【作者】 李谦；

【导师】 唐大伟；

【作者基本信息】 中国科学院研究生院（工程热物理研究所） ， 工程热物理， 2009， 硕士

【摘要】 碳纳米管阵列作为一种全新的热界面材料受到广泛关注和研究。本文通过实验测量结合理论分析的方法,对碳纳米管阵列的导热性能进行了研究。理论分析给出了交流加热谐波特性,给出了3ω谐波通用表达式,通过液体导热系数的测量验证了实验系统的可行性及稳定性。采用化学气相沉积FePc的方法得到了平均长度约1.5微米的多壁碳纳米管阵列,并估算出阵列的基本参数:碳管平均外径D≈75纳米,平均内径d≈49纳米,碳管间距δ≈75纳米,碳纳米管阵列的面积（体积）分数φ≈13%。分别使用40微米和8微米宽度Au金属膜探测器对样品进行了测试。得到不同电压下θ_2ω-f关系变化趋势以及分段出现直线段的情况都比较相似。进行归一化处理后,得到本测试结构在不同电压下的热阻抗与频率的关系完全重合。为40微米探测器测试结果设计符合其热量传递特点的一维导热模型,使用以模拟退火算法为核心的多参数拟合方法,得到在本实验条件下,样品沿厚度方向的有效导热系数为17W/mK。根据Borca得到二维多层薄膜-基底结构解析解对8微米探测器测试结果进行描述,采用一种可以自行判断各物性敏感度的基于斜率的最小二乘法进行多参数拟合。由于参数过多,拟合过程得不到确定的结果,但定性的可以得到如下结果:类金刚石绝缘膜内存在较强二维导热现象;绝缘膜与阵列接触热阻较大,催化剂层和氧化层的存在造成较大接触热阻;阵列内部几乎没有横向传热。考虑到测试样品暴露在空气中,对阵列内存在横向传热的可能性以及横向传热方式进行分析:碳管的相互纠缠,空气对流和导热以及热辐射的存在都可能导致横向传热。对碳纳米管阵列内部的热传递过程进行了详细的分析。建立起碳纳米管阵列作为整体的有效热阻抗与阵列中碳管形成的热流网络有效热阻抗之间的关系。在一定的假设和简化下,利用一个简洁的“中心碳管-同心圆环”模型,得到了环绕在与绝缘膜接触的中心碳管外围的其他相邻碳管的总有效热阻抗Z_s,（eff）的递归循环关系式,并借此得到阵列等效热容和等效热阻与碳管间热阻的关系,通过数值计算,发现当碳管间热阻R_l^’’>10^-3m²KW^-1时,碳纳米管阵列有效热容及有效热阻已与单根碳管无异,此时,阵列内部横向传热完全可以忽略,该结论与二维模型计算结果,即阵列内的横向传热完全可以忽略相吻合。由此得到一个非常简单有效的判断碳纳米管阵列材料导热系数的公式K_eff=K_CNT*φ。以该公式为基础,同样可以判断碳管末端与外界的接触状况。更多还原

【Abstract】 Carbon nanotubes array is receiving extensive attention as a novel thermal interface material. We investigated the thermal conductivity of this material by both experimental and theoretical method.We theoretically analyzed the characters of harmonic in metal under ac heating and obtained the general expression of third harmonic. The feasibility and stability of the experimental system was inspected and verified by measuring the thermal conductivity of liquid and nanofluids.The multi-wall carbon nanotubes array with a length of 1.5 microns was produced by chemical vapor deposition. We got the following parameters of our array: the average outer diameter of the carbon nanotubes is 75 nanometers, the average inner diameter is 49 nanometers, the average CNT spacing is 75 nanometers and the area or volume fraction of the carbon nanotubes array is about 13%.We measured our sample by the 40 microns wide and 8 micron wide Au strips under different heating voltage, separately. The relations betweenθ_2ω and f are similar. The relations between thermal impede and testing frequencies are exactly the same. We designed a one-dimensional thermal conduction model for the results of the 40 microns wide Au strip. Then we obtained that the longitudinal thermal conductivity of the array is 17W/mK by fitting the data with multi-parameters least square method based on simulated annealing algorithm. And then we used the analytical solution of two-dimensional multi-layers model to deal with the data of 8 microns wide Au strip. Although we used a new gradient-based least square method to fit the data, we didn’t obtain a precise result due to too many unknown parameters. After a reasonable analysis we knew that: there is strong two-dimensional heat conduction inside the insulation layer; the thermal contact resistance is large between the insulation layer and the array, the catalyst layer and the oxide layer introduce a large thermal resistance; the heat transfer inside the array can be ignored.Since the sample was exposed in air condition, we analyzed the possible heat transfer paths inside the array: heat conduction between intertwining tubes; heat convection and heat conduction caused by air; heat radiation between tubes and atmosphere. We set up the relation between the effective thermal impede of the array as a whole and the effective thermal impede of the carbon nanotubes thermal network in the array. We designed a central tube-concentric rings model to describe the relation between a central tube which is in good contact with the insulation layer and other tubes surrounding it that are not in contact with the insulation layer. The recursive relation to describe the total effective thermal impede of the surrounding tubes was obtained. After numerical simulations, we found that when the intertube thermal resistance is larger than 10^-3 m² KW^-1 the heat transfer between carbon nanotubes inside the array can be completely ignored. This conclusion exactly matches the result we got from the two dimensional model. So we can obtain a very simple but effective relation, which is K_eff=K_CNT^*φ, to calculate the thermal conductivity of carbon nanotubes array.更多还原