Wetting between prospective crucible materials and the Ba-Cu-O melt
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The reaction between the B a - C u - 0 melt (with a Ba to Cu ratio of 3-5 or 3-7) used for crystal pulling of Y-123 and several prospective crucible materials (MgO, Y2O3, A12O3, and YSZ) has been investigated by dipping these materials into the melt at 985 °C and 1050 °C in air. For comparison, MgO, YSZ, LaA10 3 , and SrTiO3 single crystals were also investigated. In the case of MgO single crystals, no wetting or reactive layer has been observed, although with increasing time an increasing amount of MgO has been dissolved into the melt. The contact angle between melt and MgO single crystal can be estimated to be about 40°, assuming a surface tension of 0.3 N/m. For all other materials, increasing time was accompanied by increasing wetting and/or an increasing reaction layer. For MgO polycrystals increased wetting with increasing time has been observed. For example, it takes about 30 min to wet the first 10 mm. The melt completely permeates samples with an apparent density of up to 85%. However, the permeated depth becomes very thin in samples with an apparent 95% density. It was found that Y 2 O 3 is a better barrier to the melt at lower densities because even at 85% density the permeated layer is very thin, but the surface wetting occurs at a slightly faster rate for Y 2 O 3 than for MgO.
I. INTRODUCTION The wetting and the reaction between melt and crucible material are of technological importance and of scientific interest, especially in the case of crystal pulling. For the crystal pulling in the YBCO system, strong wetting in addition to strong reaction with almost any oxide makes it difficult to contain the melt in the crucible over a longer range of time. Therefore, it is difficult to maintain the height of the melt constant. Furthermore, the contamination of the melt from the crucible material has to be taken into consideration. All these problems have recently led to several investigations in this field.1"4 In this paper we will first discuss a method to determine the contact angle between melt and wetted crystal via the equilibrium wetted height by a simple model that takes into account only the surface tension and the gravity. Next, we will evaluate the wetting behavior of MgO, Y 2 O 3 , A12O3, YSZ, LaAlO 3 , SrTiO3, and SiO 2 with the B a - C u - 0 melt. II. EXPERIMENTAL PROCEDURE To characterize the wetting behavior between the B a - C u - 0 melt and the above-mentioned materials, we looked at the time dependence of the wetted height, which we define as the distance from the original melt surface position up to the height of the surface wetted front. For the measurements, the single and polycrysJ. Mater. Res., Vol. 9, No. 6, Jun 1994 http://journals.cambridge.org
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talline specimens were attached to a ceramic rod by ceramic glue or platinum wire and dipped into the melt for different times, while the position was controlled by a stage. After the exposure to the melt, the specimens were cooled in air. All experiments were done under ambient atmospheric conditions. As crucible mat
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