First-Principle Calculations On Thermoelectric Coefficients Of Silicon-Nanowire-Cages And Layerd Carbon Nitrides

In recent years,increasing consumption of fossil fuel triggered huge waste of energy and pollution.The majority of fossil fuel is wasted in the form of heat,thus it's of great significance to reuse wasted heat.Thermoelectric(TE)materials which can transform low-grade heat to electricity have attracted many interests.The efficiency of TE materials can be evaluated by ZT,the ratio of power factor over thermal conductivity.Constructing material which possess low dimensional structure can not only introduce boundary scattering of phonon but also utilize the resonant hybridization wave effect to decrease thermal conductivity and to enhance ZT.Moreover,low dimensional structure helps to increase power factor.For all those above,we propose to utilize Silicon-nanowire-cage(SiNWC)to reduce thermal conductivity and to increase Seebeck coefficient.On one hand,the resonant hybridization wave effect of phonon results in thermal conductivity which is even lower than that of Silicon nanowire(SiNW)with same size.On the other hand,SiNWC structure force electrons traveling across nanowire between adjacent nano-cross-junction,that can provide density of states(DOS)similar to SiNW and enhance power factor.Besides,SiNWC is one kind of bulk material which possesses nano-structure,providing broaden space in application compared to SiNW.With the help of first-principle and Boltzmann transport equation,we study the band structure of SiNWC and thermoelectric transport properties of SiNWC under room temperature.We find that maximum of ZT of SiNWC can reach 0.96.These results show that SiNWC possess huge potential in thermoelectric application.Except low dimensional structure,low dimensional electric transport in bulk material can also improve ZT.Thus,we propose to utilize the inter-planar orbit coupling of layered carbon nitrides(PCN)to realize high performance of thermoelectric.Combining first-principle and Boltzmann transport equation,we study the band structure of PCN and thermoelectric transport properties of PCN under room temperature and find out that peak of ZT of PCN can reach 0.52.Comparing charge distribution of PCN and g-CN1,g-CN2,we give insight to the mechanism of low dimensional electric transport in PCN and attribute it to strong inter-planar p_z orbit coupling and discontinuous in-plane p_z orbit distribution.