柱形介孔材料内近场辐射和界面散射对导热的影响

发布时间：2018-04-24 20:34

本文选题：介孔材料 + 近场辐射　；参考：《北京科技大学》2015年博士论文

【摘要】：随着航空事业的发展、电子元器件的高集成化和微电子机械系统(MEMS)的发展等,微型芯片集成电路的热设计、航天器材的温控和微电子器件的冷却等都要求材料具有特殊的热特性。本文针对有序介孔异质复合材料,通过理论分析和实验测量,系统开展了介孔基材、填充纳米线及复合结构的热输运机理的研究,以及材料内存在的近场辐射及界面散射对其导热系数的影响,为介孔复合材料的热设计提供理论支撑。首先针对有序介孔基材,建立了孔道内曲面边界的近场辐射换热模型,理论计算柱形孔／球形孔内的近场辐射换热,并进一步分析孔径和温度的影响。同时,以典型介孔二氧化硅MCM-41、SBA-15和介孔氧化铝AAO为例,进行了结构重构和表征,开展了介孔基材壳壁导热系数的分子动力学模拟及理论分析,并进行了实验测量。采用理论模型将介孔基材壳壁导热系数、孔道内近场辐射及孔道内受限气体导热系数三者耦合,得到介孔基材的有效导热系数。研究表明：基材壳壁导热系数随着孔径的增大而减小,随着壁厚的增大而增大,且表现出各向异性；孔道内近场辐射热流及当量导热系数随着孔径的增加呈指数衰减,随着温度的升高缓慢增加,近场辐射热流要比远场辐射高2～7个数量级,且孔径越小,近场效应越显著；介孔基材的有效导热系数的理论值与实验值吻合得较好,随着孔径的增大逐渐减小,随着温度的升高缓慢增加,考虑了近场辐射的有效导热系数比未考虑的高0.2-10%左右。其次,针对填充金属纳米线,采用平衡分子动力学方法来模拟纳米线的声子导热系数,并采用玻尔兹曼输运理论来计算纳米线的电子导热系数,得到了材料表面散射对对纳米线导热系数的影响,同时考虑了晶界界面散射对纳米线导热系数的作用,进一步分析了纳米线尺度和温度的影响。同时,采用线性响应理论、电子密度矩阵对纳米线截面(XY面)内的导热系数进行量子修正。研究表明：电子输运对金属纳米线的导热占主导地位,而声子导热系数的贡献也不容忽视；晶界散射导致导热系数减小,尤其对电子导热系数作用显著；纳米线总导热系数随着温度的升高而降低；随着截面尺寸减小而减小,但声子导热系数所占份额有所增加；随着长度的增加逐渐增加,但当长度增至200nm时,导热系数趋于一稳定值。当纳米线长度大于截面尺寸时,XY面内的导热系数低于轴向(Z方向)上的导热系数。与此同时,采用两平面模型计算两纳米线端面间的近场辐射换热,与文献中的实验数据进行验证,并分析了间距和温度的影响。研究表明：近场辐射热流和当量导热系数随着温度的增加逐渐增加,且理论计算值与文献实验结果吻合得较好；两纳米线间近场辐射换热的当量导热系数随着间距的增加先增加后减小,在间距为30nm左右,存在一个峰值。在介观尺度下,对辐射起主要贡献的是s偏振近场电磁波,但是随着间距的减小,p偏振近场电磁波急剧增加。当间距大于1000nm时,近场作用逐渐消失,远场开始起主导作用。再次,采用非平衡分子动力学模拟介孔基材与纳米线间的界面热阻,并与声学失配模型、散射失配模型进行比较。研究表明：随着温度的升高,界面热阻逐渐减小；界面两端材料质量差异越大,界面热阻越高。最后,建立热阻网络模型来耦合介孔基材导热系数、纳米孔隙内气体导热系数、考虑界面散射的填充纳米线导热系数、介孔基材与纳米线间的界面热阻、孔道内近场辐射、纳米线间近场辐射等,得到复合材料的综合导热系数,同时采用双流计法和瞬态热源法对材料的导热系数进行实验测量,与理论研究进行验证,分析近场辐射、界面散射对材料综合导热系数的影响。研究表明：介孔复合材料的综合导热系数呈现各向异性,随着孔径的增加,X、Z方向上的导热系数逐渐降低,Y方向上的导热系数存在峰值；随着纳米线间距的增加或纳米线长度的减小,即填充率的减小,各个方向上的导热系数均逐渐减小；在X、Y方向上,由界面散射产生的导热系数降低值高于由近场辐射产生的导热系数升高值,考虑了界面散射和近场辐射的导热系数比未考虑的要低；Z方向上刚好与之相反。孔道内填充金属纳米线,只有当填充率很高时,才表现出对导热系数的明显提高；界面热阻导致介孔复合材料的导热系数降低了20%以内。
[Abstract]:With the development of aviation industry , the high integration of electronic components and the development of microelectronic mechanical system ( MEMS ) , the thermal design of micro - chip integrated circuits , the temperature control of aerospace equipment and the cooling of microelectronic devices require special thermal characteristics .

The near - field radiation heat transfer model is established for the ordered mesoporous substrate , the near - field radiation heat transfer model is established for the inner surface boundary of the pore channel , and the influence of pore size and temperature is further analyzed . The thermal conductivity of the shell wall of the mesoporous base material is calculated by using the theoretical model . The thermal conductivity of the shell wall of the mesoporous base material is reduced with the increase of the aperture , and the anisotropy is exhibited .
The near - field radiation heat flux and the equivalent thermal conductivity of the tunnel are exponentially decaying with the increase of the aperture . As the temperature increases slowly , the near - field radiation heat flow is 2 - 7 orders of magnitude higher than the far - field radiation , and the smaller the aperture , the more significant the near field effect ;
The theoretical values of the effective thermal conductivity of the mesoporous base material are in good agreement with the experimental values . As the pore size increases , the effective thermal conductivity of the near - field radiation is increased slowly , and the effective thermal conductivity of the near - field radiation is considered to be about 0.2 -10 % .

Secondly , aiming at filling metal nanowires , using equilibrium molecular dynamics method to simulate the phonon thermal conductivity of nano - wire , and using Boltzmann transport theory to calculate the electron thermal conductivity of nano - wire , the influence of surface scattering on the thermal conductivity of nano - wire is obtained .
Grain boundary scattering results in the decrease of thermal conductivity , especially the effect of electron thermal conductivity .
the total thermal conductivity of the nanowires decreases with increasing temperature ;
As the cross - sectional dimension decreases , the proportion of phonon thermal conductivity increases ;
As the length increases , the thermal conductivity tends to be a stable value when the length increases to 200 nm . When the length of the nanowire is greater than the cross - sectional dimension , the thermal conductivity in the XY plane is lower than the thermal conductivity in the axial direction ( Z direction ) .

At the same time , the near - field radiation heat transfer between the end faces of the two nanowires is calculated by using two - plane model , and the influence of pitch and temperature is analyzed . The results show that the near - field radiation heat flux and equivalent thermal conductivity gradually increase with the increase of temperature , and the theoretical calculation value is in good agreement with the experimental results .
The equivalent thermal conductivity of the near - field radiation heat transfer between the two nanowires decreases with the increase of the spacing , and there is a peak at the interval of 30 nm . Under the mesoscopic scale , the near - field electromagnetic wave of the s - polarized near - field electromagnetic wave is mainly contributed to the radiation , but the near - field effect gradually disappears when the spacing is greater than 1000nm , and the far field starts to play a leading role .

Thirdly , the interface thermal resistance between the mesoporous substrate and the nanowire is simulated by the non - equilibrium molecular dynamics , and compared with the acoustic mismatch model and the scattering mismatch model . The results show that the thermal resistance of the interface gradually decreases with the increase of temperature ;
The greater the difference between the material quality at both ends of the interface , the higher the thermal resistance of the interface .

Finally , a thermal resistance network model is established to couple the thermal conductivity of the mesoporous substrate , the thermal conductivity of the gas in the nano - pores , the thermal resistance of the filled nanowires , the near - field radiation in the pores , near - field radiation in the pores , and the like , so as to obtain the comprehensive thermal conductivity of the composite material .
With the increase of the spacing of the nanowire or the reduction of the length of the nanowire , that is , the filling rate is reduced , the thermal conductivity in each direction is gradually reduced ;
In the X and Y directions , the decrease of the thermal conductivity caused by the interface scattering is higher than that of the heat conduction coefficient generated by near field radiation , and the thermal conductivity of the interface scattering and near field radiation is considered to be lower than that of the non - field radiation ;
in that Z direction , the metal nano wire is filled in the pore canal , and only when the filling rate is high , the heat conduction coefficient is obviously improved ;
the thermal resistance of the interface leads to the reduction of the thermal conductivity of the mesoporous composite material within 20 percent .

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.4

【参考文献】