利用时域核磁共振技术研究木材孔隙分布及水分运动
发布时间：2019-03-05 20:47
【摘要】：本研究基于时域核磁共振技术理论,以木材中水分1H质子为探针,应用定场时域核磁共振波谱仪和快速场循环核磁共振弛豫仪对三种阔叶材和两种针叶材进行核磁共振弛豫性质的测定。研究了饱和吸水状态下的木材孔隙分布,不同温度下的多尺度孔隙内的水分运动,以及多种湿度下的木材干燥/吸湿过程的水分迁移和孔径动态变化。1.利用定场时域核磁共振仪测定了 5种木材的孔隙分布,结果表明:(1)5种木材的孔隙度大约在55%～70%之间,其中细胞壁孔隙约为4%～12%,而细胞腔孔隙约为46%～65%。(2)5种木材细胞壁平均孔径较为接近,约为23～54 nm,而细胞腔孔径差异较大,范围为5.5～156.7 μm。此外,针叶材的平均细胞壁孔径比阔叶材的小。2.通过快速场循环核磁共振仪对木材低温/高温下的水分运动规律的研究表明:(1)核磁共振R1(自旋晶格弛豫率)分布能够区分不同孔隙中水分的动力学机制,高频区表征的是大孔隙中弱的水分子间的偶极相互作用,而低频区对应的是小孔隙中相对较强的分子间偶极相互作用。(2)不同温度下R1的变化源自于不同的弛豫机制。零度以下,温度的降低导致结合水向细胞腔中迁移而使R1增大。而室温以上,温度的升高使得水分子快速碰撞从而加速弛豫导致R1增大。此外,高温(90和100 ℃)下R1分布的峰点反映了孔隙中体积水和表面水的快速转换。(3)温度升高加速了水分子运动,平均分子运动相关时间减小,不同种类的木材之间活化能差异较大。3.对不同湿度下木材干燥/吸湿过程水分的弛豫特征的研究表明:(1)无论是干燥过程还是吸湿过程,FID(自由感应衰减)信号量与平均含水率均线性相关;此外,平衡含水率随着相对湿度的增大而呈指数增加。(2)由T2(自旋-自旋弛豫时间)可知,干燥过程中,即使含水率低于FSP(纤维饱和点),木材中依然有部分自由水存在;在干燥末期,吸附在细胞腔表面的水分会表现出同结合水类似的弛豫特征。相比较而言,密度高的木材在干燥过程中更易发生不同状态水分间的转化。此外,木材吸湿过程中的水分累积全部为结合水,即使达到吸湿平衡也不会向自由水转化。(3)不同湿度下,自由水的干燥在主要干燥区间内符合线性函数,结合水的干燥符合Logistic函数。对于低密度木材,自由水的干燥速率约为结合水的4倍,而对于高密度木材,约为20～30倍。此外,对于木材吸湿,水分平均吸湿速率随着相对湿度的增大呈现指数增长。(4)对于干燥/吸湿过程中孔径的变化研究表明,干燥过程中,相对湿度的增加延缓了细胞壁孔径减小的速率。木材饱和吸水时的平均细胞壁孔径约为绝干后达到吸湿平衡时的细胞壁平均孔径的2.4倍。
[Abstract]:Based on the theory of time-domain nuclear magnetic resonance (TNMR), the water proton in wood was used as probe. The nuclear magnetic resonance relaxation properties of three broad-leaved and two conifers were measured by using a fixed-field time-domain nuclear magnetic resonance spectrometer and a fast-field cyclic nuclear magnetic resonance relaxation instrument. The pore distribution of wood under saturated water absorption, the moisture movement in multi-scale pores at different temperatures, and the water migration and pore size dynamic changes of wood drying / hygroscopic process under various humidity were studied. The pore distribution of five wood species was measured by fixed-field time-domain NMR. The results show that: (1) the porosity of five kinds of wood is about 55% ~ 70%, and the pore size of cell wall is about 4% ~ 12%. (2) the average pore size of the cell wall of the five wood species was close to that of 23 / 54 nm, and the pore size of the cell cavity varied from 5.5 渭 m to 156.7 渭 m in the range of 5.5 渭 m to 156.7 渭 m, while the pore size of the cell cavity was about 46% and 65% respectively. In addition, the average cell wall pore diameter of coniferous wood is smaller than that of broad-leaved wood. 2. The water movement of wood at low temperature / high temperature was studied by fast field magnetic resonance (NMR). The results show that: (1) the distribution of nuclear magnetic resonance R1 (spin lattice relaxation rate) can distinguish the dynamic mechanism of water in different pores. The high frequency region characterizes the weak intermolecular dipolar interaction in macropores, while the low frequency region corresponds to the relatively strong intermolecular dipole interaction in micropores. (2) the variation of R1 at different temperatures is due to different relaxation mechanisms. Below zero, the decrease of temperature results in the migration of bound water into the cell cavity and the increase of R1. Above room temperature, the rapid collision of water molecules leads to the acceleration of relaxation, which leads to the increase of R1. In addition, the peak point of R1 distribution at high temperature (90 鈩，
本文编号：2435272
[Abstract]:Based on the theory of time-domain nuclear magnetic resonance (TNMR), the water proton in wood was used as probe. The nuclear magnetic resonance relaxation properties of three broad-leaved and two conifers were measured by using a fixed-field time-domain nuclear magnetic resonance spectrometer and a fast-field cyclic nuclear magnetic resonance relaxation instrument. The pore distribution of wood under saturated water absorption, the moisture movement in multi-scale pores at different temperatures, and the water migration and pore size dynamic changes of wood drying / hygroscopic process under various humidity were studied. The pore distribution of five wood species was measured by fixed-field time-domain NMR. The results show that: (1) the porosity of five kinds of wood is about 55% ~ 70%, and the pore size of cell wall is about 4% ~ 12%. (2) the average pore size of the cell wall of the five wood species was close to that of 23 / 54 nm, and the pore size of the cell cavity varied from 5.5 渭 m to 156.7 渭 m in the range of 5.5 渭 m to 156.7 渭 m, while the pore size of the cell cavity was about 46% and 65% respectively. In addition, the average cell wall pore diameter of coniferous wood is smaller than that of broad-leaved wood. 2. The water movement of wood at low temperature / high temperature was studied by fast field magnetic resonance (NMR). The results show that: (1) the distribution of nuclear magnetic resonance R1 (spin lattice relaxation rate) can distinguish the dynamic mechanism of water in different pores. The high frequency region characterizes the weak intermolecular dipolar interaction in macropores, while the low frequency region corresponds to the relatively strong intermolecular dipole interaction in micropores. (2) the variation of R1 at different temperatures is due to different relaxation mechanisms. Below zero, the decrease of temperature results in the migration of bound water into the cell cavity and the increase of R1. Above room temperature, the rapid collision of water molecules leads to the acceleration of relaxation, which leads to the increase of R1. In addition, the peak point of R1 distribution at high temperature (90 鈩，
本文编号：2435272
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