Recent Technological Advances in Transparent Ceramics

1. Introduction

Transparent and translucent ceramics (TCs) represent a relatively recent development in the long history of ceramics—while silicate ceramics have existed for approximately 30,000 years, transparent ceramics have been developed only within the past 65 years. Together with certain glasses and single crystals, they form a class of materials known as transparent inorganic solids (TISs). A wide array of components and devices, including many high-tech applications, are derived from TISs [1,2,3,4,5]. The origins of the TC field can be traced to advancements in sintering science pioneered by R. L. Coble at General Electric in the late 1950s [1] (Figure 1). His work led to the development of translucent alumina walls for high-pressure sodium vapor lamps, which became a commercially successful product [6] and helped establish the field’s significance.

Many new materials and products have been developed within this evolving domain. For a long time, these materials were primarily based on cubic crystal lattices (unlike alumina), which significantly reduce light scattering and consequently enhance optical transmission. Notable examples include MgO [7,8], MgF₂ [9,10], perovskite-structured PLZT [11,12], ALON [13,14,15], MgAl₂O₄ [16,17], diamond [18], BN [19,20], and SiC [21]. These materials have enabled the development of primarily passive components, whose functionality depends on high optical transparency (T > 70%) combined with desirable mechanical, thermal, electrical, or magnetic properties, alongside excellent chemical stability. Subsequently, more sophisticated materials were developed, in which transparent ceramics serve as hosts for intentional doping with transition-metal (TM) and/or rare-earth (RE) cations. These doped materials, often referred to as active transparent ceramics, can modify the spectral characteristics of incident light. Notable examples include MgAl₂O₄ [22], ZnAl₂O₄ [23,24], CaF₂ [25] and other fluorides [26], YAG [27,28,29], Gd₃Ga₅O₁₂ [30] and other garnets, cubic ZrO₂ [31], Ca₅(PO₄)₃F [32], Y₂O₃ [33] and other bixbyite-structured sesquioxides [34], as well as oxide and FOX-type glass-ceramics (e.g., BaAl₄O₇) [35].

Passive transparent ceramics have enabled products such as transparent armor, infrared and LIDAR windows, cell phone displays, metal halide lamps, jewelry, and flash goggles. Active transparent ceramics, on the other hand, have led to the creation of solid-state lasers, optical filters, passive Q-switch absorbers, scintillators, and solid-state phosphors. A key objective for the near future is to reduce fabrication costs—particularly for spinel-based products. However, only modest progress is expected through incremental improvements to existing processes. Significant cost reductions are likely to be achieved only through the implementation of innovative, science-driven technologies. New materials are also expected to emerge, especially in the field of nanostructured, non-cubic transparent ceramics. Performance enhancements of current devices are anticipated in the short to medium term, alongside the discovery of new applications. A fundamental requirement for these advancements is the deepening and broadening of scientific knowledge underpinning the entire field.

2. Dr. Adrian Goldstein Author of Some Pioneering Contributions to the Transparent Ceramics Field

This Special Issue honors Dr. Adrian Goldstein, former head of the Israel Ceramics and Silicates Institute, in recognition of his groundbreaking contributions to the field of transparent ceramics. Dr. Goldstein was among the first to demonstrate that significant enhancement in densification rate could be achieved by microwave sintering of green bodies composed of monodisperse, spheroidal particles, such as amorphous silica, arranged in a highly compact configuration. His work showed that such accelerated densification enables the formation of transparent bulk specimens without inducing crystallization during the process [36]. Using a similar approach, Dr. Goldstein successfully fabricated transparent spinel with submicron grain sizes via low-temperature sintering of monodisperse, spheroidal powders at approximately 1300 °C—several hundred degrees lower than the temperatures typically required for conventional densification [37]. Extending these innovations, Dr. Goldstein also developed a novel method for producing bulk, polycrystalline, transparent ZnAl₂O₄ ceramics, introducing a new class of optical ceramics with a high hardness of 10.5 GPa [38].

Beyond fabrication, Dr. Goldstein conducted fundamental studies on the oxidation and coordination behavior of transition metal dopants in inverted ultrabasic silicate glasses. These investigations led to key discoveries, including the remarkable stability of Co³⁺ in ultra-high-basicity glass and a redox interaction mechanism between Cr³⁺ and Cu²⁺ ions [39]. Further advancing optical device technologies, Dr. Goldstein contributed to the development of Co²⁺:MgAl₂O₄ transparent ceramics as saturable absorbers for passive Q-switching of Er³⁺ lasers at 1.534 μm. This work established Co²⁺-doped MgAl₂O₄ ceramics as effective passive Q-switching elements [40]. In collaborative research, Dr. Goldstein also investigated the influence of cation inversion on the optical absorption behavior of transition-metal-doped spinel ceramics, notably showing that increased inversion levels in Ti-doped MgGa₂O₄ significantly alter its optical absorption spectrum, deepening our understanding of the structure–property relationships in spinel-type lattices [41]. Through these seminal contributions, Dr. Goldstein has profoundly impacted both the fundamental science and practical applications of transparent ceramics. His work continues to shape the future of ceramic materials research.

Dr. Adrian Goldstein, together with Dr. Andreas Krell and Prof. Zeev Burshtein, co-authored the excellent and comprehensive book Transparent Ceramics: Materials, Engineering and Applications, published by John Wiley & Sons, New York, in 2020.

The book Transparent Ceramics: Materials, Engineering and Applications, represents a long-awaited and much-needed comprehensive overview of transparent ceramics. From the outset, the book clearly communicates the breadth and depth of its content. Its accessible and engaging writing style avoids overly technical language, making it suitable not only for students, scientists, and engineers in materials science but also for newcomers to the field. The material is richly supported by numerous tables, diagrams, and illustrations, enhancing the reader’s experience and facilitating comprehension. While significant progress has been made in fabricating high-quality transparent ceramics, many challenges and open questions remain. These are thoughtfully addressed in the final chapter, Future Developments, which outlines the author’s perspective on the field’s future directions. This book is an invaluable resource and holds great potential to become the definitive textbook for anyone interested in transparent ceramics.

3. Works Collected in This Special Issue

Recognizing the growing importance of transparent ceramics, the MDPI journal Ceramics is committed to publishing research focused on their science and engineering. This Special Issue brings together ten such contributions. A brief overview of their content is provided below.

The first contribution [42] examines the effect of sintering atmosphere on the densification behavior of Yb-doped Y₂O₃ transparent ceramics, suited for high-power laser applications. It discusses the potential advantages of flowing oxygen versus traditional vacuum environments during the sintering process. The second paper [43] presents a large-scale commercial application of a sophisticated transparent ceramic coating designed to protect rose wines from degradation caused by UV and blue light exposure. The coatings were prepared from a sol–gel precursor capable of forming silica-based gels. Commercial organic UV-absorbing molecules were incorporated into the gel, and two different organic additives were evaluated. The resulting coatings were colorless and thin, demonstrating superior UV protection compared to gels with inorganic additives. The most effective formulation, containing approximately 1.5% of the commercial additive SemaSORB 20109, exhibited the highest efficacy in preventing anthocyanin loss. The next study [44] also focuses on high-power lasers, investigating the fabrication of Yb-doped components using a CaF₂ host. Despite its relatively low mechanical strength, CaF₂ offers several advantages over oxide hosts, such as its low phonon energy. Moreover, CaF₂-based transparent ceramics can be sintered at temperatures below 1000 °C, whereas oxide ceramics typically require sintering temperatures near 2000 °C. The authors employed a two-step densification involving pressureless sintering in air followed by hot isostatic pressing (HIP) at 600 °C. The resulting specimens achieved approximately 91% optical transmission at 1.2 μm. Under quasi-continuous wave (QCW) operation at ~1030 nm, with an output coupler mirror transmission of 18.1%, the laser delivered a maximum output power of 1.51 W and a slope efficiency of 9.2%.

The study presented in [45] was a collaborative effort among researchers from Algeria, Spain, and France. It addressed the restoration of sand-blasted Mg-spinel ceramic specimens—fabricated via spark plasma sintering (SPS)—by applying a thin, high-hardness SiO₂–ZrO₂ protective layer. Sand blasting simulated natural weathering effects in desert environments. The protective coating was synthesized and deposited via sol–gel processing. Interestingly, sand blasting was found to improve UV-region transmission (around 200 nm) in specimens with initially low transparency, while degrading transmission in highly transparent samples. Larger defects from blasting mainly reduced transmission and increased surface roughness at longer wavelengths. The applied coating exhibited notable healing capabilities, successfully restoring the original transmission characteristics. Paper [46] explores the development of transparent ceramics (TCs) from non-cubic materials, specifically Ca₁₀(PO₄)₆(OH)₂, a phosphate-based hydroxyapatite. Such materials have already demonstrated potential as solid-state laser hosts. The study focused on optimizing processing conditions using the spark plasma sintering (SPS) technique. A comparison was made between commercial nano-powders and in-house synthesized powders via hydrothermal methods. Initial tests led to the successful fabrication of 1 mm thick disks with approximately 60% optical transmission at 880 nm.

The following contribution [47] addresses an important characterization method: scanning optical microscopy. This technique enables a detailed examination of porosity and other defects in transparent ceramics. Modern digital instruments enable the acquisition of high-resolution images across multiple depths and relatively large surface areas. Although small pores are better revealed through electron microscopy, optical scanning provides valuable data on pore volume, size, and shape distribution. Yb-doped YAG ceramics were analyzed, with 2.2 × 10⁷ µm³ examined per sample. The study revealed clear differences in porosity among specimens fabricated using different techniques. Samples that appeared similar under visual inspection or SEM exhibited distinct pore populations, with these differences correlating well with their respective transmission spectroscopy data.

Paper [48] focuses on the development of eye-safe lasers, specifically examining Ho³⁺-doped Lu₂O₃ transparent ceramics fabricated via spark plasma sintering (SPS). The goal was to compare the properties of these SPS-fabricated ceramics with those of single crystals and ceramics densified by hot isostatic pressing (HIP). Using in-house-prepared active powders, specimens with characteristics comparable to single crystals were successfully produced. However, as the Ho³⁺ concentration approached 10%, auto-quenching became a concern. Energy losses were attributed to trace impurities, but cross-relaxation processes enhanced emission near 2 μm. Paper [49] investigates the challenge of joining transparent Mg-spinel plates. It is now well understood that although fabricating large spinel windows is technically possible, it remains economically impractical. Consequently, joining smaller plates appears to be the most viable approach for producing large, transparent armor components. The study explored bonding components using specially designed glass interlayers, aiming to develop a joining glass with refractive index and thermal expansion closely matched to spinel. Additionally, the glass should minimize spinel dissolution during thermal treatment and resist both crystallization and environmental degradation. A particularly interesting and practically significant phenomenon was observed when two semi-polished spinel plates were bonded using this glass. In this context, “semi-polished” refers to one plate face being well polished and the other subjected only to coarse grinding, with the glass applied to the ground face. Remarkably, the resulting bonded specimens exhibited optical transmission exceeding the theoretical value for fully polished, uncoated plates—representing a highly valuable achievement in the field.

Paper [50] details the fabrication of transparent ceramic spinel (TCS) that combines high optical transparency with excellent resistance to sand erosion—primarily achieved through high hardness. The study identifies spark plasma sintering (SPS) as a powerful method for producing components with the desired properties. Importantly, it was found that the highest sintering temperature did not yield optimal results. Instead, 1350 °C was identified as the best compromise, balancing competing factors such as pore coalescence, grain growth, graphite contamination, and diffusion kinetics. The resulting transparent samples achieved Vickers hardness of 16 GPa and Young’s modulus of 270 MPa, surpassing the properties of parts densified by hot isostatic pressing (HIP). The subsequent paper [51] examines the speciation of transition metals (TMs) incorporated into transparent ceramics. Understanding speciation—together with insights from ligand field theory—is essential for interpreting optical spectra and predicting spectral line profiles. This study investigated how differences in host lattice structures affect TM speciation, using corundum and spinel as model systems. It was observed that the spinel structure favors the stabilization of TMs in the 2+ oxidation state (substituting Mg²⁺ in tetrahedral sites), while corundum tends to favor 3+ cations occupying octahedral sites. Additionally, variation in ionization potential along the 3d transition series contributes to the stabilization of lower oxidation states for late transition metals. Normal crystallographic sites were found to be significantly more favorable for TM incorporation than inverted ones.