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.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

Microstructures and optical properties of 6Li,Ce:YAG ceramics for thermal neutron detection

The microstructures and optical properties of 5%⁶Li: Ce_3xY_3(1-x)Al₅O₁₂ (x = 0.001, 0.003, 0.05, 0.01, 0.02) transparent ceramics prepared by solid-state reaction and vacuum sintering were investigated in this paper. The results revealed that the grain size of ⁶Li,Ce:YAG ceramics at this ration conditions is 4 μm–20 μm. With the doping of Ce³⁺, the transmittance of ⁶Li,Ce:YAG ceramics decreases from 82% (x = 0.001) to 67% (x = 0.02) at 800 nm, and the intensity of transmittance peak at 340 nm and 460 nm increases. The emission peaks show red shift at around 530 nm with the increasing of Ce³⁺ concentration.     

Sintering of Yb3+:Y2O3 transparent ceramics in hydrogen atmosphere

Highly transparent Yb³⁺:Y₂O₃ ceramics with doping concentration up to 40.0 at.% had been fabricated successfully via hydrogen atmosphere sintering, where the raw powders were synthesized by co-precipitation method. The sintering temperature is about 600 °C lower than its melting temperature. SEM investigation revealed the average grain size of Yb³⁺:Y₂O₃ ceramics sintered at 1850 °C for 9 h was about 7 μm. The highest transmittance of as-prepared 1 mm thickness samples around wavelength of 1050 nm reached 80%, which is close to the theoretical value of Y₂O₃. The optical spectroscopic properties of Yb³⁺:Y₂O₃ transparent ceramics have also been investigated, which shows that it is a very good laser material for diode laser pumping and short pulse mode-locked laser.     

Preparation and characterizations of Yb:YAG-derived silica fibers drawn by on-line feeding molten core approach

Three Yb:YAG transparent ceramics with Yb₂O₃ doping concentrations of 1, 10, and 15 at%, respectively were made into silica-clad hybrid fibers using an on-line feeding molten core approach. The diffusion of silica was mitigated such that the lowest SiO₂ concentration was 36.4 wt%, and consequently, the Yb₂O₃ content could reach 8.93 wt% in the fiber core. The fiber core transformed from a YAG ceramic to an yttrium aluminosilicate glass, and the formation of abundant Q² silicate species implied that the structure of the core glass maintained some environments similar to that of YAG with Q²–AlO₄ tetrahedra. The absorption and emission spectra of the obtained fibers were compared to those of Yb:YAG ceramics, and the self-absorption effect was analyzed in detail. All three fibers could output lasers under 940 or 970 nm pumping. The maximum output power of the Yb:YAG-derived fibers was higher than that of ceramic wafers owing to the cladding pump technology, which offered a new method to improve the application of ceramics.     

Structural-phase state and lasing of 5–15 at% Yb3+:Y3Al5O12 optical ceramics

Yttrium aluminum garnet (Yb³⁺:Y₃Al₅O₁₂) laser ceramics doped by 5, 10 and 15 at% of ytterbium ions were obtained by reactive sintering. Optimal sintering temperature range for the formation of highly-dense transparent Yb³⁺:Y₃Al₅O₁₂ ceramics under normal recrystallization conditions was found to be T = 1750–1800 °C. The influence of Yb³⁺ ions on structural-phase state, phase composition, microstructure, optical and luminescent properties of sintered samples was experimentally investigated. It was shown that lattice parameter a of Yb³⁺:Y₃Al₅O₁₂ ceramics decreases linearly with increasing of Yb³⁺ concentration in a good agreement with L. Vegard’s rule, that indicates to the formation of (Y_1−xYb_x)₃Al₅O₁₂ (х = 0.05–0.15) substitutional solid solutions. No concentration quenching of Yb³⁺ luminescence was observed in Yb³⁺:Y₃Al₅O₁₂ within the 5–15 at% doping range. Quasi-CW lasing of Yb³⁺:Y₃Al₅O₁₂ ceramics was studied under diode-pumping at 970 nm. A highest slope efficiency of about 50% was obtained for 15 at%-doped Yb³⁺:Y₃Al₅O₁₂ ceramics sintered at T = 1800 °C for 10 h.     

Influence of heating rate on optical properties of Nd:YAG laser ceramic

Using commercial α-Al₂O₃, Y₂O₃ and Nd₂O₃ as raw materials, 0.8 at% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering technology, with tetraethoxysilane (TEOS) as sintering aid. The Nd:YAG ceramics were obtained by sintering at 1750 °C for 20 h under vacuum. The sintering process with different heating rate of the Nd:YAG ceramics have been studied during the present work. The grain sizes, pores and secondary phase amounts increased versus increasing the heating rate. The optical properties of the Nd:YAG ceramics were closely related to the microstructures of the specimens. The lasing performance of the Nd:YAG ceramics changed drastically with change in pores and secondary phase amounts.     

Toward vacuum sintering of YAG transparent ceramic using divalent dopant as sintering aids: Investigation of microstructural evolution and optical property

Transparent YAG ceramics were fabricated by solid state reaction sintering using divalent dopants (CaO and MgO) as sintering additives without TEOS doping, and the effects of divalent dopants on their microstructure evolution and optical properties were investigated. It was found that CaO was more effective with respect to inhibiting grain growth than MgO, but not as effective as MgO in promoting densification. Fully dense, transparent YAG ceramics with excellent optical qualities could be achieved by optimizing the doping concentrations of CaO and MgO; the transmittance at 1064 nm was as high as 84.5% for 3 mm thick sample at the molar ratio of Ca: Mg=1:4, after sintered at 1840 °C for 8 h in vacuum.     

Effect of cerium doping on optical and scintillation properties of transparent YAG ceramic

Cerium doped transparent YAG ceramics 0.15–2 at% cerium) were prepared using nano-powder technique and vacuum sintering. The effective solubility limit of cerium in YAG was found to lie between 1.0 and 1.5 at%. The PL intensity increases with the cerium content and attains a maximum for 1.0 at% doping. Lifetime of PL is not very sensitive to cerium content, however, a slight decrease in the life-time from 66 ns to 55 ns was observed with increase in cerium content from 0.15 to 2.0 at%. This decrease in PL intensity and life-time is attributed to concentration quenching for the YAG ceramics with cerium content higher than 1.0 at%. A red-shift in the PL peak position was observed and attributed to the local symmetry distortion in YAG matrix.     

High optical quality Y2O3 transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment

High optical quality Y₂O₃ transparent ceramics with fine grain size were successfully fabricated by air pre-sintering at various temperature ranging from 1500 to 1600 °C combined with a post-hot-isostatic pressing (HIP) treatment using co-precipitated powders as the starting material. The fully dense Y₂O₃ transparent ceramic with highest transparency was obtained by pre-sintered at 1550 °C for 4 h in air and post-HIPed at 1600 °C for 3 h (the pressure of HIP 200 MPa), and it had fine microstructure and the average grain size was 0.96 μm. In addition, the in-line transmittance of the ceramic reached 81.7% at 1064 nm (1 mm thickness). By this approach, the transparent Y₂O₃ ceramics with fine grain size (<1.6 μm) were elaborated without any sintering aid.     

Influence of heat treatment of powder mixture on the microstructure and optical transmission of Nd:YAG transparent ceramics

Transparent Nd:YAG ceramics were fabricated by solid-state reactive sintering of Y₂O₃, α-Al₂O₃ and Nd₂O₃ powders with TEOS and MgO as sintering aids. The powders were ball-milled, dried, sieved and calcined at different temperatures. Samples sintered at 1745 °C for 10 h were utilized to observe the microstructure and the optical transmission. It is found that heat treatments of the powder mixtures above 600 °C for 1 h are necessary to remove the carbon contamination but below 800 °C for 4 h can avoid strong aggregation of the powder. So there is a room for heat-treatment, between 600 °C and 800 °C that can obtain Nd:YAG ceramics with almost pore-free microstructures and high transparency. Highly transparent Nd:YAG ceramic with 84.3% in-line transmission at 1064 nm was fabricated by sintering the 800 °C-1 h-heat-treated powder mixture at 1745 °C for 50 h. Even at wavelength of 400 nm, the transmittance of the sample reached 82.9% and the optical scattering coefficient was as low as 0.71% cm⁻¹.     

Energy transfer,tunable emission and optical thermometry in Tb3+/Eu3+ co-doped transparent NaCaPO4 glass ceramics

Tb³⁺/Eu³⁺ co-doped glass ceramics containing NaCaPO₄ nanocrystals were successfully synthesized via traditional melt-quenching route with further heat-treatment and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectroscopy. The energy transfer process of Tb³⁺→Eu³⁺ was confirmed by excitation and emission spectra and luminescence decay curves, and the energy transfer efficiency was also estimated. The results indicated that the efficient emission of Eu³⁺ was sensitized by Tb³⁺ under the excitation of 378 nm, realizing tunable emission in the transparent bulk glass ceramics containing NaCaPO₄ nanocrystals. Furthermore, optical thermometry was achieved by the fluorescence intensity ratio between Tb³⁺:⁵D₄→⁷F₅ (~542 nm) and Eu³⁺:⁵D₀→⁷F₂ (~612 nm). The maximum absolute sensitivity of 4.55% K⁻¹ at 293 K and the maximal relative sensitivity of 0.66% K⁻¹ at T=573 K for Tb³⁺/Eu³⁺ co-doped transparent NaCaPO₄ glass ceramic are obtained. It is expected that the investigated transparent NaCaPO₄ glass ceramics doped with Tb³⁺/Eu³⁺ have prospective applications in display technology and optical thermometry.     

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.     

Optical and mechanical properties of amorphous bulk and eutectic ceramics in the HfO2–Al2O3–Y2O3 system

Bulk glasses containing HfO₂ nano-crystallites of 20–50 nm were prepared by hot-pressing of HfO₂–Al₂O₃–Y₂O₃ glass microspheres at 915 °C for 10 min. By annealing at temperatures below 1200 °C, the bulk glasses were converted into transparent glass-ceramics with HfO₂ nano-crystallites of 100–200 nm, which showed the maximum transmittance of ～70% in the infrared region. An increase of annealing temperature (>1300 °C) resulted in opaque YAG/HfO₂/Al₂O₃ eutectic ceramics. The eutectic ceramics contained fine Al₂O₃ crystallites and showed a high hardness of 19.8 GPa. The fracture toughness of the eutectic ceramics increased with increasing annealing temperature, and reached the maximum of 4.0 MPa m^1/2.     

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd³⁺ cation diffusion into transparent polycrystalline YAG (Y₃Al₅O₁₂) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd³⁺ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies.The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol^?1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol^?1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol^?1.     

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.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.     

Photoluminescence properties of Tb3+ doped Al2O3 microfibers via a hydrothermal route followed by heat treatment

Uniform Al₂O₃:Tb³⁺ microfibers were synthesized via a hydrothermal route and thermal decomposition of a precursor of Tb³⁺ doped ammonium aluminum hydroxide carbonate, and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD results indicate that various crystallographic phase Al₂O₃:Tb³⁺ microfibers are obtained by postannealing at different temperatures. SEM results show that the length and diameter of these Tb³⁺ doped α-Al₂O₃ microfibers are about 6–8 μm and 300 nm, respectively. The PL spectra indicate that the ⁵D₄ → ⁷F₅ (545 nm) electric dipole transition is the most intensive when excited at 240 nm. It is shown that the 2.0 mol% of doping concentration of Tb³⁺ ions in α-Al₂O₃:Tb³⁺ is optimum. According to Dexter's theory, the critical distance between Tb³⁺ ions for energy transfer was determined to be 12.7 Å. It is found that the decay curves follow the single-exponential decay.     

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.     

Spectrum regulation of YAG:Ce transparent ceramics with Pr,Cr doping for white light emitting diodes application

YAG:Ce transparent ceramics with high luminous efficiency and color render index were prepared via a solid state reaction-vacuum sintering method. Cr³⁺and Pr³⁺ were applied to expand the spectrum of YAG:Ce transparent ceramics. As prepared ceramics exhibit luminescence spectrum ranging from 500 nm to 750 nm, which almost covers full range of visible light. After the concentration optimization of Ce³⁺, Pr³⁺ and Cr³⁺, high quality white light was obtained by coupling the YAG:Ce,Pr,Cr ceramics with commercial blue LED chips. Color coordinates of the YAG:Ce,Pr,Cr ceramics under 450 nm LED excitation vary from cold white light to warm white light region. The highest luminous efficiency of WLEDs encapsulated by transparent YAG:Ce,Pr,Cr ceramic was 89.3 lm/W, while its color render index can reach nearly 80. Energy transfers between Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺ were proved in co-doped ceramic system. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power WLEDs application.