Solid-state Thermal Switches Modulated By Electric/magnetic Fields,or The Heating Frequency

A thermal switch actively regulates heat flow by modulating its thermal resistance dynamically.High performance thermal switches could have extensive application prospects ranging from decreasing the energy consumption of buildings to thermal regulation of thermal logic.In this thesis,solid-state thermal switches modulated by electric or magnetic fields are experimentally demonstrated,and a conceptual solid-state thermal switch which is modulated by the heating frequency is explored.Bismuth ferrite（Bi Fe O₃）,a ferroelectric and also an antiferromagnetic material,is selected to explore the microscale mechanism of the solid-state thermal switch modulated by electric or magnetic fields.The effective thermal conductivity of bulk Bi Fe O₃ is measured using the traditional 3ωmethod under an electric field ranging from 0 to 4 k V/cm and a magnetic field ranging from 0 to 9 T.The relative variation is approximately linear with the field strength,with maxima of 2.4%and 2.5%,respectively.Applying an external electric field,the lattice structure of Bi Fe O₃ changes because of its ferroelectricity.This leads to the variation of the heat transport capability of lattice waves（phonons）.Likewise,applying a magnetic field,the ordered spin arrangement of Bi Fe O₃ changes because of its ferromagnetism,which leads to the variation of the heat transport capability of spin waves（magnons）.The concept of a thermal switch modulated by the heating frequency is proposed.This mechanism is built upon the fact that the thermal conductivity is frequency-dependent under high enough heating frequencies.The theoretical limit of the switch ratio is over 100,which is calculated by the one-dimensional Boltzmann transport equation under the framework of the two-flux model.The key merit of this mechanism is that heat flux can be continuously modulated by varying the heating frequency,without any extra energy input.To demonstrate this mechanism,a“current-sweep”3ωmethod is developed to extract the frequency-dependent thermal properties.It is validated using control experiments on an amorphous silica substrate in the temperature range of 78 K to 300 K.The results show that the relative errors of thermal conductivity and heat capacity at room temperature are within 0.3%and 4.8%,respectively.