Characteristics of Y3Al5O12:Ce phosphor powders prepared by spray pyrolysis from ethylenediaminetetraacetic acid solution

Nanometer and submicron-sized YAG:Ce phosphor powders were prepared by spray pyrolysis from the spray solutions with ethylenediaminetetraacetic acid (EDTA). The precursor powders with hollow and thin wall structure turned to the fine-sized YAG:Ce phosphor powders after post-treatment at high temperatures of 1400 and 1500 °C. The mean size of the phosphor powders post-treated at a temperature of 1500 °C was 0.72 μm. The white LEDs formed from the YAG:Ce phosphor powders post-treated at 1400 and 1500 °C showed (0.2781, 0.2871) and (0.2731, 0.2795) on the CIE chromaticity diagram, and about 78.20 and 79.04 of Ra. The luminous efficiency of the white LED formed from the commercial YAG:Ce phosphor powders was 84.36 lm/W. However, the luminous efficiencies of the white LEDs formed from the YAG:Ce phosphor powders post-treated at 1400 and 1500 °C were 47.74 and 76.64 lm/W.     

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.     

Porous Ce:YAG ceramics with controllable microstructure for high-brightness laser lighting

In order to meet the increasing demand for high-power laser diode lighting and displays, phosphor converters with high-brightness and high-directionality ought to be constructed to enhance the luminance and luminous efficacy. However, the pores formed during the sintering of phosphor ceramics affect the scattering effect and directionality of light. Therefore, porosity optimization and pore size regulation need to be explored. In this work, a series of Ce:YAG ceramics with various porosities and pore sizes were prepared. The influences of porosity and pore size on the microstructure, light confinement ability, and optical properties of Ce:YAG ceramics were studied. The ceramic phosphor with a porosity of 10 vol.% and a pore size of 3 μm exhibits a good spot confinement ability and shows a high luminous flux value of 3430 lm and a central luminance (1669 592 cd/m²) under blue laser excitation. The 10 vol.% Ce:YAG ceramic phosphor with a pore size of 5 μm has the highest emission intensity and gives a maximum luminous efficacy of 268 lm/W and a luminous flux of 4020 lm under 30 W/mm² blue laser excitation. Thus, the porous Ce:YAG ceramics are expected to be a promising candidate for high-brightness laser lighting and projection applications.     

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.     

Preparation of a Φ60 mm Nd:YAG transparent ceramic disk

A high quality Nd:YAG transparent ceramic disk (Φ60 mm×2 mm) has been prepared by the solid-state reactive sintering technique, which was sintered satisfactorily at 1780 °C for 20 h. The high transparency of the sample has been assured by its pore-free microstructure and pure YAG phase composition with the in-line transmittances up to 84.5% at 1064 nm and 82.5% at 400 nm, respectively. The optical homogeneity of the disk was evaluated by the interference stripes with the PV and RMS values being 0.099 λ and 0.013 λ, respectively. The maximum output power of 2052 W with the slope efficiency of 39.0% was obtained at 500 Hz repetition rate. Results indicated that the Nd:YAG transparent ceramics could be a potential option for high power disk laser.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

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.     

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.     

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.     

Post-treatment of nanopowders-derived Nd:YAG transparent ceramics by hot isostatic pressing

Neodymium doped yttrium aluminum garnet (Nd:YAG) transparent ceramics were fabricated from Nd:YAG nanopowders synthesized via a reverse precipitation method by vacuum sintering and successive hot isostatic pressing (HIP) post-treatment. The powders obtained by calcining the precursor at 1100 °C for 4 h and then ball milling for 2 h with 0.5 wt% TEOS as sintering aid were used to fabricate Nd:YAG ceramics. The green bodies were vacuum sintered at 1500–1800 °C for 10 h, followed by the HIP at 1600 °C for 3 h in 200 MPa Ar atmosphere. Influence of the calcination temperature on the phase, morphology and particle size evolution of the nanopowders, as well as the optical transparency and microstructure of the obtained Nd:YAG ceramics before and after the HIP post-treatment was investigated in detail. It was found that for the post-treated 1800 °C-vacuum-sintered Nd:YAG ceramic sample, the in-line transmittance increased from 48.0% up to 81.2% at the lasing wavelength of 1064 nm.     

Multilayer YAG/Re:YAG/YAG laser ceramic prepared by tape casting and vacuum sintering method

This paper reports the use of tape casting and vacuum sintering process for the fabrication of optical grade YAG/Re:YAG/YAG (Re = Yb or Nd) composite laser ceramic. The influence of dispersant content on the rheological behavior of the slurry, the microstructure, the optical and laser performances, were studied. The experimental results showed that laser output at 1030 nm was generated for YAG/Yb:YAG/YAG with threshold absorbed pump power of 4.33 W and slope efficiency of about 12% when the transmission of output coupler (T_oc) was 2.3%. For YAG/Nd:YAG/YAG ceramic, 1064 nm laser output was obtained, and the slope efficiency increased from 30% to 38% while T_oc changed from 2.3% to 10%.     

Co-precipitation synthesis route to yttrium aluminum garnet (YAG) transparent ceramics

Yttrium aluminum garnet (YAG) precursor was synthesized via a coprecipitation method with aluminum nitrate and yttrium nitrate as raw materials, using ammonium hydrogen carbonate (AHC) as the precipitant. Fine and low-agglomerated YAG powder was obtained by calcining the precursor at 1200 °C. The primary crystallites were measured to be ～120 nm in size and weakly agglomerated to a particle size of ～500 nm, indicating a high degree of sinterability. With 0.5 wt% tetraethyl orthosilicate (TEOS) and 0.1 wt% magnesia as sintering aids, transparent YAG ceramics were fabricated by vacuum sintering at 1730–1790 °C for various hours. The influences of sintering temperature and holding time on the microstructure and transmittance of YAG ceramics were discussed.     

Process parameters analysis of direct laser sintering and post treatment of porcelain components using Taguchi's method

The effects of laser sintering parameters (laser power, scan speed and hatching space) and post sintering process (heating rate, sintering temperature and holding time) on the physical and mechanical properties of porcelain components have been investigated. The study has been carried out using the Taguchi's method for the experimental design. In the laser sintering process, lower laser energy density and higher hatching space will increase the final mechanical properties of the porcelain components. A stress relief principle has been put forward to explain the different influence of the factors. The appropriate laser sintering parameters are attained in this paper: laser power 50 W; scan speed 85 mm/s; and hatching space 0.6 mm. Sintering temperature has been determined to be the most important factor in the post sintering process. Appropriate sintering temperature for the laser sintered porcelain bodies is in the range of 1425–1475 °C regarding the mechanical properties of the porcelain components. The maximum bending strength, 34.0 ± 4.9 MPa, is reached.     

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.     

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.     

Scintillation,TSL and RPL properties of MgF2 transparent ceramic and single crystal

We have prepared MgF₂ transparent ceramics by spark plasma sintering (SPS) and investigated the scintillation and dosimeter properties compared with those of a single crystal. Under X-ray irradiation, a transparent ceramic showed scintillation with a broad feature over 300–600 nm with decay times of 310 and 1940 μs while the single crystal did not show such emission. It was found that MgF₂ show radio-photoluminescence (RPL), which is a generation of new luminescence center by ionizing radiation. The RPL was only observed in the transparent ceramic samples, which appeared as two emission bands peaking around 415 and 700 nm under 340±40 nm excitation. During the PL reading, the emission band at 415 nm effectively increased over 50 s. This result suggested that the RPL in MgF₂ shows a build-up phenomenon. From PL emission/excitation characteristics and decay time, it was suggested that the RPL peak around 415 nm was due to M(C_2h) center.     

Transparent YAG obtained by spark plasma sintering of co-precipitated powder. Influence of dispersion route and sintering parameters on optical and microstructural characteristics

A fine-grained (330 nm) yttrium aluminium garnet (YAG) ceramic, presenting a non-negligible transparency (66% RIT at 600 nm), was obtained by spark plasma sintering. The YAG powder was manufactured by co-precipitation, starting from a yttrium and aluminium chlorides solution. A soft precursor was obtained, whose phase evolution was studied by X-ray diffraction. Calcined powders were dispersed by either ball milling or by ultrasonication and then subjected to spark plasma sintering at several temperatures (1200–1400 °C) and for a reduced time (15 min). It is shown that the dispersion method plays a key role in enhancing the optical characteristics of YAG ceramics, in order to obtain a material with a small grain size, transparent in both the visible and the infrared range.     

Melting salt assisted sol–gel synthesis of blue phosphor Y2SiO5:Ce

High brightness Y₂SiO₅:Ce phosphor powders with spherical shape and fine size were synthesized by melting salt assisted sol–gel method (MS&Sol–Gel). Commercial tetraethyl orthosilicate was used as the silica source and rare earth oxides were used as rare earth source. The prepared Y₂SiO₅:Ce powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser particle sizer, and fluorescentometer techniques. Y₂SiO₅:Ce powders were obtained at significantly lower temperature than that by conventional techniques. When sintered at 1200 °C for 2 h with 5 wt.% LiF and 2 wt.% KH₂PO₄ as fluxes, particles with spherical shape and narrow particle distribution could be obtained. Moreover, the grain size of the powders prepared through this process was in the range of 2–7 μm, strongly depending on the thermal treatments and the species of fluxes. PL intensity of the prepared Y₂SiO₅:Ce phosphor using 5 wt.% LiF and 2 wt.% KH₂PO₄ as fluxes was similar to that of commercial product.     

Upconversion luminescence of high content Er-doped YAG transparent ceramics

The various high content Er-doped YAG transparent ceramics with excellent transparency up to nearly 84% at the visible band and the near-infrared band were prepared by the solid-state reaction and the vacuum sintering technique. It is found that the samples exhibit pore-free structures and there are no secondary phases both at the grain boundaries and the inner grains. The average grain size of the Er:YAG ceramics is about 30 μm. The green and red upconversion luminescences in the Er:YAG ceramics pumped by a 980 nm LD were observed. The different upconversion mechanisms depending on Er content in the Er:YAG ceramics and the LD power were also discussed.     

Direct thermal diffusion bonding of Nd:YAG/YAG composite transparent ceramics by vacuum sintering

Nd:YAG/YAG transparent ceramics were prepared by vacuum sintering at 1780 °C for 40 h and annealed at 1450 °C for 20 h in air. Two separately polished Nd:YAG/YAG samples were bonded into monolithic and uniform composite-material followed by vacuum sintering at 1790 °C for 50 h under uniaxial pressure of 60 MPa, and then annealed at 1450 °C for 100 h in air. The fracture strength of bonded samples at the bonding interface is higher than that of as-prepared Nd:YAG/YAG samples. Meanwhile, the extinction coefficient of bonded samples is 0.0305 cm⁻¹ which is an improvement over as-prepared samples. The microstructure of the contact interface reveals the disappearance of the contact layer at the bond due to the grain growth and coalescence mainly based on grain boundary diffusion at higher temperatures and longer heat-treated time, which indicates that the bonding technology is beneficial to the fabrication of the thick composite materials.