Research On The Construction And Application Of Nanofiber And Its Physical And Mechanical Properties

Nanofiber molding technology involves the synthesis,processing,manufacturing and application of nano fibers.Nanofiber technology has a wide application prospect.It can assemble one-dimensional and two-dimensional nanostructures on a large scale,and realize the large-scale production of related products of nanomaterials in different scales such as microscopic,mesoscopic,and macroscopic.In addition,nanofibers can also be used to directly assemble micro-nano devices.This dissertation mainly takes PVP/Ti O₂ nanofiber tube film and Na₂Ti₃O₇ nanofiber film as the research objects.The physical and mechanical behavior of nanofiber-based films are studied,and they are applied to LIBs for the design of high-performance LIBs separators and application to provide reference.The main research work of this paper is as follows:1.A PVP/TiO₂ nanofiber tube film with shape memory behavior has been prepared by coaxial electrospinning technology.The film exhibits a unique temperature on/off effect with excellent rate performance and cycle life.This film is constructed of nanofiber tubes and has higher porosity and stronger capillary adsorption characteristics.Compared with commercial PP separators,it has better wettability towards for organic liquid electrolytes and higher ionic conductivity.In addition,LIBs assembled with composite separators have an on/off function at 60℃（when the operating temperature reaches 60℃,the battery will be suspended,and then can be recovered after returning to room temperature）.It is worth noting that the membrane has no significant shrinkage in its macroscopic size in air up to180°C,and it still has high thermal stability even at a temperature of up to 500°C under vacuum or atmosphere protection.With these excellent performance and safety properties,PVP/Ti O₂ film is expected to be the next-generation LIBs separator with good commercial application prospects.This research also provides ideas for the development of safe on/off separators.2.Under the framework of finite deformation continuum thermodynamics theory,a thermal constitutive model of polymer shape memory considering microstructure evolution is established,which explains the temperature on/off effect of PVP/Ti O₂ film.The model is based on glass transition physics and the entropy elastic theory of rubber-like materials,and uses statistical mechanics methods to derive a new frozen volume fraction evolution expression,which is used to express the phase transition from the rubber phase to the glass phase.Unlike the empirical expression used in the literature,this expression provides a strong physical basis for explaining phase transitions.In addition,this work also established the relationship between the frozen volume fraction of the polymer network and the configuration entropy,and derived a new chain evolution equation based on the elastic theory of finite strain entropy based on Langevin statistics.3.A high temperature resistant and flexible Na₂Ti₃O₇ nanofiber film for advanced lithium storage has been prepared by improving wood fiber film forming technology.When it is used in LIBs separators,the LIBs exhibit excellent rate performance and cycle life at both low and high temperature.The flexible,lithium dendritic puncture-resistant and binder-free pure inorganic fiber film shows high ionic conductivity and good liquid electrolyte retention due to its high porosity and good wettability.And this kind of Na₂Ti₃O₇nanofiber film can maintain its structural integrity at a temperature below 1000℃,showing excellent high temperature resistance.Higher thermal stability and excellent electrochemical performance enable Na₂Ti₃O₇ nanofiber film to meet the needs of the next generation of safe LIBs.4.Based on the weak interaction between the fibers at the micro-nano scale,a film composed of nanofibers of pure inorganic compound（Na₂Ti₃O₇）has been successfully prepared,and the forming process of the nanofibers film and the mechanical properties of the nanofibers film were studied.Interaction methods between fibers have been developed to handle contact forces on the scale of tens of nanometers.The DLVO theory based on van der Waals force and electrostatic force controls the fiber-fiber interaction,and develops a repulsive force to ensure that the numerical stability of fiber motion is solved with discrete time steps.The interaction method is used as one of the four sub-models of the fiber suspension model.The other three sub-models include a fluid model controlled by Navier-Stokes equations,a fiber model controlled by beam theory,and a fluid-fiber and fiber-fluid interaction model based on the immersion boundary method and experimental resistance expressions.This paper also proposes a numerical simulation method based on the discrete fiber network model.The contact points between the fiber and the fiber are respectively expressed as the nodes and edges nodes of the model to simulate the mechanical properties of the nanofibers film.