几种磁制冷材料的室温磁热效应

用高频悬浮炉(冷坩埚)熔炼了Gd5Si2Ge2、Gd4Sb1.5Bi1.5、Gd3Al2、Mn5Ge3等金属化合物.在磁场H=1.3T下,用XHY磁热效应测量仪直接测量它们的绝热温变△Tad,得到了这几种材料的居里点,并和金属Gd进行了比较.     

Gd5Si2Ge2巨磁熵变合金的粒度效应研究

GdSiGe系合金的发现为磁致冷的常温范围实用化发展提供了可能,但目前其成型方面还有一定的问题.根据合金成型特点,本文研究了磁制冷材料Gd5Si2Ge2合金的颗粒尺寸对其磁热效应的影响,从而为磁制冷材料的加工成型和粒度范围提供了实验依据.结果表明,Gd5Si2Ge2合金的磁热效应在微米级随着粒度的增大而减小.     

放电等离子体烧结（SPS）制备Mn1.2Fe0.8P0.75Ge0.25化合物及其磁热效应研究

采用机械合金化结合放电等离子体烧结技术,成功制备了Mn1.2Fe0.8P0.75Ge0.25室温磁制冷材料。采用XRD、VSM对烧结样品晶体结构和磁热效应（MCE）进行了研究。结果表明该化合物具有六方Fe2P型晶体结构,其热滞为4K,居里温度为292K,并且在居里点附近有较大的磁熵变,当外加磁场为1.5T时,最大磁熵变达到18.0J/（kg.K）,绝热温变达到2.7K。     

Gd2Co2Al合金的磁相变及磁热效应

电弧炉熔炼的Gd2Co2Al合金在铸态条件下即为W2Co2B型单相正交结构。变温磁化曲线表明,当外加磁场为0.01和0.1T时,可以在40、77和215K附近观察到磁相变;而外加磁场增加到1T以上时,40和215K温度处的磁相变消失。在排除第二相相变的前提下,推测215和77K处的相变对应Gd2Co2Al合金中Co和Gd次晶格的磁有序相变,而40K处的相变可能是由于自旋重取向产生。在0～5T磁场变化下,Gd2Co2Al合金在77K附近的最大磁熵变(-ΔSMmax)为10.7J/kgK,相对制冷量的值为5.4×102J/kg,表明该合金适合作为工作在液氮温区附近的磁致冷工质。     

Si对LaFe_(12.3-x)Co_(0.7)Si_xB_(0.25)系列合金的磁热效应影响

通过对LaFe_(12.3-x)Co_(0.7)Si_xB_(0.25)(x=1.1,1.2,1.3,1.4,1.5,1.6)合金的磁热效应的研究,发现随着合金元素Si含量的增加可以提高材料的居里温度,但会使材料的绝热温变值有所下降。针对绝热温变值最大的LaFe_(12.3-x)Co_(0.7)Si_xB_(0.25)合金进行进一步研究,发现该合金能够促进主相(NaZn13相)的生成和扩散反应进行。且该合金的最大磁熵变为7.3 J/kg·K,是工业纯Gd的2.2倍,具有一定的研究价值。     

低磁场条件下Gd5（Si1-xGex）4（x=0，0.2，0.3）合金磁热效应研究

用高频真空悬浮炉在氩气保护下制备了Gd5（Si1-xGex）4（x=0，0.2，0.3）系列样品，用直接测量方法测定了在永磁体提供的低磁场H=1.3T）条件下样品的磁热效应曲线（△Tad-T）．结果表明，用元素Ge分别替代Gd5Si4合金中的Si原子，不能增强合金的磁热效应，但可在333～300K范围改变材料的居里温度，且磁热效应峰值范围变宽．     

Gd5SixSn4-x合金的结构与磁热性能

利用XRD和振动样品磁强计研究了Gd5SixSn4-x(x=2.4、2.6、2.8)合金的结构与磁热性能.结果表明,在x=2.8时合金具有Gd5Si4形正交结构,x=2.4和2.6时则为Gd5Si4和Gd5Si2Gi2型两种结构,此外均存在少量其它杂相.随着Si含量的增大,居里温度分别为276、290.5和301.5K,逐渐升高,1.8T外加磁场变化时的磁熵变分别为1.88、2.26和1.69J/(kg·K),x=2.6时最大.低温XRD分析表明合金发生磁相变的同时没有结构相变发生.     

LaFe11.17-xCo0.78Si1.05Bx合金磁热效应的研究

用工业纯原料研究了LaFe11.17-xCo0.78Si1.05Bx(x=0、0.1、0.2、0.3、0.4、0.5)合金在室温附近的磁热效应.B元素作为间隙原子进入晶胞,随着B含量的增加,磁相变点从x=0时的277K升高到x=0.3时的289K.同时,磁熵变和直接测量的绝热温变也随着B含量的增加而增大了,在低磁场具有较大的磁热效应.测量表明合金的热滞和磁滞均很小.     

低磁场条件下（Gd1-xREx）5Si4（RE：Dy,Tb；x=0、0.1、0.125、0.15）合金磁热效应研究术

用高频真空悬浮炉在氲气保护下制备了（Gd1-xREx）5Si4（RE：Dy,Tb；x=0、0.1、0.125、0.15）和（Gd1-xREx）5Si4（x=0.1、0.125）合金样品，用直接测量方法测定了在永磁体提供的低磁场（H=1．3T）条件下样品的磁热效应曲线（△Tab-T）；结果表明，用稀土金属Dy、Tb分别部分替代Gd5Si4合金中的Gd原子，不能增强合金的磁热效应，但可在333～300K范围改变材料的居里温度，且磁热效应峰值范围变宽．     

快淬低纯Gd5Si2Ge2合金的结构和磁熵变

采用纯度为99%的低纯稀土金属Gd经电弧炉熔炼获得Gd5Si2Ge2 母合金,然后通过快淬研究快淬工艺对低纯Gd5Si2Ge2 合金结构与磁熵变的影响。粉末衍射结构分析表明,铸态Gd5Si2Ge2 合金由Gd5Si2Ge2 相、Gd5 (Si、Ge)3 相和Gd(Si、Ge)相所组成;而甩带速度为25、40m/s的合金均为Gd5Si4 型正交相。用最小二乘法计算得两种不同快淬速度合金的晶格常数没有明显差异。甩带速度为40m/s 的Gd5Si2Ge2 合金的居里温度(Tc=305K)在室温区间,等温磁熵变为7.25J/kg·K(0~5T)。     

Estimation on Magnetic Refrigeration Material （Gd1-xREx）5Si4 （RE=Dy, Ho）

A systematic (Gd1-xREx)5Si4 (RE=Dy, Ho) alloys are investigated to estimate their magnetocaloric effect.The Curie points of (Gd1-xREx)Si4 alloys can tunable from 266 K to 336 K when RE=Dy, Ho; x=0～0.35 and 0～0.15, respectively, and decrease nearly linearly with increasing x. These alloys keep orthorhombic structures Ge5Sm4 and exhibit second order transition when they experience in a change magnetic field at about Curie when magnetic field changes 0～2 T. The adiabatic temperatures changes (△Tad) of these alloys at Curie points are larger than 1 K in a field change 0～1.4 T, the curve of △Tad is wide as that of Gd. The relative cooling power is about 0.8～0.9 J/cm3 when field changes 0～2 T, 55% of that of Gd. Comparing with Gd5(Si1-xGex)4, these alloys do not contain expensive element Ge, so that their cost are lower than the former. Because they could work at temperature region 260～340 K due to their Curie points can be tuned, which is an advantage comparing with Gd, these alloys are potential magnetic refrigerants working in a magnetic refrigerator with a low magnetic field at room temperatures.     

Direct Measurements of Magneto-caloric Effect of Gd5Si2Ge2 Alloys in Low Magnetic Field

Direct measurements of the magneto-caloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated by measuring adiabatic temperature change (ΔT _ad) in a magnetic field of 1.5 T using a homemade magnetocaloric effect measuring apparatus. The crystal structure, microstructure as well as the chemical composition of the alloys are measured using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersive X-ray Detector (EDX). It is found that the microstructure of the alloys could be fully homogenized and the impurities in the alloys could be remarkably removed via appropriate heat treatment. As a result, the maximum adiabatic temperature change (ΔT _ad) of the alloy annealed at 1573 K increases by 105% from 1.9 to 3.9 K for a magnetic field change from 0 to 15 kOe when compared to the as arc-melted material, while the magnetic ordering temperature slightly reduced.     

Magnetocaloric effect in Gd(1−y)DyyAl2

In this work we calculate the magnetic properties and the magnetocaloric effect in Gd_1−yDy_yAl₂ (y = 0, 0.25, 0.5, 0.75 and 1). The model Hamiltonian includes contributions from the Zeeman effect, the crystalline electrical field anisotropy and the exchange interactions among Gd–Gd, Gd–Dy and Dy–Dy ions. To obtain a composite with ΔS_T as constant as possible in the temperature range from T = 60 K to 170 K, the appropriate concentrations of the five compounds investigated were calculated using the Smaili and Chahine method. The magnetization and ΔS_T dependences on temperature in the composite were simulated and compared with the partial contributions of the single magnetic component materials. Also, the magnetic field dependence on magnetization was investigated in Gd_0.25Dy_0.75Al₂, where the discontinuous spin reorientation transitions were predicted for magnetic fields lower than 2 T, applied along <110> direction.     

A comprehensive two-dimensional numerical study on unsteady conjugate heat transfer in magnetic refrigerator with Gd plates

In the present work, the comprehensive two-dimensional numerical model is constructed with ANSYS Fluent for the unsteady conjugate heat transfer of magnetic refrigerator with Gd plates. To express the reciprocating piston movements and magneto-caloric effect, etc., several user-defined functions are compiled. With current model, the reciprocating magnetic refrigerator in the literature is simulated under room temperature conditions and its refrigeration mechanism is analyzed. Besides, the effect of cycling period (τ) on maximal temperature span (ΔT_max) is numerically investigated and different channel widths (w) are adopted for the effects of heat transfer enhancement. Computation results indicate that current comprehensive model could match well with public experimental data and the ΔT_max is effectively improved with smaller w at the cost of substantial increase of pressure difference. Moreover, the ΔT_max generated by narrower channels varies convexly with the τ, and the peak ΔT_max increases with a decreasing w.     

高能球磨Gd5Si1.8Ge1.8Si0.4合金的磁性能及相组成

采用电弧熔炼的方法制备了具有一级磁晶相变Gd5Si1.8Ge1.8Si0.4母合金,然后在不同时间条件下对其进行高能球磨,并研究球磨时间对该磁热合金的磁性能及相组成的影响.粉末XRD结果显示球磨没有改变Gd5Si2Ge2相,但拓宽了相应的布拉格衍射峰,这表明球磨后的合金的晶粒尺寸较小.另外,球磨后的合金中形成了一些非晶相.随着晶粒尺寸和磁畴的减小,与母合金相比,希望其滞后现象减小.     

The structure and magnetocaloric effect of rapidly quenched Gd5Si2Ge2 alloy with low-purity gadolinium

A magnetic refrigerating material Gd₅Si₂Ge₂ was prepared by arc-melting method under argon atmosphere with low-purity commercial gadolinium. The structure and magnetocaloric effect (MCE) of the as-cast and rapidly quenched Gd₅Si₂Ge₂ alloys were investigated by means of X-ray diffraction and magnetic measurements. The as-cast Gd₅Si₂Ge₂ consists of Gd₅Si₂Ge₂-type, Gd₅Si₄-type, Gd₅(Si,Ge)₃ and Gd(Si,Ge) phases from powder XRD results. The alloys quenched at a velocity of 25 m/s or 40 m/s both adopt in a single phase with orthorhombic Gd₅Si₄-type structure. The lattice parameters under different quenched velocities calculated by least-squares method have no significant difference. After being rapidly quenched at 40 m/s, the Curie temperature of Gd₅Si₂Ge₂ is about 303 K and its maximum magnetic entropy change is 7.25 J/kg K (0-5 T).     

Gd2(Co,Si)17化合物的结构与磁晶各向异性

采用真空电弧熔炼法制备了Ｇｄ２Ｃｏ１７－ｘＳｉｘ化合物。通过Ｘ－射线衍射和磁性测量手段,研究了化合物的晶体结构和内禀磁性。首次在Ｇｄ２Ｃｏ１７－ｘＳｉ（ｘ〉０）化合物中观察到自旋重取向转变,获得了Ｃｄ２Ｃｏ１７－ｘＳｉｘ的磁相图。     

(La0.52Gd0.15)Sr0.33MnO3多晶颗粒的磁热效应

利用非晶态分子合金作前驱体,在相对低的热分解温度800℃、10h成功合成了钙钛矿结构(La0.52Gd0.15)Sr0.33MnO3多晶颗粒.TEM观察表明,颗粒的尺寸范围为100～150nm.研究了多晶颗粒(La0.52Gd0.15)Sr0.33MnO3的居里温度和磁熵变化(MCE).在多晶颗粒(La0.52Gd0.15)Sr0.33MnO3中,居里温度(343K)附近观察到较大的磁熵变和较宽的峰值温度范围,较大的磁熵变来源于磁场条件下的铁磁转变贡献.这些结果表明,该材料是室温附近磁制冷合适的工作物质.     

Anisotropic and excellent magnetocaloric properties of La0.7Ca0.3MnO3 single crystal with anomalous magnetization

Magnetic properties and the magnetocaloric effect (MCE) have been investigated in La_0.7Ca_0.3MnO₃ single crystal with applied field along both the ab-plane and the c-direction. Due to the magnetocrystalline anisotropy, the crystal exhibits anisotropy in the MCE. Upon application of a 5 T field, the magnetic entropy changes (ΔS_M), reaching values of 7.668 J/(kg K) and 6.412 J/(kg K) for both the ab-plane and the c-direction, respectively. A magnetic entropy change of 3.3 J/(kg K) was achieved for a magnetic field change of 1.5 T at the Curie temperature, T_C = 245 K. Due to the absence of grains in the single crystal, the ΔS_M distribution here is much more uniform than for gadolinium (Gd) and other polycrystalline manganites, which is desirable for an Ericsson-cycle magnetic refrigerator. For a field change of 5 T, the relative cooling power, RCP, reached 358.17 J/kg, while the maximum adiabatic temperature change of 5.33 K and a magnetoresistance (MR) ratio of 507.88% at T_C were observed. We analysed the magnetization of La_0.7Ca_0.3MnO₃ single crystal at T_C and estimated several parameters of spin fluctuation on the basis of a self-consistent renormalization theory of spin fluctuations, with reciprocal susceptibility above T_C. We found that the magnetic property of La_0.7Ca_0.3MnO₃ is weakly itinerant ferromagnetic. A large reversible MCE and no hysteresis loss with a considerable value of refrigerant capacity indicate that La_0.7Ca_0.3MnO₃ single crystal is a potential candidate as a magnetic refrigerant.