Abstract
Solid-state lighting (SSL) sources are a new type of green lighting source in the 21st century. SSL includes both using LED and laser chips to excite phosphor-converted materials to produce white light. Among these, Ce doped Y3Al5O12 ceramic phosphorsstand out due to their stable chemical properties, good mechanical performance, high thermal conductivity and excellent luminous efficiency. This thesis systematically investigates the sintering behavior, spectral characteristics, microstructure, and
luminescent properties of Ce:YAG-based ceramic phosphors, addressing issues such as complex preparation processes with high energy consumption, luminescence saturation, moderate thermal stability, and low color rendering. The chemical composition was optimized and established through the following key research content:
(1) The Ce:YAG ceramics sintered in vacuum without heat-treatment of the raw materials displayed better transparency than the samples sintered with raw materials after heat-treatment. 1750 ℃ was the optimum vacuum sintering temperature. With vacuum sintering method, the optimized 1mm Ce:YAG sample exhibited a transparency exceeding 80% at 550 nm. Sintering under flowing oxygen, fully dense impurities-free Ce:YAG ceramics were obtained. The simpler sintering equipment and the absence of the need for annealing make oxygen sintering more energy-efficient.
(2) The integration of Sc into Ce:YAG lattice resulted in a blue-shift of the peak location in the PL from 554 nm to 538 nm, and a narrowing of FWHM from 120 nm to 112 nm. On the other hand, the inclusion of Ce3+ resulted in the observation of red-shifted PL
spectra and an increase in the FWHM. The optimized Ce:YScAG ceramic phosphors3 exhibited a luminous intensity at 150°C that retains 88.7% of its value at room temperature. The fluorescence saturation threshold reached 3.5 W, and the luminous efficacy achieved 164 lm/W. Additionally, high-concentration doping of Ce ions (>3 at%) was achieved without causing fluorescence quenching, potentially addressing the chromaticity coordinate drift issue in the high-power white light LEDs/LDs.
(3) Mn2+ and Cr3+ were doped into YAG lattices successfully, exhibiting homogeneous microstructures, uniform doping distributions and good in-line transparency. By optimizing the doping concentration of Cr3+ and Mn2+, it is possible to attain deep red ( Cr3+: 2E→4A2) and orange emission (Mn2+: 4T1→6A1). The doped Ce3+ can act as both a sensitizer and an activator. With optimum Mn2+/Cr3+ doping concentration ratio, the CRI was boosted from 62.9 to 75.3.
(4) Ce/Mn/Cr: (Re,Y)3Al5O12 (Re = Gd, Tb and Lu) phosphor ceramics were fabricated. The crystal field splitting of [GdO8] and [TbO8] was more extensive than that of [YO8], causing a massive redshift in the Ce3+ emission peaks from 542 to 561 and 595 nm. The
white-light LED devices encapsulated with (Y0.664Gd0.333Ce0.003)3(Al0.9748Mn2+0.024Cr3+0.0012)5O12 phosphor ceramics exhibited a high CRI of 83.97.
The work presented thus achieved the objectives of YAG-based transparent ceramics with high transparency, energy-saving sintering processes, improved thermal stability, adjustable spectrum, high CRI value. It has the potential, after further improvements to be a promising phosphor-converted materials in future.
| Date of Award | Jul 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jun Jiang (Supervisor), Svenja Hanson (Supervisor) & Cheng Heng Pang (Supervisor) |