Effects Of Al, Zn, B Doping On The Structure And Magnetocaloric Of Mn-Fe-P-(Si, Ge) Compounds

The Mn-Fe-P-（Si,Ge）series compounds with Fe₂P-type hexagonal structure are considered to be one of the most excellent magnetic refrigeration materials,due to their continuously adjustable Curie temperature,large magnetocaloric effects,being environmentally friendly,cheap and non-toxic raw materials.In this thesis,the effects of Al,Zn and B doping on Mn-Fe-P-（Si,Ge）compounds have been studied.The incorporation of small amounts of elements into the compound using different processing techniques are an innovation in this paper.The preparation（quenching and oven-cool effects,etc.）,crystal structure,magnetocaloric effect and mechanical properties,and electrical transport behavior of Al and Zn doped Mn-Fe-P-Si compounds have been studied.The other part is focusing on the crystal structure and magnetocaloric effects of B-doped Mn-Fe-P-Ge compound.The performance of the alloys,including crystal structure,magnetic and thermal effects,electrical and mechanical properties,have been studied by using X-ray diffractometer（XRD）,scanning electron microscope（SEM）,vibrating sample magnetometer（VSM）,micro-control electronic universal testing machine,Vickers hardness tester,etc.The main contents of this paper are as follows:In chapter 1,the research background,principle,current and research status of magnetic refrigeration materials,the purposes and content of the thesis have been introduced.In chapter 2,the theoretical basis,including the magnetocaloric effect and its thermodynamic basis,measurement and calculation methods,the mechanical basis and electronic transport of materials have been introduced.In chapter 3,the synthesis,measurement process and selected instruments used during experiment have been introduced.In chapter 4,the effects of Al doping on the crystal,microstructure,magnetocaloric,mechanical properties and electrical transport behavior of Mn_1.25Fe_0.65P_0.5Si_0.5-xAl_x（x=0,0.01,0.04,0.06）compounds have been studied.All samples crystalize in the Fe₂P-based hexagonal structure,indicating that small amount of Al doping has little effect on the crystal structure.Magnetic measurement shows a decrease in Curie temperature from 277 K（x=0）to 234 K（x=0.06）.The magnetic entropy changes have been obtained by using the Clausius-Clapeyron equation.The magnetic entropy change increases from 5.8 J·kg^-1·K^-1 for Al-free sample to14.4 J·kg^-1·K^-1 for the sample with x=0.01 under a magnetic field change of 0-3 T.The mechanical properties can be significantly improved by Al doping,which has been confirmed by compressive strength and Vickers hardness study,electrical resistivity behaviors upon cycling and variation of compactness in the SEM images.The compressive strength of Al-doped samples increased by about 140%from P=114.7 MPa for Al-free sample to P=278.3 MPa for the sample with x=0.01.The average Vickers hardness of the Al-free sample is about 593.6 HV,and is about 640.8 HV for Al-doped sample.The SEM image shows that the compactness of the sample increases as the content of Al doping increases.In chapter 5,the structure and magnetic properties of Zn-doped Mn_1.25Fe_0.65P_0.5Si_0.5-xZn_x（x=0,0.005,0.01,0.03,0.05）series compounds have been studied.A small amount of Zn doping has little effect on the crystal structure of the compound.The effects of heat-treatment（quenching v.s.oven-cool）on magnetic and magnetocaloric properties have been investigated.The Curie temperature gradually decreases with the increase of Zn content,indicating that the addition of Zn weakens the ferromagnetic interaction.There is no significant change in thermal hysteresis with increasing Zn content.The maximum magnetic entropy change shows an increasing trend with Zn doping,indicating that the addition of a small amount of Zn is beneficial for enhancing the magnetocaloric effect.In chapter 6,the B doping effects on the crystal and magnetocaloric properties of Mn_1.1Fe_0.9P_0.78Ge_0.22 compound have been studied.The Mn_1.1Fe_0.9P_0.78-xGe_0.22B_x（x=0,0.02,0.04 and 0.06）series of compounds were prepared using mechanical alloying and solid state sintering（followed）techniques.The results show that all the Mn_1.1Fe_0.9P_0.78-xGe_0.22B_x（x=0,0.02,0.04 and 0.06）compounds crystallize in Fe₂P-type hexagonal structure.With increasing of B content,the lattice constant c first increases and then decreases,and the lattice constant a first decrease and then increases.All the compounds undergo a ferromagnetic-paramagnetic（FM-PM）state phase transition.The Curie temperature decreases first and then increases,while the thermal hysteresis show decreasing trend.B-doped compounds show larger magnetic entropy changes than that of the Boron-free one.All these features show that the B doping is beneficial for improving the magnetocaloric properties of Mn-Fe-P-Ge compound.