Electrical and thermal conductivity of Earth's core and its thermal evolution——A review

The Earth’s core is composed of iron,nickel,and a small amount of light elements(e.g.,Si,S,O,C,N,H and P).The thermal conductivities of these components dominate the adiabatic heat flow in the core,which is highly correlated to geodynamo.Here we review a large number of studies on the electrical and thermal conductivity of iron and iron alloys and discuss their implications on the thermal evolution of the Earth’s core.In summary,we suggest that the Wiedemann-Franz law,commonly used to convert the electrical resistivity to thermal conductivity for metals and alloys,should be cautiously applied under extremely high pressure-temperature(P-T)conditions(e.g.,Earth’s core)because the Lorentz number may be P-T dependent.To date,the discrepancy in the thermal conductivity of iron and iron alloys remains between those from the resistivity measurements and the thermal diffusivity modeling,where the former is systematically larger.Recent studies reconcile the electrical resistivity by first-principles calculation and direct measurements,and this is a good start in resolving this discrepancy.Due to an overall higher thermal conductivity than previously thought,the inner core age is presently constrained at~1.0 Ga.However,light elements in the core would likely lower the thermal conductivity and prolong the crystallization of the inner core.Meanwhile,whether thermal convection can power the dynamo before the inner core formation depends on the amounts of the proper light elements in the core.More works are needed to establish the thermal evolution model of the core.

New Type of Nitrides with High Electrical and Thermal Conductivities

The nitrogen dimer as both a fundamental building unit in designing a new type of nitrides, and a material gene associated with high electrical and thermal conductivities is investigated by first principles calculations.The results indicate that the predicted Si N4 is structurally stable and reasonably energy-favored with a striking feature in its band structure that exhibits free electron-like energy dispersions. It possesses a high electrical conductivity（5.07 × 10^5 S/cm） and a high thermal conductivity（371 W/m·K） comparable to copper. The validity is tested by isostructural Al N4 and Si C4. It is demonstrated that the nitrogen dimers can supply a high density of delocalized electrons in this new type of nitrides.

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study

We study the effect of pressure on electronic and thermoelectric properties of Mg_2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties（Seebeck coefficient, electrical conductivity, electronic thermal conductivity） have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg_2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity.The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg_2Si at zero pressure. The maximum value of figure of merit is attained 1.83 × 10^（-3） at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.

First principle investigation of the electronic and thermoelectric properties of Mg2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties（Seebeck coefficient, electrical conductivity, and thermal conductivity） of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor（PF） increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.

Directional mechanical and thermal properties of single-layer black phosphorus by classical molecular dynamics

Black phosphorus （BP） has received attention due to its own higher carrier mobility and layer dependent electronic properties, such as direct band gap. Interestingly, the single layer black phosphorus （SLBP） has had large popularity in applications related to thermoelectric, optoelectronic, and electronic devices. Here, we investigate the phonon spectrum, thermal conductivities, and stress strain effects. Robust anisotropy was mainly observed in the thermal conductivities together with the alongside zigzag （ZZ） direction value, compared to the armchair （AC） directions. We also investigated the attitude of stress that was anisotropic in both directions, and the stress effects were two times greater across the ZZ path than those in the AC direction at a low temperature. We obtained a ~oung＇s modulus of 63.77 and 20.74 GPa in the AC and ZZ directions, respectively, for a strain range of 0.01. These results had good agreement with first principle calculations. Our study here is useful and significant for the thermal tuning of phosphorus-based nanoelectronics and thermalelectric applications of phosphorus.

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg2Si:A density functional theory study

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory（DFT） and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity,Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg2Si under uniaxial strain have been explored first time in this report.