Departamento de Física Aplicada, Escuela de Ingeniería de Bilbao, Universidad del País Vasco UPV/EHU, Plaza Torres Quevedo 1, 48013 Bilbao, Spain.
Department of Chemistry, Moscow State University, Lenisnkie Gory, House 1, Building 3, Moscow, GSP-2, 119991, Russia.
Dalton Trans. 2023 May 2;52(17):5780-5797. doi: 10.1039/d3dt00223c.
A complete experimental study of the physical properties playing a relevant role in the magnetic refrigeration application (structural, magnetic, magnetocaloric and thermal) has been performed over nine selected FeP-type RTX (R = Gd, Tb, Dy; T = Mn, Fe, Co, Ni; X = Sb, Te) intermetallic compounds, to work close to room temperature. Two magnetic phase transitions are observed for these materials: a paramagnetic to ferromagnetic transition in the range of 182-282 K and a spin reorientation transition in the range of 26-76 K. As a consequence, two peaks related to a direct magnetocaloric effect (DMCE) appear with the magnetic entropy change, generating a wide table-like plateau region in between both peaks, which is required to improve the efficiency of refrigerators following an Ericsson cycle. The highest magnetic entropy peak value for Δ = 5 T is found for TbDyFeSb, with 7.72 J kg K around 182 K. For the same applied field the other compounds show moderate values around room temperature (2.88-4.53 J kg K). However, the superposition of the two peaks results in huge refrigerant capacity values, up to RC(5 T) = 1103.04 J kg in the case of TbDyFeSb. The thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity have been measured at room temperature, and the temperature dependence of the former has been obtained around the relevant magnetic phase transition region, with values in the range of 1.3-2.3 mm s, which are good for magnetic refrigerators at high working frequencies. The study is completed with a rigorous critical behavior analysis of the second order PM-FM transition. The critical exponent points to long range order interactions, in general, while values are in the range of 0.59-0.90, indicating a deviation from theoretical models as a reflection of the magnetic complexity in these compounds. The critical exponents have been used to confirm the scaling relations of magnetocaloric properties, and the scaling of refrigerant capacity (RC) values in materials exhibiting two magnetic phase transitions is addressed, concluding that for a correct scaling of RC the magnetic entropy change peak must be considered symmetric. The role of each atom in the properties of the compounds is discussed.
已经对九种选定的 FeP 型 RTX（R = Gd、Tb、Dy；T = Mn、Fe、Co、Ni；X = Sb、Te）金属间化合物进行了完整的物理性质实验研究，这些性质在磁制冷应用中具有重要作用（结构、磁性、磁热和热），研究工作接近室温。这些材料观察到两个磁相变：在 182-282 K 范围内的顺磁到铁磁转变和在 26-76 K 范围内的自旋重定向转变。因此，磁熵变化出现两个与直接磁热效应（DMCE）相关的峰值，在两个峰值之间产生一个宽的类似平板的平台区域，这是提高遵循埃里克森循环的制冷机效率所必需的。在 Δ = 5 T 下，TbDyFeSb 的最大磁熵峰值为 7.72 J kg K，在 182 K 左右。对于相同的外加磁场，其他化合物在室温附近显示出中等值（2.88-4.53 J kg K）。然而，两个峰值的叠加导致制冷剂容量值非常大，在 TbDyFeSb 的情况下，RC（5 T）高达 1103.04 J kg。在室温下测量了热扩散率、热扩散率、热导率和比热容，并在相关磁相变区域周围获得了前者的温度依赖性，其值在 1.3-2.3 mm s 范围内，这对于高工作频率的磁制冷器来说是很好的。该研究通过对二阶 PM-FM 转变的严格临界行为分析得到了补充。临界指数 表示长程有序相互作用，一般而言，而 值在 0.59-0.90 范围内，表明偏离理论模型，反映了这些化合物中磁复杂性。临界指数已用于确认磁热性质的标度关系，并且讨论了在表现出两个磁相变的材料中制冷剂容量（RC）值的标度，得出的结论是，为了正确标度 RC，必须考虑磁熵变化峰值的对称性。讨论了每个原子在化合物性质中的作用。
