Fabrication and magneto-optical property of yttria stabilized Tb2O3 transparent ceramics

(Tb_0.5Y_0.5)₂O₃ transparent ceramics have been prepared by wet chemical co-precipitation route and flowing H₂ atmosphere sintering. The optical quality, microstructure and magneto-optical properties of the ceramics were investigated. No sintering aids or milling were adopted in the ceramic fabrication processing. The cold isostatic pressed green compact could be sintered to be transparent (Tb_0.5Y_0.5)₂O₃ ceramic at 1800 ℃ in flowing H₂ atmosphere. The mechanical polished ceramic showed the good transmittance from visible to near infrared wavelength, corresponding to a 71.9% transmittance at 1400 nm wavelength. The Verdet constant measured at 632.8 nm of the (Tb_0.5Y_0.5)₂O₃ transparent ceramics was -220.19 rad T⁻¹ m^-1, which was 1.64 times that of Tb₃Ga₅O₁₂ single crystal.     

Fabrication of transparent Tb3Al5O12 ceramics by hot isostatic pressing sintering

Tb₃Al₅O₁₂ (TAG) transparent ceramics were prepared by a reactive sintering method using presintering in a muffle furnace combined with hot isostatic pressing (HIP) sintering. The dilatometric, differential scanning calorimetry‐thermogravimetric (DSC‐TG) curves and optical quality were investigated. The microstructure evolution of the TAG ceramic samples was clarified. Two successive transformations were found to generate a TAG phase, as observed in the dilatometric and DSC‐TG curves and XRD patterns of TAG ceramics sintered at different temperature. The changes in average grain size and densification suggest that a 1600°C presintering temperature is suitable for HIP. The optical transmittance of the obtained 0.4 wt% TEOS:TAG transparent ceramics, which were fabricated by a new two‐step sintering of presintering at 1600°C in a muffle furnace followed by HIP at 1650°C, can reach above 80% in the visible (vis) and near‐infrared (NIR) regions. Its transmittance was very close to the theoretical limit. To the best of our knowledge, this is the first time that TAG transparent ceramics with ideal optical quality were obtained without vacuum sintering.     

Conventional HP sintering of asymmetric hexagonal structure Yb3+-doped Sr5(PO4)3F transparent ceramic without additives

The 2 at.% Yb³⁺:Sr₅(PO₄)₃F (S-FAP) polycrystalline transparent ceramic with asymmetric hexagonal structures has been synthesized by vacuum hot-pressing the nanoparticles prepared via coprecipitation method. X-ray diffraction results of powder and ceramic indicate that their phase peaks are well matched to the crystal structure of S-FAP. The average particle size of 35.5 nm has been exhibited by powder scanning electron microscopy images, and subsequent images of the ceramic cross section and surface morphology revealed a homogenous and compact microstructure with an average grain size of around 220 nm. The relationship between the optical loss caused by the scattering of anisotropic ceramic grains and the optical transmittance of ceramics was revealed in the hexagonal S-FAP transparent ceramics with different thicknesses. The in-line transmittance of hot-pressed ceramics with 1.5-mm thickness achieved 79.95% at 1100-nm wavelength, and the room-temperature absorption and emission spectra of Yb³⁺ in S-FAP polycrystalline ceramic matrix were measured using a spectrofluorometer.     

Vacuum sintering of transparent Cr:Y2O3 ceramics

0–0.7 at% Cr:Y₂O₃ transparent ceramics were prepared by vacuum sintering. The optimum in-line transmittance in the visible and near infrared region is 78%, and the Vickers hardness of the sintered 0.1 at% Cr:Y₂O₃ is 10.1 GPa, respectively. The mechanism of Cr-doped and the optical properties has been discussed. The results indicated that the Cr:Y₂O₃ transparent ceramic is a promising laser material with enhanced mechanical property.     

Transparent ultrafine Yb3+:Y2O3 laser ceramics fabricated by spark plasma sintering

Conventional ceramic processing techniques do not produce ultrafine‐grained materials. However, since the mechanical and optical properties are highly dependent on the grain size, advanced processing techniques are needed to obtain ceramics with a grain size smaller than the wavelength of visible light for new laser sources. As an empirical study for lasing from an ultrafine‐grained ceramics, transparent Yb³⁺:Y₂O₃ ceramics with several doping concentrations were fabricated by spark plasma sintering (SPS) and their microstructures were analyzed, along with optical and spectroscopic properties. Laser oscillation was verified for 10 at.% Yb³⁺:Y₂O₃ ceramics. The laser ceramics in our study were sintered without sintering additives, and the SPS produced an ultrafine microstructure with an average grain size of 261 nm, which is about one order of magnitude smaller than that of ceramics sintered by conventional techniques. A load was applied during heating to enhance densification, and an in‐line transmittance near the theoretical value was obtained. An analysis of the crystal structure confirmed that the Yb³⁺:Y₂O₃ ceramics were in a solid solution. To the best of our knowledge, this study is the first report verifying the lasing properties of not only ultrafine‐grained but also Yb‐doped ceramics obtained by SPS.     

Phase-separation engineering in fluorozirconate glass for designing and fabricating of transparent perfluorinate glass ceramic

Perfluorinate glass ceramics with ultra-low phonon energy are very important optical and photonic materials. Unfortunately, there is no suitable method to obtain transparent perfluorinate glass ceramics due to poor thermal stability of fluoride glass. As a result, wide applications of glass ceramics in advanced infrared systems are restricted. Here, an effective method based on phase-separation engineering is used to develop transparent perfluorinate glass ceramics. In this article, a novel transparent Er³⁺-doped ZnZrF₆-Ba₆Zn₇F₂₆ perfluorinate glass ceramic was designed and fabricated by phase-separation engineering. The sample exhibits low phonon energy (564 cm⁻¹), ultra-wide transmission range (0.33–8.2 μm, T ≥ 50 %), and strong infrared emission, which is better than that of ZBLAN glass, oxide-, and oxyfluoride-glass ceramics. These good properties of the perfluorinate glass ceramic demonstrate that phase-separation engineering not only offers an effective approach to obtain perfluorinate glass ceramics but also provides wide-ranging opportunities for advanced infrared optical and photonic materials.     

Photochromic effect of transparent lead-free ferroelectric KSr2Nb5O15 ceramics

Transparent KSr₂Nb₅O₁₅ (KSN) lead-free ferroelectric ceramics have been synthesized via modified pressureless sintering method. A significant photochromic effect was observed for the transparent KSN ceramics prepared without rare-earth dopant modification. The piezoelectric properties depend on the grain orientations were investigated. The optical transmittance of the KSN ceramics is greater than 40% in the wavelength range of 530–800 nm. After NUV irradiation, the absorbance was enhanced by more than 40% in a broad visible range (more than 79%). The absorbance returned to the initial value after a thermal bleaching process. The results of the cycling tests and response experiments showed the stability and saturation of the photochromic effect. In addition, the possible photochromic mechanism of the KSN ceramics is discussed and the photochromic centers are identified. This transparent KSN ceramics exhibits an obvious photochromic effect and is a potential candidate materials for optical data storage and information recording applications.     

New KNbTeO6 transparent tellurate ceramics

New transparent defect pyrochlore KNbTeO₆ ceramics were successfully prepared by Spark Plasma Sintering (SPS) of same composition polycrystalline powders elaborated by classic solid-state reaction from oxide precursors (K₂CO₃, Nb₂O₅, TeO₂) and followed by high energy milling powders. As such precursors are not available as commercial nanopowders, a suitable process has been developed by combining solid-state reactions and high energy milling. The determination of appropriate consolidation conditions and sintering parameters of the green body such prepared, are described in this paper. The resulting ceramic is transparent in both the visible and near infrared range (up to 5.5 μm). The maximum of transmittance is reached in the near infrared region around 2500 nm with a value of 78 % (1 mm thick sample), close to the maximum theoretical value of transmittance. This transparent KNbTeO₆ ceramic demonstrates a homogeneous and dense microstructure with an average grain size less than 500 nm. A small content of secondary phase has been detected by nanoscale observations without drastic effects on transparency. This ceramic exhibits very good mechanical properties similar to the Y₂O₃ transparent ceramic, as well as interesting dielectric properties in the microwave range. This innovative method should drive the development of new transparent materials with technologically relevant applications.     

Transparent AlON ceramic combined with VO2 thin film for infrared and terahertz smart window

Transparent AlON ceramics are intriguing window materials with excellent mechanical strength and superb optical transparency from the near ultraviolet to the mid-infrared range. However, previous studies focused their investigations in the visible range; therefore, the application of transparent AlON ceramics to tunable windows has yet to be reported. In this work, a VO₂ thin film with a characteristic semiconductor-metal phase transition (SMT) was fabricated on a transparent AlON ceramic, which exhibited remarkable tunable switching properties in the infrared and terahertz ranges. The transparent AlON ceramic was prepared by a two-step method, which included carbothermal reduction and pressureless sintering. The resulting ceramic exhibited high transparency of over 70% in the visible-infrared range and a notable THz transmission of 64.5–73.9% at 0.1–1.5?THz. The VO₂ thin film was prepared on a transparent AlON ceramic using the sol-gel method and showed excellent optical and electric switching performance. The square resistance variance was close to four orders of magnitude, and an infrared switching ratio of over 40% was obtained. Furthermore, the combined structure showed an efficient THz switching ratio of approximately 70.9%. This study proposes a composite material combined with a transparent ceramic and a phase transition oxide and provides great insights into their application in infrared and terahertz smart windows as well as in switching devices.     

Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl₂O₄ powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl₂O₄ powder was modified by introducing Ga³⁺, and stable MgAl_1.9Ga_0.1O₄ aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s^-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl_1.9Ga_0.1O₄ and MgAl₂O₄ transparent ceramics were systematically compared. It is indicated that the transparent MgAl_1.9Ga_0.1O₄ ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm^-1 at 5 µm, which could be ascribable to the fact that Ga³⁺ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al³⁺. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.     

Enhanced optical transmittance and energy-storage performance in NaNbO3-modified Bi0.5Na0.5TiO3 ceramics

Dielectric ceramics with both excellent energy storage and optical transmittance have attracted much attention in recent years. However, the transparent Pb-free energy-storage ceramics were rare reported. In this work, we prepared transparent relaxor ferroelectric ceramics (1 − x)Bi_0.5Na_0.5TiO₃–xNaNbO₃ (BNT–xNN) by conventional solid-state reaction method. We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺ to Ti³⁺. As a result, a high recoverable energy-storage density of 5.14 J/cm³ and its energy efficiency of 79.65% are achieved in BNT–0.5NN ceramic at 286 kV/cm. Furthermore, NN-doping can promote the densification to improve the optical transmittance of BNT, rising from ∼26% (x = 0.2) to ∼32% (x = 0.5) in the visible light region. These characteristics demonstrate the potential application of BNT–xNN as transparent energy-storage dielectric ceramics.     

Improved ultraviolet -visible optically transmittance and luminescent properties in Yb,Lu: Sr5(PO4)3F transparent ceramics via Lu3+ co-doping

The asymmetric hexagonal Sr₅(PO₄)₃F (S-FAP) crystal material is considered to be the most suitable solid state laser gain medium for small laser diode pumping in the future due to its large absorption, emission interface, and long fluorescence lifetime. However, the mediocre optical transmittance of S-FAP transparent ceramics and the degradation of luminescence properties due to the doping of Yb activated ions seriously hinder its application prospects. In view of this, a series of 0.02Yb, xLu: S-FAP (x = 0–0.02) transparent ceramics with excellent optical properties were synthesized by hot pressing sintering. The powder SEM results show that Lu doping has no obvious effect on the morphology, grain size, and dispersion of powder. The linear transmissivity curves show that the ultraviolet (200 nm) and visible (500 nm) transmissivity increase by 54 % and 17 %, respectively, with Lu doping compared with the undoped ceramic samples. The surface SEM of ceramics revealed that Lu³⁺ promoted the increase of ceramic grain size significantly. The emission spectrum and fluorescence decay curves at room temperature also show that the emission intensity and fluorescence lifetime of ceramic samples increase significantly with Lu co-doping.     

Synthesis and luminescence characterization of Pr3+, Gd3+ co-doped SrF2 transparent ceramics

Pr³⁺, Gd³⁺ co-doped SrF₂ transparent ceramic, as the potential material for visible luminescent applications, was prepared by hot-pressing of precursor nanopowders. The microstructure, phase compositions, and in-line transmittance, as well as the photoluminescence properties were investigated systematically. Highly optical quality Pr,Gd:SrF₂ transparent ceramic with nearly pore-free microstructure was obtained at 800°C for 1.5 hours. The average in-line transmittance of the x at.% Pr, 6 at.% Gd:SrF₂ (x = 0.2, 0.5, 1.0, 2.0) transparent ceramics reached to 87.3 % in the infrared region. The photoluminescence spectra presented intense visible light emissions under the excitation of 444 nm, the main intrinsic emission bands located at 483 and 605 nm, which were attributed to the transitions of Pr³⁺: ³P₀ → ³H₄ and ¹D₂ → ³H₄, respectively. With the co-doping of Gd³⁺ ions, the emission intensity of the Pr:SrF₂ transparent ceramic was greatly enhanced. All the emission bands of x at.% Pr, 6 at.% Gd:SrF₂ transparent ceramics exhibited the highest luminescence intensity with the 1.0 at.% Pr³⁺ doping concentrations, whereas the lifetimes decreased dramatically with the Pr³⁺ doping contents increasing from 0.2 to 2.0 at.% due to its intense concentration quenching effect. The 1 at.% Pr, 6 at.% Gd:SrF₂ transparent ceramic is a promising material for visible luminescent device applications.     

Enhanced transmittance of Y2O3 transparent ceramics from near-UV to near-infrared through reduced atmosphere annealing

Y₂O₃ transparent ceramics were annealed under different atmospheric conditions. The samples annealed in H₂ containing atmosphere were colorless and had high in-line transmittances from the near-UV to the mid-infrared wavelength range. This is due to the elimination of carbon contamination and preventing the formation of high concentration oxygen interstitial defects in the sintered samples. Annealing in oxygen containing atmosphere resulted in stronger optical absorption in the visible wavelength region. High temperature annealing in O₂ or hot isostatic pressing under high partial pressure of O₂ (O₂ HIP) led to obviously declining of transparency in a broader wavelength range of 230–800 nm. The Er:Y₂O₃ ceramics annealed in H₂ containing atmosphere had high in-line transmittance of about 80% at 400 nm as well. Room temperature laser oscillation at 2.7 µm was also obtained on the 5%H₂/95%Ar atmosphere annealed Er:Y₂O₃ ceramics.     

Direct tape casting of Al2O3/AlN slurry for AlON transparent ceramic wafers via one-step reaction sintering

In this work, transparent aluminate oxynitride (AlON) ceramic wafers were successfully fabricated by the direct non-aqueous tape casting of Al₂O₃/AlN slurry and the one-step reaction sintering for the first time. The reaction sintered AlON ceramic wafer exhibits high transmittance of 73.2 % at the wavelength of 1600 nm. This fabricating route realizes smooth and flexible tape without cracks or pinholes in Al₂O₃/AlN system and efficiently shortens the preparation cycle of transparent AlON wafers, which is a feasible way to prepare high-quality transparent AlON ceramics with large lateral sizes and thin thicknesses by reaction sintering, might also promote the application of transparent AlON ceramic wafers.     

Optical properties of transparent polycrystalline ruby (Cr:Al2O3) fabricated by high-pressure spark plasma sintering

Doped transparent ceramics with high optical quality can serve as materials for photonic applications such as laser gain media. In that regard, transparent polycrystalline alumina has potential for high-power applications due to its excellent physical and chemical properties, combined with unique doping possibilities. However, optical birefringence of Al₂O₃ crystals make achieving sufficiently high optical transmittance a processing challenge. In the present study, we demonstrated fabrication of highly transparent 0.5 at.% Cr:Al₂O₃ ceramics by high-pressure spark plasma sintering (HPSPS). The optical properties of these polycrystalline ruby ceramics were analyzed in order to assess possible laser operation (at 694.3 nm). The obtained ceramics exhibit high in-line transmittance (～72.5 % at 700 nm), equivalent to a scattering coefficient of 2.15 cm^?1, and characteristic ruby photoluminescence. The theoretically estimated lasing threshold and percentage of absorbed pump power indicate that such ruby ceramic lasers could operate at reasonable thresholds of 80?225 mW with short lengths of 0.5?5 mm. Thus, HPSPS is a promising method for producing laser-quality doped transparent ceramics for compact laser systems.     

Influence of Yb Concentration on the Optical Properties of CaF2 Transparent Ceramics Codoped with Er and Yb

Er, Yb:CaF₂ nanoparticles with different Yb concentrations were synthesized by a coprecipitation method using nitrates as raw materials. X‐ray powder diffraction and transmission electron microscopy analysis showed that the nanoparticles were single fluorite phase and the nanoparticle size was found to decrease with increasing Yb concentrations. The obtained nanoparticles were hot‐pressed at 800°C under 30 MPa under vacuum environment to fabricate Er, Yb:CaF₂ transparent ceramics. The influence of Yb ion concentrations on the optical transmission, microstructure, and luminescence properties of Er, Yb:CaF₂ transparent ceramics were investigated. The addition of Yb ions was found effectively to reduce grain size and has a positive effect on improving the optical transmission of Er, Yb:CaF₂ transparent ceramics. The highest transmittance in the near‐infrared spectral region of the Er, Yb:CaF₂ transparent ceramic reached about 90%. The green, red, and near‐infrared emission intensities were found to increase with increasing Yb concentration.     

Fabrication of Transparent Dysprosium Aluminum Garnet (Dy3Al5O12) Ceramics via a Solid‐State Reaction Method

Polycrystalline, transparent Dy₃Al₅O₁₂ ceramics were firstly fabricated by a solid‐state reaction method using high‐purity Dy₂O₃ and Al₂O₃ powders. The fully dense Dy₃Al₅O₁₂ ceramic with an average grain size of less than 10 μm was obtained by vacuum sintering at 1820°C for 6 h. The in‐line transmittance of the optimized sample reaches 80% in the visible region. Scanning electron microscopy reveals that no secondary phases and almost no pores are observed at grain boundaries or triple junctions, and the fracture mode of the ceramic is mainly transgranular. The Dy₃Al₅O₁₂ ceramic is promising for magneto‐optical applications. Verdet constant of the Dy₃Al₅O₁₂ transparent ceramic is as high as ?0.41 min·(Oe·cm)^?1.     

Fabrication of transparent electro-optic (K0.5Na0.5)1−xLixNb1−xBixO3 lead-free ceramics

Lead-free transparent electro-optic ceramics (K_0.5Na_0.5)_1?xLi_xNb_1?xBi_xO₃ have been fabricated by hot-press sintering. Owing to the effective suppression of grain growth, the Li and Bi co-modified ceramics generally possess a dense and fine-grained structure. The co-modification also causes the ceramics to transform into a nearly cubic structure with minimal optical anisotropy. A diffuse phase transformation is also induced, causing the ceramics to become more relaxor-like and contain more polar nano-regions. These would reduce the light scattering by the grains, at the grain boundaries and at the domain walls, respectively, and thus making the ceramics become optically transparent. For the ceramic modified with 5 mol% Li⁺ and Bi⁵⁺, the optical transmittance reaches a high value of 60% in the near-IR region. The ceramics also exhibit a strong linear EO response, giving a large effective linear EO coefficient in the range of 120–200 pm/V.     

Mixed precipitants derived nanocrystalline powders and RE doped LuAG transparent ceramics

Lu₃Al₅O₁₂ (LuAG) nanocrystalline powders were synthesized by using ammonium hydroxide (NH₄OH, AH) and ammonium hydrogen carbonate (NH₄HCO₃, AHC) as mixed precipitant. In the absence of sintering aids such as TEOS, MgO or ZrO₂, the obtained LuAG powders showed good sinterability in H₂ atmosphere (PLSH) at low temperature. The in-line transmittance of LuAG ceramic reached 81% in the whole visible light band from 400 nm to 800 nm. The average grain size of obtained transparent ceramics was ranged in 1–6 μm at different sintering temperatures by PLSH. Various kinds of rare earth ions, such as Nd, Yb, Ce, Pr, and Tm doped RE:LuAG transparent ceramics could be prepared by PLSH technology without sintering aids and HIP post-treatment. Through PLSH technology, RE:LuAG transparent ceramics show high optical quality and large aperture size.