High-pressure Synthesis And Thermoelectric Performance Tuning Of Cu₂Se-based Bulk Materials

Thermoelectric materials can directly realize the mutual conversion between electrical energy and thermal energy,and are used in temperature difference power generation and solid-state refrigeration,which can effectively improve energy efficiency and scale,and provide a new way to alleviate energy shortages and environmental pollution problems.Due to its unique crystal structure,excellent thermoelectric properties,and low-cost,low-toxicity commercial value,copper-selenium-based thermoelectric materials have developed into one of the most promising thermoelectric material systems in the past decades.Preparation methods such as vacuum melting-annealing,mechanical alloy or solution synthesis and hot-pressing sintering or spark plasma sintering?SPS?have been widely used in the preparation of Cu₂Se materials,but some methods are complicated and time-consuming.High-pressure technology,as a simple,fast and novel method of thermoelectric material preparation and thermoelectric performance regulation,has been widely used in the research of Bi₂Te₃,Pb Te,Co Sb₃and other thermoelectric material systems,however,there are few literatures about the use of high-pressure technology to synthesize Cu₂Se-based materials.This paper uses high-pressure technology to carry out high-pressure synthesis and thermoelectric performance tuning of Cu₂Se-based bulk materials,and clarifies the thermoelectric performance tuning mechanism of high-pressure synthesis process parameters,Cu non-stoichiometric ratio,and Bi element doping.The main research results are as follows:?1?Synthesis and research on thermoelectric properties of Cu₂Se materials by high-pressure and room-temperature.The high-pressure technology was used to achieve the rapid synthesis of Cu₂Se materials at room temperature within 5 minutes,and the high-pressure synthesis mechanism of Cu₂Se was clarified.Studies have shown that pressure can effectively regulate the phase and microstructure,and then optimize its thermoelectric properties.Under high pressure,the synthesis reaction process of Cu₂Se is from Cu+Se,Cu₃Se₂+Cu,?-Cu₂Se+Cu to?-Cu₂Se+?-Cu₂Se.The synthesis pressure and synthesis time directly determine the phase composition of the products.The results show that the critical condition for synthesizing Cu₂Se is to keep it under the pressure of 1 GPa for5 min.Cu₂Se samples synthesized under high pressure at 1?3 GPa are mixed phases of?-Cu₂Se and?-Cu₂Se.With the increase of pressure,the Seebeck coefficient and resistivity increase,the synthesis reaction of Cu₂Se is more sufficient,the lattice defects are reduced,and the phonon scattering is weakened.Due to the extremely low lattice thermal conductivity of 0.35 Wm^-1K^-1,the maximum z T value of the sample synthesized at 1 GPa is 0.92@783 K,which is 18%higher than the z T value of the 3GPa sample.?2?Regulation mechanism of high-pressure and high-temperature on the microstructure and thermoelectric properties of Cu₂Se samples.Cu₂Se materials were successfully prepared using high-pressure and high-temperature technology at the synthesis temperature of room temperature?1000°C and pressure of 3?5 GPa.Studies have shown that the synthesis temperature can effectively regulate the material's morphology,thermoelectric transport performance and thermal stability.Under the pressure of 3 GPa,as the synthesis temperature increases,the average grain size of the sample becomes larger,the bulk density increases,as well as the thermal stability.An increase in the synthesis temperature causes an increase in the electrical conductivity.Compared with the sample obtained at room temperature,the power factor is doubled when the synthesis temperature is 1000°C.Synthesis temperature can significantly regulate the lattice thermal conductivity of Cu₂Se samples.Due to the inherent superionic characteristics and abundant grain boundaries,micropores,nanoparticles,and lattice defects generated under high pressure,the lattice thermal conductivity of all Cu₂Se samples at high temperature is as low as 0.3?0.5 Wm^-1K^-1.The combined effect of synthesis pressure and synthesis temperature determines the microstructure and thermoelectric properties of Cu₂Se.Among them,Cu₂Se sample synthesized at 3 GPa and 1000°C has a maximum z T value of 1.19@723 K.?3?Synthesis and research on its thermoelectric properties of non-stoichiometric Cu_2+?Se by high-pressure and high-temperature.High-pressure and high-temperature technology was used to prepare non-stoichiometric Cu_2+?Se?-0.1???0.05?samples at 3 GPa and 1000°C.It was found that Cu content can effectively regulate the phase,carrier concentration and lattice thermal conductivity.Cu_2+?Se?-0.1??<0?exists as a mixed phase of monoclinic?-Cu₂Se and cubic phase?-Cu₂Se at room temperature,but Cu_2+?Se?0???0.05?is a single monoclinic phase?-Cu₂Se.With the increase of Cu content,the Seebeck coefficient and resistivity of Cu_2+?Se sample increase,and the thermal conductivity decreases.At 843 K,the thermal conductivity decreases from 1.45 Wm^-1K^-1??=-0.1?to 0.47 Wm^-1K^-1??=0.05?.With the increase of Cu content,the lattice thermal conductivity of Cu_2+?Se samples decreases first?-0.1??<0?and then increases?0???0.05?.Among them,the Cu_2.0Se sample has a maximum z T value of1.46@843K,which is 100%higher than the z T value of Cu_1.90Se.?4?Synthesis and research on its thermoelectric properties of Bi-doped Cu₂Se samples by high-pressure and high-temperature.By optimizing the high-pressure and high-temperature synthesis process?segmented heating?,Bi-doped Bi_xCu₂Se?x=0,0.005,0.01,0.02,0.03?samples were synthesized at 3 GPa and 1000°C,and the composition segregation was effectively suppressed.The influence of Bi doping on the thermoelectric properties of Cu₂Se is described.With the increase of Bi content,the carrier mobility decreases significantly,and the Seebeck coefficient and resistivity of Bi_xCu₂Se samples increase.Due to the decrease of the electronic thermal conductivity,the thermal conductivity of the Bi-doped samples is significantly reduced.The Bi_0.005Cu₂Se sample has a maximum z T value of 1.57@873K,which is 25%higher than that of pure Cu₂Se?z T=1.25?.In summary,Cu₂Se-based bulk compounds were synthesized using high-pressure technology.The synthesis pressure,synthesis temperature,Cu content and element doping can effectively control the phase,crystal morphology,and microstructure of Cu₂Se-based thermoelectric materials.By optimizing the carrier concentration of the samples,the lattice thermal conductivity is reduced,and the thermoelectric properties of the Cu₂Se-based compound are optimized.The research in this paper can provide experimental references for the performance optimization of liquid thermoelectric materials such as Cu₂Se.