Abstract
Silicon nitride crucibles have the potential to replace silica crucibles and reduce the cost of silicon crystallization because of their reusability potential. Till date, crucibles’ heat treatment before each use is a prerequisite to achieve non-wetting conditions that is needed to facilitate the ingot release and hence enable reusability. Yet, no studies have examined the heat treatment influence on the crucibles’ durability. The present investigation focuses on the crucibles’ heat treatment and its impact on the crucibles’ lifetime. Repeated heat-treatments of silicon nitride crucibles in the air at above 1100 °C leads to crucible fracture. Therefore, this study identifies the cause and the mechanism of such failures by applying different heat treatment procedures in the air. The mass gain and the oxidation rates of the crucibles at different temperatures are measured via Thermogravimetry (TG) and Differential Thermal Analyzer (DTA). The results show that the porosity and phase distribution along the crucible wall thickness, play a key role in the crucible’s behavior during oxidation. Moreover, excessive internal oxidation in the tested crucibles results in severe thermal stresses which cause cracking during cooling.
Highlights
Crucibles contribute significantly to the multi-crystalline silicon ingots in terms of cost and quality [1,2,3]
The major drawbacks of silica are; (i) its reactivity with molten silicon which results in a high oxygen content that negatively affects the efficiency of the solar cell and, (ii) its single-use by virtue of the crucible cracking during cooling as a result of the two-phase transformations that silica crucibles undergo
Bellmann et al [1] reported lower oxygen and metallic impurities levels in silicon nitride crucible ingots compared to high purity electronic-grade silica crucibles
Summary
Crucibles contribute significantly to the multi-crystalline silicon ingots in terms of cost and quality [1,2,3]. The significance of silicon nitride crucibles to the future of PV market lies in its reusability and low oxygen content. Both attributes can reduce silicon production cost and yield high-efficiency solar cells. Studies have shown that the heat treatment step is crucial to avoid wetting of coating by molten silicon which may cause cracks in the ingot and the crucible [7,8]. Regarding the heat treatment step that requires the oxidation at 1100 °C, it is crucial to understand the oxidation behavior of silicon nitride crucibles at this temperature especially that heat treatment must be performed before each solidification run. The study investigates the influence of the silicon nitride properties such as the porosity and the phase distribution on the oxidation behavior and the durability of the crucibles
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