Influence of crystallization inside glass frit on seal stress in ceramic metal halide lamps

Ceramics International, Volume 39, Issue 3, April 2013, Pages 2767-2774.

Takuya Honma, Hiroshi Kamata, Junichi Tatami.

 

Research and Development Center, Toshiba Lighting & Technology Corporation, 1-201-1 Yokosuka, Kanagawa 237-8510, Japan

Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Yokohama, Kanagawa 240-8510, Japan

 

Abstract

 

Ceramic metal halide lamps use polycrystalline aluminum oxide as an arc tube material; cracks inside the glass frit—used as the seal material—have been known to occur occasionally. This study measured the stress on the lamp seals caused by changes in the cooling rate during the sealing process by a 2D stress measurement method. Seal stress decreased with reducing cooling rate. Therefore, we discuss the influences of the glass frit’s microstructure and the coefficients of thermal expansion (glass frit, capillary, and Nb wire) on the seal stress. The coefficient of thermal expansion of the annealed glass frit was essentially closer to those of the capillary and Nb wire, while that of the rapidly cooled glass frit differed greatly. Moreover, the glass frit of the rapidly cooled lamp seal contained only an amorphous phase (Dy, Si, Al, and O), while the glass frit of the annealed lamp seal contained both an amorphous phase (Dy, Si, Al, and O) and a crystalline one (Dy2SiO5 and Al2O3). Fracture toughness was found to be larger in the crystals than in the amorphous phase area. Moreover, it was larger in the area where crystalline Dy2SiO5 and Al2O3 were present compared to the area where only crystalline Dy2SiO5 was present.

Thus, the coefficient of thermal expansion of the glass frit containing the crystalline phase was found to influence the seal stress. Furthermore, the crystallization in the glass frit was found to influence crack propagation. Therefore, formation of cracks inside the glass frit can be prevented by optimizing these factors, which can be achieved by changing the cooling rate in the sealing process without changing the specifications of the glass frit (composition, composition ratio, etc.).

 

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Additional information

 

Our study measured the stress on ceramic metal halide lamp seals, caused by changes in the cooling rate during the sealing process. The seal stress decreased with a decrease in the cooling rate. Therefore, we discussed the influences of the coefficient of thermal expansion (glass frit, capillary, and Niobium wire) and the glass frit’s microstructure on the seal stress. We observed that the coefficient of thermal expansion of the glass frit containing the crystalline phase was found to influence the seal stress. Furthermore, the crystallization in the glass frit was found to influence crack propagation. Therefore, formation of cracks inside the glass frit can be prevented by optimizing these factors.

 

Fig.1 Scanning Electron Microscope image of microstructure inside glass frit of lamp seal

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