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The Effect of LiF Addition on the Sintering Mechanism of
Spinel, MgAl2O4
Hans-Joachim Kleebe, Mathis Müller, Keith Rozenberg 1 , Ivar E. Reimanis 1
1 Colorado School of Mines, Metallurgical and Materials Engineering Dept., Golden, CO 80401, USA
MgAl2O4 is widely regarded as one of the most promising optical ceramics [1,2]. It has
excellent transmissivity in the near infrared frequency range. There is an excellent body
of research on the sintering mechanisms of pure spinel for optical applications.
Unfortunately hot pressing with LiF doped spinel, which is one of the most promising
processes for producing optical quality spinel ceramic, is also one of the least
understood. Many compounds have been proposed or used as sintering aids for
MgAl2O4 spinel (hereafter called spinel). For example, Na3AlF6, AlCl3, CaCO3, and LiF
have all been shown to promote sintering in spinel, though the detailed mechanisms
have remained elusive. LiF has been used to speed up the sintering kinetics of several
other systems including SrTiO3, and BaTiO3, and MgO. LiF, is the only compound
considered for use as a sintering aid in the production of low porosity, transparent spinel
via hot pressing.
The effect of LiF on the sintering of spinel has been only qualitatively studied. An
understanding of the mechanism for removal of LiF from the system is essential as any
residual LiF or related species could destroy the optical properties of the material. It has
been shown that LiF reduces the sintering temperature of spinel by nearly 200°C, and
decreases reaction temperatures during reactive sintering. More recent studies on
dense spinel were performed to determine the location of LiF subsequent to sintering,
with the objective of better understanding its role in sintering. LiF was not identified in
any of the studies, suggesting that it may react with spinel during sintering and/or
ultimately was removed from the system. It is known that LiF is active at the interface
between spinel grains at higher temperatures. Furthermore, it has been reported that
spinel sintered in the presence of LiF has a substantially higher Al content. Large
amounts of LiF have also been observed to act corrosively with spinel. All of these
observations indicate that LiF does chemically react with the spinel at elevated
temperatures. A recent study indicates that LiF reacts with Al in spinel, forming LiAlO2,
and leaving Mg-rich regions in the matrix. This, however, leaves the question of what
becomes of the fluoride species. This spinel
project addresses the issue of how LiF can
promote the sintering performance of
magnesium aluminate. In addition to HRTEM
investigations, model experiments with high
volume fraction of LiF addition are performed.
[1] R. J. Bratton, “Translucent Sintered MgAl2O4”,
J. Am. Ceram. Soc., 57 [7] 283-86 (1974).
[2] C.-J. Ting and H.-Y. Lu, “Hot Pressing of
Magnesium Aluminate Spinel; Part-II. Microstructural
Development”, Acta mater. 47 [3] 831-
40 (1999).
Fig. 2: TEM image of the spinel sample, doped
with 5 wt% LiF and pre-sintered at 900 o C. In close
proximity of the intrinsic pores, an amorphous LiFphase
was observed (transient phase).
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