Effects of pre-sintering and annealing on the optical transmittance of Zr-doped Y2O3 transparent ceramics fabricated by vacuum sintering conjugated with post-hot-isostatic pressing

In this work, Zr-doped Y₂O₃ transparent ceramics were fabricated by vacuum pre-sintering at various temperatures ranging from 1650 to 1800 °C combined with a post-hot-isostatic pressing (HIP) treatment. The pre-sintered samples all show black opaqueness mainly due to the formation of oxygen vacancies, which can be removed by post-annealing in air. The HIP treatment can also eliminate the blackness as well as residual pores, giving rise to optical transparency. The in-line transmittance of the samples after HIP was found to depend strongly on the microstructure of the as-sintered samples. The optimal microstructure of these types of samples before the HIP treatment should be fine grained with only intergranular pores, which can easily be removed by HIP. Annealing before HIP was shown to be necessary to enhance the transmittance of the samples. The effects of the pre-sintering heating rate on the optical transmittance of the HIP-treated samples were also studied.     

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.     

Coating Y2O3 nano-particles with ZrO2-additive via precipitation method for colloidal processing of highly transparent Y2O3 ceramics

In the present work, transparent Y₂O₃ ceramics were prepared via colloidal processing method using ZrO₂-coated nano-sized Y₂O₃ powders. The chemical precipitation method was adopted for the coating of Y₂O₃ raw powder. The evolution of the coated-ZrO₂ layer upon calcination was studied. The rheological behaviors of the slurries of Y₂O₃ powders coated with different content of ZrO₂-additive were investigated. The pH_IEP of ZrO₂-coated Y₂O₃ powders shows intermediate values between that of raw Y₂O₃ and ZrO₂ powders. As the ZrO₂-coating concentration increased from 0 to 5.0 at%, the magnitude of the negative zeta potential at pH > pH_IEP shows a general trend of increment, whereas it decreased at pH < pH_IEP. The viscosity decreases pronouncedly with the increase of ZrO₂ content from 0.5 at% to 3.0 at%. The suspensions with low viscosity and high stability was achieved for a solid loading of 35.0 vol% using Y₂O₃ powders coated with 5.0 at% ZrO₂. The dispersed suspensions were consolidated by centrifugal casting method and the green bodies shown improved homogeneity. Transparent Y₂O₃ ceramics were fabricated by vacuum sintering at 1800 ℃ for 5 h. Transmittance at wavelength 800 nm (1.0 mm thick) reached 80.8%, close to the theoretical value of Y₂O₃.     

New optical ceramics: High-entropy sesquioxide X2O3 multi-wavelength emission phosphor transparent ceramics

Highly transparent X₂O₃ sesquioxide ceramics were obtained from a solid solution of five different oxides (Lu₂O₃, Y₂O₃, Yb₂O₃, Gd₂O₃, and Dy₂O₃), mixed in an equal molar ratio according to the principle of high-entropy. The fabricated (Lu, Y, Yb, Gd, Dy)₂O₃ ceramics achieved 99.97 % of the relative density and exhibited a high degree of optical transparency with the in-line transmittance of almost 80 % in the visible wavelength range. Emissions of Gd³⁺ (⁶P_J → ⁸S_7/2 at 312 nm), Dy³⁺ (⁴F_9/2 → ⁶H_15/2 at 492 nm and ⁴F_9/2 → ⁶H_13/2 at 572 nm), and Yb³⁺ (²F_5/2 → ²F_7/2 at 1031 nm) suggested a potential application of the high-entropy ceramics as multi-wavelength emission phosphor transparent ceramics. High-entropy ceramics also exhibited lower specific heat and thermal conductivity compared to single-element sesquioxide ceramics. This work demonstrated that highly transparent oxide ceramics, with complex chemical compositions and good optical properties, could be obtained using the high-entropy principle.     

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 and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Effects of ZrO2 on the photoluminescence of 0.5 at% Eu:(Y0.9La0.1)2O3 transparent ceramics

Zr-codoped 0.5 at% Eu: (Y_0.9La_0.1)₂O₃ ceramics sintered in H₂-reducing atomsphere, together with the ceramics with annealing treatment, were fabricated by solid-state reactions and the effects of Zr codoping on these materials’ photoluminescence examined. The obtained emission spectra showed that Zr codoping adjust the materials’ photoluminescence with UV excitation and a logical explanation was proposed. The results suggested that an Eu-doped, yttrium-lanthanum oxide transparent ceramic with Zr in low concentration appeared to have promising potential in modern lighting applications.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Spark plasma sintering of Sm3+ doped Y2O3 transparent ceramics for visible light lasers

Transparent Sm:Y₂O₃ ceramics were fabricated by spark plasma sintering (SPS). The effects of LiF additive and sintering temperature on the microstructure and optical transmittance of the Sm:Y₂O₃ ceramics were investigated. The optimal content of LiF additive and sintering temperature was found to be 0.3 wt% and 1500 ℃. The transmittance of Sm:Y₂O₃ ceramics with a thickness of 1.7 mm reached 75.3% at 609 nm, which is about 94% of the theoretical value. The average grain size of the sample was about 50 µm.     

A leaf-like structured ITO conductive transparent thin film from visible to near-infrared region with enhanced stability

ITO thin films as the optical and electrical windows to transform photons and charges have been applied in many areas. Here, a leaf-like structured particle is composed of small particles growing along three different orientations leading to low thermal stress accompanied by well transmittance (85%) in a wide wavelength range from visible to near-infrared region and a narrowed band gap 3.07 eV. The evolution of structure and electronic performance was studied to obtain the low resistivity (12 μΩ m) and enhanced stability of the film (1000 °C). The leaf-like structure can be maintained under 600 ℃ and the electrical properties can be modified in He and N₂ atmosphere, owing to the reduced defects, increased concentration of Sn and carrier mobility. Although the structure has changed after being annealed at 1000 °C in N₂, the thin film performs excellent electrical properties (?3.44 × 10²⁰ cm^?3 and 28 cm² V^?1 s^?1).     

Fabrication and laser oscillation of Yb:Sc2O3 transparent ceramics from co-precipitated nano-powders

Ytterbium doped scandium oxide (Yb:Sc₂O₃) transparent ceramics were fabricated by a co-precipitation and vacuum sintering method. The characteristics of the precursor and the calcined powders were investigated by BET, XRD, and SEM. Ultra-fine and low agglomerated 5at%Yb:Sc₂O₃ powders with the average particle size about 65.4 nm were obtained after calcined at 1100 °C for 5 h. Using the synthesized powders as starting materials, 5at%Yb:Sc₂O₃ transparent ceramics with the in-line transmittance of 71.1% at 1100 nm and average grain size of 145 μm were fabricated by vacuum sintering at 1825 °C for 10 h. Quasi-CW laser oscillation of Yb:Sc₂O₃ ceramics was obtained at 1040.6 nm. A maximum output power as high as 2.44 W with a corresponding slope of 35% was achieved. Finally, the tunability of the ceramic was explored measuring a tuning range up to 55 nm.     

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.     

Fabrication of Lu2O3:Eu transparent ceramics using powder consisting of mono-dispersed spheres

Mono-dispersed spherical Lu₂O₃:Eu (5 mol%) powders for transparent ceramics fabrication were synthesized by urea-based homogeneous precipitation technique. The effects of the doped-Eu³⁺ on the synthesis of Lu₂O₃:Eu particles were investigated in detail. The results show that the doping of Eu³⁺ ions into Lu system can significantly decrease the particle size of the resultant precursor spheres. Owing to the sequential precipitation in Lu/Eu system, there are compositional gradients within each of the resultant precursor spheres. Well dispersed, homogeneous and spherical/near spherical Lu₂O₃:Eu powders were obtained after calcination at 600–1000 °C for 4 h. The powder calcined at 600 °C shows better sintering behavior and can be densified into transparent ceramic after vacuum sintering at 1700 °C for 5 h. The luminescence properties of the obtained Lu₂O₃:Eu powder and transparent ceramic were also studied.     

Photoluminescence and optical properties of Eu3+/Eu2+-doped transparent Al2O3 ceramics

Transparent Eu³⁺-doped (0.05–0.15 at. %) alumina ceramics with fine-grained microstructure were prepared and studied in terms of optical properties and photoluminescence (PL). The light transmission through ceramics up to dopant concentrations 0.125 at. % is dominated by birefringence scattering at grain boundaries. As confirmed by HRTEM/EDS element mapping, high photoluminescence intensity was achieved as the result of the dopant segregation at grain boundaries. The PL emission spectra of Al₂O₃:Eu³⁺ ceramics exhibited red light emissions with the highest intensity (394 nm excitation) for material containing 0.125 at. % of Eu³⁺. The luminescence decay was single-exponential with a lifetime ～1.5 ms. The post-sintering reduction of Eu³⁺→Eu²⁺ under an H₂ atmosphere (at 1300 °C) was difficult. Two simultaneously coexisting Eu²⁺ emitting PL centers were identified, one emitting blue light with average decay constant of 150 ns, and the other green light (more intense) with average decay constant of 1.3 μs.     

Preparation of multicomponent A2Zr2O7 transparent ceramics by vacuum sintering

Recently, high-entropy oxide ceramics have become a hot topic in the field of high entropy materials. In this paper, multicomponent pyrochlore A₂Zr₂O₇ transparent ceramics were prepared via vacuum sintering using combustion synthesized nanopowders. The phase analysis results indicate that the powders exhibit defective fluorite structure and the ceramics are in pyrochlore structure. The structural order degree of ceramics varies with the increase of incorporated components. It is found that the grain size of A₂Zr₂O₇ ceramics is related with the component of A-site. The main fracture mode of final ceramics exhibit typical transgranular fracture. The multicomponent A₂Zr₂O₇ ceramics exhibit excellent optical transmittance, and the highest in-line transmittance reaches to 80% for #A2ZO ceramic at 1880 nm.     

Crystal structure evolution and luminescence property of Ce3+-doped Y2O3-Al2O3-Sc2O3 ternary ceramics

Ce³⁺-doped Y₂O₃-Al₂O₃-Sc₂O₃ ternary ceramics with varying Sc₂O₃ were prepared via solid-state-reactive method. Their constitutions, crystal structures, microstructures and luminescence properties were thoroughly investigated by powder X-ray diffraction, ⁴⁵Sc and ²⁷Al solid-state nuclear magnetic resonance (NMR), scanning electron microscope (SEM) as well as photoluminescence (PL) characterizations. Single-phased YSAG transparent ceramics with garnet structure and homogeneous microstructures can be obtained. The NMR results suggest that partial [AlO₆] octahedrons are converted into [AlO₄] tetrahedrons with increased Sc³⁺. Partial Sc³⁺ occupies the remaining octahedral sites, the other Sc³⁺ occupies the dodecahedral sites. The luminescence spectra of the as-prepared ceramics show broadened emission bands with excessive Sc³⁺ incorporation. More significantly, the solubility of Ce³⁺ in garnet ceramics extensively increases due to the incorporation of Sc³⁺, leading to a considerable red-shift and broadening of emission spectra with little luminescence decline, indicating that the new ceramic phosphors are promising for white-light illumination with improved color rendering index.     

Fabrication and characterization of highly transparent Y2O3 ceramics by hybrid sintering: A combination of hot pressing and a subsequent HIP treatment

We succeeded in the optimization of highly transparent Y₂O₃ ceramics with a submicrometer grain size approximately 0.6?μm by hot pressing (1300–1550?°C) and a subsequent HIP (1450?°C) treatment using commercial Y₂O₃ powders as starting powders and ZrO₂ as a sintering additive. The optimum microstructure for the HIP treatment was prepared by hot pressing at a temperature as low as 1400?°C for 3?h with a relative density of 99.3%. The thus HIP-treated specimen showed the best transmittance (2?mm thick) ever reported of 83.4% and 78.3% at 1100 and 400?nm, respectively. Specifically, the transmittance using this hybrid sintering method improved substantially in the visible range compared to that of the counterpart using hot pressing only. A simulation of the transmittance based on the Beer-Lambert law and Mie scattering theory has proved that this improvement is mainly due to the elimination of nanopores below 15?nm in size.     

Highly transparent yttrium titanate (Y2Ti2O7) ceramics from co-precipitated powders

Highly transparent yttrium titanate (Y₂Ti₂O₇) ceramics were fabricated by vacuum sintering using co-precipitated powders for the first time. The effects of the powder calcination temperature on the phase composition, morphology of the calcined powders, and on the microstructure and transmittance of the Y₂Ti₂O₇ ceramics were investigated. When the calcination temperature was above 850 °C, pure phase Y₂Ti₂O₇ nanopowders with high sintering activity were obtained. Transparent Y₂Ti₂O₇ ceramics were obtained after vacuum sintered at 1600 °C for 6 h and annealed at 1100 °C for 5 h in air. The highest transmittance reached 73% at 1000 nm when the calcination temperature was 1150 °C. The measured refractive index of Y₂Ti₂O₇ ceramics was higher than 2.24 at the wavelength range of 350–1000 nm, making it a promising candidate for optical devices.     

Microstructural freezing of highly NIR transparent Y2O3-MgO nanocomposite via pressure-assisted two-step sintering

Y₂O₃-MgO nanocomposite has attracted attention for use in infrared windows and domes. However, the large difference in refractive index between the two phases of this materials phases induces inevitable grain boundary scattering. To overcome this drawback, it is indispensable to greatly reduce the grain size and eliminate residual pores. Two-step sintering has been extensively used to produce fine-grained ceramics, but long incubation time and an additional process are required. As a robust approach, pressure-assisted two-step consolidation of Y₂O₃-MgO nanocomposite ceramics is investigated to achieve full density while retaining domain size. A sintering path for pressure-assisted two-step sintering and related kinetic window are obtained for the first time. By effectively suppressing grain growth and eliminating residual pores via two-step hot-pressing, outstanding mid- and near-infrared transmittance were achieved. The results indicate that pressure-assisted two-step sintering is a promising alternative strategy that can decrease the domain size and achieve full density of ceramics.     

Characterizations of vacuum sintered Gd2Zr2O7 transparent ceramics using combustion synthesized nanopowder

Highly transparent Gd₂Zr₂O₇ ceramic was fabricated by vacuum sintering using combustion synthesized nanopowder with mean particle size of about 80 nm. The morphology and structure were analyzed by X-ray diffractometer, scanning electron microscopy, Raman spectroscopy and transmission electronic microscopy. The Gd₂Zr₂O₇ nanopowder and transparent ceramic are both in low ordered pyrochlore structure. The effects of sintering temperature on the density and transmittance of Gd₂Zr₂O₇ ceramic were investigated, and the optimum sintering temperature (1825 °C) was obtained. Gd₂Zr₂O₇ transparent ceramic sintered at 1825 °C for 6 h shows the highest transmittance of 77.3 % and the average grain size of about 80 μm.