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In this project, magnesia-alumina composite granules were prepared via the spray drying method. Next, the synthesized powder was sintered at 1400 °C–1500 °C by the spark plasma sintering method under the pressure of 100 MPa without using any sintering aids. The effects of two sintering temperatures (1400 °C and 1500 °C) on the phase evolution, density, fracture toughness, and light transmission of the samples in the visible and IR range were investigated. SEM results indicated that the magnesia-alumina composite granules had spherical morphology with a mean particle size of 7-8 μm. The XRD pattern showed that after spark plasma sintering at 1400 °C and 1500 °C, magnesium aluminate phase spinel was obtained from the penetration and reaction of alumina and magnesia nanoparticles. The disc sintered at 1400 °C had more transparency than the sample sintered at 1500 °C within the UV–Vis and middle IR region because of the lower porosity of the sample. The magnesium aluminate spinel sintered at 1400 °C had a density of 99.98% theoretical density, hardness of 18 GPa, and fracture toughness of 1.6 MPam1/2. [ABSTRACT FROM AUTHOR]

Magnesium aluminate spinel (MgAl 2 O 4) is considered as one of the most important spinels possessing desirable mechanical properties, with a wide range of applications at high temperatures. Powder granulation by spray freeze drying can lead to better powder flow and distribution in the mold, resulting in a specimen with high green density and strength. Accordingly, this study aimed at investigating the influential parameters on the granulation process of spinel powders via the spray freezing drying method. At first, spinel powders with various weight percents of 20, 25, 30, 35 and 50, and PVA binders with 0, 1, 2 and 8 wt% were investigated to prepare the optimal solution. Finally, the granules were sintered using the SPS method. The obtained bodies were tested by X-ray diffraction, SEM, FE-SEM, Viscosity and UV–Vis. The results showed that the optimal amount of the binder to obtain uniform granules in this process was 3%. Also, 35 wt% of solid loads represented the optimum amount for slurries, forming granules with the size range of 10–50 μm. By powder granulation, density experienced an increase of about 20%, leading to the growth of more than 15% in the in-line transmission for the IR range. [ABSTRACT FROM AUTHOR]

Orthodontic brackets based on transparent ceramics in the visible area (especially magnesium aluminate, MgAl 2 O 4) are considered for high strength, but the most important drawback of transparent magnesium aluminate is the lack of fracture toughness. In this research, the silicon nitride nanoparticles as a reinforcing phase were used to improve the fracture toughness of magnesium aluminate orthodontic brackets. For this purpose, firstly, MgAl 2 O 4 reinforced with 2% Si 3 N 4 nanostructured granules were prepared using the spray drying method. Then, the nanostructured granules were spark plasma sintered (SPS) at 1400 °C under pressure of 100 MPa. Finally, with the help of the diamond cutting tool, the heat-treated sample was cut into an orthodontic bracket. The results of the tetrazolium-based colorimetric assay (MTT test) showed that the produced orthodontic brackets have excellent biocompatibility with human osteoblasts cell. [ABSTRACT FROM AUTHOR]