木材微波精准爆破的水热迁移与破膜机理研究

Permeability is one of the key factors that restrict the development of wood modification. Microwave(MW) pretreatment can be applied to improve wood permeability by breaking up pit membrane openings, and to realize its high value-added application. As the uneven distribution of water within the wood and the different degree of microwave attenuation, microwave pretreatment is difficult to achieve accurate rupture of membranes, and even cause irreparable damage on the cell wall of wood. This project intended to study the interaction between non-uniform distribution of moisture inside wood and microwave radiation, explore how wood physical properties such as thermal coefficient, dielectric constant and dissipation factor respond to moisture heterogeneity, build up the water-heat distribution and migration model, reveal the change regularities on temperature and pressure in the process of microwave pretreatment, clarify the microwave precision blasting mechanism of wood with nonuniform distribution of moisture. Combine with the fiber micro-tensile and environmental scanning on line technology, digital image cross-correlation method and inverse extraction method, the damage process of the pit membrane was analyzed, the cohesive zone model of pit membrane was established, the critical conditions for the pit membrane rupture under different hygrothermal loads were also analyzed, the fracture mechanism of pit membrane of wood by microwave precision blasting was finally revealed, Which provides the theoretical basis for energy-saving drying, efficiency modification, depolymerization and reconstruction of the fast-growing plantation wood.

渗透性是制约木材功能性改良的关键因素之一，利用微波预处理可打通木材纹孔膜以改善其渗透性，实现速生人工林木材提质增效利用。由于木材内部水分分布不均、微波辐射衰减不一，微波预处理木材难以实现精准化破膜，甚至对木材细胞壁产生不可修复的损伤。本项目拟研究木材水分非均匀分布与微波辐射交互影响规律，探明木材导热系数、介电常数和耗散因子对水分非匀质性的响应关系，构建微波预处理水热分布与迁移模型，揭示微波预处理过程中木材内部温压场变化规律，阐明水分非均匀分布的木材微波精准爆破机理。结合单纤维拉伸在线环境扫描技术、数字图像相关法与场投影反解法反馈纹孔膜损伤破坏演变过程，构建纹孔膜内聚力损伤模型，解析不同湿热载荷下纹孔膜破裂临界条件，揭示木材微波精准爆破的纹孔膜破裂疏通机制，为速生人工林木材节能干燥、高效改性、解聚重构等绿色高值化利用提供重要理论支撑。

本项目旨在探明木材在微波处理过程中的水热迁移机理及木材细胞在水热载荷作用下的损伤机制，以人工林杉木、杨木、樟木及柚木为研究对象，系统研究了木材非匀质物性参数定量表征、木材非匀质效应下水热迁移机理及湿热载荷作用下木材细胞壁层损伤机理，得出以下主要结论：（1）获得杉木与杨木微观含水率定量表征方程，拟合优度分别为0.892和0.903；（2）利用Matlab软件结合物理反问题遗传算法编写数值计算程序，解译了木材水分、温度与微波吸收功率间的交互影响规律，获得水热交互作用下杨木微波吸收功率的定量表征方程；（3）构建了微波预处理木材传热的匀质与非匀质模型，结合Fortran语言编写数值模拟仿真程序。结果表明在杨木平均含水率49-134%范围内，非匀质模型预测精度较匀质模型提升了81-97%，可以定量分析非匀质效应对微波处理木材传热的影响规律；（4）构建了可移动蒸发界面的木材水分迁移模型，利用界面蒸发率与体积蒸发率方程表征移动的水分蒸发界面，量化分析了木材在高温处理过程中温度、含水率、界面蒸发率、体积蒸发率、水蒸气密度及相对湿度的时空演变规律，揭示了木材高温处理过程中水分迁移机理；（5）构建了木材细胞单壁层三素超微构型与木材细胞多壁层力学映射模型，结合Fortran语言编写数值模拟仿真程序，定量表征了木材细胞各壁层力学性能及其在复杂载荷作用下损伤机制，并利用单纤维拉伸及纳米压痕实验验证了模型的准确性；（6）微波预处理破坏杨木细胞纹孔膜的最优工艺为：控制板内温度在155-165℃区间，过高温度会损伤杨木射线薄壁细胞，导致木材整体强度降低；（7）基于复合材料理论，量化分析了不同薄壁细胞力学性能和体积含量下，木材内水分在高温驱动迁移作用下，内部湿应力的时空演变规律。