Infrared emission imaging as a tool for characterization of hydrogen storage materials

Combinatorial thin films provide an opportunity for studying a variety of properties over a wide range of compositions and microstructures on a single substrate, allowing substantial acceleration of both the fabrication and study of materials and their properties. This paper details the use of infrared (IR) emissivity imaging for studying the in situ hydrogenation of Mg_xNi_1-x films with hydrogen gas; the method is shown to be a powerful combinatorial screening tool for metal hydride storage materials. The 100 nm thick Mg_xNi_1-x composition gradient films (0.4 < x < 0.9) capped with a Pd layer of varying thickness were deposited in a combinatorial electron-beam deposition chamber using a shutter-controlled multilayer technique. The microstructure of as-deposited and 250 °C-annealed films was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM studies of the "X-ray amorphous" films show that the microstructure consists of nano-scale grains of a metastable fcc phase as well as Mg₂Ni and MgNi₂ phases over a broad range of higher Ni compositions. The metastable phase appears to be a Ni-stabilized fcc form of Mg. Hydrogenation differences between the studied films and bulk alloys are suggested to be associated primarily with crystallographic differences of the metallic and hydride phases. Hydrogen absorption and desorption of the films were monitored with an infrared camera capable of simultaneously imaging the entire composition spread. The observed changes in infrared intensity during hydrogen loading/unloading demonstrate the sensitivity of the method to hydrogen absorption behavior of different compositions and microstructures.