ZrO u2 -nucleated calcium aluminate glass-ceramics with mid-infrared transparency
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A silica-free MgO–CaO–BaO–Al2O3–ZrO2 glass was converted into glass-ceramic by adding zirconia as a nucleating agent. The crystallization and properties of the glass-ceramics were investigated. It was found that ZrO2 is an efficient nucleating agent for the present calcium aluminate glass. Addition of an appropriate amount of ZrO2 changed the crystallization behavior from surface nucleation to internal nucleation. The primary crystalline phase was Ca3Al2O6, which exhibited bulk nucleation. When the crystallizing time was increased, BaAl2O4 formed as a surface layer that grew from the surface into the interior of the sample. The hardness was increased when the parent glass was converted into glass-ceramics. The glass-ceramics without BaAl2O4 surface layer have visible-to-infrared transparency comparable with the parent glass. The transparency was substantially reduced after the development of the surface layer.
I. INTRODUCTION
Calcium aluminate (CA) glasses exhibit ultralow intrinsic scattering at 1.55 m, making them promising for low loss optical fibers.1,2 Such glasses are also photosensitive to ultraviolet radiation and thus are applicable to photometric devices for information storage purposes.3,4 The most important characteristic of CA glasses is that their infrared (IR) transmission cut-off is relatively long (∼6 m). Therefore, CA glasses are potentially important for use as IR windows, IR domes, and IR fiber optics.5–10 Moreover, rare-earth-doped CA glasses can be used as laser hosts for converting IR light to visible light (known as frequency up-conversion).11,12 However, the mechanical strength of the IRtransmitting CA glasses still needs to be improved. Glass-ceramics generally have higher mechanical strength at room temperature and elevated temperatures than their parent glasses. Therefore, one of the possible methods for improving the mechanical properties is to convert the CA glasses into CA glass-ceramics. To date, little has been reported on IR glass-ceramic materials based on calcium aluminate composition. One of the keys for fabricating glass-ceramics with uniform microstructure is the use of appropriate nucleating agents. Although several works reported investigation of the crystallization of some CA glasses,13–15 the main
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0053 J. Mater. Res., Vol. 21, No. 2, Feb 2006
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purpose of these studies was to avoid crystallization during fabrication of CA glass fibers. Therefore, no nucleating agents were used in these studies. For glassceramics based on silicate composition, ZrO2, TiO2, ZrO2 + TiO2, P2O5, Au, Ag, and Cu are often used as nucleating agents to generate uniform nucleation in parent glass.16 In this work, we selected ZrO2, which was added into an IR-transmitting calcium aluminate glass in the CaO–BaO–MgO–Al2O3 system to determine whether internal nucleation can be stimulated or not. In addition, the crystallization of the parent glass and t
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