探索一維熱傳遞現象之極限

Abstract

實驗上，量測單一奈米碳管的熱導率是個相當困難的工作。這篇論文的第一部分，將介紹如何克服樣品製備、量熱裝置製作、及量測上的種種困難。接下來就能用實驗探測一維系統熱傳遞的三個極限：(1)一維熱傳遞在彎曲應變下的極限 (2)熱導率的最低極限 (3)熱導率的最高極限。 單壁奈米碳管是典型的一維材料，而且具有極高的力學強度和很長的聲子平均自由徑。論文的第二部分為單壁奈米碳管在週期性力學彎曲下的熱導率量測。特別的是，就算聲子平均自由徑超過其彎曲的曲率半徑，奈米碳管的熱導率始終維持不變。我們的實驗結果不能用傳統的熱傳導理論來解釋。 在論文的第三部分，我們將以實驗證明，自然界中最弱的化學鍵可以有最低的熱導率，同時我們也看到了不尋常的聲子穿隧現象。當高品質的多壁奈米碳管一層層被剝離，其電導率和熱導率有巨大、階梯狀的減少。我們發現多壁奈米碳管電導率和熱導率的各向異性超過106倍。經過換算，多壁奈米碳管層與層之間的熱導率在室溫下小於2×10-3 W/m-K，比目前所知最好的熱絕緣體的熱導率還小了至少25倍。 論文的最後一部分是我們量測到熱導率的最高極限。我們在超長單壁奈米碳管中，測量到不遵守傅立葉定律、熱導率隨長度發散的現象，此發散現象使1.039公厘長的單壁奈米碳管熱導率高達8396 W/m-K。我們發現熱導率與長度的0.25次方發散，且直至1.039公厘長都沒有停止發散的趨勢。另一方面，在某些單壁奈米碳管樣品中，我們有量測到不對稱的熱傳導現象。

Experimental investigations of thermal conductivities of individual nanotube are challenging. In the introductory part of my thesis, I describe several attempts and methods to overcome the difficulties in thermodevice fabrications, sample preparations, and manipulations. Overcoming these challenges thus allows me to experimentally investigate the limits of heat transport in one-dimensional (1D) systems in three different aspects. (1) The limit of 1D heat transfer under extreme bending strain. (2) The lowest limit of thermal conductivity. (3) The highest limit of thermal conductivity. Single-wall carbon nanotubes (SWCNTs) as typical 1D materials have been shown to exhibit excellent mechanical properties and very long phonon mean free paths. In the second part of my thesis, I report in situ measurements of individual SWCNTs under cyclic mechanical bending. Surprisingly, we find that the thermal conductivity of the SWCNT remains intact even the characteristic phonon mean free path beyond the theoretical limit set by the radius of curvature. Our results strongly challenge the traditional theory of heat transfer. In the third part of the thesis, we demonstrate the lowest thermal conductivity of matters can be obtained in the weakest chemical bond of nature, in which the heat conduction displays unusual phonon tunneling behavior unfound before. Giant stepwise reductions of electrical/thermal conductivity are observed when the layers of high quality multiwall carbon nanotubes are removed one by one. We find that electrical and thermal anisotropy is more than 106. Correspondingly, the interlayer thermal conductivity of a nanotube is less than 2×10-3 W/m-K at room temperature, which is at least 25 times lower than that of the best thermal insulator known so far. In the last part of the thesis, we investigate the highest limit of thermal conductivity. The non-Fourier, divergent thermal conductivity is found in ultralong SWCNTs, leading to thermal conductivities as high as 8396 W/m-K in a 1.039 mm long SWCNT. The thermal conductivity is found to diverge with lengths to the power of 0.25 for SWCNTs. Moreover, the thermal conductivity shows no sign of saturation for sample lengths up to 1.039 mm. On the other hand, asymmetric thermal conductance is also observed in some of the SWCNT samples.