含Nb超導高熵合金研究

Nb-containing superconductive high-entropy alloys

曹德綱

指導教授 ： 陳瑞凱 ； 林樹均

國立清華大學/工學院/材料科學工程學系/碩士(2012年)

摘要

本研究在探討多元合金是否也具有超導電性，藉此進一步了解高熵合金的物理性質。選擇實驗室已配製過的含Nb多元合金，進行電性量測，有MoNbTaW, MoNbTaVW及NbSiTaTiZr三種，並觀察到在低溫時，NbSiTaTiZr疑似有超導現象；因此對此合金進行成分改變，有NbTaTiZr, HfNbTaTiZr, NbGeTaTiZr, NbSiTaTiZrV, NbGeTaTiZrV, NbSiTaTiZrGe及NbSiTaTiZrGeV等七種，對以上共十種四至七元合金進行電性質、磁性質、霍爾效應等研究。以真空電弧熔煉，製備各種合金；試片經切割及研磨後，進行SEM、EDS、XRD、低溫到室溫(5 K ~ 300 K)電阻的四點量測、磁化量對溫度(2 K ~ 300 K)、磁滯曲線(5 K及300 K)、及「變溫與變磁場之霍爾效應」等量測與分析。
鑄造態MoNbTaW、NbTaTiZr、MoNbTaVW及HfNbTaTiZr的微結構，都是單一的、簡單固溶的「擬一元BCC結構」，且皆分成樹枝相及樹枝間相。其餘六種鑄造態合金內，都含有多結構的多相；但重要的是，含有擬一元BCC結構「富Nb-Ta固溶相」的合金，均具有超導電性。
有零電阻現象的合金及其臨界溫度，分別是NbTaTiZr (8.98 K), HfNbTaTiZr (7.93 K), NbGeTaTiZr (9.16 K), NbSiTaTiZrV (4.99 K), NbGeTaTiZrV (9.10 K)及NbSiTaTiZrGe (8.10 K)；而NbSiTaTiZr在低到約5 K時，電阻開始急遽降下，但因儀器限制，溫度無法再降低，故疑似在低於5 K時，有零電阻現象。
單一BCC結構的MoNbTaW與MoNbTaVW合金，係由BCC結構元素組成的，兩者的電阻率(約22 ~40 μΩ-cm)，較以往量測過的高熵合金電阻率(約100 ~200 μΩ-cm)為小。而七元NbSiTaTiZrGeV合金正常態電阻率(約200 μΩ-cm)是本研究合金正常態中最大的。而所有合金之殘餘電阻比值(RRR =ρ290/ρ10)在1.05 ~ 1.36之間，表示ρ10非溫度項影響因素，大於ρ290溫度項影響因素，符合高熵合金內擁有大量缺陷存在之過往結論。
從外加磁場為1 kOe下的M(T)曲線觀察，在以上電性量測中，具有及疑有超導電性的合金，都出現反磁現象；合金及臨界溫度為NbTaTiZr (7.98 K), HfNbTaTiZr (6.30 K), NbSiTaTiZr (4.92 K), NbSiTaTiZrV (4.73 K), NbGeTaTiZr (8.61 K), NbGeTaTiZrV (6.34 K), NbSiTaTiZrGe (5.94 K)，由此可驗證超導電性存在的現象；而在電性量測中，不發生超導電性的合金，皆看不出反(抗)磁的M(T)曲線。
對5 K下M(H)磁滯曲線進行分析，具有超導電性的合金，磁滯曲線均形成四個象限分布對稱的形狀，顯示為第二類超導體。藉磁滯曲線，可觀察到各合金的臨界磁場值。以Hc1來看，NbTaTiZr (400 Oe), NbSiTaTiZr (400 Oe), NbGeTaTiZr (300 Oe), NbGeTaTiZrV (300 Oe), NbSiTaTiZrGe (100 Oe)及HfNbTaTiZr (< 100 Oe)；而Hc2幾乎都超過1 T (因本實驗中，最大只能量測到1 T)，只有NbSiTaTiZrGe (6 kOe)較小。在300 K下，所有合金的磁滯曲線，都呈超順磁性或軟鐵磁性。
本研究合金5 K與300 K霍爾效應的量測顯示，載子多為「類電洞」，濃度約1022 cm-3，與過往之多元合金結果相似。各合金之載子遷移率在正常態下，比一般純金屬低一至二個數量級，變溫度的霍爾效應量測分析則顯示，隨著溫度漸高，霍爾電阻率也跟著升高。綜合霍爾效應之量測結果，也說明高熵合金的擬一元晶格中，確有大量的缺陷存在。
硬度最小的是NbTaTiZr合金(322 Hv)，加入Hf、Si、Ge等元素後，都使合金硬度上升；加入V後，合金硬度下降；而NbSiTaTiZrGeV合金具有最大的硬度(760 Hv)，推測係其內之相數最多，固溶強化效應顯著所致。

關鍵字

高熵合金 ； 超導

並列摘要

This study aims to explore if the high-entropy alloys (HEAs) also possess superconducting behaviors as conventional metals and alloys possess. By doing this, one may be able to understand further the properties of HEAs. Before systematic investigation, we first selected some known Nb-containing alloy and HEAs, such as MoNbTaW, MoNbTaVW, and NbSiTaTiZr, to check the R(T) behavior down to 5 K; and found the superconducting-like behavior of NbSiTaTiZr near that temperature. Therefore, we continued to design and to prepare NbTaTiZr, HfNbTaTiZr, NbGeTaTiZr, NbSiTaTiZrV, NbGeTatiZrV, NbSiTaTiZrGe, and NbSiTaTiZrGeV alloys; and performed the electrical and magnetic properties, and Hall measurements of these ten 4- to 7-component alloys. Alloy samples were vacuum-arc-remelted, cut, and ground, and then they were performed SEM, EDS, XRD, and 5-300 K 4-probe resistivity (T), 2-300 K magnetization M(T), and 5 K- and 300 K-hysteresis M(H) measurements, as well as varied-temperature and -magnetic field Hall measurement and analysis.
Microstructure of as-cast MoNbTaW, NbTaTiZr, MoNbTaVW, and HfNbTaTiZr are of single, simple, solid-solution, pseudo-unitary BCC with dendrite-interdendrite substructure. Other 6 as-cast alloys contain a structure with multi-structure and multi-phase in them. Of the most important is that all structures that contain Nb-Ta-riched, pseudo-unitary BCC, solid-solution phase possess superconductivity.
Alloys with zero electrical resistance and their corresponding critical temperatures are NbTaTiZr (8.98 K), GeNbTaTiZr (9.16 K), HfNbTaTiZr (7.93 K), NbSiTaTiVZr (4.99 K), GeNbSiTaTiZr (8.10 K), and GeNbTaTiVZr (9.10 K). As to NbSiTaTiZr, it has a drastic drop in resistance near 5 K. Owing to the limitation of measurement that experimental temperature cannot be lowered further; it therefore shows zero-like resistance in the electrical resistance measurements.
Single BCC-structured MoNbTaW and MoNbTaVW alloys are composed of all BCC elements, their electrical resistivity ranges 22~40 μΩ-cm, which is lower than that (100~200 μΩ-cm) of other non-all-BCC multi-component alloys measured as before. The 7-component NbSiTaTiZrGeV alloy has the highest resistivity of ~200 μΩ-cm among the ten alloys in this study. The value of residual resistivity ratio, RRR  290K/10K, is in 1.05 ~ 1.36, manifesting that the non-thermal effect, i.e., lattice defect, is greater than the thermal factor. The latter conclusion is in consistent with the one ever made in the similar experiments.
From M(T) curves at 1 kOe, alloys that have superconductivity or the like in resistivity measurements show definite diamagnetism. These alloys with their corresponding critical temperatures are NbTaTiZr (7.98 K), NbSiTaTiZr (4.92 K), NbSiTaTiVZr (4.73 K), GeNbTaTiZr (8.61 K), GeNbTaTiVZr (6.34 K), GeNbSiTaTiZr (5.94 K), and HfNbTaTiZr (6.30 K). These M(T) experiments demonstrate the existence of superconductivity in these alloys. On the other hand, the alloys without zero resistivity show no diamagnetic behavior in M(T).
Measurements of M(H) hysteresis at 5 K with superconducting alloys show a loop extended in 4 quadrants, demonstrating that these alloys are of the type II superconductors. By M(H), one is able to determine the critical magnetic fields of superconducting alloys. The lower critical magnetic field, Hc1, in alloys and their corresponding values are NbTaTiZr (400 Oe), NbSiTaTiZr (400 Oe), GeNbTaTiZr (300 Oe), GeNbTaTiVZr (300 Oe), GeNbSiTaTiZr (100 Oe), and HfNbTaTiZr (< 100 Oe). The higher critical magnetic field, Hc2, in almost alloys is exceeding 1 T (104 G). Only GeNbSiTaTiZr show small Hc2 (6 kOe). M(H) measurements at 300 K for all ten alloys show superparamagnetism or soft ferromagnetism.
Hall measurements at 5 and 300 K and at 1 T to 9 T for these multi-component alloys demonstrate that most of the carriers are of hole-like, with concentration of 1022 cm-3 that is the same value measured as before. The mobility of the alloys is one or two orders of magnitude less than that of the pure metals. As temperature rises, the Hall resistivity increases. The summary in Hall measurements elucidates that there is a large amount of point defects in the pseudo-unitary lattice of the multi-component alloys.
Alloy that has the lowest hardness (322 Hv) is NbTaTiZr. The hardness value increases after the individual addition of Hf, Si, and Ge in NbTaTiZr, while the V addition in it decreases the hardness of the alloy. The NbSiTaTiZrGeV alloy has the largest hardness value (760 Hv) that is ascribed to its largest number of multi-phases and a significant effect of the solid solution strengthening.

參考文獻

12. 林川翔, 從Al, Co, Cr, Fe, Ni, Ti選取五至六元高熵合金之電與磁性質研究, 國立清華大學材料科學工程研究所碩士論文 (2008).

1. J.W. Yeh, S.K. Chen, S.J. Lin, J. Y. Gan, T. S. Tsau, and S.Y. Chang, Nano structured High-Entropy Alloys with Multiple Principal Elements:Novel Alloy Design Concepts and Outcomes, Advanced Engineering Materials, 6 (2004) 299-303.

7. A. Inoue, T. Zhang, and T. Masumoto, Zr-Al-Ni amorphous alloys with high glass transition temperature and significant supercooled liquid region, Materials Transactions, JIM 31 (1990) 177-183.

8. A. Inoue, T. Zhang, K. Ohba, and T. Shibata, Continuous-Cooling-Transformation (CCT) Curves for Zr-Al-Ni-Cu Supercooled Liquids to Amorphous or Crystalline Phase, Materials Transactions, JIM 36 (1995) 876-878.

9. W. Stevenson, Metals Handbook, 9th ed., p.773, (ASM, Ohio, 1985).