Synthesis and Investigation of Phosphate Phosphors for Light Emitting Diodes
Date Issued
2007
Date
2007
Author(s)
Lin, Chun-Che
DOI
zh-TW
Abstract
The energy crisis awareness is improving gradually and many countries diverted their research propaganda towards the development of the novel lighting equipments. In this regard, the new trend has been developed to use light-emitting diodes (LEDs) as a source of light, which is considered to be advantageous in many respects such as energy saving, lower contamination, lower heat generation, higher response speed and longer lifetime. In 1996, the first white LED was developed by Nichia Chemical Company, Japan. They utilized a blue LED chip (InGaN) in combination with a yellow phosphor (cerium doped yttrium aluminum garnet; YAG:Ce) to generate white light. This opens up the new directions for the development of LEDs with great advantages that will leap forward and march towards its optimum use in illumination.
However, the above white LED presented the major drawbacks of lower color rendering index (CRI), cool light and lower thermal stability. To overcome this problem, there is a kind of phosphor technology being developed at leading companies, which focuses on improving their efficiency, power stability for prolonged period, color temperature and thermal stability. The aim of this work is to develop the novel phosphors based on phosphate host matrix, which can produce superior blue, green and red lights under excitation by UV-LED or blue range and exhibit thermal stability at higher temperature.
In this study, we prepared the phosphate compounds by using solid state reaction. The general chemical formula of these materials is ABPO4 (A = Li, K；B = Sr, Ba). The host matrixes were doped with different rare earth elements (Eu, Tb and Sm). We investigated the influence of change in crystal field which composed of different alkaline metal (Li, K) or alkaline-earth metal (Sr, Ba) cations. The effect of sintering of these materials at different temperatures to explore their PL and chemical properties has also been investigated. In addition to this we also studied their luminescent mechanism and concentration quenching effect.
The characterization of the phosphors was carried out by X-ray diffraction (XRD) to study the crystal structure and photoluminescence (PL) measurements were carried out to show excitation and emission properties. The emission spectra were transformed to the Commission International de I ‘Eclairage (CIE) by the Colortt program to realize their color coordinates. The reflective ultraviolet/visible spectrometer (UV-Vis) revealed the absorption mechanism of phosphor in consistent with the PL measurements. We also used scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX) to obtain particle size, morphology, and chemical composition respectively. The thermo gravimetric analysis (TGA) and differential scanning calorimetric (DSC) analyses were performed to examine the weight loss and chemical reactions occur during the heat treatment. Finally, the thermoluminescence (TL) measurements suggested luminescence stability of the powders at higher temperature. The part of our work has been published in Appl. Phys. Lett. [90, 151108 (2007)] and also we have filed a patent.
Subjects
螢光粉
磷酸鹽類
白光發光二極體
phosphor
phosphate
white light emitting diodes
Type
thesis