Low Magnetic Field Magnetocaloric Effects And Tailoring Mechanism Of EuTiO₃-based Compounds In The Liquid Helium Temperature Range

Refrigeration in the liquid helium temperature range plays an imperative and strategic role in many scientific research frontiers and engineering fields.However,the efficiency of Gifford-Mc Mahon（GM）refrigerators,currently the mainstream refrigeration technology in the liquid helium temperature range,is as low as 1%of that of Carnot cycle.Hybrid GM magnetic refrigerator using high-performance magnetic refrigerants can achieve a higher cooling efficiency and a larger cooling power.Therefore,it is of strategic importance to develop high-performance magnetic refrigeration materials operating in the liquid helium temperature range.In this dissertation,antiferromagnetic（AFM）EuTiO₃ possessing considerable magnetocaloric effect（MCE）was selected as the research object.The magnetic phase transition,magnetic properties and MCEs as well as their tailoring mechanism of EuTiO₃-based compounds were systematically investigated.In addition,the thermal conductivity and strengthening mechanism of EuTiO₃-based magnetic refrigerants were studied,and an exploration of their magnetic refrigeration applications was carried out.Elemental substitution is a common and effective way to tailor the structure and performance of a functional material.In this dissertation,the influence of elemental substitution on the structure and performance of EuTiO₃-based compounds was investigated by combining first-principles calculations and experiments.Results show that an appropriate substitution of Ti⁴⁺by Zr⁴⁺ions can cause significant lattice expansion without changing the crystal configuration.The lattice expansion contributes to enhancing the ferromagnetic（FM）coupling and leads to a reversal of the sign of the exchange constant J₁,thus promoting the AFM-FM transition in EuTiO₃.Meanwhile,elemental substitution can change the electronic interactions and suppress the super-exchange between the Eu 4f and Ti 3d states,weakening the AFM exchange.In addition,an appropriate substitution of Nb introduces itinerant electrons,which can induce RKKY interaction and thus enhance the FM coupling.In summary,elemental substitution reduces the AFM exchange and enhances the FM coupling by inducing lattice expansion,changing electronic interactions and introducing itinerant electrons,to achieve an FM ground state in the EuTiO₃ magnetic system.Eu（Ti,Zr）O₃,Eu（Ti,Nb,Zr）O₃ and Eu（Ti,Nb,Al）O₃ compounds all exhibit significant FM behavior,with a dominance of long-range ferromagnetic interactions.Elemental substitution causes a faint change in the phase transition temperature,but a significant decrease in the critical magnetic field for these compounds,most of which approach magnetic saturation in a field of 1 T.Meanwhile,the type of magnetic phase transition changes from first-order for the parent EuTiO₃ to second-order for these compounds.As expected,giant low-field MCEs have been observed in the compounds due to the FM state induced by elemental substitution.In a field change of 0-1 T,the maximum magnetic entropy change of the Eu（Ti,Nb,Zr）O₃ compounds reaches 19.6 J·kg^-1·K^-1,being the peak ever reported for the materials operating in the liquid helium temperature range.The corresponding refrigeration capacity and maximum adiabatic temperature change are 82.7 J·kg^-1and 5.4 K,respectively.It is worth noting that the peak value of their magnetic entropy changes reaches 11.8 J·kg^-1·K^-1 under a field change of 0-0.5 T,making them promising candidates for magnetic refrigeration in the liquid helium temperature range.Although the EuTiO₃-based compounds mentioned above exhibit outstanding low-field MCEs,the low thermal conductivity limits their practical applications.Introducing components with high thermal conductivity,such as copper or graphene,can significantly improve the thermal conductivity without remarkably reducing the magnetocaloric effect.The thermal conductivity of EuTiO₃-based composites with the addition of 5 wt.%Cu and 0.18 wt.%C increases by 260.0%and 170.0%,respectively,compared to the base material.A one-step reducing technology has been developed for the preparation of EuTiO₃-based compounds,which enables a batch preparation of the compounds using graphite as the reducing agent.The optimal amount of graphite addition is determined to be 2.5 wt.%,and the best synthesis parameter is sintering at 1300°C for 2 h.Furthermore,high-performance EuTiO₃-based spherical particles can be prepared by using an improved three-fluid spray drying technique,which can be used as a filler of the active magnetic regenerator of hybrid GM magnetic refrigerators to achieve excellent refrigeration performance.