| The rapid development leads to the higher energy demand,and it is more terrible for the energy supply and the environmental pollution followed.Developing the new-clean energy materials is the immediate areas of research focus,therefore,eco-friendly thermoelectric(TE)materials have intensively attracted worldwide attention due to its ability to convert heat into electricity directly and vice versa.Traditional TE materials have been systematically studied such as Bi2Te3,PbTe and SiGe.The exploration of new TE materials has attracted the attention of researchers in recent years.Cu2GeSe3 TE material we discussed belong to one of these materials,but there are few references for this new Cu-based material.Cu2GeSe3 has the same structure and similar band structure and phonon spectrum with Cu2SnSe3,and Though both have rather low lattice thermal conductivity,a big difference on the reported thermoelectric performance is found between Cu2SnSe3 and Cu2GeSe3:a record high zT of 1.42@823 K in Cu1.85Ago.15Sn0.9In0.1Se3,yet the highest zT is 0.65@758 K reported in Cui.95GeSe3.There must be a large space for the optimization of TE performance in Cu2GeSe3.The structures of Cu2GeSe3 compound is divided into two part,the Cu-Se framework atoms build up the skeleton of the system and bonds provide channels for the carrier transport;while Ge filler atoms act as phonon scattering centers inside the materials.Based on this theory,we chose the framework atoms and filler atoms of Cu2GeSe3 materials to alloying and doping for obtaining the better thermoelectric property in this paper.The samples were prepared by the solid phase transition and spark plasma sintering.The phase composition,microstructure and thermoelectric properties of Cu2GeSe3 system were studied in detail.The major research work and results are listed below:1)We prepared Cu2-xAgxGeSe3(x=0,0.1,0.2,0.3,0.4)samples through isoelectronic substitution of Cu for Ag element in Cu2GeSe3.The relatively high solubility(-10%)of Ag on Cu sites allows for the strong point defect scattering for phonons,which causes remarkable reduction in lattice thermal conductivity.Ag-riched precipitates emerge when the amount of Ag is higher than the solubility on Cu site,which however do not have significant effect on the lattice thermal conductivity since it is already very close to the lower limit of kinetic theory.Eventually,the samples with x = 0.2 obtain the highest value of zT = 0.87 at 786K,which enhanced about 156%compared with the pristine Cu2GeSe3.2)The Te atoms were dissolved in place of the Se atoms and synthesized the Cu2Ge(Se1-xTex)3(x=0,0.1,0.2,0.3,0.4)samples.It was found that the lattice thermal conductivity was reduced drastically for sample with 10%Te alloying,which may be attribute to point defects caused by the alloying effect.However,for samples with x ? 0.2,the lattice thermal conductivity increases with increasing x.The approaching to a less distorted structure and the 2c/a shows a tendency to approaching 1 should be responsible for this.Finally,a zT of 0.55@750 K was obtained for the sample with x-0.3,about 62%higher than that of the pristine sample.3)Based on Cu2GeSe3,Mn,Zn-doped and Ge absence samples were prepared and studied the phase,microstructure and thermoelectric properties.It is found that the filling atoms Ge can effectively regulate the carrier concentration and enhance the electrical properties,however,the TE properties have not achieved the desired effect.4)Double doping in different sites was carried out for the first time in this material system.Cui.9Ag0.1iGeSe3 sample acts as the start matrix,who has the low lattice thermal conductivity and relative high carrier mobility.And Ga-doping,an effective way to tune the hole concentration,leads to optimization of power factor in the whole temperature range.The maximal zT obtained in Cui.9Ag0.1Ge0.997Ga0.003Se3 is 1.03@786 K,about 58%higher than that in previous report,and the value of zT breaks through l for the first time.In addition,the average zT in the temperature range from 320 K to 786 K is 0.58,implying great potential for fabrication of thermoelectric devices. |
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