In photovoltaic industry, silica crucible has an important influence on the quality of single crystal silicon. To obtain a silica glass crucible with large diameter, high uniformity, and low bubble content, two series of crucibles were prepared by the arc melting method, one with various melting parameters (initial power, melting power, and melting time) and crucible sizes, and the other with various high purity quartz crucible. The bubbles inside the crucible wall and pores on the inner surface were all measured using a polarised optical microscope and a portable microscope. The results show that all crucibles have a bubble aggregation area in their inner surface (0–0.4 mm), in which the density and size of bubbles are affected by melting time, melting power, and the distance between the crucible and the graphite electrode. The uniformity of the crucible decreases as the crucible diameter increases (16–28 inches), and the crucible is relatively stable when the initial power is below 400 kW. In final, a silica crucible with large size (diameter of 28 inches) and low bubble content on inner surface (∼50% reduction than that of traditional crucibles) was successfully prepared, which is of great value to the photovoltaic industry.
The main goal of this study is to obtain a large crucible with a uniform structure by adjusting the preparation parameters, and with a low bubble content in the transparent layer using high-purity quartz. At the same time, the influence of various melting parameters and impurity element contents on the formation of bubbles in the process of crucible preparation are summarized. To achieve these goals, we prepared two series of fused quartz crucible by graphite arc furnace, and applied them to prepare monocrystalline silicon through CZ method. Then, we examine the structure of silica crucibles prepared with varying initial power, melting power, melting times, and purities of raw materials. The bubbles inside the crucible wall and the pores on the inner surface of the crucibles were observed and measured using a polarised optical microscope and a portable microscope.
As shown in table 1, the raw material used for the preparation of the silica crucible was quartz sand. The quartz ampoule bottle in this experiment was divided into two categories according to its purity: high purity (HP) and standard purity (SP), all produced by Covia (formerly Unimin, USA), and their product numbers are IOTA-6 and IOTA-CG, respectively. The chemical compositions of two kinds of quartz sand were measured using an inductively coupled plasma mass spectrometer (ICP-MS, Agilent 7500 Ce, Agilent, USA). The total content of impurity elements in HP quartz sand is 6.1 (μg·g−1), less than that of SP quartz sand. Other possible elements (P, Ni, Ba, Mg, Cr, Mn, and Cu) were also tested; they are not shown in table 1 because their contents were less than 0.01 μg·g−1. The block polysilicon used in Cz process, with bulk purity of 99.999999%, was purchased from Wacker Chemie AG, Germany.
The particle size distribution of quartz sand was evaluated using a laser particle size analyser (MASTERSIZER 2000, Malvern UK), as shown in figure 1. The refractive indices of the particles and dispersant are 1.544 and 1.000, respectively. SP quartz sand was graded into two categories: SP fine quartz sand and SP coarse quartz sand, which were used to prepare the transparent layer and BC layer of the silica crucible, respectively. HP quartz sand has only one granularity (HP fine quartz sand), which was used to prepare the transparent layer of the silica crucible. In terms of raw materials, the main variable of this experiment is the content of impurities. To eliminate the influence of the particle size of different grades of quartz sand on bubble formation, the particle size of quartz sand was strictly controlled using a mesh screen, so that the particle size distributions of HP fine quartz sand and SP fine high permeability quartz square cylinder. The D50 (50% passing size) of HP fine quartz sand, SP fine quartz sand, and SP coarse quartz sand are approximately 169, 155, and 242 μm, respectively.
The silica crucible manufacturing process is shown in figure 2(a). First, a coarse fused quartz crucible sand was poured into a rotating steel mould. The quartz sand adheres tightly to the mould walls under centrifugal force, which ensures an even distribution and thickness of quartz sand. The external diameter of the crucible is 16–28 inches, and the thickness of the crucible wall is 8–13 mm. A second layer of finer quartz sand was introduced on top of the coarse quartz. Air was then extracted from the hole in the steel mould for 2.0 min to keep the