Joining transparent spinel ceramics using refractive index–matched glass

Ceramic joining by glass is a promising method of the preparation of large transparent ceramics from small blocks. The chemical composition of glass was optimised to match the coefficient of thermal expansion and refractive index of transparent magnesium aluminate (MgAl2O4) ceramics. A two-step joining method was developed to join MgAl2O4 ceramics with a reduced number of bubbles in the joint, and the thermal properties of the optimised glass were evaluated to determine the joining temperature. Two transparent MgAl2O4 blocks were joined by a glass layer that was approximately 20 µm thick. The joint area could not be distinguished with a naked eye. The transmittance of the joined body vertical to or parallel through the glass layer was approximately the same as that of the ceramics. The average three-point bending strength of the joint reached 202 ± 33 MPa, which was 64% that of the ceramic body.