High-temperature And High-pressure Preparation And Performance Of Bi₂S₃-based Thermoelectric Materials

The consumption of fossil fuels in both daily human activities and industrial production has witnessed a steady increase in recent years.The growing economic growth rate and its energy consumption tend to grow proportionately,energy shortage and environmental pollution is one of the most critical problems in modern society,the limited reserves of fossil fuels highlight the importance of energy in sustainable development.The increase in environmental pollution and fuel consumption can be overcome by exploring green energy sources,diversifying the energy matrix and reducing environmental impact.It is worth noting that during the combustion of fossil fuel energy,about 60-70%of the waste heat is released into the environment,and heat is an abundant but often wasted energy source.Therefore,it is reasonable to reuse waste heat into electricity using thermoelectric conversion technology,which is very beneficial for energy conservation.Thermoelectric materials and devices based on thermoelectric materials can achieve direct conversion of heat and electrical energy,require less maintenance,and have a long service life.Thermoelectricity is a promising technology that can convert waste heat energy into useful electricity and make an important contribution to reducing fuel consumption by cooling the environment without the use of moving parts and harmful chemicals.Most TE-based materials（Bi₂Te₃,Pd Te,Sn Te）have excellent thermoelectric properties,but the expensive nature and toxicity of Te limits the large-scale application of TE-based materials,and a long-term challenge in the widespread use of thermoelectric devices is to find high-performance materials from abundant and low-cost elements.Recently,there have been significant developments in environmentally friendly materials that are abundant and low toxic in nature.Bismuth sulfide（Bi₂S₃）has attracted the attention of researchers as a promising n-type candidate material due to its cost-effectiveness,low toxicity,high Seebeck coefficient and low thermal conductivity of sulfur.Sulfur substitution doping has been proved to be a feasible method to improve the thermoelectric properties of Bi₂S₃.However,doping at bismuth sites is rarely reported,and more studies on cationic doping will help to understand the thermoelectric transport properties of Bi₂S₃compounds more comprehensively.In this dissertation,Bi₂S₃materials are taken as the research object,and the transition metals are integrated into the crystal structure of Bi₂S₃in two different ways,namely gap or substitution,through HPHT preparation process combined with doping,so as to optimize and improve the deficiencies in the electrical conductivity of Bi₂S₃materials.Combined with high temperature and high pressure means to regulate the thermal transport properties（phonon scattering mechanism）relatively independently,the influence of Bi bit doping mode on the electroacoustic transport of Bi₂S₃-based materials was explored.The main research contents and main innovations were summarized as follows:1.This part of the work is mainly aimed at the lack of reports on the use of metal dopants directly acting on the thermoelectric field of Bi₂S₃materials.In order to investigate the existence mode of Ni in the bismuth sulfide lattice and the effect of Ni doping on electroacoustic transport of Bi₂S₃,we selected a small size transition metal Ni-doped bismuth sulfide material.XRD and XPS test results show that Ni²⁺exists in the Bi₂S₃lattice at a gap position.With the increase of nickel doping content,the conductivity is higher than 75 Scm^-1,and the absolute value of Seebeck coefficient is lower than-189μVK^-1,which is mainly due to the increase of carrier concentration caused by Ni gap doping.The optimal power factor value of Ni_0.005Bi₂S₃sample at room temperature is about 315μWm^-1K^-2.A large number of defects such as lattice distortion and dislocation induced by pressure exist in the sample,which is conducive to reducing the mean free path,resulting in increased phonon scattering.However,Ni and Bi have a large mass difference and ionic radius difference.When Ni is introduced into the lattice,it will disturb the propagation path of conventional phonons,and this disturbance will also cause the scattering of phonons to increase,and finally make the doped samples have lower lattice thermal conductivity than the undoped samples.However,when the amount of Ni reached 0.010 and 0.015,the thermal conductivity was higher than that of pure Bi₂S₃samples due to the significant increase in the electron thermal conductivity.At low Ni content,the x=0.005 sample has a minimum thermal conductivity of 0.63 Wm^-1K^-1and a maximum z T value of0.23 at 543 K,which is about 4 times that of the pure Bi₂S₃sample.2.In order to improve the overall thermoelectric performance of the material in Ni work,Fe,which is similar to Ni and cheaper,is selected to characterize the doped sample.With Fe³⁺in the interstitial position,Fe doping effectively increases the conductivity over the entire temperature range by an order of magnitude over the pure sample(18 S cm^-1at 303 K).Through SEM characterization,it was observed that all the samples showed layered structure,and the layered grain size of Bi₂S₃was gradually refined with the increase of Fe doping amount.Fe_0.015Bi₂S₃has a maximum power factor of 326μWm^-1K^-2at room temperature.The defects of intergap doping points and the special microstructure of the sample（twisted lattice,dense dislocation）significantly reduce the heat transfer ability of the material after high temperature and high pressure,so that the thermal conductivity of Fe-doped samples(0.85-0.55Wm^-1K^-1)is lower than that of Bi₂S₃samples(0.9-0.6 Wm^-1K^-1).At 543 K,the x=0.015 sample has a higher z T value reaching 0.29.The results above confirm the feasibility of Fe doping to optimize the thermoelectric properties of bismuth sulfide materials.3.The first two works mainly studied the existence of doping elements in the Bi₂S₃lattice in the form of gaps.This part of the work innovatively replaced metal Bi with metal Nb.The sample of Nb-doped bismuth sulfide was prepared by mechanical alloying and high temperature and pressure technology.The+5 valent Nb ions substituted+3 valent Bi in the Bi₂S₃lattice,resulting in excess electrons,resulting in increased carrier concentration in the matrix.However,when the concentration of Nb doping is 0.02 and 0.03,a small number of particles will appear on the fracture surface,which increases the interfacial resistance of the material and reduces its conductivity.The Seebeck coefficient increases with the increase of effective mass of Nb-doped samples,and the power factor of Nb-doped samples is improved by the combination of stable Seebeck coefficient and good conductivity.Bi_1.98Nb_0.02S₃has a maximum power factor value of 272μWm^-1K^-2at room temperature.The lattice thermal conductivity shows a decreasing trend with the increase of Nb content,indicating that Nb substitution for Bi reduces the mean free path of phonons,enhances phonon scattering,and introduces additional point defects.Nb doping and a large number of lattice distortions and dislocation defects caused by high pressure work together to reduce the lattice thermal conductivity.Therefore,due to the optimization of carrier concentration,effective mass and the reduction of thermal conductivity,the Bi_1.98Nb_0.02S₃sample reaches a maximum z T value of 0.22 at 543 K,which is 120%higher than pure Bi₂S₃.4.On the basis of the previous work,experiments are designed based on the effect of strong point defect scattering caused by Nb substitution doping on the thermal properties of materials.The thermal transport process of Bi₂S₃was regulated by the decomposition of Nb N to produce nitrogen pore structure and high pressure preparation,and the thermoelectric properties of Bi₂S₃were systematically studied.The results show that at low Nb N doping content（x=0,0.2,0.5）,more Bi³⁺will be replaced by Nb⁵⁺,resulting in free electrons,and the combined effect of pore and lattice defects will reduce the lattice thermal conductivity.However,a small amount of Bi heterophase was found in the XRD pattern of the sample with a high doping content（x=1.0）,and the conductivity decreased significantly at high temperatures.The increase of the electronic thermal conductivity increased the total thermal conductivity,which was not conducive to the improvement of thermoelectric properties.The optimal Nb N doping amount is x=0.5,when the sample has a high average power factor of 300μWm^-1K^-2and a low thermal conductivity of 0.44Wm^-1K^-1.The final sample with x=0.5 has the highest z T value of 0.35 at 543 K,and the average z T value in the range of 303-543 K is 0.25.It has certain competitiveness with the reported Bi₂S₃-based thermoelectric materials.In summary,a series of Bi doped bismuth sulfide based materials were prepared by high temperature and high pressure method,and their phase,microstructure,structural defects and thermoelectric properties were analyzed in detail.The study demonstrates that the transition metal（Nb,Ni,Fe）doping has different degrees of improvement on the electrical properties,using high pressure to control the sample microstructure,further regulate the phonon scattering mechanism,and collaboratively improve the electrical and thermal transport properties of bismuth sulfide materials with high thermoelectric properties.