由于其零排放、高效率的優(yōu)勢(shì),室溫磁制冷技術(shù)被認(rèn)為是新一代的新型制冷技術(shù),可應(yīng)用在在空調(diào)、家用冰箱、冷庫(kù)等儲(chǔ)能技術(shù)中。磁制冷技術(shù)是通過(guò)利用巨磁熱材料中由磁場(chǎng)可以引發(fā)溫度變化的這個(gè)特性來(lái)工作。巨磁熱效應(yīng)起源于材料的磁結(jié)構(gòu)相變,一般來(lái)說(shuō),磁結(jié)構(gòu)相變可以分為一級(jí)磁結(jié)構(gòu)相變和二級(jí)磁性相變。一級(jí)磁結(jié)構(gòu)相變可提供大的磁熵變,但通常伴隨著大的磁滯和熱滯,從而顯著降低了制冷效率。盡管二級(jí)磁性相變沒(méi)有磁滯和熱滯,但磁熵變和絕熱溫變通常還不夠大。因此,最佳的磁熱材料應(yīng)該是在一級(jí)磁結(jié)構(gòu)相變和二級(jí)磁性相變的相變臨界點(diǎn)處附近尋求,這樣可以同時(shí)獲得具有小磁滯和巨磁熵變的材料。在本論文中,系統(tǒng)地研究通過(guò)改變化學(xué)計(jì)量,退火溫度和時(shí)間以及摻雜釩,來(lái)調(diào)控鐵基和錳基合金中的相變和臨界現(xiàn)象。當(dāng)上述參數(shù)得到精確和細(xì)微的調(diào)控之后,可以制備在臨界點(diǎn)附近具有優(yōu)異性能的磁熱材料。首先,可以通過(guò)微調(diào)非化學(xué)計(jì)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比來(lái)調(diào)控從一級(jí)磁結(jié)構(gòu)相變到二級(jí)磁性相變的轉(zhuǎn)變。通過(guò)添加過(guò)量的Fe原子來(lái)獲得接近化學(xué)計(jì)量比的NaZn_(13)型相。當(dāng)x值從0.00增加到0.20時(shí),由于晶格收縮,居里溫度(T_C)從198.6上升到216.6K,磁熵變(|ΔS_M|)在0-2 T時(shí)由18.4降低到8.0 J/(kg·K),在0–5 T時(shí)由22.5降低到13.8 J/(kg·K),這是由于一級(jí)巡游電子轉(zhuǎn)變(IEM)變?yōu)槎?jí)磁相變的原因。它們的有效制冷功率在0-5 T時(shí)從347.6降至為245.9 J/kg,這與Gd_5Si_2Ge_(1.9)Fe_(0.1)的值相當(dāng)。在La_(1-x)Fe_(11.4+x)Si_(1.6)中,誘導(dǎo)從一級(jí)到二級(jí)的轉(zhuǎn)變的主要因素可能是NaZn_(13)相的Fe/La比率。當(dāng)x的值由0.00增加到0.20時(shí),熱滯減小了87%到0.3 K,磁滯減少了90%,在0-5 T時(shí)只有1.4 J/kg;在相變的臨界區(qū)域,x=0.15的材料可獲得巨磁熱效應(yīng)(T_C=205.6 K),在0-2 T時(shí)時(shí),磁熵變?yōu)?1.8 J/(kg·K),磁滯為3.01 J/kg,熱滯為1.9 K。這類相變臨界區(qū)域的材料可望在磁制冷技術(shù)中得到應(yīng)用。其次,系統(tǒng)地研究了Fe_2P型(Mn,Fe)_2(P,Si,B)基材料的微觀結(jié)構(gòu)與磁相變的關(guān)系。在這些合金中,含有Fe_2P型主相和(Fe,Mn)_5Si_3型和Fe_2MnSi型兩種雜相。研究表明,硼在電弧熔化過(guò)程中有助于形成Fe_2P型相。隨著退火溫度從1123 K升高到1423 K,Mn_(1.15)Fe_(0.85)P_(0.55)Si_(0.45)合金的T_C從302.0降低到270.5 K,|ΔS_M|隨退火溫度線性增加,表明一級(jí)相變得到強(qiáng)化。對(duì)于在1423 K下退火不同保溫時(shí)間的Mn_(1.15)Fe_(0.85)P_(0.52)Si_(0.45)B_(0.03)合金,隨著退火時(shí)間的增加,在退火48 h后|ΔS_M|達(dá)到最大值。退火溫度和時(shí)間的差異會(huì)影響合金中Fe_2P型主相的Si含量,從而進(jìn)一步?jīng)Q定了材料的居里溫度、熱滯和一級(jí)磁結(jié)構(gòu)轉(zhuǎn)變的強(qiáng)度。隨后,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退火溫度下,V取代Mn的影響。通過(guò)優(yōu)化退火溫度和V取代量,研究發(fā)現(xiàn)當(dāng)Mn被V取代時(shí),磁熱效應(yīng)得到增強(qiáng)但不會(huì)增大熱滯。V的取代使得晶體結(jié)構(gòu)中的a軸減小和c軸增加,進(jìn)而導(dǎo)致T_C降低。最優(yōu)的組合是1323 K退火的x=0.02的合金,其性能與MnFe_(0.95)P_(0.595)Si_(0.33)B_(0.075)合金的性能相當(dāng),說(shuō)明Mn_(1.2-x)V_xFe_(0.75)P_(0.5)Si_(0.5)合金是在室溫附近很有應(yīng)用前景的磁制冷材料之一。在MnFe_2(P,Si,B)體系中,其大熱滯后會(huì)嚴(yán)重阻礙熱交換效率,因此限制了磁制冷在高頻下的應(yīng)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,由于潛熱降低,用Mn代替V可以進(jìn)一步降低MnFe_2(P,Si,B)體系的熱滯。引入V會(huì)增加由磁場(chǎng)誘導(dǎo)相變溫度的漂移和每分子單位的磁矩。因此,通過(guò)這種方式,接近了臨界相變點(diǎn),達(dá)到了降低熱滯的下降卻不損失巨磁熱效應(yīng)的目的。最后獲得了在1 T下具有超低滯(0.7 K)和2.3 K的巨絕熱溫變的材料,使得這類合金有望成為使用永磁體的商用磁制冷冰箱的候選者。
【學(xué)位單位】：華南理工大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位年份】：2018
【中圖分類】：TB34
【部分圖文】：

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]圖 1-21. 多卡制冷的循環(huán)曲線。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.圖 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)合金晶格參數(shù) a 和 c、c/a 與雜質(zhì)相分?jǐn)?shù)和 V 含量的關(guān)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

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