Research On Preparation And Heat Transfer Mechanism Of Puffed Wood Based-metal Composite

As a kind of sustainable natural green material,wood material is widely used in various fields.However,its low thermal conductivity limits its use as a thermally conductive material,and low-melting point alloy(LMA)has high thermal conductivity and low melting point.Therefore,in order to improve the thermal conductivity of wood,the environment-friendly LMA can be impregnated into the wood matrix to form puffed wood based-metal composite(WMC).However,the permeability of untreated wood matrix needs to be improved,so the puffed wood treated by high-strength microwave is used as the matrix and impregnated by different methods.The WMC not only has high thermal conductivity,but also the existing macro cracks can promote the heat transfer.?n this paper,the composite mechanism and properties of WMC are systematically studied,and the 3D microscale heat transfer model of WMC cells is established.The heat transfer properties of WMC are analyzed from the micro and macro scale level to form a relatively systematic WMC heat transfer mechanism.The obtained main research contents and conclusions are as follow:(1)Three types of puffed wood were impregnated with LMA by high pressure impregnation method.The puffed wood treated by high-strength microwave was impregnated in a high-temperature and high-pressure equipment,with the impregnation pressure of 2 MPa,1 h and145?.The microstructure of WMC was observed by SEM and found that the cracks between tracheids and pits were filled with LMA.EDS showed that the components of LMA were Sn and Bi.Because of the filling of LMA in puffed wood,the density,modulus of rupture(MOR),compressive strength parallel to the grain(CS),thermal conductivity and heat transfer properties of WMC were higher than that of untreated.The density and MOR of WMC ? were 6.1 times and 3.8 times of the untreated,the CS and thermal conductivity of WMC ? are 2.2 times and28.5 times of the untreated.The CS and MOR of WMC are higher than LMA,and the thermal conductivity can reach 3.42 W/(m·°C).(2)Based on the prepared WMC,a 3D microthermal model of WMC was established and the correlation was analyzed between the model and the experimental test to obtain a good linear relationship(R~2=0.97).By changing the shape of the WMC cell,the effect of the cross-sectional area on the heat transfer properties was investigated,and it was found that the average temperature and the temperature alteration ratio(TAR)of WMC increase with the increase of cross-sectional area,while the heat flux and the heat transfer time decrease with the increase of the area,the conclusions were explained by the existing formula.The transverse heat transfer rate was higher than the longitudinal heat transfer rate due to the effect of cell area.Lower applied temperatures resulted in lower TAR values and heat transfer time.When the applied temperature increases from 30?to 70?,the TAR increases by 381.8%,and the heat transfer time increases by 25.2%.(3)The vacuum impregnation method was used to impregnate three types of puffed wood with different vacuum and treatment time.The optimum process conditions were 0.1 MPa,16 h,and type ? which determined by the weight percent gain.Most of the LMA in WMC did not enter the tracheid and other tissues of wood,but only filled in the cracks of the puffed wood.?n addition,the meshing structure between LMA and the uneven grooves,holes and other structures at the cracks of puffed wood surface is formed in the way of"mechanical interlock",which makes the combination of LMA and the cracks more closely.DMA also showed that WMC had higher storage modulus and loss modulus than puffed wood,which indicated that the addition of LMA could improve the mechanical properties of puffed wood.The study also found that there was only physical combination between LMA and puffed wood,and no chemical reaction occurred.LMA impregnation did not change the unit cell parameters of the puffed wood,and also the original physical structure of the wood was not destroyed,but the crystallinity of WMC increased.Due to the high temperature impregnation treatment,the thermal stability of WMC is better than untreated and puffed wood,and the surface contact angle is also increased.This paper also uses CT to visually show the spatial distribution of LMA in WMC,and uses CT images to reconstruct a 3D rendering cut map,showing the beautiful texture of WMC.(4)The macroscopic heat transfer mechanism of WMC was studied,and the heat transfer properties of WMC was analyzed using thermal instrument.Under different grain direction and applied temperature,the stable maximum temperature,heating rate and cooling rate of WMC were higher than the untreated.With the change of WMC 1 to WMC 3,the stable maximum temperature,heating rate and cooling rate of WMC had all decreased,while the temperature unevenness had a tendency of increasing first and then decreasing.With the increase of applied temperature from 30°C to 70°C,the stable maximum temperature,heating rate,cooling rate and temperature unevenness of WMC 1 continued to rise.Similarly,as the heating area increases,the stable maximum temperature,heating rate,and cooling rate of WMC all increase slightly.When the heating area increases from 10 cm~2 to 20 cm~2,the stable maximum temperature increases only by 1.19?,the heating rate increases only by 0.0018?/s,and the cooling rate increases only by 0.0005?/s.