Due to the shortage of energy, the research and development and substitution of energy-saving products all over the world have become the trend of future industrial development, and lighting and display devices in daily life account for most of the power consumption, so LED (photodiode) semiconductor lighting technology has become the focus of competitive development all over the world. Traditional phosphors used in white LED are mainly inorganic compound materials such as silicate and aluminate. At present, another kind of high-performance oxynitride/nitride phosphor has outstanding advantages in color rendering index, luminous efficiency, color temperature, lifetime, chemical stability and thermal stability, so it has gradually become an important phosphor material system for white LED, and has been widely used in LED lighting and LED backlight display devices.
At present, the main method for preparing oxynitride/nitride phosphors is air pressure synthesis, which requires high equipment, consumes a lot of electricity and increases the maintenance cost accordingly. In terms of product quality, the powder is easy to agglomerate after being calcined at high temperature (above 1700 degrees), and the surface of particles is destroyed by later crushing, resulting in a large number of surface defects, which directly affects the luminous performance. In addition, the distribution of particle size is uneven, which makes the packing density of powder small, increases the scattering coefficient and reduces the luminous efficiency. The metal or metal nitride raw materials needed for the synthesis of nitride phosphors are not only expensive, but also extremely unstable in the air, which leads to the complicated preparation process and high production cost of these nitride phosphors. At present, only a few Japanese and American companies can provide this kind of phosphor stably. Therefore, it is urgent for China to develop green, reliable and low-cost synthesis technology to prepare high-quality oxynitride/nitride phosphors.
At present, Liu Lihong, a postdoctoral fellow, researcher Huang Qing and Dr. Xie Rongjun, a ceramic research and development team with structural and functional integration in Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, have successfully prepared high-quality nitride phosphors at low temperature and normal pressure by microwave method. Its basic heating principle is to use the coupling of raw material molecules and electromagnetic fields to convert microwave energy into heat energy to heat the object, so it can realize zero gradient uniform heating in a large area of the material, and the heating speed is fast and the sintering time is short, which is beneficial to obtain powder with uniform particle size distribution. For example, high-quality Ca-α-sialon: EU+phosphor for white LED was successfully synthesized at 1550℃ and atmospheric pressure. Compared with the conventional atmospheric pressure method (1700℃, 0.5MPa nitrogen protection), the synthesis temperature was reduced by 150℃, and compared with the luminescent characteristics of the powder synthesized by atmospheric pressure method at the same temperature, its luminescent intensity, quantum efficiency and thermal stability were significantly improved. The external quantum efficiency of the unoptimized phosphor tested under the excitation of 450nm wavelength can reach 48%, which is expected to reach or exceed the luminous efficiency of commercial powder after screening and purification. Fig. 1 is a picture of the physical luminescence of Ca-α-sialon: EU+phosphor synthesized by microwave method; Fig. 2 is a physical luminescent picture of SrSi2O2N2:Eu²+ green phosphor synthesized by microwave method.
The successful development of green low-cost technology of nitride phosphor will greatly reduce the cost of high-power LED lighting devices and backlight display, and will accelerate the promotion of semiconductor lighting technology in China on a large scale. At present, this technology has applied for China invention patent (20110152682.4) and PCT patent protection, and is promoting pilot production.