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Characterization Of Thermal Conductivity Of Carbon Nanotubes Array Used In Thermal Management Of Electrical Devices

Posted on:2010-02-07Degree:MasterType:Thesis
Country:ChinaCandidate:Q LiFull Text:PDF
GTID:2121360275978456Subject:Engineering Thermal Physics
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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 betweenθ2ω 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 Keff=KCNT*φ, to calculate the thermal conductivity of carbon nanotubes array.
Keywords/Search Tags:carbon nanotubes array, thermal conductivity, 3ωmethod, one dimensional thermal conduction model, two dimensional thermal conduction model
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