White light emitting diodes directly driven by alternating current (AC-LEDs) become increasingly popular in lighting. However, the harmful flicker occurs because the blue LED chips only work over the threshold voltage. Transparent afterglow phosphor-glass composites are thus proposed as excellent color converters to eliminate the flicker. In this paper, Gd_2.94Al₂Ga₃O₁₂:0.06Ce³⁺-SiO₂ (GdAGG:Ce-SiO₂) glass ceramics were prepared via cold isostatic pressing molding and pressureless sintering at a low temperature. The transparency, structures, microscopic morphologies and luminescence properties were investiagted. Besides, the samples were assembled on the commercial blue LED chips or blue laser diodes (LDs) to explore the potential application.

For the preparation of GdAGG:Ce phosphor, stoichiometric Gd₂O₃, Al₂O₃, Ga₂O₃ and CeF₃ (99.9%) were weighed and placed in a mortar. After fully ground for 30 min, the obtained powder was transferred into a 20 mL corundum boat and placed in a tube furnace. The gas flow of 95%N₂/5%H₂ (in volume ratio) was introduced, and the temperature was hold at 1 450 ℃ for 4 h. After cooling to room temperature, the powder was ground in a mortar.

For the preparation of GdAGG:Ce-SiO₂ glass ceramics, 0.9 g GdAGG:Ce phosphor and 9.1 g amorphous SiO₂ nanoparticles were weighed and fully ground in a mortar for 30 min. The obtained powder was sieved with a 200 mesh standard sieve. Then, 0.8 g sieved powder was poured into a stainless-steel mold with an inner diameter of 20 mm, and pressed at 40 MPa for 30 s. The obtained blocks were placed in a cold isostatic press and kept at 250 MPa for 5 min. The green compacts were sintered in a muffle furnace in air at 600-800 ℃ for 5 h. After cooling down to room temperature, the samples were double-side polished.

For the preparation of lighting devices, Gd₃Al₂Ga₃O₁₂:Ce-SiO₂ glass ceramics of different thicknesses were encapsulated on the surface of 1 W commercial 450 nm LED lamp to assemble white LED devices. Besides, 1.0 mm thick Gd₃Al₂Ga₃O₁₂:Ce-SiO₂ glass ceramic was excited by a power-adjustable 450 nm laser to simulate a reflective white LD device.

The translucent GdAGG:Ce-SiO₂ glass ceramics are yellowish green, and the color becomes lighter with increasing sintering temperature. The transmittance of glass ceramics (1.0 mm thickness) in the visible region is approximately 60% due to the mismatch of refractive index between GdAGG:Ce phosphor and SiO₂. The X-ray diffraction patterns show that the glass ceramics exhibit both bulge peak of glass phase and diffraction peaks of Gd₃Al₂Ga₃O₁₂ (JCPDS 46-0448). The scanning electron microscopy images and laser scanning confocal microscopy images indicate that GdAGG:Ce phosphor powders with the sizes of 1-10 μm are uniformly dispersed in SiO₂ glass matrix. The transmission electron microscopy image indicates that the interfacial reaction is well suppressed at a low sintering temperature.

Under 450 nm excitation, the glass ceramics show typical Ce³⁺: 5d₁→4f broadband emissions peaked at 536 nm, whose luminescence characteristics are similar to those of GdAGG:Ce phosphor. The luminescent intensity firstly increases and then decreases, reaching the maximum value at 700 ℃. After removing the excitation source, the glass ceramics show a bright yellowish green afterglow. The longest afterglow appears from GdAGG:Ce-SiO₂ glass ceramic obtained at 700 ℃. Besides, the lifetime when the afterglow intensity decays to 80% of initial one is 27 ms, which meets a requirement of AC-LED devices (no less than 0.4 ms).

The internal quantum efficiency of glass ceramics is 63%, which meets the practical application standard of white LED devices (no less than 60%). The emission intensity at 150 ℃ maintains 85.3% of that at room temperature, indicating that the glass ceramics have an excellent thermal stability.

In coupling the glass ceramic with a 450 nm blue chip, the resulting white LED device exhibits a luminous efficiency (LE) of 81.7 lm/W, a correlated color temperature (CCT) of 5 805 K and a color rendering index (CRI) of 70.8 when driven by 150 mA direct current (DC). Besides, the flicker index decreases to 68.9% when driven via alternating current (AC). Coupled with a 450 nm blue laser, the white LD device shows the LE of 132.6 lm/W, the CCT of 6 528 K and the CRI of 70.6.

Translucent GdAGG:Ce-SiO₂ glass-ceramics were prepared via cold isostatic pressing and low temperature sintering (i.e., 700 ℃/5 h). The self-made GdAGG:Ce afterglow phosphor was uniformly embedded in SiO₂ glass matrix. The glass ceramic retained the luminescent properties of GdAGG:Ce phosphor due to little interfacial reaction. The glass ceramic exhibited a broad emission band peaked at 536 nm, whose quantum efficiency was 63% after excited at 450 nm. The emission intensity at 150 ℃ was 85.3% of that at room temperature. Besides, the glass ceramic exhibited a bright yellowish green afterglow, and the lifetime was 27 ms when the intensity decayed to 80% of initial one. The glass ceramic based white LED device had the LE of 81.7 lm/W, the CCT of 5 805 K, and the CRI of 70.8 when driven by 150 mA DC, and the flicker index reduced to 68.9% when driven by AC. The white LD device showed the LE of 132.6 lm/W, the CCT of 6 528 K and the CRI of 70.6. The results indicated that GdAGG:Ce-SiO₂ glass-ceramic could have a great application potential for AC-LEDs.