Transparent Glass Ceramics

In the past few decades, glass ceramic (GC) has revolutionized the application of glass [1,2]. Glass ceramic is a composite material of crystalline phases dispersed in the glass matrix [3]. In other words, GC materials provide a unique combination of the ordered crystalline structure within the disordered glass matrix. Glass ceramics provide improved properties, such as mechanical, electrical, thermal, etc., compared to their parent glass materials [3,4]. The chemical nature of the crystalline phases in the GC varies between metallic, semiconductor, oxide and non-oxide crystalline phases. However, retaining the transparency of the GC in comparison to the glass is a big challenge [3,5]. As the crystals’ size increases, the transparency decreases due to scattering losses, and finally, it becomes opaque at a bigger crystallite size (in the micron to millimeter range). The loss of transparency limits the application of glass ceramics; as such, the development of transparent glass-ceramics is essential for many applications. The transparent glass ceramics are cost-effective in their production, in comparison to the single crystals and transparent ceramics materials.

In general, transparent glass ceramics are produced through two methods: the melt-quenching of glass and subsequent heat treatment for crystallization, and the sol-gel technique [5]. The size of the crystal can be controlled through nucleation and crystal growth kinetics [6]. A few glasses are self-nucleating, whereas in most cases, crystallization is governed by different nucleating agents [5]. Nucleating agents are melted uniformly in the glass melting step, and precipitate through reheating the glass. These nucleating agents provide the surface for the nucleation and growth of the crystals, hence nucleating agents facilitate the favorable kinetic path for crystallization. Moreover, this process helps to develop homogeneous and well-defined crystals in the glass matrix. The crystallization process is carried out using the conventional method of thermal treatment in the electrical furnace, selective crystallization using laser sources of different energies, and electron beam irradiation.

The production of transparent glass ceramics is a very tricky process [7]. Transparent glass ceramic has to fulfill two criteria to retain its transparency [3,5]. The first condition is to match the refractive indices of the dispersed crystal phase and the host glass matrix. The matching of the refractive index decreases the optical scattering when light passes through the glass and crystal phases. The second condition is to have a very fine crystal size, much smaller than the wavelength of visible light (typically < 50 nm). The smaller size of crystals down to the nanometer regime will cause less scattering of the light. Generally, these transparent glass ceramics are produced from mullite, spinel and oxyfluoride-based compositions [5,8]. They are widely used in low-expansion materials, cooktops, cookware, gyroscopes, etc. In addition to these applications, there are many emerging applications of transparent glass ceramics, such as luminescent materials for optical and photonics use, spectral converters for solar application, solid-state light-emitting diodes, optical amplifiers for optical communication, electrochromic window, microwave absorber, etc. [8,9,10,11,12,13,14]. This demand for transparent glass ceramics is increasing with the advancement of technologies. Additive manufacturing (AM) technology is another futuristic technology for producing the complicated shapes of transparent glass ceramics [15,16,17]. However, the presence of pores is a major challenge in the additive manufacturing of transparent glass ceramic materials, which decreases their transparency. As such, it is crucial to address the elimination of pores from the transparent glass ceramic produced by AM. Furthermore, new properties of the transparent glass ceramic can be found if the crystallization kinetics can be controlled under extreme conditions of processing. Transparent glass ceramics with improved mechanical properties and comparatively low weight, used for displays, is another growing field of interest. Gorilla^® glass ceramic is a well-known transparent glass ceramic being used in smartphones [1,8].

The glass research community is very small compared to other emerging research areas, such as 2D materials, high-entropy materials, semiconductors, nanocrystalline materials, etc. [1]. As we have discussed above, transparent glass ceramics have plenty of uses in real applications, ranging from day to day life to strategic sectors. As such, it is essential to integrate glass research in academia with the industries, so as to explore many new avenues of its applications.

The present issue on “Transparent Glass Ceramics” has included articles on the absorption library of rare-earth orthophosphates, the crystallization of GeO₂-Al₂O₃-Bi₂O₃ glasses, and the luminescence behavior of GdVO₄: Tb nanocrystals in silicate glass ceramics. We believe these articles will be useful for the materials community in several fields of application.