Ultra-fast Synthesis Of Cu₂S Thermoelectric Material Under Pulse Electric Field

With the vigorous development of the world's economy,human demand for energy is increasing.In the past 100 years,the exploitation of fossil energy,such as coal and petroleum,which has been widely used,has approached the limit of the earth's content,meanwhile the use of these energy sources will cause serious environmental pollution.Therefore,developing new renewable and clean energy has become an urgent task for human beings.In the process of ultilizing energy,most of the energy is wasted in the form of useless heat,so it is of great significance to use and recycle these wasted heat.As a new energy material,thermoelectric materials can realize the direct conversion between electric energy and thermal energy,which provides a solution to the energy problem.Among thermoelectric materials,there is a kind of material with special structure,represented by copper-based chalcogenides.This kind of material has complex crystal structure.When it is above a certain temperature,the cations in the structure will break away from the lattice and become free-moving ions,migrating between the lattice interstitial sites and any possible positions.These liquid-like ions can dramatically scatter phonons so the lattice thermal conductivity of the material is significantly reduced.The electronic transport properties of these materials retain the characteristics of crystals,but they have the heat conduction characteristics similar to liquids.Therefore,they are called Phonon Liquid-Electron Crystal?PLEC?materials.Cu2S is one of the typical types.Chemometric Cu2S has three different crystal structures at different temperatures.The low temperature phase of monoclinic system is below 370 K,the medium temperature phase of hexagonal system is between 370 K and 700 K,and the high temperature phase of cubic system is transformed over 700 K.When the Cu₂S is above 700 K,it shows typical superionic state.The copper ions in the material will move disorderly in the sub-lattice,showing liquid-like properties.Phonons being strongly scattered makes the lattice thermal conductivity of Cu₂S material as low as 0.3 Wm^-1K^-1,which is equivalent to the thermal conductivity of some liquids.In this paper,with Cu₂S compounds taken as the research object,aiming innovating the preparation method and the improving thermoelectric performance,Pulsed Eletric Current Sintering?PECS?is innovatively introduced to complete the activation,synthesis and densification of Cu₂S in one step.The phase composition micro-structure and thermoelectric properties of Cu₂S samples prepared by different methods were systematically studied.On the basis of this new method,the thermoelectric properties of Cu2-xS were optimized by adjusting the stoichiometric ratio of Cu_2-xS.The thermal stability of Cu_2-xS at high temperature was also studied systematically by selecting the compounds with the best performance.The main contents and conclusions are as follows:?1?Intensive pulsed electric field?pulse electric field parameters:voltage value50 V,pulse peak value 1000A,pulse duty cycle 0.967?can complete the activation,synthesis and densification of mixed uniform Cu powder and S powder within 30seconds,thus forming compact and uniform bulk Cu₂S material.Compared withtraditional SPS processes including SPA+DCS,PECS avoids harmful ion migration and greatly inhibits grain growth due to its lower reaction temperature and shortreaction time.In addition,the samples prepared by PECS have abundant multi-scale microstructures,which have obvious scattering effects on phonons.The synthesis of Cu₂S under pulsed electric field can be further identified as three process:activation process,synthesis and densification process and complete reaction process.?2?The process parameters of PECS have obvious effects on the microstructures of materials.Changing the size of pulse current can make the grains inside thematerials change from amorphous to 0.5² m grains.Sintering pressure has animportant influence on the density and thermoelectric properties of materials.Samples prepared under 80 MPa sintering pressure perform lower conductivity and lowerthermal conductivity.Compared with samples prepared under 40 MPa sinteringpressure,higher thermoelectric value can be obtained in Samples prepared under 80MPa.?3?The effects of three different preparation methods of PECS,SPA+DCS and Melting-Quenching+Annealing+Spark Plasma Sintering on the thermoelectricproperties of Cu₂S materials were studied.The PECS samples have the lowest carrier mobility and the highest carrier concentration due to the scattering effect ofpolycrystalline boundary structure on carriers.Because of its multiscale phononscattering structure,PECS samples obtained the largest ZT at 873 K,0.49,which was88%higher than that of MQ+AN+SPS samples.?4?On the basis of PECS preparation method,the thermoelectric properties were optimized by adjusting the stoichiometry of Cu₂S.It was found that the carrier concentration of the material was significantly increased by the absence of Cu,and the electric conductivity of material can be optimized by the increase of carrierconcentration.At the same time,the crystalline transformation occurs with theincrease of the amount of copper missing.The total thermal conductivity increases with the increase of the crystalline transformation and the electronic thermalconductivity of the material.The maximum ZT of Cu1.97S is 0.72 at 873 K,which is50%higher than that of the intrinsic sample.?5?The thermoelectric properties of Cu_1.97S bulk samples annealed underdifferent conditions were systematically studied.At 773 K,the samples exposed to air were easy to oxidize to form CuO,and its internal structure became loosening.Meanwhile,although the samples annealed under vacuum conditions could avoidoxidation of the materials,the service properties of the materials were affected by the volatilization of S.The samples annealed with BN coating within 120 h could avoid the oxidation of material and the volatilization of S,but when the time reaches 168 h,the oxidation of the material can not be prevented.The study of thermoelectricproperties shows that the carrier concentration of annealed samples has beenincreased in varying degrees under the protection of BN coatings.Meanwhile,the thermal conductivity of annealed samples was increased because they became more compact.The increase of thermal conductivity is higher than that of power factor.Therefore,the ZT of annealed samples is lower than that of unannealed samples,but the ZT of annealed samples under the protection of BN coatings is closest to that of unannealed samples,the highest ZT is achieved at 873K in samples annealed under the protection of BN coatings for 120 h,which is 0.69.