Minna Niittymäki

Dielectric properties of HVOF sprayed ceramic coatings

Thermally sprayed ceramic coatings can be used as electrical insulators for example in high temperature applications (e.g. fuel cells) or in other demanding conditions. In electrical insulation applications the mostly used coating materials are aluminum oxide, magnesium oxide and magnesium aluminate. In general, only few reports of dielectric properties of thermally sprayed ceramic coatings can be found in literature and further analysis is thus needed. In addition, the measurement methods and conditions in previous research are often not fully documented, complicating the evaluation and comparison of the properties of different coatings. The aim of this paper was to characterize dielectric properties of thermally sprayed ceramic spinel coating sprayed with high-velocity oxygen fuel (HVOF) technique. The studied dielectric properties are DC resistivity, DC dielectric breakdown strength, as well as permittivity and dielectric losses at different frequencies. All measurements were made at temperature of 20 °C and at relative humidity of 20 %. Dielectric properties and the composition of coating material are presented and analyzed.

Electric field dependency of dielectric behavior of thermally sprayed ceramic coatings

High temperature applications e.g. fuel cells require ceramic based insulation solutions instead of polymers. The aim of this paper was to characterize the dielectric properties of thermally sprayed ceramic coatings; especially the electric field dependency of AC and DC behavior of thermally sprayed ceramic coatings. One of the spinel samples and one of the alumina samples have quite similar lamellar microstructure which may partly explain their similar type of behavior in DC resistivity as well as in AC loss indexes at low frequencies. These two samples had smaller lamellar size than the other alumina and spinel samples which also had quite similar behavior of AC losses at low frequencies, respectively. In addition to the lamellar size and structure, also micro cracks in the coating microstructure are proposed to have an effect on the dielectric behavior and its electric field dependency.

Influence of humidity and temperature on the dielectric properties of thermally sprayed ceramic MgAl 2 O 4 coatings

Thermally sprayed ceramic coatings can be used as electrically insulating materials for example in high temperature applications (e.g. fuel cells) or in other demanding conditions where ceramic-based solutions are needed instead of e.g. polymers. The dielectric properties of thermally sprayed ceramic coatings are strongly affected by external conditions. The aim of this paper is to characterize the dielectric properties of thermally sprayed ceramic MgAl2O4 coatings; especially the effects of ambient conditions on certain dielectric properties of thermally sprayed coatings are studied. DC resistivity at various electric field strengths as well as permittivity and losses at different frequencies is reported in the paper for MgAl2O4 samples made by three different thermal spray techniques. These measurements were performed at three temperatures as well as at two different relative humidities. The DC breakdown strength was studied at one condition. Due to the slightly open porous microstructure of the studied coatings, increasing humidity particularly increases the dc conductivity and relative permittivity.

Role of microstructure in dielectric properties of thermally sprayed ceramic coatings

Thermally sprayed insulating ceramic coatings can be utilized in conditions where polymers are inapplicable. The coatings exhibit a special lamellar microstructure with interfaces and some defects (e.g. voids, cracks) in between. The aim of this study was to analyze the relationship between the microstructural features and the dielectric properties of various thermally sprayed ceramics. The structural characterization of the ceramic coatings was made based on following properties: porosity, volumetric gas permeability and the characteristic size of crystalline lamella. High gas permeability of the coatings decreased the breakdown strength but similar effect cannot be seen in the DC resistivity and the permittivity results. Decrease of DC resistivity at high humidity did not correlate to microstructural properties; rather it is speculated to indicate the hydrophilic nature of the coatings. The characteristic crystalline domain sizes showed no clear correlation with the dielectric properties.

Differences in AC and DC large-area breakdown behavior of polymer thin films

A large-area multiple breakdown measurement using self-healing gold sputtered electrodes was evaluated in determining the AC breakdown characteristics of insulating biaxially oriented polypropylene (BOPP) and BOPP-silica nanocomposite thin films. Results were compared with conventional small area measurements and it is shown that the large-area approach could be extended into AC measurements as long as good quality film-electrode interface in ensured. Special electrode arrangements are needed due to the absence of electrostatic force. AC small-area breakdown strength of pilot-scale films was ramp rate dependent but no such effect was clearly noticed for a commercial product. Conventional DC small-area breakdown measurements were also conducted and the results were compared to previously published large-area multiple breakdown data. The consistent results support the dependency between ramp rate and DC breakdown strength in the studied BOPP-silica nanocomposite.

DC conduction and breakdown behavior of thermally sprayed ceramic coatings

In this study, the DC conductivity from low electric fields up to breakdown fields is studied for several different thermally sprayed ceramic coatings. Although the DC conductivity of bulk alumina ceramic has been observed to follow the space charge limited current conduction mechanism, the studied ceramic coatings do not follow or follow only partly this mechanism. Possible reason for this is their different microstructure since bulk alumina exhibits fully crystalline microstructure while the ceramic coating consists of crystalline and amorphous regions as well as voids, defects and numerous interfaces. A possible conduction mechanism of the ceramic coatings based on the different conductivities of the amorphous and crystalline regions of the coatings is proposed. The microstructural features (e.g. volumetric porosity) are found to affect the breakdown strength for some of the studied coatings. The step-test breakdown strengths of the coatings were lower than the ramp-test ones due to the longer stress durations in step tests giving an indication of effects of electrical stress duration and possible short-term degradation of the coatings.