Performance Optimization And Interfacial Behavior Of NbFeSb-based Thermoelectric Materials

NbFeSb-based half-Heusler(HH)compounds have recently attracted wide interest because of the excellent thermoelectric(TE)performance,stability and mechanical property applied in the field of medium-to-high temperature electricity generation.Especially,the figure of merit zT value of p-type NbFeSb has exceeded 1.5.However,in view of large-scale commercial application,there are still several problems needed to solve.For example,the price of p-type dopant Hf is too high to reduce the device cost.The zT value of n-type(V,Nb)FeSb is too low to match the corresponding p-type one.The contact resistivity of the junction between Ag and half-Heusler TE matrix is too high,degenerating the output power of TE devices.This work mainly focuses and researches on the NbFeSb-based half-Heusler compounds.The materials'TE performance and the devices'interfacial performance are studied deeply.On the one hand,we realize the optimization of TE performance through composition control.On the other hand,we choose suitable high temperature metal eletrodes to joint with TE matrix in order to minimize the contact resistivity and improve the device service behavior.The main results are listed as following:1)Dual-doping is an effective way to simultaneously realize the optimization of carrier concentration and suppressing the lattice thermal conductivity.We choose Hf and Ti as the two p-type dopants in the NbFeSb matrix.The results show that compared with single Ti pdoping,dual-doping of Hf/Ti suppresses the lattice thermal conductivity about 30%while the optimization of carrier concentration is realized simultaneously.Theoretical calculation indicates that the point-defect scattering is strongly enhanced by mass and strain field fluctuation,leading to the decline of lattice thermal conductivity.Besides,further research shows that partial stoichiometry,such as excess Fe and deficient Nb,is not an effective method to enhance the point-defect scattering on phonons.Furthermore,as a kind of p-type dopant of NbFeSb,the doping efficiency of Sc is lower than Hf and Ti.Therefore,the zT value of p-type Hf/Ti dual-doped NbFeSb reaches 1.32 at 1200 K.Due to the reduce of the content of Hf,the cost of the TE materials is successfully saved.2)The theoretical calculation of band structure of VFeSb and NbFeSb indicates that NbFeSb has a smaller conduction band effective mass than VFeSb,which is beneficial to the improvement of carrier mobility.We experimentally realize the n-type doping in the NbFeSb alloy using Co/Ir as two dopants to substitute on the Fe site.SPB model shows that the band effective mass of NbFeSb reduces to 0.76me and the high temperature carrier mbility improves.However,the room temperature mobility does not show an improvement.The influence of ionized impurity scattering on the mobility is excluded by calculating Debye screening length.Deeply research suggests that the optimal carrier concentration of n-type NbFeSb is lower than p-type one.The carrier with low energy is easier to be scattered by grain boundary.Therefore,the room temperature mobility is reduced.By comparing the lattice thermal conductivity between Ir doped and Co doped NbFeSb,we find that the Ir doping shows a stronger point-defect scattering,which suppresses the lattice thermal conductivity of NbFeo.92Ir0.02Sb to 3.7 Wm-1K-1 at 1000 K.Besides,the larger band gap of NbFeSb can also prevent the excitation of minority carrier.Finally,the zT value of n-type NbFe0.94Ir0.06Sb reaches up to 0.5 at 1000 K,a 50%increase compared with n-type(V0.6Nb0.4)FeSb.3)In view of commercial application of TE devices,it is required to choose suitable high temperature electrodes to joint with NbFeSb alloys in order to give the full play of materials5 TE performance.We experimentally choose Ag,Fe,Nb,Mo and FeSi as the matel electrodes to joint with p-type NbFeSb with one-stpe SPS method and studies the interfacial performance.The results show that except Mo and FeSi,the other metals can not joint with p-type NbFeSb successfully.An Ohmic contact between Mo and NbFeSb is realized and the contact resistivity is lower than 1??·cm2.A deep analysis shows that such a low value is attributed to the matching of work function between Nb0.8Ti0.2FeSb and FeMo.Due to the high doping level and relatively low dielectric constant of p-type Nb0.8Ti0.2FeSb,the interfacial carrier transport is dominated by field emission.Therefore the contact resistivity is further reduced by the strong tunneling current.After 32 days'aging,the contact resistivity rises up to 18.4 ??· cm2 resulting from the relatively high electrical resistivity of FeSb2 phase,the increasing content of Nb3Ti and the crack at both sides of Nb3Ti interlayer according to the interfacial microstructure analysis.Besides,the FeSi electrode owns a more complicated interface,which leads to the high contact resistivity of 37.1 ??·cm2 after 2 days'aging.4)By measuring the mechanical property of NbFeSb alloys with different components prepared by different processes,we find that the micro Vickers hardness and elastic modulus of NbFeSb is about 9.6 GPa and 207 GPa,respectively.Compared with many other typical TE materials,HH alloys show an excellent mechanical property.The influence of the doping elements on the mechanical property of NbFeSb is negligible while the samples prepared by arc melting shows both higher hardness and elastic modulus than that of levitation melting.The bending test suggests that the bending strength of p-type Nb0.8Ti0.2FeSb is about 113 MPa.The value of the corresponding n-type Hfo.3Zro.7NiSn0,98Sb0.02 based HH compound is also about 113 MPa,which further verifies that the mechanical property of HH alloys is excellent.