Abstract: Single crystal sapphire is conventionally employed for mid-wave IR windows and domes exposed to high aerodynamic heating environments. Single crystal sapphire, however, suffers a loss of compressive strength in the c-axis of 95% on heating from ambient to 800.degree. C. This loss of compressive strength on heating results in poor thermal shock resistance. Poor thermal shock resistance jeopardizes the reliability of the windows and domes. The process of the present invention strengthens sapphire and improves its resistance to thermal shock by introducing magnesium (Mg) ions into the crystal lattice of sapphire to form a solid solution. The solid solution increases the strength of the sapphire. Additionally, a larger increase in strength results from converting the Mg into second phase precipitates comprising magnesium aluminate spinel.

Abstract: Methods of arc suppression connecting a PTC material in parallel with a pair of contacts but in series with a second pair of contacts. The PTC material could be doped-BaTiO.sub.3 -ceramics, conductive polymer, or metallic PTC materials. The two pairs of contacts should be so mechanically associated that the second pair of contacts must be always opened right after the opening of the first pair. It is enough for some applications to connect one pair of contacts in parallel with a polyswitch or BaTiO.sub.3 -ceramics. For medium and high voltage circuit breakers, more than two pairs of contacts may be needed, and all these contacts should be opened sequentially during a circuit interruption. According to the methods, simple structured circuit breakers can be made to protect circuits from a short circuit, an overload and a ground fault. The circuit breaker invented here can provide an electronic alarm signal when a fault current occurs.