Ferroelectric glass-ceramics, due to their excellent dielectric properties, high-temperature stability, and rapid charge-discharge capabilities, have strong potential for dielectric energy storage applications. These materials exhibit a unique combination of high energy density and the ability to withstand extreme operating conditions, making them indispensable in lightweight and miniaturized electronic devices, such as capacitors, sensors and piezoelectric devices. By combining the processability of glass with the electrical advantages of ceramics, ferroelectric glass-ceramics address the growing demand for high-performance energy storage systems in a wide range of applications, from consumer electronics to renewable energy storage. As the world continues to focus on energy efficiency and environmental sustainability, ferroelectric glass-ceramics stand out for their potential to support the development of advanced energy storage solutions. Their ability to deliver both high energy density and reliability, coupled with their adaptability to various manufacturing methods, makes them key to overcoming current limitations in energy storage technologies. The global push for more environmentally friendly materials, particularly the need to replace lead-based systems with lead-free alternatives, has accelerated the search for ferroelectric materials that meet both performance and sustainability criteria.

Recent advancements in ferroelectric glass-ceramics have focused on optimizing their composition, microstructure and preparation methods to enhance energy storage performance. Two prominent systems, niobate-based and titanate-based materials, have been extensively studied for their favorable dielectric and energy storage properties. Niobate-based glass-ceramics, such as those derived from SrO-BaO-Nb₂O₅, exhibit high breakdown strength and low dielectric loss, making them ideal for high-frequency applications. These materials have demonstrated superior energy storage densities, particularly in high-power applications that require rapid charge-discharge cycles. For example, SrO-BaO-Nb₂O₅-based systems have achieved energy densities above 17 J·cm⁻³, making them suitable for power capacitors and other energy-intensive systems. Titanate-based glass-ceramics, including BaTiO₃ and its derivatives, are noted for their high dielectric constants and tunable properties. These materials are widely used in capacitors and high-frequency circuits, due to their ability to achieve large capacitance values. Advances in doping strategies, such as incorporating rare-earth elements, have been particularly beneficial in improving the breakdown strength and polarization characteristics of these systems. By enhancing their dielectric properties, titanate-based glass-ceramics can be optimized for use in both high-energy and high-power applications. However, challenges remain in maintaining a balance between dielectric constant and breakdown strength, particularly when incorporating with glass additives. In addition to material composition, advancements in preparation methods have played a crucial role in improving the performance of ferroelectric glass-ceramics. The melting method, a conventional technique, allows for the production of highly homogeneous materials but requires precise temperature control, which can be difficult to maintain at large scales. The sintering method, while cost-effective and scalable, often leads to issues with structural homogeneity, limiting its applicability for certain high-performance applications. In contrast, sol-gel method offers exceptional control over the materials microstructure, enabling the production of high-purity nanomaterials at lower temperatures. However, this process is more time-consuming and complex, making it less suitable for large-scale production. Recent research has also focused on the role of ferroelectric glass-ceramics as additives in traditional ceramics. By incorporating specific glass systems, such as B₂O₃-SiO₂ or PbO-SiO₂, into ceramic matrices, the breakdown strength and energy density of these materials can be significantly improved. Additionally, the reduced sintering temperatures associated with glass additives make them an energy-efficient alternative for manufacturing ferroelectric ceramics. This strategy has shown great potential in enhancing the performance of traditional ceramics used in energy storage devices.

Ferroelectric glass-ceramics have shown immense potential in addressing the demands of high-performance energy storage devices. Their combination of high energy density, excellent dielectric properties and thermal stability positions them as key candidates for use in energy storage systems that require rapid charge-discharge cycles, such as those in capacitors and piezoelectric devices. However, the current limitations of these materials, such as the trade-off between dielectric constant and breakdown strength, as well as the challenges associated with lead-based systems, require further innovation and exploration. The development of lead-free ferroelectric glass-ceramics is becoming increasingly important, driven by environmental regulations and the global push for sustainable technologies. Niobate-based systems show promise in meeting these requirements, offering a viable alternative to traditional lead-containing systems, while still providing high energy density and breakdown strength. Furthermore, the continued optimization of titanate-based systems through compositional and structural modifications will be essential in achieving the necessary balance between performance and environmental sustainability.

Future research should focus on exploring novel composite systems that combine the properties of glass and ceramics to optimize energy storage performance. The development of new preparation methods, such as advanced sintering techniques and additive manufacturing, could help overcome the challenges of achieving uniformity and scalability. Additionally, exploring the integration of ferroelectric glass-ceramics into multifunctional devices, such as energy harvesters and sensors, presents an exciting avenue for expanding their applications beyond traditional energy storage. With ongoing advancements in material science and manufacturing technologies, ferroelectric glass-ceramics are poised to play a transformative role in energy-efficient and sustainable electronics. By focusing on improving their dielectric properties, enhancing energy storage densities and developing environmentally friendly compositions, these materials will contribute significantly to the next generation of energy storage solutions.

Transparent polycrystalline alumina ceramics (PCA) have attracted extensive attention as a viable alternative to traditional glass and single crystal materials, due to their excellent mechanical properties (high hardness, fracture toughness and flexural strength), making them widely studied materials in industrial and defense applications. Although progress has been made in sintering technology, it is challenging to achieve high optical transmittance for PCA, due to the birefringence effect related to its non-cubic crystal structure, thus requiring simultaneous reduction in porosity (< 0.05%) and grain size (< 1 μm) during the preparation process. In addition, in the SPS process, the development of inhomogeneous microstructure induced by temperature gradient and pressure gradient is widely considered to be the main factor hindering the further improvement of mechanical and optical properties. In this work, transparent alumina was prepared by using a two-step SPS process using untreated commercial powders at 1150-1350 ℃. Effects of SPS parameters on sintering behavior and optical properties of the prepared samples were systematically studied.

Commercial α-Al₂O₃ powder with an average particle size of 0.17 μm (99.99% purity, TM-DAR, Taimei Chemicals Co., Tokyo, Japan) was used in this study. The as-received powder was directly poured into a 16 mm diameter graphite die without any special treatment or additives. The sintering experiments were conducted using an SPS equipment (SPS-20T-10-IV, Shanghai Chenhua Science and Technology Co., Ltd., China). The temperature was measured by using an optical pyrometer with a non-penetrating hole positioned at the top of the graphite mold. Sintering was conducted at temperatures of 1150-1350 ℃ at an applied pressure of 50 MPa. The samples were rapidly heated to 600 ℃ in 1 min, then a heating rate of 50 ℃·min^-1 was used up to 800 ℃, where it was held for 3 min. During this holding stage, the pressure was increased from 40 MPa at room temperature to 50 MPa. Subsequently, the temperature was increased at a same heating rate of 50 ℃·min^-1 up to the 1150℃, with dwell times of 30-100 min. Then, the samples were cooled at a rate of 50 ℃·min^-1 to 600 ℃, while the pressure was reduced to 40 MPa, thus completing the entire process. The obtained disc-shaped samples were ground and polished to an optical level and had final dimensions of 16 mm in diameter and 0.8 mm in thickness. TFT measurements in the wavelength range of 200-2500 nm were conducted using a double-beam spectrophotometer (Lambda1050+, PerkinElmer, Inc., USA), with the samples directly placed in contact with a 5×5 mm square aperture in front of the integrating sphere. RIT was measured in the wavelength range of 200-800 nm using the same spectrophotometer by inserting a pinhole (diameter of 2 mm) in front of the detector to allow the measurement of only the specularly transmitted portion of the incident light beam. The distance between the sample and the detector was sufficiently far to exclude scattered light > 0.5°. The RIT data were normalized at similar thickness of d₂=0.80 mm using the equation RIT(d₂)=(1-R_S)[(RIT(d₁)/(1-R_S)]^d2/d1, where R_S denote the total normal surface reflectance (0.14 for transparent alumina), RIT (d_i) is the RIT for a sample with given thickness. For microstructure observation, the samples were polished and thermally etched in air at a temperature of 200 ℃ below the SPS sintering temperature for 5 h, by using a scanning electron microscope (SEM, SU8100, Hitachi High-Tech Corporation, Japan).

According SEM results, the non-uniformity of the sample is greatly dependent on the sintering temperature, while the samples SPS sintered at 1150 ℃ is more uniform. When the temperature is increased to 1250 ℃ and 1350 ℃, grain coarsening phenomenon expands from the central region of the sample, so that the central region of the sample gradually becomes opaque. The two-step SPS process at 1150 ℃ for 60 min can fully densify and improve uniformity of the samples, while the grain size is 0.25 μm. Infrared transmittance of the sample sintered at 1250 ℃ for 60 min reached 80%. The sample with 150 ℃ heating and cooling rate at 1250 ℃ for 60 min and additional SPS annealing step (1000 ℃/20 min) shows a higher total forward transmittance, with a maximum value of 83%, which is close to the theoretical value. The RIT spectrum has a great correlation with the microstructure of the sample. The sample sintered at 1150 ℃ for 60 min has the highest RIT, which is 46.5%. After heat treatment at 950 ℃ for 5 h in air, this value is increased to 49%. Moreover, the additional SPS annealing step can effectively improve the TFT and infrared transmittance, but it may also deteriorate the RIT.

Transparent sub-micron PCA ceramics were prepared by using two-step SPS process with untreated commercial alumina powder. The relative density of all samples is > 99%, while the uniformity of microstructure and the number of defects are dependent on the SPS parameters. The combination of two-step SPS sintering and optimized SPS parameters can be used significantly control the grain size and improve uniformity of the submicron PCA ceramics. The fine grains and pores make the transmittance curves of the samples in the near-infrared and infrared bands show similar characteristics. Among them, the sample SPS sintered at 1250 ℃ for 60 min at a heating and cooling rate of 50 ℃·min^-1 has the highest infrared transmittance, which is close to the level of sapphire. TFT of the sample sintered at heating/cooling rate of 150 ℃·min^-1 at 1250 ℃ for 60 min, followed by additional SPS annealing at 1000 ℃ for 20 min, reached 80% at 1150 nm and 83% at 2000 nm, which is very close to the theoretical value of 86%. The grain size (0.25 μm) of the sample sintered at 1150 ℃ for 60 min with a heating and cooling rate of 50 ℃·min^-1 is finer and more uniform. The sample with a thickness of 0.8 mm achieves 46.5% RIT at 640 nm, which is increased to 49% after thermal annealing. The use of annealing steps in SPS can improve TFT performance, but prolonged sintering time will lead to grain coarsening and adversely affect RIT. Therefore, the introduction of annealing process in SPS requires careful design and consideration of sintering kinetics and microstructure stability.