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

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.