Low Temperature Sintering of Magnesia-Chrome Ceramics Through Smart Uses of Nano-Materials

Chromium oxide (Cr₂O₃) and Cr₂O₃ containing compounds are well-known as refractory materials, while they are also widely used as catalyst support structures. Among Cr₂O₃ containing compounds, magnesia-chrome ceramics are widely used in non-ferrous and steel industry where no substitution has been found for them yet. However, difficult sintering of magnesia-chrome ceramics continues to impede their development and applications. In particular, the very high temperatures currently required to process such ceramics (up to 1800°C) produces significant amounts of GHG emissions. This work proposes new processing methods for magnesia-chrome ceramics, to significantly decrease their processing temperatures. In this regard, magnesia grains were coated with nano-chromia precursors and  calcined at 650°C in order to in-situ synthesize nano-chromia on magnesia grains. The coated grains then pressed and fired at various temperatures up to 1500°C. XRD and SEM were used in order to study the phase evolusion and microstructure development during sintering. Impedance spectroscopy (IM) was also used to evaluate the progress of microstructural development of these ceramics, and to study the sintering mechanisms. The results show that addition of 2 wt. % nano-chromia particles doubled the densification at 1450°C compared to the mixes containing only micron size chromia particles. Impedance spectroscopy results indicated that the microstructure of nano-chromia containing matrix consists of magnesia grains and a continuous secondary phase at the grain boundaries. This is in contrast to the micro-chromia containing ceramics which consist of magnesia grains and a second phase discontinuous (i.e. as separated islands) at the grain boundaries.  We propose that smart processing method developed in this work assures uniform distribution of nanomaterials on coarser grain of the target materials, such that the composite can contain relatively small amount of nanomaterials (here: 2 wt%) and still experience significant improvement in processability and performance.