Study On Thermal Conductivity, Adhesive Interphase And Pyrolysis Properties Of Wood At Microscale By Scanning Thermal Microscopy

Thermal conductivity is one of the most important physical properties of materials. Wood is typical porous material, at macroscale thermal conductivity of wood is affected by solid, liquid and gaseous substance in wood. By the development of processing wood material at microscale or even at nanoscale, it will be very significative to obtain thermal conductivity property of wood material at microscale, and it will help to understand how the heat transfer in wood at macroscale. Meanwhile, thermal conductivity is determined by the structure and properties of materials, thermal conductivity changes with the change of structure and peoperties of materials, thus the structure and properties can be understood by researching on the thermal conductivity of material. Scanning thermal microscopy(SThM) can be used to study thermal conductivity of material efficiently.This work studied on the thermal conductivity of wood material at microscle by using SThM and by researching thermal conductivity at micrcoscale the microstructure property of wood-based materials were analyzed. This research included there parts mainly. In the first part, the theraml conductivity properties of wood cell wall and how these properties affected the thermal conductivity of wood at macroscale were focused on. Based on the thermal conductivity studies, the microstructure of wood were analyzed, especially the differences of sturcture and composite between different layers of wood cell wall were interpreted. The key researches of second part were about studied on the structure properties of interphase between wood and adhesive resin by mean of SThM. The penetration of adhesive in wood structure was investigated based on the difference of thermal conductivity between wood and adhesive. By using SThM, this part established a new research approach for structure studies of wood based composites. In the third part, combined with spectrum technologies, X ray diffraciton, nanoindentation et al., SThM was used to study how the microstructure and properties of wood changes when pyrolyzed at different temperatures. By these studies the transition process of wood and the correlation between chemical, physical properties and structure of wood duing pyrolysis were investigated. This work developed the researches of thermal conductivity of wood-based materials at microscale, established the method of studing structure and propertis of wood at microscale by SThM. The main study results were conclued as below.(1) By using SThM scanned fiber cell wall of oak in cross section, the results indicated that the thermal conductivity of S2 layer was higher than compound middle lamella(CML) and cell corner(CC). Combined with the results from the researches by Raman spectrum, the reason why thermal conductivity of S2 was higher than CC and CML was that in S2 layer the content of cellulose was higher, the fibril angle in S2 layer was about 11°and it meant cellulose oriented nearly parallel to longitudinal direction of wood cell, so when heated S2 layer in cross section the heat transferred along the grain; in CC and CML the components were random arrangement and this characteristic would not benfit heat transfer. When scanned the radial section of wood cell wall, the rsults showed that the thermal conductivity of S2, CML and CC were almost the same. This due to when heated cell wall in radial section, in S2 layer heat transferred along the direction that perpendicular to the grain, so the advantage for increaseing heat transfer was vanished and caused that the thermal conductivity of S2 was almost the same to CC and CML where components were random arrangement. It could be conclued that the main reason caused the differenc of thermal conductivity between different layers of wood cell wall was the difference of sapce structure in different layers. At macroscale the thermal conductivity of wood anisotropy, this part study confirmed that at microscale the thermal conductivity of wood cell wall was also anisotropy by SThM.(2) SThM was applied to study on structure properties of interphase between wood and adhesive resin, the bond characteristics of wood cell wall and nano cellulose added phenolic resin(PF) were investigated. The results indicated that at microscale the thermal conductivity of PF was lower than wood cell wall, the added of nano-fibrillated cellulose and cellulose nanocrystals shown no effect on the thermal conductivity of PF. For the difference of thermal conductivity between cell wall and PF, in bondline from the resin to cell wall there was a transition area where the probe current of SThM increased. It regarded that the transition area include the areas where wood and PF affected each other and where the wood and PF contacted directly. The length of transition area was determined by analyzing SThM image. The length of transition area from cellulose nanocrystals added PF to wood cell wall was 1.92±0.32μm, and when consider nano-fibrillated cellulose added PF resin the length was 1.76±0.277 μm, in the transition area the length of the area where PFs and cell wall contact directly were 0.73±0.144μm and 0.7±0.092μm. SThM images also shown PF resin penetrated into wood cell lumen. By analyzing probe current of SThM, the results indicated there was difference of thermal conductivity between PF in lumen and in bondline, the difference maybe was caused by in lumen PF was mixed with extractives.(3) SThM was used to investigate how the structure of wood cell wall change after pyrolyzing. The results shown, with increase pyrolysis temperature, the layer structure of cell wall could be visible until 300 ℃. When the pyrolysis temperature increased to 325 ℃, the layer structure was invisible and wood cell wall became uniform. Different layers could be visible in SThM images for the different thermal conductivity that caused by different sturcture in cell wall layers. In S2 layer, cellulose was the main component and cellulose fibril parallel to longitudinal direction of wood cell, so the S2 layer had higher thermal conductivity. When wood pyrolyzed at temperature higher than 325 ℃, Raman Spectrum and Fourier Transform Tnfrared Spectrum examination shown that cellulose in cell wall was decomposed, X ray diffraciton investigation also indicated the structure of cellulose fibril parallel to longitudinal direction vanished, thus the strucutre and component of the whole cell wall became uniform. When used Nanoindentation to examine the change of micromechanical properties of wood cell wall, the results showd elastic modulus of cell wall changed dramaticly if the temperature higher than 300 ℃. This part researches showed that physical, chemical properties and microstructure of wood cell wall changed correlatively in pyrolysis process, SThM could be used to investigate microstructure and micro-properties of wood cell wall efficiently.