摘要:
木材是一种多尺度且具有高度各向异性的天然高分子材料,其由纤维素、半纤维素、木质素等组分组成复杂三维网络结构。这些结构是影响木材微宏观物理力学性能的关键因素,木材的微观结构和主要特征参数对认识木材性质以及木材改性和加工利用具有重要意义。本文综合分析了木材微观结构分析的方法,并归纳总结了其主要特征参数。在分析方法方面,显微观察、X射线照相及衍射分析、计算机断层扫描等技术被广泛应用,结合数据采集和二维、三维成像方法,可以重建木材微观结构模型,测量微观结构特征参数,重建的结构模型可以揭示出更为清晰的微观形貌特征。木材微观特征参数主要包含木材的细胞结构特征参数、孔隙率、纤维素微纤丝等参数。目前微观结构的观测研究多局限在微米尺度,对于纳米尺度和更小分子尺度结构的分析主要以理论模拟为主,可以进行直接观察表征的结构分析方法有待进一步研发。
Abstract:
Wood is a multi-scale and highly anisotropic natural polymer material, which consists of cellulose, hemicellulose, lignin and other components of a complex three-dimensional network structure. These structures are the key factors affecting micro-macroscopic physical and mechanical properties of wood, and the microstructure and main characteristic parameters of wood are of great significance to the understanding of properties of wood as well as wood modification and processing and utilization. This paper comprehensively analyzes the methods of wood microstructure analysis and summarizes its main characteristic parameters. In terms of analytical methods, microscopic observation, X-ray radiography and diffraction analysis, computed tomography and other technologies were widely used, combined with data acquisition and two-dimensional and three-dimensional imaging methods, can reconstruct wood microstructural models, measure microstructural parameters, and reconstructed structural models revealed clearer micro-morphological features. The microstructural parameters of wood mainly include cellular structure parameters, porosity, cellulose microfilaments and other parameters of wood. At present, the observation and study of microstructure is mostly limited to the micron scale, and the analysis of nanoscale and smaller molecular scale structure is mainly based on theoretical simulation, and the structural analysis methods that can be directly observed and characterized need to be further studied in depth.
图 1 木材多层级结构示意图(a)木材; (b)针、阔叶材结构示意图; (c)木材细胞; (d)典型木材细胞壁的壁层构造示意图[1]
Figure 1. Schematic diagram of multi-layer structure of wood (a) Schematic diagram of wood; (b) Schematic diagram of needle and broadleaved wood structure; (c) Schematic diagram of wood cells; (d) Schematic diagram of wall layer structure of typical wood cell walls[1]
图 2 木材组织的虚拟切片、生成体积渲染解剖图[43]
Figure 2. Virtual slices of wood tissue, generating volume-rendered anatomical images[43]
表 1 二维平面结构特征分析方法
Table 1 Analysis method for two-dimensional planar structural features
显微观察设备 测试参数类型 测试样品树种 样品制备方法 测试样品尺寸 分辨率/表 2 木材纤维素晶体结构及微纤丝角参数分析方法
Table 2 Analysis method for crystal structure and microfibril angle parameters of wood cellulose
设备 测量参数类型 测试样品树种 样品制备方法 测试样品尺寸 引用 X射线衍射仪 纤维素结晶结构,包括纤维素结晶尺寸、纤维素微纤丝中纤维素分子的排列以及结晶纤维素的比例 将木结构建筑中花拱的样本磨成木粉进行测试 粉末 [11] 纤维素晶体的晶格间距 杉木(Cryptomeria japonica)、桧木(Chamaecyparis obtusa) 将木材制成横截面和四等分锯切的样本,在密封的干燥室中用蒸馏水调节约一个月至达到纤维饱和点,将样本分成3组进行不同的干湿处理周期 5 mm × 15 mm × 15 mm表 3 三维空间结构特征分析方法
Table 3 Analysis method for three-dimensional spatial structural features
设备 测试参数类型 测试样品树种 样品制备方法 测试样品大小 最佳分表 4 数据收集分析及成像方法
Table 4 Data collection analysis and imaging methods
测试方法 数据收集、分析及成像方法 测量参数 引用 同步加速器显微断层成像 重建垂直于旋转轴的单切片图像;使用滤波后投影,并将所有负值设为零 管胞大小/形状、管壁厚度、孔隙率、纹孔 [29] 显微计算机断层扫描 开发了μCTanalysis 软件,利用灰度图像阈值对二维图像进行更快、更简单的图像分割 导管内径、导管横截面积、导管密度和孔隙率 [37] 使用 datos|reconstruction® 软件将投影图像叠加转换成容积模型,使用 Avizo 的区域生长工具对感兴趣的区域进行分割 真菌引起的早期木质部细胞变形和晚期木质部细胞厚壁中的孔洞 [32] 利用各种形态学处理方法获得血管孔隙的二值图像,并提取其特征进行分类 导管孔隙的数量、圆度、面积、周长和其他特征参数 [36] 使用 Drishti 对NaI浸渍前后的木塑样品断层图中的相位进行可视化,并根据体素强度创建了二维图像和三维动画 木质颗粒的体积、表面积和长宽比 [25] 使用 Dragonfly 软件进行可视化和定量分析,从图像中分割出细胞裂缝和孔腔 杉木和杨木纤维素提纯过程中三维结构的变化,细胞壁厚度和腔体体积比 [34] LBP变形形成均匀LBP、旋转不变LBP和旋转不变均匀LBP,并与 GLCM 特征融合,形成3种融合特征 木材三切面显微图像 [41] 同步辐射相衬X射线断层显微镜 使用灰度阈值法将断层扫描图像分割成二进制的木材和空气体素数据集 测定木材膨胀/收缩应变 [23] 相位对比 X 射线成像 利用分析或迭代算法计算不同 X 射线吸收路径的衰减系数,以重建图像 吸收对比度不够的样品内部结构 [40] 配备LCD相机的显表 5 木材细胞壁厚度、细胞直径、导管/管胞参数、孔隙率
Table 5 Wood cell wall thickness, cell diameter, vessel/tracheid parameters and porosity
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