Chromium crystal chemistry mullite-spinel refractory ceramics

A small amount of chromium oxide was added to a mullite-spinel refractory m ixture to improve its thermal and mechanical properties. Two different comp ositions of mullite-spinel refractory were studied to define the crystal st ructures hosting the chromium cations, and the chromium solubility in spine l (MgAl2-xCrxO4) was determined. Powder X-ray diffraction (XRD), X-ray abso rption near edge spectroscopy (XANES), and scanning electron microscopy (SE M) elemental distribution maps were used to determine the chromium crystal chemistry in the system. The observed maximum solubility of chromium in spi nel was found at x = 1.2, but the presence of mullite in the mixture caused a strong decrease of this value. The chromium distribution among the cryst al phases reflects the different reaction paths of the two samples: a stage involving spinel and melt drives all present chromium in the spinel, while a simultaneous crystallization of spinel-mullite distributes chromium cati ons between mullite, spinel, and secondary corundum. (C) 1999 Elsevier Scie nce Ltd.