Roles of alkali ions in densification process of cold sintered lithium disilicate glass materials

Cold sintering process is one of the low-environmental load technologies that are attracting attention in recent years. Herein, KOH/NaOH solutions with 5–20 M were utilized to aid densification of cold sintered lithium disilicate (Li2O·2SiO2 or Li2Si2O5, written as LD) glass materials, and roles of K+/Na+ ions in densification process were investigated. Under the sintering conditions of 300 MPa, 180 °C and 30 min, delayed densifications were observed for cold sintered LD glass samples prepared with KOH solutions than those prepared with NaOH solutions. The optimized concentration was 20 M for KOH solutions used to aid densification, and sharply reduced to 10 M for NaOH solutions. Correspondingly, the highest relative densities and flexural strengths of sintered samples prepared with 20 M KOH and 10 M NaOH solutions were 89.3%, 90.7 MPa and 89.0%, 88.9 MPa, respectively. In comparison with K+ ions ionized from KOH solutions, smaller ionic size and higher concentration gradient made Na+ ions ionized from NaOH solutions more easily diffuse into hydrolyzed silicate structures including K+ ions, and thereby Na+ ions might play better roles in catalyzing hydrolyzation process induced by OH− ions and aiding densification process of cold sintered LD glass materials than K+ ions.