Sintered transparent polycrystalline ceramics: the next generation of fillers for clarity enhancement in corundum

Abstract

A significant proportion of mined natural corundum (ruby and sapphire) contain fractures, which negatively affects a gemstone’s clarity and value. Over the past decades, heat treatment techniques have been developed for either fracture healing, or filling to make such gems marketable. The clarity enhancement processes are mainly based on techniques which are either not durable, as in the case of lead silicate fillers, or do not yield perfect transmittance through a fracture, as in the case of borax based fluxes. Therefore, the gemstone treatment community is actively in pursuit of better techniques for clarity enhancement in corundum. Given that application of pressure is a recent advancement in the heat treatment processes of natural sapphire, it is essential to explore the possibilities regarding different outcomes such treatments can have. In this perspective paper, we have briefly described how application of pressure during heat treatments can lead to in-situ sintering of transparent polycrystalline ceramics within the fractures of corundum, which can result in clarity enhancement. Spinel-structure based fillers can be tailored to mimic corundum in terms of tribological, chemical and optical properties. Therefore, gemstones treated with such fillers will be durable, unlike currently used glass-based filler material. We also provide a possible explanation for ghost-fissures in sapphires heated under pressure, as being a by-product of in-situ sintering process of ceramic fillers that are thermodynamically compatible with Al2O3. The prospect of transparent polycrystalline ceramics in the gem and jewelry industry opens a new field of research in this area, given that ceramic fillers can outperform currently used methods and material for clarity enhancement in gemstones. In essence, we present a novel application for sintered transparent polycrystalline ceramics.

Effects of pH and calcination temperature on gel-combustion synthesizability of BaZr0.8Y0.2O3 perovskite

“…Among the other activities carried out to achieve high density at minimum sintering temperature and time, the BaZr 0.8 Y 0.2 O 3-δ perovskite was sintered by the spark plasma sintering, but the graphite phase was observed in the perovskite structure [54]. It is worth mentioning that dopants and additives have an important role in the sintering of oxide ceramics and their characteristics [55][56][57][58][59]. In this paper, BaZr 0.8-x Sr x Y 0.2 O 3 (x=0, 0.05, 0.1, and 0.15) perovskites are synthesized by the gel-combustion method and then sintered at different temperatures.…”

Effects of clay and fireclay addition on the properties of magnesia–forsterite–spinel refractories synthesized at different firing temperatures

“…Refractory oxides are ceramics that can be employed for elevatedtemperature applications [1][2][3][4][5]. Magnesia with a melting point of 2800 °C is a member refractory materials that is broadly used for lining the industrial steel and metallurgical furnaces because of its properties like good slag resistance and superior refractoriness [6][7][8][9].…”