Research On The Hydrogenation Characteristies Of Magnetic Regenerative Material Er3Ni And The Properties Of Its Hydrides

Cryocoolers act an important role in military defence and modern industries, with the applications in space aeronautics, energy industry, electronic information systems, biological medicine and other fields. The thermophysical properties of regenerative material are key factors for the refrigeration performance of a regenerative cryocooler. The specific heat of traditional regenerative materials for a cryocooler drops to a very low value at low temperatures, even lower than that of gas helium at a temperature below10K, which results in a low regenerator efficiency and large difficulty for the cryocooler to reach liquid helium temperature. Many magnetic materials, such as Er3Ni, Ho2Cu and GdAlO3, have been consecutively proposed as the regenerative materials for cryocoolers, thanks to their abnormal peak values of specific heat caused by magnetic phase transition. The successful use of these materials brought many breakthroughs for regenerative cryocoolers in improving refrigeration performance at low temperatures.Theoretical analysis indicated that a higher cooling power and coefficient of performance could be achieved with He-H2mixture as working gas than those with pure He for a pulse tube cryocooler working around30K temperature region, which has been experimentally verified. However, the experiments on a pulse tube cryocooler with He-H2mixtures as the working gas and Er3Ni as the regenerative material of regenerator’s second stage demonstrated a20%extra increase in refrigeration performance than the results from thermodynamic analysis, which was attributed to the formation of hydride Er3NiHx from the Er3Ni-H2reaction and the hydrid has a higher specific heat than that of Er3Ni. Till now, the research on the hydrogenation of Er3Ni and the thermophysical properties of Er3NiHx is seldom reported, except for a few studies on its electromagnetic properties. This thesis emphasizes on the hydrogenation of magnetic regenerative materiai and the properties of Er3NiHx, especially in the system of materials science and cryophysics. The work includes four aspects as follows:1) The mechanism of hydrogen absorption for Er3Ni is expounded. The process how hydrogen molecules split up into atoms and enter Er3Ni particles is studied from the dynamic point of view, referring to the hydrogenation mechanism of other hydrogen-adsorption alloys. The hydrogen absorption capacity of Er3Ni is then deduced.2) The experimental apparatus for hydrogen absorption of Er3Ni is designed and fabricated. Some further improvements are also made for different reaction conditions, such as a reacting temperature lower than ambient temperature.3) The activation characteristics, hydrogenation process (including reversibility and stability) and hydrogenation properties at low temperatures are experimentally investigated and qualitatively analyzed.4) The magnetization and the specific heat of Er3NiHx are measured by Magnetic Property Measurement System (MPMS) and Physical Property Measurement System (PPMS), respectively. The influence of hydrogenation on the magnetic phase transition and specific heat of Er3Ni, as well as its underlying mechanism, is analyzed and discussed.