磁偶極共振與漸逝波耦合之材料近場熱輻射研究

Abstract

本論文將討論磁偶極共振與漸逝波耦合之近場熱輻射行為，極化材料(polar materials)與金屬材料在中紅外光波段的光學性質上，皆有消光係數大於折射率的部分，也就是介電常數的實部為負值，此時極化材料或金屬材料與空氣的介面為表面波存在的條件。當表面波模態行走於介面時，表面的法線方向會有漸逝波(evanescent wave)存在，其為一個快速衰減的電場，但並沒有能量在此方向上傳遞，不過漸逝波可與其他物體於近場處耦合，進而傳遞能量，過去大多使用同樣有表面波模態的材料作為耦合對象，而我們認為擁有磁偶極共振現象的矽粒子同樣也可以成為耦合能量的媒介。 熱輻射可以使物體不需任何媒介而將能量傳遞至另一方，但能量轉移的幅度受到放射率的限制。不過近年來許多研究指出兩物體間距在次波長尺度時，其輻射熱轉移會大於黑體輻射(blackbody radiation)的理論強度。雖然此方法可以讓能量大幅地在物體間傳遞，可是能量進入另一物體後仍需要藉由傳導或對流的方式將熱能排出至環境，而此研究中的矽粒子之磁偶極共振不僅可將能量耦合，亦能直接散射熱能至環境當中，達到提升遠場放射率的效果。 由於碳化矽晶體與鋁膜皆有表面波的模態，因此我們將論文將分為兩個部分，第一個部分討論矽粒子對碳化矽表面輻射能力的影響，第二個部份我們則將碳化矽晶體換為鋁膜來討論。於此兩部分中我們都透過模擬與實驗來驗證我們的假設，在模擬中，我們用三維有限時域差分法(three dimensional finite difference time domain, 3D-FDTD)模擬碳化矽表面在放出熱輻射(紅外光)時，矽粒子在表面增加輻射的能力。於實驗上，我們分別將有無矽粒子的試片加熱至同一溫度，接著比較其降溫速度，觀察散熱能力的變化，以此表示輻射的強度。於碳化矽基材的比較上，未經過任何處理的碳化矽基板、矽粒子覆蓋率為0.845%的碳化矽試片以及矽粒子覆蓋率為2.205%的碳化矽試片，三種樣品個別在真空腔體內加熱至攝氏70度並持溫一段時間後，觀察樣品的降溫的時間，三者從攝氏70度降到平衡溫度所需的時間分別為3203秒、1237秒、1104秒。於鋁膜的比較上，未經過任何處理的鋁膜、矽粒子覆蓋率為2.19%的鋁膜試片以及矽粒子覆蓋率為5.72%的鋁膜試片，三者從攝氏70度降到平衡溫度所需的時間分別為4895秒、2787秒、1432秒。由此可知矽粒子對於碳化矽或是鋁膜的放射率之影響是非常大的。此外，為了證實矽粒子與碳化矽基板間的熱交換是近場熱輻射所造成，我們將矽粒子塗佈於另一個矽基板並覆蓋於碳化矽基板上，其中用光阻在矽基板上做出四根柱子使兩基板的間距可由光阻厚度調控。當光阻厚度為3.1、4.0、5.2及5.9微米時，碳化矽基板從攝氏70度降到平衡溫度所需的時間分別是1178、1569、1778、1937秒，熱轉移的速度隨光阻厚度增加而快速減小。另一方面，我們也將試片加熱到不同溫度下，用熱像儀觀察試片平均放射率的變化，在攝氏180度，當矽粒子覆蓋率僅有2.205%的情況下，其平均放射率為0.685，而未處理過的碳化矽表面平均放射率只有0.648而已，放射率增加的幅度已超過矽粒子所覆蓋的面積比例。當增益對象改為鋁膜時，未經過任何處理的鋁膜之放射率約為0.03，矽粒子覆蓋率為5.72%的鋁膜試片之放射率則約為0.05，在如此低的覆蓋率下提升了60%以上。透過各種實驗的結果驗證磁偶極共振確實能與表面波模態互相耦合，並再散射能量到遠場環境中。 由研究可以觀察到矽粒子與表面波模態互相耦合，當能量進入矽粒子內部，且在該矽粒子大小下有磁偶極現象發生時，能量將被大量的散射至遠場。實驗上雖然球體的大小不一，形狀不規則，此現象仍然非常明顯，未來可應用於散熱工程上。

In this thesis, the behavior of near-field thermal radiation generated from the coupling between magnetic dipole resonance and evanescent wave are investigated. Generally the extinction coefficients of the polar materials and the metallic materials are greater than their refractive indices in the middle infrared band. That is, the real part of the dielectric constants of the polar and metallic materials is negative. At this condition, the surface wave mode would be supported on the boundary between air and polar materials (or metallic materials). The presence of surface waves is accompanied with evanescent wave, an electric field decay rapidly along the normal direction of surface. Though an evanescent wave cannot deliver energy directly, the energy could be coupled to nearby objects within sub-wavelength gap. Previous studies used polar materials based superstrates or structures having surface mode to couple with the evanescent wave. Furthermore, these studies illustrated that the energy transfer between two objects could be larger than the black body radiation when the gap between two objects was very close. However, these method cannot expel the thermal energy to surrounding through radiation. In this thesis, the magnetic dipole resonance of silicon particles was used as a coupling medium of surface waves. The thermal energy can further emit to environment through resonant scattering, induced by magnetic dipole resonance, and further achieving emissivity enhancement. Silicon carbide and aluminum both can support surface wave mode. The first part of this thesis discuss the silicon particle affecting the emissivity of the silicon carbide. The second part changed the substrate material from silicon carbide to aluminum. We used three dimensional finite difference time domain (3D-FDTD) method to simulate the influence of radiation when the silicon particles put on the silicon carbide substrate or aluminum film. In the experimental part, we heated the samples with/without silicon particle to the 70 oC, then measured the cooling time to compare the difference of the emissivity. In the study of silicon carbide substrate, the cooling time of a bare silicon carbide substrate, a silicon carbide substrate with surface coverity 0.845% of silicon particles and a silicon carbide substrate with surface coverity 2.205% of silicon particles were 3203, 1237, and 1104 seconds, respectively. In the study of aluminum film, the cooling time of a bare aluminum film, an aluminum film with surface coverity 2.19% of silicon particles and an aluminum film with surface coverity 5.72% of silicon particles were 4895, 2787, 1432 seconds, respectively. The huge influence of the low surface coverage of silicon particles on the thermal emission of silicon carbide substrate or aluminum film were observed. In addition, in order to confirm that the radiative heat transfer between the silicon particles and the silicon carbide or aluminum film substrate resulted from the near-field thermal radiation, we capped the silicon carbide substrate with a silicon superstrate coated with few silicon particles. Four photoresist pillars were made on the silicon superstrate to control the gap size between superstrate and silicon carbide or aluminum film substrate. When the height of pillars were 3.1, 4.0, 5.2 and 5.9 microns, the cooling time of silicon carbide substrate from 70 oC to the equilibrium room temperature were 1178,1569,1778, and 1937 seconds, respectively. The heat transfer reduced rapidly when the height of pillars was increased. Moreover, we recorded the average emissivity with temperature by infrared camera. At 180 oC, the emissivity of bare silicon carbide was 0.648 and the emissivity of silicon carbide coated surface coverity 2.205% of silicon particles was increased to 0.685. The emissivity enhancement is obviously greater than the surface coverage of silicon particle. When the substrate was replaced by the aluminum film, the emissivity of bare aluminum film was 0.03 and the emissivity of aluminum film with 5.72% silicon particle coverity was increased to 0.05. It’s surprising that emissivity enhancement up to 60% at low coverity of silicon particles. The experimental results revealed that the magnetic dipole resonance of silicon particles could effectively couple with surface wave and scatter electromagnetic energy to the surrounding. In this study, we demonstrated that silicon particles could couple effectively energy with surface wave of polar materials or metallic film within the near field regime. When the energy of surface waves coupled into the silicon particles, the magnetic dipole resonance effect could further scatter the surface wave to far-field. Though the size and shape of silicon particles are not uniform, the phenomenon of thermal emission enhancement is still obvious. Therefore the technique has great potential to apply on advanced cooling engineering in the future.