Growth of mullite crystals in MgO/Al2O3/TiO2/SiO2/B2O3/CaO glasses by electrochemically induced nucleation

The aim is the preparation and characterisation of oriented ceramics. Mullite is the desired crystalline phase which crystallised from a melt with the mol% composition 52.6SiO(2)center dot 18.7Al(2)O(3)center dot 14.3MgO center dot 7.7TiO(2)center dot 4.7B(2)O(3)center dot 2.0CaO after electrochemically induced nucleation. For that purpose a platinum wire (phi=0.35 mm) was inserted into the melt located inside a platinum crucible (phi=50 mm) and a current (I=5 mA (current density=0.45 mA/mm(2)), t(1)=5 min) was supplied. The wire was the cathode while the crucible acted as anode. This led to nucleation at the cathode and subsequent growth of mullite needles. The latter were oriented with their crystallographic c-axis perpendicular to the electrode surface. The nucleation is caused by the reduction of Ti4+ to Ti3+ ions which takes place at the cathode. The resulting assemblies consisting of the platinum wire, the glassy matrix with embedded mullite crystals were characterised by means of XRD, SEM, optical microscopy, image analyse and EPR spectroscopy. Optical microscopy and the image analysis enabled us to determine crystal growth velocities depending on the crystallographic direction. Optical microscopic images showed a staircase structure of the mullite crystal tips with heights between 0.5 and 1.5 pm, and step lengths between 20 and 50 mu m.