Improve The Thermoelectric Properties Of SnTe-Based Alloy Materials By Nanocomposited

Tin telluride(SnTe),which is also a group Ⅳ-Ⅵ compound,is considered as an ideal replacement material for lead telluride(PbTe),which has the same face-centered cubic crystal structure and similar bivalent band energy band structure.However,the intrinsic SnTe thermoelectric properties are poor due to the high carrier concentration,small energy gap,large light-heavy band energy difference,and high thermal conductivity.The problem of energy band structure is usually solved by the strategies of energy band convergence and introduction of energy level resonance.It is found that manganese(Mn)has the ability to cause the simplification of the light and heavy valence bands of SnTe,and indium(In)can induce the surge of density of states near the Fermi energy level EF,both of which can improve the Seebeck coefficient and enhance the electrical transport performance of SnTe alloys.As for the modulation of thermal properties,the thermal conductivity of SnTe can be effectively reduced by introducing multi-scale microstructures to enhance phonon scattering.This thesis optimizes the SnTe energy band structure through elemental doping to enhance electrical properties;and introduces scattering sources such as heterogeneous interfaces,refined grains,nano-defective structures,high thermal resistance second phases,and pores by compound compounding to reduce lattice thermal conductivity,thus achieving synergistic optimization of electrical and thermal transport properties,improving the thermoelectric properties of SnTe alloys,and obtaining SnTe-based thermoelectric materials with high output power and excellent thermoelectric conversion efficiency.The research of this thesis includes:1.Improvement of the thermoelectric properties of SnTe alloys by manganese doping composited with carbon nanotubes.A series of Sn0.9Mn0.1Te-x wt%carbon nanotube(CNT)(x=0,0.05,0.1,0.15,0.2,0.25)composited samples were designed and prepared using a solidphase reaction and hot-pressure sintering process.It was found that the heterogeneous interface introduced by carbon nanotubes effectively scatters low-energy carriers and produces an energy filtering effect.Based on the energy band simplification,the effective carrier mass of SnTe is further enhanced to achieve a significant enhancement of the Seebeck coefficient.Among them,the x=0.15 composite sample Seebeck reaches～122 μVK-1 at 823 K,and its power factor is enhanced to 22.71μWK-2cm-1,which is 33%higher than that of Mn-doped SnTe.Meanwhile,the composited of carbon nanotubes inhibits the grain growth due to hot-pressure sintering,resulting in internal grain refinement,which,together with the appearance of carbon nanotube/SnTe heterogeneous interfaces,effectively reduces the lattice thermal conductivity.Among them,the sample with x=0.25 has a total thermal conductivity of only 2.7 Wm-1K-1 at 823 K,which is 23%lower than that of the substrate.Ultimately,the sample with x=0.25 achieves a thermoelectric optimum of 0.66 at 823 K,which is a 70%improvement relative to Sn0.9Mn0.1Te.2.The average thermoelectric properties in the full temperature region are of practical importance,and in order to enhance the thermoelectric properties in the low and medium temperature regions of SnTe The strategy of indium doping composited with tourmaline was selected for optimization.First,through 1%In doping,a resonance energy level was introduced near the Fermi energy level of SnTe,which effectively increased the Seebeck coefficient in the low and medium temperature regions,and then led to a significant increase in the power factor in the low and medium temperature regions,with the power factor of Sn0.99In0.01Te reaching 22μWK-2cm-1 at 373 K and 23.5 μWK-2cm-1 at 873 K.The average power factor in the full temperature region was 22.62 μWK-2cm-1.The average power factor in the full temperature range is 22.62 μWK-2cm-1,which is 2.8 times higher than that of SnTe.In addition,the composite of high thermal resistance tourmaline effectively reduces the thermal conductivity of the material,and the lowest lattice thermal conductivity of the sample with 0.08 wt%tourmaline composite is 0.9 Wm-1K-1,which is only 80%of that of Sn0.99In0.01Te.Interestingly,although the introduction of tourmaline paradoxically increases the carrier concentration of Sn0.99In0.01Te,leading to a decrease in the Seebeck coefficient and an increase in the electronic thermal conductivity,the significantly higher conductivity greatly optimizes the theoretical output power density of the device.Among them,the theoretical output power density of the single-leg thermoelectric module constructed from the sample with 0.02 wt%tourmaline composite can reach～354 μWcm-2,which is nearly 120%higher than that of SnTe.3.In order to further optimize the thermoelectric properties of SnTe,experiments on the composite of In-doped and metal-organic compound copper phthalocyanine(CuPc)were carried out,and a series of Sn0.99In0.01Te-x wt%CuPc(x=0,0.25,0.5,1,2,3,4)samples were prepared.On the one hand,the composite of copper phthalocyanine can effectively modulate the Sn0.99In0.01Te carrier concentration,and a high power factor of 21 μWK-2cm-1 and a low electronic thermal conductivity of 1.03 Wm-1K-1 were obtained in the sample with x=1.On the other hand,the high temperature during the sintering process caused part of the organic matter to volatilize,leaving a large number of pores in the samples and forming dense phonon scattering sources.The lattice thermal conductivity of the composite samples decreased significantly,with the lattice thermal conductivity of the x=0.5 sample as low as 0.25 Wm-1K1@873 K,which is close to the SnTe amorphous limit.Finally,the Sn0.99In0.01Te-1 wt%CuPc sample achieved a thermoelectric optimum of 0.9,which is nearly 50%better than Sn0.99In0.01Te.