Growth of Nickel Ferrite Spinel Crystals by Skull MeltingHeydy Ramirez - Purdue Graduate Student
A modified Bridgman technique known as skull melting has been applied to the growth of nickel ferrite spinel (Fe(2-x)NixO4) crystals in a controlled oxygen atmosphere. Previous phase equilibria studies show that in air the NiO-saturated spinel composition decreases from the stoichiometric nickel fraction Ni/(Ni + Fe) = 0.33 below approx. 1000oC to 0.30 at its congruent melting point at approx. 1700oC, limiting the ability to grow stoichiometric spinel directly from the melt. Preferential loss of NiO, most likely due to evaporation during skull melting, presents further challenges for producing single-phase spinel. More than twelve crystal growth experiments using 99.9% pure NiO and Fe2O3 powders with starting nickel fraction from 0.25 to 0.37 have been run. The charges were melted in the water-cooled copper finger crucible using high frequency induction heating and withdrawn at approximately 1.5 cm/h for a total melt time of approx. 6 h. Starting Ni fractions ¡Ü0.27 resulted in single-phase, coarse-grained polycrystalline boules containing spinel crystals up to approx. 1 cm in size. Higher Ni fractions resulted in fine NiO-Spinel eutectic structures, consistent with the phase diagram. Characterization of the spinel crystals was conducted by x-ray diffraction (XRD), energy dispersive spectroscopy (EDS) in SEM, and optical microscopy.
It is desirable to achieve larger Nickel Ferrite Spinel crystals, so a method was tried involving varied cooling rate by withdrawing the skull melt from the induction heat at 2 cm/hr for the first 3 mm, then 1 cm/hr for the next 6 mm, and completing the withdrawal at the regular 1.5 cm/hr. This method achieved crystals of 5 to 6 mm in size as opposed to the 1 mm crystals achieved at the constant 1.5 cm/hr rate. Another approach to be explored will be to anneal crystals pre-formed by skull melting in order to achieve grain growth. Additional work need to be conducted to better approach the stoichiometric composition using the method.