Bismuth borate zinc glass braze for bonding sapphire in air

Reliable brazing of sapphire was successfully achieved by using a novel 50Bi O-2(3)-40B(2)O(3)-lOZnO (mol%) glass braze and joining cycles that peak at temperatures of 600 - 700 degrees C for 20 min in air. The effect of the brazing temperature on microstructure evolution as well as mechanical properties of the sapphire/sapphire joints was investigated. The results showed that Bi4B2O9 and Bi24B2O39 were formed in the joints brazed at 600 and 625 degrees C, respectively. ZnAl2O4 particles were formed in the joints when the brazing temperature was 635 degrees C due to a reaction between the glass and the sapphire substrate. With the brazing temperature increasing, ZnAl2O4 particles exhibited steady growth, from similar to 160 nm at 635 degrees C to similar to 455 nm at 700 degrees C. Besides, ZnAl2O4 particles began to aggregate when the temperature was 675 degrees C. The strength increased sharply when the brazing temperature increased to 635 degrees C, which was mainly attributed to the formation of ZnAl2O4 particles in the joint domain. The maximum shear strength of 85 MPa was obtained for the joints brazed at 650 degrees C for 20 min. The joint shear strength decreased due to the aggregation of ZnAl2O4 particles in the joint.