High temperature single crystal properties of mullite

https://doi.org/10.1016/S0955-2219(99)00124-7Get rights and content

Abstract

Extensive neutron diffraction and Rietveld studies of dense, hot pressed mullite (3Al2O3·2SiO2) have been conducted up to 1650 °C in air, yielding a complete set of lattice parameters and axial thermal expansion coefficients. Unconstrained powders of the same stoichiometric composition were also analyzed by X-ray diffraction and Rietveld techniques up to 900 °C in air, from which lattice parameters and thermal expansion coefficients were obtained. An earlier reported structural discontinuity was confirmed by XRD to lie in the temperature range 425 to 450 °C. Single-crystalline mullite fibers of composition 2·5Al2O3·SiO2 were grown from the melt by a laser-heated, float zone method. A partial set of the single-crystal elastic moduli were determined from various sections of fiber, by Brillouin spectroscopy, from room temperature up to 1400 °C. They indicated a roughly 10% reduction in stiffness over that temperature range.

Introduction

In the design of oxidation resistant, high temperature ceramic composites, it is wise to know the intrinsic properties of the component materials, independent of their structural configuration in the composite. Detailed theoretical predictions of composite behavior, particularly at elevated temperatures, would rely heavily on a knowledge of single crystal properties. Furthermore, since the fabrication and mechanical evaluation of composites is a difficult, time consuming and costly procedure, any data on the intrinsic properties of a component material is extremely valuable in accelerating the design, modelling and development of high temperature composites.

Mullite, of nominal composition (3Al2O3·2SiO2), is a highly attractive candidate for oxide composites.1 As a matrix it is a widespread ‘workhorse’ refractory material, having very good creep resistance and chemical stability up to 1600 °C. The current, widely accepted phase diagram indicates that the equilibrium phase grown by solid state reaction has a narrow solid solution range around the 3Al2O3·2SiO2 (abbreviated 3:2) composition.2 However, when grown from the melt, mullite crystallizes in 2Al2O3·SiO2 or 2:1 composition.2 The solid solution range can further be extended to 3Al2O3·SiO2 or 3:1 composition by rapid cooling in a closed container, in the absence of alumina nuclei.2, 3

Currently, steady progress is being made in the production of mullite fibers. Directionally solidified mullite fibers can be grown by a laser heated, float-zone method.4 They appear to consist of columns of single crystals with [c] axes parallel to the fiber direction. Polycrystalline mullite–alumina fibers (Nextel 720, from 3M Company, St. Paul, MN) have also been fabricated by a sol–gel process from an 85 wt Al2O3–15 wt. SiO2 composition, which subsequently converts on firing at 1350 °C to 59 vol% mullite and 41 vol% Al2O3.5 The microstructure is extremely fine grained with ∼0·5 μm mullite grains containing <100 nm largely intragranular alumina grains. This microstructure is unstable above 1300 °C however, due to grain growth and drastic loss of mechanical strength from an initial 2·4 MPa.6, 7 More recently, homogeneous, aluminosilicate, glass fibers (of mullite composition) and amorphous yttrium aluminate fibers (of Y3Al5O12 or ‘YAG’ composition) have been pulled from deeply undercooled melts via a containerless processing technique.8, 9 The high tensile strengths of the aluminosilicate glass fibers (∼6 GPa) however, are again drastically reduced to 1 GPa at best, due to uncontrolled random crystallization to mullite or YAG, on annealing above 1100 °C. Work is therefore in progress to develop textured or single crystal fibers from the amorphous solid precursors.

Measurements of the thermal expansion of various mullite compositions are mainly reported for temperatures up to 900 °C and are either based on dilatometric measurements or on X-ray diffraction.10, 11, 12, 13, 14 For higher temperatures reliable data are rare.13 It has to be kept in mind that the former, i.e. the dilatometric method provides results which are always of both structural and microstructural origin and therefore, an average, which is hard to interpret on an atomic length scale. X-ray diffraction allows, at least in principle, a separation of both aspects via an evaluation of the positions of the reflections in a diffraction pattern and by a line profile analysis. The X-ray method has its limits if the sample material is coarse grained, which is sometimes unavoidable, and because of absorption effects. In particular in the high temperature regime around and above 1000 °C, the X-ray diffraction method becomes additionally tedious for experimental reasons.

An alternative diffraction method uses neutrons. In the case of mullite neutron diffraction, results are not affected by absorption and the relatively large sample volume allows for good statistics, even in the case of coarse grained samples. Moreover, the oxygens have a relatively high scattering power for neutrons so that oxygen related structural details can be studied in oxide compounds more reliably. We performed in-situ neutron diffraction experiments with the 3:2 mullite described above up to a temperature of 1650 °C. The overall aim of the investigation was a full structure refinement of mullite at high temperatures. Only the thermal expansion results of neutron diffraction measurements will be given here, since full information will be published in a forthcoming paper.15

The bulk elastic properties of mullite have been measured on sintered compacts of relatively pure, raw materials.16, 17 More recent work on hot pressed stoichiometric 3:2 mullite, which was hydrothermally grown without any glassy phase, indicated an unusual decrease in Poisson's ratio with increasing temperature up to room temperature.18 This was attributed to the incommensurate modulation in the mullite crystal structure. Early high temperature measurements16 of relatively pure raw materials indicate a drop in elastic modulus for both 3:2 and 2:1 mullite above 600 °C, strongly suggesting the presence of intergranular glass. With the availability of highly textured, polycrystalline fibers,4 it was decided to measure the elastic moduli of mullite up to temperatures of 1400 °C.

The complete elasticity tensor can be measured by the Brillouin light scattering technique.19 This technique presents several advantages over other methods, the most important being that only very small samples are required, no contact is needed with the sample, and it is suitable for low-symmetry crystals. The sample size is limited only by the spot size of the laser used as the excitation source, making Brillouin scattering well-suited for measurements on the thin mullite fibers used in this study. Also, only optical access to the sample is required for Brillouin measurements, which allows much flexibility in furnace design.

Brillouin scattering involves the inelastic scattering of light from phonons in a crystal. If V is the velocity of the phonon, φ and φ′ are the incident and scattered angles, respectively, and ν and ν′ are the frequencies of the incident and scattered photons, respectively, then the equation relating phonon velocity to the frequency shift of the scattered photon, Δν=ν′−ν, isV=Δννc2nsinφfor the case of symmetric scattering where φ and φ′ are very nearly equal. c is the speed of light in vacuum and n is the refractive index for the direction of photon propagation. In this experiment we use a ‘platelet geometry’,20 which allows nsinφ to be replaced by nosinθo, where no is the index of the surrounding medium, and θo the incidence angle of the laser light. Platelet geometry allows access to all phonon directions in the plane of the sample.

The velocities of acoustic phonons are determined entirely by the elastic moduli and density of the material. The velocity in eqn (1) for plane monochromatic waves must satisfy Christoffel's equation [eqn (2)].21Ciklmqiqm−ρV2δkl=0where qi and qm are unit vectors in the phonon propagation direction, ρ is the density, and Cijkl is the elastic tensor.

In anticipation of the successful growth of single crystal or aligned textured fibers of mullite, the work described here was undertaken. The goal of a mullite matrix reinforced with mullite fibers coated by a suitable debonding oxide interphase would be well served by a precise knowledge of crystallographic lattice parameters as a function of temperature up to 1600 °C, as well as axial and volumetric thermal expansion coefficients. These data would enable the residual stresses arising during high temperature cycling to be estimated. Since dense mullite composites could be used as shingles to line an aircraft combustion chamber, for example, (Ref. 22 and H. Schneider, German Aerospace Center, pers. comm.) the thermal expansion of dense, hot pressed, polycrystalline mullite samples were measured, rather than of loose powders, as is customary in crystallographic measurements by X-ray or neutron diffraction. In addition, 3:2 mullite powder would be studied by X-ray diffraction up to 900 °C, and the data would be analyzed by Rietveld methods.

To complement the crystallographic data, and enable better modelling of the high temperature behavior of a mullite-containing composite, single crystal elastic moduli for orthorhombic mullite would be determined experimentally. Brillouin spectroscopic measurements would be made both at room temperature, and up to 1400 °C. Information gathered from this work would then indicate the feasibility and limits of using mullite fiber-reinforced composites in a load-bearing structural application, at high temperature in an oxidizing environment.

Section snippets

Sample fabrication

For diffraction studies, hydrothermally grown, stoichiometric 3:2 mullite powder (Kyoritsu Ceramic Materials Co. Ltd., Tsukisan-cho 2-41, Minato-ku, Nagoya 455-91, Japan) was used as a starting material. For X-ray diffractometry, the powder of average particle size 0·3 μm was inserted into 0·3 mm diameter quartz capillaries which were kept in rotation during the measurements at all temperatures to reduce preferred orientation effects. For neutron diffraction, a polycrystalline mullite ceramic

X-ray diffraction

Rietveld refinement was performed for all data sets to extract the temperature dependence of the cell dimensions. The refinement was performed in space group Pbam, and published room temperature atom coordinates27 were used as starting values. The resulting cell dimensions and the corresponding cell volumes are given in Table 1 and are shown Fig. 2(a)–(d). About 2 wt% of Al2O3 was detected as a minority phase, which could be neglected for the purpose of a cell parameter Rietveld refinement.

The

X-ray diffraction

The fact that our data also showed the discontinuities of the lattice parameters a and b around 425 °C supports the general validity of this effect in mullite. Explanations were given in terms of the complex dependencies between thermal expansion and structural arrangement of mullite and the related phases andalusite and sillimanite. A relation to domains originating from oxygen vacancies was also discussed by Schneider et al.14 The somewhat larger deviation of our cell parameter a may be

Conclusions

In this work, the lattice parameters and thermal expansion coefficients for stoichiometric mullite (3Al2O3·2SiO2) powder were measured by X-ray diffractometry up to 900 °C. The data was analyzed by the Rietveld technique, and the strutural discontinuity reported by Schneider et al.12 was confirmed. The temperature range of its occurrence was narrowed to between 425 and 450 °C.

A dense, hot pressed sample of the same mullite made from hydrothermally grown, stoichiometric 3Al2O3·2SiO2 powder was

Acknowledgements

W. M. Kriven gratefully acknowledges the Institut für Kristallographie und Angewandte Mineralogie, in München, Germany, for hosting her sabbatical leave of six months in 1997. The work of J. W. Palko was supported in the United States, by a Fannie and John Hertz Foundation graduate Fellowship. The X-ray work in conjunction with the elastic constant measurements was carried out at the Center for Microanalysis of Materials at UIUC. The WDS electron microprobe analysis was conducted by Dr. I.

References (34)

  • H. Schneider et al.

    Mullite and Mullite Ceramics

    (1994)
  • I.A. Aksay et al.

    Stable and metastable equilibria in the system SiO2–Al2O3

    J. Am. Ceram. Soc.

    (1975)
  • W.M. Kriven et al.

    Solid solution range and microstructures of melt-grown mullite

    J. Am. Ceram. Soc.

    (1983)
  • Sayir, A. and Farmer, S. C. Directionally solidified mullite fibers. In Ceramic Matrix Composites—Advanced High...
  • D.M. Wilson et al.

    Microstructure and high temperature properties of NEXTEL 720 fibers

    Cer. Eng. Sci. Proc.

    (1995)
  • J. Göring et al.

    Creep and subcritical crack growth of Nextel 720 alumino silicate fibers as-received and after heat treatment at 1300°C

    Cer. Eng. Sci. Proc.

    (1997)
  • H. Schneider et al.

    Thermal stability of Nextel 720 alumino silicate fibers

  • W.M. Kriven et al.

    Synthesis and microstructure of mullite fibers grown from deeply undercooled melts

  • Weber, J. K. R., Cho, B., Hixon, A. D. Abadie, J. G., Nordine, P. C., Kriven, W. M., Johnson, B. R. and Zhu, D., Growth...
  • G. Oehlschlegel et al.

    Anisotrope Mischkristallbildung einiger Verbindungen des ternären Systems BaO–Al2O3–SiO2. Teil II. Messungen an Strukturen mit zweidimensionaler Verknüpfung von (Si,Al)O4-Tetrahederen und Modellvorstellungen von deren Wärmedehnungsanisotropie (Anisotropic mixed crystal formation of a compoundfrom the ternary system BaO–Al2O3–SiO2. Part II. Measurement of structures with two dimensional joining of (Si,Al)O4-tetrahedra and proposed model of their thermal expansion anisotropy)

    Glastech. Ber.

    (1974)
  • S. Winter et al.

    Thermal expansion and high-temperature crystal chemistry of Al2SiO5 polymorphs

    Am. Mineral

    (1979)
  • H. Schneider et al.

    Thermal expansion of mullite

    J. Am. Ceram. Soc.

    (1990)
  • Margalit, J., Thermal expansion of mullite up to 1500°C. Ph.D. thesis, Verlag Mainz, Wissenschaftsverlag, Aachen,...
  • H. Schneider et al.

    Thermal expansion discontinuities of mullite

    J. Amer. Ceram. Soc.

    (1993)
  • Brunauer, G., Boysen, H., Frey, F., Hansen, T. and Kriven, W. M., High temperature crystal structure of a 3:2 mullite...
  • J.E. Fenstermacher et al.

    High temperature mechanical properties of ceramic materials: IV, sintered mullite bodies

    J. Am. Ceram. Soc.

    (1961)
  • Davis, R. F. and Pask, J. A., Mullite. In High Temperature Oxides, Vol. 4., ed. A. M. Alper. Academic Press, New York,...
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