A non-invasive microwave method for assessing solid-state ammonia storage

• A proposed measurement process to non-invasively measure dielectric properties of materials in conjunction with other measurement techniques.
• Analysis of the measurement setup as an effective gas sensor.
• A study of dynamic changes in halide materials due to ammonia absorption process.
• Identify relationships between the changes in dielectric and structural properties of a material.

A 2.45 GHz microwave cavity resonator is used to measure the change in dielectric properties of alkali halide salts (such as CaI2) when exposed to ammonia gas. This technique is based on the change in electric dipole moment of the material that occurs as a function of ammonia content, and so can be used to determine ammonia concentration in solids in a non-invasive way. When a powdered sample is placed in the electric field of the TM010 mode of a resonant cylindrical cavity, we find that ammonia absorption gives a first order change in material polarisation (i.e. real permittivity), related to the ammonia sequestered within the solid. The associated dielectric losses (i.e. imaginary permittivity) exhibit second order transitions, which we believe are due to order–disorder transitions between the different coordination complexes of the halide salt.