赫斯勒鐵磁形狀記憶合金之磁特性與磁性相變化之研究

本論文首先研究Mn-Ni-Sn系列鐵磁形狀記憶合金，探討改變晶格常數及價電子濃度對其馬氏體相變溫度(TM)和磁卡效應(MCE)的影響。首先，在Ni48Mn39Sn13-xGex(x=0-3)及Mn49Ni41Sn10-xGex (x=0-2)薄帶中，研究Ge替代Sn對其TM及磁卡效應的影響。結果顯示，對高Mn含量(Mn>45at%)之樣品，Ge原子會與Mn原子產生析出相而阻礙馬氏體相變，使得合金薄帶之ΔSMmax.大幅衰退。接著研究Co置換Mn，即研究價電子濃度變化對Mn49-xCoxNi41Sn10 (x=0-4)薄帶磁卡效應之影響。隨著Co添加量x≥1.5時，改善了TM附近馬氏體與奧氏體之間的磁化量差(∆M)，因此可以在不同應用溫區中獲得在10 kOe場下量測平均磁熵變為12.5 J/kg·K的優異MCE特性。另一方面， Fe-Mn基系列合金亦是一種具備豐富物理特性的磁性材料。在此部分，藉由改變Fe-Mn-Ga三元之間成分，來研究成份對相組成及磁特性的影響。當薄帶成分為間Fe50Mn25Ga25時，其FCC相在低溫下具有類自旋玻璃的磁性態，透過磁場冷卻會呈現巨交換偏置(Exchange bias)效應。在5 K下，其交換偏置場(HEB)可達2.5 kOe 而 iHc則高達11 kOe。於本研究中，亦將在銅輪轉速15 m/s下製備的Fe-Mn-Ga三元不同比例的薄帶繪成三元相圖，統整其成分差異所導致的相組成關係。當Fe原子含量大於50 at%，且Mn原子含量小於25 at%時，其相傾向形成奧氏體的B2相；當Mn原子含量大於25 at%時，樣品則傾向形成FCC相。本研究中也嘗試將B2相的鐵磁轉順磁行為應用於MCE上。研究Ni取代Fe對Fe50-xNixMn25Ga25(x=0-7)合金薄帶磁性與磁卡效應之影響。結果顯示隨著Ni含量的提升，結構從FCC相轉變為B2相，且0-30 kOe量測平均磁熵變為1.5 J/kg·K。此合金操作溫度範圍相當大約可達90 K，是相當適合在室溫附近應用的磁製冷材料。關鍵字：赫斯特合金、磁製冷材料、磁卡效應、交換偏壓

關鍵字

磁卡效應；磁製冷材料；赫斯特合金；交換偏壓

並列摘要

The effects of lattice constant and valence electron concentration e/a (electrons per atom) on the magnetocaloric effects of Ni–Mn-Sn Heusler alloys are invetigated. At first, the effect of Ge substitution for Sn on phase evolution and magnetocaloric effects (MCEs) of Ni48Mn39Sn13-xGex (x=0-3) and Mn49Ni41Sn10-xGex (x=0-2) ribbons has been studied. When Mn content is over 45 at%, the Ge atoms prefer to form a Mn-rich phase with Mn atoms. This Mn-rich phase might suppress the martensitic transition of the alloy ribbons, resulting in the decrease of maximal magnetic entropy change, ΔSMmax. Secondly, for Co-substituted Mn49-xCoxNi41Sn10 (x=0-4) ribbons, the ferromagnetic behavior of austenite is improved with incresing Co content originated from the enhancement of ferromagnetic exchange interaction. For these series samples, an average ΔSMmax value of 12.5 J/kg·K is easily obtained in different operation temperature region. On the other hand, the Fe-Mn based ferromagnetic shape memory alloys (FSMAs) is also an interesting functional material to study. In order to observe the relationship of exchange bias (EB) effect with phase composition, tunning the composition of Fe-Mn-Ga ternary alloy is nessessary. For Fe50Mn25Ga25 ribbon, the results showed that if its FCC phase undergoes field cooling, giant EB properties could be observed. The HEB=2.5 kOe and iHc=11 kOe were obtained at 5 K. Furthermore, if the content of Fe atoms is over 50 at% and Mn atoms lower than 25 at%, ribbons are perfer to form a B2 phase. In contrast, if Mn content is higher than 25at%, the ribbons are perfer to form a pure L12 phase (FCC). Next, the effect of Ni substitution for Fe on the crystal structure, magnetic state and magnetocaloric effect of the Fe-Ni-Mn-Ga Heusler alloy ribbons is also presented. Results show that with the increase of Ni content, the phase changed from FCC structure into B2 structure. An average ΔSMmax value is 1.5 Jkg/K in these samples. A relatively broader temperature range at the half maximum of ΔS¬M peak (~90 K), low-cost and nontoxic elements make Fe-Ni-Mn-Ga-based ribbons the promising candidates for magnetic refrigeration applications close to room temperature. Keywords: Heusler alloys, Magnetic refrigerant materials, Magnetocaloric effect and Exchange bias.

並列關鍵字

Magnetocaloric effect ； Magnetic refrigerant materials ； Heusler alloys ； Exchange bias

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赫斯勒鐵磁形狀記憶合金之磁特性與磁性相變化之研究

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