高熵合金在兆赫波段之導電率和光學常數探討及其應用

Abstract

金屬材料對人類的發展一直都扮演著不可或缺的角色。在過去為了提升金屬的某些特性，我們以一種主要金屬，再添加少量的元素改變其原來性質，使之優化並可應用在更多更廣的領域。隨著添加越多難易度也越高，甚至合金也越容易脆化。近年來，由於高熵合金的出現，更開啟了設計新材料的方向。 「熵」在物理學裡指亂度，在一主要金屬裡混合越多元素意指亂度越高，一般定義由五種或五種以上的元素所構成的合金亦可稱為高熵合金。除了可以合成我們一般所認知的塊材外，亦能以固溶基體的方式呈現。由於至今精進的技術，使之劃分為兩代；第一代屬於各元素約等原子比，第二代則是元素間非等原子比，兩代差異在於後者可以提供更優越的性能。不同元素、不同比例、不同尺度下的高熵反應會造成有不同的物理特性，例如：磁特性、高強度、高硬度、耐磨性、耐高溫、耐腐蝕…等。 現今在兆赫波光譜可快速得到很多光學資訊。例如：折射率、光電導率、電子遷移率…等。高熵合金有千變萬化的可能性，為了方便提升量測效率，這次想藉由THz來探討此材料的特性。THz量測的好處在於非接觸性、光子能量低(4.1meV)且快速。對於分部不均勻的高熵合金來說，傳統接觸性的量測方法，可能在量測過程中破壞材料結構，因此可以透過此非接觸性的方法量測。由於THz對金屬有很強的反射，所以本論文是用奈米等級的NbMoTaW薄膜，使用DC磁控濺鍍的方式鍍在高阻質的矽基板上作為樣品，針對不同的厚度去做量測。再利THz打在此金屬薄膜上，看訊號相位、振幅的變化，經運算成電導率、穿透率與探討其性質。Metallic materials always play an important role to our life. In the past, we have tried to enhance the certain characteristics of the metal, so we would have used one of the main metal with smaller amounts of other elements to change its original properties for more applications in many fields. However, the experiments showed that the higher mixing make higher difficulty, even make the materials easier to become brittle. In recent years, there is a novel material called High-Entropy Alloys(HEAs) that has opened up the whole new developments to the world. In physics, Entropy indicated some kinds of chaos. As more other elements increase, the chaos of the principal element becomes more confusing. HEAs is defined as five or more than five elements, but widely it can also be three or four principal elements. HEAs can not only synthesize bulks but also synthesize thin films. Due to advanced technology, there are two types of HEAs. One is that each element has an equal atomic ratio, the other is unequal. The difference between them that is the latter provides the higher capabilities. Different kinds of elements,proportions, scales can produce different attributes. For instance, high density,corrosion resistance, high temperature resistance, wear resistance, high toughness, magnetic properties, formability…etc. Nowadays, many optical information can be obtained quickly in THz spectroscopy. For instance, refractive index, photoconductivity, mobility…etc. High entropy alloys come in a variety of types. In order to improve the measurement efficiency, THz is used to explore the properties of this HEAs. The advantage of THz measurement are non-contact, low photon energy(4.1meV) and even more fast. For non-uniform HEAs, the traditional contact measurement method may destroy the material structure in the measurement process, so it can be measured by THz TDS. Even though THz has a strong reflection on metals, in this paper we used nano-scale NbMoTaW film with DC magnetron sputtering method to deposit on high resistivity silicon substrate as a sample, and to measure different thickness. And then the THz wave is passed through the sample by normal incident. We can see the change of phase and amplitude of the signal. Finally take the data and analyze. The conductivity, transmittance and properties of the signal are calculated.