Construction and functionality of heat conversion wood composite materials based on transition metal compounds
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摘要: 热能在社会活动中扮演着不可或缺的角色并存在多种转化形式，过渡金属化合物(Transition metal compound，TMC)因其强关联电子体系和固有的电荷、自旋、轨道等自由度和有序相之间存在着竞争与共存关系，可以在光、电、磁和热能之间实现高效转化。然而，以粉末和晶体形式存在的TMC在使用过程中会出现易氧化聚合、体积变化、转化热能易消散及收集困难等问题，限制其热转化效率。木材具有天然的层级孔隙结构和稳定的力学支撑，借助木材中的化学组分可以与TMC形成共价键、离子键、氢键、范德华力等结合方式，促使TMC均匀负载至木材微纳表面或多孔结构中，形成TMC@木材复合材料。此外，木材具有优异的热管理能力，能够调节热能以提高热转化效率。本文基于木材的木质—纤维素大分子网络构造，详细讨论了TMC与实体木材、脱木质素木材、碳化木材的构筑方法和界面结合机制，进一步分析了基于TMC的非辐射衰变、弛豫损耗和金属-绝缘体转变的热转化机制，概述了TMC@木质复合材料在海水淡化、油水分离、建筑节能和火灾预警领域的功能应用。最后，分析了当前基于TMC构建热转化木材的优势和所面临的挑战，以期为木材的先进功能和能量转化提供一定的思路。Abstract: Thermal energy plays an indispensable role in social activities and can be transformed among various energy sources. Transition metal compound (TMC) enables the efficient conversion of light, electrical and magnetic energy into thermal energy due to the competition and coexistence between its strongly correlated electronic systems and inherent charge, spin, orbital and other degree of freedom and ordered phase. However, used in the form of powder and crystal, TMC owns the problems of oxidation polymerization, volume change, high heat dissipation and collection difficulties, which limits its heat conversion efficiency. Wood has natural hierarchical pore structure and could supply stable mechanical support for TMC. TMC can be uniformly loaded into wood micro-nano surfaces or porous structures by forming covalent bond, ionic bond, hydrogen bond, van der Waals forces with the chemical components of the wood to form TMC@wood composite materials. In addition, wood's excellent thermal management ability can regulate thermal energy to improve heat conversion efficiency. Based on the lign-cellulosic macromolecular network configuration of wood, we discussed the combination method and interfacial binding mechanism of TMC with raw wood, delignification wood and carbonized wood. We further discussed the thermal transformation mechanisms of non-radiative relaxation, relaxation loss and metal-insulator transition of TMC, the functional applications of TMC@wood composite materials in the fields of desalination, oil-water separation, building energy conservation and fire warning are presented. Finally, in view of providing ideas for advanced functionalization and energy conversion of wood, the current advantages and challenges of thermal conversion of TMC@wood are summarized.
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热能是人类文明发展史中极其重要的能量形式，更是当今工业发展的基础，占当前世界能源消耗的60%~70%，其利用过程中涉及热能的转化、传输、管理和消耗4种途径[1-2]。其中，热转化是指通过某种介质在一定条件下将光能、磁能和电能等形式的能量与热能进行单向或可逆转化。转化介质主要包括等离子体贵金属[3]、窄带隙半导体[4]、碳基纳米材料[5]等光热材料，掺杂Fe[6]、Co[7]、Ni[8]等元素的磁热材料及以负温度系数热敏电阻为代表的热电转化材料[9]。而过渡金属化合物(Transition metal compound，TMC)作为半导体的一类，具有不同光学、磁学、电学和热学特性被广泛应用的热转化介质。木材作为经济可持续的天然材料并广泛应用于人类的生产生活，其天然结构赋予木材优异的热性能，因此，将TMC引入木材，有望开发集TMC的性能优势及木材的结构和功能于一体的TMC@木材先进功能材料，丰富其功能和适用性。
1. 过渡金属化合物与木材
TMC是指从ⅢB到ⅡB族的过渡金属元素(其中包括3d、4d和5d过渡金属元素)与其他非金属元素(C、O、S、N、P等)化合形成晶体结构 [10]。由于3d过渡金属元素具有电子-电子、电子-晶格、电荷-自旋-轨道之间的强关联性，可通过关联电子系统中电荷、自旋、轨道等自由度和有序相之间存在的竞争与共存关系，产生包括半金属、半导体、超导性和铁磁性的电子相，从而在光能、电能、磁能与热能形式之间实现相互转化[11-13]。目前，基于TMC热转化已经实现了光热、磁热和热电转化(图1)。然而，以粉末和晶体形式存在的TMC在使用过程中会出现易氧化聚合、体积变化严重、转化的热能易消散及收集困难等问题，使热转化水平和进一步的实际应用受到影响[14]。因此，在热转化过程中，合理的材料和器件设计与构建是实现热能高效利用、赋予材料先进功能的关键所在[15]。其中，具有优异结构稳定性和热管理能力的基体材料，可以保护负载的TMC晶体，保持其固有的尺寸结构和分散性，增强热转化水平，并减少热能的耗散，进一步拓宽TMC的应用范围。
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图 1 过渡金属化合物(TMC)热转化的物理学原理、TMC对木材的构筑方式及基于热转化的应用领域Figure 1. Physics of thermal transformation of transition metal compounds (TMC), the way TMC are constructed to wood and the areas based on thermal transformationA/D—Analog digital; MO—Methyl orange; TCY—Tetracycline木材具有天然的层级孔隙结构、紧凑的细胞壁和10~40 μm细胞腔孔面积(图2(a)、图2(d))、稳定的力学支撑性能，能够满足负载TMC颗粒、防止其团聚及保持其结构稳定性的要求[16]。木材结构中，各向异性的垂直排列通道及较大的细胞壁内表面积(图2(e))为TMC提供了足够的收容空间[17]。TMC的官能团和金属离子可以与木材细胞壁的纤维素、半纤维素与木质素上存在的亲水基团(主要为羟基及少量的羰基、羧基等)通过离子键、共价键、分子间氢键或范德华力等方式结合(图2(b)、图2(f))[18]，从而有效负载到木材表面(结合深度毫米级)或通道内壁(细胞壁)上，形成稳定的TMC@木材复合材料。此外，当热流沿木材径向和纵向传输时，木材化学成分(主要是纤维素和半纤维素)会产生较大的多界面声子散射效应，提供较高的声子阻力，使木材具有优异的隔热性能(0.04~0.4 W/(m·K))[19-20]，有利于TMC@木材在使用过程中良好的热管理，并实现热能的理想调节提高热能利用效率。因此，将具有不同光学、磁学、电学和热学特性的TMC引入木材，有望开发集TMC的性能优势及木材的结构和功能于一体的TMC@木材先进功能材料，丰富其功能和适用性。
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本文基于木材天然结构(包括各向异性、多层级性、孔隙构造)、木质-纤维素大分子网络组织，结合TMC非辐射衰变、弛豫损耗和金属-绝缘体转变3种热转化的物理学原理，概述了TMC对热转化木材的构筑方式，并重点分析了TMC与木材结合的机制，进一步展望了TMC@木材复合材料在海水淡化、水油分离、建筑节能和火灾预警领域的应用实践和面临的挑战(图1)。
2. TMC热转化的物理学原理
2.1 光热转化
光热转化指在阳光照射下，TMC吸收光子能量产生光诱导电场驱动光激发电荷，最终将太阳能转化为热能[26]。因此光热转化材料应具有宽波长光响应，太阳光中紫外线(UV 10~400 nm)仅占全光谱的7%，因此扩展光吸收至可见光(VIS 400~700 nm)和近红外光区(NIR 700~2500 nm)能够提高光能利用效率[27]。TMC由于其强大的可见与近红外光吸收和等离子体特性，能够将光能以下列两种不同的电子转移机制转化为热能：
(1) 窄带隙引发的非辐射复合产热
过渡金属氧化物半导体材料能够吸收能量大于带隙的入射光子，产生光诱导电场驱动晶体内价带电子激发到导带，形成电子和空穴两种载流子，随后通过非辐射衰减释放出声子形式的额外能量将光能转化为热能(图3)[28]。带隙宽度对电子跃迁起到主要作用并同时反向控制半导体材料光热转换性能[29]。如TiO2(能带宽度Eg≈3.2 eV)[30]、ZnO(Eg≈3.2 eV)[31]和Fe2O3(Eg≈2.75 eV)[32]等宽带隙半导体仅能吸收短波长紫外光。而且，宽带隙会导致激发到带隙边缘附近的电子-空穴对浓度低，且容易复合为光子重新释放出来(图3(a))。这种辐射复合会显著降低TMC的光热转换效率，因此宽带隙半导体的太阳能利用效率低于20%[33-35]。通过缩小带隙，可以在扩大吸收光谱范围的同时提高激发载流子浓度，从而提高非辐射复合的概率(图3(b))，提高光热转换效率。可以通过引入杂质能级的方法减小带隙，Qi等[36]通过掺杂杂质碳在宽带隙In2O3(Eg=3.2 eV)中引入了杂质能级，从而显著减小了带隙，宽波长和强烈的光吸收促进了有效的光热转换。还可以通过引入本征缺陷减小带隙，Qi等[37]通过部分还原法将Ti用作界面还原金属在高温下与ZrO2中的氧反应形成氧空位，生成ZrO2-x之后再将Ti的产物TiOy分离，通过引入本征缺陷有效地改变ZrO2的电子结构，使带隙从5.47 eV减小到1.38 eV，并在体外用作光热治疗剂，以上方法可以提高光热转化效率至50%以上[29]。
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Figure 3. ((a)-(d)) Different photothermal conversion mechanisms[29,50-51]; ((e)-(g)) Magnetothermal conversion mechanisms of ferromagnetic materials with different particle sizes[44]; ((h)-(j)) Thermoelectric conversion mechanisms[9,52]CB—Conduction band; VB—Valence band; h—Planck constant; v—Frequency; M—Magnetization intensity; Ms—Specific magnetism; Mr—Residual magnetism; H—Magnetic field intensity; Hc—Coercive force; NPs—Nano particles; NWs—Nanowires(2) 自由载流子诱导的等离子体震荡效应
过渡金属硫族化合物纳米粒子在价带中可产生载流子(空穴)的集体振荡，从而导致局域表面等离子体震荡效应(Localized surface plasmon resonance，LSPR)，支持近红外区域的光响应。载流子的集体振荡称为表面等离子体激元，在光照射下获得足够的能量保持激发的“热”状态，随后表面等离子体激元会衰变产热，在NIR区进行光热转化(图3(c))[38-39]。TMC半导体相比于贵金属的导带电子集体振荡产生的LSPR，其电荷密度较低，需要通过产生本征缺陷或引入杂质调控空穴载流子浓度来调节LSPR响应频率。Hessel等[40]制备了具有Cu缺陷的高空穴载流子浓度的非化学计量Cu2-xSe半导体，空穴载流子浓度达到~1021 cm−3，表现出近红外吸收峰。
半金属特性二维材料，如过渡金属硫族化合物和碳/氮化物(MXene)，具有可调的带隙和高激子结合能，具备很强的光吸收系数和优异的集光性能。其自由电荷密度可以在~1021~1022 cm−3介于金属和半导体之间，能够为NIR区的LSPR提供足够的电荷浓度，高效地产生热载流子(图3(d))[41]。Lin等[42]证明了Ti3C2纳米片在c轴上的收缩可以提供很强的电荷约束，且价带和导带的重叠，使其具有足够的载流子浓度以诱导在750~850 nm近红外区的强吸收，表现出强烈的LSPR效应，有助于快速收集太阳能进行光热转换和储存。
提高光吸收能力除减小带隙外，还要求TMC具有较低的透射率和反射率，当TMC粒子直径小于光的波长(nm级)时，粒子间的光散射增强且光子光路增加，导致捕获和吸收入射光的能力提高，从而降低TMC粒子的透射率和反射率[29]。纳米TMC粒子极易团聚，影响光吸收和光热转化水平，因此可以借助木材分级孔隙结构、大比表面积和丰富的羟基结合位点，使纳米TMC粒子均匀加载到木材通道的内壁上，保持良好的纳米尺寸。
2.2 磁热转化
磁热转化是指铁磁性材料及其周围介质在外部交变磁场中通过磁能损耗使部分磁能转化为热能[43-44]。某些铁氧化物被广泛应用于交变磁场中的磁热材料，例如，铁氧化物[45]、钴铁氧体[46]、锰铁氧体[47]等。磁性颗粒的发热机制与粒径相关，Aono等[48]合成了微米Y3Fe5O12粉体，发现在微米级的多磁畴结构铁磁材料中，磁矩在磁畴内排列，通以强磁场，磁矩被磁场定向，达到饱和磁化。再施加一个具有相反方向精确强度的磁场，会产生铁磁体的磁滞回线，如图3(e)所示，磁滞回线的面积为磁滞损耗产生热量的能力。当材料的体积减小至纳米尺寸时，粒子会出现单畴结构，表现出超顺磁性(图3(f))，更容易在磁场中磁化，因此相比于多磁畴结构，在低频磁场中的磁致热有很好的应用前景[44]。Tong等[49]发现40 nm铁磁颗粒的加热效率最高，可以在最小剂量下有效加热肿瘤组织，对于6~11 nm的Fe2O3颗粒，磁矩旋转克服能垒导致的奈尔弛豫产热占主导地位，而15~40 nm的Fe2O3颗粒，由自身机械旋转与环境产生摩擦导致布朗弛豫的产热贡献显著(图3(g))。超顺磁性材料由于高效的磁热转换和连续的磁热效应，很难有效地管理磁热能。而木材具有的多孔结构、大比表面积和丰富的羟基位点及优异的隔热性能，可以很好地保持磁性颗粒的纳米尺寸防止团聚，同时实现高效磁热转换和热能管理，提高热能的利用效率[20]。TMC@木材复合材料通过利用木材的层次结构作为自组装磁性纳米颗粒的模板，可以生产形状和尺寸不受限制的复合材料，实现其在建筑节能材料领域的应用。
2.3 热电转化
热电转化指热量影响TMC载流子浓度或晶体结构的变化，从而影响能带结构或费米能级，引起金属与绝缘体间的相互转变(Metal-insulator transition，MIT)，进而可以将热能转变为电信号输出[53]。过渡金属氧化物半导体的电子迁移率和电子漂移速度都很高，具有显著的MIT的特性[54]，可作为热敏开关[55]、热敏电阻[56]应用于可燃材料的火灾预警。Zhang等[52]基于Fe3O4纳米线、Ag纳米粒子开发了一种热敏传感器。在火焰下热能增强载流子的动能，有助于克服晶界的高势能释放出被束缚的电荷载流子，从而提高导电性。火焰移除后，载流子的浓度和动能降低，电子不能跨越晶界实现定向流动，恢复绝缘特性(图3(h))，连接报警灯后可以提供火灾预警功能。
除了影响载流子浓度外，温度还能引起晶体结构变化，从而改变能带的相对位置，使绝缘体的满带和空带发生能量重叠，禁带消失，变成导体；或者相反，使重叠的能带分开，出现禁带，从导体变成绝缘体[57]。Zhang等[52]将Ti3O5通过逐层组装的方式制备了火灾报警智能涂层，在受热过程中的Ti3O5的原子结构、电子带和轨道结构从电荷有序的β相(电绝缘状态)转变为α相(导电状态)，呈现热诱导相变行为(图3(j))。其电阻随着温度从室温升高到MIT温度(460 K)而降低几个数量级(图3(i))。由于其MIT温度与一些可燃材料的着火温度(210~340℃)具有很好的对应，因此可满足建筑中木材、棉织物、聚乙烯和聚丙烯等材料的火灾预警涂层的应用。
3. TMC对不同形式木材的功能构筑方式
3.1 实体木材构筑
实体木材(Solid wood，SW)的主要构筑对象是细胞壁(纤维素、半纤维素和木质素)，由于木材细胞壁的大比表面积和丰富的羟基结合位点，TMC纳米粒子可以均匀分布于木材基体，保持纳米尺寸，有利于热转化过程，同时木材的原始孔隙也不会被破坏，有利于热管理。TMC颗粒可直接构筑在木材表面，Liu等[58]通过真空浸渍将CuFeSe2颗粒构筑在木材表面，颗粒与木材丰富的羟基之间形成Fe–O相互作用，使其牢固结合于木材横切面(图4(a))。还可以对前驱体溶液进行氧化还原后沉积在木材的表面，Li等[59]将泡桐浸泡于KMnO4溶液，并通过木材表面羟基的还原生成MnO2，Mn原子通过共价配位键或氢键沉积在木材横切面。TMC还可以构筑在木材纵切面用于智能涂层，Shen等[60]将松木浸泡于氟钛酸铵和硼酸的混合溶液中，待反应3天后真空干燥，木材与TiO2通过氢键相互作用在木材纵切面上获得原位沉积TiO2涂层(图4(b))；除表面构筑外，TMC还可以浸渍至木材内部，Gan等[61]将杨木浸入在FeSO4和CoCl2溶液中，通过水热生长在木材模板中转化为钴铁氧体颗粒(图4(c))。木材和CoFe2O4颗粒之间通过氢键结合，可以通过调整木材中作为成核位点的可结合羟基的数量控制颗粒的含量和分布。TMC对于木材表面沉积和浸渍至木材内部的主要区别在于与木质细胞的结合深度，表面沉积只是与表层木材的细胞壁结合(毫米级)，而浸渍则是与木材基体内部的细胞壁结合。
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3.2 脱木素木材构筑
天然木材通道的内壁光滑并含有填充物质(侵入物和树脂)，不利于TMC负载(图5(a)~5(b))[18]可通过预处理去除填充物及木质素和部分半纤维素，得到脱木素木材(Delignified wood，DW)。脱木素会使木材细胞分离，纤维素微纤维之间产生大量中孔，增加了细胞壁的孔隙率，而不会改变木材的微通道结构(图5(c)~5(d))[64]。细胞壁上暴露的活性基团使DW更有利于与TMC结合，为TMC提供更大的收容空间和羟基结合位点。TMC可直接负载于DW内部，Zheng等[65]通过真空浸渍和冷冻干燥制得Ti3C2Tx装饰木材气凝胶，通过范德华力和羟基之间丰富的氢键作用，在纤维素骨架周围形成粗糙、致密的Ti3C2Tx层(图5(e)~5(f))。为了提高复合材料的热能存储与利用，TMC颗粒可与有机物(固液相变材料)共同构筑于DW内腔空间及细胞壁的内部，Yang等[66]将Fe3O4纳米颗粒与熔解后的1-十四醇(TD)混合，通过真空浸渍将DW样品浸泡在分散良好的溶液中。DW和TD之间存在氢键和范德华力，而Fe3O4纳米颗粒位于管腔中(图5(g)~5(h))，聚合物浸润后在细胞腔内发生聚合反应并完全填充到木材的微孔中，纳米颗粒分布并固定在聚合物中，有机物能够充当“胶水”的作用，增强TMC粒子与木材之间的亲和力、均匀性和稳定性。木材去木素后优化了TMC@木材整体的隔热性，润湿性和多孔性，而且低弯曲度的通道和良好的压缩性，可以通过降低传递阻力提高木材的液体吸收能力，使其成为优异载体的理想材料。需要考虑的是由于脱木素木材需要通过化学处理，无形的增加了能源消耗和碳排放。
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3.3 碳化木材构筑
木材碳化(Carbonized wood，CW)处理也是保留木材原始自支撑分级多孔结构的一种功能化方式。高温使部分半纤维素降解，羟基等亲水结构减少，使木材颜色加深、强度增加，并且过程中产生的甲酸、乙酸、酚类化合物可以延缓腐化延长保存期限。TMC对CW的修饰与原始木材相似，无论是沉积在CW的表面或是浸渍至内部，均是与木质细胞壁上的羟基结合。Xi等[67]将银氨溶液涂刷于CW，再加入Na2HPO4溶液，在CW细胞壁上沉积Ag3PO4(图6(a)~6(c))。CW通道结构呈直线排列，具有各向异性的流体渗透性，在对齐的轴向上的刚度高(约10 GPa)[68]。CW相比于原始木材不仅可以改善木材的尺寸稳定性、耐久性、导电性、导热性和磁性外，更重要的是，碳基材料具有许多共轭π键，促进了几乎所有波长的太阳光激发的电子的π-π*跃迁，相比天然木材具有更强的光吸收能力[69]。因此，CW可以协同TMC的光热转化作用，将具有热转换特性的TMC构筑于CW表面，可以在整体上提升热转化性能。
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本节总结了TMC与不同处理形式木材的合成方法，并详细讨论了TMC与木材之间的结合方式。表1归纳总结了目前已经报道的TMC@木材的合成方法、操作、原理、优点和缺点。每种合成方法对木材的不同利用形式(SW、DW、CW)和构筑深度(表面或内部)均适用，也可以根据不同的反应条件单独或组合使用。
表 1 TMC@木材复合材料的制备方法Table 1. Methods of TMC@wood composite preparationComposite
methodOperational approach Reaction principle Advantage Disadvantage Ref. Painting Apply directly to the wood surface and dry Combination of hydroxyl groups on the wood surface by charge attraction or cross-linking Simplicity of operation Bond between TMC and wood is weak, usually by introducing other substances to act as a binder [62, 70-73] Soaking Wood is soaked in
TMC mother liquor
and then agedWool absorption
of
wood to load TMCSimplicity of operation and mild reaction conditions For raw wood TMC can only dip into the surface load, for delignified and carbonized wood it is possible to load deep inside [59, 67, 74] Vacuum/ultrasonic
impregnationWood is soaked in
TMC mother liquor, and the entire reaction occurs in a vacuumLow-pressure effect
allows TMC to grow evenly in woodMore uniform
growth
of TMCRelatively complex operation [58, 66, 75] Solvothermal
methodWood and TMC
mother liquor is placed in a stainless-steel high-pressure reactor with polytetrafluoroethylene
and then reacted at high temperaturesTMC grow evenly in wood under high
temperature and
pressureUniform and firm growth of the TMC required for the synthesis of the precursor solution in
the wood at high temperaturesComplex reaction
conditions[69, 76-77] 4. TMC热转化功能性木材的潜在应用
4.1 海水淡化
通过光热转换将太阳能转换成热能，实现太阳能界面蒸发是一种解决缺水问题的新兴技术[78]。木材蒸发器由超亲水隔热木材底层和光热转化疏水顶层构成。不同于传统的太阳能蒸发(加热大量水)，木基太阳能界面蒸发器可以将热量集中在空气/水界面而减少热量损失，通过木材的毛细吸收提供持续的水补充，显著加快蒸汽生成(图7(a)~7(b))[70, 76]。Song等[72]用Fe3+盐溶液/聚乙烯醇(PVA)涂刷于脱木素的椴木横截面上合成了Fe3O4/PVA/DW蒸发器。DW内部丰富的各向异性通道构成了一个低曲折度的多孔基质，能够保证充分的水传输。窄带隙(0.8 eV)的Fe3O4在整个波长范围内表现出高达97%的强光吸收，转化效率为73%，蒸发速率为1.3 kg·m−2·h−1。
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图 7 (a) 蒸发器工作原理；(b) 蒸发器的盐再溶解过程；(c) 天然木材(NW)、涂有高缺陷MoS2的木材(WM-H)合成和微观结构示意图[76]；(d) 碳化木-TiO2@TiN超支纳米线蒸发器示意图及TiO2转变为TiN的形貌变化[69]Figure 7. (a) Working principle of evaporator; (b) Salt redissolution process of evaporator; (c) Schematic diagram of synthesis and microstructure of natural wood (NW), wood coated with highly defective MoS2 (WM-H)[76]; (d) Schematic diagram of carbonized wood-TiO2@TiN hyperbranched nanowires evaporator and morphology change of TiO2 to TiN[69]由于脱木素木材需要通过化学处理，因此为了避免复杂的提取和制造过程而进一步尝试直接使用天然木材。He等[76]通过水热反应合成了缺陷MoS2颗粒并涂覆在巴沙木横切面上，再进一步冷冻干燥。通过调整MoS2中的S缺陷进而调节载流子浓度，实现NIR光区吸收。该装置的太阳能热转换效率为82.5%。水热和冷冻干燥过程使木材孔径由主要分布在~10~25 μm增加到~10~50 μm(图7(c))，蒸发通量可达1.46 kg·m−2·h−1。
为了在整体上提升太阳能蒸发器的热转化效率，Ren等[69]将TiO2在NH3中氮化转变为TiN沉积到碳化榉木表面，合成了具有不同取向的超支纳米线结构(图7(d))，可以有效地捕获太阳光谱中几乎所有波长的光。CW中的共轭π键，协同促进了光激发的电子的π-π*跃迁，相比原始木材具有更强的光吸收能力。蒸发器表现出97.42%的光吸收和94.01%的热转换效率，在1 kW·m−2模拟太阳辐照下水蒸发率为1.5252 kg·m−2·h−1。
值得注意的是，太阳能蒸发器中的盐分逐渐积累会阻塞水通路和光照区域，从而严重降低太阳能海水淡化的性能。因此，有效避免太阳能海水淡化过程中盐分积累的适当材料和结构设计对于保持稳定的太阳能蒸汽发电性能也非常重要。
4.2 油水分离
随着人类社会对原油及其产品越发依赖，海上大规模采油导致的原油泄漏问题日益严重，对海洋环境和国民经济造成了严重破坏。由于原油的流动性差，室温下黏度在103~105 mPa·s之间，严重限制了原油的吸附。引入热转换材料后能够通过原位升温的方式降低体系黏度，实现对污染物料的高效收集处理。太阳能的光热转换可作为一种可持续的经济和成本效益高的解决方案。近年来，具有超疏水/超亲油性多孔材料在处理油水混合物方面的高分离效率引起了广泛关注。Wang等[74]将脱木素的巴沙木浸入Ti3C2Tx溶液中并涂覆聚二甲基硅氧烷(PDMS)调整润湿性，得到一种多孔的具有出色的压缩性和疏水/亲油性的PDMS@木海绵/MXene(WSM)，木材海绵相比于其他海绵减少了海绵中典型的锯齿形孔隙对吸附石油的限制。由于MXene优良的吸光性和木海绵的弹簧状片层结构，在200~800 nm范围内具有显著的宽吸收带和低反射率带。MXene-Ti3C2T
x通过电力和太阳能的辅助原位产生热量，焦耳热和光热交替使用可实现全天候作业，如图8所示。模拟阳光照射1.5 kW·m−2，温度在3 min内上升到42℃，6 min内最大的吸附容量为11.2×105 g·m−3，在150 s内可实现25 mL原油的主动收集。 ![]()
相比于其他光热材料(贵金属、碳基纳米材料)，TMC半导体的成本低廉，可在水处理领域进行广泛应用。其他具有优异光热/磁热特性的TMC同样被应用于处理高黏度原油污水，例如Fe3O4[79]、CuO/CuS[80]、MoS2[4]、TiO2[81]等，负载于各种多孔海绵状基体中。使用传统海绵基材不可降解，可能会造成二次污染，而木基材料可以避免污染问题，且表现出良好的性能。值得注意的是，木材加工通常需要大量的化学品，并且只能在实验室规模上进行。因此，在进一步的研究中，应该考虑使用绿色化学品以避免污染环境，并注重扩展工业实践和商业化领域。
4.3 建筑节能
建筑材料为了降低能源消耗，高效的热能存储十分重要，以减少电力使用和气候化成本[82]。磁热材料与木材复合可以在磁场下升温，用于建材可提高建筑物的热能效率与储存。然而，高频磁场(300 kHz~1 MHz)对人体带来极大的危害。因此，研究磁性材料在低频磁场下的感应加热具有重要意义。Gan等[20]通过水热法和真空浸渍制备了一种木材/Fe3O4磁热材料，如图9(a)所示。在35 kHz的低频交变磁场中，温度显著上升(10 min内从25.9℃上升到70.1℃)。此外，磁性木材具有优异的隔热作用(图9(b))，减缓热能从高温区域向周围环境输送，有利于热能的合理利用。为了更好地将热能储存，具有光热-磁热特性的TMC纳米粒子可以与有机相变材料(PCM)复合，PCM在相变过程中可以以潜热的形式储存和释放热能。高潜热和储存过程的等温性质在热能储存方面具有巨大优势。Yang等[66]将1-十四醇和Fe3O4纳米颗粒组成的化合物浸渍到脱木素的巴沙木(DW)中，DW作为支撑材料有利于形成稳定的复合相变材料(图9(c))。黑色Fe3O4具有光热和磁热效应，与PCM复合能够提高热能储存能力。材料具有较大的潜热(179 J/g)、100次加热-冷却循环后的良好热可靠性和优异的形状稳定性。多功能木基复合相变材料有望实现磁热转换和光热转换与储存，用于节能建筑大幅降低能耗。
由于木材复杂的层级孔隙结构及TMC粒子易团聚的特性，难以控制TMC在大尺寸木材内部的均匀分散，并保留其良好的纳米尺寸，导致复合材料的热转换性能受到限制。目前对于建筑领域应用的TMC-木材复合材料都集中在小尺寸应用。因此未来的挑战包括需要更大规模的木材尺寸和结构，使其在实际应用中仍然保持优异的性能。
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4.4 火灾预警
木材的易燃性限制了其在燃料之外等其他领域的应用。由于木质材料频发火灾，因此需要及时的火灾预警，降低生命危险和财产损失。TMC半导体热敏电阻材料具有稳定性、灵敏性和可重复性等优点，本课题组通过层层自组装将Ti3O5与聚磷酸铵(APP)和聚乙烯亚胺(PEI)结合在榉木表面制备火灾预警阻燃智能涂层(图10(a))[9]。涂层基于Ti3O5的MIT特性引起的局部态之间的热电子跃迁导致电阻下降，在外接30 V电压下将热量转变为电信号通过互联网应用发送到各种外部端口进行实时火灾信号传输，系统具有较低的热响应温度(190℃)、灵敏的火灾响应时间(~3.78 s)和100次良好的响应重复性。为了进一步赋予木材涂层低压触发预警功能，Zhang等[83]又以聚多巴胺(PDA)作为粘结层、自组装APP阻燃层和纤维素纳米晶(CNC)插层MXene杂化物(C-MXene)制备预警涂层(图10(b))。Ti3C2Tx在火焰下转化为与锐钛矿型TiO2相的混合晶体。受热产生的载流子在TiO2@C导电网络中传输，促进电阻下降引发火灾预警。CNC/MXene涂层可在2.5 V低电压下显示出约2.1 s的敏感火灾响应。由于Ti3C2Tx边缘氧化产生少量表面TiO2具有光催化活性，智能木材还具有去除挥发性有机物(VOCs)的潜在光催化氧化作用，拓宽了其在工程和环境应用中的实践。由于外接电源会增加火灾报警系统的不稳定性，因此为了更进一步实现无线自供电系统，可基于具有热电效应的TMC将热能直接转换为电信号。Xie等[73]将具有热电性质的Ag2Se与AgNW/PVB混合均匀喷涂在樟子松表面，制备热电(TE)层。当涂层表面产生温差时，TE层的载流子可以沿着温度梯度迁移，直接产生电信号(图10(c))。涂层的火灾报警响应时间仅为2 s，重新燃烧时在2.8 s内触发火灾报警装置，表现出出色的防火安全性。Ti3C2Tx (MXene)同样具有热电特性，Zeng等[84]将蒙脱土(MMT)和功能化纤维素(UPC)逐层组装(图10(d))，涂层能够在4 s内触发火灾报警系统，并增加了自愈合能力与压敏传感等功能。
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Figure 10. (a) Preparation and fire warning mechanism of Ti3O5 smart coatings[9]; (b) Fire warning mechanism for MXene coatings[83]; ((c)-(d)) Thermoelectric response mechanism and fire warning[73]CNC—Cellulose nanocrystals; RT—Room temperature; MMT—Montmorillonoid; UPC—2-ureido-4[1H]-pyrimidinone-containing cellulose; FR—Fire retardant; TE—ThermoelectricityTMC基火灾预警涂层的发展为实现凝聚态物质作为实时火灾监测提供了一个创新概念，为互联网时代建筑的消防安全应用提供了新的视角。但对于TMC基的温度传感涂层的研究还有待扩大材料种类的选择、预警响应灵敏度的提高与深度的机制研究。
TMC以不同的制备工艺与不同树种结合可以表现出特殊性能，表2归纳总结了已报道的各种TMC与不同树种通过不同的方式结合，制备的TMC@木材功能复合材料的性能特征与应用领域，为未来TMC@木材复合材料进一步发展提供参考。
表 2 TMC@木材复合材料的组成、特点和应用领域Table 2. Components, characteristics, and applications of TMC@wood compositesWood TMC Combination method TMC@wood Characteristics Applications Basswood
(DW)Fe3O4 Fe3O4 cross-linking of wood surface hydroxyl groups and affinity enhancement by polyvinyl alcohol (cross-section) Fe-D-Wood [72] Evaporation rate 1.3 kg·m−2·h−1, 97% strong light absorption over the entire wavelength range, 73% photothermal conversion efficiency Desalination Balsa wood MoS2 with S defects —
(cross-section)WM-H [76] Evaporation rate of 1.46 kg·m−2·h−1
and heat conversion efficiency of 82.5%Desalination Beech wood (CW) TiO2@TiN —
(cross-section)TO@TNBNs-
CW [69]Evaporation rate 1.5252 kg·m−2·h−1, absorbs 97.42% of the sunlight, thermal conversion efficiency 94.01% Desalination Poplar wood FeNi —
(cross-section)W/FeNi/RGO [71] Evaporation rate 1.5 kg·m−2·h−1, thermal transfer efficiency 99.64% Desalination Paulownia wood MnO2 Mn interacts with O in the hydroxyl group of the wood through covalent coordination or hydrogen bonding (cross-section) K-wood [59] Evaporation rate 1.22 kg·m−2·h−1, efficiency 81.4%, sunlight absorption 94% Solar steam power, seawater desalination Basswood CuFeSe2 By forming Fe-O interactions with wood surface hydroxyl groups
(cross-section)Black wood [58] From 20°C to 51.5°C in 400 s irradiation time, solar thermal efficiency 86.2% Solar steam power, seawater desalination Paulownia wood Ti3C2−
OHTi3C2 surface hydroxyl modification and covalent bonding between the wood via isocyanate (cross-section) Ti3C2-wood [62] Evaporation rate of 1.465 kg·m−2·h−1 and solar energy conversion
efficiency of 96%Solar steam power, seawater desalination Poplar wood VO2 —
(cross-section)W/VO2-Ba [70] Evaporation rate 1.57 kg·m−2·h−1,
solar evaporation efficiency 93.45%Desalination Pinewood (CW) Black TiO2 —
(cross-section)BTW [77] Evaporation rate of 2.04 kg·m−2·h−1
and high solar steam efficiency of 90.06%Seawater desalination and degradation of organic pollutants Balsa wood (CW) Ag3PO4 —
(Impregnation to the interior)Ag3PO4@CW [67] photothermal conversion efficiency
of ~88.0% and a water generation
rate of 1.59 kg·m−2·h−1Desalination of sea water, removal of organic dyes, bacteria, heavy metal ions Balsa wood
(DW)Ti3C2Tx Hydrogen bonding via van der Waals forces and abundant hydroxyl groups (Impregnation to the interior) PDMS@WSM [74] 1.5 kW·m−2 simulated sunlight heats
to 66°C, maximum adsorption capacity of 11.2×105 g in 6 min,
25 mL of crude oil collected in 150 sCrude oil spills, energy conditioning and desalination of high brine Poplar wood (CW) Fe3O4 Formation of Fe-O bonds with
oxygen-containing functional groups of wood (Carbonized wood powder)Fe3O4-GNS/
CWF/
PCMC [75]Temperature rise above 65°C in
150 s, enthalpy of phase change greater than 95 J/g, phase change temperature about 55°C, thermal stability below 300°CEnergy harvesting, conversion and storage Balsa wood (DW) Fe3O4 Hydrogen bonding and van der Waals forces between wood and organic matter, Fe3O4 is located in the lumen of the tube (Impregnation to the interior) Fe3O4/TD/
DW [66]Large latent heat (179 J/g) and good thermal stability below 112°C Multifunctional thermal energy storage Poplar wood Fe3O4 Formation of Fe-O bonds with
oxygen-containing functional groups of wood (Impregnation to the interior)MW [20] From 25.9°C to 70.1°C in 10 minutes
at 35 kHz magnetic fieldArchitectural, decorative and massage furniture Pinus sylvestris Ag2Se Plasma treated substrates, introducing polar groups to provide good adhesion (longitudinal section) TE-FR [73] Fire alarm response time of only
2.0 s and excellent fire resistanceSelf-powered fire warning Fir wood/Beech wood Ti3O5 Interaction with PEI/APP by electrostatic attraction, covalent
cross-linking, hydrogen and ionic bonding (longitudinal section)PEI/APP/
Ti3O5 [52]Fire response time of approx. 3.78 s and significant fire, smoke and weather resistance Fire warning Beech wood Ti3C2Tx Stable bonding of the wood surface to MXene through an intermediate bonding bridge with hydrogen bonding of polydopamine, van der Waals forces and mechanical interlocking interactions
(longitudinal section)PA/C-MXene-Wood [83] Fire reaction time 2.1 s,
photocatalytic removal of VOCsFire warning, photocatalytic removal of VOCs Pinewood Ti3C2Tx Hydrogen bonding interactions (longitudinal section) MFNC [84] Fire alarm triggered within 4 s of combustion, coating with self-
healing and piezoresistive sensing capabilityFire warning, self-healing and pressure sensitive sensors Notes: PEI—Polyetherimide; PA—Polydopamine/ammonium polyphosphate; APP—Ammonium polyphosphate; TE-FR—Thermoelectric flame retardant; MFNC—Multifunctional fire protection nanocoating; TD—1-tetradecanol; GNS—Graphene nanosheets; CWF—Carbonized-wood-flour; PCMC—Phase-change-material composite; BTW—Black titanium dioxide loading on the surface of wood; RGO—Reduced graphene oxide; VOCs—Volatile organic compounds. 5. 结论与展望
随着人们对自然资源枯竭和环境恶化的担忧日益加剧，人们越来越关注可持续发展。森林资源可以提供日常生活所需要的材料，以取代不可持续和能源密集型的人造材料。这种环保、可再生、可持续的木基原材料引起了人们极大的兴趣，并且在设计木基层次结构方面取得了重大进展。在对过渡金属化合物(TMC)@木材复合材料的热转换等性能研究进行回顾以后，得到以下几点结论：
(1) 木材先进功能化仍处于发展阶段，包括分子和纳米尺度，甚至向更小的尺度发展，在前期研究的基础上需要在过程控制和简化方面做进一步的努力，才能达到工程材料应用的水平，工业技术仍是最终的挑战；
(2) TMC是基于光、热、磁、电转化性质丰富的物质，在与木材复合的热转换方面崭露头角，仍有必要开发具有适当性能范围的新材料，以有效利用现有的热资源；
(3) 可以设想同时采用多种能源转换机制的办法，实现互相协同的效应。今后应致力于将TMC优异的理化性质与结构复杂的实体木材结合起来，保留和功能化细胞壁纳米结构，有助于结构和承载性能，使新的功能可以添加到木材的原始特性中；
(4) 更加注重TMC热转换机制研究的同时联系起与木材特性之间的内在联系，如木材的一些理化性质包括各向异性、组成成分、三维结构等，可以结合仿生手段，极大发挥两者的协同作用，提高木材的增值利用空间，才能够极大地推进木材科学领域的发展。
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图 1 过渡金属化合物(TMC)热转化的物理学原理、TMC对木材的构筑方式及基于热转化的应用领域
Figure 1. Physics of thermal transformation of transition metal compounds (TMC), the way TMC are constructed to wood and the areas based on thermal transformation
A/D—Analog digital; MO—Methyl orange; TCY—Tetracycline
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图 3 ((a)~(d)) 不同光热转换机制[29,50-51]；((e)~(g)) 不同粒径铁磁性材料磁热转化机制[44]；((h)~(j)) 温度-电信号转化机制[9,52]
Figure 3. ((a)-(d)) Different photothermal conversion mechanisms[29,50-51]; ((e)-(g)) Magnetothermal conversion mechanisms of ferromagnetic materials with different particle sizes[44]; ((h)-(j)) Thermoelectric conversion mechanisms[9,52]
CB—Conduction band; VB—Valence band; h—Planck constant; v—Frequency; M—Magnetization intensity; Ms—Specific magnetism; Mr—Residual magnetism; H—Magnetic field intensity; Hc—Coercive force; NPs—Nano particles; NWs—Nanowires
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图 7 (a) 蒸发器工作原理；(b) 蒸发器的盐再溶解过程；(c) 天然木材(NW)、涂有高缺陷MoS2的木材(WM-H)合成和微观结构示意图[76]；(d) 碳化木-TiO2@TiN超支纳米线蒸发器示意图及TiO2转变为TiN的形貌变化[69]
Figure 7. (a) Working principle of evaporator; (b) Salt redissolution process of evaporator; (c) Schematic diagram of synthesis and microstructure of natural wood (NW), wood coated with highly defective MoS2 (WM-H)[76]; (d) Schematic diagram of carbonized wood-TiO2@TiN hyperbranched nanowires evaporator and morphology change of TiO2 to TiN[69]
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图 10 (a) Ti3O5智能涂层的制备和火灾预警机制[9]；(b) MXene涂层的火灾预警机制[83]；((c)~(d)) 热电响应机制与火灾预警[73]
Figure 10. (a) Preparation and fire warning mechanism of Ti3O5 smart coatings[9]; (b) Fire warning mechanism for MXene coatings[83]; ((c)-(d)) Thermoelectric response mechanism and fire warning[73]
CNC—Cellulose nanocrystals; RT—Room temperature; MMT—Montmorillonoid; UPC—2-ureido-4[1H]-pyrimidinone-containing cellulose; FR—Fire retardant; TE—Thermoelectricity
表 1 TMC@木材复合材料的制备方法
Table 1 Methods of TMC@wood composite preparation
Composite
methodOperational approach Reaction principle Advantage Disadvantage Ref. Painting Apply directly to the wood surface and dry Combination of hydroxyl groups on the wood surface by charge attraction or cross-linking Simplicity of operation Bond between TMC and wood is weak, usually by introducing other substances to act as a binder [62, 70-73] Soaking Wood is soaked in
TMC mother liquor
and then agedWool absorption
of
wood to load TMCSimplicity of operation and mild reaction conditions For raw wood TMC can only dip into the surface load, for delignified and carbonized wood it is possible to load deep inside [59, 67, 74] Vacuum/ultrasonic
impregnationWood is soaked in
TMC mother liquor, and the entire reaction occurs in a vacuumLow-pressure effect
allows TMC to grow evenly in woodMore uniform
growth
of TMCRelatively complex operation [58, 66, 75] Solvothermal
methodWood and TMC
mother liquor is placed in a stainless-steel high-pressure reactor with polytetrafluoroethylene
and then reacted at high temperaturesTMC grow evenly in wood under high
temperature and
pressureUniform and firm growth of the TMC required for the synthesis of the precursor solution in
the wood at high temperaturesComplex reaction
conditions[69, 76-77] 
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表 2 TMC@木材复合材料的组成、特点和应用领域
Table 2 Components, characteristics, and applications of TMC@wood composites
Wood TMC Combination method TMC@wood Characteristics Applications Basswood
(DW)Fe3O4 Fe3O4 cross-linking of wood surface hydroxyl groups and affinity enhancement by polyvinyl alcohol (cross-section) Fe-D-Wood [72] Evaporation rate 1.3 kg·m−2·h−1, 97% strong light absorption over the entire wavelength range, 73% photothermal conversion efficiency Desalination Balsa wood MoS2 with S defects —
(cross-section)WM-H [76] Evaporation rate of 1.46 kg·m−2·h−1
and heat conversion efficiency of 82.5%Desalination Beech wood (CW) TiO2@TiN —
(cross-section)TO@TNBNs-
CW [69]Evaporation rate 1.5252 kg·m−2·h−1, absorbs 97.42% of the sunlight, thermal conversion efficiency 94.01% Desalination Poplar wood FeNi —
(cross-section)W/FeNi/RGO [71] Evaporation rate 1.5 kg·m−2·h−1, thermal transfer efficiency 99.64% Desalination Paulownia wood MnO2 Mn interacts with O in the hydroxyl group of the wood through covalent coordination or hydrogen bonding (cross-section) K-wood [59] Evaporation rate 1.22 kg·m−2·h−1, efficiency 81.4%, sunlight absorption 94% Solar steam power, seawater desalination Basswood CuFeSe2 By forming Fe-O interactions with wood surface hydroxyl groups
(cross-section)Black wood [58] From 20°C to 51.5°C in 400 s irradiation time, solar thermal efficiency 86.2% Solar steam power, seawater desalination Paulownia wood Ti3C2−
OHTi3C2 surface hydroxyl modification and covalent bonding between the wood via isocyanate (cross-section) Ti3C2-wood [62] Evaporation rate of 1.465 kg·m−2·h−1 and solar energy conversion
efficiency of 96%Solar steam power, seawater desalination Poplar wood VO2 —
(cross-section)W/VO2-Ba [70] Evaporation rate 1.57 kg·m−2·h−1,
solar evaporation efficiency 93.45%Desalination Pinewood (CW) Black TiO2 —
(cross-section)BTW [77] Evaporation rate of 2.04 kg·m−2·h−1
and high solar steam efficiency of 90.06%Seawater desalination and degradation of organic pollutants Balsa wood (CW) Ag3PO4 —
(Impregnation to the interior)Ag3PO4@CW [67] photothermal conversion efficiency
of ~88.0% and a water generation
rate of 1.59 kg·m−2·h−1Desalination of sea water, removal of organic dyes, bacteria, heavy metal ions Balsa wood
(DW)Ti3C2Tx Hydrogen bonding via van der Waals forces and abundant hydroxyl groups (Impregnation to the interior) PDMS@WSM [74] 1.5 kW·m−2 simulated sunlight heats
to 66°C, maximum adsorption capacity of 11.2×105 g in 6 min,
25 mL of crude oil collected in 150 sCrude oil spills, energy conditioning and desalination of high brine Poplar wood (CW) Fe3O4 Formation of Fe-O bonds with
oxygen-containing functional groups of wood (Carbonized wood powder)Fe3O4-GNS/
CWF/
PCMC [75]Temperature rise above 65°C in
150 s, enthalpy of phase change greater than 95 J/g, phase change temperature about 55°C, thermal stability below 300°CEnergy harvesting, conversion and storage Balsa wood (DW) Fe3O4 Hydrogen bonding and van der Waals forces between wood and organic matter, Fe3O4 is located in the lumen of the tube (Impregnation to the interior) Fe3O4/TD/
DW [66]Large latent heat (179 J/g) and good thermal stability below 112°C Multifunctional thermal energy storage Poplar wood Fe3O4 Formation of Fe-O bonds with
oxygen-containing functional groups of wood (Impregnation to the interior)MW [20] From 25.9°C to 70.1°C in 10 minutes
at 35 kHz magnetic fieldArchitectural, decorative and massage furniture Pinus sylvestris Ag2Se Plasma treated substrates, introducing polar groups to provide good adhesion (longitudinal section) TE-FR [73] Fire alarm response time of only
2.0 s and excellent fire resistanceSelf-powered fire warning Fir wood/Beech wood Ti3O5 Interaction with PEI/APP by electrostatic attraction, covalent
cross-linking, hydrogen and ionic bonding (longitudinal section)PEI/APP/
Ti3O5 [52]Fire response time of approx. 3.78 s and significant fire, smoke and weather resistance Fire warning Beech wood Ti3C2Tx Stable bonding of the wood surface to MXene through an intermediate bonding bridge with hydrogen bonding of polydopamine, van der Waals forces and mechanical interlocking interactions
(longitudinal section)PA/C-MXene-Wood [83] Fire reaction time 2.1 s,
photocatalytic removal of VOCsFire warning, photocatalytic removal of VOCs Pinewood Ti3C2Tx Hydrogen bonding interactions (longitudinal section) MFNC [84] Fire alarm triggered within 4 s of combustion, coating with self-
healing and piezoresistive sensing capabilityFire warning, self-healing and pressure sensitive sensors Notes: PEI—Polyetherimide; PA—Polydopamine/ammonium polyphosphate; APP—Ammonium polyphosphate; TE-FR—Thermoelectric flame retardant; MFNC—Multifunctional fire protection nanocoating; TD—1-tetradecanol; GNS—Graphene nanosheets; CWF—Carbonized-wood-flour; PCMC—Phase-change-material composite; BTW—Black titanium dioxide loading on the surface of wood; RGO—Reduced graphene oxide; VOCs—Volatile organic compounds.
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目的
热能在社会活动中扮演着不可或缺的角色并存在多种转化形式。过渡金属化合物(Transition metal compound，TMC)因其强关联电子体系，具有高效的热转化能力。为了避免以粉末和晶体形式存在的TMC在使用过程中出现氧化聚合、体积变化、转化热能消散以及收集困难等问题，可利用木材的自支撑多孔结构有效固定粉末状TMC以达到进一步提高能量之间转化效率的目的。此外，木材具有优异的热管理能力，能够调节热能以提高热转化效率，在热能利用过程中减少热损失。TMC@木材功能复合材料在跨学科研究的方向上显示出巨大的潜力。
方法木材结构中，各向异性的垂直排列通道以及较大的细胞壁内表面积为TMC提供了足够的收容空间。TMC的官能团和金属离子可以与木材细胞壁的纤维素、半纤维素与木质素上存在的亲水基团(主要为羟基，以及少量的羰基、羧基等)通过离子键、共价键、分子间氢键或范德华力等方式结合，从而有效负载到木材表面或通道内壁上，形成稳定的TMC@木材复合材料。将具有不同光热转化、磁热转化和热电转化特性的TMC引入木材足以丰富其功能和适用性，有望开发集TMC的性能优势以及木材的结构和功能于一体的TMC@木材功能复合材料。
结果TMC可以构筑于不同处理形式的木材中，主要包括原始木材、脱木素木材和碳化木材。TMC@木材热转化复合材料具有以下几个应用领域：① 海水淡化：基于TMC半导体的窄带隙引发的非辐射复合产热或自由载流子诱导的LSPR产热的物理学原理，将其结合在木材横切面上，利用木材内部丰富的各向异性通道的毛细吸收作用提供充分的水传输，在复合材料界面处进行高效的太阳能水蒸发。② 油水分离：将TMC构筑于木材海绵基体中并进行疏水处理，基于TMC半导体的光热转化效应减小原油粘度，从而实现对含油污水的处理。③建筑节能：基于铁磁性材料(铁氧体)的弛豫损耗产热物理学原理，满足其在低频交变磁场中的磁热转化，并与木材基体复合，实现高效的热能存储，以减少电力使用和气候化成本。④ 火灾预警：基于TMC半导体的金属绝缘体间的转变(Metal-Insulator Transition，MIT)特性，并将其沉积于木材表面制备智能涂层，将温度信号转变为电信号输出，实现实时火灾监测。
结论木材先进功能化仍处于发展阶段，需要在过程控制和简化方面做进一步的努力，才能达到工程材料应用的水平。TMC是基于光、热、磁、电转化性质丰富的物质，在与木材复合的热转换方面崭露头角，仍有必要开发具有适当性能范围的新材料，以有效利用现有的热资源。更加注重TMC热转换机理研究的同时联系起与木材特性之间的内在联系，如木材的一些理化性质包括各项异性，组成成分、三维结构等，可以结合仿生手段，极大发挥两者的协同作用，提高木材的增值利用空间，才能够极大地推进木材科学领域的发展。可以设想同时采用多种能源转换机制的办法，实现互相协同的效应。今后应致力于将TMC优异的理化性质与结构复杂的实体木材结合起来，保留和功能化细胞壁纳米结构，有助于结构和承载性能，使新的功能可以添加到木材的原始特性中。
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