Influence of high temperature annealing on the dielectric properties of alumina films prepared by the aerosol deposition method

The aerosol deposition method (ADM) is a novel coating technique that allows to fabricate dense and nanocrystalline ceramic films at room temperature. To investigate the dielectric properties of aerosol deposited alumina films at high temperatures and the influence of annealing on them, the temperature was increased in steps of 100 degrees C from 200 degrees C to 900 degrees C and subsequently cooled down stepwise again. At each step, the dielectric properties were measured by impedance spectroscopy between 50 mHz and 200 kHz. During the heating steps, the relative permittivity and also the loss tangent showed a disordered behavior with various maxima in the loss tangent. After reaching 900 degrees C, during cooling, the behavior was more ordered, and the loss tangent exhibited only one maximum that appeared at lower frequencies. Overall, the annealing reduces the loss tangent at low frequencies and low temperatures. The origin of this behavior could lie in the annealing of defects, which are incorporated into the layer when the particles hit the surface of the substrate and crack while being deposited via ADM following the room temperature impact consolidation mechanism (RTIC).