Pore-Free 12CaO·7Al2O3 Single-Crystal Growth by Melt State Control using the Floating Zone Method

Pore-free ⟨110⟩ oriented 12CaO·7Al2O3 (C12A7) single crystals with sizes up to 7 mm (Phi) × 40 mm are grown by controlling the melt states in a floating zone (Fz) method. Control is achieved by increasing the melt temperature to ∼1495 °C and using a high-density C12A7 polycrystalline feed rod, which reduces oxygen bubbles in the melt almost completely due to a distinct decrease in O2-solubility with temperature increase. The optimization of the melt temperature, coupled with ⟨110⟩ oriented single crystal as a seed and the decreased growth speed, allows high-quality crystal growth. The grown crystal exhibits a good internal transparency (above 98% in the visible region) and does not contain magnetic impurities such as Ir, which are inevitably incorporated in Czochralski grown crystals. The crystal can be used as a precursor for superconductive C12A7:e−.