Chemical interaction between LiF and MgAl2O4 spinel during sintering

Article abstract of DOI:10.1111/j.1551-2916.2007.01723.x

Reactions between LiF and MgAl2O4 at temperatures up to 1500°C are examined with a variety of tools, including differential scanning calorimetry, thermo-gravimetric analysis, X-ray diffraction, and scanning electron microscopy. LiAlO2 and MgF2 are found to be the active reaction products at these temperatures. A transient liquid phase comprising MgF2 and LiF forms at intermediate temperatures, but then is consumed at higher temperatures during the reformation of MgAl2O4. If processed as an uncompacted powder mixture, all of the initial LiF in the system eventually vaporizes at temperatures exceeding 1300°C. A new reaction sequence relevant to the densification of LiF-doped MgAl2O4 spinel is proposed.

Full text of DOI:10.1111/j.1551-2916.2007.01723.x

J. Am. Ceram. Soc., 90 [7] 2038–2042 (2007)
DOI: 10.1111/j.1551-2916.2007.01723.x
r 2007 The American Ceramic Society
ournal
J
Chemical Interaction Between LiF and MgAl2O4 Spinel During
Sintering
K. Rozenburg* and I. E. Reimanis*w
Metallurgical and Materials Engineering, Colorado School of Mines, Golden, Colorado 80401
H.-J. Kleebe*
Institut fur Angewandte Geowissenschaften, Technische Universitat Darmstadt, 64287 Darmstadt, Germany
¨
¨
Ron L. Cook
TDA Research Inc., Arvada, Colorado 80033
Reactions between LiF and MgAl2O4 at temperatures up to
15001C are examined with a variety of tools, including differ-
ential scanning calorimetry, thermo-gravimetric analysis, X-ray
diffraction, and scanning electron microscopy. LiAlO2 and
MgF2 are found to be the active reaction products at these tem-
peratures. A transient liquid phase comprising MgF2 and LiF
forms at intermediate temperatures, but then is consumed at
higher temperatures during the reformation of MgAl2O4. If
processed as an uncompacted powder mixture, all of the initial
LiF in the system eventually vaporizes at temperatures exceed-
ing 13001C. A new reaction sequence relevant to the densificat-
ion of LiF-doped MgAl2O4 spinel is proposed.
tures.18,19,37 Furthermore, it has been reported that spinel sin-
tered in the presence of LiF has a substantially higher Al
content.17–19,37,38 Large amounts of LiF have also been ob-
served to act corrosively with spinel.20,21 All of these observa-
tions indicate that LiF does chemically react with the spinel at
elevated temperatures. A recent study19 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 be-
comes of the fluoride species. The present study addresses these
issues through a series of thermal analyses and a quench exper-
iment.
II. Experimental Procedure
I. Introduction
Baikalox S30CR spinel, (Baikowski, Annecy, France), was used
in this study. The main impurities in the spinel powder, given by
the supplier, are (in wt. ppm): Na, 10; K, 200; Fe, 20; Si, 50; and
Ca, 30. Baikowski also reports d50 5 350 nm with a mean
agglomerate size B2.5 mm and a specific surface area of
30 m2/g. Sixty grams of the spinel was mixed with 40 g of LiF
(Puratronic; Alfa Aesar, Ward Hill, MA) in a ball mill with al-
umina media in a 250 mL polyethylene container for 20 min to
make a 60 wt% spinelꢀ40 wt% LiF mixture. Normally, doping
amounts would be between 0.5 and 2.0 wt% LiF, a relatively
large proportion of LiF was used amplify the effect and make
any reaction products more detectable. This mixture was then
divided for thermal analysis experiments and quenching exper-
iments. A Netzsch thermal analyzer (STA 409; Netzsch Instru-
ments Inc., Burlington, MA), which simultaneously conducts
differential scanning calorimetry (DSC) and thermo-gravimetric
analysis (TGA) experiments, was used for the thermal analysis.
Data were collected on 60 mg of the mixture heated at a rate of
101C/min to 10001C and held for 15 min before cooling at 101C/
min to 2001C. This same sample was subsequently heated to
15001C at a rate of 101C/min and held for 15 min before cooling
at 101C/min to room temperature. After an appropriate tem-
perature (10001C) was found through thermal analysis, 2 g
of the mixture was placed in an alumina crucible (AD-99;
Coors, Golden, CO) and heated to 10001C and held for 5
min. The crucible was then withdrawn from the furnace and
its contents were emptied onto a copper bar to accelerate the
cooling rate. This sample was then divided with half being used
to perform X-ray diffraction (Phillips X-Pert; PANalytical Inc.,
Natick, MA) and half being used for scanning electron micros-
copy and energy-dispersive X-ray spectroscopy (JEOL USA,
Peabody, MA).
gAl2O4 is widely regarded as one of the most promising
optical ceramics. It has excellent transmissivity in the
M
near-infrared frequency range. There is a substantial body of
research on the sintering mechanisms of pure spinel for optical
applications.1–15 Unfortunately, hot pressing with LiF-doped
spinel, which is one of the most successful processes for pro-
ducing 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,z Na3AlF6, AlCl3, CaCO3, and LiF have all been shown
to promote sintering in spinel,16–26 although the detailed mech-
anisms have remained elusive. LiF has been used to speed up the
sintering kinetics of several other systems including SrTiO3,25
BaTiO3,26–30 and MgO.31,32 LiF is the only compound consid-
ered for use as a sintering aid in the production of low porosity,
transparent spinel via hot pressing.16,33–36 The effect of LiF on
the sintering of spinel has been only qualitatively studied.17–23
An understanding of the mechanism for removal of LiF from
the system is essential as any residual LiF or related species
could impair the optical properties of the material.20 It has been
shown that LiF reduces the sintering temperature of spinel by
nearly 2001C17,18,33–36 and decreases reaction temperatures dur-
ing reactive sintering.17,20 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.19,29 LiF was not identified in any of the studies, sug-
gesting 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 tempera-
Manuscript No. 22271. Received September 19, 2006; approved December 15, 2006.
zIt should be noted that Huang et al. and Ganesh et al.
18,22,23
are concerned with re-
fractory grade spinel not the ultra high-purity material needed for high-performance optical
applications.
*Member, American Ceramic Society.
HSC Chemistry 5.1 (Outokumpo Research Oy, Pori, Fin-
land) was used to predict thermodynamically the products of a
reaction between LiF and MgAl2O4 spinel. HSC uses published
wAuthor to whom correspondence should be addressed. e-mail: reimanis@mines.edu
2038

July 2007
Chemical Interaction Between LiF and MgAl2O4
2039
0.8
0.6
Heating
Cooling
0.4
0.2
0.0
–0.2
–0.4
–0.6
–0.8
100 200 300 400 500 600 700 800 900 1000
Temperature, °C
Fig. 3. Differential scanning calorimetry results from heating (solid
line) the 40% LiF-Spinel mixture to 10001C at 101C/min and then cool-
ing (dashed line) at the same rate.
Fig. 1. Equilibrium composition of a mixture of LiF and spinel as cal-
culated through the HSC program. It should be noted that the model of
equilibrium assumes that mass is conserved; this is not the case as it will
be shown that LiF vaporizes shortly after melting at 8401C.
III. Results
Figure 3 reports the first stage (to 10001C) of the DSC results.
The solid curve shows the heat flow into the sample as the tem-
perature is increased. The linear character of the endotherm in-
dicates melting of a pure material. The melting of a pure
material is characterized by the onset of the peak, in this case
8401C. The solidification exotherm shown in the dashed line of
Fig. 3 has a concave aspect to its tail: this is characteristic of the
solidification of an alloy. The temperature at the peak of the
exotherm characterizes solidification of an alloy, which is 8151C
in the present case. The small peak at 5551C may represent a
solid exsolution reaction. Figure 4 shows the DSC results for the
second stage of the experiment (to 15001C). The solid line de-
scribes heat flow through the sample as the temperature is in-
creased. A small peak observed at 5901C most likely results from
the dissolution of MgF2 into the LiF. The melting endotherm
has a concave character to it again indicating the melting of an
alloy at 8351C. The exothermic rise at 10501C is likely the result
of a reaction between MgF2 and LiAlO2. The endotherm ap-
pearing with a peak at 12841C is likely the result of the vapor-
ization of a species. Cooling, represented by the dashed line in
Fig. 4, is uneventful, indicating that no reactive species remain.
The TGA results for the first stage are shown in Fig. 5. A
small mass loss is observed until 8401C, likely indicating the re-
lease of adsorbed gasses from the powders. The sharp increase in
mass loss after 8401C is associated with the melting of a phase,
at which point its vapor pressure would increase leading to en-
hanced mass loss. Mass loss continues to be substantial on cool-
ing (the dashed line) until 8001C where solidification is shown to
occur in Fig. 3. TGA results from the second stage are shown in
Fig. 6. In this stage, mass loss begins at 8301C where melting was
reported in Fig. 4. Mass loss rate is highest at 12941C (deter-
mined by examining the derivative), where Fig. 4 shows a peak
on an exotherm, indicating the two are related. Overall, the
sample experiences a 40% mass loss. It is noted this mass loss
corresponds to the wt% of LiF in the system. X-ray diffraction
performed on the specimen, which was heated to 10001C and
then quenched to room temperature (Fig. 7), reveals MgAl2O4
and LiF as well as the products LiAlO2 and MgF2. Notably
absent are AlF3, MgO, and Al2O3 and other compounds, which
have been suggested in prior work as intermediate reaction
products. Scanning electron microscopy of this sample shows
highly faceted grains growing out of fluoride-rich melt (Fig. 8).
Based on the grain morphology and X-ray diffraction results,
thermodynamic data (usually from the NIST-JANAF tables of
thermo-chemical data) to calculate equilibrium compositions
over an range of temperatures. The initial compounds are fixed;
in the present case they are MgAl2O4 and LiF. The calculation
was performed both with an atmospheric ratio of gasses and in a
vacuum. Although AlF3, MgO, and Al2O3 (which have been
proposed as reaction products by other studies21,22) were al-
lowed as products, the only compounds that were predicted in
any appreciable amount were LiAlO2 and MgF2 (Fig. 1). The
LiF–MgF2 phase diagram (Fig. 2) indicates MgF2 is soluble in
LiF up to B35% with a eutectic temperature near 7401C.39
Based on this result, model experiments were designed to iden-
tify potential reactions in the system.
Fig. 2. LiF–MgF2 phase diagram showing a eutectic at B7401C and
extensive solid solubility above 8001C. Source: W. E. Counts, R. Roy,
and E. F. Osborne, ‘‘Fluoride Model Systems: II, The Binary Systems
CaF2–BeF2, MgF2–BeF2, and LiF–MgF2,’’ J. Am. Ceram. Soc., 36 [1]
12–22 (1954).

2040
Journal of the American Ceramic Society—Rozenburg et al.
Vol. 90, No. 7
0.1
100
95
90
85
0.0
–0.1
–0.2
–0.3
–0.4
Heating
Cooling
80
75
70
65
60
55
Heating
Cooling
–0.5
–0.6
200
400
600
800
1000 1200
1400
200
400
600
800
1000
1200
1400
Temperature, °C
Temperature, °C
Fig. 4. Complete differential scanning calorimetry trace of a 60 wt%
mixture of MgAl2O4 and LiF re-heated to 15001C. The circled portion is
indicative of an exothermic decomposition following melting.
Fig. 6. Thermo-gravimetric analysis results of a 60 wt% mixture of
MgAl2O4 and LiF heated to 10001C cooled to 2001C (the initial hump)
and then heated to 15001C.
these grains (arrowed in Fig. 8) must be spinel. X-ray diffraction
of the specimens heated to 15001C (not shown here) do not re-
veal the presence of LiF.
EDS was conducted by moving a single spot across the image
seen in Fig. 8 to determine the chemistry of the different fea-
tures, i.e., the large faceted grains and the melt-like zones. Five
of the large octahedral grains were examined and found to be
consistent with stoichiometric MgAl2O4 spinel. The melt is
mostly fluorine and magnesium with some oxygen and alumi-
num, and a slight composition dependency on location.
ent work identifies the presence of the intermediate phases MgF2
and LiAlO2. It is noted that the phase LiAl4FO6 may also form
between Al2O3 and LiF.40 Belov et al.40 heated LiF and Al2O3
8301C for 10 h and then heated to 12001C for 2 h. They con-
ducted single-crystal X-ray diffraction to determine its structure
and lattice parameter. It is noteworthy because LiAl4FO6
ꢀ
exhibits the spinel structure, Fd3m, and has nearly the same lat-
40
tice parameter as MgAl2O4 (7.925 A for LiAl4FO6 compared
˚
with 7.9–8.1 A for MgAl2O441). This makes the phase nearly in-
˚
visible in the X-ray diffraction patterns of the samples in this
study. Unfortunately, current NIST-JANF thermodynamic ta-
bles (used in the present HSC thermodynamic calculations) do
not include LiAl4FO6, and thus it is not possible to predict
whether or not it would be present in the experiments performed
here. The results of this study suggest a series of possible reac-
tions, which describe how LiF reacts with MgAl2O4 spinel.
IV. Discussion
The results presented above confirm earlier conjectures that LiF
and spinel chemically react at elevated temperatures. The pres-
99.8
99.6
99.4
99.2
99.0
98.8
98.6
98.4
Heating
Cooling
100 200 300 400 500 600 700 800 900 1000
Temperature, °C
Fig. 5. Thermo-gravimetric analysis results showing the solidification
starting at the melting point of LiF and melting beginning at the LiF–
MgF2 eutectic temperature.
Fig. 7. X-ray diffraction of 60 wt% MgAl2O4 LiF mixture heated to
10001C in air and quenched. S, MgAl2O4 Spinel; F, LiF; L, LiAlO2; M,
MgF2. JCPDF: Spinel-771193; LiAlO2-750905; MgF2-722231.

July 2007
Chemical Interaction Between LiF and MgAl2O4
2041
liquid phase enhance mass diffusion, but the reformation of
spinel at higher temperatures would be expected to occur in
high-energy areas such as small pores and necks between sinte-
ring grains. Furthermore, just the presence of LiAlO2 (or
LiAl4FO6) would be expected to enhance atomic diffusion in
the manner described above. More work is required to establish
the details of these proposed sintering enhancement mecha-
nisms. It is noted that the above reaction sequence (Eqs. (1)–
(3)) also supports the ameliorative effect LiF has on reactive
sintering in the MgO–Al2O3 system. The work by Huang et
al.,17,18 shows that LiF lowers the reaction temperatures during
the sintering of MgO and Al2O3 powders to form spinel. They
report that this occurs because of enhanced diffusion through
the melt, consistent with the present experimental results and the
series of reactions proposed above.
V. Conclusions
An elevated temperature reaction sequence that involves a tran-
sient liquid phase between LiF and MgAl2O4 is proposed. It is
based on thermal, structural, and microstructural analyses and
is consistent with thermodynamic calculations. It is further pro-
posed that understanding this reaction sequence underpins the
mechanism for sintering of MgAl2O4 spinel in the presence of
LiF. In particular, the presence of the transient liquid phase and
the reformation of MgAl2O4 at higher temperatures are believed
to be the reasons that the presence of LiF increases the driving
force for sintering in spinel. Additionally, the reaction sequence
explains why large amounts of LiF act corrosively against the
spinel. Finally, the reaction sequence helps to explain why the
yield of spinel can increase in the reactive sintering of MgO and
Al2O3. Future work must focus on the kinetic details of the
reaction and how they are connected to microstructural devel-
opment in transparent spinel.
10 µm
Fig. 8. Scanning electron micrograph of 40 wt% LiF-spinel sample
quenched from 10001C in air. Arrows indicate highly faceted spinel
grains growing from a fluoride-rich melt.
It is suggested that just before the melting point of LiF, it
begins to react with the spinel. Through this reaction either
LiAlO2 or LiAl4FO6 forms along with MgF2. Near the melting
point of LiF, MgF2 combines with LiF to form a eutectic flu-
oride liquid (Fig. 2). At this temperature LiAlO2 becomes
Li2Al4O7, which, as a derivative of spinel, has intrinsic oxygen
vacancies and is expected to enhance diffusion. Therefore, its
formation at MgAl2O4 grain boundaries would be expected to
promote sintering or coarsening. In earlier work on hot-pressed
MgAl2O4 spinel, Auger spectroscopy was used to detect the
presence of Li several hundred nanometers into the surface of
the MgAl2O4.42 Others have also noted that higher Al/Mg ratios
are present near the interfaces of spinel grains sintered in the
presence of LiF.37 Based on these observations the following
series (Eqs. (1)–(3)) of interactions between MgAl2O4 and LiF
above the melting point of LiF is described
Acknowledgments
K.R. and I.E.R. thank the U.S. Army Research Office for funding this work
under grant # W911NF-06-1-0311. R. L. C. acknowledges Mr. Jim Kirsch, U.S.
Army Aviation and Missile Command SBIR Contract W31P4Q-05-C-R137.
3LiFðlÞ þ MgAl2O4ðsÞ ! LiF
: MgF2ðlÞ þ 2LiAlO2ðsÞ
(1)
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