Nucleating role of P₂O₅ in nepheline‐based transparent glass‐ceramics

Abstract

The nepheline-based transparent glass-ceramics are promising candidates for cover glass applications in electronic displays owing to their superior mechanical properties (than glasses) and ability to be chemically strengthened. However, our poor understanding about the kinetic and thermodynamic drivers controlling their crystallization processes usually results in their opacification and development of large internal stresses. The present work focuses on the development of nepheline-based nanocrystalline transparent glass-ceramic designed in the Na 2 O-Al 2 O 3 -SiO 2 ternary system nucleated with P 2 O 5 . The temporal evolution of the phosphate and nepheline nanocrystal formation has been followed using X-ray diffraction, scanning/transmission electron microscopy, and energy-dispersive spectroscopy. The incorporation of P 2 O 5 in the glass structure leads to the phase separation resulting in the crystallization of nanocrystalline Na 3 PO 4 as an intermediate phase; thus, acting as a nucleating site for volume crystallization of nepheline. The optimization of nucleation and growth profile in the designed composition results in the formation of a transparent glassceramic with high optical transmittance (91.5 ± 0.1%).

Phase transition and thermal stability of ceramics from Na‐based geopolymers

“…Similarly, Ahmad et al 38 . found the nepheline formation during the sintering process from 900°C to 1200°C of kaolin‐based geopolymer, where the characteristic peaks of nepheline became narrower and intensified as the temperature increased 22 . Padilla et al.…”

“…The characteristic peaks at 13.9 • and 24.26 • attributed to sodalite only exist in the XRD patterns obtained at 80 • C and 600 • C, which are replaced by the new peaks at 21.199 • and 35.077 • (PDF #52-1342) of nepheline (NaAlSiO 4 ) first appearing at 800 • C. It is worth noting that, for different Si/Al ratios, XRD patterns display high consistency in the number of nepheline peaks over the temperature range from 800 • C to 1200 • C. Similarly, Ahmad et al 38 found the nepheline formation during the sintering process from 900 • C to 1200 • C of kaolin-based geopolymer, where the characteristic peaks of nepheline became narrower and intensified as the temperature increased. 22 Padilla et al reported that the conversion from sodalite (a cubic crystal) to pure nepheline (a hexagonal crystal) took place in the 800 • C-900 • C interval. 29 Phase transition from kaolinite to sodalite and further to nepheline is unified and the main composition of sintered geopolymers with different Si/Al ratios is similar at the same temperature, except for the increasing intensity of quartz due to the additional silica.…”

“…explored the kinetics of nepheline crystallization in a rice husk ash (RHA) system, revealing a high activation energy of 391 kJ/mol and the onset temperature was 700°C for the formation of nepheline. Li et al 22 . also found the beneficial effect of phosphate nanocrystals on the nucleation of nepheline due to reduced activation energy, resulting from the additional heterogeneous grain boundaries in phosphosilicate systems.…”

“…20 Martín et al 21 explored the kinetics of nepheline crystallization in a rice husk ash (RHA) system, revealing a high activation energy of 391 kJ/mol and the onset temperature was 700 • C for the formation of nepheline. Li et al 22 also found the beneficial effect of phosphate nanocrystals on the nucleation of nepheline due to reduced activation energy, resulting from the additional heterogeneous grain boundaries in phosphosilicate systems. Since the progress in deeply investigating the mechanism of nepheline formation were achieved mainly at the temperature less than 1000 • C, the thermal behavior at higher temperature are required to be discovered.…”

Nanoscale composition fluctuations and crystallization process: Case study in Li2O–SiO2‐based glasses

“…Recently, Li et al 122 . examined the nucleating role of P 2 O 5 in nepheline (NaAlSiO 4 )‐based transparent glass–ceramics with the composition of 21Na 2 O–5.7K 2 O–4.7P 2 O 5 –19.4Al 2 O 3 –49.2SiO 2 (mol%) with [Na 2 O+K 2 O]/[Al 2 O 3 ] = 1.37 and proposed that Na 3 PO 4 nanocrystals formed initially promote the volume crystallization of nepheline K 0.12 Na 0.87 Al 0.99 Si 1.01 O 4 nanocrystals.…”