Zhang Ming-fu

Thermal Stresses and Cracks During the Growth of Large-sized Sapphire with SAPMAC Method

Abstract The finite-element method has been used to study the thermal stress distribution in large-sized sapphire crystals grown with the sapphire growth technique with micro-pulling and shoulder-expanding at cooled center (SAPMAC) method. A critical defect model has been established to explain the growth and propagation of cracks during the sapphire growing process. It is demonstrated that the stress field depends on the growth rate, the ambient temperature and the crystallizing direction. High stresses always exist near the growth interfaces, at the shoulder-expanding locations, the tailing locations and the sites where the diameters undergo sharp changes. The maximum stresses always occur at the interface of seeds and crystals. Cracks often form in the critical defect region and spread in the m-planes and a-planes under applied tensile stresses during crystal growth. The experimental results have verified that with the improved system of crystal growth and well-controlled techniques, the large-sized sapphire crystals of high quality can be grown due to absence of cracks.

Optical Temperature Sensor Using Infrared-to-Visible-Frequency Upconversion in Er3+/Yb3+-Codoped Bi3TiNbO9 Ceramics

An optical temperature sensor based on infrared-to-visible upconversion emission in Er3+/Yb3+ co-doped Bi3TiNbO9 (BTN) ceramics is reported. The fluorescence intensity ratio of the green upconversion photoluminescence (UC-PL) around 524 nm and 545 nm depends on temperature. The operating temperature range and the maximum sensitivity of Er3+/Yb3+ co-doped Bi3TiNbO9 ceramics are 123–693 K and 0.0032 K−1, respectively. BTN:Er3+/Yb3+ ceramic has good thermal, physical and chemical stability, great UC-PL intensity and low cost fabrication. The results imply that Er3+/Yb3+ co-doped Bi3TiNbO9 ceramic is promising for applications in wide-temperature-range sensors.