含鈮鋯鈦多元合金超導性質研究

Superconductivity in NbZrTi-bearing Multi-element Alloys

劉宗翰(Tsung-Han Liu)

指導教授 ： 陳瑞凱 ； 吳振名

國立清華大學/工學院/材料科學工程學系/碩士(2014年)

摘要

本研究探討以非等莫耳Nb-Zr-Ti，少量添加Hf、V、Ta、Ge，所得之三元至六元鑄造態與熱處理態合金(以下簡稱本合金)的超導特性。
本合金主要為單一無序富Nb的BCC固溶體，元素間(尤其是Ta與Ge)的結合焓差異及熱處理驅動力，促使富Nb的BCC固溶體析出Zr，而形成富Zr(Ge)相，導致在富Nb的BCC固溶體相的Nb/Zr比例因而改變，從而影響本合金的超導特性。
本合金臨界溫度Tc介於8 K至11 K之間。鑄造態試片的室溫電阻率介於21 μΩ-cm至35 μΩ-cm之間；與其他的多元合金相比，本合金有較低的電阻率。由殘餘電阻率比值RRR大小在1.2至1.3之間得知，電阻率主要由合金內部的雜質原子所主導。若單純以Nb加 Zr的e/a值討論，本合金Tc值大致符合二元合金的Matthias經驗法則；然而影響Tc值的其他因素，尚有多元添加所造成的晶格扭曲及本合金個別元素的特性。
本合金為典型的type II超導體，上臨界磁場Hc2的估計值，介於5 T至9 T之間。在2 K之溫度下，及5 T外加磁場的臨界電流密度Jc值，仍高達105 A/cm2；此特性與合金中的析出相有關，而於元素添加造成的晶格缺陷無關。

關鍵字

第二類超導體 ； 鈮鋯鈦多元合金 ； 超導臨界溫度 ； 超導臨界磁場 ； 超導臨界電流密度

並列摘要

This study investigates the superconductivity of as-cast and as-heat-treated ternary to 6-element multi-component alloys (hereafter abbreviated as the alloys) that are made of non-equal molar Nb-Zr-Ti by addition of minor Hf, V, Ta and Ge.
The alloys are principally single random BCC Nb-rich solid solutions. Difference in formation enthalpies between elements, especially for pairs with Ta and Ge, and the driving forces by heat treatments induce a Zr-bearing precipitation from the Nb-rich BCC phase to form a Zr(Ge)-rich phase. This results in a change of Nb/Zr ratio in the Nb-rich BCC phase, and thus affects the superconductivity of the alloys.
The critical temperature of the alloys, Tc, ranges from 8 K to 11 K. The room-temperature resistivity of the as-cast alloys varies from 21 μΩ–cm to 35 μΩ–cm. Compared with the electrical resistivity of other multi-component ones, the alloys have a lower electrical resistivity. The residual resistivity ratio RRR value is from 1.2 to 1.3, which mentions that the resistivity is principally controlled by the impurity atoms in the alloys. If one simply emphasizes the e/a ratio from the content of Nb and Zr in the alloys, the Tc of the alloys approximately follows the Matthias empirical rule. However, factors affecting Tc, besides the e/a, include lattice distortion due to multiple element addition, and the characteristics of individual elements in the alloys.
The alloys are typically type II superconductors. The upper critical magnetic field Hc2 is estimated to be in the range of 5 T to 9 T. At 2 K & 5 T, the critical current density Jc has the value of approximately 105 A/cm2. This property has something to do with the precipitates and has yet nothing to do with the lattice distortion of the alloys.

參考文獻

[1] 曹德綱，國立清華大學材料科學工程學系碩士論文 (2012)。

[2] 吳克元，國立清華大學材料科學工程學系碩士論文 (2013)。

[4] C. Kittel and P. McEuen, Introduction to Solid State Physics, Wiley (New York, 1986) 75, 154-158, 269-305.

[5] 林川翔，國立清華大學材料科學工程學系碩士論文 (2008)。

[7] G.T. Dyos and T. Farrell, Electrical Resistivity Handbook, IET (London, 1992).