UHS Preparation of MgAl₂O₄ Spinel: NonEquilibrium Grain Boundaries and FineGrain Driven Densification

Magnesium aluminate spinel (MgAl₂O₄) is a technologically important transparent ceramic with a broad optical window, high hardness, excellent thermal stability, and good chemical durability, making it attractive for infrared windows, transparent armor, and related optoelectronic components. However, pressureless densification of spinel remains difficult because the diffusion of Mg²⁺ and Al³⁺ ions is intrinsically sluggish, so conventional sintering typically requires long dwell times or pressure-assisted routes such as hot pressing and hot isostatic pressing. In this work, pressureless ultrafast high-temperature sintering (UHS) was employed to densify MgAl₂O₄ spinel ceramics. Starting from a green compact with an initial relative density of about 55%, the samples were rapidly heated to 1750 ℃ at rates exceeding 1000 ℃·min^-1, held for only 5 s, and then cooled in vacuum. The sintered body retained a single cubic spinel phase, with no detectable secondary phases in XRD. Archimedes measurements gave a relative density of 98.2%, corresponding to a low residual porosity of 1.8%. The microstructure consisted of fine equiaxed grains with an average size of 0.62 ± 0.13 μm, together with a small amount of residual pores located mainly at grain boundaries and triple junctions. After polishing to a thickness of about 1 mm, the specimen exhibited an in-line transmittance of 64.7% at 5.3 μm and showed a monotonic increase in transmittance with wavelength, consistent with residual pore scattering in the mid-infrared region.

The enhanced densification achieved by UHS is attributed to two coupled effects. First, the extremely fast heating schedule suppresses the low-temperature stage dominated by surface diffusion, thereby limiting unnecessary grain coarsening and preserving a high sintering driving force at the peak temperature. This fine-grain retention is critical because the densification rate in grain-boundary-diffusion-controlled sintering depends strongly on grain size. Second, the severe thermal shock associated with UHS may freeze grain boundaries into high-energy, non-equilibrium states, which could accelerate grain-boundary diffusion and facilitate pore elimination, although this mechanism is presently supported only by indirect evidence and requires direct atomic-scale verification in future work. For comparison, a conventional pressureless sintering schedule of 10 ℃·min^-1 to the same peak temperature followed by a 5 s hold produced a much lower relative density of 74.2% and a coarser grain size of about 2.31 μm. These results demonstrate that UHS provides a powerful pressureless route for rapidly densifying diffusion-limited spinel ceramics and offers a viable basis for the preparation of small-sized transparent MgAl₂O₄ components.