Journal of Minerals & Materials Characterization & Engineering, Vol. 3, No.2, pp 105-108, 2004
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105
An Investigation Into T he Sint e ring Of
Magnesium Fluoride Optical Material By Microwave
Shangzhao Shi, Jiann-Yang Hwang, Bowen Li, Xiaodi Huang
Michigan Technological University
Houghton, MI 49931
Abstract:
The presented work was an investigation on the sintering of magnesium fluoride
with microwaves. The sintering was conducted in a 2.45-GHz microwave applicator
under an argon atmosphere. Sintering shrinkage and density were measured. The
microstructure of the sintered samples was examined. Feasibility and advantages
regarding microwave sintering of magnesium fluoride were discussed.
Introduction
Magnesium fluoride is an optical material utilized in fabrication of infrared
transmission windows. The conventional techniques to produce magnesium fluoride
ceramics include single crystal growth1, pressureless sintering2, hot pressing3 as well as
hot isostatic pressing4. Microwave sintering is a volumetric and fast densification
technique, and has demonstrated many advantages in ceramics sintering. This method,
however, has not been attempted in the sintering of Magnesium fluoride. The presented
study is the first reported attempt of microwave sintering of the MgF2 ceramics.
Experimen tal
As-received Mg F2 powder was calcinated in argon at 600°C for 2 hours and then
ground with a mortar to pass a 325-mesh screen. Th e ground powder w as mixed with 2%
gum as binder. Distilled water of appropriate amount was added to the binder to develop
strength. Uniaxial compacting was performed with a steel die of 0.5 inch in diamet er, and
the compaction pressure was 5000 psi. The disk dimensions were φ½” × ¼”.
Microwave sinteri ng was performed with a 4-kW microwav e furnace. M gF2 disks
were placed at the center of the hot chamber surrounded by SiC susceptors. Zirconia
beads were placed on the bottom of the chamber, in order to avoid possible reactions of
the refractory insulation with the specimen as well as the SiC susceptor. A k-type
thermocouple was used to measure the chamber temperature. The distance from the
specimen top to the thermocouple tip was ½”.
The hot chamber of the furnace was airtight, and had ports to connect a vacuum
pump and an argon cylinder. For each run, the chamber was vacuumed and then filled
with argon to reach atmospheric pressure. The procedure of vacuuming/argon-filling was
repeated several times to ensure complete removal of oxygen.

106 Shangzhao Shi, Jiann-Yang Hwang, Bowen Li, Xiaodi Huang Vol. 3, No.2
The sintering temper ature profi le was determined with the consideration of binder
decomposition, which may otherwise result in collapsing, cracking or other problems.
The temperature was controlled with the controller, which is capable of continuously
adjusting the microwave power intensity.
The specimen dimensions were measured with a caliper before and after sintering.
Linear shrinkage was calculated by comparing of the data. The sintered specimens were
weighed. Densities were calculated based on the weight and volume of the sintered
specimens. SEM was employed to examine the microstructures.
Results and Discussion
Fig.1 shows the sintering
temperature profile. The shrinkage and
density of #1 and #2 specimens are given
in Table 1.
Compared to the theoretical
density (3.18 g.cm-3), the densities of #1
and #2 specimens are substantially low.
Sintering at 1100°C (#4) and 1075°C (#5)
was therefore attempted in order to
improve the density. However, the
sintering behavior was so sensitive to
temperature, that these two specimens
were partially melted even though no
holding time was used.
Table 1
Shrinkage (%)
specimen
sSintering conditions Heightdiameter Density (g.cm-3)
#1 1000°C×20min -9.79 -9.702.22
#2 1050°C×20min -11.11 -11.472.34
Figure 2 shows the SEM images taken from the MgF2 samples sintered at
different temperatures and holding times. Image a and b were taken from sample #1 and
#2, respectively. The images indicate that the higher degree of sintering and denser
microstructures were achieved when sintering was performed at 1050°C for 20min. The
SEM observation is in good agreement with the shrinkage and density measurement as
shown in Table 1. Compared to Image a, Image b features straight grain boundari es, less
porosity and substantial grain growth. The transgranular cracking indicates the strong
bonding between grains had been developed.

Vol.3, No.2 Investigation of Sintering of Magnesiu m Fluoride Op t ical M aterial b y Microwave 107
Image c and d were taken form simple #4 and #5, respectively. They indicate
substantial liquids formed in sintering at these temperatures. Grains in d have almost lost
their shape and merged into a shapeless matrix. Although there are a few distinguishable
grains, their shape chan ged i nto spheri cal. Im age c reve als a number o f cr aters distribut ed
in the liquid matrix. It indicates gaseous species formed during sintering at 1100°C.
There are also rod-like pa rticles, one of which is shown in higher magnification in Fig.3.
Their formation is believed to involve with an evaporation-condensation process.
Conclusions
Sintering of magnesium fluoride is difficult. We have searched several well-
documented literature databases for microwave sintering of magnesium fluoride, but have
found no publication dealing with this subject. Even for conventional sintering, very few
references can be found dealing with this subject. It seems that successful microwave
sintering of pure MgF2 requires delicate sintering conditions, which needs extensive
research.

108 Shangzhao Shi, Jiann-Yang Hwang, Bowen Li, Xiaodi Huang Vol. 3, No.2
The result from this study suggests that
the optimum sintering temperature would be
1050°C. Lowering the sintering temperature
would not produce a densified microstructure.
Increase the sintering temperature would result
in melting. At the optimum sintering
temperature, a prolonged holding period seems
necessary for higher density. However,
structural coarsening appears significant. In
order to obtain a fine microstructure while
achieving full densification, an additional
densification method, such as hot pressing,
needs to be added.
Reference
1. Recker, Kurt; Leckebusch, R.; “Vapor phase growth of single crystals of high-melting
fluorides. I. Magnesium fluoride”; Journal of Crystal Growth, (1969), 5(2), 125-31.
2. Rice, Hal H.; Garey, Maurice J.; “Sintering of magnesium fluoride”; American
Ceramic Society Bulletin, (1967), 46(12), 1149-53.
3. Mal'tsev, M. V.; Udalova, L. V.; Goryachev, A. Ya.; Levina, N. K.; Perminova, N.
B.; “Fabrication of optical-ceramic preforms without mechanical treatments”;
Opticheskii Zhurnal, (1993), (1), 69-72.
4. Shirakawa, Youichi; Harada, Tamotsu; Sashida, Norikazu; Miyata, Noboru;
“Preparation of MgF2 sintered body by normal sintering combined with capsule-free
hot-isostatic pressing treatment”, Nippon Seramikkusu Kyokai Gakujutsu
Ronbunshi/Journal of the Ceramic Society of Japan, v 107, n 1252, Dec, 1999, p
1137-1139.