Preparation Of Flax Cellulose Nanofibrils And Study On Properties Of Their Reinforced Thermoelectric Composites

Cellulose materials are considered to be ideal and environmentally friendly substrates for the construction of flexible thermoelectric materials due to their inherent low thermal conductivity and rich macro-or micro-porous structures.Therefore,cellulose has great potential to be used as a matrix material for conductive composites and as a thermoelectric material.Graphene has excellent electrical conductivity,mechanical strength,and good photothermal conversion,electrothermal conversion,and photoelectric conversion efficiency,and has high application value in energy conversion of composite materials.Bi2Te3-based thermoelectric material is the best thermoelectric material at room temperature.It has good chemical stability,a large Seebeck coefficient,and low thermal conductivity.It is one of the earliest and most mature thermoelectric materials studied.However,its processing is complicated,its rigidity is large,and its flexibility is poor,so it cannot be applied well in the field of flexible thermoelectricity.In this thesis,flax cellulose nanofibrils(FF-CNF)with a gradation scale were produced using flax fibers as raw materials.Then,the prepared FF-CNF and graphene were mixed to prepare a graphene/FF-CNF composite film with thermoelectric properties.In addition,on the basis of the composite film,the thermoelectric performance of the composite film was optimized by adding Bi2Te3 material.The main work of this paper can be divided into three parts:(1)This paper first prepares FF-CNF by using TEMPO-mediated oxidation and mechanical treatment.The prepared FF-CNF was then dispersed in deionized water and dried at room temperature to form a film,which was characterized by a variety of detection methods.Through characterization,we obtained that under the same conditions,the FF-CNF produced by the oxidation of flax fibers treated with high-concentration NaOH solution had fewer CNF bundles with large sizes.Besides,the prepared FF-CNF film is composed of dense nanofibers,which will reduce its thermal conductivity,indicating that FF-CNF can be used as a thermoelectric material.(2)We prepared graphene/FF-CNF composite films with different graphene concentrations by a simple solution mixing method.Through some listing tests,it was proved that graphene and FF-CNF were successfully mixed.And because of the excellent mechanical properties and good electrical conductivity of graphene,the higher the graphene concentration in the composite film,the better the electrical conductivity of the composite film,and the mechanical properties have also been significantly improved.In addition,the thermoelectric performance test shows that the composite film does have thermoelectric properties,but the thermoelectric properties are poor.(3)Due to the poor thermoelectric properties of the composite films prepared in Chapter 2,we prepared graphene/FF-CNF/Bi2Te3 composite films by adding Bi2Te3 powder and hot pressing on the basis of the solution mixing.The SEM characterization of the composite film showed that we successfully added Bi2Te3 to the composite film.Thermoelectric characterization tests show that the addition of Bi2Te3 can greatly improve the thermoelectric properties of composite films.In addition,the prepared thermoelectric composite film is simply cut and pasted into a simple thermoelectric generator.We hope that the prepared thermoelectric device can be applied to the field of low-temperature energy collection such as human waste heat and environmental waste heat.