Microstructural Control of CaO–Al₂O₃–SiO₂ Glass Ceramics by Oxidation and Mixing with Nucleation Agents

Abstract

Microstructural control of CaO–Al 2 O 3 –SiO 2 (CAS) glass ceramics (GCs) was achieved by oxidation and mixing with nucleation agents. CAS GCs were precipitated with hexagonal platy particles of metastable CaAl 2 Si 2 O 8 layered crystals (CAS GC-H), which are typically prepared under a reductive atmosphere that forms metallic Mo or W particles as nucleation agents. The average particle size of crystals decreased significantly from 50 to 11 μm when the CAS GC-H containing metallic W particles was prepared under an oxidative atmosphere. Compared to this CAS-GC-H, the crystal particle size increased from 8–20 to 10–30 μm when the CAS GC-H was prepared by mixing glass cullet containing metallic Mo and that containing metallic W particles. These results indicate that one microstructure of CAS GC-H is controlled on the micrometer scale from a parent glass with one composition by varying the experimental conditions related to the glass melting state.

Effect of carbon on the co-presence of metallic tungsten as a nucleation agent and Eu2+ in glass: crystallization of CaO–Al2O3–SiO2 glass probed with Eu2+ luminescence

“…1, left), in good agreement with our previous studies. 19–21,24,27,31 By contrast, Cullet-Eu 2+ was yellowish (Fig. 1, right), consistent with the results of the previous studies in which Eu 2+ -doped glass specimens were reported to be yellow.…”

“…28 However, such coloration was not observed in the colorless CAS glass prepared by the oxidative melting of glass cullet containing metallic W particles in our previous study. 19,21 Therefore, the oxidation of metallic W particles in the CAS glass with the present composition likely dose not generate a glass specimen with W cluster coloration. Meanwhile, the colorless glass that never showed bulk crystallization was obtained when the raw material of CAS glass was melted without addition of carbon.…”

“…17 Furthermore, various coloration of glasses containing metallic W particles exhibited crystallization of metastable CaAl 2 Si 2 O 8 . [19][20][21] Therefore, metallic W particles likely act as heterogeneous nucleation sites.…”

“…Notably, the amounts of crystalline phases generally depend on the number of nucleation agents. [19][20][21]50 In the case of a low carbon content (e.g. 0.40 wt%), metallic W particles would also help suppress Eu 2+ oxidation to reduce the number of nucleation agents.…”

“…10 Concerning the precipitation of metallic W and Mo particles in the CAS glass, tungsten oxide (WO 3 ) and molybdenum oxide (MoO 3 ) are reduced by carbon during glass melting. 10,19–25 In our previous research, metallic Mo particles, whose oxidation free energy is similar to that for metallic W particles 27 were absent in the CAS glass with a yellowish color caused by Eu 2+ when the glass raw material containing MoO 3 , carbon, and europium oxide (Eu 2 O 3 ) was melted, suggesting that metallic Mo was oxidized when Eu 3+ was reduced. 10 The efficient reduction of both Eu 3+ and MoO 3 is thus necessary.…”