THE INFLUENCE OF BORON ON THE CRYSTAL STRUCTURE AND PROPERTIES OF MULLITE Investigations at Ambient, High-Pressure, and High-Temperature Conditions

Abstract

Mullite is one of the most important synthetic compounds for advanced structural and functional ceramic materials. The crystal structure of mullite with the composition can incorporate a large variety of foreign cations, including (amongst others) significant amounts of boron. However, no chemical or crystal structure analyses of boron-mullites (B-mullites) were available prior to this work, thus representing the key aspects of this thesis. Furthermore, the influence of boron on selected properties of mullite under ambient, high-temperature, and high-pressure conditions are addressed. Starting from a 3:2 mullite composition (Al4.5Si1.5O9.75), the initial hypothesis for this study was a 1:1 isomorphous replacement of silicon by boron according to the coupled substitution mechanism. Based on a series of compounds synthesized from sol-gel derived precursors at ambient pressure and 1200°C, the formation conditions and physical properties of B-mullites were investigated. The formation temperature for B-mullites decreases with increasing boron-content, as revealed by thermal analyses. An anisotropic development of lattice parameters is observed: Whereas lattice parameters a and b only exhibit minor changes, a linear relationship between lattice parameter c and the amount of boron in the crystal structure was established, on the basis of prompt gamma activation analyses (PGAA) and Rietveld refinements. According to this relationship about 15% of the silicon in mullite can be replaced by boron yielding single-phase B-mullite. B-mullites with significantly higher (~ factor 3) boron-contents in the mullite structure were also observed but the respective samples contain alumina impurities. Fundamental new details regarding the response of B-mullite to high-temperature and high-pressure are presented in this thesis. On the one hand, long-term thermal stability at 800°C was proved for B mullite, whereas on the other hand, complete decomposition into boron-free mullite and corundum is observed at 1400°C. Furthermore, the incorporation of boron into the crystal structure reduces the mean metric thermal expansion coefficient by 15% in comparison to boron-free mullite. Such a reduction by chemical substitution makes B mullites a potential candidate for technical applications in the temperature range below 1000°C. Boron incorporation is associated with the formation of additional oxygen vacancies which reduces the mechanical stability of the mullite structure at high-pressure. Moreover, a slight increase of the overall (volume) compressibility of B mullite compared to boron-free mullite is observed. The compressibility in mullite is anisotropic with the a-axis being the most and the c-axis being the least compressible one. The increasing divergence with pressure between the compressibilities in a- and b-direction can be explained by a rotation of the octahedra and the increasing inclination angle ω. One major outcome of this thesis is the crystal structure of B-mullite, synthesized at 1200°C and ambient pressure. The refinements in space group Pbam based on neutron diffraction and 11B MAS NMR data clearly confirm the suggested silicon boron substitution mechanism and yield a composition of Al4.64Si1.16B0.2O9.58. Boron resides in planar BO3 groups crosslinking the mullite-type AlO4 octahedral chains perpendicular to the c-axis. The position and the intrinsic rigidity of the BO3 group imposes local distortion of the AlO6 octahedra. As a consequence split positions of the oxygen atoms are required in the first coordination sphere of boron, which in turn lead to significantly shortened oxygen-oxygen distances in c-direction and only minor shortenings in the a- and b directions. Herewith, the crystallographic model provides an explanation for the anisotropic behavior of lattice parameters upon boron-incorporation described above. Single-phase B mullite with 40% replacement of silicon by boron was synthesized at 10 kbar and 875°C representing a marked increase in boron-content compared to the B-mullites synthesized at ambient pressure and 1200°C. The composition Al4.5Si0.9B0.6O9.4 was derived from refinements based on X-ray diffraction data in combination with the established silicon-boron substitution mechanism. Besides the three-coordinated boron, the chemical shifts in the 11B MAS NMR spectra clearly resolve additional replacement of some aluminum in the AlO4 tetrahedra by boron which is in good agreement with the PGAA results.