以二氧化釩奈米粒子調變矽化鎂熱電材料之性能;Modulate the Properties of Magnesium Silicide Thermoelectric Materials with Vanadium Dioxide Nanoparticles

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Title:	以二氧化釩奈米粒子調變矽化鎂熱電材料之性能;Modulate the Properties of Magnesium Silicide Thermoelectric Materials with Vanadium Dioxide Nanoparticles
Authors:	藍健華;Lan, Jian-Hua
Contributors:	材料科學與工程研究所
Keywords:	矽化鎂化合物;熱電性質;二氧化釩;火花電漿燒結;Magnesium silicide;Thermoelectric propertie;Vanadium dioxide;Spark plasma sintering
Date:	2020-11-26
Issue Date:	2021-03-18 17:04:02 (UTC+8)
Publisher:	國立中央大學
Abstract:	考量到低成本與環保等優點，本研究選擇了價格低廉且無毒之矽與鎂二元素化合所得矽化鎂(Mg2Si)。鉍的摻雜已被證明有助於提升純矽化鎂之低電導率並且降低熱導率，然而鉍摻雜有其溶解度限制，超過其溶解度便會析出鉍化鎂(Mg3Bi2)。若以上述為基礎並引入奈米結構，例如奈米析出物或奈米複合物，利用增加之大量界面對聲子的散射，能進一步降低熱導率。文獻中常見的奈米複合物多為氧化物，如二氧化矽(SiO2)、二氧化鈦(TiO2)等，而二氧化釩(VO2)經研究指出，其具有68 ℃時金屬－絕緣體相轉變（MIT）的特性，且金屬相二氧化釩具有電導率大幅增加、熱導率卻幾乎不變的優勢。因此本研究嘗試利用二氧化釩作為奈米複合物加入矽化鎂，添加不同比例之二氧化釩奈米粒子，進一步降低其熱導率，而有效提升整體熱電優值(ZT)。本研究以矽、鎂、鉍三種粉末原料依比例秤重混和，使用管型爐在氬氣氣氛下進行固相反應，製備鉍摻雜矽化鎂化合物(Mg2SiBi0.02)。而二氧化釩粉末需經過行星式球磨細化，再依比例秤重經滾動式研磨機充分混合，隨後再研磨至粒徑小於44 ?m後進行火花電漿燒結(SPS)。試片量測的部分以X光繞射分析(XRD)和掃描式電子顯微鏡(SEM)測量並觀察完整試片的組成和微觀結構。利用雷射閃光法熱傳導分析儀(LFA)、阿基米德、ZEM-3和高溫示差掃描熱分析儀(DSC)研究熱電性能，獲得包括熱擴散係數、電導率和Seebeck 係數等參數，經過計算以獲得最終的熱導率和ZT值。本研究在二氧化釩加入3%的部分，擁有常溫下熱電優值ZT = 0.086，已接近未添加之2倍。最終目標是製造高ZT值的矽化鎂化合物，以作為具有高性能轉換效率的熱電材料應用。 ;In this study, magnesium silicide (Mg2Si) synthesized from low-cost, non-toxic silicon and magnesium was selected. The doping of Bi has been proven to increase the conductivity of pure magnesium silicide and reduce the thermal conductivity. However, doping of Bi has its solubility limit, and magnesium bismuthide (Mg3Bi2) will be precipitated if the solubility is exceeded. Adding nanostructures, such as nanoprecipitation or nanocomposite, to the above powders can further reduce thermal conductivity by increasing phonon scattering on the interface. The common nanocomposites in the literature are mostly oxides, such as silicon dioxide (SiO2) and titanium dioxide (TiO2). Vanadium dioxide (VO2) has been studied and pointed out that it has the characteristics of metal-insulator phase transition (MIT) at 68 ℃. Metallic vanadium dioxide has the advantage that the electrical conductivity is greatly increased, but the thermal conductivity is almost unchanged. Therefore, in this work, we tried to use vanadium dioxide as nanocomposite and mixed with magnesium silicide. The addition of vanadium dioxide nanoparticle in different proportions will further reduce its thermal conductivity and effectively increase the figure of merit(ZT). Bi-doped magnesium silicide compounds(Mg2SiBi0.02) were prepared by solid state reaction in tube furnace under argon atmosphere after mixing starting materials with the rolling machine. The vanadium dioxide powder needs to be refined by planetary ball milling. The vanadium dioxide and magnesium silicide powders are weighed in proportions and mixed with the rolling machine. Spark plasma sintering (SPS) was later operated after grinding and sieving for densification. The composition and microstructure of the sample were measured and observed by using X-ray diffraction and SEM, respectively. The thermoelectric properties were studied by Laser flash apparatus (LFA), Archimedes, ZEM-3, and Differential scanning calorimetry (DSC) to obtain the parameter including thermal conductivity, electrical conductivity, and Seebeck coefficient in order to get the figure of merit(ZT). The final result of this study is that the addition of 3% vanadium dioxide has the highest thermoelectric performance ZT = 0.0857 at room temperature, which is twice that without addition.. The final goal is to manufacture magnesium silicide compounds with high ZT values as thermoelectric materials with high-performance conversion efficiency.
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