The Effect Of The Crystalline Phase On The Dielectric Properties Of Plasma Sprayed Al₂O₃ Coatings

The low cost Al2O3 ceramics,rich in natural resources, is widely used in the field of electronic technology such as ceramic shell, tube vacuum capacitor, the high-power metal ceramic tubes, microwave tubes for lose energy window of the ceramic components, ceramic substrates and semi-conductor ceramic package shell and so on.However. For the poor conditions,the dielectric properties of alumina changes in a fairly wide range, resulting in poor consistency of product performance. The coatings, widely used in a vacuum device, may appear fire in ceramic surface, decreasing in electrical insulaticn, and overheating of too large dielectric loss, and suriously, the device fails.Lots of references and researches show that the dielectric properties of Al2O3 ceramics coating depends on the physical characteristics of the main crystalline phase.Circumferential capacitors made from alumina coatings with low dielectric constants were prepared using atmosphere plasma spraying on the SUS316 substrate at different input power levels by changing the arc currents from 300 A to 600 A with a step of 150 A to create different crystalline phases. The results show that the phase of the original powders was a-Al2O3 without y-Al2O3. Theγ-Al2O3 phase, however, was present in the coatings, and the amount of the y phase increased with increasing arc currents. The dielectric constant of the alumina coatings increased with increasing amounts of theγ-Al2O3 phase in the coatings, and the electric resistance decreased. The capacitance is not only related to the phase of alumina in the coating but also the measurement frequency. The phase state present in the coatings is the primary determinant of the coatings’ dielectric constants compared with the air humidity. The density and hardness of the coatings were also measured.