| The problems of energy shortage and environmental pollution are increasingly serious,so developing new energy materials becomes very attractive.Thermoelectric(TE)materials can implement the solid-state energy conversion between heat and electrical energy which can be used in power generation and refrigeration.The thermoelectric devices have the advantages of small size,no pollution and no moving parts,and have been playing an irreplaceable role in the field of space power supply and refrigeration technology.Among many thermoelectric materials,Bi2Te3 is studied and used most widely with the best performance near room temperature.In recent years,great progress has been made to improve thermoelectric properties of p-type(BiSb)2Te3 materials by element doping and nano-composite.However,the thermoelectric properties of n-type Bi2(TeSe)3 materials are not high compared to p-type materials,which limits the conversion efficiency of thermoelectric devices.The common methods to obtain nano-particles are hydrothermal,ball milling,melt spinning and so on.The bulk samples can be made by SPS or hot pressing the nano-powder,by which the micro-morphology of the materials can be controlled.In this paper,n-type Bi2Te2.7Se0.3 nano-powders were prepared by the hydrothermal method.By adjusting the reaction conditions,the nano-powders with sheet or flower morphology were obtained,and were hot-pressed to bulk samples to determine the optimal hot-pressing temperature and time.The method of element doping and nano-composite,as well as the combination of the two methods were used to improve the thermoelectric properties of n-type materials.Bi2Te2.7Se0.3 nano-powder was prepared by the hydrothermal method.When BiC13,Te and Se powder were used as starting materials,surfactant EDTA could accelerate the growth rate of a-and b-axis,and NaOH could provide alkaline medium for the reaction process and participate in the reaction.Both EDTA and NaOH can affect the morphology of nano-powders.When the ratio of 8 g NaOH and 2.5 g EDTA was selected,the nano-powders with dispersed flake morphology can be obtained by means of rapidly nucleation and dispersed growth.When the ratio of 3.5 g NaOH and 3.5 g EDTA was selected,the nano-powder with flower morphology can be obtained by means of slow nucleation and concentrated growth.When the hot pressing temperature was 480? and the hot pressing time was 15min,the sample had the best electric transport performance.The anisotropy of the material is studied,and the results show that thermoelectric properties perpendicular to the direction of hot pressing was better.The thermoelectric properties of Lu-doped LuxBi2-xSe0.3Te2.7 samples were studied.The results show that the size and thickness of the flake nano-powder are changed by doping because bond energy is affected by the substitution of Lu atom for Bi,and thus affects the growth rate in various directions.In addition,point defects and lattice distortion are produced when Lu atoms are doped into the lattice,because of the impurity atomic radius,atomic mass and stress.The thermal conductivity is decreased by effectively enhancing phonon scattering.The Seebeck coefficient and resistivity are improved by the carrier scattering.The power factor has no obvious improvement,but the value of ZT is greatly increased by the decrease of thermal conductivity.When x=0.1,and the maximal value of ZT reach 1.37 at 373 K,which is higher than that of commercial Bi2Te3.The ZT value of the sample with the doping amount of x=0.2 can reach 0.8 in the whole testing temperature range,and it is an excellent thermoelectric material which can be used in the large temperature range.It is also proved that Lu doping is an effective method to improve thermoelectric properties.In order to further improve the thermoelectric properties,the flower-like Lu-doped Lu0.1Bi1.9Te3 nano-powders were prepared and different amount of carbon nano-tubes(CNTs)was dispersed into the nano-powders.After hot-pressing,the thermoelectric properties were studied.The results show that both the point defects introduced by Lu doping and the interface introduced by CNTs are helpful for scattering phonons,which can reduce the thermal conductivity.A small amount of CNTs provide a conductive path in the lattice,resulting in an increase in mobility and a decrease in resistivity for the sample with 0.05 wt%CNTs.However,when most CNTs are dispersed around the grain boundary,there is a potential barrier with the substrate grain boundary,which hinders the migration of carriers and increases the resistivity.When the amount of CNTs is too much,agglomeration occurs,which is very disadvantageous to the thermoelectric properties of the materials.Therefore,all the ZT values of the samples are improved except for the sample with 1 wt%CNTs.The maximal ZT value of the Lu0.1Bi1.9Te3+0.05 wt%CNTs sample is 1.05 at 423 K.Therefore,the combination of element doping and CNTs dispersion is an effective method to improve thermoelectric properties,which has important research significance.Bi2Te2.7+xSe0.3(x=0,0.2,0.4,0.6,0.8)nano-powders containing excess Te were prepared by the hydrothermal method.Because the melting point of Te is lower than the sintering temperature and the excess Te is easily extruded during hot pressing.The liquid phase promotes plane slip and recrystallization and dislocations occur at the grain boundary.Small hexagonal grains are found around the grown grains,which is conducive to strengthening phonon scattering and decreasing lattice thermal conductivity.Excess Te can form more vacancies and electrons,and the carrier concentration increases.The larger grain size of the material increases the carrier mobility and the appropriate excess of Te can effectively reduce the resistivity and raise the power factor.The best performance of the sample x=0.4 is obtained with the ZT value of 0.78 at 450 K.In order to further improve the performance of Bi2Te2.7+xSe0.3,carbon nano-dots(CDs)were dispersed in samples during hydrothermal reaction and the effects on thermoelectric properties were studied.The results show that the nano-sized CDs are uniformly dispersed in the samples without changing the layered microstructure.CDs introduce potential barriers at the interface and increase the Seebeck coefficient.As a result,the power factor increases by 20%.CDs dispersion introduces the nanometer grain boundary,effectively scattering the mid-frequency phonon,reducing the thermal conductivity,and increasing the ZT value of the material.The ZT values of the two samples with CDs of 0.05 wt%and 0.1 wt%are close to each other,and reach the maximum of 0.91 at 423 K. |
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