Preparation, Magnetic Properties And Magnetocaloric Effects Of （Mn,Fe）₂（P,Si） Compounds

Based on the magnetocalotic properties of magnetic materials, magnetic refrigeration hasbeen applied to cooling system. Magnetic refrigeration has been attracted much attention dueto its high efficiency, energy-saving, and environmental friendliness. Nowadays, theresearches of magnetic refrigerant at room temperature mainly focus on rare earth and itsalloys, rare earth-transition-metal-based compounds, transition-metal-based alloys andperovskite oxide materials, etc. Especially，transition-metal-based Fe₂P-type MnFeP_1-xAs_xcompounds become one of the best candidate materials for room temperature application,because of their giant magnetocalotic effect （GMCE），low-cost of raw materials and simplepreparation. In order to avoid use of As, a great deal of work has been done to replace As byother elements. The results show that As can be replaced totally by Ge or Si, but Ge is moreexpensive than Si and the partially or totally replacing As by Si will lead to large thermalhysteresis （Thys），which is unfavorable for magnetic cooling applications. In this dissertation,the structure and magnetic properties of （Mn,Fe）₂（P,Si） compounds have been studied byemploying various preparation technologys, adjusting the ratios of Mn/Fe, P/Si and adding Bto decrease the thermal hysteresis and to improve magnetocaloric effect.Firstly，Mn_1.2Fe_0.8P_0.6Si_0.4B_0.05compound was prepared by ball milling and melt spinningtechniques.The effects of two different techniques on the structure, phase transformation andmagnetocaloric properties were systematically investigated by XRD, SEM and VSM methods.Then, a series of Mn_2-xFe_xP_0.55Si_0.45（x=0.8,1.0,1.1） compounds were prepared by ball milling.A second phase of （Mn,Fe）₃（P,Si） was detected in all compounds. The Curie temperatureranges from181to229K with inereasing Fe content from0.8to1.1, but the thermalhysteresis of these compounds are almost unchanged. The maximum magnetic-entropychange （|ΔSM|） of the compound with x=1.1reaches29.6J·kg^-1K^-1in a field change from0to5T. The|ΔSM|of Mn_2-xFe_xP_0.55Si_0.45compounds is larger than that of Gd in the same magneticfiled change. However, the second phase and the existence of large thermal hysteresis isunfavorable to the practical application of this compound.In order to decrease the thermal hysteresis and improve magneto-caloric properties,interstitial atom of B was added into （Mn,Fe）₂（P,Si） compounds. Mn_1.2Fe_0.8P_0.6Si_0.4B_x（x=0,0.03,0.05） series of compounds were prepared by melt-spinning technique. The results showthat these samples consist of single phase with two different compositions.This phase hasFe₂P-type hexagonal structure, which belongs to P62mspace group. The grains are uniformly distributed and have regular hexagonal shape. The Curie temperature increasesfrom152to197K and thermal hysteresis decreases from20to9K with increasing B contentfrom0to0.05. The maximal magnetic-entropy change are60.2、22.8、15.6J·kg^-1K^-1respectively in a field change from0to5T. The type of this transition is the first-order phasetransition in these compounds.The thermal hysteresis of Mn_1.2Fe_0.8P_0.6Si_0.4B_xcompounds can be significantly reducedby adding B element. To improve the magnetocaloric properties, a series ofMn_1.2Fe_0.8P_1-xSi_xB_0.03（x=0.4,0.5,0.55） compounds were prepared by melt-spinning. The XRDand SEM results show that the compounds keep Fe₂P-type hexagonal structure with spacegroup P62m.With increasing Si content, the small amounts of （Mn,Fe）₃（P,Si） phase appearsand the Fe₂P phase grow from regular hexagonal shape to large stripy shape. The Curietemperature also increases with increasing Si content. The maximum magnetic-entropychange in Mn_1.2Fe_0.8P_0.45Si_0.55B_0.03compound reaches11.1J·kg^-1K^-1in a field change from0to5T. The thermal hysteresis of this compound is less than3K. The value of RC is255.1J·kg-1.The results indicate that the low thermal hysteresis and large RC can be obtained in the（Mn,Fe）₂（P,Si） compounds with high Si content.In summary, As or Ge elements is successfully substituted by Si element. Giantmagnetocalotic effect near room-temperature is obtained. The low-cost raw materials, thesimple preparation techniques, the large magnetocaloric effect, the smaller thermal hysteresisand the ideal cooling temperature make （Mn,Fe）₂（P,Si） compounds an excellent candidatematerial for room-temperature magnetic refrigeration applications.