Recently, plasma display panels (PDPs) have been used for high-definition television (HDTV) applications. However, there are some problems to be solved in terms of luminous efficiency and life time of PDPs which are directly related to the performance of phosphors used in PDPs. Among the phosphors for PDPs applications, the $BaMgAl_{10}O_{17}:Eu^{2＋}$ is an important blue phosphor. However, the luminance decrease and color shift of this phosphor are the well-known problems. Many researchers have tried to improve the stability of $BaMgAl_{10}O_{17}:Eu^{2＋}$. It has been found that the degradation process is caused by thermal treatment during PDP manufacturing, irradiation by VUV photons (< 200 nm), and ion sputtering. The thermal degradation during the manufacturing process is a particularly serious problem. It has been reported that these problems are probably caused by the crystal structure of $BaMgAl_{10}O_{17}:Eu^{2＋}$ (β-alumina) phosphor. Based on these information, we have selected $CaMgSi_2O_6$ as a host crystal for the $Eu^{2＋}$ activator. The $CaMgSi_2O_6$ has a one-dimensional like structure and its Ca site is tightly enclosed by eight oxide ions. Thus, the $CaMgSi_2O_6$ structure has a stable structure in comparison with $BaMgAl_{10}O_{17}$. In this study, we have optimized synthesis conditions of blue-emitting $CaMgSi_2O_6:Eu^{2＋}$ phosphor powder and evaluated its thermal stability after baking process.
The $CaMgSi_2O_6:Eu^{2＋}$ phosphor was synthesized by solid-state reaction with two-step firing process. In order to control the particle shape of $CaMgSi_2O_6:Eu^{2＋}$ phosphor, we checked the vapor pressure of starting materials at 1200 ℃. Because it means that the material of low-vapor pressure is the most stable one in this temperature. Thus, we could predict the particle growth based on stable particles. At last, we could control the shape of the final product by changing the shape of raw $SiO_2$. And we performed experiments on parameters such as a flux, $SiO_2$ mol ratio, activator concentration so as to improve brightness of $CaMgSi_2O_6:Eu^{2＋}$ phosphor. Finally, it is obtained that its PL intensity is as much as 63 % that of commercial $BaMgAl_{10}O_{17}:Eu^{2＋}$ phosphor.
To investigate the thermal stability, both $BaMgAl_{10}O_{17}:Eu^{2＋}$ and $CaMgSi_2O_6:Eu^{2＋}$ phosphors were baked in air at 500 ℃ and 600 ℃ for 20 min, respectively. And after baking process VUV/PL, ESR, XPS were measured for analysis. In the case of $BaMgAl_{10}O_{17}:Eu^{2＋}$ phosphor, the VUV emission spectra showed that the luminance was significantly reduced after the baking at 500 ℃ and 600 ℃, the color shift toward green was observed. Moreover, the ESR spectra showed that the $Eu^{2＋}$ signal was decreased after the baking process. And XPS spectra of Eu3d showed that the $Eu^{2＋}$ ions changed to $Eu^{3＋}$ ions after the baking process. These results suggest that the cause of luminance decrease be related with the oxidation of $Eu^{2＋}$. On the other hand, in the case of $CaMgSi_2O_6:Eu^{2＋}$ phosphor, the VUV/PL intensity was increased after the baking process and spectra of $Eu^{2＋}$ ion in both ESR and XPS analyses also slightly increased. It was believed that due to charge compensation $Eu^{3＋}$ ions reduced to $Eu^{2＋}$ ions during the firing process in air. Therefore, $Eu^{2＋}$ intensity increased after the baking process and the VUV/PL intensity of $CaMgSi_2O_6:Eu^{2＋}$ phosphor also increased slightly. It was revealed the reason why $CaMgSi_2O_6:Eu^{2＋}$ phosphor has a good stability in comparison with $BaMgAl_{10}O_{17}:Eu^{2＋}$ phosphor upon going through baking process for PDPs manufacture.
