Annealing and firing in wet hydrogen are widely used steps in the processing alumina-ceramic insulators that may affect their dielectric breakdown strengths (DBS). In this study, the effects of annealing (at 1300 ℃ for 7 h) and firing in wet hydrogen on the DBS of alumina ceramics (all sintered at 1650 ℃) were studied, and the underlying mechanisms were analyzed by material characterizations. Annealing reduced the DBS of the 95% alumina ceramics due to the inter-granular phase crystallization, and the reduction in the DBS could be correlated to the reduction in mechanical strength. In contrast, annealing improved the DBS of the 99% alumina ceramic without intergranular phase transformation. Firing in wet hydrogen at 1500 ℃ caused the DBS increment, which can be ascribed to the reduction in the concentrations of point defects and electrical carriers.
Residual stress distribution in soldered structure of Kovar alloy and Al 2 O 3 ceramics was determined using XRD analyses. In order to measure the residual stress, position of the characteristic diffraction peak and stress constant were obtained using several versatile/advanced techniques after calibration. Residual stress of soldered structure was measured based on the diffraction patterns obtained for the distribution of residual stress in the soldered joint. Only diffraction peak at 149°for Kovar alloy and two diffraction peaks ranging from 140-170°for Al 2 O 3 ceramics were found to be appropriate for the residual stress determination. It was also confirmed that for Al 2 O 3 ceramics the XRD peak at 152°reflects the changes of stress more precisely than the one at 146°. The stress constant K of Kovar alloy and Al 2 O 3 ceramics was found to be −197 MPa/°and −654 MPa/°, respectively. After soldering, the maximum residual stress of the soldered joint of both materials developed at 1 mm from the soldering seam, and the values within 3 mm from the soldering seam are generally significant. Thus, it is important to pay attention to the area of 3 mm from the soldering seam in practical application.
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