Abstract
Recently, a laboratory setup for microwave-based characterization of powder samples at elevated temperatures and different gas atmospheres was presented. The setup is particularly interesting for operando investigations on typical materials for exhaust gas aftertreatment. By using the microwave cavity perturbation method, where the powder is placed inside a cavity resonator, the change of the resonant properties provides information about changes in the dielectric properties of the sample. However, determining the exact complex permittivity of the powder samples is not simple. Up to now, a simplified microwave cavity perturbation theory had been applied to estimate the bulk properties of the powders. In this study, an extended approach is presented which allows to determine the dielectric properties of the powder materials more correctly. It accounts for the electric field distribution in the resonator, the depolarization of the sample and the effect of the powder filling. The individual method combines findings from simulations and recognized analytical approaches and can be used for investigations on a wide range of materials and sample geometries. This work provides a more accurate evaluation of the dielectric powder properties and has the potential to enhance the understanding of the microwave behavior of storage materials for exhaust gas aftertreatment, especially with regard to the application of microwave-based catalyst state diagnosis.
Highlights
In order to meet the legal emission standards, customized automotive exhaust gas aftertreatment systems are required
Using the microwave cavity perturbation (MCP) method, the dielectric properties of the sample are determined by means of the differential measurement between the empty and a sample-filled resonator
Since powder samples typically have a high porosity and can be dosed flexibly in small amounts, it can be assumed, that the field disturbance caused by the introduction of the sample is negligible
Summary
In order to meet the legal emission standards, customized automotive exhaust gas aftertreatment systems are required. Using the microwave cavity perturbation (MCP) method, the loading state of an exhaust gas aftertreatment components can be measured operando, i.e., during operation, without contacting electrodes [1,2,3] In this method, the catalyst housing serves as a cavity resonator, in which standing electromagnetic waves are excited at discrete frequencies via coupling elements ( designated as antennas). The ammonia load of a SCR (selective catalytic reduction) catalyst [11,12,13], the nitrogen oxide (NOx ) storage of a lean NOx trap [14,15], and the soot load of a diesel particulate filter (DPF) [16,17] can be evaluated with the RF system These microwave-based systems are useful for real-world applications. The accuracy of the calculation method, alternative approaches and the transferability of the solution to other typical materials for exhaust aftertreatment are discussed
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