Abstract
Thermal expansion coefficients play the most important role in the practical application of lithium aluminosilicate (LAS) glass-ceramics. The effects of composition and thermal treatment conditions on microstructure, thermal expansion and mechanical strength of a low thermal expansion LAS glass-ceramic material were studied. Samples were prepared by the standard bulk method and heat-treated by two-step thermal programs to achieve a vitreous-crystalline transition. The analytical techniques to be emphasized on this work were: X-ray powder diffraction（XRD）, hard X-ray absorption fine structure（XAFS） and SEM. In addition, thermal expansion coefficients were measured by thermal mechanical analysis（TMA）. Mechanical strength of samples was tested by a standard three-point bending machine. Our goals for the above analysis is to figure out the formation of crystallites under different thermal treatment conditions and to understand the effects of variation of specific component（B2O3, P2O5, MgF2 and ZnO）, including crystallization behavior and the local order of glassy phases. Then, try to find out the correlations among thermal expansion, mechanical strength and microstructures. According to the experimental results, the transparency and color of LAS glass-ceramic samples are varied significantly by different heat treatment schemes and constitutes. It is suggested that:(1) β-quartz solid solutions are formed below 880℃ in most of the samples. When the heating temperature is raised above 880℃, the phase transformation of hexagonal β-quartz solid solutions to tetragonal β-spodumene solid solutions usually occurs. The additives, especially B2O3, often result in a lower transformation temperature due to the decrease of viscosity of the glass matrix.(2) Various secondary phases are usually precipitated at higher temperatures (especially at 1150℃). The quantity, composition and stoichiometry of these phases depend on the kind and amount of additives. A needle-like Ti-rich phase can be found at grain boundaries of main phases.(3) The local environment of Zr, Zn and Ti is changed significantly by different thermal treatment conditions. The variation of XANES spectra confirmed that the Zr, Zn and Ti involve in the crystallization process and the XANES features also certify the existence of some minor phases, which sometimes cannot be clearly seen in the XRD data. The elements studied usually change from lower to higher coordination state when the sample transforms from an amorphous to a highly crystalline solid.（Ti from 4 to 6; Zr from 6 to 8）(4) The linear thermal expansion coefficients of most samples, except those containing P2O5, are 1-2×10-6 /℃ (25-500℃). Near zero or negative expansion coefficient can be obtained when about 4％ P2O5 is added.(5) Flexural strengths of samples are usually higher than 50 MPa (500 kgf/cm2). When about 4.7% B2O3 and 6% ZnO are added in the samples, the strengths increase from about 62 MPa (630 kgf/cm2 ) to 88 MPa (900 kgf/cm2) and 78 MPa (800 kgf/cm2), respectively. MgF2 and P2O5 bearing samples accompany with a decreased flexural strength.