奈米流體導電性質的實驗與理論探討

Abstract

奈米流體是液體中具有均勻分散且穩定懸浮奈米粒子(粒徑1~100nm)的懸浮液。如要將奈米流體發展成能夠以電力來操控的智慧流體，必須要了解奈米流體的導電性質，但相關文獻不多，且部份實驗結果指出奈米流體的導電率會大幅高於(達兩個量階)基底流體及傳統等效介質理論之預測值，其中機制尚未明瞭。因此本文擬以實驗及理論方法探討奈米流體的導電性質，並期能建立一個簡易的理論模型來預測導電率。 本研究共使用了TiO2、Al2O3及兩種不同粒徑的SiO2奈米顆粒和去離子水及乙二醇(EG)兩種基底流體來配製奈米流體，從實驗結果所歸納之結論有：(1)不同奈米顆粒的加入都會造成奈米流體的導電率增加，且隨著顆粒體積分率的上升，導電率也會隨之上升。(2)隨著體積分率的上升，Al2O3-water奈米流體導電率會大於Al2O3-EG奈米流體者。(3)以SiO2-water奈米流體為例，初始顆粒單體粒徑較小者其導電率的增益大於初始單體粒徑較大者(40nm)。就理論部份言，文獻中現有馬克士威爾(Maxwell)的等效介質理論模式、及Shen等人(2012)所提出包含奈米顆粒電泳及布朗運動效應的模式。我們量測了奈米流體的pH值，發現不同奈米流體的pH值均與基底流體者有甚大差異，因此推斷奈米流體中含有相當數量的H+及伴隨而來的OH-離子。此等離子在電場驅策下會往相反電極方向泳動，產生電流而為導致奈米流體導電率大幅增益的主因。本研究所發展的理論模式除上述離子的泳動外，尚包否一般狀況下較次要的基底流體的電滲效應、Shen等人模式中的奈米顆粒電泳效應、和等效介質效應。該模式能定量地預測TiO2-water奈米流體的導電率，但只能定性地預測Al2O3-water及SiO2-water者(理論與實驗值在同一量階)，不過仍遠優於Shen等人理論所預測者。

Nanofluid is a liquid suspended uniformly and stably with nano-sized (1-100nm) solid particles. If one can control nanofluids via external means, nanofluids can be regard as smart fluids. In order to develop nanofluids into smart fluids by controlling nanofluids via electrical means, one need to know the electrical conductivity of nanofluids. However, the literatures are limited, and it was found that the electrical conductivities of nanofluids are much greater than (up to two orders) that of the base fluid and the prediction under the classical effective medium theory. The underlining mechanisms are still not clearly understood. So, the goal of this thesis is to perform experimental and theoretical studies of electrical properties of the nanofluids, and aim to build up a simple theory for prediction. Four kinds of nanoparticles and two kinds of base fluids were used in this study. The nanoparticles that were TiO2, AL2O3, and two different sizes of SiO2, and the two base fluids were deionized water and ethylene glycol (EG). According to the present experiments, we found: (1) Electrical conductivity of nanofluids with different nanoparticles are greater than those of the base fluids, and increase with volume fraction. (2) Al2O3-water nanofluid has greater electrical conductivity than that of the Al2O3-EG nanofluid. (3) In the case of SiO2-water nanofluids, we found that the nanofluids with smaller size (15nm) has greater electrical conductivity increment than that with larger size (40nm). There are Maxwell’s equivalent media model, and the model by Shen et al. (2012) taking into account the effect of electrophoresis the Brownian motion of nanoparticles. We measured the pH of the nanofluids and found that the pH values of different nanofluids were all very different from those of the basal fluids. Therefore, it was inferred that the nanofluids contained a considerable amount of H and the associated OH- ions. These ions move toward opposite electrodes under an applied electric field, induces current, and is responsible mainly to the large increment of electrical conductivity of nanofluids. The theory developed in this thesis not only includes the ion movement mentioned above, but also includes the electroosmossis of the liquid phase, electrophoresis of suspended particles, and the Maxwell’s effective medium effect. It can predict quantitatively the electrical conductivity of TiO2-water nanofluids within 30% discrepancy, but only qualitatively the electrical conductivity of Al2O3-water and SiO2-water nanofluids (theoretical and experimental values are in the same order), and is far better than that predicted by the model of Shen et al..