Porous Lithium Disilicate Glass-Ceramics Prepared by Cold Sintering Process Associated with Post-Annealing Technique

Using melt-derived LD glass powders and 5-20 M NaOH solutions, porous lithium disilicate (Li₂Si₂O₅, LD) glass-ceramics were prepared by the cold sintering process (CSP) associated with the post-annealing technique. In this novel technique, H₂O vapor originating from condensation reactions between residual Si-OH groups in cold-sintered LD glasses played the role of a foaming agent. With the increasing concentration of NaOH solutions, many more residual Si-OH groups appeared, and then rising trends in number as well as size were found for spherical pores formed in the resultant porous LD glass-ceramics. Correspondingly, the total porosities and average pore sizes varied from 25.6 ± 1.3% to 48.6 ± 1.9% and from 1.89 ± 0.68 μm to 13.40 ± 10.27 μm, respectively. Meanwhile, both the volume fractions and average aspect ratios of precipitated LD crystals within their pore walls presented progressively increasing tendencies, ranging from 55.75% to 76.85% and from 4.18 to 6.53, respectively. Young's modulus and the hardness of pore walls for resultant porous LD glass-ceramics presented remarkable enhancement from 56.9 ± 2.5 GPa to 79.1 ± 2.1 GPa and from 4.6 ± 0.9 GPa to 8.1 ± 0.8 GPa, whereas their biaxial flexural strengths dropped from 152.0 ± 6.8 MPa to 77.4 ± 5.4 MPa. Using H₂O vapor as a foaming agent, this work reveals that CSP associated with the post-annealing technique is a feasible and eco-friendly methodology by which to prepare porous glass-ceramics.