Contactless thermal diffusivity characterization of second order magnetocaloric materials under magnetic field using modulated photo-thermal radiometry method with very low power probe beam
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引用次数: 0
Abstract
The thermal diffusivity of magnetocaloric materials has a transition point at a given temperature that depends on the intensity of the applied magnetic field. Consequently, a fine temperature resolution on the material sample is needed to obtain an accurate determination of the thermal diffusivity variation with temperature. The coupling between the external and the internal fields has to be carefully mastered since the pertinent operating condition is fixed by the actual internal field which is not directly measurable and may be heavily affected by any element in contact with the sample. Therefore, contactless methods such as Photo-Thermal Radiometry (PTR) are privileged. The latter is based on a radiative excitation of the front face of a thin sample and the detection of the thermal effect on the opposite face. However, the powerful radiative source may significantly increase the sample temperature which is not suitable for caloric materials. In this work, a low power modulated PTR method is proposed to characterize second order magnetocaloric materials under magnetic field. It was compared to high energy thermo-flash PTR and validated on common materials such as steel and stainless steel, and then applied to gadolinium which is the reference magnetocaloric material for magnetic refrigeration and heat pumping study. The thermal diffusivity of gadolinium samples is measured in the 285.1 K to 305.1 K temperature range, including the magnetic transition temperature without and under an external magnetic flux density of 0.5 T in the 13 mm air gap of the permanent magnet magnetic circuit. The low power probe beam ensures a temperature stability with a negligible sample temperature fluctuation less than 0.05 K on the incident sample surface and less than 0.03 K on the measurement surface. The experimental results without magnetic field align with those using other methods including the magnetic transition temperatures determination. This low-power optical method proved its efficiency to characterize highly temperature dependent materials such as magnetocaloric materials sensitive to magnetic field. The data obtained partly fills the lack of information in the literature on excited gadolinium.
磁致性材料的热扩散率在给定温度下有一个过渡点,该点取决于外加磁场的强度。因此,要准确测定热扩散率随温度的变化,需要对材料样品进行精细的温度分辨。外部磁场和内部磁场之间的耦合必须仔细掌握,因为相关的工作条件是由实际内部磁场决定的,而实际内部磁场是无法直接测量的,并且可能会受到与样品接触的任何元素的严重影响。因此,光热辐射测量法(PTR)等非接触式方法就显得尤为重要。后者基于对薄样品正面的辐射激发和对反面热效应的检测。然而,强大的辐射源可能会显著提高样品的温度,这并不适合热量材料。本研究提出了一种低功率调制 PTR 方法,用于表征磁场下的二阶磁致热材料。该方法与高能量热闪烁 PTR 进行了比较,并在钢和不锈钢等常见材料上进行了验证,然后应用于磁制冷和热泵研究的参考磁致性材料钆。钆样品的热扩散率是在 285.1 K 至 305.1 K 的温度范围内测量的,包括永磁磁路 13 mm 气隙中无外部磁通密度 0.5 T 和外部磁通密度 0.5 T 时的磁转变温度。低功率探针光束确保了温度稳定性,入射样品表面的样品温度波动小于 0.05 K,测量表面的样品温度波动小于 0.03 K。无磁场实验结果与使用其他方法(包括磁转变温度测定)得出的结果一致。这种低功率光学方法证明了它在表征高温度依赖性材料(如对磁场敏感的磁致性材料)方面的效率。所获得的数据部分填补了激发钆文献资料的空白。
期刊介绍:
Experimental Thermal and Fluid Science provides a forum for research emphasizing experimental work that enhances fundamental understanding of heat transfer, thermodynamics, and fluid mechanics. In addition to the principal areas of research, the journal covers research results in related fields, including combined heat and mass transfer, flows with phase transition, micro- and nano-scale systems, multiphase flow, combustion, radiative transfer, porous media, cryogenics, turbulence, and novel experimental techniques.