利用TD-NMR技术研究杨木高温干燥过程水分分布

Water Distribution in Poplar during High-Temperature Drying Process Studied by Time-Domain Nuclear Magnetic Resonance

木材中水分状态变化和迁移对木材的物理性质有重要影响.通过时域核磁共振技术(TD-NMR)可以从分子层面解读木材与水分的关系,可以为木材干燥、木制品加工提供理论依据和实践参考.该研究以北京杨为研究对象,通过对高温干燥过程中木材内部水分变化的自由感应衰减(FID)曲线和横向弛豫时间(T_2)进行测定与分析,探究木材干燥过程中水分状态变化及迁移过程.研究结果表明,FID和T_2信号量与木材含水率高度线性相关,由此可以计算木材在干燥过程中任意时刻的含水率.通过对干燥过程中水分T_2分布的分析表明:心材试件在干燥过程中,长弛豫时间自由水(c状态水分)的拟合面积出现了先减小后增大然后再减小的趋势,而边材试件中则不存在这种现象.在北京杨心材试件中含量最多的是弛豫时间为10 ms数量级的水分,而在边材试件中各状态水分含量差异较小,含量最多的是弛豫时间为100 ms数量级的水分.在高温干燥过程中,边材试件内各状态水分百分含量减少的速度快于心材,各试件中自由水的蒸发速度明显快于结合水.

Moisture in wood affects its physical properties. Time-domain nuclear magneticresonance （TD-NMR） technique can be used to monitor water state changes and migration,providing a powerful tool to study changes of water in wood during drying and productprocessing. In this study, water states and migration in Beijing poplar were monitored byTD-NMR during high-temperature drying. Free induction decay （FID） and transverserelaxation time （T2） of water were measured. The results showed that both FID and T2signals were linearized with moisture content, hence the moisture content in the wholddrying process can be calculated. Free water with long relaxation time in heart woodshowed a trend of first decrease and then increase and decrease again at last, thisphenomenon did not happen in the sap wood. In the heart wood, the most abundantmoisture is the water with magnitude of 10 ms relaxation time. But the moisture content ofeach water state has small difference in sap wood, and the most abundant moisture is thewater with magnitude of 100 ms relaxation time. During high temperature drying process,the moisture percentage decrease faster in sap wood than that in heart wood, and theevaporation rate of free water is significantly faster than the bound water.