MECHANISMS OF SINTERING AND SECOND PHASE FORMATION IN BAYER ALUMINA

Open Access
Author:
Frueh, Tobias
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 25, 2017
Committee Members:
  • Gary Lynn Messing, Dissertation Advisor
  • Gary Lynn Messing, Committee Chair
  • James Hansell Adair, Committee Member
  • John Richard Hellmann Jr., Committee Member
  • Douglas Edward Wolfe, Outside Member
  • Charles E. Compson, Special Member
Keywords:
  • Sintering
  • Alumina
  • second phase formation
  • grain boundaries
Abstract:
The vast majority of industrial Al2O3 applications use powders derived from Bayer feedstocks with purity levels of 99.0 - 99.9%. This dissertation explores how the extrinsic powder chemistry of Bayer alumina influences densification, microstructure evolution, and fundamental sintering mechanisms. To determine how grain-boundary composition affects the sintering of MgO-free Bayer Al2O3 dilatometry and sintering kinetics experiments of samples with Na2O and SiO2 concentrations up to 1029 ppm and 603 ppm, respectively, were performed. It was shown that Na2O and SiO2 form a nanometer thick siliceous grain boundary film. The estimated viscosities (20 - 400 Pa·s) indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase, and densification strongly depends on the Na2O/SiO2 ratio in the samples. For low Na2O/SiO2 ratios, densification of Bayer Al2O3 at 1525°C is controlled by diffusion of Al3+ through the grain-boundary liquid, whereas for high Na2O/SiO2 ratios, densification can be governed by either the interface reaction of Al2O3 or diffusion of Al3+. Increasing Na2O in SiO2-doped samples increases diffusion of Al3+ and Al2O3 solubility in the liquid, and thus densification increases by 1-2.5%. Based on these findings, it is concluded that Bayer Al2O3 densification is controlled by the Na2O to SiO2 ratio. The stages at which MgO influences densification of Al2O3 were identified by comparing dilatometry measurements and sintering kinetics of MgO-free and 380 ppm MgO-doped Bayer Al2O3 powder. Using high-resolution TEM it was shown that MgO reduces the grain boundary thickness during final stage sintering. It was shown by EDS that MgO increases the solubility of SiO2 in Al2O3 grains near the boundaries. First-principles DFT calculations demonstrated that the co-dissolution of MgO and SiO2 in Al2O3 is thermodynamically favored over the dissolution of MgO or SiO2 individually in Al2O3. For the first time, it was experimentally demonstrated that removal of SiO2 from the grain boundary during final stage sintering is a key process by which MgO enhances the sintering of Al2O3. A physical model that describes the dynamic development of grain boundary chemistry and thickness from initial to final stage sintering as a function of chemical and physical powder parameters was developed. The liquid glass phase initially accumulates in the particle contacts due to capillarity and during densification the glass phase distribution changes due to the formation of grain boundaries and grain growth, leading to a dynamic change in grain boundary thickness during densification. Grain boundary thicknesses measured from high-resolution TEM images agree well with predicted film thicknesses. The model shows that the grain boundary thickness is determined by either the concentration of the glass phase in the sample or by an equilibrium film thickness, which is controlled by a balance between attractive and repulsive interparticle forces. Variations in powder chemistry, such as changing the Na2O/SiO2 ratio or changing the MgO concentration, can affect the grain boundary thickness by influencing the interparticle force balance or by changing the amount of glass phase in the grain boundaries. The formation of beta-Al2O3 (Na2O·11Al2O3) is sometimes observed during sintering of Bayer Al2O3, and the conditions and mechanisms of formation were investigated. Formation kinetics showed that the majority of beta-Al2O3 grains form between 0 and 1 hour at 1525°C, at densities >90%. The amount of beta-Al2O3 increases with increasing Na2O and MgO concentrations, and decreases with increasing SiO2 concentration. SiO2 increases the solubility of Na2O in the grain boundaries by forming a glassy film, and thereby counteracts the formation of beta-Al2O3. At final sintering stage SiO2 is removed from the grain boundaries because SiO2 and MgO form a solid solution in alpha-Al2O3, which leads to the supersaturation of grain boundaries and the nucleation and growth of beta-Al2O3. The Master Sintering Curve (MSC) approach and the factors affecting its accuracy were explored for analyzing the complete sintering profile of Bayer Al2O3. MSC analysis is sensitive to shrinkage anisotropy, which can be accounted for by directional shrinkage measurements, and to powder chemistry and forming technique effects that result in mechanistic changes that cannot be sufficiently described by MSC analysis. It is concluded that Q should not be interpreted as the sintering activation energy, or used to interpret mechanistic differences between powders since Q includes several mechanisms that influence densification throughout the sintering cycle. Despite these limitations, the MSC approach is a useful and practical tool for predicting the effects of thermal load (i.e. time and temperature) on densification of a specific powder and forming process.