Development of mid-infrared-transmitting cryolite ceramics via a cold sintering process below 200 °C

This study demonstrates, for the first time, the successful fabrication of mid-infrared-transmitting cryolite (Na 3 AlF 6 ) ceramics via a cold sintering process (CSP) below 200 °C. Cryolite powders were synthesized by the solution precipitation and followed thermal treatment to control particle size ranging from 140 nm to 1.5 µm and moisture content that effects on the IR transmittance. Bulky dense cryolite ceramics could be obtained by the CSP using optimized synthesized cryolite powders. The resulting ceramics achieved high relative density (~90%) and IR transmittance (over 80% at 1 mm thickness) in the 3.8–5.6 µm wavelength range although relatively low sintering temperature below 200°C. Furthermore, it was clarified that a critical balance exists between pore size and hydroxyl content for the transmission properties of cryolite ceramics. The present findings thus establish a sustainable, low-temperature route for producing optical fluoride ceramics suitable for infrared applications. • High-purity, hydroxyl-free single-phase cryolite (Na 3 AlF 6 ) powders could be successfully synthesized through aqueous solution precipitation method at room temperature followed by heat treatment. • Particle size was controlled within a range of 190 nm to 1,500 nm depending on heat-treatment temperature of 200 to 600 °C. • Cryolite ceramics, a sintered body of fluoride compounds, were successfully fabricated using cold sintering methods below 200 °C, with the addition of small amounts of deionized water. • Through optimized fabrication conditions, we achieved densification of over 90% with minimal pore volume and size below 140 nm. • The maximum infrared (IR) transmittance of 83% (average value at wavelength between 3.8 and 5.6 µm) with the relative density of 88% was achieved under optimized low-temperature sintering conditions (180 °C, 585 MPa, 10 min with 20 wt% deionized water). • Present research addresses not only the science of fluorides ceramics and the need for efficient optical ceramic components, but also presents an innovative approach aligned with growing environmental concerns.