【摘要】：本研究基于時(shí)域核磁共振技術(shù)理論,以木材中水分1H質(zhì)子為探針,應(yīng)用定場時(shí)域核磁共振波譜儀和快速場循環(huán)核磁共振弛豫儀對三種闊葉材和兩種針葉材進(jìn)行核磁共振弛豫性質(zhì)的測定。研究了飽和吸水狀態(tài)下的木材孔隙分布,不同溫度下的多尺度孔隙內(nèi)的水分運(yùn)動,以及多種濕度下的木材干燥/吸濕過程的水分遷移和孔徑動態(tài)變化。1.利用定場時(shí)域核磁共振儀測定了 5種木材的孔隙分布,結(jié)果表明:(1)5種木材的孔隙度大約在55%～70%之間,其中細(xì)胞壁孔隙約為4%～12%,而細(xì)胞腔孔隙約為46%～65%。(2)5種木材細(xì)胞壁平均孔徑較為接近,約為23～54 nm,而細(xì)胞腔孔徑差異較大,范圍為5.5～156.7 μm。此外,針葉材的平均細(xì)胞壁孔徑比闊葉材的小。2.通過快速場循環(huán)核磁共振儀對木材低溫/高溫下的水分運(yùn)動規(guī)律的研究表明:(1)核磁共振R1(自旋晶格弛豫率)分布能夠區(qū)分不同孔隙中水分的動力學(xué)機(jī)制,高頻區(qū)表征的是大孔隙中弱的水分子間的偶極相互作用,而低頻區(qū)對應(yīng)的是小孔隙中相對較強(qiáng)的分子間偶極相互作用。(2)不同溫度下R1的變化源自于不同的弛豫機(jī)制。零度以下,溫度的降低導(dǎo)致結(jié)合水向細(xì)胞腔中遷移而使R1增大。而室溫以上,溫度的升高使得水分子快速碰撞從而加速弛豫導(dǎo)致R1增大。此外,高溫(90和100 ℃)下R1分布的峰點(diǎn)反映了孔隙中體積水和表面水的快速轉(zhuǎn)換。(3)溫度升高加速了水分子運(yùn)動,平均分子運(yùn)動相關(guān)時(shí)間減小,不同種類的木材之間活化能差異較大。3.對不同濕度下木材干燥/吸濕過程水分的弛豫特征的研究表明:(1)無論是干燥過程還是吸濕過程,FID(自由感應(yīng)衰減)信號量與平均含水率均線性相關(guān);此外,平衡含水率隨著相對濕度的增大而呈指數(shù)增加。(2)由T2(自旋-自旋弛豫時(shí)間)可知,干燥過程中,即使含水率低于FSP(纖維飽和點(diǎn)),木材中依然有部分自由水存在;在干燥末期,吸附在細(xì)胞腔表面的水分會表現(xiàn)出同結(jié)合水類似的弛豫特征。相比較而言,密度高的木材在干燥過程中更易發(fā)生不同狀態(tài)水分間的轉(zhuǎn)化。此外,木材吸濕過程中的水分累積全部為結(jié)合水,即使達(dá)到吸濕平衡也不會向自由水轉(zhuǎn)化。(3)不同濕度下,自由水的干燥在主要干燥區(qū)間內(nèi)符合線性函數(shù),結(jié)合水的干燥符合Logistic函數(shù)。對于低密度木材,自由水的干燥速率約為結(jié)合水的4倍,而對于高密度木材,約為20～30倍。此外,對于木材吸濕,水分平均吸濕速率隨著相對濕度的增大呈現(xiàn)指數(shù)增長。(4)對于干燥/吸濕過程中孔徑的變化研究表明,干燥過程中,相對濕度的增加延緩了細(xì)胞壁孔徑減小的速率。木材飽和吸水時(shí)的平均細(xì)胞壁孔徑約為絕干后達(dá)到吸濕平衡時(shí)的細(xì)胞壁平均孔徑的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，

本文編號：2435272