【作者】 王丽；

【导师】 王喜明；

【作者基本信息】 内蒙古农业大学 ， 木材科学与技术， 2020， 博士

【摘要】 利用天然立体多通道且富含活性官能团-OH、-COOH的木材,与集导电、疏水及机械性能为一体的二维石墨烯有机结合,制备出一种绿色、触觉特性优良的三维导电木材,为我国人工林木材功能性改良及高附加值利用提供新途径,并为导电领域提供一种新型绿色材料。本文以实体木材为基质模板,氧化石墨烯(GO)前驱体进行浸渍处理,采用绿色化学法、间歇式机械力热压法、隔氧热还原法致使还原性氧化石墨烯(rGO)在木材基质模板中原位生长,制备出三种新型三维导电木材,并对rGO在三种条件下的生长机理、材料的导电机理、材料的电磁屏蔽-吸波性能及物理力学性能进行了探讨,取得如下研究成果:1.实体木材基质模板的制备工艺优化。对比了杨木素材(PPW)、多次水循环处理木材(WEW)及碱法抽提木质素制备木材(AEW)的孔隙形貌结构及成分变化,将三种木材与GO前驱体结合,并通过抗坏血酸(AA)还原出木材机体内部具有导电性的rGO,得到导电木材,对三种导电木材的导电性能、形貌结构及成分进行了分析,得出WEW保持了 PPW原有的三维各向异性骨架结构,孔隙连通性好,GO可畅通地进入并原位生长出连续性的rGO,形成完整的导电线路,利于电子的穿行。2.石墨粉粒度与GO分散液的关系分析。采用5种不同粒径的石墨粉制备出5种GO前驱体,对其片层尺寸、氧化程度及含氧官能团种类进行了分析,并将5种GO前驱体与WEW复合,经AA还原制备出5种导电木材,对其导电性、rGO的分布、与WEW的结合进行了分析。结果表明,D4.0μm石墨粉制备的GO(GO4)含有的游离态-COOH和-OH最多,-O-最少,最有利于与木材中的游离态-OH及-COOH发生化学键合,其横向尺寸约为1.6nm,进入到木材基质模板的三维结构中原位生长的rGO还原程度最大,构建的导电线路最完整。3.绿色化学还原法制备木材/石墨烯导电材料。首先确定出本研究的最佳还原剂为AA,之后对AA处理方式导电性能的变化进行了系统研究。结果表明,GO前驱体浓度5mg·mL-1,AA浓度5mg·mL-1,水热反应时间4h,反应温度为100℃时,材料的纵向电阻率为36.7Ω·cm,弦向电阻率为591.4Ω·cm,径向电阻率为3231.3Ω·cm。导电材料弦向的电磁屏蔽效应在高频段39.8Ghz为18.4dB,径向在中频段26.5GHz为17.6dB,纵向在低频段13GHz为18.5dB。导电材料径向的吸波损耗在拟合厚度为4.0mm,10-15Ghz波段为-58dB,弦向为-33.5dB,纵向在拟合厚度5.0mm,35-40Ghz波段为-11.4dB。导电材料的吸水率比素材降低25%,体积膨胀率降低45%,体积干缩率降低73.3%,径向硬度提高26.4%,抗弯强度及抗弯弹性模量均值分别增大了 25%及13%。4.间歇式机械热压力还原法制备木材/石墨烯导电材料。考察了机械热压过程中,GO前驱体浓度、热压力、热压温度及时间对rGO还原度及导电线路重新构建的影响,并对材料的电磁屏蔽-吸波性能及物理力学性能进行了分析。结果表明,当浸渍的GO浓度为3mg·mL-1,热压温度为200℃,热压时间为45min,试件压缩率达到45%时,电阻率数值分别为:纵向3.8Q·cm,径向48.04Ω·cm,弦向70.70Ω·cm。导电材料弦向的电磁屏蔽效应在26.5-40GHz范围内最大值为26.8dB,径向为18.5dB,纵向为16.3dB。导电材料弦向的吸波损耗在拟合厚度为4.5mm,35-40GHz范围内为-33.5dB,径向在拟合厚度为5.5mm时,25-30GHz范围内为-52.5dB,纵向在拟合厚度为4.5mm,35-40GHz范围内为-38.5dB,导电材料的吸水性能降低43%,体积膨胀率由素材的22.12%降低到8%,体积干缩率由素材的18.14%下降至4.43%,静曲强度和抗弯弹性模量由素材的83.185MPa,7310.8MPa提高至168.921MPa,17563.8MPa。5.隔氧热还原法制备木材/石墨烯导电材料。用两种热法还原的方式对材料进行处理,一是在保留木材原有力学性能前提下低温隔氧热还原法制备的导电木材,评价了木材基质模板中GO还原度对材料三维导电性能的影响,并分析了 rGO还原度与材料电磁屏蔽性能、吸波性能及物理力学性能之间的关系。结果表明:GO前驱体浓度为3mg·mL-1,温度为210℃,时间为2h时,导电材料的弦向电阻率为2100Ω·cm,径向为6073Ω·cm,纵向为1903Ω·cm,导电材料弦向的电磁屏蔽效应在26.5-40GHz范围内最大值为13.8dB,径向为9.8dB,纵向为14.6dB。导电材料弦向的吸波损耗在拟合厚度为5.5mm,35-40GHz范围内为-31.1dB,径向为-22.8dB,纵向为-26.4dB,导电材料的吸水率降低18.6%,体积吸湿膨胀率降低78.70%,体积干缩率降低76.10%,导电材料的径向硬度降低26.25%,静曲强度提高26.56%,抗弯弹性模量提高20.75%。二是在在不考虑木材原有力学性能变化的前提下,将木材/GO复合材料在高温隔氧条件下进行处理,考察材料导电性能的变化,并对导电机理及高温条件下rGO与木材基质模板之间的关系进行了分析。结果表明,GO前驱体浓度在4mg·mL-1,炭化温度750℃、时间30min的条件下,材料在纵向、弦向、径向三个不同方向的体阻率分别为0.641Ω·cm,2.153Ω·cm,2.932Ω·cm,导电材料的三维各向异性差距明显缩小,电磁屏蔽性能提高至40dB,吸波性能提高至-12dB。更多还原

【Abstract】 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.更多还原