Fabrication and microstructure characterizations of transparent Er:CaF2 composite ceramic

A novel layered transparent Er:CaF₂ composite ceramic was proposed in the present study. Er:CaF₂ nanoparticles were synthesized by a chemical coprecipitation method. The crystal structures and morphologies of synthesized nanoparticles were performed by X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) measurements, respectively. Transparent composite ceramic was fabricated by the combination of multistep dry pressing and hot-pressed sintering method without any sintering aids or binders. The average grain size of 2% Er-doped and 5% Er-doped layers were about 30 and 55 μm, respectively. The thickness of interfacial between two different Er-doped layers was 150-200 μm. For a 1.5 mm thickness transparent Er:CaF₂ composite ceramic, the optical transmittance reached 44.9% at 500 nm and 53.6% at 1200 nm. The luminescence spectra and thermal conductivities of transparent ceramic specimens were also discussed.     

Fabrication and upconversion luminescence properties of Er:SrF2 transparent ceramics compared with Er:CaF2

SrF₂ transparent ceramic is a promising upconversion material due to the low phonon energy. The effect of different sintering temperatures on Er:SrF₂ transparent ceramics was investigated. The suitable sintering temperature for Er:SrF₂ transparent ceramics was 900 °C by hot-pressed sintering in this study. High quality of Er:SrF₂ transparent ceramics with different doping concentrations were obtained. The upconversion luminescence spectra and decay behavior were compared between Er:SrF₂ and Er:CaF₂ transparent ceramics with different Er³⁺ doping concentration. The green emission of 5 at.% Er:SrF₂ ceramic was much stronger than that of 5 at.% Er:CaF₂ ceramic, while the red emission of Er:SrF₂ ceramic was almost the same as that of Er:CaF₂ ceramic. The upconversion luminescence lifetime of Er:SrF₂ transparent ceramics was longer than that of Er:CaF₂.All the results indicated Er:SrF₂ transparent ceramics was a candidate for green fluorescent upconversion materials.     

Microstructural,spectroscopic and mechanical properties of hot-pressed Er:SrF2 transparent ceramics

Raw SrF₂ powders were synthesized by the chemical precipitation method, and the mean particle size was 58.48 nm. Er:SrF₂ transparent ceramics were obtained by hot-pressed (HP) technique, and the effect of ErF₃ levels on the transparency, microstructure, luminescence spectroscopic and microhardness were studied. The ratio of emission intensities R (Red/Green) increased with the ErF₃ doping levels. The addition of ErF₃ was found effectively to reduce grain size and has a positive effect on improving the microhardness. The SrF₂ ceramic doped with 5 wt.% ErF₃ (2 mm thick) showed the best optical transparency, the transmittance at 500 nm and 1200 nm are 87.9 % and 89.5 %, respectively. The average grain size, Vickers hardness (Hv), and fracture toughness (K_IC) for the SrF₂ ceramic were 21.1 ± 4.5 μm, 1.73 ± 0.04 GPa, and 0.52 ± 0.08 MPa·m^1/2, respectively.     

Fabrication and spectroscopic investigations on Er3+, Ho3+: SrF2 transparent ceramics for 2.7 μm emission

3 at.% Er³⁺, x at.% Ho³⁺: SrF₂ (x = 0, 0.05, 0.1, 0.5, 1, 2) transparent ceramics, as the potential material for the 2.7 μm solid-state laser, were fabricated by hot-pressed sintering. XRD, TEM, SEM, and EDS measurements were used to investigate the phase composition, morphology, microstructure, and distribution of the elements of the nanoparticles and transparent ceramics. Results showed that the Er³⁺ ions and Ho³⁺ ions do not alter the SrF₂ crystal structure, and they are distributed uniformly in the sample. With the increase of the Ho³⁺ doping concentration, the lattice parameter decreased from 5.799 Å to 5.784 Å, and the average grain size decreased gradually. The maximum transmittance of as-obtained ceramics is approximately 93 % which is close to the theoretical transmittance of SrF₂. Moreover, the absorption spectra, emission spectra, and the lifetime of Er³⁺ and Ho³⁺ were investigated. The energy transfer processes between Er³⁺ and Ho³⁺ were discussed. After co-doping Ho³⁺, the lifetime difference between Er³⁺:⁴I_11/2 and Er³⁺:⁴I_13/2 levels was shortened from 8.50 ms to 1.12 ms. All the results show that the incorporation of Ho³⁺ with proper doping concentration is beneficial for achieving 2.7 μm laser output in Er³⁺: SrF₂ transparent ceramics.     

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.     

Efficient energy transfer of Ce to Nd in SrF2 transparent ceramics for enhancing NIR emission

High optical quality Nd³⁺ and Ce³⁺ co-doped SrF₂ (Nd³⁺, Ce³⁺: SrF₂) transparent ceramics were fabricated successfully by a simple hot-pressing (HP) method. The phase composition, in-line transmittance, absorption and emission spectra, as well as the detailed energy transfer of Nd³⁺ and Ce³⁺ were investigated. In addition, the Judd- Ofelt (J-O) theory was adopted to evaluate the luminescence property. The SrF₂ transparent ceramic samples exhibited excellent optical properties, up to 82 % at 400 nm and 92.5 % at 1054 nm. The fracture surface of SrF₂ transparent ceramic proved nearly dense microstructure and EDS results demonstrated uniform doping. The addition of cerium ions changed the crystal field environment of neodymium ions and shifted the emission peak to higher wavelengths at 796 nm excitation. Moreover, through the energy transfer process of Ce³⁺ to Nd³⁺, the occurrence of concentration quenching phenomenon was avoided under 298 nm excitation, and the emission cross-section of ⁴F_3/2→⁴I_11/2 increased to 3.1 × 10⁻²⁰ cm².     

Application of Judd–Ofelt theory in analyzing Nd3+ doped SrF2 and CaF2 transparent ceramics

Nd³⁺ doped SrF₂ and CaF₂ transparent ceramics were fabricated by vacuum hot-press sintering and the absorption spectra, emission spectra as well as luminescence decays of the samples were measured. Judd-Ofelt (J–O) theory was used to analyze the optical performance of Nd³⁺ in these two isostructural hosts. The Nd: SrF₂ transparent ceramic was found to have smaller line strength, larger radiative lifetime and smaller Ω₂ value (corresponding to more ionic Nd³⁺-ligand bonding and more symmetry of Nd³⁺ environment). These features made it easier for Nd: SrF₂ to realize population inversion and strong emission, thus doing good to laser performance. The strong emission of ⁴F_3/2 → ⁴I_9/2 transition in Nd: SrF₂, which was predicted by J–O theory and demonstrated by luminescence spectrum, made it possible to achieve effective laser output around 900 nm. The intensity parameters and radiative lifetimes of ceramics were found comparable with their corresponding single crystals.     

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.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Effects of Nd Concentration on Microstructure and Optical Properties of Nd: CaF2 Transparent Ceramics

Highly transparent Nd‐doped calcium fluoride (Nd: CaF₂) ceramics with different Nd‐doped concentrations were fabricated by hot‐pressed method using Nd: CaF₂ nanopowders synthesized by coprecipitation method. SEM observations indicated that the average grain size of nanopowders was about 16–30 nm and the average grain size of the ceramics was between 200 nm and 1 μm. The grain boundaries of the ceramics were clean and no pores or impurities were detected. For 2‐mm‐thickness sample, the transmittance of the as‐fabricated 5 at.% Nd: CaF₂ ceramic at 1200 nm was about 85%. The absorption coefficient and emission intensity of the Nd: CaF₂ ceramics were measured and discussed. From the Nd: CaF₂ ceramics fluorescent spectra and the decay curves, it was found that the fluorescent quenching effect became more evident with the increase in the Nd³⁺ ions‐doped concentration.     

Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y₂O₃ transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y₂O₃ ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y₂O₃ ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y₂O₃ transparent ceramic at 2.7 μm was demonstrated.     

Fabrication and properties of transparent Nd-doped BaF2 ceramics

Nd:BaF₂ nanoparticles have been prepared via co-precipitation and a pumping filtration wash method. The phase composition and morphology of the synthesized nanoparticles were investigated by X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FE-SEM) analyses, respectively. SEM observations revealed the powder's particle size to be approximately 100-200 nm after calcination at 600°C for 5 hours in Ar. The transparent Nd:BaF₂ ceramics were then fabricated by the one-step vacuum sintering method at a temperature of 1200°C for 10 hours. SEM observations of the polished and thermally etched cross sections of the sintered ceramic revealed a highly homogenous microstructure with average grain size of 420 μm. Optical property characterization revealed that the transmittance of the ceramic reaches a maximum of ~70% in the infrared wavelength range, and an emission peak located at 1058 nm, excited by 808 nm light.     

Fine grained Nd3+:Lu2O3 transparent ceramic with enhanced photoluminescence

Fine-grained Nd³⁺:Lu₂O₃ transparent ceramic was developed by a two-step sintering method in flowing H₂ atmosphere at T₁ = 1720 °C for 15 min and T₂ = 1620 °C for 10 h. The initial nanopowders were synthesized by a wet chemical processing with a uniform particle size of about 40 nm. The average grain size of the obtained 3 at.% Nd³⁺:Lu₂O₃ ceramic was 406 nm, which is ∼150 times smaller than the coarse-grained ceramic by normal H₂ sintering. The emission intensity of the fine-grained transparent ceramic is 3 times of its coarse-grained counterpart, indicating higher Nd concentration without serious quenching in fine-grained transparent ceramic is possible, which agreed well with the prediction of an atomistic modeling work with YAG. EXAFS research demonstrated that with decreasing grain size, higher degree of disorder factor of the local environment of doped Nd atoms was discovered.     

High Transparency Nd: Y2O3 Ceramics Prepared with La2O3 and ZrO2 Additives

High transparency Nd: Y₂O₃ ceramics were prepared by vacuum sintering with La₂O₃ and ZrO₂ sintering additives. The optimum in‐line transmittance of the sintered Nd: Y₂O₃ is 80.98% at the wavelength of 1100 nm, for which the content of La₂O₃ and ZrO₂ are 10.0 and 3.0 at.%, respectively. This specimen demonstrates homogeneous microstructure with the average grain size of 8.3 μm. The mechanism of sintering with La₂O₃ and ZrO₂ aids and the optical properties was discussed. The absorption, emission cross section, and fluorescence lifetime have been estimated as 1.62 × 10^?20 cm², 5.13 × 10^?20 cm², and 232 μs, respectively. Vickers hardness and the fracture toughness were measured of 9.18 GPa and 1.03 Mpa·m^1/2, respectively. All the results indicate that Nd: Y₂O₃ transparent ceramic is a promising candidate for laser material.     

Fabrication of transparent MgAl2O4 ceramics by gelcasting and cold isostatic pressing

Highly transparent MgAl₂O₄ ceramics have been fabricated by aqueous gelcasting combined with cold isostatic pressing (CIP), pressureless sintering and hot isostatic pressing (HIP) from high purity spinel nanopowders. The gelling system used AM and MABM as monomer and gelling agent. The influences of dispersant and PH on the rheological behavior of the MgAl₂O₄ slurries were investigated. The spinel slurry with low solids loading (25 vol%) and low viscosity (0.15 Pa s) was obtained by using 6 wt% Duramax-3005 (D-3005) as dispersant. After CIP, the green body had a relative density of 48% with a narrow pore size distribution. The influence of sintering temperature on densification and microstructure was studied, choosing 1500 °C as the sintering temperature. After HIP (1650 °C/177 MPa/5 h), transparent MgAl₂O₄ ceramic with the thickness of 3 mm was obtained, whose in-line transmittance was 86.4% at 1064 nm and 79.8% at 400 nm, respectively. The ceramic exhibited a dense microstructure with the average grain size of 23 μm. The Vickers hardness and flexure strength of the sample reached 13.6 GPa and 214 MPa, respectively.     

Effect of sintering temperature on the microstructure and transparency of Nd,Y:CaF2 ceramics

1 at% Nd, 3 at% Y doped CaF₂ transparent ceramics were obtained by hot pressing at the sintering temperature varing from 500 to 800 °C under vacuum environment with co-precipitated CaF₂ nanopowders. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grain size around 26 nm. Scanning electron microscopy (SEM) observations of the Nd, Y: CaF₂ ceramics indicated that the mean grain size of the ceramic sintered at 800 °C was about 748 nm. The influence of the temperature on the grain size, microstructure and optical transmittance was investigated. For the ceramic sintered at 800 °C, the transmittance was 85.49% at the wavelength of 1200 nm. The room temperature emission spectra of Nd: CaF₂ and Nd, Y: CaF₂ ceramics were measured and discussed.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Sol-gel synthesis and characterization of ZnAl2O4 powders for transparent ceramics

The sol-gel synthesis of ZnAl₂O₄ ceramic powders from alkoxide and acetate sources of metals, as well as the microstucture and the hardness of the hot-pressed ZnAl₂O₄ specimens were considered. ZnAl₂O₄ powders were prepared by the hydrolysis of an alcohol solution of aluminium isopropoxide using an aqueous solution of zinc acetate followed by heat treatment. The thermal evolution of the ZnAl₂O₄ precursor was investigated. The effect of calcination temperature on the morphology and the specific surface area of ZnAl₂O₄ powders were also studied. The sintering of the resultant powders to the high transparent ceramic using a hot pressing with 1?wt% ZnF₂, as a sintering additive was successfully demonstrated. The in-line transmittance of ZnAl₂O₄ ceramics (1?mm thickness) achieved 80% in the visible region and 85% at 5?µm; Vickers hardness was 11.6?GPa.     

Vacuum sintering of highly transparent La1.28Yb1.28Zr2O7.84 ceramic using nanosized raw powders

Highly transparent La_1.28Yb_1.28Zr₂O_7.84 ceramic was prepared by vacuum sintering using nanosized raw powders, which were synthesized by a simple solution combustion method using rare earth nitrate as the raw materials. The as-burnt powders were calcined at 1200?℃ and then ball-milled for 24?h with resultant particle size of about 60?nm. The two phases, cubic pyrochlore and defective fluorite, are uniformly distributed in the ceramic. La_1.28Yb_1.28Zr₂O_7.84 transparent ceramic with the maximum in-line transmittance of 83.9% was successfully prepared at 1850?℃ for 6?h in a vacuum furnace.     

The phase,microstructure evolution and the Nd3+ function in the fabrication process of LuAG transparent ceramics

Nd:LuAG transparent ceramics were fabricated by the solid-state reaction under vacuum sintering using SiO₂ and MgO as sintering aids, commercial Lu₂O₃, Al₂O₃ and Nd₂O₃ as raw materials. The Nd doping concentration was adopted from 0 at. % to 1.3 at. %. The phase transformation and microstructure evolution of 1.3 at. % Nd:LuAG ceramics under different sintering temperature was investigated in detail. Meanwhile, the effects of Nd₂O₃ on the grain growth were surveyed. The results shown that when the samples were sintered at 1780?°C, the 1.3 at. % Nd:LuAG ceramic had clean gain boundary, and the transmittance of it reached 83.8% at 1064?nm.