铁基和锰基磁热材料中相变和临界点研究
发布时间：2020-11-13 19:13
由于其零排放、高效率的优势,室温磁制冷技术被认为是新一代的新型制冷技术,可应用在在空调、家用冰箱、冷库等储能技术中。磁制冷技术是通过利用巨磁热材料中由磁场可以引发温度变化的这个特性来工作。巨磁热效应起源于材料的磁结构相变,一般来说,磁结构相变可以分为一级磁结构相变和二级磁性相变。一级磁结构相变可提供大的磁熵变,但通常伴随着大的磁滞和热滞,从而显着降低了制冷效率。尽管二级磁性相变没有磁滞和热滞,但磁熵变和绝热温变通常还不够大。因此,最佳的磁热材料应该是在一级磁结构相变和二级磁性相变的相变临界点处附近寻求,这样可以同时获得具有小磁滞和巨磁熵变的材料。在本论文中,系统地研究通过改变化学计量,退火温度和时间以及掺杂钒,来调控铁基和锰基合金中的相变和临界现象。当上述参数得到精确和细微的调控之后,可以制备在临界点附近具有优异性能的磁热材料。首先,可以通过微调非化学计量La_(1-x)Fe_(11.4+x)Si_(1.6)(x=0,0.05,0.10,0.15和0.20)合金中的Fe/La比来调控从一级磁结构相变到二级磁性相变的转变。通过添加过量的Fe原子来获得接近化学计量比的NaZn_(13)型相。当x值从0.00增加到0.20时,由于晶格收缩,居里温度(T_C)从198.6上升到216.6K,磁熵变(|ΔS_M|)在0-2 T时由18.4降低到8.0 J/(kg·K),在0–5 T时由22.5降低到13.8 J/(kg·K),这是由于一级巡游电子转变(IEM)变为二级磁相变的原因。它们的有效制冷功率在0-5 T时从347.6降至为245.9 J/kg,这与Gd_5Si_2Ge_(1.9)Fe_(0.1)的值相当。在La_(1-x)Fe_(11.4+x)Si_(1.6)中,诱导从一级到二级的转变的主要因素可能是NaZn_(13)相的Fe/La比率。当x的值由0.00增加到0.20时,热滞减小了87%到0.3 K,磁滞减少了90%,在0-5 T时只有1.4 J/kg;在相变的临界区域,x=0.15的材料可获得巨磁热效应(T_C=205.6 K),在0-2 T时时,磁熵变为11.8 J/(kg·K),磁滞为3.01 J/kg,热滞为1.9 K。这类相变临界区域的材料可望在磁制冷技术中得到应用。其次,系统地研究了Fe_2P型(Mn,Fe)_2(P,Si,B)基材料的微观结构与磁相变的关系。在这些合金中,含有Fe_2P型主相和(Fe,Mn)_5Si_3型和Fe_2MnSi型两种杂相。研究表明,硼在电弧熔化过程中有助于形成Fe_2P型相。随着退火温度从1123 K升高到1423 K,Mn_(1.15)Fe_(0.85)P_(0.55)Si_(0.45)合金的T_C从302.0降低到270.5 K,|ΔS_M|随退火温度线性增加,表明一级相变得到强化。对于在1423 K下退火不同保温时间的Mn_(1.15)Fe_(0.85)P_(0.52)Si_(0.45)B_(0.03)合金,随着退火时间的增加,在退火48 h后|ΔS_M|达到最大值。退火温度和时间的差异会影响合金中Fe_2P型主相的Si含量,从而进一步决定了材料的居里温度、热滞和一级磁结构转变的强度。随后,研究了Mn_(1.2-x)V_xFe_(0.75)P_(0.5)Si_(0.5)(x=0.00,0.01,0.02,0.03,0.04,0.05)合金在1323,1373和1423 K退火温度下,V取代Mn的影响。通过优化退火温度和V取代量,研究发现当Mn被V取代时,磁热效应得到增强但不会增大热滞。V的取代使得晶体结构中的a轴减小和c轴增加,进而导致T_C降低。最优的组合是1323 K退火的x=0.02的合金,其性能与MnFe_(0.95)P_(0.595)Si_(0.33)B_(0.075)合金的性能相当,说明Mn_(1.2-x)V_xFe_(0.75)P_(0.5)Si_(0.5)合金是在室温附近很有应用前景的磁制冷材料之一。在MnFe_2(P,Si,B)体系中,其大热滞后会严重阻碍热交换效率,因此限制了磁制冷在高频下的应用。最后,研究表明在Mn_(1-x)V_xFe_(0.95)P_(0.593)Si_(0.33)B_(0.077)和Mn_(1-x)V_xFe_(0.95)P_(0.563)Si_(0.36)B_(0.077)合金中,由于潜热降低,用Mn代替V可以进一步降低MnFe_2(P,Si,B)体系的热滞。引入V会增加由磁场诱导相变温度的漂移和每分子单位的磁矩。因此,通过这种方式,接近了临界相变点,达到了降低热滞的下降却不损失巨磁热效应的目的。最后获得了在1 T下具有超低滞(0.7 K)和2.3 K的巨绝热温变的材料,使得这类合金有望成为使用永磁体的商用磁制冷冰箱的候选者。
【学位单位】：华南理工大学
【学位级别】：博士
【学位年份】：2018
【中图分类】：TB34
【部分图文】：
Ph.D Dissertation of South China University of Technology( )ThotTcoldRC S T dT (1here Thotand Tcoldcorrespond to the temperature range at which the |ΔSM| value is hpeak value. However, equation (1.13) concerns materials with second order which h reversible |ΔSM| and |ΔTad| value.
Figure 1-21. Multicaloric refrigeration cycle.[142]图 1-21. 多卡制冷的循环曲线。n the past decades, the iron-based amorphous alloys have been extensively studse of their high electrical resistivity, good mechanical properties, broad |ΔSM| peak, land tunable TCaround room temperature.[135,136]These alloys have a relatively labecause of a SOMT. However, the ΔTadvalue is relative low compared to other Mals. Fe-Cr-B-(La, Ce) ribbons were investigated and ΔTadvalue of 1.4 K was shoa magnetic field change of 1.8 T.[137,138]Future application of the iron-based amorph are still controversial. The MCE in the iron-based amorphous alloys needs toved significantly through some new preparation method or chemical change.e-Rh alloys show a giant FOMT from a ferromagnetic state to a antiferromagnetic s
68Figure 4-2. Relationship between the lattice parameters a and c, c/a, and the phase fraction ofimpurity and the V content of Mn1.2-xVxFe0.75P0.5Si0.5(x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) alloysafter annealing at 1323, 1373 and 1423K.图 4-2. 在 1323, 1373和 1423K 退火后的 Mn1.2-xVxFe0.75P0.5Si0.5(x=0.00, 0.01, 0.02, 0.03,0.04, 0.05)合金晶格参数 a 和 c、c/a 与杂质相分数和 V 含量的关系。4.4 Room temperature neutron diffraction and atom siteoccupancy change of Mn1.2-xVxFe0.75P0.5Si0.5alloysFor the Mn-Fe-P-Si alloy, it is reported that the Fe atoms preferentially occupythe 3f site, the Mn atoms the 3g site, the P atoms and Si atoms the 2c or 1b sites andthe Si atoms have a high preference for the 2c site.[174]X-ray absorption and powder
本文编号：2882529
【学位单位】：华南理工大学
【学位级别】：博士
【学位年份】：2018
【中图分类】：TB34
【部分图文】：
Ph.D Dissertation of South China University of Technology( )ThotTcoldRC S T dT (1here Thotand Tcoldcorrespond to the temperature range at which the |ΔSM| value is hpeak value. However, equation (1.13) concerns materials with second order which h reversible |ΔSM| and |ΔTad| value.
Figure 1-21. Multicaloric refrigeration cycle.[142]图 1-21. 多卡制冷的循环曲线。n the past decades, the iron-based amorphous alloys have been extensively studse of their high electrical resistivity, good mechanical properties, broad |ΔSM| peak, land tunable TCaround room temperature.[135,136]These alloys have a relatively labecause of a SOMT. However, the ΔTadvalue is relative low compared to other Mals. Fe-Cr-B-(La, Ce) ribbons were investigated and ΔTadvalue of 1.4 K was shoa magnetic field change of 1.8 T.[137,138]Future application of the iron-based amorph are still controversial. The MCE in the iron-based amorphous alloys needs toved significantly through some new preparation method or chemical change.e-Rh alloys show a giant FOMT from a ferromagnetic state to a antiferromagnetic s
68Figure 4-2. Relationship between the lattice parameters a and c, c/a, and the phase fraction ofimpurity and the V content of Mn1.2-xVxFe0.75P0.5Si0.5(x = 0.00, 0.01, 0.02, 0.03, 0.04, 0.05) alloysafter annealing at 1323, 1373 and 1423K.图 4-2. 在 1323, 1373和 1423K 退火后的 Mn1.2-xVxFe0.75P0.5Si0.5(x=0.00, 0.01, 0.02, 0.03,0.04, 0.05)合金晶格参数 a 和 c、c/a 与杂质相分数和 V 含量的关系。4.4 Room temperature neutron diffraction and atom siteoccupancy change of Mn1.2-xVxFe0.75P0.5Si0.5alloysFor the Mn-Fe-P-Si alloy, it is reported that the Fe atoms preferentially occupythe 3f site, the Mn atoms the 3g site, the P atoms and Si atoms the 2c or 1b sites andthe Si atoms have a high preference for the 2c site.[174]X-ray absorption and powder
本文编号：2882529
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