磁热效应材料的研究进展

磁制冷技术的发展取决于具有大磁热效应磁制冷材料的研发进展.经过长期的工作积累,特别是近20年来的努力,许多新型磁制冷材料的探索和研究极大地促进了磁制冷技术的进步.本文介绍了磁热效应的基本原理和磁制冷研究的发展历史,系统综述了低温区和室温区具有大磁热效应的磁制冷材料的研究进展,重点介绍了一些受到较为关注的磁热效应材料的最新研究成果.低温区磁制冷材料主要包括具有低温相变的二元稀土基金属间化合物(RGa,RNi,RZn,RSi,R_3Co以及R_(12)Co_7)、稀土-过渡金属-主族金属三元化合物(RTSi,RTAl,RT_2Si_2,RCo_2B_2,RCo_3B_2)以及四元化合物RT_2B_2C等,其中R代表稀土元素,T代表过渡金属.这些材料一般都具有二级相变,具有良好的热、磁可逆性,也因其合金属性具有良好的导热性.室温区磁制冷材料主要包括Gd-Si-Ge,La-Fe-Si,Mn As基,Mn基Husler合金,Mn基反钙钛矿,Mn-Co-Ge,Fe-Rh以及钙钛矿氧化物等系列.这些材料一般都具有一级相变,多数在室温具有巨大的磁热效应而受到国内外的极大关注.其中,La-Fe-Si系列是国际上普遍认为具有重要应用前景的磁制冷工质之一,也是我国具有自主知识产权的材料.本文还对磁制冷材料的发展方向进行了展望.     

钆硅锗确有巨磁热效应

室温磁制冷在近几年取得重要进展,尤其是造出了高性能的样机和发现了巨磁热材料,但近来因为巨磁熵变发生在一级相变点,在磁熵变的计算上国际诸研究小组出现了分歧,本文对此进行了详细的讨论,给出了统一的处理方法,结果表明钆硅锗的确具有巨磁热效应。     

磁热效应演示装置的设计与制作

介绍了磁热效应演示装置的工作原理及设计方案,将磁热材料Mn1.28Fe0.67P0.48Si0.54作为磁工质对样机进行测试,实验结果与文献值相比基本一致.通过重复测试验证了演示仪的可靠性.该装置因其结构简单、便于操作、测量用时短、成本低廉等优点,为磁热效应的研究提供了补充测量手段.     

Eu_(0.9)M_(0.1)TiO_3(M=Ca,Sr,Ba,La,Ce,Sm)的磁性和磁热效应

EuTi0_3是直接带隙半导体材料,在液氦温度附近呈现反铁磁性,且具有较大的磁熵变,但是当其转变为铁磁性时,可以有效提高低磁场下的磁熵变.本文通过元素替代,研究晶格常数的变化和电子掺杂对磁性和磁热效应的影响.实验采用溶胶凝胶法制备EuTiO_3和Eu_(0.9)M_(0.1)TiO_3 (M=Ca, Sr, Ba, La, Ce, Sm)系列样品.结果表明:大离子半径的碱土金属离子替代提高了铁磁性耦合,有利于提高低磁场下的磁热效应.电子掺杂可以抑制其反铁磁性耦合从而使其表现为铁磁性.当大离子半径的稀土La和Ce离子替代Eu离子时,既增大了晶格常数也实现了电子掺杂,表现出较强的铁磁性.在1 T的磁场变化下,Eu_(0.9)La_(0.1)TiO_3和Eu_(0.9)Ce_(0.1)TiO_3的最大磁熵变分别为10.8和11 J/(kg·K),均大于EuTi0_3的9.8 J/(kg·K);制冷能力分别为39.3和51.8 J/kg,相对于EuTi0_3也有所提高.     

La_(2/3/)Sr_(1/3)MnO_3单晶的低场磁热效应

本文对La2/3Sr1/3MnO3单晶中b和a(c)轴方向的磁热效应进行了研究,由于晶体的磁晶各向异性,磁热效应表现出一定的各向异性.通过不同温度下的等温起始磁化曲线的实验分析表明,在外加场为10kOe的情况下,b和a(c)轴方向的磁熵变-ΔSM(H)在370K达到最大值,分别为1.942Jkg-1K-1和1.873Jkg-1K-1.低场下较大的磁熵变是由于磁化强度随外场迅速变化以及自旋-晶格的耦合造成的.La2/3Sr1/3MnO3单晶在低场下表现出相对较大的磁热效应,表明其在磁制冷方面有一定的潜在应用价值.     

一级磁结构相变材料Mn0.6Fe0.4NiSi0.5Ge0.5和Ni50Mn34Co2Sn14的磁热效应与磁场的线性相关性

磁熵变（△S_M）与磁场（μ₀H）的相关性已在很多二级相变材料中被研究并报道,但一级相变材料的磁热效应与磁场相关性还少有报道.本文在具有一级磁结构相变的Mn_0.6Fe_0.4NiSi_0.5Ge_0.5材料中研究发现△S_M与μ₀H存在线性相关性,并通过麦克斯韦关系式的数值分析详细讨论了这一线性相关性的来源.同时,进一步发现在低磁场时,△S_M近似正比于μ₀H的平方.该线性相关性同样在一级磁结构相变Ni₅₀Mn₃₄Co₂Sn₁₄材料中得到了印证.但由于一级磁弹相变LaFe_11.7Si_1.3材料相变温度具有更强的磁场依赖性,不具有△S_M的线性相关性,因此,本研究表明,当磁结构相变材料的相变温度具有弱磁场依赖性时,△S_M与μ₀H具有线性相关性.进而,在磁场未达到相变饱和磁场以下,利用△S_M与μ₀H的线性相关性可以有效推测更高磁场下的△S_M.     

磁热效应与磁冰箱

冰箱是日常生活中必不可少的家用电器，原来的普通冰箱大都以氟里昂为工作介质，采用气体压缩式制冷，这种致冷技术不仅工效低，噪声大，而且会排出大量的温室气体，对环境造成难以恢复的危害．能否找到一种更好的替代品呢72001年12月，美国能源部科研人员在艾奥瓦州立大学埃姆斯实验室制成了世界上第一台能在室温下工作的磁冰箱．这是一种利用铁磁性制冷工质进出磁场引起温度变化而在室温范围进行制冷的新方法，无污染、无噪音、使用寿命长，是21世纪最具发展前景的制冷技术．下面简要介绍一下磁制冷冰箱的工作原理．     

非晶合金的磁热效应及磁蓄冷性能

非晶合金的功能物性开发是突破非晶合金应用瓶颈的关键点之一.磁相变是非晶合金的一个重要特征.利用非晶合金的磁相变所带来的独特效应,可以将其应用于制冷领域.一方面非晶合金的磁热效应可以作为磁制冷材料应用于磁制冷机,另一方面非晶合金的比热突变可以作为磁蓄冷材料应用于低温制冷机.本文就非晶合金的磁热效应和磁蓄冷性能的原理、特征及其应用前景进行了详细介绍.     

相变调控、磁热效应和反常热膨胀

相变作为广泛存在于自然界中的一种现象很早就受到了广泛的关注,并且已经被应用于相变制冷、相变存储、相变储能和负热膨胀等领域中.基于磁热、电热和机械热效应不断发展起来的固态制冷技术具有环保、高效、低噪声和易小型化等优点,被视为替代汽压缩制冷的新型制冷技术.其中,磁热效应是研究历史最悠久的一种.然而,单磁场驱动磁热效应的诸多不足限制了其固态制冷应用,如热效应幅度不够高、滞后损耗大、制冷温跨窄等,因此多场调控和多卡效应应运而生.本文主要介绍笔者团队近期开展的多场调控磁热效应、以及磁热材料的反常热膨胀行为的研究.     

巨磁热效应与磁冰箱

 今天,电冰箱即使在我们这样发展中国家的城市中,也已经是很普通的家电,日常生活中缺之不得。无疑,电冰箱给人们带来了很多的便利。但是,电冰箱存在着两大缺点,一是使用压缩机,需要消耗不少的能量;二是其普遍使用氟立昂作为致冷剂,氟立昂(R12)的化学分子式是CCl2F,它是通常称之为氟氯烃(CFCs)的含氯源气体的一种,是破坏大气臭氧层的元凶之一。这也表明,科学技术是一把双刃剑。那么,能不能除去电冰箱这两大缺陷呢?科学技术的发展肯定地回答了这个问题,这就是5年前发现的巨磁热效应以及近一二年发明的室温磁冰箱。     

Giant low-field magnetocaloric effect in EuTi_(1-x)Nb_xO_3(x = 0.05, 0.1, 0.15, and 0.2) compounds

The magnetic properties and magnetocaloric effect(MCE)of EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds are investigated.Owing to electronic doping,parts of Ti ions are replaced by Nb ions,the lattice constant increases and a small number of Ti⁴⁺(3d^0)ions change into Ti³⁺(3d^1).It is the ferromagnetism state that is dominant in the derivative balance.The values of the maximum magnetic entropy change(-?SM^max)are 10.3 J/kg·K,9.6 J/kg·K,13.1 J/kg·K,and 11.9 J/kg·K for EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds and the values of refrigeration capacity are 36,33,86,and 80 J/kg as magnetic field changes in a range of 0 T–1 T.The EuTi(1-x)NbxO3(x=0.05,0.1,0.15,and 0.2)compounds with giant reversible MCE are considered as a good candidate for magnetic refrigerant working at lowtemperature and low-field.     

Giant magnetocaloric effect in Tb5Ge2-xSi2-xMn2x compounds

The crystal structure,magnetic and magnetocaloric characteristics of the pseduo ternary compounds of Tb5Ge2 xSi2 xMn2x(0 ≤ 2x ≤ 0.1) were investigated by x-ray powder diffraction and magnetization measurements.The x-ray powder diffraction results show that all compounds preserve the monoclinic phase as the majority phase and all the synthesized compounds were observed to be ferromagnetic from magnetization measurements.Magnetic phase transitions were interpreted in terms of Landau theory.Maximum isothermal magnetic entropy change value(20.84 J.kg-1.K-1) was found for Tb5Ge1.95Si1.95Mn0.1 at around 123 K in the magnetic field change of 5 T.     

Review of magnetocaloric effect in perovskite-type oxides

We survey the magnetocaloric effect in perovskite-type oxides (including doped ABO 3-type manganese oxides, A3B2O7-type two-layered perovskite oxides, and A2B'B'O6-type ordered double-perovskite oxides). Magnetic entropy changes larger than those of gadolinium can be observed in polycrystalline La1-xCaxMnO3 and alkali-metal (Na or K) doped La0.8Ca0.2MnO3 perovskite-type manganese oxides. The large magnetic entropy change produced by an abrupt reduction of magnetization is attributed to the anomalous thermal expansion at the Curie temperature. Considerable magnetic entropy changes can also be observed in two-layered perovskites La1.6Ca1.4Mn2O7 and La2.5-xK0.5+xMn2O7+δ (0 x 0.5), and double-perovskite Ba2Fe1+xMo1-xO6 (0 ≤ x ≤ 0.3) near their respective Curie temperatures. Compared with rare earth metals and their alloys, the perovskite-type oxides are lower in cost, and they exhibit higher chemical stability and higher electrical resistivity, which together favor lower eddy-current heating. They are potential magnetic refrigerants at high temperatures, especially near room temperature.     

Observation of giant magnetocaloric effect under low magnetic fields in EuTi_(1-x)Co_xO_3

The magnetic properties and magnetocaloric effect(MCE) in EuTi_(1-x)Co_xO_3(x = 0, 0.025, 0.05, 0.075, 0.1) compounds have been investigated. When the Ti~(4+) ions were substituted by Co2+ions, the delicate balance was changed between antiferromagnetic(AFM) and ferromagnetic(FM) phases in the EuTiO_3 compound. In EuTi_(1-x)Co_xO_3 system, a giant reversible MCE and large refrigerant capacity(RC) were observed without hysteresis. The values of -?S_M~(max) were evaluated to be around 10 J·kg~(-1)·K~(-1) for EuTi_(0.95)Co_(0.05)O_3 under a magnetic field change of 10 kOe. The giant reversible MCE and large RC suggests that EuTi_(1-x)Co_xO_3 series could be considered as good candidate materials for low-temperature and low-field magnetic refrigerant.     

Giant magnetocaloric effect in the Gd5Ge2.025Si1.925In0.05 compound

The magnetocaloric properties of the Gd 5 Ge 2.025 Si 1.925 In 0.05 compound have been studied by x-ray diffraction,magnetic and heat capacity measurements.Powder x-ray diffraction measurement shows that the compound has a dominant phase of monoclinic Gd5Ge2Si2-type structure and a small quantity of Gd 5(Ge,Si) 3-type phase at room temperature.At about 270 K,this compound shows a first order phase transition.The isothermal magnetic entropy change(△SM) is calculated from the temperature and magnetic field dependences of the magnetization and the temperature dependence of MCE in terms of adiabatic temperature change(△Tad) is calculated from the isothermal magnetic entropy change and the temperature variation in zero-field heat-capacity data.The maximum S M is 13.6 J·kg-1·K-1 and maximum △Tad is 13 K for the magnetic field change of 0-5 T.The Debye temperature(θD) of this compound is 149 K and the value of DOS at the Fermi level is 1.6 states/eV·atom from the low temperature zero-field heat-capacity data.A considerable isothermal magnetic entropy change and adiabatic temperature change under a field change of 0-5 T jointly make the Gd5Ge2.025Si1.925 In 0.05 compound an attractive candidate for a magnetic refrigerant.     

Table-like magnetocaloric effect and enhanced refrigerant capacity in crystalline Gd55Co35Mn10 alloy melt spun ribbons

Gd₅₅Co₃₅Mn₁₀ ribbons were prepared by melt-spinning and subsequent crystallization treatment. Crystallization resulted in the precipitation of the Gd₃Co-type and Gd₁₂Co₇-type phases in the amorphous matrix. Under a magnetic field change of 0–5 T, a table-like magnetocaloric effect, with a maximum magnetic entropy change ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ of $5.46\text{J}{\text{kg}}^{?1}{\text{K}}^{?1}$ in the temperature range of 137–180 K and enhanced refrigerant capacity (RC) of $536.4\text{J}{\text{kg}}^{?1}$, was achieved in Gd₅₅Co₃₅Mn₁₀ ribbons crystallized at 600 K for 30 min. The table-like ${(?\mathrm{\Delta }{S}_{\mathrm{M}})}^{\max }$ feature and enhanced RC values make Gd₅₅Co₃₅Mn₁₀ crystallized ribbons promising for Ericsson-cycle magnetic refrigeration in the temperature range from 137 to 180 K.     

Hydrogenation, structure and magnetic properties of La(Fe0.91Si0.09)13 hydrides and deuterides

Hydrogenation, crystal structure and magnetic properties of La(Fe0.91Si0.09)13H(D)y have been studied by pressure-composition isotherms (PCI), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and magnetization measurements. The maximum absorption capacity is found to be 1.9 H(D) atoms per formula unit as a solid solution. All hydrides and deuterides crystallize in the NaZn13-type cubic structure with the lattice parameter increasing linearly with H(D) concentration. The H(D) absorption enhances the Curie temperature significantly. The magnetic entropy change of the highly H-absorbed compound La(Fe0.91Si0.09)13H1.81 reaches ～26 J/kg·K under a magnetic field change of 5 T near the Curie temperature TC = 350 K. No observable isotope effect seems to imply that only the magnetovolume effect is responsible for the strong interplay between magnetism and lattice.     

自制霍尔效应演示仪

利用霍尔效应原理制作了霍尔效应演示仪,详细介绍了该演示仪的制作过程,定性地演示了霍尔电压与通电电流和磁感应强度的关系.