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An Investigation Into The Sintering Of Microwave

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An Investigation Into The Sintering Of

Magnesium Fluoride Optical Material By Microwave

 

 

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.

 

 

 

Experimental

 

As-received MgF2  powder was calcinated in argon at 600°C for 2 hours and then ground with a mortar to pass a 325-mesh screen. The ground powder was 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 diameter, and the compaction pressure was 5000 psi. The disk dimensions were ö½” × ¼.

 

Microwave sintering was performed with a 4-kW microwave furnace. MgF2  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 temperature profile 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

specimen


 

Sintering


Shrinkage (%)                                              -3


s                conditions                 Height                   diameter            Density (g.cm  )

#1

1000°C×20min

-9.79

-9.70

2.22

#2

1050°C×20min

-11.11

-11.47

2.34

 

 

 

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 boundaries, 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 Magnesium Fluoride Optical Material by 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 changed into spherical. Image c reveals a number of craters distributed

in  the  liquid  matrix.  It  indicates  gaseous  species  formed  during  sintering  at  1100°C. There are also rod-like particles, 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.

  
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