16726
J. Phys. Chem. B 2008, 112, 16726–16733
Structure of High Alumina Content Al2O3-SiO2 Composition Glasses Richard Weber,*,†,‡ Sabyasachi Sen,§ Randall E. Youngman,| Robert T. Hart,⊥ and Chris J. Benmore‡ Materials DeVelopment, Inc., Arlington Heights, Illinois 60004, Department of Chemical Engineering and Materials Science, UniVersity of California-DaVis, DaVis, California 95616, Corning, Inc., Corning, New York 14831, Shepherd Chemical Company, Norwood, Ohio 45212, and AdVanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: October 15, 2008
The structure of binary aluminosilicate glasses containing 60-67 mol % Al2O3 were investigated using highresolution 27Al NMR and X-ray and neutron diffraction. The glasses were made by aerodynamic levitation of molten oxides. The 67% alumina composition required a cooling rate of ∼1600 °C s1- to form glass from submillimeter sized samples. NMR results show that the glasses contain aluminum in 4-, 5-, and 6-fold coordination in the approximate ratio 4:5:1. The average Al coordination increases from 4.57 to 4.73 as the fraction of octahedral Al increases with alumina content. The diffraction results on the 67% composition are consistent with a disordered Al framework with Al ions in a range of coordination environments that are substantially different from those found in the equilibrium crystalline phases. Analysis of the neutron and X-ray structure factors yields an average bond angle of 125 ( 4° between an Al ion and the adjoining cation via a bridging oxygen. We propose that the structure of the glass is a “transition state” between the aluminarich liquid and the equilibrium mullite phase that are dominated by 4- and 6-coordinated aluminum ions, respectively. 1. Introduction High alumina content aluminosilicate materials are used in applied ceramics and industrial refractories1,2 due to their stability and high melting temperature. Small amounts of aluminum oxide homogenize the distribution of laser active dopant ions in silicate glasses used in optical device applications.3,4 The aluminosilicates are one binary member of the important group of geological materials comprising the oxides of aluminum-calcium-magnesium-silicon.5 The high temperature phase stability of the aluminosilicates is controversial in spite of several investigations of the system. Aramaki and Roy6 report congruent melting of 3:2 mullite at a temperature of approximately 1850 °C. Prochazka and Klug7 report a mullite phase containing ∼67 mol % Al2O3 that melts congruently at 1900 °C. Aksay and Pask8 and Risbud and Pask9 showed that mullite undergoes a peritectic decomposition reaction at 1830 °C to form a silica-rich liquid and nearly pure Al2O3. Thermodynamic models9 indicate large regions of liquid phase separation in the undercooled liquids. The differing results from various samples suggest that the sample preparation and processing methods influence phase stability in the aluminosilicate compositions. High alumina content glasses are difficult to synthesize. Compositions that contain more than ∼40 mol % Al2O3 form fragile, low viscosity liquids. These liquids become viscous when they are deeply undercooled, and Kriven et al.10 demonstrated fiber drawing from liquids close to the mullite composi* To whom correspondence should be addressed. E-mail: info@ matsdev.com. † Materials Development, Inc. ‡ Argonne National Laboratory. § University of California-Davis. | Corning, Inc. ⊥ Shepherd Chemical Company.
tion. This finding is consistent with the liquid having a viscosity of ∼103 poise at about 200 °C below the melting point of the corresponding crystal. Sen and Youngman11 investigated glasses containing 0.3-28 mol % Al2O3 using a combination of high resolution 27Al, 17O, and 29Si NMR. Glasses with low Al2O3 content (e7.5 mol % Al2O3) were found to be homogeneous whereas those with higher Al2O3 content were phase separated. All the glasses investigated contained AlIV, AlV, and AlVI species, where the superscripted number denotes the number of oxygen ions with aluminum. The results of this study indicated that the tetrahedral [AlO4/2]-1 units in glasses with
