Study On Chalcogenide Thermoelectric Fibers And Their Properties Fabricated By An Optical Fiber Template Method

Thermoelectric materials are a kind of energy materials that can realize the mutual conversion of thermal energy and electric energy and have significant meanings for the revolution of traditional electric generation and energy recovery.Their properties are dependent on three key parameters:Seebeck coefficient?S?,electrical conductivity???and thermal conductivity???.In general,a dimensionless thermoelectric figure of merit?ZT=S²?T/?,T is the absolute temperature?is used to evaluate the properties of thermoelectric materials.To improve the ZT value of the materials,we must attempt to increase their power factor?PF=S²??,and reduce their thermal conductivity.However,it is a challenge to get a large ZT value in thermoelectric materials,being a hot topic in energy materials research,because thermal and electrical transport properties of the materials are interrelated while they are restricted by microstructure,carrier concentration,and lattice vibration.In recent years,the study of the thermoelectric materials theory has made some breakthroughs which indicated that low-dimensional and nanostructured inorganic thermoelectric materials could significantly improve their thermoelectric performance.It provides a clear direction for better thermoelectric materials.In the research direction of one-dimensional inorganic thermoelectric materials,there is an urgent need for the efficient preparation of high-quality inorganic thermoelectric fibers.Herein,a new preparative technology of the optical fiber template method is studied to farbricate thermoelectric fibers.Glass is selected as the cladding,and thermoelectric material is selected as the fiber core to obtain inorganic thermoelectric fibers with controllable diameter scale by using the commercial fiber drawing technique.It is systematically researched for the new technology that what are its basic theory,technological parameter,and specific influence about the structure and properties of thermoelectric fibers.This technology supplies a possible idea to explore high-performance thermoelectric materials.The specific innovations and achievements of this paper are as follows:?1?The general rules and crucial techniques of preparing multi-material thermoelectric fibers are systematically studied.The matching rules of core and cladding materials are included:melting point-viscosity matching,high temperature wettability,thermal expansion coefficient,element diffusion,etc.The technique of low-oxygen-content fiber preform fabrication and optical fiber template drawing method of multi-material thermoelectric fibers are studied.The control law of layered chalcogenide semiconductors core and glass-clad thermoelectric fibers is analyzed.High-performance chalcogenide-core thermoelectric fibers are yielded in low cost.?2?Fabrication and properties of thermoelectric fibers are researched.Sn-Se,In-Se and Bi₂Te₃ core thermoelectric fibers are fabricated.These three kinds of thermoelectric fibers are prepared by powder-in-tube and molten-core drawing techniques,that is to say,thermoelectric fibers are drawn with melting core and softening glass at high temperature,going through a quick cooling and recrystallization process.Then the fiber core is polycrystalline.In terms of Sn-Se core thermoelectric materials,the material performance of different components varies greatly.By regulating the diffusion of elements between the fiber core and the cladding during fiber drawing,the fiber core is mixed polycrystalline SnSe and SnSe₂ in an adjustable proportion.At 900 K,the electrical conductivity of 78SnSe-22SnSe₂core fiber is close to SnSe single crystal?6×10⁵ S/m?.Usually,the thermoelectric materials of In₄Se₃ are difficult to be synthesized directly,but with a suitable heat treatment after drawing the drawing In-Se core fiber could adjust the core components and realize a transformation from InSe and residual In to In₄Se₃ single phase.At 300 K,the Seebeck coefficient of In₄Se₃core fiber?^-350?V/K?was 85%higher than that of InSe core fiber.In addition,two kinds of glass-clad Bi₂Te₃ core thermoelectric fibers are successfully prepared by the molten-core drawing and vacuum-molten-core drawing technique,respectively.The oxygen content of the fiber core drawn by the vacuum-molten-core drawing technique is significantly lower than that of the fiber drawn by the molten-core drawing technique,and its power factor is 0.99mWm^-1K^-2,which is 157%higher than that of the molten core drawing fiber.?3?Tailoring comprehensive performance of thermoelectric fibers is researched.Bi₂Te₃core thermoelectric fibers are prepared by vacuum-molten-core and rod-in-tube element-prediffusion drawing techniques and the fiber core is polycrystalline.When the core is cooled and recrystallized after a high temperature melting process,it suffers a great tensile stress from the cladding and presents a preferred nanosheet structure with high electrical transport.If the oxygen element of the cladding diffuses to the fiber core properly at high temperature,the thermoelectric fiber core with oxygen-rich surface can be formed,and its thermal conductivity can be reduced while retaining the high electrical conductivity.The results show that the Bi₂Te₃ core fibers at 300 K temperature has a high power factor of 1.27mWm^-1K^-22 and extremely low thermal conductivity of 0.52 Wm^-1K^-1,and obtains ZT⁰.73,which is higher than ZT⁰.42 of the undoped Bi₂Te₃ bulk at room temperature.By adjusting the cladding with different size and expansion coefficient further,we can optimize the tensile stress and microstructure in the core to obtain the optimal power factor of Bi₂Te₃ fiber(1.54mWm^-1K^-2).And it can be expected to enhance the fiber thermoelectric performance after component doping.?4?Performance test of micro-nano thermoelectric fiber has been studied.The sample processing and measuring technique of micro-nano Bi₂Te₃ core thermoelectric fibers are explored.Casing draw technique is used to prepare the micro-nano fibers.The micro-nano fiber core exhibits an ordered structure in a polycrystalline state,whose preparative technique could be extended to high-efficient preparation of other chalcogenide materials with ordered structure.This paper designs a measuring microchip and system in common use for the micro-nano thermoelectric fibers,and the ZT of these fibers is measured being 0.30 at 300 K.The size effect on the properties of thermoelectric fibers is studied.Except for ultrahigh ZT values of one-dimensional thermoelectric fibers in theory,ZT value of one micro fiber is found to be a maximal value in fibers ranging from micro-nano to bulk scale.All the results provide a theoretical basis and technical support for the further development of high-performance thermoelectric fibers.