Dielectric Properties of Materials Used for Microwave-Based NO x Gas Dosimeters

Nitrogen oxides (NO ), primarily generated from combustion processes, pose significant health and environmental risks. To improve the coordination of measures against excessive NO emissions, it is necessary to effectively monitor ambient NO concentrations, which requires the development of precise and cost-efficient detection methods. This study focuses on developing a microwave- or radio frequency (RF)-based gas dosimeter for NO detection and addresses the optimization of the dosimeter design by examining the dielectric properties of LTCC-based (Low-Temperature Co-fired Ceramics) sensor substrates and barium-based NO storage materials. The measurements taken utilizing the Microwave Cavity Perturbation (MCP) method revealed that these materials exhibit more pronounced changes in dielectric losses when storing NO at elevated temperatures. Consequently, operating such a dosimeter at high temperatures (above 300 °C) is recommended to maximize the sensor signal. To evaluate their high-temperature applicability, LTCC substrates were analyzed by measuring their dielectric losses at temperatures up to 600 °C. In terms of NO storage materials, coating barium on high-surface-area alumina resolved issues related to limited NO adsorption in pure barium carbonate powders. Additionally, the adsorption of both NO and NO was enabled by the application of a platinum catalyst. The change in dielectric losses, which provides the main signal for an RF-based gas dosimeter, only depends on the stored amount of NO and not on the specific type of nitrogen oxide. Although the change in dielectric losses increases with the temperature, the maximum storage capacity of the material decreases significantly. In addition, at temperatures above 350 °C, NO is mostly weakly bound, so it will desorb in the absence of NO . Therefore, in the future development of a reliable RF-based NO dosimeter, the trade-off between the sensor signal strength and adsorption behavior must be addressed.