Solid-state lasers have the advantages of realizing laser outputs with a high energy and a high peak power, having potential applications in civil and military fields (i.e., remote sensing, environmental monitoring, medical treatment and optoelectronic countermeasure). Rare-earth ions doped sesquioxide materials as one of the most promising gain media have attracted recent attention due to their low phonon energy, low thermal expansion coefficient and high thermal conductivity. However, large-size sesquioxide single crystals with a good optical quality are difficult to grow because of the high melting points (i.e., >2 400 ℃) and phase transition point at 2 280 ℃. Fortunately, the sintering of sesquioxide ceramics as an alternative way to prepare laser host materials. It is possible to obtain the materials with a large volume and a high doping concentration, showing a superiority in large-scale production, a feasibility of shape control and better mechanical properties.

To achieve high-efficiency and high-power laser oscillation from sesquioxide ceramics, it is crucial to eliminate the main scattering centers inside microstructures, i.e., residual pores and secondary phases. Raw powders with less agglomeration and high sinterability, combined with appropriate molding methods are fundamental for producing compacts with small pore size and uniform microstructures. This is favorable to avoid differentiate densification, degrading the optical homogeneity and transparency. During the sintering process, it is essential for pores to remain at grain boundaries until the final full densification. This prevents the formation of intragranular pores that are hard to remove. The effective control of grain boundary migration can be achieved to prevent pore-boundary separation through the addition of suitable sintering additives, regulation of sintering atmospheres (i.e., vacuum, hydrogen and oxygen), and the use of advanced sintering techniques (i.e., microwave sintering, spark plasma sintering or two-step sintering, etc.). In addition, pressure assisted sintering, including hot pressing and hot isostatic pressing can also improve the densification rate and facilitate the effective removal of residual pores. This even enables the full densification of sesquioxide ceramics without using sintering additives. Zirconia is once widely used as a sintering aid for sesquioxide transparent ceramics. However, it can severely degrade the laser performance of sesquioxide ceramics due to the occurrence of photodarkening phenomenon under high-power pump beam excitation. This may be attributed to the charge imbalance between Zr⁴⁺ and host cation ions. Consequently, the introduced point defects act as acceptors for the electrons of excited laser ions, forming Zr³⁺ color centers that cause a broad absorption in a wavelength range of 400-700 nm. This seriously affects the laser oscillation efficiency. Therefore, in addition to achieving laser-grade quality compared to single crystals, minimizing the use of sintering additives or regulating the lattice defects are also considered in the fabrication process of sesquioxide ceramics.

In general, chemical co-precipitation process for synthesizing well-dispersed powders, combined with vacuum sintering and hot isostatic pressing sintering, is considered as an effective way for fabricating high-quality sesquioxide transparent ceramics. This approach provides a great driving force for densification through well controlling grain boundary diffusion and migration. However, it is necessary to determine suitable sintering curves and microstructure morphology before hot isostatic pressing.

Conventional transparent materials can be primarily divided into four categories, i.e., glasses, polymers, single crystals, and transparent ceramics. Growing large single crystals with high melting points is difficult, and glass and polymers have poor mechanical and chemical stability. These shortcomings limit their applications. Transparent ceramics are favored because of their high refractive index, high hardness, high strength, high thermal conductivity, and high chemical stability. In 2005, La₂Hf₂O₇ was fabricated into transparent ceramics with a high density and a high atomic number, having a great potential in the detection field of scintillators. Since then, A₂B₂O₇ transparent ceramics has attracted much attention. The structure of A₂B₂O₇ ceramics is mainly determined by the radius ratio of A-site and B-site ions (r_A/r_B), and includes three structures (i.e., fluorite structure (r_A/r_B<1.46), pyrochlore structure (1.46<r_A/r_B<1.78) and monoclinic structure (r_A/r_B>1.78)). While, only the fluorite- and pyrochlore-structure A₂B₂O₇ ceramics could be transparent monolith when sintered at proper temperature and pressure.

A₂B₂O₇ ceramics with fluorite and pyrochlore two-phase coexisting structures have the advantage of smaller grains, compared with conventional A₂B₂O₇ transparent ceramics with single-phase structures. The mechanical properties of two-phase coexisting structures ceramics can be improved due to the smaller grains. LaLuZr₂O₇ transparent ceramics with two phases of fluorite and pyrochlore were fabricated in 2014. Two phases are uniformly distributed in the ceramic and the average grain size of the ceramic is only 2-3μm. The in-line transmittance of LaLuZr₂O₇ transparent ceramic is 73.4% at 1 100 nm due to the close refractive index between the two phases of fluorite and pyrochlore, and the scattering of light across the interface between the two phases is negligible. In 2019, (La_0.2Nd_0.2Sm_0.2Gd_0.2Yb_0.2)₂Zr₂O₇ was fabricated into transparent ceramics with a fluorite and pyrochlore two-phase coexisting structure. Compared with conventional transparent ceramics, high-entropy transparent ceramics may have better mechanical properties, corrosion resistance, and high-temperature resistance. The enhanced atomic disorder due to multi-element doping in high-entropy ceramics makes it a promising material for high-temperature windows with a lower thermal conductivity. However, there are some challenges in the preparation of two-phase coexistence transparent A₂B₂O₇ ceramics. In some previous reports on A₂B₂O₇ transparent ceramics with two-phase coexistence, the transmittance of the ceramics was severely reduced when there was a large difference in the grain sizes of the fluorite and pyrochlore phases. It is necessary to ensure that the grain growth rates of the pyrochlore and fluorite phases are similar during sintering.

Transparent ceramic scintillators of La₂Hf₂O₇:Ti⁴⁺ were fabricated in 2005. The transmittance of La₂Hf₂O₇:Ti⁴⁺ transparent ceramics at 600-800 nm is close to 60%. La₂Hf₂O₇:Ti⁴⁺ transparent ceramics gain a high X-ray stopping power due to its high density and high atomic number. The integrated emission intensity of La₂Hf₂O₇:Ti⁴⁺ transparent ceramics is 1.5 times greater than that of Bi₄Ge₃O₁₂ single crystals for the same size and the same excitation conditions. In 2019, Tb₂Hf₂O₇ was fabricated into transparent ceramics. Tb₂Hf₂O₇ transparent ceramic ceramics perform well in the Faraday magneto-optical effect tests, having higher measured Verdet constants than commercial Tb₃Ga₅O₁₂ single crystals and a potential application as an excellent magneto-optical material. It is indicated that the magneto-optical properties of magneto-optical materials are related to the concentration of Tb³⁺ in the materials, and the higher the Tb³⁺ concentration is, the better the magneto-optical properties will be. Tb₂Hf₂O₇ transparent ceramics can be doped with higher concentrations of Tb³⁺ due to the structural advantages of A₂B₂O₇, thus having better magneto-optical properties rather than garnet-structured Tb₃Ga₅O₁₂ single crystal.

For the favorable physicochemical properties, high fluorescence conversion efficiencies, a relatively short fluorescence lifetime, and an efficient spectral congruence between excitation and ultraviolet/blue LED emission spectra, Ce³⁺-doped garnet-based fluorescent materials can be used in encompassing lighting, displays, medical imaging, etc.. For leveraging the diverse cation sites within a garnet structure, the emission wavelength of Ce³⁺ can be continuously adjusted from 460 nm to 610 nm via the incorporation of varied matrix chemical constituents. This versatile tunability broadens the horizons of their potential applications and augments the repertoire of materials available for advancing Ce³⁺ luminescence theory. However, recent work on Ce³⁺-doped garnet-based fluorescent materials has unveiled certain anomalies and even contradictions within the characteristics and the emission wavelength modulation theory, becoming one of the prominent bottlenecks hindering the development of this material. To address this issue, this review represented the relative theories of substrate chemical component substitution and the modulation of Ce³⁺ emission wavelengths. The review also summarized recent advancements in Ce³⁺-doped garnet-based fluorescent materials, and the influence of ion substitution on Ce³⁺ luminescence performance with the distortion factor of [CeO₈].

This review discussed the Ce³⁺-doped yttrium aluminum garnet structure as a prototype and comprehensively analyzed the impacts of commonly used doping ions that occupy the [AO₈], [BO₆], and [CO₄] sites on the emission wavelength, thermal quenching resistance, and the [CeO₈] distortion factor of Ce³⁺.

The ion substitution of a single lattice generally necessitates that the substituting ion and the substituted ion have the same valence state and exhibit a minimal disparity in atomic radius. In the case of [AO₈] lattice substitution, Y³⁺ can be replaced with rare-earth ions, i.e., Lu³⁺, Tb³⁺, Gd³⁺, and La³⁺. When the radius of the doped rare-earth ions is greater that that of Y³⁺, the emission wavelength of Ce³⁺ shifts towards the red end of the spectrum. Conversely, when the radius is smaller, the shift occurs in the opposite direction.

The ion substitution of octahedral lattice sites mainly involves the replacement of Al³⁺ by Ga³⁺, Sc³⁺, and In³⁺. With an escalation in the concentration of doped ions like Ga³⁺, Sc³⁺, and In³⁺, the peak wavelength of the Ce³⁺ emission spectrum has a blue shift. In addition, the incorporation of Sc³⁺ effectively enhances the thermal quenching resistance of Ce³⁺, while the inclusion of Ga³⁺ diminishes the thermal quenching resistance of Ce³⁺.

The main substitutions of dodecahedral-octahedral lattice ion pairs include Ca²⁺-Hf⁴⁺ and Ca²⁺-Zr⁴⁺ replacing Y³⁺-Al³⁺. The emission and excitation peak wavelengths of Ce³⁺ gradually blueshifts as Ca²⁺-Hf⁴⁺ and Ca²⁺-Zr⁴⁺ doping contents increase.

The substitution of dodecahedral-tetrahedral lattice ions is mainly achieved by M²⁺-Si⁴⁺ (M=Mg, Ca, Sr, Ba) replacing Y³⁺-Al³⁺, with alkaline earth metals occupying the dodecahedral lattice and Si⁴⁺ occupying the tetrahedral lattice. In the garnet system, when M²⁺-Si⁴⁺ replaces Y³⁺-Al³⁺, the emission wavelength of Ce³⁺ undergoes a blue shift with the increase of M²⁺ ion radius, resulting in improving the thermal stability.

The substitution of octahedral-tetrahedral lattice ions mainly involves Mg²⁺-Si⁴⁺/Ge⁴⁺ replacing Al³⁺-Al³⁺, where Mg²⁺ occupies the octahedral lattice and Si⁴⁺/Ge⁴⁺ occupies the tetrahedral lattice. Among them, Mg²⁺-Si⁴⁺ replacing Al³⁺-Al³⁺ is an effective method to achieve a large redshift of Ce³⁺ emission wavelength. The introduction of Mg²⁺-Ge⁴⁺ also leads to the redshift of Ce³⁺ emission wavelength, but the extent of this redshift is considerably less than that achieved by Mg²⁺-Si⁴⁺.

The chemical components in dodecahedral-octahedral-tetrahedral lattice co-substitutions are more intricate, with Ca²⁺ and Mg²⁺ occupying the dodecahedral lattice, Mg²⁺, Sc³⁺, and Hf⁴⁺ occupying the octahedral lattice, and Si⁴⁺ and Ge⁴⁺ in the tetrahedral lattice. Different lattice ions on the co-substitution affect the luminescence performance of Ce³⁺.

Solid-state lighting is one of the most promising technologies in the 21st century due to its high luminous efficiency, faster responding, energy-saving, environmentally friendly, and longevity. It relies on solid-state electronic components to achieve the conversion of electrical energy to light energy. At present, solid-state lighting mainly includes light-emitting diodes (LEDs) and laser diodes (LDs). The luminous efficiency of existing LEDs decreases with the auger effect as the input power increases. They cannot achieve a higher light efficiency output. However, LDs have a higher efficiency, a stronger brightness and a longer lighting distance for the efficiency degradation of LEDs at high power densities. The existing color conversion materials that can be used for laser diodes are single crystals, phosphor ceramics, phosphor-in-glasses, phosphor films, etc.. Phosphor ceramics are the most promising color conversion materials for LDs with high thermal conductivity, good optical properties, and controllable microstructures.

This review discussed the current research status of forming methods, sintering processes, component selection, and structural design of phosphor ceramics for laser diodes. This review summarized the design requirements of LDs for phosphor ceramics, such as thermal saturation characteristic, optical saturation characteristic, luminous efficiency, color rendering index, mechanical properties, etc.. This review also elaborated the performance requirements of phosphor ceramics for practical applications. The preparation technologies of phosphor ceramics were summarized. The preparation methods of the powders included chemical coprecipitation method, hydrothermal method, sol gel method, etc.. The forming methods of powder included dry pressing, cold isostatic pressing, molding by slip casting process, casting molding, gel casting, etc.. The sintering methods of ceramics include pressureless sintering, gas pressure sintering, vacuum sintering, spark plasma sintering, hot pressed sintering, hot isostatic pressing, etc..

For selecting and optimizing raw materials, sintering aids are one of the important factors affecting the sintering of phosphor ceramics. In the design of the composition of phosphor ceramics, the luminescence performance is adjusted via designing the types of matrix elements in oxide and nitrogen (oxide) phosphor ceramics. The luminescence behavior of phosphor ceramics is analyzed via introducing different luminescent centers into the same matrix. In addition, the introduction of a second phase is also chosen to improve both the blue light absorption rate and the heat dissipation performance of ceramics in order to further enhance the luminescent performance of phosphor ceramics. In the structural design of phosphor ceramics, some methods such as introducing pores, multi-layer structures, and surface modifications are used to further improve the luminescence efficiency and color rendering index of phosphor ceramics.

Infrared imaging and precision guidance systems are distinguished by their high imaging accuracy, robust concealment, and resilience to interference. These characteristics make them a pivotal area of contemporary and future warfare. The infrared window is an integral part of the infrared guidance system, being responsible for transmitting target signals, preserving aerodynamic configurations, and protecting internal precision optoelectronic components. Consequently, the window material must possess a high optical transmittance across the operational band and exhibit the superior strength and hardness.

Hypersonic vehicles have challenges to infrared window materials, including extreme thermal shock, aerodynamic overload, degradation of optical and mechanical performance at high temperatures, and self-heat radiation that can interfere with target signals. At present, no material fully satisfies the demands of cutting-edge systems, making it imperative to develop a window material that offers a low radiation, a high strength, and a broadband transmittance at elevated temperatures.

Commonly utilized infrared window materials encompass sapphire, spinel, AlON, magnesium fluoride, yttrium oxide, and Y₂O₃-MgO nanocomposite ceramic materials. Sapphire also as α-alumina monocrystal is characterized by its high strength and broad high transmittance properties. It boasts a well-established industrial chain and is extensively employed in various aircraft models. While polycrystalline alumina transparent ceramics offer lower production costs, compared to sapphire, and they have yet to reach the same level of application. Spinel and AlON both possess cubic structures and demonstrate great transmission rates from near-ultraviolet to mid-infrared spectrums. AlON exhibits superior mechanical properties akin to those of sapphire, making them popular choices for aircraft infrared windows. However, these three materials share a significant drawback, i.e., an excessively high heat emissivity at high temperatures. This can lead to thermal barrier complications due to spontaneous radiation interference with target signals under intense aerodynamic heating. In addition, their shorter infrared absorption cut-off edges also result in pronounced transmission rate reductions at 5 micrometers, following a high-temperature blue shift. Note that sapphire and spinel both present challenges related to their inadequate mechanical performance at high temperatures. Researches on zirconia ceramics as infrared windows are limited, but their high bending strength and decent transmission performance are noteworthy. Y₂O₃ and MgF₂ with the cubic structure exhibit superior transmission in the 3-5 micrometer band due to their low phonon energy and minimal spontaneous radiation coefficient. Their further infrared absorption cutoff edge mitigates an impact of high temperature on the mid-infrared wave transmission. However, their mechanical properties do not withstand the intense heat shock encountered at a hypersonic speed. Y₂O₃-MgO nanocomposite ceramics, characterized by extremely small grain size from the pinning effect, combine a high transmission in mid-infrared, low radiation and high strength at high temperatures. This makes them an ideal material for infrared windows in hypersonic aircraft.

Electrochemical energy storage (EES) is an essential technology in modern lifestyle. As two typical EES devices, batteries and supercapacitors have their own advantages and disadvantages. Pseudocapacitive materials have attracted recent attention. Such materials store charges via battery-like redox reactions, but have supercapacitor-like electrochemical behaviors. Therefore, pseudocapacitive materials have a considerable potential to simultaneously achieve both high energy and high power density as well as excellent stability, merging a unbridgeable gap between batteries and supercapacitors. However, the development of pseudocapacitive materials still encounter some serious challenges such as unclear energy storage mechanism, lack of alternative pseudocapacitive materials and vague future development directions.

This review represented recent development on pseudocapacitive materials and briefly discussed their energy storage mechanism and the imperfections of the existing issues. Furthermore, we proposed a concept of bulk pseudocapacitance and realized a goal for the future development of pseudocapacitive materials. In addition, we also discussed some key factors and strategies for achieving such a bulk pseudocapacitance.

The energy storage mechanisms of pseudocapacitive materials were discussed. The first pseudocapacitive material, i.e., RuO₂ thin film, was reported by Trasatti et al. in 1971. The thin film electrode exhibited a supercapacitor-like cyclic voltammetry curve but differed from the electric double layer in faradic origin. Although the electrochemical performance of RuO₂ electrodes were enhanced via fabricating various nanostructures and introducing structural water, the energy storage mechanism of this electrode was not fully understood, and limited to reversible redox reaction at the surface, the content of structural water in crystal, fast proton transport and crystallinity of electrode, etc.. MnO₂ is another typical pseudocapacitive electrode, which was firstly reported by Lee and Goodenough in 1999. Simon and Gogotsi reported the pseudocapacitive response of this electrode to successive multiple surface redox reactions. Recently, Augustyn et al. indicated the pseudocapacitive response to the existence of nanoconfined interlayer structural water, mediating an interaction between charge carrier and electrode. Dunn et al. investigated the pseudocapacitive response for T-Nb₂O₅ electrode and proposed a model of pseudocapacitance, i.e., intercalation pseudocapacitance. Mxenes is one of pseudocapacitive materials with a unique structure that is responsible for its pseudocapacitive behaviors. Although it is still controversial whether it is a pseudocapacitive electrode, Co(OH)₂ demonstrates a coupled performance of batteries and supercapacitors. The energy storage mechanism of this electrode was investigated by in-situ X-ray adsorption spectroscopy. It was found that the high specific capacitance originates from a battery-mimic bulk reaction, while the high stability performance roots in the crystal structure stability in charge-discharge process. Furthermore, we highlighted that the Co(OH)₂ electrode blurs the distinction between supercapacitors and batteries, which is in accordance with the continuous transition from electric double layer to battery proposed by Fleischmann and co-workers. In future, more intensive studies should be conducted on the energy storage mechanism of pseudocapacitive materials, especially on the unique characteristics of charge carrier and electron transportation, charge transfer, interaction between charge carrier and electrode in an atom-level.

The energy density of pseudocapacitive materials is still far below than the actual demand, even the batteries. To address this issue, a concept of bulk pseudocapacitance is proposed. In fact, intercalation pseudocapacitance depicted by Dunn et al. is a typical example of bulk pseudocapacitance. Bulk pseudocapacitance refers to the pseudocapacitive response that happens throughout the bulk of the electrode material rather than the surface. Meanwhile, the proposal of bulk pseudocapacitance can provoke a research on boosting the pseudocapacitive response of battery materials to alleviate the lack of the alternative pseudocapacitive materials.

Reducing the size of active materials to nano-scale, i.e., nanostructured material, as one of the strategies, can be used to achieve the bulk pseudocapacitance. For instance, LiCoO₂ nanocrystalline with the sizes of ˂11 nm exhibits a pseudocapacitive response. In addition to nanostructured materials, a strategy named diffusionless-like conversion reaction to achieve pseudocapacitive response in conversion reaction was proposed. Based on the conversion reaction between Fe(OH)₂ and δ-FeOOH, a battery-like conversion reaction achieves a high specific capacitance, and a diffusionless-like transformation between charge and discharge phases without a massive atom movement presents high rate and stability performance.

For battery materials, their pseudocapacitive response can be enhanced via manipulating the charge carrier transport, electron transfer, and designing the interaction between charge carrier and electrode in atom-scale. In addition, such a provoking pseudocapacitive response in battery materials extends the scope of bulk pseudocapacitive materials. Li and co-workers enhanced the Li⁺ transport by pre-intercalation of NH₄⁺ into interplay of Mo₂CT_x. The mechanism of FeHCF storing NH₄⁺ is clarified, indicating that the weak Fe—N interaction mitigates the volumetric expansion induced by NH₄⁺ insertion. These findings demonstrate that modulating on charge carrier transport and interaction between charge carrier and electrode can improve the pseudocapacitive response of battery materials.

Lithium-ion batteries (LIBs) dominate the field of energy storage due to their high specific energy density and long cycle life. However, the scarcity of lithium resources, potential safety issues, and high cost severely restrict their further large-scale energy storage applications. It is thus urgent to investigate other battery systems beyond LIBs. Alkali metal ions (Na⁺ and K⁺) with higher abundance and multivalent charge carriers (i.e., Zn²⁺, Mg²⁺, Al³⁺, etc.) have attracted much attention. Although sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) have the similar chemical properties to Li, they have lower energy density (i.e., 100-120 W·h/kg, 150-170 W/kg), toxic and flammable electrolytes, high operating costs, and safety hazards. Magnesium-ion batteries (MIBs) and aluminum-ion batteries (AIBs) involve multi-electron redox reactions. Although they can theoretically achieve greater specific capacity and energy density, the positive electrode materials available are just a few compounds, affecting their overall development. In addition, the passivation of the Mg anode greatly prevents a further transport of Mg²⁺. AIBs are in the primary development stage because of the formation of Al₂O₃ layers on the anode, leading to the corrosion of aluminum electrodes, decreased battery efficiency and low cycling stability.

Among all available energy storage systems, aqueous zinc-ion batteries (AZIBs) are considered as a most promising large-scale energy storage due to their lower cost, higher cycling stability, aqueous electrolyte, and safe battery manufacturing process. AZIBs can directly use metallic zinc as a negative electrode material, and they also have unique advantages, such as high theoretical capacity (i.e., 820 mA·h/g and 5 855 mA·h/cm²), low redox potential (i.e., -0.76 V vs. SHE), abundant crustal reserves (i.e., 300 times greater than that of lithium), and good stability in the presence of oxygen and humid atmosphere. Multivalent AZIBs allow multiple electron transfers during the electrochemical reaction, providing an opportunity for achieving high energy and power density. In addition, compared to flammable organic electrolytes, the aqueous electrolytes typically used in AZIBs have lower cost, safer characteristics, and higher ionic conductivity.

AZIBs have been developed since the invention of the Voltaic Pile (Zn-Ag) by Alessandro Volta. In 1870-1969, a large-scale production led to the invention of zinc-carbon batteries (i.e., Zn|NH₄Cl|MnO₂) and alkaline zinc-manganese batteries (i.e., Zn|KOH|MnO₂), which are still used. In 1986, Yamamoto used weak acidic electrolytes instead of alkaline electrolytes to improve the reversibility of zinc-manganese batteries. In late 2011, Kang proposed a concept of "zinc-ion batteries" and confirmed the reversible insertion/extraction reaction of zinc ions in manganese dioxide in a zinc sulfate electrolyte system. Since then, various rechargeable battery systems based on near-neutral aqueous electrolytes (i.e., zinc-manganese, zinc-vanadium, zinc-cobalt, and zinc-iodine) have been developed. The existing work on AZIBs deals with aqueous solutions containing ZnSO₄, ZnCl₂, or Zn(CF₃SO₃)₂ as an electrolyte with a high safety and a reversibility.

There are numerous reports on the extensive research on AZIBs, i.e., zinc metal anode, cathode materials, electrolyte engineering, and potential applications. However, there exist still some challenges in the development of these batteries. For instance, the cathode materials suffer from the issues like structural instability, poor conductivity, and dissolution. The zinc anode has inevitable dendrite formation, hydrogen evolution reactions, surface corrosion, and passivation. The aqueous electrolyte has a low voltage window and corresponding parasitic reactions. In addition, most studies are also conducted under mild laboratory conditions without considering the entire battery system, including inactive components.