Defect Free Spinel Ceramics of High Strength and High Transparency

A Review on the Sintering and Microstructure Development of Transparent Spinel (MgAl2O4)

“…A complete systematic study on the relative importance of LiF distribution within the spinel powder has not been performed, but the present authors have observed that ultrasonic mixing with a solvent such as ethyl alcohol before forming and/or firing facilitates the production of more uniform and transparent microstructures. Different homogenization methods, for example, utilizing fluoride salts to coat spinel particles, have also been shown to be successful in producing a uniform LiF distribution that leads to transparent material 68–70 …”

Hydrolysis-Induced Aqueous Gelcasting of Magnesium Aluminate Spinel

“…Magnesium aluminate (MgAl 2 O 4 ) spinel (MAS) has attracted much attention from materials scientists and ceramic engineers due to its interesting set of properties such as high melting point (2135°C), high hardness (16.1 GPa), relatively low density (3.58 g/cm 3 ), excellent transmittance in the range of 0.25–5.0 μm wavelength, high strength at room and at elevated temperature (180 MPa), high chemical inertness, relatively low thermal expansion coefficient (9 × 10 −6 /°C between 30°C and 1400°C), high thermal shock resistance, etc 1–14 . Furthermore, MAS does not react with SiO 2 until 1735°C, with MgO or CaO until 2000°C, with Al 2 O 3 until 1925°C and it can be in contact with all metal except alkaline earths 5 .…”

“…Furthermore, MAS does not react with SiO 2 until 1735°C, with MgO or CaO until 2000°C, with Al 2 O 3 until 1925°C and it can be in contact with all metal except alkaline earths 5 . Owing to these properties, MAS has been used as optical windows, pressure vessels, and bullet‐proof vehicles, 6–8 alternative materials to replace the conventional carbon anode in aluminum electrolytic cells, 9 humidity sensors, 10 refractory materials for cement rotary kilns and steel ladles, 11,12 and as a catalyst support for several reactions 13,14 . Despite these advantages, dense MAS bodies are hardly obtainable through a single‐stage reaction sintering process due to a severe volume expansion (∼8%) associated with the formation of MgAl 2 O 4 phase from alumina and magnesia 15 .…”

“…These two consecutive firing cycles significantly increase the production costs of dense MAS ceramics. Moreover, the preparation of MAS components for certain strategic applications such as infrared domes and radomes is also quite expensive due to the involvement of extensive and costly postsintering machining operations 6,7,18 . At present, MAS domes and radomes are being fabricated by hot isostatic pressing (HIPing) (∼1500°C and 200–400 MPa pressure) followed by machining to obtain the desired final shape 6,7 …”

High-strength transparent spinel with fine, unimodal grain size

“…As transparent armor, spinel is superior to glass because of its higher hardness 1,2 and strength 3,4,5 . Spinel has also shown better ballistic performance, in certain cases, than sapphire.…”