Acicular Mullite–Cordierite Composites with Controllable CTE Values

Acicular Mullite–Cordierite Composites with Controllable CTE Values

Daniel Grohol,w Chan Han, Aleksander J. Pyzik, Janet M. Goss, and Clifford S. Todd

The Dow Chemical Company, Core R&D, Midland, Michigan 48674

A new, two-step method for the synthesis of porous mullite–cordierite composites with controllable coefficients of thermalexpansion (CTE) values is reported. In the first step, a porousstructure of interconnected acicular mullite grains with high as-pect ratios is formed by using SiF4 gas at elevated temperatures.This intermediate contains particles of MgF2 and cristobalitethat are attached to the mullite grains. In the second step, MgF2and cristobalite react with parts of the acicular mullite structure,and they form cordierite, while maintaining the acicular natureof the resulting mullite–cordierite composite. CTE of mullite–cordierite composites synthesized using this method decreaseabout linearly with increasing cordierite content, and porositiesrange from 49% in cordierite-rich composite to 54% in a mul-lite-rich composite. The CTE value of a desired porous compos-ite can be easily controlled by choosing the appropriate ratios ofreactants Al2O3, SiO2, and MgO, which result in the desiredmullite-to-cordierite ratio with its characteristic CTE value.

I. Introduction

THE ability to control macroscopic properties such as thermalexpansion and fracture strength of ceramic materials entails

significant benefits for numerous technologies, particularlywhere materials are required to withstand high stresses associ-ated with severe thermal gradients. In dense materials, desirablevalues of thermal expansion and other properties such asstrength and modulus are often accomplished by forming phys-ical mixtures of matrices with desired amounts of embeddedfibers or particles of carefully chosen characteristic properties.1,2

These fibers or particles reinforce the matrix and affect its mod-ulus or thermal expansion characteristics in a desired manner.The preparation of porous ceramic materials of targeted thermalexpansion characteristics can be achieved by similar means as ofdense materials, except that pores are created by the additionand subsequent burnout of sacrificial porogen. Alternatively,coating of polymer foams of desired pore structures in prece-ramic slurries has been used followed by the removal oftemplating foam.3 The shaping of preceramic polymers into de-sired morphologies at low temperatures followed by thermoset-ting and burnout of the polymeric chains represents yet anotherpath toward porous ceramics.4

In the quest for control over thermomechanical properties ofporous materials, and in contrast to the more traditional me-chanical engineering approaches, we used a solid-state chemicalreaction during which one ceramic material with its set of de-sirable properties is partially converted into another materialwith another set of desirable properties. The resulting compositethen adopts the advantages of both its parent components.

In the first step, we synthesized acicular mullite, Al6Si2O13,with high aspect ratio grains. The synthesis of acicular mulliteby using SiF4 via the intermediate fluorotopaz has beendescribed previously.5–7 These high aspect ratio grains fusetogether in a truss-like manner bestowing high fracture strengthon acicular mullite, even at very high porosities (B20 MPa at65% porosity).8

In the second step, parts of the acicular mullite structure wereconverted into cordierite, Mg2Al4Si5O18, by a solid-state chem-ical reaction using in situ-formed magnesium- and silicon-con-taining particles. The most distinct benefit of cordierite inceramic applications is its low thermal expansion (B1 ppm/1C,201–8001C), which makes this material highly desirable for ap-plications requiring the ability to withstand severe thermal gra-dients. In contrast to acicular mullite, however, cordierite has notbeen known to form acicular grains with high aspect ratios. Inorder to achieve porosity in cordierite, sacrificial porogen mustbe used, which leads to the creation of neck-like connectivitybetween cordierite grains, resulting in dramatic weakening of itsporous structure. By synthesizing mullite–cordierite compositesin which predetermined fractions of structurally robust acicularmullite grains were converted into cordierite, we sought to com-bine the high structural integrity of acicular mullite with the lowthermal expansion of cordierite.

II. Experimental Procedures

Metal oxides were purchased from commercial sources: magne-sia 99.5%, 325 mesh (Strem Chemicals, Newburyport, MA),k-alumina 99% (Selecto Scientific, Suwanee, GA), powderedquartz 99.9%, 400 mesh (Alfa Aesar, Ward Hill, MA).

In the synthesis of mullite–cordierite composites,9 powders ofMgO, k-Al2O3, and quartz were first homogenized in a mixerin amounts required to obtain desired composite ratios forabout 4 min (Table I).

After the blending of the reactants was complete, 7.00 g ofbinder Methocelt (Methocel, The Dow Chemical Company,Midland, MI) and about 50 mL of distilled water were added intoeach mixture, and the mixtures were homogenized again. Flatbars were then extruded using hydraulic ram extruder HandleStS-02A-SA-265 (Handle GmbH, Muhlacker, Germany), andthey were dried in air over several days. These samples wereheated in air to 9701C to remove the organic binder, and thenreacted in the SiF4 atmosphere at 150 torr according to pro-grammed regime to a maximum temperature of 11001C. After theSiF4 reaction was complete, the samples were heated to 14001C inair for about 6 h, and cooled to room temperature.

Thermal expansion curves for all composites were measuredwith a DuPont dilatometer Model 2940 (DuPont Instruments,Wilmington, DE) in the 201–8001C temperature range using a51C/min heating rate. Porosities were measured by water ab-sorption using the Archimedes method. Powder X-ray diffrac-tion (XRD) data were acquired on a Bruker AXS D8 (BrukerAXS GmbH, Karlsruhe, Germany) diffractometer with a cop-per X-ray tube operating at 40 kV and 40 mA, using CuKa1radiation. The powder patterns were analyzed by MDI Jade 7software. Scanning electron microscopy (SEM) images were

P. Colombo—contributing editor

Presented at the 34th International Conference and Exposition on Advanced Ceramicsand Composites, Daytona Beach, FL, January 26, 2010 (Porous Ceramics: Novel Devel-opments and Applications, Paper No. ICACC-S9-013-2010).

wAuthor to whom correspondence should be addressed. e-mail: [email protected]

Manuscript No. 28173. Received June 10, 2010; approved August 13, 2010.

Journal

J. Am. Ceram. Soc., 93 [11] 3600–3603 (2010)

DOI: 10.1111/j.1551-2916.2010.04129.x

r 2010 The American Ceramic Society

3600

mailto:[email protected]

acquired using the FEI Quanta Inspect electron microscope. Anenergy-dispersive X-ray spectroscopy (EDS) image was acquiredon a sample that was embedded in epoxy, ground and polishedby using a Bruker 4010 XFlash (Bruker AXS GmbH) energy-dispersive X-ray spectrometer.

III. Results

Powder XRD data acquired on intermediates after SiF4 reactionat 11001C showed the presence of mullite, cristobalite, andMgF2 in the form of sellaite (Table II). The relative amountsof cristobalite and MgF2 increased with increasing targeted cor-dierite content, reflecting the increasing amounts of MgO andpowdered quartz used in the preparation of samples with highertargeted cordierite contents (Table I).

During heating of the intermediates in air at 14001C, theoriginal shapes of the sample bars remained unchanged. TheXRD analysis of samples heated to this temperature showedthat the samples contained only mullite and cordierite (TableIII). Traces of sapphirine were also detected in some of thepowder patterns, but they did not refine well.

An SEM image of the microstructure of intermediate I-4(Fig. 1, left) shows elongated acicular mullite grains with regularrectangular bases of an approximate width of 10–15 mm. Theselarge acicular mullite grains are intermixed with nonacicularmatter and with smaller acicular mullite grains. Energy analysisof the electron image showed that the nonacicular matter con-sisted of cristobalite, MgF2, and glass.

The structure of the composite MC-4 (Fig. 1, right) retainedmuch of its open, acicular character predicated by the templatingeffect of the mullite grains. The formerly regular, rectangulargrains of high aspect ratios observed in the intermediates are nolonger smooth in the composite, and their surfaces appear severelyetched and irregular. An EDS image of a polished cross-section ofthe same composite MC-4 (Fig. 2) shows remnants of acicularmullite grains surrounded by cordierite. Silica-rich glass regionsare also observed near the cordierite phase, but not in the directcontact with the mullite phase. Since the EDS image represents a2-D cross-section of the porous material, its acicular characterapparent in 3-D perspective of Fig. 1 is not as easily discerned.

The cordierite content found by X-ray powder diffractionincreased with increasing targeted cordierite content, but the

actual values lagged behind the targeted values (i.e., valuesexpected from stoichiometry of initial reactant ratios of Al2O3,MgO, and SiO2) by 5%–12%. Porosities decreased with increas-ing cordierite content from 58% for mullite-only containingsample MC-1 to 49% for sample MC-6 containing 88% cor-dierite. The coefficients of thermal expansion (CTE) values werefound to decrease with increasing cordierite content in the entiremullite–cordierite composition range, approximately accordingto the rule of mixtures. A plot of CTE as a function of cordieritecontent shows a roughly linear dependence (Fig. 3).

Thermal expansion curves of prepared mullite–cordieritecomposites show monotonous, nonlinear shapes in the 201–8001C temperature range (Fig. 4). All the curves have aslight concave character; the slopes are less steep in the lowertemperature region and they become steeper with increasingtemperature.

Fracture strengths of the mullite–cordierite compositesprepared by this method generally decreased with increasingcordierite content in most of the mullite–cordierite compositionrange.9 Mechanical properties of these composites, however,are outside the scope of this communication, and they are notpresented here.

IV. Discussion

(1) Synthesis

Mullite–cordierite composites were prepared in two steps. XRDdata and SEM images showed that exposure of homogenizedmixtures containing k-Al2O3, SiO2, and MgO to stoichiometricexcess of silicon tetrafluoride gas in elevated temperatures led tothe formation of acicular mullite grains that were interspersedwith particles of MgF2 and cristobalite:

k-Al2O3ðsÞ þ SiO2ðsÞ þMgOðsÞ �!SiF4ðgÞ

TAl6Si2O13ðsÞ

þMgF2ðsÞ(1)

The formation of the acicular mullite phase most likelytook place via the topaz intermediate as it has been described

Table II. X-ray diffraction (XRD) Analysis of Mullite-Containing Intermediates

Intermediate XRD analysis of crystalline phases

I-1 95% mullite, 5% cristobaliteI-2 92% mullite, 8% cristobaliteI-3 81% mullite, 16% cristobalite, 3% MgF2

I-4 69% mullite, 25% cristobalite, 6% MgF2



Table I. Amounts of Reactants Used for Synthesis of Mullite–Cordierite Composites

Reactant

mixture no.

Targeted wt%

of cordierite

Reactants (g)

MgO Al2O3 SiO2

1 0 0.00 71.80 28.202 20 2.76 64.41 32.833 40 5.51 57.02 37.474 60 8.27 49.64 42.105 80 11.02 42.25 46.736 100 13.78 34.86 51.36

Table III. Composition and Properties of Mullite–CordieriteComposites

Composite

Targeted wt%

of cordierite

XRD analysis

CTE

(ppm/1C)

Porosity

(%)% Cordierite % Mullite

MC-1 0 0 100 5.2070.16 5871MC-2 20 12 88 4.8170.15 5471MC-3 40 33 67 4.0070.12 5371MC-4 60 55 45 3.4370.10 5071MC-5 80 72 28 2.8570.09 4971MC-6 100 88 12 2.0170.06 4971

XRD, X-ray diffraction; CTE, coefficients of thermal expansion.

Fig. 1. Scanning electron microscopy images of fracture surfaces of theintermediate I-4 on left (69% mullite, 25% cristobalite, and 6% MgF2),and of the MC-4 composite on right (45% mullite and 55% cordierite).

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earlier.5,7 In contrast to the previously described synthesis pro-cedure targeting pure acicular mullite, the current experimentsinvolved a source of magnesium, which reacted with SiF4 andformed MgF2 and cristobalite:

2MgOþ SiF4 ! 2MgF2 þ SiO2 (2)

In the second synthetic step, the intermediates containingmullite, MgF2, and cristobalite were heated in air to 14001C,which resulted in the formation of the mullite–cordieritecomposites:

2Al6Si2O13 ðsÞ þ 6MgF2 ðsÞ þ 14SiO2 ðsÞ! 3Mg2Al4Si5O18 ðsÞ þ 3 SiF4 ðgÞ

(3)

During this process, the mullite grains partially served as areactant phase for the formation of cordierite: nonacicular com-ponents of the intermediates, MgF2, and cristobalite reactedwith acicular mullite to form cordierite. The evidence for such areaction became apparent from the absence of MgF2 and SiO2

in the XRD patterns of the final composites (Table III). Theamounts of cordierite that formed during the second syntheticstep, i.e. during the heating of the intermediates to 14001C werelimited by the amounts of MgF2 and cristobalite present in theintermediates. In turn, the amounts of these transition phaseswere limited by the amounts of MgO and quartz powder added

into the starting reactant mixture. Thus, the desired relativeamounts of mullite and cordierite phases in the final compositescan be easily controlled by determining the necessary ratios ofthe reactants.

The fact that lower amounts of cordierite were found in thecomposites than that expected by stoichiometry (Table III) mostlikely resulted from incomplete reaction in Eq. (3). In order forthis reaction to proceed to completion, all the reactants in theintermediates, i.e. mullite, MgF2, and SiO2 (cristobalite) must becompletely uniformly distributed. If uniform distribution is notachieved for any reason, lesser amount of cordierite will formthan that expected from stoichiometry. Since neither MgF2 norSiO2 were found in the powder patterns of the final composites,it is likely that the small amounts of ‘‘missing’’ Mg21 and Si41

ions that were not found in any crystalline phases were presentin these composites in the form of silica-rich glass (Fig. 2).

In previous studies, mullite–cordierite composites have beenprepared by various methods including sintering and hot-press-ing of the mullite and cordierite powders,10–12 by chemicalreaction from hydroxides and sols,13 or from sol–gels.14 Weare not aware of another example in which mullite served asboth a structural template and a reactant for the synthesis of thesecond composite component, cordierite. The utilization ofacicular mullite to function both as a reactant and as a compo-nent phase in the final composites in this study was predicatedby the high porosity-ensuring conditions that resulted in excel-lent contact between mullite and the intermediate phases MgF2

and SiO2. In a dense material, such a reaction would have beenmuch more difficult to achieve in reasonable yields.

(2) Properties

The most intriguing and potentially the most technologicallyuseful feature of the porous mullite–cordierite composites pre-pared by a partial mullite-to-cordierite conversion is their ther-mal expansion behavior, which impacts the ability to withstandsevere thermal shock. Dense mullite–cordierite composites andtheir thermomechanical properties have been investigatedextensively in the past. In general, dense composites with higheramounts of mullite showed higher CTE values, and also higherfracture strengths.10,11

Approximately linear decrease of CTE values in the 201–8001C temperature range with increasing cordierite content inthe current study indicates a significant degree of connectivitybetween the mullite and cordierite phases. The observeddecrease of the CTE values starts from a relatively low cordie-rite content of 12% (Fig. 3), and the slope remains about con-stant up to the composite containing 88% cordierite and 12%mullite.

If the cordierite phase was substantially separated from themullite phase, such as in a hypothetical model of cordierite par-ticles attached to the structure of connected mullite grains, the

Fig. 2. Energy-dispersive X-ray spectroscopy image of mullite–cordie-rite composite MC-4. Color coding: mullite—red; cordierite—blue;silica-rich glass—green; porosity—black.

Fig. 3. Coefficients of thermal expansion (CTE) in the 201–8001C range,and porosity values of mullite–cordierite composites as a function ofcordierite content determined by powder X-ray diffraction.

Fig. 4. Thermal expansion curves of mullite–cordierite composites.

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thermal expansion behavior of the phase with greater phasepercolation, i.e. the acicular mullite, would be expected to dom-inate. In that case, CTE values very close to that of mullitewould be expected up to relatively high cordierite contents. Thenear-linear dependence of CTE values on the mullite-to-cordie-rite fractions, however, favors a model in which acicular mulliteis discontinuous with cordierite intervening between mullitegrains. This proposed model in which the connectivity ofacicular mullite grains is disrupted by intervening cordieritewas corroborated by the EDS image of a mullite–cordieritecomposite MC-4 (Fig. 2).

The ability to control CTE values in porous mullite–cordierite composites with highly acicular character couldmake these composites attractive substrates for filtration appli-cations in which both high fracture strength and high resistanceto thermal shock are required. The relative contributions offracture strength originating from acicular mullite and of ther-mal shock resistance, bestowed by cordierite, can be easilycontrolled by controlling the relative ratios of the components.Thus an additional degree of control can be gained overthe fundamental properties of these technologically importantporous materials.

V. Conclusions

We have described a new class of porous mullite–cordieritecomposites, which adopt the advantages of both their parentmaterials: acicular character is bestowed by the mullite phase,and lowered CTE values are rendered by cordierite. In the pre-pared series of mullite–cordierite composites, CTE values de-creased about linearly with increasing cordierite content. Thecomposites were synthesized in two steps: in the first step, in-terconnecting acicular mullite grains with a truss-like structurewere formed in the presence of SiF4 at elevated temperature; themullite framework was interspersed with particles of MgF2 andSiO2. In the second step, varying amounts of the mullite frame-work were converted into cordierite; MgF2 and SiO2 particlesreacted with parts of the mullite framework that also served as astructural template. Near-linear dependence of CTE on cordie-rite content indicated that discontinuous high-CTE mullite

phase was interrupted by the low-CTE cordierite phaseessentially in the entire mullite–cordierite composition range.This feature potentially makes the acicular mullite–cordieritecomposites amenable to control of technologically importantmaterial attributes such as fracture strength and resistance tothermal shock.

Acknowledgment

We thank Nick Shinkel, Sean Pardell, and Mike Dopp for technical assistanceduring the synthesis process.

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Acicular Mullite–Cordierite Composites with Controllable CTE Values