Interfacial structure of annealed alumina-zirconia-silicate nanoceramics

D Le Messurier, N Sissouno, AR Vearey-Roberts, S Evans, DA Evans, R Winter; Mater Sci Technol 20 (2004) 975
Abstract. An alumina zirconia nanocomposite has been produced using the chloride sol-gel method and embedded into a silicate matrix by dispersing the nanocomposite into a powdered silica glass and subsequent annealing. The resultant nanoceramic was subjected to 27Al magic angle spinning (MAS) NMR, small angle X-ray scattering (SAXS), and X-ray photoelectron spectroscopy (XPS), leading to a core-shell type model of the interfacial region. Initially the particles are agglomerated with the shell containing mainly atoms of octahedral coordination and the core aluminium atoms of tetrahedral coordination. Upon annealing the agglomerates break up, causing a change in the coordination of the aluminium atoms. As the atoms diffuse into the matrix, the ones that were initially in the shell change to be tetrahedrally coordinated, and therefore increase the overall population of tetrahedrally coordinated aluminium atoms within the interface.
Fig.1: XPS spectrum. Fig. 1: XPS core level spectrum of the 50% alumina - 50% zirconia sample using Mg-K alpha source with the analyser in fixed retard ratio mode (FRR) and a channel width of 0.3eV. The remaining oxygen peaks are located outside this range and have been recorded separately.

The composites are prepared from a powder mixture of alumina and zirconia nanoparticles and crushed Na2O - 3 SiO2 glass. The nanoparticles themselves are prepared by hydrolysing the corresponding chlorides and subsequent washing and drying. The level of remaining chloride, according to XPS, is about 5%.

Fig.2: 27Al NMR spectra.
Fig.3: 27Al NMR spectra.
Fig. 2-4: 27Al MAS NMR spectra of alumina-zirconia nanocomposite (2- 90:10; 3- 70:30; 4- 10:90) embedded in a silicate glass matrix: a) raw material, b) after annealing.

Fig.4: 27Al NMR spectra.
The peak near 0ppm (200Hz) is due to 27Al nuclei in octahedral environments while the peak near 55ppm (5700Hz) corresponds to tetrahedrally coordinated aluminium. The relative strength of the two peaks changes during the annealing process: although both types of site are present in the raw alumina nanoparticles (a), the relative strength of the tetrahedral line increases in the annealed samples (b).

Fig.5: 27Al NMR dfference spectra. Fig. 5: 27Al MAS NMR difference spectra between raw and annealed compounds of varying percentages of alumina and zirconia: a) 90:10, b) 70:30, c) 10:90 alumina-zirconia ratio.

This change in environment becomes clearer when the difference spectra, obtained by subtracting the raw spectrum from the product spectrum, are considered. The reduction of the intensity of the octahedral line is largely compensated by an increase in the tetrahedral intensity. In addition, there is a positive contribution to the difference spectra to the right of the octahedral line, corresponding to a line broadening of the octahedral component which is due to increased disorder as the particles begin to dissolve in the glass matrix.

Fig.6: SAXS patterns. Fig. 6: a) Pre- and b) post-annealing scattering patterns obtained from an in-situ SAXS experiment conducted on a sample of 50% alumina - 50% zirconia embedded into a silicate glass.

The SAXS pattern of the raw powder mixture consists of a q-4 slope owing to the macroscopic grain surfaces of the silicate glass grain surfaces, with a long superimposed contribution of the nanoparticles, which produces a q-2-sloped tail, thus indicating that the nanoparticles have a rough surface. During annealing, an intermediate plateau is introduced between the q-4 and q-2 flanks.

Fig.7: Model (schematic). Fig. 7: Schematic model of dissolving agglomerates: a) raw material, b) after annealing; white circles are the AlO6 groups in the shell of the agglomerates and black circles are the AlO4 groups in the core of the agglomerates and also diffused into the glassy matrix.

We propose a model of the annealing process based on the NMR and SAXS results as follows: The alumina nanoparticles, originally consisting of tetrahedral and octahedral environments, gradually dissolve in the glass matrix. This accounts for the plateau observed in the SAXS pattern after annealing. The fact that the octahedral site fraction is reduced during annealing suggests that the octahedral sites are located near the grain surface originally. As the grain dissolves, the aluminium atoms occupying these sites change into a tetrahedral geometry typical of a network constituent in the glass.

Acknowledgements. We would like to acknowledge Chiu Tang and Chris Martin, the beamline scientists on beamline 6.2 at the SRS in Daresbury.
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