Abstract
The fabrication of well densified wollastonite with smooth appearance by direct sintering method is still a challenge due to the competitive behaviors between sintering and crystallization. In this study, the coarser glass frits with a size of 1–4 mm are subjected to heat treatment at different temperatures. An attempt of integrating differential thermal analyzer with a slag melting temperature characteristic tester was exploited to monitor the heat and geometry changes during the heating. The results showed that the addition of CaF2 can significantly promote the crystallization of wollastonite at 940 °C, while hindering the sintering ability. At higher temperature, the increase of CaF2 acts as flux and favors the formation of eutectics, leading to a decline in the precipitation amount of wollastonite. The predominated liquid sintering brought fast shrinkage. It was found out that high content of CaF2 narrows the dense sintering temperature range and results in uneven surfaces. In order to obtain wollastonite glass-ceramics with smooth appearance, the maximum content of CaF2 in sintering glass-ceramics should be limited to 2 wt.%.
Highlights
IntroductionThe low amount of fluorides could promote the nucleation and crystallization of wollastonite [7], whereas the high concentration of fluorides leads to the precipitation of fluorosilicates (KMg3AlSi3O10F2, KNaCaMg5Si8O22F2), fluorapatite (Ca5(PO4)3F), and CaF2, depending on its glass composition [4,6,7,8]
Wollastonite is a calcium inosilicate mineral (CaSiO3), which belongs to the pyroxenoid group of white inorganic materials with a ratio of Si:O = 1:3
Wollastonite glass-ceramics possess outstanding characteristics, i.e., high whiteness, low moisture absorption, low thermal expansion, low shrinkage, and low dielectric constant, which are suitable for wide applications in ceramics, chemicals, electronic devices, dental implant, construction, and polymers [2,3,4,5]
Summary
The low amount of fluorides could promote the nucleation and crystallization of wollastonite [7], whereas the high concentration of fluorides leads to the precipitation of fluorosilicates (KMg3AlSi3O10F2, KNaCaMg5Si8O22F2), fluorapatite (Ca5(PO4)3F), and CaF2, depending on its glass composition [4,6,7,8]. The preparation of wollastonite-based glass-ceramics with well densified structure is still a challenge via direct sintering process. Compared with the traditional melting process, direct sintering is capable of initiating crystallization without adding nucleation agents, reducing the processing temperature and energy consumption [8]. The crystallization and sintering mechanisms are usually in competition when direct sintering method is adopted for glass-ceramics’ preparation, which should be delicately balanced to produce densified structure with smooth surface. The key when fabricating dense glass-ceramics is to make the glass particles sinter to almost their full density by viscous flow before crystallization
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