Preparation and optical properties of transparent Ce:YAG ceramics for high power white LED

Dual precipitating reagents-assisted deep blue-emitting borate and near-white oxide-based luminescent materials

We explored dual precipitating reagents-assisted Ce-based deep blue-emitting borate and near-white oxide-based luminescent materials. The first precipitating reagent (aqueous sodium borohydride) was used until gelation. The second precipitating reagent (ammonia solution) was employed to prepare as-synthesized powders. XRD analysis, FTIR spectroscopy, and Raman spectroscopy of calcined powders ranging from 1000 °C to 1400 °C confirmed the development of a primary borate phase (i.e., nearly polyhedral-shaped YBO3) with secondary phases of (Y,Al)BO3 and oxide. However, the preliminary oxide phase (i.e., elongated/rod-like YAG) and secondary phases of YBO3, Al2O3, and CeO2 were developed at 1500 °C with different holding times. The emission behavior and CIE coordinates confirmed that the borate (i.e., YBO3-based) sample was deep blue. However, the oxide (i.e., YAG-based) sample showed tunable color emissions from light blue to near white with increased holding time. The average lifetime of prepared samples varied between 788 ns and 891 ns, which indicated a long decay time. The quantum yield of the sample calcined at 1400-1500 °C varied between ∼61% and ∼77%. Based on the emission behavior, CIE diagram, CCT, powder color, average lifetime, and quantum yield, the developed deep blue-emitting borate and light blue/near white oxide-based luminescent materials could be used in lighting industries.

Nano Wave Plates Structuring and Index Matching in Transparent Hydroxyapatite-YAG: Ce Composite Ceramics for High Luminous Efficiency White Light-Emitting Diodes

Replacing traditional luminous silicone or resins with phosphor in ceramics (PiCs) as color converters has been proposed as an efficient way to improve thermal stability of high-power white light-emitting diodes (WLEDs). However, excessive light scattering in existing PiCs results in enormous phosphor-converted light losses, which makes the luminosity of current PiCs color converters less efficient and means that they can only be used in devices working in reflective mode. By introducing nano wave plate structuring and Rayleigh scattering, luminous hydroxyapatite (HA)-YAG: Ce ceramics are prepared from mesoporous HA nanorods and YAG: Ce phosphors at 850 °C, enabling for the first time WLEDs equipped with PiC color converters in transmission mode. With low-temperature sintering and a highly transparent matrix, the quantum yield of HA-YAG: Ce retains ≈90% of the raw phosphor, and WLEDs with the color converters exhibit a record luminous efficiency of 170 lm W^-1 and a correlated color temperature below 4500 K. A facile and practical strategy of using nano structural modulation to eliminate birefringence-induced light scattering for fabricating high-performance ceramic converters suitable for multiple mode luminaires is demonstrated.

Warm White Light with a High Color-Rendering Index from a Single Gd3Al4GaO12:Ce3+ Transparent Ceramic for High-Power LEDs and LDs

Transparent ceramics (TCs) are promising for high-power (hp) white light-emitting diode (WLED) and laser diode (LD) lighting. However, comfortable warm white light has not been achieved only using a single TC in hp-WLEDs/LDs. Herein, highly transparent Gd₃Al₄GaO₁₂:Ce³⁺ (GAGG:Ce³⁺) TCs (transmittance, T = 55.9-80.2%) were prepared via a solid-state reaction. Ce³⁺ as a doped activator center in grains plays a positive role in luminescence based on the microstructural investigations by scanning electron microscopy and the cathodoluminescence system. T decreases upon increasing the Ce³⁺ concentration and/or the ceramic thickness, whereas the luminous efficacy of hp-WLEDs/LDs goes up. For blue hp-LEDs driven at 350 mA or LDs of 2 W, warm white light with a low correlated-color temperature of ∼3000 K was achieved by a single GAGG:Ce³⁺ TC, benefiting from its broad emission band (full width at half maximum, FWHM = 133-137 nm) and abundant red components (peaking at about 568-574 nm). The color-rendering index of hp-WLEDs reaches 78.9. These results are much better than the performance of the traditional Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) TC, indicating that GAGG:Ce³⁺ TCs are promising color converters for hp-WLEDs/LDs with a comfortable warm white light.

Hybrid 2D photonic crystal-assisted Lu3Al5O12:Ce ceramic-plate phosphor and free-standing red film phosphor for white LEDs with high color-rendering index

This paper reports the combined optical effects of a two-dimensional (2D) SiNx photonic crystal layer (PCL)-assisted Lu3Al5O12:Ce (LuAG:Ce) green ceramic-plate phosphor (CPP) and a free-standing (Sr,Ca)AlSiN3:Eu red film phosphor to enhance luminous efficacy, color rendering index (CRI), and special CRI (R9) of LuAG:Ce CPP-capped white light-emitting diodes (LEDs) for high-power white LEDs at 350 mA. By introducing the 2D SiNx PCL, the luminous efficacy was improved by a factor of 1.25 and 1.15 compared to that of the conventional flat CPP-capped LED and the thickness-increased CPP-capped LED (with a thickness of 0.15 mm), respectively, while maintaining low color-rendering properties. The combining of the free-standing red film phosphor in the flat CPP-capped, the 2D PCL-assisted CPP-capped, and the thickness-increased CPP-capped LEDs led to enhancement of the CRI and the special CRI (R9); it also led to a decrease of the correlated color temperature (CCT) due to broad wavelength coverage via the addition of red emission. High CRI (94), natural white CCT (4450 K), and acceptable luminous efficacy (71.1 lm/W) were attained from the 2D PCL-assisted LuAG:Ce CPP/free-standing red film phosphor-based LED using a red phosphor concentration of 7.5 wt %. It is expected that the combination of the 2D PCL and the free-standing red film phosphor will be a good candidate for achieving a high-power white CPP-capped LED with excellent CRI.