Structure and optical properties of transparent Er3+-doped CaF2–silica glass ceramic prepared by controllable sol–gel method

Transparent Er³⁺-doped CaF₂–silica glass ceramics were prepared by the direct physical introduction of Er³⁺ doped CaF₂ nanocrystals into acid-catalyzed sol–gel silica glass. The physical methods of ball milling, ultrasonic baths, and stirring were investigated to disperse Er³⁺ doped CaF₂ nanocrystals in the silica sols. The CaF₂–silica sol mixture went through gelation and heat-treatment to form Er³⁺-doped CaF₂–silica glass ceramics. The morphology of Er³⁺ doped CaF₂ in silica glass did not change after heat-treatment at 600 °C for 10 h. The experimental results showed that Er³⁺ doped CaF₂ in the glass ceramic prepared with the assistance of ball milling possesses the best dispersity and homogeneity. The highest in-line transmittance of the glass ceramic reached up to 85% in visible region. Glass ceramic exhibits efficient up-conversion emissions corresponding to the Er³⁺:⁴F_9/2→⁴I_15/2 transition and long lifetime of ⁴F_9/2 level (1.73 ms) under 980 nm excitation.     

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.     

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.     

Effects of deformation rate on properties of Nd,Y-codoped CaF2 transparent ceramics

Different deformation rates of Nd,Y-codoped CaF₂ transparent ceramics were prepared by ceramization of single crystals. The deformation rate effects on the crystallization behaviors, microstructures, mechanical properties, and optical performances were investigated for the first time. The results indicate that the comprehensive performances of Nd,Y-codoped CaF₂ ceramic (△a?=?62%) are the most optimal compared with other ceramics having different deformation rates (△a?=?34%, 40%, 50%, and 75%). In further investigations of the optical properties, the Nd,Y-codoped CaF₂ ceramic (△a?=?62%) sample exhibited a high transparency (T_a?>?91%, 3-mm thick，250?～?1200?nm), low light scattering, superior fracture toughness (K_1c?～?0.71?MPa·m^1/2), strong fluorescence emission, long lifetime (τ?=?348.72?μs), and broad FWHM (29.2?nm), promising a good candidate for high-power laser material.     

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.     

Optical Characterizations of Hot‐Pressed Erbium‐Doped Calcium Fluoride Transparent Ceramic

Nanoparticles of erbium‐doped calcium fluoride were synthesised by the coprecipitation method. Micromorphology of the obtained nanoparticles was observed by transmission electron microscopy. The nanoparticles were hot‐pressed in a vacuum environment to achieve Er:CaF₂ transparent ceramic. X‐ray diffraction analysis confirmed the crystallization of a single fluorite phase after sintered. Transmittance spectrum of Er:CaF₂ ceramic sample was measured, and the transmittance at 1200 nm reached about 87%. Microstructures were characterized using field‐emission scanning electron microscopy. The luminescence spectrum of Er:CaF₂ transparent ceramics under 488‐ and 978‐nm excitation was measured and discussed. It was evidenced that strong cross‐relaxation processes between Er³⁺ ions occur at high dopant concentration, and favoring the red emission at the expense of the green one.     

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.     

The effect of Nd3+ concentration on fabrication,microstructure, and luminescent properties of anisotropic S-FAP ceramics

Nd³⁺ doped strontium fluorophosphate (S-FAP), with chemical formula Sr₅(PO₄)₃F, nanopowders were prepared using the co-precipitation method. The prepared powders had no impurity phase with a grain size of about 30 nm and the doping limit of Nd³⁺ ions in strontium fluorophosphate is about 9 at.%. The morphology and particle size were determined by the doping concentration of Nd³⁺. Anisotropic Nd: S-FAP transparent ceramics with different Nd³⁺ doping concentrations were fabricated successfully by the simple hot-pressing method. The grain size of prepared S-FAP transparent ceramics decreased first and then increased with the increase of Nd³⁺ concentration. The 2 at.% Nd: S-FAP ceramic presented the highest optical transmittance at all wavelengths range. The characteristic transitions from the ground state to the excited states of Nd³⁺ ions were observed from the absorption spectra, and the absorption cross-section was calculated at 3.71 × 10^–20 cm². The influence of Nd³⁺ ion concentration on luminescence intensity and fluorescence lifetime was studied under 796 nm excitation. The strong emission of ⁴F_3/2→⁴I_9/2 transition in Nd: S-FAP was calculated by Judd–Ofelt (J-O) theory.     

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.     

Nd3+‐Doped Silicate Glasses as an Efficient Color Filter to Improve the Color Gamut of a White LED

Nd³⁺‐doped silicate glass (Nd‐glass) was employed as a color filter for a white LED based on red and green phosphor (RG‐LED), to manipulate the photoluminescence spectral shape and thus to provide a wider color gamut. The hypersensitive transition of Nd³⁺:⁴I_9/2→⁴G_5/2,²G_7/2 was adjusted via glass composition and Nd concentration, and improved absorbance as well as reduced the absorption bandwidth. The effective absorption of the Nd‐glass at ~580 nm reduced the spectral linewidth of the green and red emissions, improving the color reproduction range. The color gamut of the RG‐LED was improved from 75.3% to 81.6% NTSC by the introduction of Nd‐glass as a color filter. Reliability under high operating current and high temperature were also examined and discussed.     

Preparation of carbon/carbon‐ultra high temperature ceramics composites with ultra high temperature ceramics coating

In order to increase the oxidation resistance of carbon/carbon (C/C) composites at long‐term high temperature, C/C‐Ultra High Temperature Ceramics composites (UHTCs) with a dual‐layer UHTCs oxidation coating was successfully designed and fabricated. The microstructure and ablation resistance were investigated and discussed. After ablation in arc‐heated wind tunnel with temperature being 2200°C for 1000s, the mass ablation rate and linear ablation rate were ?1.9 × 10^?2 mg/cm²s and 2.9 × 10^?5 mm/s, respectively. The formation of thermodynamically compatible oxide scale including ZrO₂ skeleton and SiO₂ or Zr–Si–O glass on the surface were mainly contributed to the excellent ablation resistance of the composite.     

Multi-phase glass-ceramics containing CaF2: Er3+ and ZnAl2O4:Cr3+ nanocrystals for optical temperature sensing

Oxyfluoride transparent glass-ceramics (GC) containing CaF₂ and ZnAl₂O₄ nanocrystals have been fabricated with melt-quenching method. By carrying out the heat treatment of the precursor glass (PG), Er³⁺ and Cr³⁺ were selectively partitioned into CaF₂ and ZnAl₂O₄ nanocrystals, respectively. The obtained multi-phase GC exhibited strong upconversion (UC) fluorescence of Er³⁺ as well as intense down-conversion (DC) fluorescence of Cr³⁺. Under 980 nm excitation, the green UC fluorescence of Er³⁺ due to ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition and the red DC fluorescence lifetime of Cr³⁺ due to ²E, ⁴T₂ → ⁴A₂ transition were found to be highly dependent on the temperature and makes them possibly suitable for Optical Thermometry. With least-square fitting methods, the FIR of Er³⁺ from thermally coupled energy states (²H_11/2 and ⁴S_3/2) produced maximum temperature sensing sensitivity values of 0.33% K⁻¹ at 437 K and 0.36% K⁻¹ at 267 K, respectively. Similarly, fluorescence lifetime of Cr³⁺ attributed to the parity forbidden (²E → ⁴A₂) and spin allowed (⁴T₂ → ⁴A₂) produced the maximum temperature sensor sensitivity value equal to 0.67% K⁻¹ at 535 K.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.     

Tribological behaviors of hot-pressed Al2O3/TiC ceramic composites with the additions of CaF2 solid lubricants

Al₂O₃/TiC ceramic composites with the additions of CaF₂ solid lubricants were produced by hot pressing. The effect of the solid lubricant on the microstructure and mechanical properties of the ceramic composite has been studied. The friction coefficient and wear rates were measured using the ring-block method, and the tribological behaviors were discussed in relation to its mechanical properties and microstructure. Results showed that additions of CaF₂ solid lubricants to Al₂O₃/TiC matrix led to a decrease in the flexural strength, fracture toughness, and hardness compared to a conventional Al₂O₃/TiC composite. The friction coefficient of Al₂O₃/TiC/CaF₂ ceramic composites when sliding against both cemented carbide and hardened steel decreased with an increase in CaF₂ content up to 15 vol.%. The reason is that the CaF₂ released and smeared on the wear surface, and acted as solid lubricant film between the sliding couple. When the content of CaF₂ solid lubricant is less than 10 vol.%, the wear rate of Al₂O₃/TiC/CaF₂ composites decreases with an increase in CaF₂ content, with further increases in CaF₂ content, the wear rate of Al₂O₃/TiC/CaF₂ composites increases rapidly. This is due to the large degradation of mechanical properties in samples with high CaF₂ contents.     

Novel Synthesis of Low Hydroxyl Content Yb3+‐Doped Fluorophosphate Glasses with Long Fluorescence Lifetimes

A novel dehydration method for preparing rare‐earth ions‐doped fluorophosphate glasses was presented using raw materials soaking and rapid heating process, and a series of water‐free Yb³⁺‐doped fluorophosphate glasses (FPC) were obtained. In contrast with other dehydration methods, we found that the Yb³⁺‐doped FPC has wide transmission range and long fluorescence lifetime. The hydroxyl groups (‐OH) absorption was barely observed in the range 260–3750 nm, and the UV absorption cut‐off wavelength was blue shifted by 100 nm. Under the optimal conditions, the maximum value of emission cross section and the fluorescence lifetime of FPC reached 0.82 pm² and 2.67 ms, respectively. These excellent optical properties implied a potential application of the FPC glass in multicomponent glass fibers, IR lasers, and optical amplifiers.     

Oxyfluoride glass-silica ceramic composite for low temperature co-fired ceramics

Glass/ceramic composite materials based on CaF₂–AlF₃–SiO₂ oxyfluoride glass and silica ceramic filler were prepared. The sintering behavior, phase composition and dielectric property of oxyfluoride glass/silica ceramic composites, as well as its compatibility with Ag electrode, were investigated. The results show that the glass/ceramic composite system can be sintered at 825 °C. When the amount of SiO₂ increased from 0 to 20 wt.%, the shrinkage decreased from 17.0 to 14.5%, and the dielectric constant decreased from 5.9 to 5.4, while the thermal expansion coefficient (20–200 °C) increased from 6.0 to 10.1 ppm/°C. The sintered samples had low dielectric losses less than 0.002 and high flexural strengths. This novel glass/ceramic composite system exhibits good sintering compatibility with silver paste, which makes it a promising candidate for low temperature co-fired ceramic application.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Efficient adsorption and in-situ ceramics immobilization of simulated trivalent actinides (Nd) by amino-functionalized mesoporous zirconia-silica

It is of great significance to develop a kind of adsorbent which can adsorb and in-situ immobilize radionuclides from aqueous solution. Herein, new amino-functionalized mesoporous zirconia-silica (ZNSi) composites were prepared and applied to adsorb and in-situ immobilize the simulated trivalent actinides (Nd) from aqueous solution. The obtained ZNSi composites exhibited high Nd adsorption capacity (31.14 mg/g) owing to the formation of Nd(OH)₃ via the reaction between Nd³⁺ and OH⁻ derived from the protonation of amino groups. The spent adsorbents with adsorbed Nd³⁺ were successfully changed to stable ZrSiO₄-based glass ceramics by simple sintering treatment. The ZrO₂ and Nd contents had great effect on the phase composition, microstructure evolution and aqueous stability of the obtained ZrSiO₄-based glass ceramics waste forms. The immobilized Nd showed excellent aqueous stability (10⁻⁷ g m⁻² d⁻¹) due to the crystal lattice immobilization of ZrSiO₄. Owing to the efficient adsorption and in-situ immobilization ability, the obtained ZNSi could be potential materials for radioactive wastewater treatment.     

Phosphor‐in‐fluorescent‐glasses for high color rendering white light emitting diodes

Fluorescent glass frits were prepared and used to synthesize phosphor‐in‐fluorescent glass composites (PiFGs) to realize stable white light emitting diodes with high color‐rendering properties. Commercial red, green, and blue phosphors were co‐sintered and red phosphors were partially replaced by Eu³⁺ in glass frits. Phosphor‐in‐glass composites were placed on UV‐light emitting diodes (UV‐LEDs) to generate white light. Pure white light with a luminous efficacy=58.4 lm/W, general color rendering index R_a=87 and special color rendering index for strong red R₉=73 was realized with glass frits containing 7 mol% Eu₂O₃ and RGB ratio of 35:20:15. Luminous efficacy, R_a and R₉ increased as red phosphors were replaced by red‐fluorescent glass frits.     

Recrystallization of Er3+:CaF2 in Transparent Fluorophosphate Glass‐Ceramics with the Co‐Firing Method

Transparent glass‐ceramics containing Er³⁺:CaF₂ crystallites were prepared with the co‐firing method. The formation process of the glass‐ceramics was investigated by means of SEM, XRD, and DSC. The results reveal that the Er³⁺:CaF₂ nanocrystals do not dissolve into the fluorophosphates (FP) glassmelt until the co‐firing temperature increase higher than about 920°C. Below this temperature, Er³⁺:CaF₂ survives the co‐firing process and the nanocrystals just grow to spherical crystals of micrometers in size. Co‐firing temperature higher than this temperature leads to the dissolution of Er³⁺:CaF₂ and the dissolved Er³⁺:CaF₂ recrystallized during quenching process and takes the shape of dendrite.