| In order to prepare a kind of nanocellulose based electric heating membrane with uniform and controllable electric heating effect,nanofibrillated cellulose(NFC) as substrate incorporating into graphene was used to prepare NFC/graphene electric heating membrane by ultrasonic dispersion.At first,NFC/GO/graphene electric heating membrane was prepared by ultrasonic dispersion via making full use of self-assembled forming performance of NFC and graphene oxide(GO),as well as their dispersion effect.Under the fixed dosage ratio 1:1 between NFC and GO,the structure,chemical properties,electric heating properties and its stability of the electric heating membrane with different graphene contents and grammages were studied.Then the influence of GO content on its preparation,sheet resistance,morphology,structure,chemical properties,thermal stability,temperature rise and temperature uniformity of the electric heating membrane was analyzed at the condition of constant graphene dosage.Moreover,the ultrasonic dispersion process of NFC and graphene to prepare NFC/graphene membrane was mainly and systematically studied.And by discussing the influence of ultrasonic power and dispersion time on its dispersibility,surface morphology,phase structure,chemical properties,mechanical properties,surface conductivity uniformity,surface electric heating temperature uniformity and its electric heating stability,the better ultrasonic dispersion process was obtained after the comprehensive analysis.Finally,the study focused on the electric heating properties of NFC/graphene electric heating membrane with different graphene contents and different grammages.Specifically,the time-temperature relationship,change rule of resistance during the electrification process and surface electric heating temperature uniformity was comprehensively explored to indicate the change rule and mechanism of electric heating properties.The main results are as follows:(1)In the NFC/GO/graphene electric heating membrane,NFC,GO and NFC/GO dispersion showed negative potentials.Their Zeta potentials were -46.8,-42.7 and -37.9m V,respectively,which indicated they can be used as dispersants.Mutual repulsion arose from the negative charge on their surface can promote uniform dispersion of graphene and improve the static stability of their dispersion.With the increase of the graphene content and the grammage of the heating membrane,the sheet resistance of the NFC/GO/graphene heating membrane showed a regular downward trend.When the graphene content was 40,50 and 55wt%,the sheet resistivity was 5.13,1.98,1.49kΩ/sq,respectively.Raman and XRD analysis results showed that the addition of NFC and GO affected the regularity of stacked structure in electric heating membrane,and the regularity had been improved after synergistic effect of the Joule heat and electricity.During electrification at 1500W/m~2 power density,the stable temperature rise on the surface of the electric heating membrane increased with the increase of graphene content and grammage.When the graphene content was 55wt%,the stable temperature rise was about 15℃ higher than that at 30wt% and 35wt%.This was because that with the increase of graphene content and grammage,the resulted increase of lap joints among graphene sheet made the conductive network structure more perfect.And the thermal resistance effect was more obvious in the energization process,which resulted in that the reduction rate of resistance at the moment of power off showed an increasing trend.It caused a significant increase in the electrical conductivity of the electric heating membrane and an increase in the actual input power density.The reduction rate decreased significantly during the second power-on,indicating that the stability of the electric heating membrane was improved after the first electrification,which was consistent with the results of Raman and XRD analysis.The stable temperature rise of the electric heating membrane increased linearly with the increase of power density,and the stable temperature rise on its surface can reach 90℃ under the power density of 2500W/m~2.It can be seen that the resistance change of the electric heating membrane and the electric heating performance are regular.(2)In the NFC/GO/graphene electric heating membrane,when the graphene content was fixed at 50wt%,with the increase of GO content(ratio of GO accounting for NFC/GO),the time for filtrating to be membrane increased significantly,and the sheet resistivity increased approximately linearly.When there was not GO,the sheet resistivity was 1.188±0.214kΩ/sq.When the GO content was 60wt%,the sheet resistivity increased to 2.274±0.313kΩ/sq.The GO with a large radius-thickness ratio was easier to hinder the stack and assembly connection of graphene sheets than the NFC with a large aspect ratio layer,which weakened the conductive performance.The reduction of GO amount was conducive to improving the structure of the electric heating membrane,and enhancing the flatness and electric heating stability of the electric heating membrane.(3)In the ultrasonic dispersion preparation process of the NFC/graphene electric heating membrane,under 1000W ultrasonic power,when the ultrasonic dispersion time was prolonged,the Zeta negative potential of the pure NFC dispersion was slightly weakened,and the particle size dropped significantly,then tended to be stabilized after 35min.As well as that the dispersibility and standing stability of pure graphene dispersion was also improved slightly.By increasing the ultrasonic dispersion parameters,the shearing effect caused by ultrasonic cavitation effect was enhanced,as well as the synergistic effect of the NFC dispersant.Then the dispersibility of graphene sheet in dispersion can be significantly improved.With the increase of ultrasonic dispersion power and time,it was conducive to improving the surface conductive uniformity and tensile strength of the electric heating membrane,but the infrared thermal image showed that the uniformity of the surface temperature was not obvious difference.This fact attributes to the excellent radial thermal conductivity of graphene sheet.The reduction rate of resistance of the electric heating membrane made by the dispersion process of 400W-20min,700W-50min and1000W-65min after the first electrification was 59.32%,51.67%,46.67%.And that was 28.57%,19.51%,24.00% after the second electrification.The results showed that the enhancement of ultrasonic dispersion conditions can promote the interweaving of NFC and graphite sheets to form a stable structure,which was conducive to improving electric heating stability.(4)In the electric heating performance of NFC/graphene electric heating membrane,as increasing the content of graphene and the grammage of the electric heating membrane,the resistance between the two electrodes of the electric heating membrane exhibited a certain exponential function curve decline rule.And the stable temperature rise of the electric heating membrane showed an increasing trend,and the relationship between power density-temperature rise and time-temperature rise exhibited regular variation.Among them,under the power density of 2500W/m~2,the stable temperature rise of the electric heating membrane at the grammage of 20g/m~2 was nearly 50℃higher than that at the 8g/m~2.The change rate of resistance of the electric heating membrane after electrification,also increased with adding the graphene content and grammage,and then tended to be stable.During the energization process,the resistance between the two electrodes of the electric heating membrane exhibited a creep phenomenon,which decreased rapidly at the beginning of energization and then tended to be stable.(5)In summary,electric heating performance of NFC/graphene electric heating membrane exhibited regularity and controllability,which will have application prospect in the electric heating product. |
PDF Full Text Request