Nilsson Martin, Frenning Göran, Gråsjö Johan, Alderborn Göran, Strømme Maria
The Angström Laboratory, Department of Engineering Sciences, Uppsala University, Box 534, SE 751 21 Uppsala, Sweden.
J Phys Chem B. 2006 Oct 19;110(41):20502-6. doi: 10.1021/jp063835q.
The present study aims at contributing to a complete understanding of the water-induced ionic charge transport in cellulose. The behavior of this transport in loosely compacted microcrystalline cellulose (MCC) powder was investigated as a function of density utilizing a new type of measurement setup, allowing for dielectric spectroscopy measurement in situ during compaction. The ionic conductivity in MCC was found to increase with increasing density until a leveling-out was observed for densities above approximately 0.7 g/cm3. Further, it was shown that the ionic conductivity vs density followed a percolation type behavior signifying the percolation of conductive paths in a 3D conducting network. The density percolation threshold was found to be between approximately 0.2 and 0.4 g/cm3, depending strongly on the cellulose moisture content. The observed percolation behavior was attributed to the forming of interparticulate bonds in the MCC and the percolation threshold dependence on moisture was linked to the moisture dependence of particle rearrangement and plastic deformation in MCC during compaction. The obtained results add to the understanding of the density-dependent water-induced ionic transport in cellulose showing that, at given moisture content, the two major parameters determining the magnitude of the conductivity are the connectedness of the interparticluate bonds and the connectedness of pores with a diameter in the 5-20 nm size range. At densities between approximately 0.7 and 1.2 g/cm3 both the bond and the pore networks have percolated, facilitating charge transport through the MCC compact.
本研究旨在促进对纤维素中水分诱导的离子电荷传输的全面理解。利用一种新型测量装置，研究了松散压实的微晶纤维素（MCC）粉末中这种传输行为随密度的变化，该装置允许在压实过程中进行原位介电谱测量。发现MCC中的离子电导率随密度增加而增加，直到在密度高于约0.7 g/cm³时观察到趋于平稳。此外，结果表明离子电导率与密度遵循渗流型行为，这意味着在三维导电网络中导电路径的渗流。发现密度渗流阈值在约0.2至0.4 g/cm³之间，强烈依赖于纤维素的水分含量。观察到的渗流行为归因于MCC中颗粒间键的形成，并且渗流阈值对水分的依赖性与压实过程中MCC中颗粒重排和塑性变形对水分的依赖性有关。所得结果有助于理解纤维素中密度依赖性水分诱导的离子传输，表明在给定水分含量下，决定电导率大小的两个主要参数是颗粒间键的连通性和直径在5 - 20 nm尺寸范围内孔隙的连通性。在约0.7至1.2 g/cm³的密度下，键网络和孔隙网络都已渗流，促进了电荷通过MCC压实体的传输。
