| A kind of green three-dimensional conductive wood with excellent tactile properties was prepared by organic combination of natural three-dimensional multi-channel wood riching in active functional groups-OH,-COOH,and two-dimensional graphene with electrical,hydrophobic and mechanical properties,and it could provide a new way for the functional improvement and high value-added utilization of plantation wood in China,and provided a new green material for the field of electrical conductivity.In this paper,wood was used as matrix template,graphene oxide(GO)precursor impregnated it by pulse vacuum,then adopted green chemical method,dynamic mechanical hot pressing method and oxygen isolation thermal reduction method for growing reduced graphene oxide(rGO)in wood matrix respectively,and it could gain three novel 3-dimensional conductive wood materials.The growth mechanism,conductive mechanism,electromagnetic shielding absorbing properties and physical and mechanical properties of rGO under three conditions were discussed,the results were as follows:1.Optimization of solid wood matrix template preparation process.The changes of morphology,pore structure and composition of poplar wood(PPW),wood treated by multiple water cycles(WEW)and wood prepared by alkali extraction(AEW)were compared.The three kinds of wood were combined with GO precursor and reduced by ascorbic acid(AA)to produce rGO,then analyzed the electrical conductivity,morphology,structure and composition of three kinds of conductive wood.It was concluded that WEW kept the original three-dimensional anisotropic skeleton structure of PPW,and had good pore connectivity,GO could enter and grow continuous rGO,forming a complete conductive circuit in wood pores,which was conducive to the passage of electrons.2.Analysis of the relationship between graphite particle size and GO dispersion.5 kinds of GO precursors were prepared by using 5 kinds of graphite powder with different particle sizes.Their lamellar size,oxidation degree and oxygen-containing functional groups were analyzed,and combined them with wood matrix template,reduced by AA,developed 5 conductive wood.The electrical conductivity,distribution of rGO and the combination with WEW were studied.The results showed that GO prepared by D4.0μm graphite powder(GO4)contained most free-COOH and-OH,and least-O-content.It was most conducive to chemical bonding with a large number of free-OH and-COOH in wood.Its transverse size was about 1.6 nm,and rGO in situ growing in the three-dimensional structure of wood matrix template,and had the largest reduction degree,the most complete conductive circuit.3.Wood/graphene conductive materials were prepared by green chemical reduction method.Firstly,the best reducing agent in this study was determined as AA,then the change of conductivity of AA treatment was studied systematically.The results showed that the optimum conditions are as follows:GO concentration 5mg·mL-1,AA concentration 5mg·mL-1,hydrothermal reaction time 4h,reaction temperature 100℃,the longitudinal resistivity was 36.7 Ω·cm,the tangential resistivity was 591.4Ω·cm,and the radial resistivity was 3231.3 Ω·cm.The electromagnetic shielding effect of conducting material is 18.4dB at 39.8GHz in high frequency band,17.6dB in radial direction at 26.5GHz in middle frequency band,and 18.5dB in longitudinal direction at 13GHz in low frequency band.The radial absorption loss of the conductive material 4.0 mm,10-15 GHz band is-58 dB,chord direction is-33.5dB,longitudinal fitting thickness 5.0 mm,35-40 GHz band is-11.4dB.Compared with natural wood,the water absorption of the conductive material reduced 25%,the volume expansion rate reduced 45%,the volume shrinkage rate reduced 73.3%,the radial hardness increased 26.4%,the average bending strength and flexural modulus of elasticity increased 25%and 13%respectively.4.Wood/graphene conductive materials were prepared by batch mechanical reduction.The effects of GO dispersion concentration,thermal pressure,hot pressing temperature and time on the reduction degree of rGO and reconstitution of conductive circuit during batch mechanical hot pressing were investigated.The results showed that,when the concentration of GO was 3 mg·mL-1,the hot pressing temperature was 200℃,and the hot pressing time was 45 min,the shrinkage reached 45%,the resistivity values were 3.8 Ω·cm in longitudinal direction,48.04 Ω·cm in radial direction and 70.70 Ω·cm in tangential direction.The maximum electromagnetic shielding effect is 26.8dB in the range of 26.5-40GHz,18.5dB in radial direction and 16.3dB in longitudinal direction.When the fitting thickness is 4.5mm,the absorbing loss of the conductive material is-33.5dB in the range of 35-40GHz,5.5mm in the radial direction,the absorbing loss of the conductive material is-52.5dB in the range of 25-30GHz,4.5mm in the longitudinal direction,the absorbing loss of the conductive material is-38.5dB in the range of 35-40GHz.The water absorption property of the conductive material reduced 43%,the volume expansion rate reduced from 22.12%to 8%,and the volume shrinkage reduced from 18.14%to 4.43%.The static bending strength and flexural modulus of elasticity increased from 83.185MPa,310.8MPa to 168.921MPa,17563.8MPa respectively.5.Preparation of wood/graphene conductive materials by adiabatic thermal reduction method.Two methods of thermal reduction were used.One was the conductive wood prepared under the premise of retaining the original mechanical properties of wood.The influence of GO reduction degree in wood matrix template on the three-dimensional electrical conductivity of the materials was evaluated.The results showed that:when the concentration of GO precursor is 3 mg·ml-1,the temperature is 210℃,and the time is 2 h,the tangential resistivity is 2100Ω·cm,the radial resistivity is 6073Ω·cm,and the longitudinal resistivity is 1903 Ω·cm,the maximum electromagnetic shielding effect of conductive material in tangential direction is 13.8dB in the range of 26.5-40GHz,9.8dB in radial direction and 14.6dB in longitudinal direction,the microwave absorption loss of the conductive material in tangential direction is-31.1 dB in the range of 35-40GHz with the fitting thickness of 5.5mm,-22.8dB in radial direction and-26.4dB in longitudinal direction,the water absorption rate of the conductive material reduced 18.6%,the volume hygroscopic expansion rate reduced 78.70%,the volume dry shrinkage rate reduced 76.10%,the radial hardness of the conductive material reduced 26.25%,the static bending strength increased 26.56%,and the bending modulus of elasticity increased 20.75%.Secondly,the wood/GO composites were treated under high temperature and oxygen isolation condition without considering the original mechanical properties of wood.The electrical conductivity mechanism and the relationship between rGO and wood matrix template at high temperature were analyzed.The results show that when the concentration of GO precursor is 4mg·ml-1,carbonization temperature is 750℃ and time is 30 min,the bulk resistances of the material in longitudinal,tangential and radial directions are 0.641 Ω·cm,2.153 Ω·cm and 2.932 Ω·cm,respectively.The difference of three-dimensional anisotropy of conductive materials is significantly reduced,the electromagnetic shielding performance is improved to 40dB,and the microwave absorbing property is improved to-12dB. |
PDF Full Text Request