Liquid-Phase Sintering of Alumina Coated with Magnesium Aluminosilicate Glass

Liquid-Phase Sintering of Alumina Coated withMagnesium Aluminosilicate Glass

Akira Nakajima*,† and Gary L. Messing

Particulate Materials Center and Department of Materials Science and Engineering,The Pennsylvania State University, University Park, Pennsylvania 16802

Densification of alumina coated with a MgO–Al2O3–SiO2(MAS) glass was investigated from 1400° to 1460°C. Spinelwas observed to form at the liquid-alumina interface,whereas mullite crystallized uniformly in the liquid. Spineland mullite crystallization kinetics were accelerated bysmaller alumina particle size. Mullite and spinel crystalli-zation retarded densification by forming a percolating net-work. Boron doping suppressed spinel formation, and thusalumina sintered to higher densities at low temperature.The concept of glass basicity is proposed as a useful guidefor selecting dopants for low-temperature sintering.

I. Introduction

ALUMINA (Al2O3) is a useful ceramic, because of its highmelting point, excellent mechanical strength, electrical re-

sistivity, and chemical durability. As a result, it is used forabrasives, milling media, refractories, spark plugs, and catalystsupports. In addition to these applications, alumina has beenrecently employed for electrical, optical, and biomedical appli-cations such as integrated circuit packaging, laser media, fibersfor composite materials, and artificial tooth roots.

Since solid-state sintering of pure alumina generally requireshigh temperatures, lowering the sintering temperature is animportant research objective for alumina-based ceramics.Although there are several approaches for reducing the sinter-ing temperature of alumina, the most common method is toinduce liquid-phase sintering by the addition of CaO–Al2O3–SiO2 (CAS) or MgO–Al2O3–SiO2 (MAS) glass-forming sys-tems.1–11When the alumina–glass system is heated at tempera-tures higher than the melting temperature of the glasscomposition, the glass melts and the liquid spreads over theparticle surface. The liquid becomes richer in Al2O3, due to thedissolution of alumina particles during sintering. The tempera-tures at which alumina is the only solid phase in thermal equi-librium with a liquid for these systems are >1500°C (CAS) and>1600°C (MAS).12

To lower the liquid-phase sintering temperature of alumina,it is necessary to sinter in the phase region where alumina is notthe only solid phase in thermal equilibrium with the liquid. Inthis case, additional phenomena are expected to be importantduring liquid phase sintering, such as partial crystallization ofthe liquid, solid-state reaction or formation of a solid skeleton

between precipitated phases and alumina, and incongruentmelting of precipitated phases. In this case, densification isexpected to occur by a transient and/or reactive liquid-phasesintering process. Such processes have been investigatedmainly for powder metallurgy13–16 and nonoxide ceram-ics,17–20 whereas investigations of transient liquid-phase sin-tering of alumina, when the objective is a lower sintering tem-perature, are still limited.

Kostic and Kiss21 investigated the densification behavior ofalumina–eutectic MAS glass (forsterite–cordierite–protoenstatite,eutectic temperature4 1365°C) mixtures (5 to 15 vol%) at1450° and 1550°C from 10 to 240 min. They found spinel inthe sintered samples and showed that it inhibited full densifi-cation. They concluded that chemical equilibrium was notreached in this system, and spinel formation between the meltand alumina made mullite formation impossible. They alsoinvestigated the effects of silica and magnesia on the sinteringof this system at 1400° to 1450°C.22 Both mullite and spinelformed in the magnesia–MAS glass system (MgO:MASglass4 90:10), and only mullite formed when silica was addedto an alumina–MAS glass mixture (Al2O3:SiO2:MAS glass469.7:20.3:10). They reported that density decreased due to mul-lite crystallization. However, their system was initially hetero-geneous due to the mixing of alumina (3.0 m2/g) and MASglass powders. Experiments were carried out with one aluminaparticle size, and the particle size distributions of the aluminaand MAS glass powders were not described. Consequently, theeffect of particle size on spinel formation kinetics and densi-fication kinetics during sintering of such systems is stillambiguous.

Spinel formation by a chemical reaction between aluminaand a magnesium-containing liquid around 1450°C has beenobserved by several researchers.4,23–27 Sandhage andYurek23–25 observed that spinel formed at the interface be-tween a sapphire substrate and a CMAS (CaO–MgO–Al2O3–SiO2) liquid at 1450°C. They showed, by a thermodynamicsanalysis, that spinel forms by the reaction of a magnesia-containing liquid with sapphire at 1450°C when the activity ofmagnesia is more than 0.056. Based on the analysis and results,they proposed a model for spinel formation in which aluminaindirectly dissolved into CMAS liquid through a spinel layerwhen the liquid is not saturated with alumina. Although thechemical reaction between MAS liquid and alumina is gener-ally believed to be the main mechanism for the spinel forma-tion around 1450°C, spinel can form on alumina by the simpleheterogeneous nucleation, because of an epitaxial relationshipbetween these two materials.28 The particle size of alumina canbe an important parameter for the balance between densifica-tion and liquid crystallization or reaction during sintering, be-cause it determines pore size, densification kinetics, nucleationor reaction site number, and dissolution kinetics.

Tarutaet al.29 investigated the densification of bimodal pow-der mixtures of alumina and glass powders at 1400°–1600°C.The glass powder was the ternary eutectic composition of tridy-mite–protoenstatite–cordierite, which forms a liquid at the low-est temperature (1355°C) in the MAS system. However, theirglass powder was heated at 1300°C for 10 h and crystallized to

L. C. DeJonghe—contributing editor

Manuscript No. 191676. Received July 12, 1996; approved July 24, 1997.Presented in part at the 98th Annual Meeting of the American Ceramic Society,

Indianapolis, IN, April 15, 1996 (Basic Science Division, Paper No. B-35).Based in part on the thesis submitted by A. Nakajima in partial fulfillment of the

requirements for a Ph.D. Degree in the Department of Materials Science and Engi-neering, The Pennsylvania State University, 1997.

*Member, American Ceramic Society.†Now with Japan Energy Corporation, Tokyo, Japan.

J. Am. Ceram. Soc., 81 [5] 1163–72 (1998)Journal

1163

a-alumina, cristobalite, enstatite, and cordierite prior to mixingit with the alumina powder.

Since the phenomena during liquid-phase sintering, over thetemperature range where alumina is not the only solid phase inthermal equilibrium with the liquid, have not been extensivelystudied, the understanding of the effect of the processing vari-ables on lower temperature liquid-phase sintering of alumina isincomplete. In the present study, the effect of liquid composi-tion and particle size on the densification of alumina was in-vestigated. The MAS system was selected as a model liquid-phase system because it is commonly used in industry, and itsdensification, when alumina is the only solid phase in thermalequilibrium with the liquid, is well characterized.4,9 To sim-plify comparison of our results with the earlier studies,4,9,29

the same ternary eutectic composition, 21.5MgO?17.5Al2O3?61.0-SiO2 (wt%), as used in previous studies was selected.Sintering was carried out over the temperature range of 1400°to 1460°C because the tie-line relationship in the MAS systemis fixed in this temperature range.30

The effect of boron and sodium doping on crystallizationduring liquid-phase sintering in the alumina–MAS glass systemwas also investigated at 1400° and 1460°C. Kawazoe31 showedthat the coordination number of magnesium gradually increaseswith increasing boron concentration in an oxide glass. It is wellknown that the addition of sodium to aluminosilicate glassesincreases the concentration of four-fold coordinated aluminumatoms in the silica network with coupling sodium ions.32 Be-cause sodium and boron affect the aluminum and magnesiumcoordination in the glass, they may modify crystallization dur-ing liquid-phase sintering. To study the above phenomena fora homogeneous system and thus avoid the effects of liquidspreading and redistribution of the liquid-phase sintering ki-netics, MAS glass-coated alumina was prepared by a sol–geltechnique.

II. Experimental Procedure

(1) MaterialsThe starting powders were commercial high-purity

(>99.99%) aluminas (AKP-50, AA-2, Sumitomo Chemical,Tokyo, Japan) with median particle sizes of 0.2 and 1.8mm.The standard deviation in size of the 0.2- and 1.8-mm aluminas,determined for 200 particles, were 0.04 and 0.31mm, respec-tively, and thus these powders were narrowly sized. The oxideglass or liquid compositions derived from the gels are denotedby 1355MAS (MgO–Al2O3–SiO2 system), MASB (MgO–Al2O3–SiO2–B2O3 system) and MASN (MgO–Al2O3–SiO2–Na2O system). The 1355MAS glass is the ternary eutectic com-position in the MAS system and has a melting temperature of1355°C. The molar ratio of B2O3 and Na2O in the MASB andMASN glasses were 1355MAS:B2O3 or Na2O 4 10:3. Thus,MASB and MASN refer to 23.8MgO, 7.7Al2O3, 45.4SiO2, and23.1B2O3 or 23.1Na2O (mol%), respectively.

The initial mixed metal alkoxide solutions for particle coat-ing were prepared from reagent-grade magnesium ethoxide(Mg(OC2H5)2) (Aldrich Chemical Co., Milwaukee, WI), alu-minum butoxide (Al(OC4H9)3) (Alfa Products, Danvers, MA),sodium ethoxide (NaOC2H5) (Aldrich), tributylborate((C4H9O)3B) (Alfa), and tetraethyl orthosilicate (Si(OC2H5)4)(Aldrich). These chemicals were dissolved in 2-methoxyethanol(Aldrich) in the appropriate ratio and refluxed in nitrogen at125°C for 16 h.33

(2) Powder Preparation for SinteringAlumina suspensions were prepared by mixing the alumina

powder in 2-methoxyethanol. This suspension was dispersedby ultrasonic agitation for 5 min, and the mixed alkoxide so-lution was added to the alumina suspension. The concentrationof the mixed alkoxide was adjusted to yield 9 vol% oxide glasswith respect to alumina at room temperature. The suspensionwas hydrolyzed with an equimolar amount of water and gelled

at 70°C. The suspension changed from a low-viscosity fluid toa yogurtlike gel in a few hours. After drying and calcination at700°C for 2 h, the powders were ground and sieved to <90mm.

Green bodies of the powders were prepared by uniaxialpressing and subsequent cold isostatic pressing at 276 MPa.Green densities of the pressed samples were measured by ge-ometry. Theoretical densities of the two-phase compacts werecalculated by assuming the theoretical densities of alumina as3.986 g/cm3, and 2.60, 2.32, and 2.61 g/cm3 for the 1355MAS,MASB, and MASN glasses, respectively.3,4,34Green bodies ofpure alumina were also prepared by the same procedure for thesintering studies. The average relative densities of the greenbodies were 52.9%, 49.7%, 49.1%, and 49.8% for the 0.2-mmalumina powder with no glass, 1355MAS, MASB, and MASNglasses, respectively. The average density of the 1355MASsamples prepared from the 1.8-mm alumina powder was55.6%.(3) Sintering and Characterization

Samples with a green density of ±1% average density wereselected for sintering. The mass of each sample was 0.3∼ 0.4g. Isothermal sintering kinetics were obtained for samplesheated in air between 1400° and 1460°C in a horizontal tubefurnace (Type 54434, Lindberg, Watertown, WI). A thermo-couple and an indicator with zero-point compensation wereused for temperature measurement before and after the experi-ments at each temperature. Typical sample loading and unload-ing times were about 1 s. When a sample was inserted into thefurnace, the temperature in the furnace decreased∼10°C andincreased to the control temperature in≈20 s. The stability ofthe control temperature was ±1°C. Sintered samples were air-quenched to room temperature.

The alumina coated with the 1355MAS glass was observedby TEM (H-9000UHR, Hitachi, Tokyo, Japan). The bulk den-sities of sintered samples were measured by the Archimedesmethod. Crystalline phases in coated powders and sinteredsamples were identified by X-ray diffraction (Geigerflex,Rigaku, Tokyo, Japan). The microstructures of the sinteredsamples were observed by SEM (SX-40A, Akashi Beam Tech-nology, Tokyo, Japan). For SEM observation, sintered sampleswere ground and polished with 1-mm diamond paste, thenchemically etched with 1% HF for 30 min and thermally etchedat 1250° to 1400°C for 30 min.(4) Solid/Liquid Interface Observation andContact Angle Measurement

To observe the wetting of alumina by the liquids and theliquid crystallization behavior in the presence of alumina, aseries of alumina/glass interface studies were performed. Aglass was prepared from the mixed alkoxide, which was hy-drolyzed with an equimolar amount of water and gelled at70°C. The gel was calcined at 700°C for 2 h to obtain a glassfrit. High-purity alumina with a median particle size of 0.2mmwas uniaxially pressed and subsequently cold isostaticallypressed at 242 MPa. The green bodies were sintered at 1600°Cfor 30 min in air, and the sintered samples were ground andpolished with 1-mm diamond paste. The sintered alumina wasalmost fully dense, and its grain size was 3 to 10mm. A pieceof the glass frit (approximately 0.01 g) was placed on thepolished surface of the sintered alumina. The sample was in-serted into the furnace for 5 and 15 min at 1400°C, and for 100and 400 min at 1450°C. All samples were heated at 1400°C,and the 1355MAS glass was heated at both 1400° and 1450°C.After heating, the samples were air-quenched to room tempera-ture. Although 15 min is not long for spreading of the alkalineearth aluminosilicate glass on sapphire at this temperature,35 ashort hold time was used to avoid boron and sodium volatil-ization. Therefore, the measured values of the contact angle areapproximate, but nevertheless indicative of wetting for the con-ditions studied.

Sample cross sections were ground and polished with 1-mmdiamond paste, then etched with 1% HF for 30 s. Contact anglemeasurements and solid/liquid interfaces were examined by

1164 Journal of the American Ceramic Society—Nakajima and Messing Vol. 81, No. 5

both optical microscopy (EPIPHOT-Time, Nippon KogakuK.K., Tokyo, Japan) and SEM with an energy-dispersive X-rayspectrometry (EDS, Delta 1, Kevex, Foster City, CA).

(5) Viscosity EvaluationThe viscosity of the glass was evaluated by a capillary pen-

etration technique. For the viscosity approximation, high-purityalumina with a median particle size of 1.8mm was uniaxiallypressed and subsequently cold isostatically pressed at 242MPa. The compacts were sintered at 1480°C for 30 min in airto obtain a porous alumina with an average pore size of 0.33mm (71.6% relative density). The pore size of the porous alu-mina was measured by mercury porosimetry (Autopore II9220, Micromeritics, Norcross, GA). A glass frit (approxi-mately, 0.01 g) was placed on the surface of the porous alu-mina, and the sample was inserted into a furnace at 1400°C.After holding the samples at the temperature for 5 or 15 min,they were quenched to room temperature.

The cross section of the sample was ground and polishedwith 1-mm diamond paste, and the penetration depth of theliquid into the porous alumina was measured by optical mi-croscopy. Assuming Poiseuille’s equation for capillary-drivenflow, the viscosity for each liquid system was approximatedfrom the penetration depth4 by assuming a tortuosity factor of4 and a surface tension of 0.405 J/m2.36,37Since glass frits wereobserved to start melting in less than 15 s, 15 s was subtractedfrom the total penetration time to compensate for the time delaydue to glass melting.

III. Results

(1) DensificationThe TEM micrograph of 0.2-mm alumina particles coated

with the 1355MAS glass in Fig. 1 shows that the particles areuniformly coated with the glass. No crystalline phases, excepta-Al2O3, were identified by XRD in the coated powders aftercalcination at 700°C. Apparent contact angles of the1355MAS, MASB, and MASN glasses on alumina were 24°(average) ± 4° (maximum deviation), 22° ± 2°, and 17° ± 3°,respectively. Apparent viscosities calculated from the penetra-tion depth after 5 and 15 min were nearly constant for eachglass. The glass viscosities at 1400°C were 14 to 15, 50 to 58,and 1 to 3 mPa?s for 1355MAS, MASN, and MASB glasses,respectively.

The effect of 1355MAS glass coating on the densification of0.2-mm alumina is shown in Fig. 2. The densification rate ofthe coated 0.2-mm alumina at 1400°C was almost the same asthe pure 0.2-mm alumina when the hold time was less than 3min. However, after 3 min the densification rate of the coated0.2-mm alumina decreased to less than the pure 0.2-mm alu-mina. The final density of the MAS glass-coated 0.2-mm alu-mina after 100 min at 1400°C was less than the pure alumina.At 1460°C, the densification kinetics of the coated 0.2-mmalumina were faster than the pure 0.2-mm alumina. Figure 3shows the densification kinetics for 0.2- and 1.8-mm aluminascoated with 1355MAS glass and sintered at 1400° and 1460°C.Both the relative density and the densification rate increased

Fig. 1. TEM micrograph of alumina coated with 1355MAS glass.

May 1998 Liquid-Phase Sintering of Alumina Coated with Magnesium Aluminosilicate Glass 1165

with increasing sintering temperature and decreasing particlesize. The time before the densification rate decreased for thecoated 1.8-mm alumina (30 min) was longer than that for the0.2-mm alumina (3 min) at 1400° and 1460°C. Densification of1.8-mm alumina seems to stop after 30 min, although the rela-tive density is less than 90%.

The densification kinetics for 0.2-mm aluminas coated withthree kinds of glass and sintered at 1400°C are shown in Fig. 4.The alumina coated with MASB glass sintered faster than theother samples at both sintering temperatures. The densification

kinetics of the alumina coated with 1355MAS glass decreasedsignificantly after only 3 min. The relative density of the alu-mina with 1355MAS glass was 92.8%, which was slightlylower than the 93.7% density for the alumina with MASN glassafter 100 min at 1400°C.

SEM micrographs of the 0.2-mm alumina-based samples sin-tered at 1400° and 1460°C for 100 min are shown in Figs. 5 and6. The microstructure of the alumina with 1355MAS glass (Fig.5(a)) sintered at 1400°C was composed of grains of 0.6-mmaverage size. However, grain growth to 2.7mm was observedwhen this sample was sintered at 1460°C (Fig. 5(b)). The grainsize of the alumina with the MASB glass was 2.6mm at1400°C and 4.6mm at 1460°C. The alumina grain size with theMASN glass was 0.4mm at 1400°C, and 0.6mm at 1460°C.(2) Crystallization

Mullite (Al 6Si2O11), spinel (MgAl2O4), sapphirine(Mg4Al10Si2O23) and cordierite (Mg2Al4Si5O18) crystallized

Fig. 4. Influence of coating composition on the densification kineticsof 0.2-mm alumina at 1400°C.

Fig. 5. SEM micrographs of 0.2-mm alumina coated with 1355MASglass: (a) 1355MAS glass, 1400°C, 100 min, (b) 1355MAS glass,1460°C, 100 min, (c) alumina, 1460°C, 100 min.

Fig. 3. Densification kinetics of 0.2- and 1.8-mm aluminas coatedwith 1355MAS glass at 1400° and 1460°C.

Fig. 2. Effect of 1355MAS glass coating on densification kinetics of0.2-mm alumina at 1400° and 1460°C.


during sintering (Table I). Based on the increase in X-ray peakintensities, mullite and spinel content increased with increasingsintering time and temperature. The relative amounts of thecrystalline second phases were calculated from the integratedpeak intensities in the following manner:

Crystalline second-phase ratio4(Imullite + Ispinel & sapphirine+ Icordierite)/Ialumina

Intensity ratio of mullite4 Imullite/Ialumina

The following peaks used for this calculation were free fromoverlap and the intensity was easily detected with respect to the

background level: (024) for alumina (CuKa/2u 4 53.5°), (122)for mullite (2u 4 41°), (222) for cordierite (2u 4 28.5°), and(004) or (400) for sapphirine and spinel (2u 4 45°), respec-tively. Since many spinel and sapphirine peaks overlap, thesetwo phases were evaluated from the same peak. The crystallinesecond-phase ratio is abbreviated as CSPR.

Figure 7 shows that the CSPR for the 1355MAS-coatedalumina sintered at 1400°C is dependent on particle size. Thistendency is the same when sintering was carried out at 1460°C,although the value of CSPR increased with increasing tempera-ture. The CSPR of 0.2-mm alumina coated with 1355MASglass began to increase after 3 min at 1400°C, whereas that of1.8-mm alumina coated with 1355MAS glass began to increase

Fig. 7. Change in crystalline second-phase ratio at 1400°C for 0.2-and 1.8-mm aluminas coated with 1355MAS glass.

Table I. Crystalline Second-Phase Formation as a Functionof Glass Composition and Sintering Conditions

Particle size(mm)

Glasscomposition

Sinteringtemperature (°C)

Hold time(min) Phases*,†

0.2 1355MAS 1400 1 Sa(Sp) (Co)1355MAS 1400 100 SaMu (Sp)1355MAS 1420 1 Sa(Sp)1355MAS 1420 100 SaMu (Sp)1355MAS 1450 1 SaMu (Sp)1355MAS 1450 3 SaMu (Sp)1355MAS 1450 10 SaMu (Sp)1355MAS 1450 100 SpMu1355MAS 1460 100 SpMuMASB 1400 100 MuMASB 1460 100 MuMASN 1400 100 SpMASN 1460 100 Sp

1.8 1355MAS 1400 100 SaMu (Sp) Co1355MAS 1460 100 SpMu

*Sa: sapphirine (Mg4Al10Si2O23). Co: cordierite (Mg2Al4Si5O18). Sp: spinel(MgAl2O4). Mu: mullite (Al6Si2O13).

† : main phase. ( ): possible.

Fig. 6. SEM micrographs of 0.2-mm alumina coated with MASB or MASN glass: (a) MASB glass, 1400°C, 100 min, (b) MASB glass, 1460°C,100 min, (c) MASN glass, 1400°C, 100 min, (d) MASN glass, 1460°C, 100 min.


after 10 to 30 min at this temperature. The intensity ratios ofsecondary phases for aluminas coated with 1355MAS glasssintered at 1400° and 1460°C are shown in Fig. 8. The sinteredsample contained mullite, spinel, sapphirine, and a smallamount of cordierite. Although the mullite concentration forthe 0.2-mm alumina increased significantly after 3 min at1400°C, a high concentration of spinel and sapphirine hadformed in the initial stage of sintering. Similar results wereobtained for the 1355MAS-coated 1.8-mm alumina, althoughthe concentration of these crystalline secondary phases in-creased after 30 min.

The dependence of the CSPR on glass composition is shownin Fig. 9 for the 0.2-mm alumina. When the sintering time wasless than 10 min at 1400° and 1460°C, the CSPR decreased inthe order of MASN > 1355MAS > MASB. However, the CSPRfor the MASN sample was the lowest after 100 min at bothtemperatures. The CSPR for the alumina coated with theMASB glass was lowest during the initial stage of sintering andgradually increases with sintering time. The CSPR for the alu-mina coated with the MASN glass was constant during sinter-ing, although it was higher than the other alumina–glass sys-tems during the initial stage of sintering. Intensity ratios for0.2-mm alumina coated with MASB and MASN are shown inFig. 10. Crystalline second-phase compositions in the sinteredsamples depended on glass composition. Only mullite wasidentified in the alumina with the MASB glass after sintering at1400°C for 100 min, whereas only spinel was identified in thealumina-plus-MASN glass. This trend of intensity ratios forMASB–alumina and MASN–alumina systems was the same at1460°C.

Figure 11 shows SEM micrographs and EDS analyses of thesample cross sections at the alumina/1355MAS glass interfaceafter heating at 1450°C for 100 and 400 min. After 100 min, aninterface layer, identified as spinel, was observed between theliquid and alumina. In addition to this interface layer, mulliteprecipitates were observed in the MAS glass in the samplecross section heated for 400 min. Mullite crystallized in theMASB–alumina system and spinel formed in the MASN–alumina system with similar morphologies and locations as inthe MAS–alumina system. However, the thickness of the spinel

layer in the MASN–alumina sample was thicker than that forthe 1355MAS–alumina sample.

IV. Discussion

(1) 1355MAS Glass–Alumina SystemBased on the phase diagram,12 the 1355MAS liquid forms at

the sintering temperature. The calculated theoretical density for

Fig. 8. Change in the intensity ratio of crystalline second phases in the 1355MAS glass-coated alumina: (a) 0.2mm, 1400°C, (b) 1.8mm, 1400°C,(c) 0.2 mm, 1460°C, (d) 1.8mm, 1400°C.

Fig. 9. Change of crystalline second-phase ratio in the aluminacoated with glass: (a) 1400°C, (b) 1460°C.


Fig. 11. SEM micrograph and EDS analysis of sample cross section: (a) 1450°C, 100 min, (b) 1450°C, 400 min.

Fig. 10. Change of the intensity ratio of crystalline second phase in the 0.2-mm alumina coated with glasses: (a) MASB 1400°C, (b) MASB1460°C, (c) MASN 1400°C, (d) MASN 1460°C.


the alumina–1355MAS glass composite is 3.86 g/cm3. Assum-ing all MgO in the initial glass composition becomes spinel andSiO2 becomes mullite, the calculated theoretical density is 3.85g/cm3. Based on these densities, the difference of relative den-sity can be estimated to be less than 0.3%. The time at whichthe densification rate starts to decrease corresponds to whencrystallization becomes significant at 1400°C (Fig. 12). Thisindicates that crystallization of the liquid impedes densificationof the alumina at 1400°C.

When the sintering temperature was increased to 1460°C,the densification rate of the MAS-coated 0.2-mm alumina in-creased significantly and there was significant grain growth.These results indicate that liquid-phase sintering was more ef-fective for 0.2-mm alumina at 1460°C than at 1400°C. Possiblereasons for the temperature dependence are increased solidsolubility12 and lower liquid viscosity.38 The derivative curvesof the densification kinetics and CSPR at 1460°C are shown inFig. 12(c). Since the densification of coated 0.2-mm alumina isalmost complete in 3 min, the densification rate had alreadystarted to decrease before the liquid started to crystallize.Therefore, there is little correlation between densification andcrystallization for coated 0.2-mm alumina at 1460°C. Fromthese experiments, it is clear that the effectiveness of liquid-

phase sintering is significantly reduced once crystallization ofthe liquid phase begins.

Based on the change in intensity ratio, the dominant phasesthat inhibit densification are spinel and/or mullite. The samplecross section revealed that spinel forms more easily than mul-lite, and this trend agrees with the change in intensity ratio. Thesurface area dependence of spinel formation can be expectedbecause it forms at the interface between the alumina and theMAS liquid. Although mullite crystallizes uniformly in the1355MAS liquid, its kinetics are particle size dependent, also.However, the effect is due primarily to the particle size depen-dence of spinel formation, because when spinel forms by thechemical reaction of the liquid with the alumina particles, themagnesium concentration in the MAS liquid decreases. Thiscomposition change depends on the particle size of alumina. Inaddition, alumina concentration in the MAS liquid increasesdue to alumina dissolution. The dissolution kinetics of aluminaare also expected to depend on the difference of surface area.Thus, when spinel forms in the system, the initial 1355MAScomposition approaches the Al2O3–SiO2 system. It has beenshown many times that mullite crystallizes at 1200°C startingfrom mullite gels.39,40 Although the degree of supersaturationfor mullite nucleation in the MAS system is not clear, bothmagnesium consumption by spinel crystallization and aluminadissolution lead to mullite nucleation.

Crystallization of the liquid phase during sintering can re-duce densification by one or more of the following mecha-nisms:

(1) Increase the liquid viscosity due to change in compo-sition.

(2) Decrease interface reaction due to the decrease in liq-uid content.

(3) Inhibit solution/precipitation of the major phase bycoating it with a crystallized material.

(4) Inhibit particle approach by creating a percolating crys-talline network.

When spinel forms from the MAS liquid, the viscosity of theremaining liquid should be higher.38 Since the densificationkinetics of the alumina coated with the MAS glass did notdecrease in the initial stage of sintering compared to the un-coated alumina (Fig. 2), it is believed that the change in liquidviscosity is not the dominant reason for the reduction of thedensification kinetics by crystallization. The densification ki-netics of liquid phase sintering generally depend on the 2/3 to1 power of the liquid volume.4 The decrease in liquid contentwill decrease densification kinetics; however, this should notstop the densification. Because the densification kinetics of thecoated alumina appeared to stop after crystallization, the dom-inant mechanisms for the decrease in the densification kineticsby crystallization are believed to be either (3) and/or (4). Basedon the location of crystalline phases in the MAS liquid, spinelcontributes to both (3) and (4), and mullite contributes to (4)only. Both crystalline phases can contribute to the decrease ofdensification kinetics during liquid-phase sintering.

(2) MASN and MASB Glass–Alumina SystemsBased on the literature,38 the viscosity of 1355MAS at

1400°C should be 9.2 to 15 mPa?s. Although spinel formationor alumina dissolution may affect these results, the calculatedviscosities from the penetration experiments for 5 and 15 minare close. Therefore, the rank order of the viscosities, MASN >1355MAS > MASB, is expected to be reasonably correct. Thecontact angles for the three glasses on alumina ranged from 17°to 24°. This small difference suggests that wetting or particlerearrangement should not be significantly different between thethree systems.

The significant decrease in the densification kinetics for theMAS-coated alumina after 3 min at 1400°C was attributed toliquid crystallization prior to full densification. In contrast, thealumina coated with MASB glass densifies to >95% densitywithin 3 min, and the CSPR at that time is 0.03, which is lowerthan the CSPR of 0.08 for the alumina with 1355MAS glass. In

Fig. 12. Derivative curves of densification kinetics and CSPR for thealumina coated with 1355MAS glass: (a) 0.2mm, 1400°C, (b) 1.8mm,1400°C, (c) 0.2mm, 1460°C.


the alumina–MASB system, spinel formation is suppressed andmullite crystallization is not significant until 3 min. The vis-cosity of the liquid decreases by boron addition. Because of thelower liquid viscosity and avoidance of crystallization duringinitial sintering, liquid crystallization does not affect the den-sification of the alumina coated with MASB glass.

In the MASN system, only spinel crystallizes, but its con-centration remains constant during sintering. The CSPR after100 min at 1400°C for MASN glass (0.11) is lower than thatfor 1355MAS glass (0.17), although the CSPR at the initialstage of sintering for MASN glass (0.11) is higher than that for1355MAS glass (0.08). Due to a constant CSPR during sinter-ing, the rapid decrease in the densification kinetics in the1355MAS–alumina system at 1400°C does not occur for thealumina coated with MASN glass, although the densificationkinetics for the rearrangement stage of the alumina–MASNsystem are lower than the alumina–1355MAS system becauseof the high liquid viscosity. Thus, the densities after 100 min at1400°C are almost the same between the alumina–1355MASsystem and the alumina–MASN system. However, the densityreaches more than 93%, and densification does not seem tostop for the MASN system after 100 min at 1400°C. Based onthese results, spinel may reduce solution/precipitation but doesnot inhibit it completely. Thus, it is difficult to believe that (3)in the above discussion is the dominant reason for the rapiddecrease of densification kinetics by crystallization of liquid inthe MAS system. Instead, it is believed that the formation of apercolating crystalline network is the dominant reason for thereduced densification kinetics by liquid crystallization. Bothmullite and spinel can contribute to formation of a percolatingcrystalline network.

Grain growth is influenced by solid solubility, dissolutionkinetics (direct or indirect), and the rate of material transport(viscosity). The solid solubilities for MASN and MASB sys-tems are not clear, because of the absence of suitable phasediagrams. The dissolution kinetics can be estimated as MASB(direct) > 1355MAS (from direct to indirect) > MASN (indi-rect) from the thickness of spinel layer. The viscosity order isMASB < 1355MAS < MASN. Grain growth of the MASBsystem can be attributed to the increased material transport, dueto both a lower liquid viscosity and no spinel formation at thesolid–liquid interface. In contrast, small grain size for MASNsystem is due to high liquid viscosity and a thick spinel layerformed on the alumina surfaces.

Hong and Messing41 studied the effect of boria on the crys-tallization of mullite and showed that the activation energy formullite crystallization is decreased by approximately 300 kJ/mol by the addition of 3 wt% boria. The results obtained withthe alumina–MASB glass system are consistent with their ob-servations. Fahrenholtz and Smith42 investigated the densifica-tion of sodium-doped colloidal mullite. They verified that so-dium (1 wt%) did not decrease the mullite crystallizationtemperature but, in fact, decreased the mullite crystallization.In the alumina–MASN glass system, only spinel was identifiedafter isothermal sintering, and mullite did not crystallize evenafter 100 min at 1400° and 1460°C.

Because of the existence of a solid phase in the system, theeffect of glass structure on crystallization during sintering ismuch more complex than crystallization from a simple glass.However, some of the crystallization behavior can be explainedby understanding the effect of the dopants on the glass struc-ture. In turn, this understanding can provide guidelines for thedesign of the glass phase to resist crystallization during sinter-ing. The concept of glass basicity is one approach to under-stand the effect of chemistry on glass structure.

Herzog investigated spinel layer formation between refrac-tories and molten aluminosilicate slag.27 He showed that thewear of MgO bricks increases with decreasing slag basicity,due to the formation of spinel layer on the brick surface. Duffyand Ingram proposed the concept of optical basicity as a mea-sure of Lewis basicity for oxide and halide crystals, glasses,and solutions. It is defined by the following equations.43

L (optical basicity)4 S(zir i/2gi)

zi is the oxidation number of cation i,r i is the molar ratio ofcation i with respect to total oxygen, andgi is the basicityparameter introduced by Duffy and Ingram.

The basicity parameter expresses the tendency of the cationto suppress the donor properties of oxygen. Calculated opticalbasicities of the glasses used in this study are 0.552 for1355MAS, 0.509 for MASB, and 0.638 for MASN, respec-tively. Kawazoe31 showed that there is a correlation betweenthe coordination structure of Mg2+ in solid-state metal oxides(both crystals and glasses) and their optical basicity by analyz-ing the chemical shifts of Mg. The results of this analysis areshown in Fig. 13. Mg2+ has 4-fold or 6-fold coordination in thesolid state. Based on Kawazoe’s analysis, Mg2+ prefers 6-foldcoordination when the optical basicity decreases. For the

Fig. 13. Relationship between chemical shift of MgKa1,2 and the optical basicity of oxides (after Kawazoe31). Dots4 crystalline oxides, Opencircles4 glasses.


glasses used in this study, the coordination preference for Mg2+

is expected to be

4-fold CN → MASN → 1355MAS→ MASB → 6-fold CN

In spinel, Mg2+ is in tetrahedral coordination. The optical ba-sicity for the glasses used in this study is consistent with theease of spinel formation. It is known that the addition of alu-mina to sodium aluminoborosilicate glasses inhibits the crys-tallization of cristobalite because the concentration of 4-foldaluminum atoms increases in the silica network with couplingsodium ions.44–46This structural change makes the nucleationof mullite, which is a pure aluminosilicate phase, very difficult.

Although further investigation is necessary to clarify thedetailed mechanism of the change in crystallization behaviordue to the addition of boron and sodium to MAS glass system,it is suggested that it is the change in the liquid compositionand in liquid structure (i.e., the coordination number for thecation) which affects crystallization in these systems.

V. Summary

The densification behavior of alumina coated with MAS-based glasses was investigated between 1400° and 1460°C tounderstand the liquid-phase sintering of alumina ceramics atlower temperature. The major crystal phases during sintering,beside alumina, are spinel and mullite. Crystallization of theglass inhibits densification of alumina when the inherent den-sification rate is low. Spinel forms at the interface betweenalumina and the MAS liquid first, then mullite uniformly crys-tallizes in the liquid during sintering. Crystallization kineticsare accelerated with finer particle size.

The effects of boron and sodium on densification and glasscrystallization of MAS-coated alumina were also investigated.The densification of MAS-coated alumina increases at 1400°Cwhen boron was added. The addition of boron to the MASglass decreases the viscosity and inhibits the crystallization ofspinel, whereas sodium enhances the crystallization of spinelwhile inhibiting the crystallization of mullite. It is believed thatthe dominant reason for reduced densification kinetics whenthe liquid crystallizes during sintering is that a crystalline per-colating network forms and it constrains shrinkage.

From this study, it was determined that key factors influenc-ing the balance between densification and crystallization at thesintering temperature are phase assemblage and particle size.The basicity of the glass could be a useful guide for selectingdopants for low-temperature liquid-phase sintering of alumina.

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Liquid-Phase Sintering of Alumina Coated with Magnesium Aluminosilicate Glass

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