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Mn基和Fe置换Ni-Mn-Sn基 Heusler合金 晶体结构、磁与磁热性能研究
论文作者:童鞋论文网  论文来源:www.txlunwenw.com  发布时间:2019/9/20 8:40:52  

摘要:磁制冷技术是基于磁热效应的新型制冷技术。传统制冷技术的制冷效应是通过氟氯昂等气体的压缩-膨胀循环实现的,但氟氯昂的使用会造成臭氧层孔洞和温室效应,并且气体压缩制冷的工作效率较低。随着能源危机和全球变暖的加剧,传统制冷工业面临的主要挑战为降低能源消耗和减少有害气体排放。这促使了新型制冷技术的研究。磁制冷被认为是一种有希望取代传统气体压缩制冷的技术。相比传统技术,磁制冷设备不使用有害气体,仅用固体材料,工作效率更高。对于室温磁制冷设备,需要磁热材料在室温附近具有较大的磁热效应,没有磁滞和热滞,同时组成元素无毒无害,价格相对低廉。

Heusler合金及化合物因为形状记忆效应和磁热效应而引起广泛关注。这类化合物的性能受价电子浓度影响,可以通过元素掺杂和置换获得期望的性能,因而在磁制冷领域拥有巨大的应用前景。

本文选用Mn基Heusler合金和Ni-Mn-Fe-Sn基Heusler合金为研究对象。通过电弧熔炼和后续热处理制备样品,其中Mn基Heusler合金使用甩带法制成薄带形状。使用X射线衍射仪和振动样品磁强计等分析仪器对不同样品的晶体结构、磁及磁热性能进行了测试,并将实验结果与理论模拟结果进行了对比,分析了合金成分对材料晶体结构、磁和磁制冷性能的影响。

对Mn2-xSn0.5Ga0.5 Heusler合金的研究发现,合金在常温下为六方结构。所有样品在室温附近仅发生一次二阶磁性转变,无明显磁滞和热滞。Mn2-xSn0.5Ga0.5的居里温度和饱和磁化强度对合金中Mn的含量非常敏感,随着Mn含量的升高,合金的居里温度和饱和磁化强度均出现下降,分别由Mn1.2Sn0.5Ga0.5的304 K和64.1 emu/g降至Mn2Sn0.5Ga0.5的262 K和46.7 emu/g,这表明合金中的磁矩呈亚铁磁形态分布。所有样品的磁熵变都比较小,Mn1.5Sn0.5Ga0.5在30 kOe外场下的最大磁熵变为1.7 J/kgK,但是具有与其他非稀土二阶相变材料相当的相对制冷能力,Mn1.2Sn0.5Ga0.5在30 kOe外场下的相对制冷能力为121.1 J/kg。

对Ni43Mn46-xFexSn11 Heusler合金的研究发现,该系合金在室温下为立方结构,晶体内存在A-2型原子无序,在含有Fe的样品中存在γ第二相。随着Mn不断被置换为Fe,合金中的马氏体相变温度由Ni43Mn46Sn11的223 K降低到Ni43Mn38Fe8Sn11的115 K,当x值达到10时,马氏体相变被完全抑制。合金的居里温度和饱和磁化强度随Fe含量的上升而上升,分别由Ni43Mn40Fe6Sn11的300 K和42.3 emu/g升至Ni43Mn36Fe10Sn11的325 K和72.4 emu/g。这表明合金中磁矩呈反铁磁性的Mn被Fe置换,强化了Fe-Mn间的铁磁性相互作用。Ni43Mn46-xFexSn11矫顽力较小,均为软磁性材料。仅发生一次二阶相变的Ni43Mn36Fe10Sn11的磁熵变较小,为2.0 J/kgK,相对制冷能力为129.0 J/kg。

由于没有磁滞和热滞,室温附近较大的工作温度区间以及比其他不含稀土元素的二阶相变材料具有更高的相对制冷能力,因此,这两类材料在磁制冷领域具有较大的应用前景。

Magnetic refrigeration is a new coolingtechnology based on magnetocaloric effect. In a conventional refrigerationdevice, the cooling process is achieved by employing vapor-condense cycles ofsome fluids. However, the utilization of these fluids is the main response tothe ozone layer depletion and global warming. Also, their working efficiency isrelatively low. With the growing concern about energy scarcity and globalwarming, the main challenge to the current refrigeration industry is thereduction of energy consumption and harmful gas emission. All these stimulatethe development of new refrigeration technology. Magnetic refrigeration is seenas a promising alternative to the gas-compression cooling technology. For theapplication on the functional room temperature magnetic refrigeration devices,the materials should exhibit large magnetocaloric effect occurring near ambienttemperature with large magnetic entropy and adiabatic temperature change. Also,the material should be hysteresis-free which will destroy the lifespan andworking efficiency of the substance and constitute with non-toxic and feasibleelements.

Heusler alloys and compounds draw recentattention due to their shape memory effect and magnetocaloric effect. Theproperties of these materials are dominated by the valence electronconcentration. So, the properties can be adjusted by elemental substitution anddoping as desired. They are seen as promising candidates for room temperaturemagnetic refrigeration.

Mn-based Heusler alloys and Fe substitutedNi-Mn-Sn Heusler alloys have been researched in this paper. The samples weresynthesized by arc-melting and subsequent heat treatment. For the Mn-basedHeusler alloys, they were also made in ribbon form by melt-spinning. Theeffects of the elemental composition on the crystal structures, magnetic andmagnetocaloric properties have been researched.

It has been found that Mn2-xSn0.5Ga0.5crystalized in hexagonal structure at the room temperature. Generally, thehexagonal structure in Mn2Ga-type alloys is not stable and the results indicatethat partial substitution of Ga with Sn can make the hexagonal structure morestable. All samples undergo only second order phase transition at their Curietemperature near room temperature with nearly zero hysteresis. The Curietemperature and the saturation magnetization of the Mn2-xSn0.5Ga0.5 are verysensitive to the composition where both Curie temperature and saturationmagnetization decrease with the increase in Mn concentration, from 304 K and64.1 emu/g to 262 K and 46.7 emu/g for Mn1.2Sn0.5Ga0.5 and Mn2Sn0.5Ga0.5,respectively, which indicates the ferrimagnetic order in these materials. Themagnetic entropy changes are relatively small with maximum value of 1.7 J/kgKat 30 kOe for Mn1.5Sn0.5Ga0.5, but the relative cooling power are comparable toother non-rare-earth materials showing second-order phase transition, whereMn1.2Sn0.5Ga0.5 shows a highest RCP of 102.3 J/kg at 30 kOe. Higher magnetic entropychange may be obtained by doping suitable elements.

Ni43Mn46-xFexSn11 is mainly cubic instructure with A-2 type atomic disorder. A partial substitution of Fe for Mnsignificantly changed the structure and properties of the parent alloyNi43Mn46Sn11. The temperature of the martensitic phase transition decreasedwith the increase in Fe concentration and when x reaches 10, the austenitebecomes stable. Both Curie temperature and saturation magnetization increasewith increase in Fe concertation, from 300 K and 42.3emu/g to 325 K and 72.4emu/g for Ni43Mn40Fe6Sn11 and Ni43Mn36Fe10Sn11, respectively, which areattribute to the enhancement of Fe-Mn ferromagnetic interaction which resultfrom the substitution of antiferromagnetic Mn atoms with Fe. The coercivitiesof Ni43Mn46-xFexSn11 are small and show soft magnetic behavior. The maximumvalues of magnetic entropy change and RCP of Ni43Mn36Fe10Sn11 which showssingle second order phase transition are 2.0 J/kgK a.

关键词:Heusler合金;磁热效应;磁制冷;磁熵变;相对制冷能力

Heusler alloys;magnetocaloriceffect;magnetic refrigeration;magneticentropy change; relative cooling power

上一篇:低氧压铁基合金选择性氧化及相关相平衡研究     下一篇:TWIP钢选择性氧化及Fe-Si-Mn-Sb部分相关系的研究
 
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