Neutron Scattering Study Of The Structure And Dynamic Properties Of Thermoelectric Materials, Amorphous Materials And Magnetic Skyrmion Materials

Currently,the energy crisis is one of the main problems facing the development of human society.With its improvement,energy consumption is growing rapidly,and the traditional fossil energies including oil,coal and natural gas,etc.are facing the danger of exhaustion.Moreover,the exploit and use of fossil energies have also caused big environment pollution,which has severely challenged the sustainable development of human civilization.Consequently,global scientists are actively searching for new renewable energy materials.Thermoelectric materials,working based on thermoelectric effect,which can be used to directly convert thermal energy and electrical energy,are new clean energy materials with broad application prospects.At present,the main problem in the research field of thermoelectric materials is that the material's thermoelectric conversion efficiency is low,e.g.the small ZT value.Thus,how to improve the ZT value of existing thermoelectric materials and find new materials with higher ZT values,is one of the main research directions in this field.In the 1930s,the theoretical framework of lattice dynamics in crystalline materials was established by creating the quantized concept of phonon.This theory successfully explained the specific heat of solids.In addition,the electron-phonon scattering is one of the sources of solid resistance,and the electron-phonon coupling is the main essence of BCS superconducting theory.On the other hand,there are still many problems in the construction of the dynamic theory of atomic vibrations in amorphous materials.The main reason is that the broken of spatial translational symmetry caused by atoms disorder,makes the hard construction of a mathematical model.On the other hand,the experimental research data is relatively lacking,which cannot provide clear evidence to theorists.The study of atomic vibrational dynamics in amorphous materials not only contributes to the construction of its atomic vibration theory model but also helps to understand the nature of the glassy states.Magnetic skyrmion is a kind of topological protected nanoscale microscopic magnetic vortex structure which has the topological protection,low driving current density,and multiple physical manipulation properties,such as magnetic field,current,electric field,etc.Making it becomes a potential application material for future high density,high speed,and low energy consumption storage devices.The main microscopic experimental techniques for studying magnetic skyrmions are Lorentz transmission electron microscopy?LTEM?,atomic force microscope,and spin-polarized scanning tunneling microscope,etc.,and the main macroscopic experimental techniques are small angle neutron scattering?SANS?and magnetic Hall measurements etc.At this time,the research on the formation mechanism,transformation,movement and interaction,etc.,physical properties of skyrmion are not sufficient,which obstruct research progress of these new materials.Neutron scattering plays an important role in the study of crystal structure,magnetic structure,lattice dynamics,and spin dynamics.In recent years,the neutron scattering technology has been developed rapidly with the state-of-art neutron sources are continuous constructed and operated in the world,such as spallation neutron source?SNS?at America,Japan proton accelerator research complex?J-PARC?at Japan,and China spallation neutron source?CSNS?at China,which significantly promoted the study of all research fields in condensed matter.In this thesis,we mainly using the neutron scattering instruments from all the worldwide neutron sources studied the problems of the above three types of materials,e.g.,MgAgSb-based thermoelectric materials,Zr₄₆Cu₄₆Al₈ amorphous materials,and MnNiGa magnetic skyrmion materials.The thesis contains three main parts as follows:1,The crystalline structure and lattice dynamic properties of high performance near room temperature Mg Ag Sb-based thermoelectric materials are studied by neutron diffraction and inelastic neutron scattering?INS?.Combining with ab initio calculation,two main sources of the super-low thermal conductivity were found,which are the strong transverse acoustic phonon scattering by the intrinsic local structure distortion and phonon anharmonic effect in materials.2,The atomic dynamic properties in Zr₄₆Cu₄₆Al₈ amorphous materials are studied by INS and molecule dynamic?MD?simulation.The existence of phonon-like atomic vibrational modes was found in amorphous materials.We first separated the transverse phonons from the oscillational overlapped longitudinal and transverse modes.A universal one-to-one correspondence between the full width at half maximum?FWHM?of transverse modes and the static structure factor was demonstrated by MD simulation.3,The biskyrmion states in Mn Ni Ga are studied by SANS.Combining with numerical simulation,the biskyrmions forming a tube-like 3D structure was found.The in-plane biskyrmions are not long-range ordered.Surprisingly all of the disordered biskyrmions have their in-plane symmetry axis aligned along certain directions.The above studyings have the following significances.The study of Mg Ag Sbbased thermoelectric materials revealed that the phonon scattering by a distorted local structure and phonon anharmonic are the origin of super-low thermal conductivity in this kind of materials,which has great significance for the future design and manufacture of thermoelectric materials with lower thermal conductivity.The study of phonon-like properties and Boson peak in Zr₄₆Cu₄₆Al₈ amorphous materials broadens people's perception of atomic dynamics in amorphous materials.The accurate detection of the phonon-like dispersions and the discovery of the universal relationship between the FWHM of the transverse acoustic phonon and the static structure factor provide an experimental basis for the future construction of the theoretical model of atomic dynamics in amorphous materials.The determination of the biskyrmion in the bulk Mn Ni Ga establishes the basis of its potential application.Such in-plane oriented symmetry nature,representing an extra degree of freedom,may be exploited as an information carrier in the future.