of grain boundary region (white spot inFig. 2) is higher than the mullite grain region (gray spot inFig. 2). No porosity and intergranular spacing is observed inthe picture of electron micrograph, which indicates a closepacked structure of mullite region surrounded by zirconia atits grain boundary region. This may be one of the reasons forshowing higher bulk density value well comparable to thecommercial available MUZ. The distribution of intergrainand intragrain region is not homogeneous. To imply theexact distribution of elements present in the intergrain andgrain boundary region, the individual mapping of eachelement has been done using the EPMA instrument.The X-ray image map of each element like Al, Zr,

and Si are shown in Fig. 3. The picture predicts that Alpercentage is more in the mullite region, whereas Zr is muchmore in the grain boundary region. A small percentage ofzirconia is present in the intragrain region of mullite. Siis equally distributed both in mullite and zirconia region.It confirms that all silica is not associated with alumina

Figure 3.—X-ray image map of Al, Zr, Si elements in fused zirconia mullite(MUZ) composite.

Figure 4.—Qualitative spectrum of fused zirconia mullite (MUZ) compositefor the identification of Al, Zr, and Si.

to form 100% mullite phase. There is a possibility ofpresence of crystalline silicon with zirconia in the grainboundary region. Such observation indicates that the mainmatrix of mullite (rich in alumina and silica) is closepacked with the mullite grain boundary region of zirconia.Hence, variation of Al, Si, Zr composition within grains isobserved from the X-ray mapping. The presence of zirconiain the grain boundary inhibits the grain growth process andtoughens the mullite structure, which enhances the bulkdensity of the MUZ material [9]. The qualitative spectrumshown in Fig. 4 confirms the presence of Al, Zr, and Siin MUZ. Beside these elements, minor impurity of Fe isfound in the sample. The quantitative data given in Table 1form suggest that zirconia percentage is much more in thesurface rather than alumina and silica in comparison to theconcentration of alumina, silica and zirconia ratio taken inthe raw components. Zirconia concentration is higher ingrain boundary region and a small percentage of zirconiais present in the intergrain of mullite. The quantitative dataobtained from the elemental compositional analysis fromEPMA is quite obvious that zirconia is in higher percentagein comparison to the actual concentration fed to the reactorfor the synthesis of MUZ.

Table 1.—Quantification result of alumina, zirconia,and silica in fused MUZ composite obtained fromEPMA point chemical analysis.

Composition Raw sample EPMA result

Al2O3�%� 67 59.159SiO2�%� 23 17.175ZrO2�%� 10 20.756

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PROPERTIES OF FUSED ZrO2-MULLITES 285

Figure 5.—XPS wide survey spectrum of fused zirconia mullite (MUZ)composite.

In order to have further insight into the nature of thecomponents present in the MUZ, XPS analysis was carriedout. Figure 5 shows the survey scan of MUZ composites.All the elements like Al, Zr, Si, and O are observed inthe sample surface. XPS spectra corresponding to O 1s,Al 2p, Si 2p, and Zr 3d core level of MUZ compositesare calibrated using the binding energy of C 1s (284.6eV,shown in Fig. 6) as the internal reference spectra [15]. Apartfrom all the core level peaks, auger peak of O and carbonare found in the specimen.As our study is much more concerned to the Al, Si, and

Zr elements, so the impurities present in the samples arenot taken care of. The 1s carbon peak at 284.6eV shown inFig. 6 is taken as a reference binding energy spectrum foranalyzing the other elements present in the specimen. Anycharging shifts were also removed by fixing the bindingenergy of carbon peak [15–17]. There is no observationof any shoulder peak in C1s peak, which implies thatno metallic carbide is forming during the synthesis. The

Figure 6.—XPS spectrum of C 1s core level of fused zirconia mullite (MUZ)composite.

Figure 7.—XPS spectrum of Si 2p core level of fused zirconia mullite (MUZ)composite.

signature of free carbon is found from the contamination ofsamples with the graphite crucible and graphite electrode.Similarly, Si 2p peak shown in Fig. 7 is deconvoluted intofour components [16]. The low binding energy componentsat 99.85eV are attributed to the Si 2p peak. The mediumbinding energy and higher binding energy componentsat 102.13, 103.67, and 104.72eV are attributed to thealuminosilicate (mullite) compounds. It is evidenced fromthe literature that the Si 2p peak observed within 100 to105eV is assigned as aluminosilicate phase [17, 18]. Thereis prominent signature of Si in the XRD pattern shownin Fig. 1, which implies that the small percentage of Sipresent in the grain boundary region is in crystalline stateand remains unreacted. While dissociation of zircon occursinto monoclinic zirconia and silica, a small percentage ofsilica reacts with the carbon (coming from graphite crucible)at about 1900�C, converts silica to crystalline Si. Mostlythis small percentage of Si is found in the grain boundaryregion.The XPS spectrum of O 1s shown in Fig. 8 is broad and

asymmetrical in nature due to the various coordinations ofoxygen in MUZ composite. One main emission line O 1s isfound in MUZ composite at 531.2eV. The binding energycomponent is attributed to the alumina matrix composed ofSi-containing phase, i.e., alumino-silicate (mullite) [19, 20].The broadened peak above 531.2eV indicates the oxygenenvironment in mullite only. There is no observation ofalumina peak in the oxygen spectra which envisage thatthe alumina used for the synthesis of mullite is completelyused. It is also confirmed from the Al 2p spectra [16].The main Al 2p peak is observed at 75.13eV with a

shoulder peak at 74.06eV and is shown in Fig. 9. Bothpeaks are corresponding to the mullite only. There is nosignature of alumina peak. The asymmetry nature of thecurve suggests that alumina is not in its pure form. It is in acomposite state, which means alumina is reacted with silicaand forms mullite material. Similarly, the XPS spectrumof Zr 3d is shown in Fig. 10. The peaks at 182.93 and187.69eV correspond to the Zr 3d5/2 and Zr 3d3/2 peak of

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Figure 8.—XPS spectrum of O 1s core level of fused zirconia mullite (MUZ)composite.

Figure 9.—XPS spectrum of Al 2p core level of fused zirconia mullite (MUZ)composite.

zirconia compound, which is well matched to the referencespectra of zirconia. The binding energy of Zr 3d5/2 peaksupports that Zr is in +4 coordination state [15].It is observable from the EPMA that mullite phase exists

with the surrounding of monoclinic zirconia and crystallinesilicon. The silica associated with alumina forms mulliteand the small percentage of Si is associated in the zirconiaregion (grain boundary region). The presence of crystallineSi is confirmed from the XPS and XRD pattern.Fused MUZ sample was studied by diffuse reflectance

spectroscopy, in order to examine the coordinationenvironment of metal cations. Spectrum of Al, Zr, and Sicontaining sample is shown in Fig. 11. It is well knownthat Al, Si, and Zr cations exist in the valence state of+3�+4, and +4, and are located in sites with octahedralor tetrahedral coordination. Due to the presence of various

Figure 10.—XPS spectrum of Zr 3d core level of fused zirconia mullite(MUZ) composite.

Figure 11.—Diffused reflectance spectrum of fused zirconia mullite (MUZ)composite.

coordination compounds in the matrix, multistep energyband gaps are observed in the spectrum. The lowest andhighest band gap observed for the DRS spectrum is 2.06eVand 3.49eV. This multistep band gap energy predicts thepresence of mullite compounds with other oxide compoundslike zirconia.Our experimental results confirm that the fused product

is in mullite phase with monoclinic zirconia phases at itsgrain boundary region.

Conclusions

In summary, thermal plasma is a novel technique tosynthesize fused MUZ within a few minutes (5 minutes)using extended arc thermal plasma reactor with the desiredphysical properties. The electromicrograph picture clearly

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PROPERTIES OF FUSED ZrO2-MULLITES 287

exhibits the close compacted structure of intergrain andintragrain region. The XPS analysis documents the presenceof unreacted crystalline Si in the grain boundary region withmonoclinic zirconia.

Acknowledgments

Authors are grateful to Director, IMMT (CSIR),Bhubaneswar for providing research support. Authors arethankful to Mr. Santosh Choudhary, Institute of Physics,Bhubaneswar for helping in XPS measurement.

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