关键词: 木材细胞壁, 导热特性, 扫描热显微镜, 共聚焦显微拉曼光谱

Abstract: [Objective] Scanning thermal microscopy (SThM) has been proved to be an effective method for studying thermal conductivity and structure of materials at microscale. In this study, SThM was applied to scan the cross section and radial section of oak fiber cell wall, and the result were used to study thermal conductivity of wood cell wall at microscale.[Method] In order to make the surface of samples smooth enough for SThM and confocal Raman microscopy scanning, diamond knife was used to do microtomy. Thermal conductivity contrast mode of SThM was applied to scan oak fibre cell both in cross and raidal section.[Result] The scan result of SThM show that when scanned cross section of wood cell wall, the probe current of SThM in compound middle lamella (CML) and cell corner (CC) was lower than in S₂ layer apparently and it meant the thermal conductivity of CML and CC were lower than S₂ layer. When scanned the radial section of cell wall, the difference of probe current between CML, CC and S₂ was almost invisible, thus the thermal conductivity of CC, CML and S₂ were almost the same. The components of different layers of wood cell wall were analyzed by confocal Raman microscopy. Raman spectra indicated the intensity of peaks that assigned to lignin were higher in CML and CC than in S₂ layer, in S₂ the intensity of Raman peaks assigned to cellulose was relatively higher. This study used wave number from 1 520 cm^-1 to 1 680 cm^-1 that assigned to lignin to image cell wall in cross section. Raman image shows that the intensity of 1 520-1 680 cm^-1 in CC and CML was higher than S₂ layer, so the content of lignin in CC and CML was higher than S₂.[Conclusion] SThM investigation indicate the difference of thermal conductivity between CC, CML and S₂ layer was existing in cross section and was disappeared in radial section. This study believes the difference was caused by how components arranged in different layers mainly. Cellulose was the major component in S₂ layer, and the key point was that cellulose arranged with grain oriented structure. In S₂ layers cellulose microfibril was almost parallel to axial direction of wood cell, so when heated cell wall by SThM probe in cross section the heat would transferred along the grain direction, but when heated cell wall in radial section the heat would transferred perpendicular to grain direction. In CC and CML, the lignin content was higher and components in these two areas were arranged disorder, thus in cross section compared with S₂ layer where has grain oriented structure, CC and CML showed lower thermal conductivity, but when heated in radial section the advantage that heat transferred along grain in S₂ layer disappeared and S₂, CML and CC showed almost the same thermal conductivity. By mean of SThM, this work confirmed grain oriented structure of cellulose affected thermal conductivity of wood cell wall and demonstrated that SThM could be used to investigated thermal conductivity and structure of wood very efficiently.

Key words: wood cell wall, thermal conductivity, scanning thermal microscopy, confocal Raman microscopy