Chemicl Durability of Polycrystalline Materials Produced from Glasses

Abstract

Glasses of the compositions, Li₂O⋅1.5SiO₂, Li₂O⋅2SiO₂, Li₂O⋅3SiO₂, and Li₂O⋅3SiO₂⋅0.1 M_mO_n (M_mO_n=Na₂O, K₂O, Al₂O₃ or ZrO₂) in mole ratio, each with or without platinum, were prepared in a form of rectangular plate of 20×40×2mm. After polished with 1500-grit Al₂O₃ powder to make smooth surfaces, these glasses were repeated up to 900°C at the constant rate of 5°C/min. The polycrystalline materials thus obtained were exposed to various solutions, mainly to 5% HCl solution of 90°±2°C for various times. The chemical durability was reported in terms of weight loss per unit surface exposed. Identification of crystalline phases in the specimens by an X-ray diffractometer and electron microscopicobservation of the surface of specimens were made before and after the durability test in order to correlate the durability to the microstructure of the crystallized specimens.
The results obtained were summarized as follows:
1) Effects of the basic composition.
For the two component system, i.e., Li₂O⋅1.5SiO₂, Li₂O⋅2SiO₂, and Li₂O⋅3SiO₂, the durability, in general, increased with the increase in SiO₂ content, regardless of the presence of Pt. The durability of the crystallized specimens was much higher than that of their mother glasses. An exception was the Li₂O⋅2SiO₂ crystallized specimen without Pt, which showed the very poor durability.
2) Effects of the addition of Pt.
The poor durability of the specimen above described was found to be due to the presence of micro-cracks on the surface of the crystallized specimen. The addition of Pt as a nucleating agent in an appropriate amount (0.005-0.01% in weight) was found effective for suppressing the development of the cracks during reheating, thus improving the durability.
3) Effects of addition of the third components.
Of the four components, e. g., Na₂O, K₂O, Al₂O₃, and ZrO₂, Na₂O and K₂O lowered the durability markedly. This was attributed to the concentration of these components in crystal grain boundaries (matrix glass) of the crystallized specimens, which was confirmed by an electron microscope. The matrix glass was easily attacked by 5% HCl.
4) Effects of the kind of test reagent.
The durability to various test reagents such as pure water, 5% H₂SO₄, 5% NaCl, 5% NaOH and 5% Na₂CO₃, besides 5% HCl, was also measured with the glass of the composition Li₂O⋅3SiO₂ with 0.01% Pt. In general, the attack of the alkaline solutions was much more severe than that of pure water and the acid solutions.