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Specific Surface Free Energy of Crucible Material for Sapphire Crystal Synthesis

Takaomi Suzuki ,  Kou Shirotsuki ,  Toshinori Taishi ,  Keigo Hoshikawa 

Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan

Abstract

  Vertical Bridgman method is one of the effective techniques to synthesize a single crystal of sapphire.  In order to obtain high quality single crystal, removal of the voids or defects in the crystal is the important problem, and the material of crucible reflects the incidence of voids.  When we use W-crucible, sapphire single crystal accompanies large voids at the boundary between the crucible and the crystal.  On the other hand, Mo-crucible does not accompany voids at the boundary of crucible and the crystal.  We are going to explain the mechanism of the incidence of voids by the specific surface free energy (SSFE) of the crucible materials. 

W, Mo, and tungsten-molybdenum alloy (W-Mo) plates are used as the test pieces for crucible material.  Water or formamide was dropped on the test pieces using micropipette.  The contact angle of liquid droplet on the test piece was measured using a protractor and printed photos by a digital camera.   SSFE was calculated using Forks approximation and Wu’s harmonic mean equation.  The SSFEs were 46, 48, and 52 mN/m for W, W-Mo, and Mo, respectively, indicating that the SSFE of Mo is larger than that of W.  Even though the SSFEs were calculated from the contact angle of liquids at room temperature, they are considered to suggest the SSFE at high temperature.  Contact angles of molten sapphire on test pieces were also measured by photograph, and they were 30.5, 27.5, 22.3 degree for W, W-Mo, and Mo, respectively.  Molten sapphire is most wettable on the test piece of Mo.  Voids at the boundary between sapphire and crucible can be removed, if the SSFE of crucible material is large enough to make the voids in spherical, and sphere voids easily detach from the crucible wall.
 

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Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 9, by Takaomi Suzuki
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17

Submitted: 2013-03-14 01:00
Revised:   2013-03-14 01:02