Research On The Mechanism Of Microscopic Triboelectric Charging And Charge Diffusion On SiO₂ Surface Based On Atomic Force Microscope

Triboelectric charging is a ubiquitous phenomenon that has been widely used in industrial fields such as electrostatic separation,electrostatic spraying and energy harvesting,etc.Triboelectric separation technology is free from product dehydration or water treatment that are common in wet separation methods,thus it has great development potential and broad application prospects.However,current studies on the triboelectric separation mostly focused on the macroscale by investigating the overall charge of particles,while the charge transfer law of particle friction process is rarely involved,and the microscopic mechanism of triboelectric charging is still unclear.Hence,based on the atomic probe microscope,this research investigated the mechanism of the triboelectric charging and charge diffusion process on Si O₂ surface at the microscale to provide strong theoretical supports for the industrialization of triboelectric separation.The single impact triboelectric charging behavior of PET,silica and anthracite particles on the Si O₂ surface was systematically studied by using Kelvin probe force microscopy(KPFM),and the effects of particle size,collision velocity,particle material and contact type on the surface potential and topography change on Si O₂surface were quantitatively analyzed.The results showed that the increase of particle size and collision velocity can strengthen the triboelectric charging process,which involved the material transfer of small debris,resulting in the topographic change on the Si O₂ surface.After colliding by PET,quartz and anthracite particles,the Si O₂surface was negatively charged,positively charged,and randomly distributed with positive/negative charges,respectively.The composition of anthracite particles was complicated,leading to the uncertainty of charge transfer direction,thus the positive and negative charges were randomly distributed.The change of contact type could alter the amount of transferred charge,while the polarity of the transferred charge was not shifted.Besides,the increase in particle size could significantly enhance the sliding triboelectric charging process.The microscale triboelectric charging process could be realized by contact-mode atomic force microscope(AFM),and the in-situ characterization of surface potential and topography changes were performed using KPFM.The correlation between the charge transfer and the morphology change in the process of triboelectric charging was established,and the mechanism of elastic deformation and plastic deformation of contact surfaces on the triboelectric charging process was revealed.The strengthening mechanism of multiple friction cycles on triboelectric charging was clarified and the role of material transfer in the process was explored.It is confirmed that lower tip velocity during the triboelectric charging process will wear the tip of the probe and reduce the amount of charge transfer.However,higher tip velocity will reduce the contact probability and thus weaken the triboelectric charging process.The underlying mechanism of charge sign reversal caused by the synergistic effects of tip wear and sample surface pretreatment was verified.After being rinsed with acetone or ethanol,the Si O₂ surface charged negatively during triboelectric charging with the original probe while charged positively with the worn tip.Moreover,the acetone/ethanol ultrasonication process will further enhance charge transfer at the tip-sample interface.The Si O₂ surface without surface pretreatment obtained negative charges after triboelectric charging by original/worn tip.Combined with the X-ray photoelectron spectroscopy analysis,it was found that the carbon content on the Si O₂ surface was significantly reduced after the acetone/ethanol ultrasonication process,and the effect of carbon impurities on the triboelectric charging process was analyzed.A surface state model was proposed to reveal the mechanism of charge transfer direction reversal between the tip and the sample.The influence mechanisms of the relative humidity(RH)on the triboelectric charging and charge diffusion were clarified.For the charge generation,the increase in RH can significantly reduce the magnitude of the transferred charge at the tip-sample interface,due to that the surface water film can participate in the triboelectric charging process and weaken the amount of charge transfer.Meanwhile,the surface water film allowed some of the charge to be transferred to the ground.For the charge diffusion,the temporal and spatial evolution behavior of charges on the silica surface under different RH levels were explored,and the corresponding charge diffusion coefficients were obtained through simulation.When relative humidity was increased from 0 to 60%,the diffusion coefficient of surface charge on Si O₂ increased by roughly 5 orders of magnitude.The dissipative process of negative charge was not significantly different from that of positive charge.The mechanism of lateral diffusion and longitudinal dissipation for surface charges were quantitatively analyzed.It was found that surface charge was difficult to migrate into the interior of Si O₂.Water film on the Si O₂ surface was mainly responsible for the lateral diffusion of charge,and atmospheric water molecules played an important role in neutralizing surface charge.The interaction of surface charges in the process of triboelectric charging and charge diffusion is clarified.By altering the tip bias,the triboelectric charging process was manipulated quantitatively.Based on this technique,charges with different polarities and magnitudes were injected on the Si O₂ surface,and the interaction of positive and negative surface charges in the triboelectric charging process was investigated.When the initial surface potential of the sample was negative(positive),the electron affinity was decreased(increased),and the surface tended to lose(gain)electrons when the sample was triboelectric-charged again.The increase of the initial surface potential value of the sample could hinder the tendency of the charge with the same polarity to transfer to the sample surface.Based on the tip bias control technique and the simulation method of the dissipation process,the interaction law of adjacent charges on the Si O₂ surface during lateral spreading was quantitatively characterized.During lateral spreading,adjacent charges with identical polarity were overlapping,while adjacent charges with opposite polarity were mutually neutralized.The charge dissipation rate at the gap of the charged region was lower than that of the sides of the charged region,demonstrating that the adjacent charges could mutually hinder the lateral diffusion behavior.This dissertation contains 77 figures,6 tables and 183 pieces of references.