Glass-forming ability and stability of calcium aluminate optical glasses

Abstract

It has long been known that certain compositions in the CaO–Al2O3 system form glasses, although they do not contain any strong network formers such as SiO2, B2O3 or P2O5 [1–7]. In addition, there has been much interest in the calcium aluminate (CA) glasses due to their unique optical properties. The CA glasses show sapphire-like infrared (IR) transmission and an IR cutoff of about 6 μm, whereas silicate glasses present strong absorption in the 3–5 μm region [4]. The CA glasses are low-loss optical materials, showing scattering value of approximately 0.04 dB km−1 at 1.55 μm [8]. This is considerably lower compared to that of silicate glasses, which have a scattering value of 0.16 dB km−1. The CA glasses are also applicable to photometric devices because they are photosensitive to ultraviolet radiation [9]. Moreover, the CA glasses have appropriate mechanical properties for a variety of applications. For example, the CA glass fibers have a higher elastic modulus than Sand E-glass fibers [10]. The combined optical and mechanical properties give the CA glasses their high potential of application. The ease of devitrification, however, is a critical limitation not only in the production but in the application of the CA glasses in various forms. Once devitrification initiates, their optical and mechanical properties would seriously deteriorate. Some effort has been spent in improving the glass-forming ability of the CA glasses. Earlier studies [1–3] focused on the addition of SiO2 to the CA glass to improve its glass-forming ability, but such addition was found to deteriorate the optical properties due to the stretching motion of the Si–O bonds. Sun [2] and Hafner et al. [4] studied the effect of the addition of several percentage points of alkali or alkaline– earth oxides to the composition of the CA glasses. Their results showed that the production of stable CA glasses with a large volume was possible without using any strong network former. Most of their glass compositions, however, contained several percentage points of iron oxide, which affects many optical properties. Uhlmann et al. [11] prepared calcium aluminate-based glasses containing 6 vol % of Na2O, BaO and/or SrO and compared the glass-forming ability and stability of each glass. Their results revealed that most of the glasses have a much-enhanced glass-forming ability and stability compared to the CA glass (64CaO– 36Al2O3) without the additives, and among them the CANB glass (52CaO–36Al2O3–6Na2O–6BaO) was found to be the best glass-former and the most stable glass. For this study, the calcium aluminate glass (61CaO– 39Al2O3) was prepared by air quenching. This composition has been intensively studied for the glass-fiber production via inviscid melt spinning (IMS) [12–15] because it has the lowest eutectic temperature in the CaO–Al2O3 binary system and thus is easy to melt. Also, the calcium aluminate glasses containing 5 or 10% of Na2O, BaO and/or SrO were prepared for the purpose of enhancing the glass-forming ability and stability of the CA glasses. Table I lists the compositions of glasses prepared for this study. High-purity powders of Al2O3, CaCO3, Na2CO3, CaCO3 and SrCO3 from Aldrich Chemical Company (Milwaukee, WI, USA) were used for glass preparation. Each powder mixture loaded inside a platinum crucible was heated to 1550 ◦C and held for 2 h in an air atmosphere. The weight of each glass melt was 10 g. The platinum crucible containing the glass melt was taken from the furnace at 1550 ◦C and cooled on a brass plate at room temperature. Each of the glasses formed by air cooling was examined for glass-forming ability by using optical microscopy. Also, differential thermal analysis (DTA; Setaram TGDTA-92, France) scans on the glasses were performed at a heating rate of 10 ◦C min−1 up to 1550 ◦C and at a cooling rate of 80 ◦C min−1 down to 1000 ◦C to investigate the glass-forming ability and stability. The DTA scans were performed using a 50 mg fragment of each glass in an air atmosphere. A 50 mg amount of alumina powder was used as a reference material, and platinum cups were used for the DTA scans. The DTA temperatures were calibrated using pure aluminum and copper. Table II lists the results of the glass-forming ability of each glass composition according to optical microscopy. The CANS composition showed the best glass-forming ability, while the CAN composition showed the worst one, showing complete opaqueness. The CAB, CAS and CA compositions showed small isolated crystals, while the CAS and CANB showed partial crystallization with a slightly large portion. The resulting glass-forming ability of the compositions in