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Self-resemble ( 自我相似 ) NanoLetters, Vol. 7, No. 2, p 269 - 275, (2007).
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R Time Electrical connections Silver electrodes SWNT film Self-resemblance
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Clean room Summer SWNT film Thermometer 7 days
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SWNTs Peak-to-peak and valley-to-valley Positive temp coefficient of resistivity (PTCR, metallic) Heat island ( 熱島效應, 夏天 ) 室外最熱時為 36 C at 2-4 pm 最涼為 27-28 C at 4-6 am ( 中央氣象局 )
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PTCR 是可理解 因為 O 2 doping O2O2 EFEF Charge transfer Doping state, gap closing, metallic
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不能理解的是為何 SWNTs 具高熱感性, 甚至可以感應溫差只有 1 度 的變異, 而銅不能 ? PTCR α tube = 0.0043 per K and α Cu = 0.0039 per K 二者相差無幾 為何 SWNTs 之熱感性質比銅更敏銳 !
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Structure Bonding 2000-2500 W/m*K (ref1) thermal conductivity (at R.T.) sp 3 sp 2 with interlayer coupling 250 W/m*K (ref3) sp 2 3000 W/m*K (ref2)
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sp 2 3000-6000 W/m*K (ref4) 2000-3000 W/m*K (ref4) sp 2 with interlayer coupling Structure Bonding thermal conductivity (at R.T.)
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40000 W/mK K Temp (k) A single (10, 10) tube (ideal model) 100 K 400 K50 K 6000 W/mK 1. Between 50-400 k, K is temp dep 2. Why a change-over emerges at 100 k? K = C v · ·l <100 k, l constant, so K is dominated by C v. >100 k, C v constant, so K is dominated by l, and l decreases as temp increase, due to umklapp process, Diamond: k = 2500 W/mK Graphene: k = 1500-2000 W/mK (PRL, 84, 4613, 2000 by Tomanek)
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Quantized phonon and phonon speed reaches 10,000 m/s Science, 289, 1730, 2000 by Hone et al Ph 1 Ph 2 Ph 3 EFEF C1C1 C2C2 C3C3 V1V1 V2V2 V3V3 Electron band structure EgEg Op 1 Op 2 Op 3 A1A1 A2A2 A3A3 Phonon band structure EgEg Ph 3 > Ph 2 > Ph 1
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RBM D and G bands 沿碳管軸向之熱傳導主要二個 phonon modes 高頻 低頻
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Phonon modes at high frequency in a CNT
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Thermal contraction of CNTs Tube length Tube volume : thermal linear expansion coefficient : thermal volumetric expansion
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以上 data 得知 a single CNT 之熱傳很快 ( 散熱也很快 ) 因此不可能出現熱感性質比銅金屬靈敏之現象 Phonon generation (ps) 快於 phonon-electron coupling (ns) ( 熱能還來不及傳給電子就已經 dissipated)
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SWNTs 以 bundle form 方式出現 1. 碳管尺寸螺旋性不一致 無法形成完美六方堆積晶格 2.Radial breath mode (RBM) 于 bundle form 中變得很重要
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完美六密堆積 一致性之 RBM 熱傳由 1D (single tube) 3D (bundle) Heat flux Phonon dispersion from 1 D into 3D structure and heat conduction continues along tube axis (normal process).
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Ph 1 Ph 2 向量合仍然在 BZ 內 (normal process) no heat resistance and heat conduction proceeds forwards
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不完美堆積 RBM 不一致
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Bundle becomes heat resistance (Umklapp process) Heat flux reduction
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Heat resistance Reciprical lattice
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Bundle bcomes heat reservoir ( 熱儲存槽 ) 熱儲存槽可合理解釋為何碳管比銅更具熱感性 環境溫度
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1.Enhanced resistive phonon 2.Heat flux reduction 3.Low thermal conductivity 4.Heat reservoir Bundle 成為熱儲存槽之三個證據 1 可由 3 來直接驗證 single carbon nanotube, k = 2500-5500 W/mK at room temperature SWNT film = 0.5 - 3.2 W/mK Phys. Rev. B, 59, 2514 (1999)
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2. Heat flux reduction Heat flux Heat flux reduction k = .M.KQ/L M: characteristic number of occupied phonon branches (=1.5πk B TR 2 /ha ) KQ: thermal conduction quantum (=π2k B 2 T/3h = 9 10-10 W/K) L: are bundle length (=1-2 μm) h: Planck’s constant R: tube radius (= 0.7 nm) A: tube-tube separation (= 1-1.5 nm) k = 1.3 10 -10 - 9 10 -11 W/K (bundle) k = 6 10 -10 W/K (single SWNT) Phys. Rev. Lett, 95, 226101 (2005).
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4.Heat reservoir 熱儲存槽意味碳管溫度比外界溫度 高的時後更高 低的時候更低
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熱儲存槽 吸熱 放熱 吸熱 放熱 / 放熱 = 96% 高效率之熱交換器
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In vacuum Negative temperature coefficient of resistivity, (NTCR, semiconductor) O 2 desorption from nanotubes O 2 doping state EFEF VBVB CBCB EgEg
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See video
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Single tube Bundle medium spacing Bundle large spacing Hardening effect Vibration spectra
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碳管間距小
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熱由左傳到右
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碳管間距大
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Phonon density of state single
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Phonon density of state 7.31
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Phonon density of state 6.71
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Heat capacity at 300K 4.013024.213024.344.41302 6.7144 6.943.5 7.1138.54042 7.3137394045 7.5138 Single38
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P cri = 2 EI/L 2, P cri :critical axial load, E: elastic modulus, L: beam length, I: inertia moment (=A 2 ) : radius of gyration A : beam cross section
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5 meV/atom 0.2 eV/atom
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1. 彎曲 bundle 變直 2.Thermal vibration (8-10 s -1, phonon wave through rigid body, 剛硬 ) 3.Bundle lengthening by 13.5%
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E1E1 E2E2 Tube displacement + Tube Rolling Applied electric field intensity = tube-tube friction
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Cohesive energy of unfolded tube (planar sheet) E 1 = 3-5 meV/atom E 2 = 0.254 eV/atom
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