Preparation And Thermoelectric Propeties Of Pb₄ln_2.8Bi_3.2Se_13-xCl_x And Mg₃Sb_2-x-0.5yBi_x-0.5yTe_y Mesotherm Materials

Thermoelectric material is a kind of new energy material which directly converts heat energy into electricity.It has the characteristics of non-pollution,non-vibration and non-noise.Under the current situation of energy shortage and environmental deterioration in the 21st century,it has a very broad application prospect.In this paper,the mesothermal thermoelectric materials Pb₄In_2.8Bi_3.2Se₁₃ and Mg₃Sb₂ are selected as the research objects.The optimum synthesis technology of Pb₄In_2.8Bi_3.2Se₁₃ is explored and its thermoelectric properties are optimized by Cl doping.In addition,the single phase Mg3Sb2-x Bix solid solution is prepared by self-propagating combustion synthesis technology for the first time.The thermodynamic process of self-propagating combustion is studied.Thermoelectric properties of Mg₃Sb_2-xBi_x solid solution are studied and optimized by Te doping..In this paper,the mesothermal thermoelectric materials Pb₄In_2.8Bi_3.2Se₁₃ and Mg₃Sb₂ are selected as the research objects.The synthesis of new materials Pb₄In_2.8Bi_3.2Se₁₃ and the application of new synthesis technology in Mg₃Sb₂ are explored.The thermoelectric properties of Pb₄In_2.8Bi_3.2Se₁₃ and Mg₃Sb₂ materials are optimized by means of solid solution and doping.Its main contents are as follows:A series of studies on the synthesis process of new thermoelectric materials Pb₄In_2.8Bi_3.2Se₁₃ show that single phase Pb₄In_2.8Bi_3.2Se₁₃ bulk materials with homogeneous phase composition and compact structure can be synthesized by process of melting-annealing-plasma activated sintering.The optimum technological parameters are:melting temperature 1073 K,melting holding time 12 h,annealing temperature 773 K,annealing holding time 72 h,sintering temperature 753 K,sintering holding time 5 min,sintering pressure 40 Mpa.The thermoelectric properties of the new material at 723 K are as follows:conductivity 478.3 Sm^-1,Seebeck coefficient-346.1 uVK^-1,thermal conductivity 0.27 Wm^-1K^-1,ZT value0.15.The results of Cl-doped Pb₄In_2.8Bi_3.2Se_13-xCl_x?x=0.065,0.130,0.195,0.260?series samples show that when Cl-doped amount x is less than 0.26,the bulk sample is single phase,and when Cl-doped amount x is more than 0.26,the second phase InCl exists in the bulk sample.The limit of Cl doping in this material is 0.26.The conductivity perpendicular to the pressure direction is greater than that parallel to the pressure direction,the Seebeck coefficients perpendicular to the pressure direction are less than that parallel to the pressure direction,and the thermal conductivity perpendicular to the pressure direction is slightly greater than that parallel to the pressure direction.The material has certain orientation in thermoelectric transport,and the more Cl doped,the more obvious the orientation is.With the increase of Cl doping amount x,the conductivity of the material increases gradually,the Seebeck coefficient decreases,and the power factor perpendicular to the pressure direction of the sample with Cl doping amount x=0.195 reaches the maximum value of 2.26 x 10^-4Wm^-1K^-2 at 723 K.With the increase of Cl doping amount x,the thermal conductivity of the material does change few,which is about 0.3 Wm^-1K^-1 at 723 K.With the increase of Cl doping amount x,the ZT of the material increases first and then decreases.The maximum ZT value perpendicular to the pressure direction of the sample with Cl doping x=0.195 is 0.54 at 723 K,which is 260%higher than that of the intrinsic material.A series of studies on the preparation of Mg₃Sb₂ and its solid solution by self-propagating combustion synthesis technology show that Mg₃Sb₂ compound and its solid solution can be synthesized rapidly by self-propagating combustion synthesis.In the process of self-propagating combustion synthesis of Mg₃Sb₂ compounds,Mg₃Sb₂ compounds are synthesized from Mg and Sb raw materials in one step without intermediate products.The combustion temperature is 1135 K and the propagation velocity of combustion wave is 13.5 mm/s.The reaction process of self-propagating combustion synthesis of Mg₃Sb_1.5Bi_0.5 solid solution is relatively complex,First,Mg reacts with Sb to form Mg₃Sb₂,and then Mg₃Sb₂ gradually dissolves with Bi to form Mg3Sb2-xBix intermediate solid solution.Finally,the single-phase product of Mg₃Sb_1.5Bi_0.5.5 was synthesized.During this reaction,with the addition of Bi element,the combustion temperature drops to 887 K,and the part of the reacted sample melts with the combustion wave,which promotes the propagation of the combustion wave with the propagation speed of 19.65 mm/s.The results of Mg₃Sb_2-xBi_x?x=0,0.25,0.5?and Mg₃Sb_1.5-0.5yBi_0.5-0.5yTe_y?y=0.01,0.02,0.03?series samples show that in Mg₃Sb_2-xBi_x?x=0,0.25,0.5?samples,when x is less than 0.5,a homogeneous single-phase product of Mg₃Sb_2-xBi_x can beobtained;When x is more than 0.5,a small amount of Bi will be found in the bulk samples.With the addition of Bi atom,the conductivity of Mg₃Sb_2-xBi_x sample increases and the Seebeck coefficient decreases.In this process,the energy band of Mg₃Bi₂ and Mg₃Sb₂ has a convergence effect,effectively reducing the width of E_g,optimizing the energy band structure of the material,and improving the electrical properties of the material.At the same time,solid solutions enhances the scattering mechanism of alloying and reduces the thermal conductivity of the material.In Mg₃Sb_1.5-0.5yBi_0.5-0.5yTe_y?y=0.01,0.02,0.03?samples,single-phase products of Mg₃Sb_1.5-0.5yBi_0.5-0.5yTe_y can be obtained by self-propagating combustion synthesis when Te doping y is 0.01,0.02 and 0.03.With the substitution of Bi and Sb atoms by Te atom,the conduction characteristics of all samples changed from intrinsic p type to n type.The carrier concentration increased greatly,as a result,the conductivity and power factor were optimized,and the ZT value was greatly improved.The carrier concentration of Mg₃Sb_1.495Bi_0.495Te_0.01 was 1.2×10²⁰cm^-3 at room temperature.At723 K,the maximum ZT can reach 0.92,which is significantly higher than that of intrinsic Mg₃Sb₂.