Performance Optimization Of Strontium Titanate-based High-entropy Oxide Thermoelectric Materials Using High-temperature,High-pressure Synthesis

One of the main manifestations of climate change is a rise in the global average temperature,which melts glaciers and polar ice sheets and causes sea levels to rise.This poses a threat to low-lying areas,coastal cities,and island countries.China’s current energy structure is dominated by coal,whose combustion is a significant contributor to global greenhouse gas emissions.Therefore,China’s actions will have a significant effect on achieving global climate change goals.In addition to reducing the use of traditional energy sources such as coal and oil,the innovative development of new energy materials,green energy conversion materials,and comprehensive recycling of energy have become important research directions.Thermoelectric materials provide a potential solution for sustainable energy conversion to solve global energy challenges by mutually interconverting electric energy and thermal energy.This is achieved by the Seebeck effect,in which the temperature difference between the two ends of a thermoelectric material generates an electric current,thus generating a voltage.Oxide thermoelectric materials such as strontium titanate（Sr Ti O₃）have become popular research subjects due to their low cost,high-temperature stability,and environmental friendliness.Strontium titanate is an intrinsic insulator,and its low electrical conductivity and high thermal conductivity limit its thermoelectric properties.Therefore,researchers are focused on optimizing the electrothermal transport properties of Sr Ti O₃-based thermoelectric materials through doping,nanostructure design,engineering microstructure,and strain engineering.These optimization methods can improve the electrothermal transport properties of Sr Ti O₃-based thermoelectric materials.Sr Ti O₃-based bulk ceramics are intrinsic insulators,and the introduction of oxygen vacancies transforms them from insulators to semiconductors.Conventional Sr Ti O₃-based thermoelectric materials must be sintered or annealed under a reducing atmosphere to improve their conductivity,but this may cause a gradient oxygen vacancy distribution in bulk materials.The high-entropy strategy has gained extensive interest in the research of thermoelectric materials.Traditional materials are generally composed of two or three elements,while high-entropy materials contain multiple main elements that are mixed uniformly to form a single-phase solid solution.High-entropy ceramics have been used in various structures and applications due to their superionic conductivity,oxidation resistance,mechanical properties,and low thermal conductivity.Based on the above,the electrothermal transport properties of strontium titanate-based thermoelectric materials have been optimized by using a high-temperature,high-pressure method combined with a high-entropy strategy and in-situ reduction under a high pressure.The introduction of multiple elements into the A sites of perovskite strontium titanate introduces a high entropy,while the use of a high pressure accelerates the reaction and expands the tolerance factor range of perovskite structure doping.The in-situ reduction of strontium titanate was realized using a closed synthesis environment and applying a high temperature and high pressure and the strong reduction ability of Ti powder at high temperatures.This approach avoided the gradient distribution of oxygen vacancies throughout the material.In this paper,strontium titanate-based oxide thermoelectric materials were studied.First,to reduce the thermal conductivity of materials,the influence of different pressures on the thermal properties of high-entropy strontium titanate-based bulk materials was explored by entropy engineering.Secondly,undoped strontium titanate-based materials and high-entropy strontium titanate-based materials were reduced in-situ by Ti powder,and the amount of Ti powder was adjusted to optimize the thermoelectric properties of strontium titanate-based bulk materials.Finally,the influence of dopants on the thermoelectric properties of the synthesized materials was explored through the in-situ reduction of high-entropy samples with Ti powder using the above two methods to optimize the electrical and thermal properties.The main innovative achievements of this paper are as follows:1.(Sr_0.2La_0.2Nd_0.2Sm_0.2Eu_0.2)Ti O₃ceramics with a high configurational entropy were synthesized by a high-pressure,high-temperature（HPHT）method.Due to the driving force of high pressure,the synthesis time was greatly shortened compared with the conventional synthesis method.XRD showed that perovskite ceramics with a cubic structure were synthesized by the high-entropy system using this HPHT method.HRTEM results showed that multi-component cations were uniformly distributed at the A sites,and the SEM and TEM images showed that adjusting the high pressure during synthesis changed the micromorphology of the synthesized samples.An ultra-low thermal conductivity was obtained by high-entropy samples synthesized under a high pressure,and the lowest thermal conductivity of high-entropy(Sr_0.2La_0.2Nd_0.2Sm_0.2Eu_0.2)Ti O₃perovskite ceramics was 0.82 Wm^-1K^-1at the test temperature of 973 K.This value was significantly lower than that of previously-reported perovskite strontium titanate-based ceramics.The above results show that the introduction of entropy engineering enhanced the short-range disorder of the A sites in strontium titanate,as well as the lattice distortion.The high-pressure synthesis method also enhanced the micromorphological changes such as lattice distortion in the sample,thus significantly enhancing full phonon spectrum scattering and reducing its lattice thermal conductivity.This work showed that a combination of high pressure and entropy engineering coordinated the thermal properties of materials to provide a new strategy for the synthesis and performance optimization of high-entropy perovskite oxides.2.A gradient distribution of oxygen vacancies may exist in samples obtained by short-term reductive annealing using conventional synthesis methods.To prevent this,Ti powder was added to the raw materials synthesized by the HPHT synthesis method and mixed evenly.Due to the closed environment during HPHT synthesis and the reducing ability of Ti powder at high temperatures,STO materials were transformed from dielectrics into semiconductors.Upon increasing the amount of Ti added,the electrical properties of the materials were improved.Due to microscopic changes such as grain refinement and lattice distortion caused by the addition of Ti and the use of the HPHT method,the thermal conductivity of the materials was reduced.The results showed that the introduction of oxygen vacancies improved both the electrical and thermal transport properties.Through HPHT’s collaborative optimization of the electrical and thermal properties of materials,the optimal amount of Ti addition was within the range of 15 wt%.The optimal z T value of 0.24 was obtained at a synthesis temperature of 973 K by optimizing the thermal properties by entropy engineering and optimizing the electrical properties by adding Ti powder.The above results show that the combination of the HPHT synthesis method and Ti powder is an effective,convenient,and novel method for adjusting the thermoelectric properties of perovskite ceramics.3.To optimize the electrical and thermal properties of the samples in the first two works,high-entropy strontium titanate was reduced in-situ.The high-entropy effect reduced the sample’s thermal conductivity,while the high-configuration entropy reduced the carrier mobility and electrical conductivity.Therefore,the bulk thermoelectric materials based on strontium titanate with different doping ratios(Sr_0.6-xLa_xNd_0.1Sm_0.1Eu_0.1)Ti O₃（x=0.1,0.2,0.3）and(Sr_0.2La_0.2Nd_0.2Sm_0.2Eu_0.2)Ti O₃were designed and synthesized by HPHT synthesis,and their thermoelectric properties were explored.The results showed that the in-situ homogeneous reduction of STO-based thermoelectric materials was realized due to the closed environment formed by the HPHT method and the strong reduction ability of Ti at high temperatures.This improved the electrical properties.The influence of high pressure on the microstructure of high-entropy ceramics was studied and analyzed.The results showed that the combination of the high-entropy effect and high pressure controlled the internal defects of the materials.Due to the design of different components,the best z T value of 0.34 was obtained at 973 K by synergistically optimizing theXelectrical and thermal properties.In this paper,a series of strontium titanate-based thermoelectric materials were synthesized by HPHT synthesis,and exploratory experiments were carried out.The results showed that the in-situ reduction of strontium titanate-based thermoelectric materials was realized by combining the HPHT method with the addition of Ti powder.The intrinsic thermal conductivity of strontium titanate-based materials was reduced by introducing a high-entropy strategy.The use of a high pressure during synthesis promoted the entrance of multiple elements into the crystal lattice,accelerated the synthesis of high-entropy samples,and broadened the range of tolerance factors.Finally,the thermoelectric properties of the sample are improved by designing the components of the sample to optimize the electrical and thermal properties of the sample.