Fabrication And Study Of N-type Ag₂Se-based Flexible Thermoelectric Composite Materials

With the rapid development of smart wearable electronic devices and other smart wearable devices,the development of flexible thermoelectric materials has been greatly promoted.According to the type of carriers,flexible thermoelectric materials are divided into p-type and n-type,among which p-type flexible thermoelectric materials are widely researched and generally have high thermoelectric properties,while compared with p-type flexible thermoelectric materials,n-type flexible thermoelectric materials are seriously lagging behind and currently have relatively low thermoelectric properties overall.However,in the preparation of thermoelectric devices,both p-type and n-type thermoelectric materials need to be used in order to obtain high thermoelectric conversion efficiency.Therefore,it is urgent to produce more types of n-type flexible thermoelectric materials with high thermoelectric properties at room temperature.This thesis is based on Ag₂Se,an n-type inorganic thermoelectric material with high thermoelectric properties at room temperature,and focused on the topic of preparation of Ag₂Se-based flexible composite thermoelectric materials,with non-free-standing thermoelectric materials and free-standing thermoelectric materials as the two forms of presentation.Firstly,Ag₂Se/Mxene flexible and non-free-standing composite thermoelectric films were prepared by combining Ag₂Se nanowires（NWs）with MXene nanosheets（NSs）,which is a novel two-dimensional material with high electrical conductivity.Secondly,A series of flexible and free-standing composite thermoelectric materials were prepared by combining PVDF dendricolloids with Ag₂Se nanowires（NWs）,aramid dendricolloids with Ag₂Se multisized nanostructures（MNSs）,and colloids containing different aramid microstructures with Ag₂Se microparticles,respectively.A series of flexible self-supporting composite thermoelectric materials were prepared and a series of n-type flexible thermoelectric materials with both excellent thermoelectric properties and flexibility were prepared.The main research contents and results are as follows:（1）Compounding thermoelectric materials of different types or with different microscopic morphologies may be an effective way to enhance the thermoelectric properties of single thermoelectric material.In this chapter,Ag₂Se/Mxene flexible and non-free-standing thermoelectric composite films were obtained by compounding Ag₂Se NWs with the novel two-dimensional nanomaterial MXene NSs through solution mixing,vacuum-assisted filtration and cold-press densification successively.The effect of different MXene content on the microscopic morphology,thermoelectric properties and flexibility of the composite thermoelectric materials were systematically investigated.The results show that with the addition of MXene,a highly conductive network of one-dimensional Ag₂Se NWs and two-dimensional MXene nanosheets lap each other and interfacial energy barriers at the heterogeneous interfaces in the composite films were formed.At room temperature,the power factor（PF value）of the composite film increased firstly and then decreased,thus the maximum PF value of 537.52μW m^-1K^-2 was obtained with the 3 wt%MXene,which is among the relatively high level in n-type thermoelectric materials.Moreover,this PF value increased by 143.62%compared with that of the pure Ag₂Se film,and meanwhile the composite film has excellent flexibility.The above results indicate that the incorporation of certain amount of MXene NSs can effectively enhance the thermoelectric properties of Ag₂Se.（2）High content of inorganic thermoelectric materials is usually required to achieve high thermoelectric properties for insulating polymer-based flexible self-supporting thermoelectric composites,which usually leads to poor processing and mechanical properties of the composites.In this chapter,a biomimetic approach is adopted to facilitate filler content up to 90.5 wt%in free-standing and flexible n-type PVDF/Ag₂Se TE films,where PVDF dendricolloids solution were mixed with Ag₂Se nanowires followed by filtration.These soft dendric nanoparticles within PVDF dendricolloids have high adhesivity and strong network-building ability,which allows the formation of“grapevine-grapes”like networks with soft dendritic particles and inorganic TE fillers as“grapevine”and“manicure finger grapes”,respectively.The maximum power factor of 189.02μW m^-1K^-2 is achieved for PVDF/Ag₂Se mass ratio of 1:9.5 at 300 K.Meanwhile,excellent flexibility was observed.At such high filler content,these properties are attributed to the long-range grapevine-like network structure formed by the soft PVDF dendritic microparticles and the entanglement between a large number of Ag₂Se NWs.（3）High-temperature sintering process,which is commonly used in the preparation of inorganic bulk materials,is prevented due to the low thermal temperature of insulating polymers in flexible and free-standing thermoelectric composites,resulting in simply physical laps of inorganic thermoelectric materials in most composites,which greatly limits the thermoelectric properties.In this chapter,Aramid with a high heat-resistant temperature was introduced and aramid dendricolloids were prepared using a similar method as in above（2）.Meanwhile,Ag₂Se MNSs with higher thermoelectric properties were also prepared by adjusting the preparation parameters based on previous（2）.The aramid/Ag₂Se flexible composite films were prepared by solution mixing followed by vacuum filtration,and the films were subsequently cold-pressed and sintered at different temperatures from115°C to 200°C.The effect of different sintering temperatures on the microscopic morphology,thermoelectric properties and flexibility of the composite films were systematically investigated.The results showed that the complete,flexible and free-standing thermoelectric films could be obtained at as low as 0.66 wt%aramid content（99.34 wt%Ag₂Se MNSs content）,which may be attributed to the strong long-range3D network structure of aramid within the composite films by combining the high strength and modulus of the aramid itself,the strong adhesion of the aramid dendritic particles to Ag₂Se MNSs and the strong long-range three-dimensional network construction capability of aramid dendritic particles.With sintering temperature increasing,Ag₂Se MNSs gradually fused with each other,and the PF value of the film gradually increased,while the flexibility became correspondingly worse.Both excellent thermoelectric properties and flexibility were achieved at the sintering temperature of 165°C,and PF value and ZT value of the film was 1224.4μW m^-1 K^-2 and 0.03355,respectively.Additionally,PF value of the film only lost 7.47%after1000 bending cycles,showing excellent thermoelectric properties and flexibility.（4）In the field of polymer-based flexible thermoelectric composites,inorganic thermoelectric materials with nanoscale are often used for the preparation of polymer-based flexible thermoelectric composites due to the advantages of easy dispersion and easy inter-lap to form highly conductive networks.However,these thermoelectric materials also have fatal drawbacks,such as,complicated preparation process and low yield,which greatly limit their application in practical production.In contrast,the method of obtaining inorganic thermoelectric microparticles by direct ball milling has the advantages of simple process and high yield.However,there is no report on the preparation of flexible thermoelectric materials by mixing inorganic thermoelectric microparticles with polymers,probably due to the fact that microparticles are difficult to disperse and lap each other to form highly conductive networks.In this chapter,aramid sheet-like colloids and aramid filament-like colloids were prepared based on（3）by adjusting the processing parameters.Then different aramid/Ag₂Se composite films were obtained by solution mixing these two colloids and the aramid dendricolloids in（3）with Ag₂Se microparticles prepared by conventional wet ball milling method,respectively,followed by vacuum filtration,cold-pressing and high-temperature sintering treatment.The effect of different aramid colloids on the microscopic morphology,thermoelectric properties and flexibility of the composite films were systematically investigated.The results show that aramid microparticles in all three aramid colloids showed superb loading capacity on Ag₂Se microparticles,which varied due to the different microscopic morphology of the aramid microparticles.Compared with the other two aramid colloids,aramid/Ag₂Se flexible and free-standing thermoelectric composite films can be obtained at a lower 0.50 wt%aramid content when aramid filament-like colloids were used.Moreover,after being sintered at 200°C,the composite film has a PF value as high as 2082.32μW m^-1K^-2at room temperature,which is comparable to the PF values of pure Ag₂Se films deposited on nylon-6 films and pure Ag₂Se bulk materials prepared from Ag₂Se microparticles in this chapter.This PF value is the highest PF value of n-type polymer-inorganic flexible thermoelectric composites available up to now.The composite film also shows excellent flexibility,and the PF value only decreases by 4.75%after 1000bending cycles,i.e.,flexible and free-standing thermoelectric films with thermoelectric properties comparable to those of inorganic thermoelectric materials have been successfully fabricated.