Experimental Measurements Of Resistivity Of Fe-N System Under High Temperature And High Pressure

The magnetic fields of the Earth and terrestrial planets of the Sun system are originated from the convection of its metallic core.Study on the convection model of the core is crucial for us to understand the formation history and evolution of the planets.The thermal conductivity of Fe-light elements system at the planetary core conditions is an important parameter with respect to the study of the core convection,which can be deduced by the electrical resistivity data via the Widemann-Franz law.In this thesis,for the first time,we innovatively combined the large-volume cubic press high-pressure apparatus and Van der Pauw method and developed a new technique for the measurements of resistivity at high pressures.Using this technique,the resistivity of pure iron at 3 GPa and 5 GPa were measured to exam the validity of the technique and the data reproducibility.Further measurements of Fe-N system were conducted at 5 GPa,including Fe-1.0 wt%N,Fe-2.0 wt%N,Fe₄N and Fe₃N_1.3,the first two are solid solutions and the later are Fe-N compounds.The resistivity data of Fe-1.0 wt%N and Fe-2.0 wt%N solid solutions showed that the stable phase at high temperature conditions is austenite.The models for interpreting the dilute alloys resistivity were considered and found that the empirical Matthiessen’s rule is not consistent with the data,whereas the‘shunt resistor’model is more valid.Phase transitions at temperatures are prevalent in the Fe-N compounds indicated by the resistivity data.Resistivity data for all the four compositions show saturation behaviors at temperatures.The influence of N on the resistivity of Fe was discussed and found that the resistivity shows saturation as the N content reaches about 10 wt%,i,e.,compositional saturation.On the basis of the measured resistivity data and assuming that the cores of the terrestrial planets consist of Fe-N compositions,the conductive heat flow along the adiabatic geotherms was analyzed.The comparison between the conductive heat flow at the top of the terrestrial planetary core and the adiabatic heat flow at the core-mantle boundary was also presented.