Research On The Hydrogenation Of Er₃ni And The Thermophysical Properties Of Its Hydrides

Regenerative cryocoolers have been widely used in cryopump, cryogenic vent-less storage and the cooling of electronic equipments such as superconducting devices and infrared detectors. Higher requirements in cooling temperature, cooling capacity and efficiency become a necessity for various application cases. The regenerative material is an important factor for the cooling performance. The specific heat of traditional regenerative materials (such as lead and stainless steel) for a cryocooler drops to a very low value (even lower than that of gas helium) at temperatures lower than 20 K, resulting in a difficulty for the cryocooler to reach a temperature as low as liquid helium temperature. Fortunately, magnetic materials, such as Er3Ni, ErNi, ErNio.9Co0.1, Ho2Cu and GdAlO3, have been used in regenerative cryocoolers, thanks to their abnormal peak values of specific heat induced by the magnetic phase transition at low temperatures. The alloy ER3Ni, with a specific heat peak at the temperature range of 4-15K, is one of the common regenerative materials used in 4 K cryocoolers.Previous investigation on a pulse tube cryocooler with He-H2 mixtures as working fluid and Er3Ni as regenerative material achieved a higher refrigeration capacity at 30 K than that from thermodynamic analysis. This improvement was preliminarily attributed to the reaction between Er3Ni and H2, i.e., hydrogenation of Er3Ni. A couple of literatures reported some research work on the hydrogenation of Er3Ni or similar alloys. However, their work mainly focused on the theoretical analysis of crystal structure and the magnetic or electronic property of the hydrides. The practical hydrogenation process and the thermophysical properties and stability of the hydrides, which are important for cryocoolers and have not been sufficiently studied, attract our enthusiasm. The hydrogenation mechanism of Er3Ni is analyzed analogically based on universal mechanism of metallic hydrogenation. A classical experimental apparatus has been built up to explore the hydrogenation process. The influences of particle size, reaction temperature and pressure on the induction time and hydrogen sorption of Er3Ni are emphatically studied, and the characteristics of hydrogenation process and its hydrides are also analyzed. The p-c-T dependence of Er3Ni hydrogenation processes is measured to investigate the stability of hydride Er3NiHx. Moreover, measurements such as XRD (X-ray Diffraction) and PPMS (Physical Property Measurement System) are carried out to study the crystal structure, the reaction mechanism and the specific heat capacity of hydride Er3NiHx, aiming to validate the feasibility of Er3NiHx as a novel regenerative material for cryocoolers.