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The Effects of Na 2 O and SiO 2 on Liquid Phase Sintering of Bayer Al 2 O 3
Author(s) -
Frueh Tobias,
Kupp Elizabeth R.,
Compson Charles,
Atria Joe,
Messing Gary L.
Publication year - 2016
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/jace.14206
Subject(s) - sintering , materials science , grain boundary diffusion coefficient , analytical chemistry (journal) , diffusion , grain boundary , phase (matter) , doping , mineralogy , dissolution , liquid phase , solubility , grain growth , grain size , microstructure , metallurgy , chemical engineering , chemistry , chromatography , thermodynamics , physics , organic chemistry , optoelectronics , engineering
To determine how grain‐boundary composition affects the liquid phase sintering of MgO‐free Bayer process aluminas, samples were singly or co‐doped with up to 1029 ppm Na 2 O and 603 ppm SiO 2 and heated at 1525°C up to 8 h. Na 2 O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as‐received, MgO‐free Al 2 O 3 , Na 2 O‐doped samples sinter to 98% density with average grain sizes of ~3 μm after 8 h. Increasing SiO 2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20−400 Pa·s) of the 0.3 to 1.8 nm thick siliceous grain‐boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain‐boundary phase. For low Na 2 O/SiO 2 ratios, densification of Bayer Al 2 O 3 at 1525°C is controlled by diffusion of Al 3+ through the grain‐boundary liquid, whereas for high Na 2 O/SiO 2 ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al 2 O 3 or diffusion of Al 3+ . Increasing Na 2 O in SiO 2 ‐doped samples increases diffusion of Al 3+ and Al 2 O 3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al 2 O 3 densification can be manipulated by adjusting the Na 2 O to SiO 2 ratio.